Design of Foils

The International Hydrofoil Society (IHS) Hydrofoil Correspondence Archives

Updated last August 20, 2006

Design of Foils: Foil-Struts-Controls-Performance

Design of Foils: Foil-Struts-Controls-Performance      Top


“1”,”950336″,”2″,”Re; Re; Revolutionizing a watersport||950336″,”Good response, Tom. When you say ” it’s much better to specify the pressure distribution and then calculate the section shape that will produce it. Would you be suggesting to use Xfoil to do the calculation?”,”2005-12-20″,”Ray Vellinga”,”nopswd”,” “,””,”949055″

“2”,”949055″,”2″,”Re; Re; Revolutionizing a watersport||949055″,”I don’t think the radius+ogive middle is a very good way to go. That was the philosophy behind my Proa 1-series.

The sudden change in curvature at the junction between the ogive and radius caused a sharp pressure spike:

This, in turn, led to laminar separation, premature stall from the leading edge, and increased drag. Separation near the leading edge is especially bad for a hydrofoil, because it leads to ventilation and the sudden loss of three-quarers of the lift.

So, while ogive sections may be easy to construct, I’m not enamored with their hydrodynamics.

Today, it’s much better to specify the pressure distribution and then calculate the section shape that will produce it. That way you can see what needs to be fixed in the hydrodynamics and go after it directly instead of shooting in the dark by modifying the geometry. “,”2005-12-17″,”Tom Speer”,”nopswd”,” “,” “,”946625”

“3”,”946625″,”2″,”Re; Re; Revolutionizing a watersport||946625″,”Tom Speer, any discussion of ogival hydrofoils sections is of interest to me. I have made and flown several such foils. They are easy to construct by welding a rolled piece of metal plate to a flat metal plate and then grinding to make the welds fair.

You have mentioned the idea of adding a radius to the leading edge of the ogival foil. This could be done by welding the forward edges of the top and bottom plates to a rounded section–a tube or a bar. Two questions: Do you have some guidelines on choosing a radius to the rounded leading edge? And, is the junction between the rounded edge and the top plate and the bottom plate a big problem? Would you think this to be a difficult transition?”,”2005-12-12″,”Ray Vellinga”,”nopswd”,” “,””,”945783″

“4”,”945783″,”2″,”Re; Revolutionizing a watersport||945783″,”Yes, it’s possible to design fore-aft symmetric foils that will work equally well in both directions. You basically have two possible approaches: sharp-edged, and rounded edges.

Examples of the sharp-edged foils are the ogival that have been used by may hydrofoil designers. They have the advantage of being simple to construct and have low drag within their design range of angles of attack. The problem with sharp leading edges is they only have a small range of angles of attack at which the flow is attached. Outside that range, they experience leading edge separation. This can lead to sudden ventilation – a charactersitic that has bedeviled many craft that use these sections. You can find section data for ogival sections published in the literature and in books like Hoern’er’s “Fluid Dynamic Drag”.

The round-edged approach promotes leading edge suction for low drag and does not necessarily suffer from leading edge stall. There will be a separated zone at the trailing edge which can cause some increased drag. Elliptical sections have been used for some stopped-rotor VTOL aircraft.

To the best of my knowledge, the only round-edged sections specifically designed for use as hydrofoils can be found at XFOIL predicts the Proa-3 series sections have performance comparable to NACA 6-series sections.

“,”2005-12-10″,”Tom Speer”,”nopswd”,” “,” “,”935018”

“5”,”942165″,”2″,”Foil works in forward or reverse direction||942165″,”You haven’t said specifically which watersport you want to revolutionize, but I’m pretty sure I know. I won’t say it outright here because you seem to be concerned with someone stealing your idea. I have had discussions with others wanting to do the same thing, and have evaluated some of the pitfalls. If you drop me a line at, I’ll send you my contact info and we can talk, I’m in Stuart Florida. I may not have the ultimate answer for you, but I think I can help.”,”2005-12-03″,”Scott Smith”,”nopswd”,” “,””,”941596″

“6”,”941596″,”2″,”Foil works in forward or reverse direction||941596″,”Nat,
I appreciate your response, but have no idea what you said. I’m not familiar with a “soft” foil. I have so many questions, I feel I’m just going to be more of a burden than anything on here. But again, I am willing to pay someone for their time. Also, maybe there is somewhere I can go for more info, I have tapped the internet for all its worth.

Thanks to all,
Derek”,”2005-12-02″,”Derek”,”nopswd”,” “,””,”941408″


“8”,”940695″,”2″,”Foil works in forward or reverse direction||940695″,”Thank you all for your help. Unfortunately, I am still unsure if the design I have in mind is possible. There are many variables that are not taken into consideration with hydrofoil boat designs that I have to think about. For example, instead of proplusion, this board will be towed, and the rider of the board will be able to manipulate the board in ways we could not with a boat. If there is anyone that would be willing to give me a little more in depth advice, possibly over the phone, or in person (I live in Orlando), I would be more than willing to pay for your time. I need to first determine whether it would be physically possible to do what I want, and then if it is, I would have to explain some of the problems that might arise that are not addressed with any other hydrofoil. If all goes well, I would like to make a few prototypes, and start a company that would revolutionize the fastest growing watersport.

Thanks for everyone’s time,
Derek Seaman
407-739-1827″,”2005-12-01″,”Derek”,”nopswd”,” “,””,”938319″

“9”,”938319″,”2″,”Foil works in forward or reverse direction||938319″,”A complete copy of this 57 page report is in my hands. Today I offered it to Barney Black to be posted on the IHS site. If he accepts, you can download it from there soon.

I have used the Ogival sections with some success. They are useful because they can be built easily using sheet metal, a welder and a metal grinder.”,”2005-11-26″,”Ray Vellinga”,”nopswd”,” “,” “,”935301”

“10”,”938089″,”2″,”Re; Hydro foil designs||938089″,”Go to a good technical library and take out a copy of “Theory of Wing Sections” by Ira Abbott and Von Doenhoff, by Dover Publications, Inc., NY c 1959.

All the airfoil sections described there will work as foils. The charts shown for lift and drag coefficients will be accurate for air or water. Just remember that water is 800 times more dense that air so the resulting speed, lift, drag, etc. will differ accordingly. “,”2005-11-26″,”Ray Vellinga”,”nopswd”,” “,” “,”935322”

“11”,”935334″,”2″,”Foil drag, size vs. angle of attack||935334″,”Ray, you seem to know what you are talking, about please look at my posting and see if you have any input.
Dan Bush”,”2005-11-20″,”Dan Bush”,”nopswd”,” “,””,”918835″

“12”,”935322″,”2″,”Hydro foil designs||935322″,”I have a 1973 Carri Craft Catarmaran. Full displacement hulls. Lenght 57″, beam 12′.If I did the calculations correctly theoritcal hull speed is close to 20 knots. I am not willing to repower or pay the fuel penalty for this speed. I should mention I have lived on /traveled in this boat for three years and the following speeds and fuel economies are from more than 1000 hours of travel,deterimined by gps. While in drydock two years ago I added three fins/stabilizers on each hull, 8″ wide and 8′ long. This solved the problem of excessive roll at anchor or docked. When I added these fins I expected to lose a knot or more due to extra drag. Much to my surprise and pleasure I actually picked up a knot in speed. Boat weighs approximately 38,000 lbs empty, has twin Isuzu 150 horse diesels, and the best speed I have gotten out of her to date has been a little over 12 knots at 2400 rpm slinging 20×20 four bladed nibrile props. I have solved an over heating problem and can now go to a continous 2700 rpm. Fuel effiency at 9 knots is(I am not a liar, normally I tell people three knots per gallon) 4 knots per gallon at approximately 50,000 lbs gross weight. Currently I am in dry dock and it occurred to me that by reshaping my stabilizers as hydro foils I could gain more speed. I need foil designs. It seems that the strenght of my stabilizers is sufficient to support the weight of the boat. Idiots ran STRAPS over the fins and lifted my boat with no damage.They moved it while I was not present from one place to another in the yard. The front fin is canted upwards three inches out of level which I suspect is the reason for my speed gain. I currently have helicopter foil designs which I may expand out to eight feet and install. It seems to me with the front fin pitched 3% higher then the middle and last foil like Burt Rutans designs the level of the boat should be limited by stalling of the front first. I am seriously contemplating adding trim control but before I do this I would like to add hydrofoils and play with it for six months.
I should add that I am a fully competent welder, actually “enjoy” fiberglass work, can wander around a machine shop and identify a tool or two. Feed back on hydro foil shapes would be greatly appreciated. This is not “pie-in the sky”, I will be modifying my stabilizers in the next few weeks. Your help will be greatly appreciated and is worth a wild time out fishing, or traveling to some exoctic port to get into exoctic trouble.”,”2005-11-20″,”Dan Bush”,”nopswd”,” “,””,”2″

“13”,”935307″,”2″,”Foil works in forward or reverse direction||935307″,”I have a photocopy of a few pages of Report No. E-79-6 “WATER TUNNEL OBSERVATIONS ON THE FLOW PAST A PLANO-CONVEX HYDROFOIL”, by R.B. Wade, February 1964, Division of Engineering and Applied Science, CALIFORNIA INSTITUTE OF TECHNOLOGY, Pasadena, California. On the cover page, it also says “Office of Naval Research Department of the Navy Contract Nonr-220(24)”, and “D.J.Nigg” in handwriting. I forget where I got it, maybe from Donald Nigg himself. Is he still making foils?

Anyways, the paper gives lift & drag data for a foil with an “ogive” section. That means straight line on the bottom, circular arc on the top. The model used for testing is 0.19″ thick, with a chord length of 2.77″. At zero degrees angle of attack CL is 0.4 and CD is 0.013. This would be the same forward or reverse.

Maybe someone with access to the whole report could get it posted on the IHS website. As a last resort, I could scan what I have, but it’s incomplete. Not sure about the copyright issues here.”,”2005-11-20″,”Mac Stevens”,”nopswd”,” “,””,”935018″

“14”,”935301″,”2″,”Foil works in forward or reverse direction||935301″,”I have a photocopy of a few pages of Report No. E-79-6 “WATER TUNNEL OBSERVATIONS ON THE FLOW PAST A PLANO-CONVEX HYDROFOIL”, by R.B. Wade, February 1964, Division of Engineering and Applied Science, CALIFORNIA INSTITUTE OF TECHNOLOGY, Pasadena, California. On the cover page, it also says “Office of Naval Research Department of the Navy Contract Nonr-220(24)”, and “D.J.Nigg” in handwriting. I forget where I got it, maybe from Donald Nigg himself. Is he still making foils?

Anyways, the paper gives lift & drag data for a foil with an “ogive” section. That means straight line on the bottom, circular arc on the top. The model used for testing is 0.19″ thick, with a chord length of 2.77″. At zero degrees angle of attack CL is 0.4 and CD is 0.013. This would be the same forward or reverse.

Maybe someone with access to the whole report could get it posted on the IHS website. As a last resort, I could scan what I have, but it’s incomplete. Not sure about the copyright issues here.”,”2005-11-20″,”Mac Stevens”,”nopswd”,” “,””,”935018″

“15”,”935018″,”2″,”Revolutionizing a watersport||935018″,”I think the use of foils may change the watersport I love. Unfortunately, I cannot seem to find the information I need to make a basic hypothesis on the design. Every hydrofoil I have seen is based upon moving in one direction (boats don’t reverse at high speeds). Is it possible to have a hydrofoil design that allows movement in opposite directions and will perform well either way? If you could imagine a symmetrical jet propelled boat, so that it could go backwards or forwards either way. Any help would be appreciated.”,”2005-11-19″,”Derek”,”nopswd”,” “,””,”2″

“16”,”931880″,”2″,”Re; Assistance wanted – foil design||931880″,”Dear Bob,

Please give me a call or send me your phone number and email contact.
You can call me on (203) 313 4061.

My company, Hydrofoil Assisted Water Craft HAWC Technologies was recently formed.

We work to help people like you, and believe we will have a solution for you. We need to assess your vessel’s basic information first in order to do a speed prediction based upon the vessel’s length, displacement weight and power amongst some other info.

Looking forward to talking with you.

Gerhard “,”2005-11-14″,”Gerhard Kutt”,”nopswd”,” “,””,”926828″

“17”,”926832″,”2″,”Assistance wanted – Foil design||926832″,”We have a 24 meter commercial Catamaran with a cruising speed of approx. 25 knots with full load. We plan to retrofit the vessel with “aasisting” foils.
We are looking for an hydrofoil expert that can assist in the design and technical specification of “assisting” foils that will be placed between the hulls of the Cat. The goal is to reach a cruising speed of approx 27-28 knots and with a lower the fuel consumption than today. (if possible)!

Best regards, Bob Email:”,”2005-11-05″,”Bob”,”swedbob”,” “,””,”2″

“18”,”926828″,”2″,”Assistance wanted – foil design||926828″,”We have a 24 meter commercial Catamaran with a cruising speed of approx. 25 knots with full load. We plan to retrofit the vessel with “aasisting” foils.
We are looking for an hydrofoil expert that can assist in the design and technical specification of “assisting” foils that will be placed between the hulls of the Cat. The goal is to be able to reach a cruising speed of approx 27-28 knots and lower the fuel consumption.

Best regards, Bob”,”2005-11-05″,”Bob”,”swedbob”,” “,””,”2″

“19”,”925912″,”2″,”Foil drag, size vs. angle of attack||925912″,”I appreciate the feedback, but it wasn’t really what I was asking. I’m not trying to determine the optimum foil size or profile at this time. I am trying to find out at a fixed speed and weight, which has less drag, a larger foil at lower angle of attack, or a smaller foil at higher angle of attack. A perfectly trimmed hydrofoil boat (without active controls) will perform quite differently if the overall weight or weight distribution changes. I see three directions to attack this problem. One is to have foils sized and trimmed for optimum performance when the boat is lightest, then increase the angle of attack when the boat is heavy. The second is to size and trim the foils for the boat at its heaviest, then run the foils at a reduced angle of attack when the boat is lighter. The third is of course to size and trim the foils at a point halfway between the weights, and then re-trim accordingly as the weight changes. I’m trying to figure out which will have the least drag penalty when run at the most commonly used weight.”,”2005-11-04″,”Scott Smith”,”nopswd”,” “,””,”920315″

“20”,”920315″,”2″,”Foil drag, size vs. angle of attack||920315″,”Check your data. I believe it is in error.
NAT KOBITZ”,”2005-10-26″,”NAT KOBITZ”,”nopswd”,” “,””,”918835″

“21”,”918835″,”2″,”Foil drag, size vs. angle of attack||918835″,”Scott Smith: Look on page 522 and 523 of “Theory of Wing Sections” By Abbot & Doenhoff for the
best Lift over Drag ratio, L/D for the wing section NACA 63412. This section is in common use.
The best L/D is at about Coefficient of Lift = .4. This occures at -6 degrees Angle of Attack.
The Excel formula attached determines that the area should be 1.32 square feet.
If your cord were 4.75 inches, the span should be 40.1 inches.
“,”2005-10-24″,”Ray Vellinga”,”nopswd”,” “,””,”917973″

“22”,”917973″,”2″,”Foil drag, size vs. angle of attack||917973″,”This is a rather simple question, and I hope there is a simple answer, but here goes: I am looking at the design of a foil wing that must support a fixed weight at a fixed speed, let’s say 1000 pounds at 30 mph. Which has less drag, a larger foil at lower angle of attack, or a smaller foil at higher angle of attack? Other considerations such as stall angle are not important.”,”2005-10-22″,”Scott Smith”,”nopswd”,” “,””,”2″

“23”,”917248″,”2″,”Re; Stevenson SportFoiler Published||917248″,”This is indeed good news, as there have been many requests over the years for these plans. IHS should ask permission to reprint them in the next hydrofoil CD-ROM”,”2005-10-20″,”Barney C Black”,”poopdeck”,” “,” “,”916786”

“24”,”916786″,”2″,”Stevenson SportFoiler Published||916786″,”Stevenson Projects produced a set of plans for the SportFoiler, a single person surface-piercing hydrofoil. Unfortunately, several years ago they abruptly discontinued the plans, although many of us have asked for them.

To my delight, Stevenson Products has published the plans (for free!!) online. The address is:

I want to thank the people at Stevenson, as this project shows just how easy hydrofoils are to build. Don’t dismiss these plans. “,”2005-10-19″,”Barry Steele”,”nopswd”,” “,” “,”2”

“25”,”908696″,”2″,”Re; Req for Technical Paper||908696″,”I don’t have a copy of the paper; however you may be interested in the following excerpt from IHS archival correspondence taken from, and you may want to try the email contact:

[18 Jan 01] We were sort of toying with the idea of using supercavitating foils. Do any of you know where I can get some good information on supercavitating foil sections, or the design of supercavitating hydrofoil vessels. I don’t remember who asked, but I am pretty sure we are just doing our hull with FastShip and then doing analysis using NavCad. If you have a better suggestion (which can be handled at an undergraduate level) Id love to hear it as well. — Earon S. Rein, MIDN USN (


[18 Jan 01] Two suggested sources:

Altman, R., “The Design of Supercavitating Hydrofoil Wings,” Technical Report 001-14, Hydronautics Inc., April 1968.
Martin, M., “The Stability Derivatives of A Hydrofoil Boat – Part II”, Technical Report 001-10(II), Hydronautics Inc., January 1963.
[18 Jan 01] The best info I’m aware of on supcav foil sections is the Carderock work in the 1970s on the “TAP-2” series of base-vented supercavitating foils. The work may have been done by Young Shen but I’m not sure. — Mark Bebar (

“,”2005-10-06″,”Barney C Black”,”poopdeck”,” “,” “,”904808”

“26”,”904808″,”2″,”Req for Technical Paper||904808″,”Where can I find this paper Altman, R., “The Design of Supercavitating Hydrofoil Wings,” Technical Report 001-14, Hydronautics Inc., April 1968[. Can somebody email me the pdf version of this paper at the following”,”2005-10-01″,”M.P. Mathew”,”nopswd”,” “,””,”2″

“27”,”889045″,”2″,” Supercavitating Foils||889045″,” I have to design supercavitating hydrofoils for a hydrofoil vessel going upto a max speed of 70 knots. I was thinking of going for Tulin’s sections. But I also know that the L/D charecteristics for this type of sections below 40 kts would be absymally poor. Am I correct? Can I use the public domain XFOIL(by Mark Drela) for getting the fully wetted Lift and Drag charecteristics for these sections for the non cavitating regime(upto 40 knots)or is XFOIL not suitable for sharp leading edge profiles.

My second question: Can I use base ventilated tulin section foils so that I can get supercavitating regime even at low speeds. How are supercavitating flows and base ventillated foils related. Can I use linearized Tulin’s theory for base ventillated foils. Are base ventillated foils approaching sigma (cavitation no.) = 0 . How do i get the lift and drag coefficients for base vented foils otherwise. Any references will be highly appreciated. Thanx
“,”2005-09-06″,”MP Mathew”,”nopswd”,” “,””,”2″

“28”,”888679″,”2″,”Re; Question on fully submerged foils||888679″,”My Dynafoils use a fixed rear foil, fully submerged. The front foil is a simple mechanical system, fully submerged foil coupled to a surface follower. There are no other controls except steering and throttle. It can be a handfull to control at times, but only because it is short, with deep foils and lots of power. At moderate power levels and reasonbly calm seas it handles just fine, with no roll control aparatus or trimming of the foils needed. On smaller boats with less roll moment, steering works just fine to control roll issues.”,”2005-09-05″,”Scott Smith”,”nopswd”,” “,””,”872569″

“29”,”888667″,”2″,”Re; Cheap ready made hydrofoils?||888667″,”I have copies of the old Popular Science articles on how to make wooden foils cheaply, with a tablesaw. Would work very well for you. Drop me a line and I’ll e-mail them to you, free.”,”2005-09-05″,”Scott Smith”,”nopswd”,” “,””,”2″

“30”,”884493″,”2″,”Re; Idea; Use Air to Bank Turns||884493″,”Grant,

Your proposal to use air feed to control the lift force on a hydrofoil is a sensible one. So sensible in fact, that it has been successfully implemented on both small and large hydrofoils!

The name most commonly applied to this method of hydrofoil stabilisation is “controlled ventilation”. In this context, the term “ventilation” refers to air being drawn down to the foils. On the other hand “cavitation” refers to water changing state to ‘steam’ due to very low pressure as sometimes occurs on hydrofoils so isn’t as accurate a description of what is happening.

My understanding is that this concept was first practically applied by the Swiss based company Supramar headed up by the hydrofoil pioneer Baron Hans von Schertel. Early experiments were carried out on a Supramar ST 3A fully submerged air-stabilised hydrofoil research craft. Later, various large passenger hydrofoils adopted the concept, in particular the Supramar PT 150 of which three were built. My understanding is that air stabilisation may have variously been used to assist with roll, pitch and heave stabilisation of hydrofoil.

Hans von Schertel wrote a number of technical papers on this concept at the time pointing out its advantages over conventional flapped hydrofoils. None the less, it never seems to have achieved widespread application. I don’t know why.

You would be able to find out more details if you can gain access to early issues of Jane’s Surface Skimmers or the journal “Hovering Craft and Hydrofoil” from the 60’s.

In more recent years, there had been renewed interest in foil stabilisation using air feed. A research project in Australia had considered this approach for use in controlling lift on motion control foils (for catamarans and the like). In that case, the concept had been referred to as “lift dumping foils”. I don’t believe this progressed to any operational systems.

I was not aware of any Italian research / patents on this concept but would be interested to hear more about that.

Good luck with your own experimentation.

Martin”,”2005-08-28″,”Martin Grimm”,”nopswd”,” “,””,”883043″

“31”,”883043″,”2″,”Re; Idea; Use Air to Bank Turns||883043″,”I believe this type of foil control is called artificial cavitation. I am not sure what or how much effect it has on foils at different speeds. It may not be enough effect to control the boat. The Italian patent was for large fast ferries carrying a couple of hundred passengers. I don’t think it was ever used. I think that Boeing may have investigated this idea too. I believe they held a few patents for artificial cavitation in other forms as well. I was thinking it might have application in smaller recreational boats.

My first test will be to try to improve the turning ability of My Volga. A 90-meter turning radius is not exactly turning on a dime (with very little banking). My first trial will be to use some 1” rubber hose and a lot of duct tape. Two hoses (port and starboard) will run from the cockpit to the bow and down to the center two struts (of 4) on the front foil the hose will end right at the top of the foil. A valve at the cockpit controls the airflow. Massive amounts of duck tape should smooth out the bump the hose will make as it goes down the strut. The strut is not hollow; it is made of 1/4’” stainless steel. Any ideas?
“,”2005-08-25″,”Grant Calverley”,”nopswd”,” “,””,”882728″

“32”,”882728″,”2″,”Re; Idea; Use Air to Bank Turns||882728″,”Revision #1 of Idea
On rethinking it seems air passages on from the struts to the foils should not be crossed from port to starboard and visa versa. On a boat unevenly loaded or running parallel to waves it would not right its self. It could even get dangerous as the heavy side of the boat would lose lift and sink even lower. The lower it sinks the greater the lift on the opposite side. Opps bit of a problem. It would be easier to make a foil without the cross over air tube feature anyway.

Another benefit to not having it cross over is after an operator initiated banked turn is complete the boat would right itself automatically. The lower (deeper) side would have more lift than the upper side creating a righting effect. The operator initiated banked turn air system would need to override or supply more air than the altitude control air system. The two systems would be somewhat fighting each other.
“,”2005-08-24″,”Grant Calverley”,”nopswd”,” “,””,”882721″

“33”,”882721″,”2″,”Idea; Use Air to Bank Turns||882721″,”Hello,

I have been kicking around a simple idea for stabilizing fully submerged foils for a long time. I did a patent search a while back and found that an Italian had patented a very similar idea for fast hydrofoil ferries before I was born in 1963. It seems like a good Idea so I will attempt to describe it. Maybe some one else can use the idea and make it real. I am not an engineer but would be interested to have some feedback.

The system would have almost no moving parts. It would use hollow struts and foils. Air supplied to the tops of the foils to reduce lift would main mechanism for stability, banked turns and attitude control. Two separate sets of holes on the port and starboard sides of the foils (like holes on a flute) would be across the top in the low-pressure area.

Banked Turns. When a banked turn to the right is desired an air is supplied to the right side of the foil decreasing its lift creating a banked turn. The mechanism could be as simple as a two tubes and valves (for port and starboard turns) near the steering wheel. Open the valve just before starting your turn. Electronically a turn signal lever like on a car would work well and is already instinctive to use. I have a Volga 70 and may try a duck tape and plastic hose version of this banked turn concept next year. (when I get it running)

Altitude Control and Stability. The banked turns would require some mechanical input to initiate. Attitude control would be automatic and may require a separate set of holes from the banked turn set. The line of holes on the top surface of the starboard side of the foil would be connected through the hollow foil to a corresponding set of holes in the side of the strut on the port side of the boat. At slow speeds all of the holes in the strut would be below the surface. As the boat gains speed the strut raises out of the water and the first of a serious of holes is exposed to the air. The low pressure of the wing sucks the air down through the hole and reduces the lift slightly. As the boat speeds up more holes are exposed and the lift is reduced even more maintaining equilibrium in altitude. Having the air lines cross from port strut to starboard foil and vies versa would aid in banked turns.

There are a few problems /questions in my mind. 1) Is there enough suction on the top surface of the wing to suck the air down the tubes and blow out the water that would be there already? Would you need compressed air?.(the Italians used compressed air and some complicated sensors from what I remember) 2)The hole’s orifices would need to be sized and located very carefully. Not to big and not too little. 3) Would there be a big lag time as the water is pushed out of the struts and hollow foils. 4) At slower speeds water would circulate through the strut and foils holes, would this effect lift? 5) Would the boat right its self after completing a banked turn?

I would appreciate some feedback and may try some simple experiments on my Volga next year if it is warranted. What do you think, does it have merit or is it flawed? I never even took Physics in high school so go easy on me.

Grant Calverley
“,”2005-08-24″,”Grant Calverley”,”nopswd”,” “,””,”2″

“34”,”872638″,”2″,”wsome Re; Re; Re; FOIL SHAPE AND AN||872638″,”Awsome answer. Thank you Tom.
I have already experienced much of what you have discussed. I actually started with a low angle, and found that the drag of the foils actually pulled the boat down. I compensated by doubleing the front angle, and that helped. I had already preset the rear foils at an angle of six degrees. So by end of testing on the first day, it came down to the fact that my jet pump could not supply enough thrust.
I have doubled the size of the pump, and have created a four bladed impeller. I also have an output reduction cone built to see if reducing the output diameter doesn’t increase the speed of the output.
I have several things to try including two more motors.

I was hoping to get 12mph, and the first pump failed miserbley.
I don’t think I will have any chances at the pond till next weekend

Thank you

“,”2005-08-06″,”Umi_Ryuzuki”,”nopswd”,” “,””,”872597″

“35”,”872597″,”2″,”Re; Re; FOIL SHAPE AND ANGLE||872597″,”You have the basic idea, but I think you’re missing a couple of things. You do get the area by assuming lift = weight and dividing by the dynamic pressure and design lift coefficient. But you have to use consistent units.

The factor F in your formula is the fluid density divided by 2. For water, the density is (using your English units) 1.939 slug/ft^3, so the Factor F should be 0.9695 for fresh water, or pretty close to 1.

The velocity has to be in ft/sec to be consistent, so I’ll take the “12” in your calculations as being 12 ft/sec (same as 8.2 mph or 7.1 kt). The velocity has to be squared, which I’m not sure you did to come up with your final result.

So at a speed of 12 ft/sec and a lift coefficient of 0.5349, I get an area of 0.067 sq ft or 9.64 sq in for the required area. Since each of your wings have an area of 7.5 sq in, getting the 5 lb of lift from 6 of them is not a problem. The extra area will let you fly at half the design speed of 12 ft/sec.

However, while the average lift coefficient may be 0.5349, that doesn’t mean the local lift coefficient will be the same over all parts of the wing. For your swept foils, the tips will be loaded more heavily than the root. This is due to the downwash in the wake of the hydrofoil and how it affects the conditions along the span.

And the angle of attack of the foils will not be 2.25 degrees as indicated by the two-dimensional section data. Those data are for a foil of infinite span, so it produces an infintessimal downwash. The shorter the span, the greater the downwash to produce the same lift, so the angle of attack has to be increased to offset the downwash. Your foils have an aspect ratio of 4, and at a lift coefficient of 0.5349, an additional 2.44 degrees of angle of attack will be needed because of the downwash. So the incidence of your foils will be more like 4.7 degrees than the 2.25 given by the section data for the same lift coefficient.

But more than that, the downwash will increase the drag substantially. You should allow for an additional drag coefficient of 0.0228 because of the lift-induced drag. This is 0.21 pound of additional thrust required. The induced drag goes down by the square of the span, so if you make your foils wider they will be much more efficient. But this runs into problems of strength and stiffness, so the span is always a compromise. The induced drag goes DOWN with speed (squared), so flying at too slow a speed can actually require more power than going fast.”,”2005-08-05″,”Tom Speer”,”nopswd”,” “,””,”2″

“36”,”872572″,”2″,”Re; Cheap ready made hydrofoils?||872572″,”Take a look at”,”2005-08-05″,”Tom Speer”,”nopswd”,” “,””,”2″

“37”,”872569″,”2″,”Re; Question on fully submerged foils||872569″,”It’s not enough to balance lift against weight. You also have to balance the moments that want to turn the craft, tip it over, or pitch it. And the problem with balancing the lift is the lift is constantly changing as a function of speed, the attitude of the craft, and the disturbances from waves, gusts, thrust changes, etc. So when it does change, there has to be a means of returning it to its original value. If you hold a broom upside down on your hand, it’s easy to compensate for the weight of the broom. But the moments are unstable so you can’t maintain that balance without actively compensating for any change.

There’re also the problems of regulating the flying height, maneuvering and accommodating different amounts of payload.

Lift at a constant speed and attitude does drop off as the foils get close to the surface. It’s possible to use this effect to stabilize the craft if you are operating in flat water. But this also robs the fully submerged foil of much of its performance advantages.”,”2005-08-05″,”Tom Speer”,”nopswd”,” “,””,”2″

“38”,”862463″,”2″,”Re; Cheap ready made hydrofoils?||862463″,”There were some articles published in the late 1950s – early 1960s in hobbyist magazines as to how to make wooden hydrofoils and add them to runabout-type boats. For example, Popular Science June 1960 has an article, “How I Fitted Oak Hydrofoils To My 14-Foot Runabout.” Science and Mechanics Feb 1960 has a similar article, with foil design for boats up to 18 feet length. Take a look at the magazine descriptions on the IHS website in the Hobbyist section of this page:

You can buy copies of old magazines by searching for them on eBay and/or Sooner or later, just about everything shows up on eBay. Google will find you magazine sellers who sell directly. I have used the Canadian company “Smelly Old Books” John Muxlow to obtain reasonably priced copies of articles back to the 1920s and earlier (S.O.B. has an almost complete collection of Mechanix Illustrated, Popular Mechanics, and Popular Science). It has been a while since I contacted them, so I hope the URL and email address are still good.”,”2005-07-18″,”Barney C Black”,”nopswd”,” “,” “,”0”

“39”,”861182″,”2″,”Re; FOIL SHAPE AND ANGLE||861182″,”So No help or confirmation on the previous calculations?
“,”2005-07-15″,”Umi_Ryuzuki”,”nopswd”,” “,””,”0″

“40”,”860748″,”2″,”Re; Question on fully submerged foils||860748″,”Maintaining a close enough balance between weight and lift without feedback control to allow a flight for more than a few seconds is currently not possible. Suggest you consider a mechanical feedback controller. The Rave, Hobie Trifoiler, and the height control on Talaria IV all use mechanical surface sensors with linkages to their foils to maintain a balance between lift and weight. “,”2005-07-14″,”Harry Larsen”,”nopswd”,” “,””,”0″

“41”,”860689″,”2″,”Re; Question on fully submerged foils||860689″,”Thanks! That puts me very close to the goal.

Andy”,”2005-07-14″,”Andy”,”nopswd”,” “,” “,”0”

“42”,”860646″,”2″,”Re; Question on fully submerged foils||860646″,”If you are using fully submerged foils for main lift, you can have a 25 to 35% lift stabilizing, surface piercing foils to supplant an autopilot. I do not know of any all fully submerged foil systems that are self stable.”,”2005-07-14″,”NAT KOBITZ”,”nopswd”,” “,””,”0″

“43”,”860356″,”2″,”Question on fully submerged foils||860356″,”I have read that fully submerged foils require flight control. My question is whether this is strictly necessary, or if I could design a submerged foil for a specific boat through experimentation that would be functional without flight control. The idea being to balance the lift against the weight of the boat.

“,”2005-07-13″,”Andy”,”nopswd”,” “,” “,”0”

“44”,”856862″,”2″,”Cheap ready made hydrofoils?||856862″,”Forgive me for my ignorance- I’m only just starting to embark on a project to add hydrofoils to a 12′ boat. I’ve been searcing for ready made aerofoil sections that could be used, and of course there are none specifically for hydrofoils-other than sailing ones, which are still expensive and probably unsuitable. When I searched under ‘aluminium aerofoil section extrusions’ I came up with extrusions intyended as sun blinds, see page 14 for an example :
I realise the sections avaiable would not be the most efficient, but would using these extrusions provide a cheap and reasonably efficient way of getting foilborne?

Any comments?

Cheers, Roland”,”2005-07-07″,”Roland Wilson”,”nopswd”,” “,””,”0″

“45”,”855770″,”2″,”Re; Re; FOIL SHAPE AND ANGLE||855770″,”Let me see if I read all this correctly.

S = L / F U^2 Cl

S = 5lbs/ (2.09)(12©÷)(0.5349*) *assuming a 2.25¡æ angle of attack.

S = 5lbs/ 160.9835

S = 0.310591 sq ft

Therefore S = 44.725104 sq inches divide by 3 for each foil

Each wing needs an area of 14.908 sq inches
¡î14.908 = 3.861 square plate cut diagonally

Am I correct in assuming a six wings one on each side of the struts with

a root of 2.5″
trailing edge of 3.875″
tip length of 1.375″

Will fly a 5 lbs (2.268k) hydrofoil?
Would this include control surfaces, or would adding control surfaces to the existing area be preferred?

Thank you in advance.



“,”2005-07-04″,”Umi_Ryuzuki”,”nopswd”,” “,””,”0″

“46”,”830353″,”2″,”Re; Foil Design Help||830353″,”hi sam,

you might want to re-think the approach to what you are trying to achieve. a hydro foil solution for wake boards has been around for ages.
it’s called the WEDGE and is a fold down foil under the stern of the boat that pulls the boat DOWN at speed to increase the wake.

boogie”,”2005-05-17″,”Boogie”,”nopswd”,” “,” “,”0”

“47”,”824680″,”2″,”Foil Design Help||824680″,”Gday mate my name is sam doolan and I am an Industrial design student from
RMIT university Melbourne Australia. I am currently designing a foil, which
creates larger wakes for wakeborders by lifting the front of the boat and
transferring the weight to the back, to give it larger wakes. However I am
sure how the foils should be designed. Wether one at the front of the boat,
or coming off both sides or both. Would you be able to answer this question
for me.”,”2005-05-06″,”sam doolan”,”nopswd”,” “,””,”0″

“48”,”824319″,”2″,”Re; Re; coordinated (banked) turns||824319″,”Could you send us a picture?”,”2005-05-06″,”Harry Larsen”,”nopswd”,” “,””,”0″

“49”,”823711″,”2″,”Re; Re; coordinated (banked) turns||823711″,”Thanks for your response and info. We call the boat “straightfastboat” as it’s very fast in a straight line, 65mph+ and requires slowing and greater immersion of the rudder to turn with a bit of inboard banking. I’ll try deepening the rudder as a first move and stay away from adjustable angle of attack in the foils. “,”2005-05-05″,”Mike Turner”,”nopswd”,” “,””,”0″

“50”,”823598″,”2″,”Re; Re; coordinated (banked) turns||823598″,”Roll can be a complicated consequence of rudder deflection. Since the rudder is located below the center of mass, a port deflection of an aft-mounted rudder will result in a rolling moment to port.

It also produces a yawing moment, of course, and as the craft yaws to port, it picks up a sideslip (leeway) angle. If the foil system has positive roll stability – like a V foil configuration – the sideslip angle will also make the craft roll to port. Roll due to sideslip is likely to be the more powerful effect of the two. As the bank develops, the sideslip angle will be reduced.

But it takes some time for the craft to rotate enough to generate the sideslip. So there’s lag between when you put in the input and when the rolling due to sideslip is experienced. The rolling moment due to the rudder deflection itself is prompt. The sideslip itself will reduce the force on the rudder, lessening the rolling moment from that source.

If the rudder is on a forward strut, then the craft will yaw in the opposite direction, the sideslip will be reversed, and the roll due to rudder deflection and the roll due to sideslip will be of opposite sign.

So the relationship between rudder and roll depends on the placement of the rudder, the stability of the craft, and the frequency of the input. The rolling due to rudder could be opposite in sign for different frequency ranges.
“,”2005-05-04″,”Tom Speer”,”nopswd”,” “,””,”0″

“51”,”823592″,”2″,”Re; Re; coordinated (banked) turns||823592″,”Yes, typically. Like ailerons on an airplane wing.

But they could be done in many ways. You could change the incidence on a whole foil, positive for the port foil and negative for the starboard foil to get a positive rolling moment. You could articulate the outer panel of a hydrofoil. Flaps are an effective and easily mechanized way to go. But not the only way.

Aeronautical experience has shown that it’s not a good idea to try to produce roll from a canard (forward wing), however. The resulting downwash has the opposite effect on the aft wing and can cancel or even reverse the intended rolling moment. The effects could be even more complicated by the way hydrfoil downwash is affected by the free surface.”,”2005-05-04″,”Tom Speer”,”nopswd”,” “,””,”0″

“52”,”821847″,”2″,”Re; Re; coordinated (banked) turns||821847″,”When you dig up the info on this I’d love to se it. I have a Volga 70 that I’d like to convert the banking in turns to inboard rather than that disconcerting outboard feeling. My rudder depth will be increased soon and I’ll report the result. It currently is as deep as the prop blade sweep. Anyone have anything to suggest? “,”2005-05-02″,”Mike Turner”,”nopswd”,” “,””,”0″

“53”,”821718″,”2″,”Re; Re; coordinated (banked) turns||821718″,”You mention ‘roll surfaces’… are those the main hydrofoil surfaces (or flaps on the main foil)?”,”2005-05-01″,”Wayne Johnson”,”nopswd”,” “,””,”0″

“54”,”821713″,”2″,”Re; Re; coordinated (banked) turns||821713″,”The mechanical… I wanted to be sure that I was not missing a simple thing like ‘roll is a consequence of rudder’, or some other simple mechanical link.”,”2005-05-01″,”Wayne Johnson”,”nopswd”,” “,””,”0″

“55”,”821318″,”2″,”Re; coordinated (banked) turns||821318″,”The definition of a coordinated turn is zero lateral acceleraion (along the Y axis). One way to achieve it is to use lateral acceleration feedback to a rudder. The rudder turns the craft about its Z axis to zero the leeway angle that results in the side force causing the acceleration.

However, a tricky aspect of this with a hydrofoil is the center of mass of the craft is well above the foils, and the crew station is typically above that. So you have an issue with how you enter the turn. If the craft rolls about the hydrofoils, there will be a significant lateral acceleration of the center of mass, and a somewhat greater acceleration yet at the crew station. Acceleration feedback at that point would turn the rudder to point the craft to the outside of the turn. So you’d have the roll control and the yaw control fighting each other, and when the two get out of phase you could lose control.

Everything will be fine for slow gentle entries that don’t develop much acceleration. But if you apply a frequency sweep to the wheel, sarting with a slow oscillation of the wheel and working up to faster and faster reversals, you will arrive at the point at which the motion becomes alarming. I had the chance to experience this when I rode on Harry Larsen’s Talaria.

A better approach would be to rotate the craft about either the center of mass or the crew station. This requires that the hydrofoils describe a pendulum motion, swinging to the outside of the turn as the craft rolls and the g-loads increase – keeping the net hydrodynamic force aligned with center plane at all times. To get such a motion probably requires a means of generating direct side force on the foils, such as both a forward and aft rudder or a flap on a main strut in addition to the rudder. An interconnect between the roll surfaces and the side force flap(s) would generate the right linear acceleration of the foils in concert with the roll acceleration. The feedbacks would then deal with the left-over motion due to imperfect match in the interconnect, and the fact that the control deflections you want initially are not necessarily proportional to what you want in the steady state.

An alternative approach is to use model following. The commands from the helm go to a dynamic model that has the ideal chaaracteristics – rolling about the crew station, etc. The ideal model produces state, rate, and acceleration commands to a feedback regulator control law that makes the hydrofoil follow the ideal motion as closely as possible. The regulator would typically be designed using multivariable control theory (Linear Quadratic Gaussian, Pole Placement, or many others).

The ideal model can be simulated separately, independent of the configuration of the hydrofoil itself, assuming perfect model following. This lets you tune the characteristics in parallel with designing the rest of the system. For example, you might want the ideal model to descend a bit at the same time that it kicks the hydrofoils to the outside of the turn so that the foil tips don’t broach because of the pendulum motion.

I recommend Thor Fossen’s books and papers for more details.”,”2005-04-30″,”Tom Speer”,”nopswd”,” “,””,”0″

“56”,”821034″,”2″,”Re; coordinated (banked) turns||821034″,”From a roll acceleration point of view a coordinated turn is no different than flying straight and level. Is your question related to the mechanical, sensor, electronic, or mathematical means of performing a coordinated turn? “,”2005-04-30″,”Harry Larsen”,”nopswd”,” “,””,”0″

“57”,”819978″,”2″,”coordinated (banked) turns||819978″,”How do you get a fully submerged type hydrofoil to bank in a turn?
Are there any documents which explore the requirements of coordinated turning? “,”2005-04-28″,”Wayne Johnson”,”nopswd”,” “,””,”0″

“58”,”795120″,”2″,”Re; Foil pressure coefficient data||795120″,”I think your best bet would be to go to something like a Navier Stokes CFD code if you really want to characterize the flow well past stall.

You might be able to get some idea by going as far as you can with an integral boundary layer code like XFOIL. The idea is separation occurs in an adverse pressure gradient. So there must be a lower pressure ahead of the separated flow that is attached, and that might be predictable with a lesser method. My guess – and it’s just a guess – is that even though you operate well past stall, the worst case as far as minimum pressure is concerned might be at or just past stall, and this could be computed with something like XFOIL.

If you application is operating near the surface, though, ventilation rather than cavitation is likely to be your real problem. They both end up with vapor on the suction side, but for completely different reasons. The separated flow will be a real bear when it comes to ventilation, because you are setting up all the necessary preconditions for ventilation. If you insist on the separation, you’ll have to concentrate on keeping the air away from the separated regions.”,”2005-03-12″,”Tom Speer”,”nopswd”,” “,””,”0″

“59”,”788896″,”2″,”Re: Foil pressure coefficient data||788896″,”Is stalling a separation of the boundary layer? Probably, the shapes with negative presure gradients will be usefull for your purpose to avoid cavitation. See my message No 7888876 and attached file.

“,”2005-03-01″,”Ihor Nesteruk”,”nopswd”,” “,””,”0″

“60”,”788876″,”2″,”Hydrofoils without cavitation||788876″,”I am looking for people or organization wich are interested in futher investigation and wind tunnel tests of hydrofoils without cavitation. Please find the
details below and in the attached file.

Dr. Ihor Nesteruk
Institute of Hydromechanics
National Academy of Sciences of Ukraine
Axisymmetric and plane shapes with negative pressure
gradients at a surface are calculated. It is shown
that the length of the pressure increase zone may be
very short. Some shapes of axisymmetric bodies tested
in a wind tunnel provide unseparated flow patterns.
Such shapes are both of theoretical and practical
interest, since the unseparated flow pattern gives an
opportunity to reduce the total drag of vehicles
moving in gas or liquid. Moreover presented shapes
probably provide no cavitation at arbitrary small
cavitation numbers (see theoretical and experimental
arguments in [1,2]). Tests in a water tunnel have to
be carried out to prove this fact. For the presented
2D profiles, there were no wind tunnel tests to
investigate their separation behavior at different
angles of attack. Such experiments could be carried
out in the wind tunnel of Kyiv Institute of
Hydromechanics. [1] Nesteruk I.: Can Shapes with
Negative Pressure Gradients Prevent Cavitation. 4th
ASME/JSME Joint Fluids Engineering Conference,
Honolulu, USA, July 2003: FEDSM2003-45323.[2]
Takahashi S., Washio S., Uemura K., Okazaki A.:
Experimental study on cavitation starting at and flow
characteristics close to the point of separation.
Fifth Symposium on Cavitation: No. Cav03-OS-3-003, 2003.

“,”2005-03-01″,”Ihor Nesteruk”,”nopswd”,” “,””,”0″

“61”,”787296″,”2″,”Foil pressure coefficient data||787296″,”Could anyone tell me how I could get hold of minimum pressure coefficient data for aerofoils operating at and beyond the stall (ideally up to twice the stalling incidence)? I am designing a lifting device for a marine application which is heavily stalled for much of its operating life, and must not cavitate.

Many Thanks

Chris Huxley-Reynard”,”2005-02-25″,”Chris Huxley-Reynard”,”nopswd”,” “,””,”0″

“62”,”779156″,”2″,”Lift formula||779156″,”Konstatin Matveev’s lift formula is encoded in Excel on this web site:

“,”2005-02-10″,”Harry Larsen”,”nopswd”,” “,” “,”0”

“63”,”776265″,”2″,”Calculating Lift||776265″,”Sorry but I’m not too good at math. Can somebody please give me a simple equation that will allow me to calculate approximate lifting force in kg (what the hell is a newton anyhow ?) at a given area (sq meter) speed (kph) and angle of attack (I want to experiment with variable angles to load or unload a vessel). Now I know that aspect ratio, foil thickness, diehedral etc etc all play a part but I just want approximate values please. “,”2005-02-05″,”Andy”,”nopswd”,” “,””,”0″

“64”,”764798″,”2″,”Re: FOIL SHAPE AND ANGLE||764798″,”It’s achievable. You’ll probably need a foil with a chord of about 750mm. At the speeds you’re talking about, just about any decent airfoil section would work. The incidence of the foil needs to be set with its zero lift lne about 5 – 7 degrees above the trim attitude of the boat. If you build it so you can bend the trailing edge up or down, that will allow you to fine tune the lift.”,”2005-01-15″,”Tom Speer”,”nopswd”,” “,””,”0″

“65”,”759759″,”2″,”Re: Foil Surfing||759759″,”there is no foil surfing allowed anymore.

You must shape your own board out of koa and paddle it – no more tow in’s mish”,”2005-01-05″,”big wave surfer”,”nopswd”,” “,”bigwavesurfer”,”0″

“66”,”756659″,”2″,”Foil shape and size||756659″,”Many thanks for your reply to my request for info on foil design,
being an absolute bunny in this area everything Ican learn is a huge benefit.
Iam hoping to achive a slightly ‘better’shape than a circle cord, I have a copy of the shape of a Speer H005 foil and am aiming at that shape if it will improve lift on the cord.
I note that the C/L increases somewhat at higher angles of attack is there some reason for not using 6,8 or 10 degree angles of attack?
Is there a minimium depth that the foil should be below the surface when travelling at speed, being a catamaran we dont draw much but I can extend the foil down by using longer end plates if required.
Once again many thanks for your time, I’ll keep you posted with the results when they occur.
Regards John”,”2004-12-28″,”JOHN PAYNE”,”nopswd”,” “,””,”0″

“67”,”756181″,”2″,”Re: FOIL SHAPE AND ANGLE||756181″,”John,

I see that the table I provided you is unreadable. It may help if you know that the first line and the first column is Angle of attack = -4.00, second column is Coefficient of lift = .0065, third column is Coefficient of drag = .0309, forth column is Coefficient of Moment = -0.1053, and the fifth column is lift over drag ratio = .210. Hopefully, with this you can read the table.


“,”2004-12-27″,”Ray Vellinga”,”nopswd”,” “,””,”0″

“68”,”756177″,”2″,”Re: FOIL SHAPE AND ANGLE||756177″,”Hello, John,

Here is a little cook-book on designing hydrofoil wings. As a welder, you may find the “ogival” AKA
Plano-convex shape easiest to fabricate out of aluminum, steel or stainless steel. The wing is created
by twice cutting a pipe lengthwise to form a vault-shaped piece. On the bottom inside weld a flat plate.
Use a grinder to make your welds fair. Fill in the pits with Bondo. Make everything smooth.
You may choose to round off the leading edge to avoid slicing marine life as you fly by.
The trailing edge should be sharp, but not so sharp as to be hazardous.
You then have a wing, the top of which is a segment of a circle with the underside flat.
For starters, make the thickness to cord ratio 10%.

You need to estimate the area needed, so get out your calculator. The formula is:

S = L / F U^2 Cl

S = Surface area in Square feet
L = Lift (this is the total weight of your boat, crew, etc., in Pounds)
/ = “divided by”
F = Factor to adjust for water density, unit measurements, etc. Use the number F = 2.09
U^2 = Velocity in MPH multipied by itself (squared)
Cl = Coefficient of lift. See the table provided below for Cl. You should cruise at a high L / D or say 2.25degrees.
Take off will be at a higher Cl & lower speed. You must make some assumptions based on experience.

I hope you didn’t sleep through math class.
Contact me if you have problems:

Ray Vellinga

Water Tunnel Observations on the Flow Past a Plano-Convex Hydrofoil By R B Wade Feb 1964
Caifornia Institute of Technology

Graph the Characteristics of Hydrofoil in Non-cavitationg Flow, Table, Page 51

v = 31.32 ft/sec = 21.35 M/H
Re = 0.75 X 10^6

Angle Coefficient Coefficient Coefficient L / D
of of Lift of Drag of Moment ratio
Series 1 Series 2 Series 3

-4.00 0.0065 0.0309 -0.1053 0.210
-3.50 0.0487 0.0262 -0.0932 1.859
-3.00 0.0995 0.0223 -0.0782 4.462
-2.75 0.1247 0.0201 -0.0723 6.204
-2.50 0.1481 0.0190 -0.0665 7.795
-2.25 0.1721 0.0178 -0.0564 9.669
-2.00 0.1921 0.0164 -0.0476 11.713
-1.75 0.2140 0.0152 -0.0400 14.079
-1.50 0.2420 0.0149 -0.0375 16.242
-1.25 0.2636 0.0146 -0.0313 18.055
-1.00 0.2778 0.0136 -0.0244 20.426
-0.75 0.3106 0.0137 -0.0192 22.672
-0.50 0.3295 0.0131 -0.0140 25.153
-0.25 0.3583 0.0133 -0.0089 26.940
0.00 0.3714 0.0129 -0.0026 28.791
0.25 0.4023 0.0122 0.0016 32.975
0.50 0.4261 0.0119 0.0089 35.807
0.75 0.4478 0.0126 0.0148 35.540
1.00 0.4539 0.0138 0.0197 32.891
1.25 0.4490 0.0141 0.0245 31.844
1.50 0.4644 0.0152 0.0303 30.553
1.75 0.4781 0.0164 0.0356 29.152
2.00 0.5031 0.0164 0.0408 30.677
2.25 0.5349 0.0167 0.0474 32.030
2.50 0.5443 0.0172 0.0540 31.645
2.75 0.5725 0.0185 0.0593 30.946
3.00 0.5797 0.0184 0.0647 31.505
3.50 0.6360 0.0217 0.0775 29.309
4.00 0.6751 0.0245 0.0896 27.555
5.00 0.7732 0.0338 0.1151 22.876
6.00 0.8352 0.0447 0.1391 18.685
8.00 0.9873 0.0735 0.1858 13.433
10.00 1.0919 0.1182 0.2044 9.238
“,”2004-12-27″,”Ray Vellinga”,”nopswd”,” “,””,”0″

“69”,”754783″,”2″,”Re: Foil Surfing||754783″,”I believe you need to really think about your design expectations. In the application you are considering, manueverablilty and stability are inherently opposite. If the board is stable, such as with surface peircing foils, you won’t have the manueverability of a board such as the ones based on an “Air Chair” or “Sky Ski”. You are going to eventually have to decide where to make the trade-off.”,”2004-12-22″,”Scott Smith”,”nopswd”,” “,””,”0″

JOHN”,”2004-12-19″,”JOHN PAYNE”,”nopswd”,” “,””,”0″

“71”,”743418″,”2″,”Re: Foil Surfing||743418″,”For your wakeboard, if you want an alternative to the inverted “T” fully submerged hydrofoil designs used on hydrofoil surfboards and sailboards today, you could consider using an arrangement of surface piercing foils… these would be self-stabilizing. Back in 1978, an individual named Michael Shannon of Birmingham MI sent a letter to Dave Keiper, who was offering foils kits for Hobie Cats. He stated that he and his partner James Coulter had successfully adapted parts from Keiper’s foil kit to a windsurfer and planned to make a production run. In connection with this correspondence, Keiper sketched and annotated his own first thoughts on how he would do the design. Unfortunately, Keiper is deceased, and the return address on Shannon’s letter no longer exists, according to the USPS database. So I don’t know if this hydrofoil windsurfer ever went into production or not. Anyway, I put a copy of Keiper’s notes and the Shannon letter up on the web at So take a look. It was common practice for the pioneering hydrofoil designers starting with Alexander Graham Bell to try out their hull/foil prototypes by towing them, so this is similar to a wakeboard being towed by motorboat, only the towed board is the end product rather than an interim test piece. Hopefully this info is of some help. Maybe someone else who checks the IHS BBS will know something of Shannon and Coulter. As to sources of foils and struts, that is another subject, but there are some: mostly in connection with human-powered vehicles, but also a company that makes them for adding to Moth class sailboats.”,”2004-11-24″,”Barney C Black”,”poopdeck”,” “,””,”0″

“72”,”742586″,”2″,”Re: Foil Surfing||742586″,”Hi Mitch,

In a first time, I think the better solution is a very simple inverted T foil on the rear (about under the rear foot) and a surface traking “patin” on the front. The rear foil must be about 0.1 square meter area.

For the front “canard” there are several solutions :

The more simple is a planing surface but you can try too somes V surface piercing foils.

Gérard”,”2004-11-23″,”Gérard Delerm”,”nopswd”,” “,””,”0″

“73”,”742553″,”2″,”Foil Surfing||742553″,”I am interested in alternate designs for foil boarding waves. Currently many surfers are using a foil like the water ski chair type with a stand up board similar to a snow board. It seems to me that there might be a more stable and/or manuverable design like some of the boat foils I have seen. The speeds are 15-30 mph and generally the weight of a surfer(180lbs). Manuverability and stability is a must. Can you make some suggestions as to designers that might help me or direct me as to how to decide on a design and type of foil for surf? “,”2004-11-23″,”Mitch Haynie”,”surfer”,” “,””,”0″

“74”,”730516″,”2″,”Re: Advanced Educational Pages||730516″,”Hi Barry,

If you try to do any Educational Pages project I can try to translate in French (in IHS there is “International” )
I am not a professional translator but I think I can make good translations if I can have “chat” with the author.

“,”2004-10-30″,”Gerard Delerm”,”nopswd”,” “,””,”0″

“75”,”730318″,”2″,”Re: Advanced Educational Pages||730318″,”An excellent suggestion. The closest the site has come is grouping correspondence by topic in the archives. Main page for accessing the master archives is //

There is also the barest start of an FAQ page at // However, no significant work was ever done on this.

As it states on the main page, the content of the IHS site reflects the interests of the members and visitors to the site who are willing to provide content. The site is very simple in design, no frames or anything, but rather sprawling. It is quite possible for someone with a particular interest or with a particular project in mind to assume responsibility for a page or pages on the site, whether the page currently exists or not, as an assistant to the webmaster. Revision and creation of pages is fairly simple with any WYSIWYG webpage creation program. File upload is easy with any FTP program such as CuteFTP. So if you were willing to undertake the project you suggest, even if it is over a considerable period of time, that is welcome, I believe, and fairly easily arranged.”,”2004-10-29″,”Barney C Black”,”poopdeck”,” “,””,”0″

“76”,”730307″,”2″,”Advanced Educational Pages||730307″,”I found Tom Speer’s discussion on foil stability facinating. I’ve not see this information explained so clearly before. Similarly, I’ve read explanations on calculating foil sizes from Tom and others which made it understandable for someone who hasn’t been in the industry for 30 years. I’ve seen so many of the questions I’ve asked years ago posted again and again.

Is there any way that somehow this explanation might be incorporated in an Advanced Educational page on the site? Perhaps as a continuation of the existing educational pages? I’ve gotten some great explanations from Tom, Mark Daskovsky, William O’Neill and Harry Larson.

I don’t know if you have a hit counter on your basic educational pages, but I know I’ve been there many times and they have been very very valuable. I don’t know how I could help, but I’d be happy to try.

Topics might be:

What shape foils? Typical NACA numbers (other better foils), what they mean and where to find the plots. The choices for submerged vs surface piercing vs strut foils.

How Big? Calculations on lift vs speed… or just a table. Wing loading.

Stability? Tom’s stability discusson, formatted would be great.

Takeoff speed vs flying speed, relationship between these and parameters in determining them.

Estimating power requirements or max speed foilborne…

Add to the glossary words and terms which are used a lot on the discussions: freude numbers, sea states … similar things which took a while to pull together.”,”2004-10-29″,”Barry Steele”,”nopswd”,” “,””,”0″

“77”,”729123″,”2″,”Re: Foil Spacing||729123″,”Sumi raises an interesting point. PLAINVIEW never lost directional stability. One of the closest we probably came was when one of the main foil incidence angle control systems experienced a structural failure while foilborne. The result was that the foil with the failed system went to full-leading-edge-down. In response to the ship beginning to drop, the control system called for full foil-leading-edge-up. This resulted in one forward foil full leading edge up and the other forward foil full leading edge down while foilborne. The helmsman chopped the throttle immediately. The ship rolled and the hull hit the water at foilborne speed at an angle of 18 degrees. After impacting the water, the ship continued to roll to 32 degrees before coming to a stop. No one was hurt and there was no damage to the ship. We continued back to port hullborne.”,”2004-10-27″,”Phil Yarnall”,”poopdeck”,” “,””,”0″

“78”,”729122″,”2″,”Re: Foil Spacing||729122″,”John, By conventional configuration, I refer to the airplane configuration. On PLAINVIEW, we showed the model test results done at the Michigan tank showing the loss of directional control to each of the skippers assigned. One could argue that the carriage helped the ship roll over, but it would still be quite a ride. I know of no instance where we actually encountered this situation since the crew was attuned to the possibilities.”,”2004-10-27″,”S. Arima”,”poopdeck”,” “,””,”0″

“79”,”729121″,”2″,”Re: Foil Spacing||729121″,”Hi Sumi, By “Conventional” configuration, do you mean “airplane” configuration where the aft foil is lightly loaded, as was in the case of Plainview? I understand there was an incident where the stern tried to replace the bow in the foilborne mode, if you know what I mean.
“,”2004-10-27″,”John Meyer”,”poopdeck”,” “,””,”0″

“80”,”729120″,”2″,”Re: Foil Spacing||729120″,”I would like to add a word of caution to Tom’s extensive and informative dissertation. The location of the struts on the hull, especially in the conventional configuration, one needs to look at the sea state and hull contact with the sea. In a quartering sea, where the bow of the ship could make contact with the wave could produce side loads that could easily overcome the ability of the aft strut to maintain directional control. Stability needs to be looked at in more than just the foilborne situation.”,”2004-10-27″,”S. Arima”,”poopdeck”,” “,””,”0″

“81”,”729119″,”2″,”Re: Foil Spacing||729119″,”Phil, You may also recall that during detail design and construction of PLAINVIEW, it became apparent that the initial location of one of the diesel generators was too far forward and that the aft foil would have been too lightly loaded and subject to broaching, so the generator was moved aft by at least one frame space.”,”2004-10-27″,”Mark Bebar”,”poopdeck”,” “,””,”0″

“82”,”729118″,”2″,”Re: Foil Spacing||729118″,”On PLAINVIEW, the concern was not so much the space between the forward and aft foil, but rather the amount of lift capacity in the forward and aft foils. 90% of the lift capacity was in the forward foils and 10% of the lift capacity in the aft foil. Knowing the precise longitudinal center of gravity became a high concern. Ultimately, there was scale at the ramp to the ship. The weight and location of each new piece of equipment and gear was recorded when it came onboard and the LCG calculated. There was volume in the aft portion of the hull which was unusable for payload due to the requirement to maintain 90% of the load on the forward foils.”,”2004-10-27″,”Phil Yarnall”,”poopdeck”,” “,””,”0″

“83”,”729117″,”2″,” Stability Has Been Investigated||729117″,” Yes, stability has been extensively investigated. The Hydronautics handbook on IHS’s AMV CD#1 has a whole chapter devoted to trim and a whole chapter devoted to longitudinal stability. You can also find papers on hydrofoil stability on the NACA technical reports server (

There are also performance aspects to hydrofoil spacing. Constantin Matveev used to have a page on his web site that showed how the rear foil should be located in the rising part of the transverse wave generated by the forward foil. This leads to a foil spacing based on the design Froude number.

With regard to “stability”, the foil spacing is just one of many important factors. I’d say there are really four areas to consider in addition to wave drag, all of which are affected by foil placement and spacing. The first is trim – the ability achieve an equilibrium where all the forces and moments balance (sum up to zero). For best performance, the least drag is obtained by the “airplane” configuration, with a large foil taking nearly all of the weight of the craft and a lightly loaded (quite possibly negatively loaded) stern foil for stability and trim. This means the main foil must be placed in the vicinity of the center of gravity, which for most boats is a little aft of midships. This only leaves half the length for foil spacing. If you look at the Carl hydrofoil, you’ll see that the hull has a slender tail to put the stern foil farther aft while keeping the weight near the main foils.

Then there’s stability itself. Stability has to do with whether the craft returns to a condition of equilibrium after having been disturbed from an initial equilibrium. So stability presupposes trim – it’s meaningless otherwise. Stability is usually further broken down into static stability, which is the instantaneous tendency to return to trim after a disturbance, and dynamic stability which deals with whether or not the motion damps out over time. The pitch damping goes by the square of the distance between the foils and the center of gravity. So there’s a definite connection between stability and foil spacing there. Heave damping is usually quite high by the nature of hydrofoils, so if the pitch heave coupling is stable, the dynamic heave stability will probably be stable.

The static stability in the longitudinal axis depends on how the moments change for a disturbance in pitch angle at constant depth, and how the moments change with depth at a constant pitch attitude. A bow-up change in pitch must generate a bow-down change in the pitching moment. As a practical matter, this requires that the forward foil be more heavily loaded – it must carry more of the boat’s weight per unit area than the aft foil. So as you change the foil spacing and placement relative to the center of gravity, you have to change the area of the foils. An increase in height (decrease in depth) must also generate a bow-down pitching moment to have stable pitch-heave coupling. This is why you see inverted T foils used so extensively for the aft foil and either surface piercing foils or flapped foils forward. Again, the spacing and placement of the foils is very important, taking into account their heave stiffness.

Yaw damping also improves with the square of the distance between the foils and the center of gravity. So there’s another effect of foil spacing. Roll damping goes by the square of the foil span, so it’s not very affected by longitudinal spacing, although it’s heavily influenced by the lateral spacing of the foils.

The next issue to consider is controllability. Control power is needed to achieve the desired trim state. Especially with surface piercing foils, there will be an optimum flying height for best performance, and the pitch attitude must be trimmed so as to achieve it. Control power is needed for stabilization if you are actively augmenting the craft’s stability, as is universally done for fully submerged foil systems. Control is also needed for maneuvering. Finally, control power may be needed for achieving the desired ride quality, as in using direct lift to counter the effects of waves. If you have a system with high static stability, you need to have more control power for trim. If you have a system that is unstable, you need more control power than a neutrally stable craft.

Once again, hydrofoil spacing comes into account because it provides the moment arm for a given change in force at the foil. If you want to generate a direct force at the center of gravity, this will require more or less control from other foils to cancel out the moments if the foil is located away from the center of gravity. If you want to generate a moment but the foil is close to the c.g., it’s like mounting a door knob near the hingeline of a door – pushing or pulling on the knob will not rotate the door. So you have to consider the foil placement with regard to what controls you intend to associate with it.

Finally, there’s the issue of ride quality. In the longitudinal plane, the hydrofoil can either platform the waves, flying at a constant elevation with respect to the earth; or it can contour the waves, flying at a constant distance above the water surface and following the wave shape. If you’re platforming, foil spacing may not be that important. Platforming requires a lot of direct lift control power, though, and the size of the wave you can platform at a given speed may be more limited by the control power than the flying height. But if you’re contouring, then the craft will be maneuvering much more aggressively in pitch, and the foil spacing issues above come into play. No hydrofoil on the ocean does exclusively one or the other. Wave heights greater than the flying height have to be contoured. And the short wavelengths have to be platformed.

In the lateral-directional axes, ride quality may dictate how the vessel rolls into and out of a turn, if it rolls at all. Hydrofoils have their center of mass well above the foils. If they do a skidding turn in a upright attitude, there’s an overturning moment toward the outside of the turn that has to be resisted. If they bank into the turn, then they have to roll first, then yaw as they carve the turn, and finally roll out. The rolling in and out of the turn causes lateral accelerations at the crew station that can be very disconcerting. The craft may actually have to apply direct side force to the foils while rolling so as to put the center of rotation near the center of gravity instead of at the foils. So there has to be a coordinated combination of rolling moment, yawing moment, side force, and lift to obtain acceptable lateral ride quality in maneuvers. Foil spacing would be a part of that equation, along with many other factors.

For example, an aft rudder will tend to produce side force to the outside of the turn, whereas a forward rudder would produce side force to the inside of the turn. It might be necessary to apply opposite forward rudder for a rapid change in aft rudder to generate the necessary side force while rolling, then wash out the forward rudder to allow the turn to develop. Depending on how sophisticated the control system is, the foil spacing may be important to tuning the interrelationship between the various forces and moments.

In most papers on hydrofoil stability, you will find equations that have a number of parameters called stability derivatives. They will describe how each derivative affects the craft’s stability and trim. But what you’ll find very difficult is coming up with good numbers for the stability derivatives to represent a given design. Getting those numbers is why companies spend so much money on testing and engineering analysis.

I hope this has given you the pointer you need. I think the Hydronautics handbook, “Hydrodynamics of Hydrofoil Craft”, is the most comprehensive source on the subject. If you can find them, there are two Hydronautics companion volumes, “The Stability Derivatives of a Hydrofoil Boat, Part I (and Part II)” that deal with estimating the numbers you need to actually calcuate the stability of a given configuration.
“,”2004-10-27″,”Tom Speer”,”poopdeck”,” “,””,”0″

“84”,”729116″,”2″,”Foil Spacing||729116″,”Does anyone know of anything that has been published on the fore-and-aft spacing between the main foils and the stabilizer as related principally to pitch stability? I have searched the IHS CD-ROMs (lists of titles and abstracts of those that were at all promising), and have looked through my own file of hydrofoil material (mostly of Grumman origin), and have found nothing. Has this ever been investigated? Or has the spacing of the foils which falls out from the proportions of the hull always provided sufficient pitch stability, and the question has never come up? I want only to be pointed in the right direction; not to have any research done.
Is this question worth answering? I don’t know.

“,”2004-10-27″,”Joe Koelbel”,”nopswd”,” “,””,”0″

“85”,”729115″,”2″,”Re: HYDROFOIL PONTOON||729115″,”Hi Ed. This same question has been asked of IHS several times over the years. Correspondence on the subject is archived on the IHS site at I have yet to see a report or photo of a hydrofoil pontoon boat project completed and working. You should review this information. In particular, Charlie Pieroth’s recollection of his work at Dynamic Development, Inc. should be of interest.”,”2004-10-27″,”Barney C Black”,”poopdeck”,” “,””,”0″

“86”,”725016″,”2″,”Re: HYDROFOIL PONTOON||725016″,”To review the photos as described above go to and copy Ed
“,”2004-10-18″,”Ed DeMoss”,”11030h”,” “,””,”0″

“87”,”725012″,”2″,”Re: HYDROFOIL PONTOON||725012″,”To review the photos as described above go to and copy Ed
“,”2004-10-18″,”Ed DeMoss”,”11030h”,” “,””,”0″

“88”,”724962″,”2″,”HYDROFOIL PONTOON||724962″,” This is my first attempt to acquire information about putting a hydrofoil system under a pontoon boat. We have a 28 Ft. tritoon pontoon boat that has been built by our volunteer rescue dive group. The photos will best describe what it looks like. It is powered by a new Mercruiser Bravo 5.7 I/O with a ProCharger. It has 400 HP and runs right at 40 mph (via gps). It weights right at 6000 lbs. with dive tanks, equipment, and fuel. Planes very quickly.
There is a hydraulic lift on the front that extends 1 1/2 ft into the water on the front end that will lift well over 1500 lbs. The lift is for raising injured persons out of the water and for divers to get out quickly.
I weld aluminum as a hobby and have built and designed the entire system. We have an idea on how to do it but we need some advice and maybe some calculations. WE cover a very large lake and think this would be faster to respond to a accident. We have cad drawings on what exists.
2 questions am I asking the right group and is their anyone that can help? Thanks Ed DeMoss “,”2004-10-18″,”Ed DeMoss”,”nopswd”,” “,””,”0″

“89”,”716776″,”2″,”Power Boat Foil Design||716776″,”I am doing some research and feasibility studies on developing surface piercing hydrofoils for a power boat in the 24-30 foot range. I have read up a little on the Talaria as well as pulled the patent documentation on one of the kits they made for the boats back in the seventies. I have a couple of basic questions for the group here. First, when calculating the lifting force of a surface piercing foil is the lifting force of the foil roughly equal to that of a fully submerged foil of the same width as the part of the foil that is under water? Also what NACA foil profiles do people reccommend? The 16-510 design Tom Lang used? I tried plotting this shape out using one of the foil programs and the foil bottom was concave. Is this right or did I mess something up? Thanks for all the help in advance. I’m sorry if some of this is a little simplistic!”,”2004-09-30″,”Jim Harrington”,”nopswd”,” “,””,”0″

“90”,”716553″,”2″,”Hydrofoil kitesurfer||716553″,”Has anyone seen a kitesurfer hydrofoil made of glass/kevlar/carbon fiber instead of the usual and heavy steel?

I am interested in building my own but steel is ruled out due to weight.


Rod”,”2004-09-30″,”Rod”,”rodrigo”,” “,””,”0″

“91”,”716442″,”2″,”Re: Looking for Scott Smith||716442″,”Look no farther, you’ve found him again! I can still be reached at Missed you Diane. Sorry Tori and Todd, been under the weather and out of touch for a few months, but I’m coming back around. For any Dynafoil enthusiasts out there, I’m cleaning out the extra projects and thinking about selling a pair of mine (4 is just too many). I’ll post it here when I get my act together again. By the way, if any of you have a Honda PWC and wondered what would happen if you ignored the warning label and engaged the reversing lever while under way, here is the video. It is almost 3 meg, so if IHS decides not to post it, e-mail me and I’ll send it to you.”,”2004-09-30″,”Scott Smith”,”nopswd”,” “,””,”0″

“92”,”715374″,”2″,”Check the Dynafoil area||715374″,”Hi Diane,
Check out the Dynafoil threads that he and I were posting on. Here’s one address that’s listed:

I’m not sure if it’s good though, and I had the same fatal crash a few weeks ago and lost his other address too.
Todd”,”2004-09-28″,”Todd Miller”,”nopswd”,” “,””,”0″

“93”,”714505″,”2″,”Looking for Scott Smith||714505″,”I am looking for Scott Smith from Florida. My computer crashed a few months ago and I lost his e-mail address. “,”2004-09-26″,”Diane Bell”,”nopswd”,” “,””,”0″

“94”,”701771″,”2″,”Re: foilboard design||701771″,”Sam,
The choice of aluminium or carbon fibre on a foam core would be a decision you need to make based on what you are most comfortable working with. Either material should in principle be strong enough for this purpose (provided your carbon skins used over the foam core are thick enough). One of the tricky bits for either construction material would be to work out how you would connect the foil to its support strut. I assume you are working on using an inverted “T” shape foil and strut? If you are able to weld aluminium or can get someone to do that for you after you make the parts, that may prove to give the best connection at the junction of the T. For carbon fibre, making a sufficiently strong junction will take a bit more design and construction effort so that it does not simply fail at that point when you do more harsh manoeuvres with the board. “,”2004-08-30″,”Martin Grimm”,”nopswd”,” “,””,”0″

“95”,”697278″,”2″,”Re; foilboard design||697278″,”I suggest you take a look at Rich Miller´s article on hydrofoil sailboards. Go to: You can also contact him directly for advice. I do not believe that he monitors this BBS.”,”2004-08-20″,”Barney C Black”,”poopdeck”,” “,””,”0″

“96”,”693486″,”2″,”foilboard design||693486″,”I am a kitesurfer and wakeboarder from England. I amm thinking of making a foil to go on the bottom of a board I have made. It would be for use at speeds of up to 20knts. I weigh about 13st, and my board is about 125cm long. I wouls be very keen to get some advice on foil design and building. Although I had been planning to make the foil from foam and carbon, I know that most production foils are aluminium. Why is this, and which is better to use. I have very little experience of hydrodynamics, but am keenm to learn. Many thanks, S.”,”2004-08-12″,”Sambo”,”nopswd”,” “,””,”0″

“97”,”692405″,”2″,”Re; Re; Attitude control system||692405″,”Walt,

1. The text book “Theory of Wing Sections” by I.H Abbott & A.E. von Doenhoff provides geometry definition of various NACA profiles.

2. NACA foil sections are appropriate for underwater use. The main difference between air and water is the density of the fluid, that is easy to account for, see elsewhere on our website for information. Another issue is cavitation. This may not be a problem if your application is for relatively slow towing speeds.

3. The center of lift of foil sections is typically a quarter of the chord chord length aft of the leading edge. It remains at a relatively constant position for small variations in angle of attack.

4. Another package you could consider using is Wing Analysis Plus by Hanley Innovations ( This would help with answering many of the above issues.”,”2004-08-10″,”Martin Grimm”,”nopswd”,” “,””,”0″

“98”,”688064″,”2″,”Re; Re; Attitude control system||688064″,”Tom, thanks for the link. I’ll take a look.”,”2004-08-01″,”Walt Allensworth”,”nopswd”,” “,””,”0″

“99”,”687887″,”2″,”Re; Attitude control system||687887″,”//”,”2004-08-01″,”Tom Speer”,”nopswd”,” “,””,”0″

“100”,”685359″,”2″,”Attitude control system||685359″,”Hi! I’m building an underwater attitude control system that is to keep a towed device nearly horizontal. This system will include two movable underwater foils of modest size and force (under 100lb). The angle of attack of the foils will be controlled by weights. Is there a program I can use to generate [X,Y] pairs of points that describe common hydrofoil cross-sections? Are NACA foil sections appropriate for underwater use? Also… knowing the exact center of lift of the foil section is a critical aspect of the design. Are there programs that identify the center of lift of common foil sections?

Thanks in advance!”,”2004-07-27″,”Walt Allensworth”,”nopswd”,” “,””,”0″

Re; Re; How foils lift

      • Terry, I agree completely. I would just like to know what proportion is due to deflection downwards by the underside compared with suction from the upper side, especially at takeoff.
      • regds, Ian

[Date/Time=05-26-2003 – 11:11 PM]

    Name:Ian Ward, [Msgid=441938]
    Re; Re; Re; How foils lift

        • Ian. You asked: “I would just like to know what proportion is due to deflection downwards by the underside compared with suction from the upper side, especially at takeoff.”
        • I guess what I’m saying is that I agree with Martin Grimm when he said (in an earlier reply to your question) that the two surfaces mutually interact in their contribution. Integrating the pressures over each surface of the wing does yield the “suction force” on the upper surface, and the “pressure force” on the lower surface–and the vertical component of the vector sum of the two will yield the lift force on the wing. These pressures/forces are important in structural considerations.
        • However, I think these numbers can be misleading in describing the aerodynamics/hydrodynamics of the situation. In general, I believe that the pressure force on the bottom side will not be the same as the force on it solely “due to deflection downwards” (of ambient fluid mass).
        • Here’s my conceptual model of how I think it works (and I’d appreciate hearing about any errors in this model):
        • A positive (or negative) angle-of-attack to a wing/foil shifts the location of the stagnation point relative to the leading edge of the wing/foil (this is the reason that a symmetrical section can generate lift at a non-zero AOA). The location and magnitude of this high pressure area determines (in part)the upwash over the leading edge of the wing (and the circulation around the section). This upwash (or the circulation), in turn, affects the magnitude of the suction force.
        • For a symmetrical wing/foil section generating a positive lift force, the stagnation point occurs below the leading edge on the lower face of the wing/foil. Hence the upward deflection (or “negative downward deflection”) by the lower surface of the wing/foil affects the suction force on the upper surface.
        • An example: Consider a (hypothetical) flat planing hull of infinite aspect ratio traveling across the surface of water. There will be no fluid circulation around the hull. Now consider a wing/foil section (i.e. also infinite aspect ratio) moving through the fluid. If integration of the pressure over the upper and lower surfaces of the wing/foil yields pressure forces in the ratio of ~ 3:1 (as is commonly suggested), then one would expect the wing/foil to have a lift-slope coefficient that is four times that of the planing hull alone.
        • But measurements show that the ratio is only slightly more than 2:1. That says to me that removal of the barrier to circulation (i.e. the air/water interface) allows the high pressusre area on the underside forward area of the hull to (in part) drive a circulation over the top of the wing/foil (i.e. generating the upwash ahead of the leading edge). This results in a reduction of the speed of the flow past the underside (decreasing both the mass flow/unit time and the downward component of momentum added to that flow–and hence the pressure force resulting from downward deflection of fluid mass) and boosting the flow over the upper side–hence increasing the suction flow. So the lower surface of the wing/foil contributes to the magnitude of the suction force on the upper side of the wing/foil.

    [Date/Time=05-27-2003 – 2:02 PM]

      Name:Terry Hendricks, [Msgid=442328]
      Re; Re; Re; Re; How foils lift


            • Terry,
            • I must agree that you have put a very good case for the synergy between increased pressure on the underside interacting to also create increased suction on the upper side, both contributing to the total downwash. So it would appear that the two cannot really be separated.
            • Hence it would appear to be erroneous to consider that an airfoil works by suction rather than deflection, as without deflection, there would be no suction and vice versa.
            • I can therefore see how symmetrical foils, barn doors and sails on boats work can all create lift, and why this is proportional to the angle of incidence.
            • The conclusion I am drawing is that there should therefore be no major benefit in using an asymetric thick airfoil, when compared with a thin curved foil such as a sail, provided the correct angle of attack and camber are maintained appropriately.
            • Any thoughts on this?

        [Date/Time=05-28-2003 – 8:30 AM]

          Name:Ian Ward, [Msgid=442866]

              • Ian,
              • I’m afraid that your question/comment about the relative benefits of a thin, curved foil (given the correct angle of attack and camber) vs an asymetric thick airfoil is entering territory incognita for me (I assume that you’re speaking from the standpoint of aerodynamic benefits–e.g. lift/drag ratio, max lift, etc.–as clearly a thick section has structural benefits). Clearly there have to be some benefits otherwise there wouldn’t be so many airfoil sections that have been developed. As far as whether they are major benefits, I guess that depends on your point of view. If one section has a max lift coefficient 0.1 greater than another, that can certainly be a major benefit if you can safely take off from a field with one, but not the other. Similarily, if the drag coefficient is 5% lower for one than another, that will be a major benefit if one with get you back to land, and the other won’t 🙂
              • Here’s some differences in properties (all at low Re) that I have seen mentioned 🙂
              • 1. Except at low reynolds numbers (Re < 85,000) the max lift coefficients for asymetric thick sections appear to be significantly higher than for a cambered thin wing, or a flat plate.
              • 2. The drag polars are also different for a cambered thin wing vs an asymetric thick wing. So I would imagine one section might be favored over the other depending on the lift coeffient necessary for the planned optimized configuration.
              • 3. The lift-slope coefficient is significantly greater (at least for Re < 420,000–the highest value in the comparison I saw) for the thin cambered airfoil (followed by a flat plate, and then followed by several thick, asymetric airfoils). I suppose that these differences could be used if one is using mixed airfoil sections for pitch stability.
              • Note: The sections compared were a flat plate, a thin cambered airfoil (“417A”), N60, N60R, 625. Original reference: Schmitz, F.W. Aerodynamics of Model Aircraft Wing Measurements I, R.T.P. Translation No. 2460, Issued by Ministry of Aircraft Production. The data I saw was a summary contained in: B.W. McCormick, Aerodyanmics, Aeronautics, and Flight Mechanics. John Wiley & Sons. NY. 1979.
              • Hope this helps.

          [Date/Time=05-28-2003 – 12:28 PM]

            Name:Terry Hendricks, [Msgid=443014]

                • Ian,
                • While I have been thinking about your questions and comments ‘offline’, I see that Jim and Terry have provided good feedback. Terry very neatly described in a few words what I was trying to say about downwash.
                • All the same, since I have already done some more number crunching on the relative contribution to lift from the pressure on the bottom and top of a typical foil (for my own benefit too!), here are my additional comments:
                • There has been another posted messages enquiry for which I have now provided data on the pressure distribution around a NACA 0015 section airfoil (or hydrofoil) but that also helps to answer your question. I hope you are able to open the Excel spreadsheet attached to my other reply. The pressure coefficients in that spreadsheet are from a numerical calculation rather than test data so would not be completely accurate.
                • The NACA 0015 hydrofoil is a symmetrical one, that is to say it has no camber and so the top and bottom surfaces are a mirror image. This section is popular for use in constructing ships rudders. The ?15? part of the designation indicates that the thickness to chord ratio is 15%, ie the maximum thickness of the section is 15% of the chord length.
                • The pressure distribution around a foil is often expressed in terms of the pressure coefficient (Cp). This can be thought of as a measure of the relative pressure around the foil. To get the actual surface pressure (P) at any location on the foil, use the following formula:
                • P = Po + 0.5 rho V^2 Cp
                • where:
                • Po = Pressure far upstream away from the foil (Pa).
                • rho = density of the fluid, which is around 1025 kg/m^3 for salt water
                • V = velocity of foil through the fluid (m/s)
                • If you integrate the pressure over the chord of the foil on both the upper and lower surface in the case of the NACA 0015 section, then you can estimate how much the pressure on each surface contributes to the total lift generated by the foil. The spreadsheet includes that calculation, though somewhat approximately. The results are shown in summary below:
                • Key:
                • “AoA” = Angle of Attack of the 2D NACA 0015 profile
                • “Top” = shows the % of the total lift generated by the low pressure distribution on the top surface.
                • “Bottom” = shows the % of the total lift generated by the high pressure distribution on the bottom surface.
                • AoA Top Bottom
                • (deg) (%) (%)
                • 2 146.5 -46.5
                • 4 101.2 -1.2
                • 6 87.4 12.6
                • 8 81.6 18.4
                • 10 78.9 21.1
                • You can see that at 4 degrees angle of attack or less, the net pressure force on the bottom surface of the foil is still trying to suck it down rather than lift it. For such small angles of attack, the top surface is therefore the only side generating any net lift force. At 10 degrees angle of attack, you can see that the bottom surface is now contributing 21.1% of the total lift, but even at that angle, the pressure distribution on the top surface is still generating 3.7 times as much lift as the pressure on the bottom surface. But of course if you change the shape of either surface of the foil, you will not just influence the pressure distribution on that side, but over the whole foil. In other words, the whole calculation needs to be repeated for the new foil geometry.
                • Many of the descriptions of how an airfoil generates lift that can be found in books are quite misleading as you have noted. There is always the suggestion that a wing needs to have a curved top surface and a flat bottom so that the fluid travelling over the top has further to travel than the fluid over the bottom surface. The reasoning is then the fluid travelling over the top needs to speed up and hence its pressure drops. This is too simplistic as Terry has also observed. As you observe, a flat plate (like the wing of a paper aeroplane) will happily generate lift if it is set at an angle of attack to the flow. This is possible because the sharp trailing edge of such a flat inclined plate forces the aft stagnation point of the fluid to move to the trailing edge in any real viscous fluid. This leads to a circulation of flow around the ?foil? and the strength of that circulation governs the amount of lift that is generated. It is not so easy to explain that in the average book (and I have far from described this properly here).
                • A flat plate foil is however not the best solution if you want to maximise your lift for the minimum amount of drag. This is why cambered foils with streamlined thickness distributions are used on aircraft. Exceptions are aerobatic aircraft such as the Pitts Special which I understand has symmetrical wing profiles since the designer is seeking equal performance with the aircraft flying either upright or inverted!
                • I don’t have a good feel for the performance of thin yacht sail type foil sections, but if you consider the developments in hang glider design, the most high performance hang gliders have reverted to ‘wings’ with separate fabric upper and lower surfaces. These help to shrowd all the support beams and so reduce drag, but that thickness distribution of the foil profile is probably better than a single surface as well.
                • Finally, returning to the discussion about the downwash effect of a wing. Terry has also noted that the amount of downwash is an effect caused by the entire wing and the amount of downwash is directly related to the amount of lift that the wing has generated. It may be worth illustrating what Terry said about Newton’s law by using an analogy with water flowing around a corner in a pipe:
                • Lets take a pipe with a one square metre cross section and assume the flow velocity of water in the pipe is a constant 5 metres per second. The volumetric flow rate of the water is therefore 1 x 5 = 5 cubic metres per second. For ease of analysis lets assume we have fresh water with a density of 1000 kg/m^3 (or one tonne per cubic metre) in the pipe. The mass flow rate of the water is therefore 5 x 1000 = 5000 kg/s (or 5 tonnes per second). Now, if the pipe has a bend so that it changes direction by only 5 degrees (similar to a typical downwash angle of fluid around wing), the fluid is now being redirected in that new direction of the pipe at a rate of 5 tonnes per second. Two of Newton?s laws have words to the effect that:
                • 1. A moving mass will keep moving in the same direction and at the same speed unless it is acted upon by a force. The rate of change of momentum is equal to the force that is applied (F = M.a).
                • 2. Every action must have an equal and opposite reaction.
                • In this case, for the water to have changed direction, the pipe must have applied a force to it. In turn the water has applied an equal and opposite force back on the pipe. The constant force that must be applied to the water is equal to the change of momentum of the water and for pipe bends with a small angular change this is given approximately by:
                • F = M?.V.sin(theta)
                • where:
                • M? = mass flow rate of the water (kg/s)
                • V = water velocity (m/s)
                • theta = the angle by which the pipe changes direction.
                • So the force exerted on the pipe by the water is:
                • F = 5000 . 5 . sin(5 deg)
                • F = 2179 N (or 0.22 tonnes)
                • Looking at this another way, if a hydrofoil travelling through water at 5 m/s was assumed to influence an area of one square metre of the water around it causing that water to have an average 5 degrees downwash far downstream of the foil, then the lift that the hydrofoil would have generated is likewise 0.22 tonnes. If the downwash angle was now increases to 10 degrees, the lift force doubles to 0.44 tonnes and so on.
                • Hope that has helped and not made things more complicated!!

            [Date/Time=05-28-2003 – 12:29 PM]

              Name:Martin Grimm, [Msgid=443016]
              Thin sails vs Airfoils

                  • Hello Terry,
                  • Thanks for your response. I will check out the detailed info you have guided me to.
                  • I am a little surpised that it does not seem clear, even to relative experts as yourself, as to which foil types are likely to give the best performance. I guess that this science is not as cut and dried as I thought it would be. In particular, it would appear to me that a thin cambered section such as a sail should give the best lift/drag, but only at the correct angle of attack and appropriate windspeed.
                  • What seems to be unique about a sail is that both camber and angle of attack can be maintained at close to optimum by using a flexible mast and manually altering the rig controls, unlike for aircraft, hang gliders and hydrofoils. This does not seem to be well recognised and is probably why solid foils have rarely shown any significant advantage over conventional sails.
                  • Thicker foil sections appear to come into their own, because they offer a good compromise of properties over a wide range of angle of attack and speed, for a fixed section shape which cannot be readily adjusted. This is exactly what is needed for fixed wing aircraft because the angle of attack is usually high at take off and reduces as the speed picks up. Also, the required camber reduces as the speed picks up to reduce drag.

              [Date/Time=06-03-2003 – 9:21 AM]

                Name:Ian Ward, [Msgid=446269]
                Counter intuitive!


                      • Hello Martin,
                      • Many thanks for your detailed information. I am still trying to digest the implications.
                      • I find it counter intuitive that there is a negative lift contribution from the underside of a symmetrical wing or foil at angle of attack below about 5 degrees, and that it remains so much less than the positive lift generated by the negative pressure on the upper surface at higher angles of attack.
                      • Will need to work through this, but I guess there is a natural tendency for the underside to displace fluid over the top, rather than being compressed and forced down.

                  [Date/Time=06-03-2003 – 9:40 AM]

                    Name:Ian Ward, [Msgid=446282]
                    Variable geometry foils


                          • I’ve seen many people ask about using various aircraft techniques to improve some aspect of a hydrofoil design. Somebody even went as far as to ask if a rotating foil design similar to a gyroplane might be made to work, although I can’t imagine gaining anything from that exercise. One problem that seems to come up a lot is the need for high lift at takeoff, and reduced drag at cruising speed. This is made more critical for limited power applications like human powered craft. A common solution is an extra foil that retracts at speed. I wonder if variable geometry might be an answer. Two common aircraft applications are the ‘swing wing’ like the f-14 Tomcat, and the ‘scissor wing’, usually an elliptical wing that rotates as a whole on a pivot point above the center of the fuselage. Would producing a variable geometry ‘swing wing’ help to reduce drag at higher speeds and give better low speed lift? Or am I barking up the wrong tree? I’ve never heard of a hydrofoil design using this feature.

                      [Date/Time=06-05-2003 – 4:35 PM]

                        Name:Scott Smith, [Msgid=447702]
                        Paddlewheel Hydrofoil


                              • My question is the subject line, i.e. : Has there ever been a paddle wheel hydrofoil? Is there a picture of such a craft?

                          [Date/Time=06-08-2003 – 4:37 AM]

                            Name:Mark Lape, [Msgid=448885]
                            Paddlewheel Hydrofoil

                                • Hi Mark, if you are talking foils applied to a Mississippi River Boat type of paddle wheel boat, I have not seen one. There is some interesting correspondence and photos about a “hydrocopter” with “autorotating foils” on our page at:


                                • (see photos below). Maybe you could describe in more detail what you are thinking of, also whether you are talking about a motor-powered or human-powered craft?

                            [Date/Time=06-08-2003 – 4:44 AM]

                              Name:Barney C. Black, [Msgid=448886]
                              Lift coeffs for surface-piercing vee foils


                                    • Can anyone provide me with values of lift coefficients for surface-piercing Vee hydrofoils of the type Gordon Baker used so successfully for his motorboats? He used the NACA 16-510 foil section, about 40 deg dihedral, and the lower corners of his foils had circular-arc curvature. Such lift values, with, of course, the associated values of aspect ratio and angle of attack, would be very much appreciated.

                                [Date/Time=06-09-2003 – 10:16 AM]

                                  Name:Eugene Clement, [Msgid=449347]
                                  Foil-Borne Draught


                                        • I have a 14′ fibreglass planing hull that I am considering adapting for use with a hydrofoil. The distance between the bottom of the keel and the horizontal centreline of the propellor is about 8″. Is this enough draught to enable a hydrofoil to operate effectively? My boat has a 50 hp outboard on it and the speedo goes right up to 60 mph (52.6 knots). I know that a well-designed foil will increase fuel economy at high speeds. Will using a foil also result in greater acceleration and/or higher top speed?

                                    [Date/Time=06-10-2003 – 4:08 PM]

                                      Name:Mike K., [Msgid=450211]
                                      Foil benefits


                                            • The good news is you do have enough water above the prop for foils, assuming mostly smooth water. Using foils will increase you fuel economy. The bad news is they may not increase your performance for this boat. Subcavitating foils are fairly simple to make, I can send you plans. But they are limited in speed. If your boat already does 60mph as indicated, you are already at about the highest speed you can expect to go on subcavitating foils. Adding them would help your fuel economy, but might slow your takoff a tad. Supercavitating foils might help you go faster, but there is less info available on them. You will be lucky to be able to make supercavitating foils that work for that boat. Had your boat had less power, say 25hp, then you would see real benefits in speed and economy. As it is, it won’t gain you much to add foils, but will cause you trouble in shallow water, docking, etc.

                                        [Date/Time=06-11-2003 – 6:37 AM]

                                          Name:Scott Smith, [Msgid=450522]
                                          Lift Coefficients for Surface Piercing foils


                                                • Hello Eugene,
                                                • There are many effects that should be taken into account in calculating the lift (and drag) of surface piercing hydrofoils. These include for example lift loss of foils operating near or cutting through the water surface.
                                                • I would recommend Chapter 3 of the book “High Speed Small Craft” by Peter Du Cane (1974 edition) as an ideal reference to use to prepare an estimate of the lift generated by a particular foil arrangement. In that chapter Michael Eames specifically considers surface piercing V foil arrangements. The various equations are too involved to repeat here but could be incorporated in a spreadsheet calculation or similar.
                                                • The alternative is to back the overall lift coefficients out of the particulars of a known hydrofoil. What you need to determine is the speed, submerged planform area (at that speed) and weight of the boat. I have tabulated that for a few surface piercing hydrofoils of the Supramar type and the typical CL values for the overall craft are in the order of 0.26 to 0.43. I don’t know the details of the foil profiles in those cases however.
                                                • Regards,
                                                • Martin Grimm

                                            [Date/Time=06-11-2003 – 12:03 PM]

                                              Name:Martin Grimm, [Msgid=450715]
                                              Paddle Wheel Hydrofoil

                                                  • Hi Mark,
                                                  • Now I just wonder whether the idea you are thinking about is the same as one I have had for many years but have not pursured sufficiently to apply on a full scale application? I coined the term “Rotorfoil” to describe the concept I had in mind and later stumbled across the article that described the very similar “Hydrocopter” concept that Barney has already pointed out to you.
                                                  • I am keen to hear more from you too…

                                              [Date/Time=06-11-2003 – 12:15 PM]

                                                Name:Martin Grimm, [Msgid=450724]
                                                Good News Really


                                                      • Thanks for the reply, Scott. I plan to run mostly on lakes, and the engine is not working as of yet, so I haven’t actually tested the boat as fit out. I got it for free. Sounds like I can get nearly the same big-engine performance with a much smaller engine and a foil. If I bought a new 25 hp engine, would it be worthwhile to get one with a longer vertical power shaft, so as to increase the distance between the hull and the foils?

                                                  [Date/Time=06-11-2003 – 6:58 PM]

                                                    Name:Mike K., [Msgid=451045]

                                                        • I don’t know if I would go so far as to say you’ll get the “same performance as the big engine”, that’s really comparing apples and oranges. Your performance with a 25HP engine and foils will certainly be better that a 25hp and no foils, but different from the 50hp. With the 25 and foils you will get good top end, excellent fuel economy, and a cushioned ride. But you have to get it all working together. With the 50hp you will have more pulling power, docking will be easier, and take-off will be faster, it just depends on what you want. Anyway, yes you can put a long shaft on the boat to get more lift. I put a 15HP extra long shaft on my 14′ aluminum boat for a hydrofoil experiment. Even with no foils it doesn’t hamper performance enought to notice. But you may need to add some strategic sheet metal deflectors to keep the extra shaft length from entraining water up the tower and into your boat. Not a problem when foilborne. Trailering and shallow water operation can be a pain if the engine is really low, like in my case. Transom plates don’t allow but a few inches of lift. In short, adding a longshaft won’t affect you much, and will help with the foils, adding an extra long shaft will require some compromises. I found the easiest and cheapest way around the extra-longshaft problems was just to use a trolling motor for docking and shallow water. BTW, if you can afford it, I love my 4-stroke, and it runs forever on a tank of gas 🙂 Yes, if you are intent on getting this flying, a long shaft outboard will work really well, but if you never finish the project, it may hamper the resale value of the boat, being smaller hp and a longshaft.

                                                    [Date/Time=06-12-2003 – 8:13 AM]

                                                      Name:Scott Smith, [Msgid=451290]
                                                      New Info on Hydrofoil Sailboard Design

                                                          • Rich Miller has just published a photo-illustrated technical paper on his hydrofoil sail board. It is accessible from a link on the IHS webpage devoted to this subject:

                                                      [Date/Time=06-15-2003 – 6:30 AM]

                                                        Name:Barney C Black, [Msgid=452967]
                                                        Hydrofoil Surfboard Details Wanted


                                                              • Can you steer me to a web site that shows a hydrofoil surfboard in detail? I want to know how this system works.

                                                          [Date/Time=06-24-2003 – 9:11 PM]

                                                            Name:Merv Rice, [Msgid=459271]
                                                            Re; Hydrofoil Surfboard Details Wanted

                                                                • I don’t have any design details unfortunately, though these surfboards appear from a distance to have something in common with the Air Chair and other hydrofoil waterskis. Laird Hamilton is the pioneer in this. He has a video out on DVD, and supposedly the bonus materials on the DVD have information about the hydrofoil surfboard. See our page at


                                                                • . I have not seen the video, so I don’t know if the hydrofoil info is a little or a lot. We have some archived correspondence on this subject on our page at


                                                                • .

                                                            [Date/Time=06-24-2003 – 9:24 PM]

                                                              Name:Barney C Black, [Msgid=459280]
                                                              Hydrofoils ? Available

                                                                  • I’ve been looking for alternative sources of foils for a small (28′) powered boat. I’ve been looking for used Robinson R-22 helicopter blades and found these larger R-44 blades available. Larger than what I need, I thought I’d pass along the info. If interested, email me and I’ll give you the seller’s contact info. Put Hydrofoil in the subject line.
                                                                  • Chord 10 “
                                                                  • Thickness 1.5″
                                                                  • They are not tapered,
                                                                  • length aprox 15 ‘ each, I have 2.
                                                                  • Leading edge is stainless steel D shape extrusion with aluminum skins bonded
                                                                  • aft of that with a aluminum honeycomb core. Very strong and smooth, removed
                                                                  • from a 1995 robinson R-44 4 passenger helicopter with 165 hrs since new due
                                                                  • to factory recall. I was planningto use on an expermintal copter. Would sell
                                                                  • pair for $500 plus shipping.
                                                                  • They are located at Napoleon OH

                                                              [Date/Time=07-14-2003 – 8:06 AM]

                                                                Name:Barry Steele, [Msgid=462263]
                                                                Re; Variable geometry foils


                                                                      • Scott
                                                                      • I have been wondering the same thing.
                                                                      • Is the fact that there are no ‘swing wing’ hydrofoils due to the operating speed of current hydrofoils not being high enough for it to be of any benifit? Aircraft speeds in the range from mach 0.5 to mach 1.0 (approx 300 to 600 knots) apparently benifit from increasing wing sweep angles.
                                                                      • Is there a good corelation between aerodynamics and hydrodynamics? Could the above speed range be converted to the equivelent for a hydrofoil wing? One assumes the speed would be much slower given waters much higher density. How is caviation affected by a swept wing?
                                                                      • It would also be of benifit for coming alongside as the wings could be over swept (as per the F-14) so the are in side the hull line.

                                                                  [Date/Time=07-11-2003 – 5:56 AM]

                                                                    Name:Graeme Paulin, [Msgid=468029]
                                                                    Re; Re; Variable geometry foils

                                                                        • Hydrofoils wings have to be very much thinner than aeroplane wings to support the same weight, so they have to be proportionally stronger. It is therefore difficult to build in the sort of features that let aeroplane wings change shape, like extending flaps and droops, as they would weaken the wing too much. That is one reason why hydrofoils have a relatively small speed range (about 2:1 full speed to take off) compared to aeroplanes (often 4:1 full speed to take off).
                                                                        • As for swing wings, they are fitted to supersonic aeroplanes to reduce drag and leading edge heating at supersonic speeds. Hydrofoils get nowhere near supersonic speeds, especially as the speed of sound in water is about 3000 mph. Hydrofoil top speed can be limited by cavitation, and swinging a wing would do very little to reduce that. Cavitation can only be reduced by making the foils even thinner, or, of course, slowing down.
                                                                        • Many submerged foil hudrofoils have foils that can be swung up out of the way to reduce draft and sometimes beam for docking, but these usually swing the whole strut and wing assembly. Even small irregularities in a wing would cause cavitation which would damage the wing, so the designers have felt that it it best to keep the mechanisms at the top of the strut and out of the water.

                                                                    [Date/Time=07-12-2003 – 3:38 AM]

                                                                      Name:Malin Dixon, [Msgid=468623]
                                                                      foiling vs planing


                                                                            • A previous post provided a link to a gentleman that buids foiling sailboards ( I have since misplaced the link) In his design the forward foil is a supercavitating section that essentially planes on the surface to control the angle of attack on the main lifting foil. My question is: in terms of drag (ignoring waves and smmothness of ride) which is more efficient, a planing surface or a submerged lifting one? what order of magnitude?
                                                                            • Steve Rhodes

                                                                        [Date/Time=07-28-2003 – 2:10 PM]

                                                                          Name:Steve Rhodes, [Msgid=477506]
                                                                          Re; foiling vs planing

                                                                              • You are probably thinking of Rich Miller. He has recently published a nice guide to his hydrofoil windsurfer design. That document will be on IHS’s Advanced Marine Vehicle CD-ROM #2 when it is released in the near future. In the meantime the document is posted on the web at


                                                                              • . There is an email link and a phone number in the document, so you could pose your question to Rich directly.
                                                                              • Another person with an active interest in this subject is GœGard Delerm. See his website at

                                                                          [Date/Time=07-31-2003 – 6:56 PM]

                                                                            Name:Barney C Black, [Msgid=479692]
                                                                            Force Calculation


                                                                                  • Hey everyone,
                                                                                  • Could anyone point me in the right direction here? Im looking to calculate the optimal surface area dimensions and pitch of a triangular hydrofoil that would generate 550 – 650 pounds of downforce at around 22-24 miles per hour? And then, how would a minor (1-2 degree) increase in pitch affect the downforce? Any help or hints here would be greatly appreciated.
                                                                                  • Cheers folks.

                                                                              [Date/Time=08-01-2003 – 2:08 AM]

                                                                                Name:Dan, [Msgid=479875]
                                                                                Re; Force Calculation

                                                                                    • Try out Xfoil.
                                                                                    • Cheers
                                                                                    • Mk


                                                                                [Date/Time=08-03-2003 – 6:19 PM]

                                                                                  Name:Mark Hursthouse, [Msgid=481140]
                                                                                  Re; foiling vs planing

                                                                                      • Hi Steve,
                                                                                      • You can read and compare the two NACA reports :


                                                                                      • and


                                                                                      • GœGard

                                                                                  [Date/Time=08-04-2003 – 6:39 AM]

                                                                                    Name:GœGard Delerm, [Msgid=481324]
                                                                                    Re; Force Calculation

                                                                                        • Do you have the AMV CD #1? Chapter 2 of “Hydrodynamics of Hydrofoil Craft Subcavitating Hydrofoil Systems” is devoted to this subject.

                                                                                    [Date/Time=09-01-2003 – 5:03 PM]

                                                                                      Name:Tom Speer, [Msgid=498264]
                                                                                      Re; foiling vs planing

                                                                                          • As usual, the answer is, “It depends.”
                                                                                          • The drag of a planing surface comes mainly from three sources: the skin friction on the wetted surface, the induced drag of the dynamic lift, and the wave drag. Hydrofoils also suffer from the same three. So for a hydrofoil to be a net benefit, you have to look at how it affects each of these drag contributions.
                                                                                          • A hydrofoil is wetted on both surfaces and the wetted area of the strut(s) has to be counted, too, so depending on the configuration, the hydrofoil may or may not reduce the wetted area below that which is immersed by the hull. Typically the area will be less unless you’re trying to fly at low speed.
                                                                                          • Drag due to lift depends on the square of the span of the lifting surface. It’s fairly easy to build a hydrofoil with an immersed span greater than the wetted width of a V-shaped planing hull. In addition, the drag due to lift at the surface is twice that of a very deeply submerged hydrofoil, so there’s a gain in just getting away from the surface. This is possibly the hydrofoil’s biggest advantage.
                                                                                          • Then there’s the wave drag. Deciding just what is wave drag and what is induced drag depends a bit on how you set up your drag accounting bookkeeping, but hydrofoils generally produce lower waves and have a vastly smaller waterplane area, so the hydrofoil undoubtedly has less wave drag, too.
                                                                                          • To really answer the question, you’d need to estimate the drag of the planing boat’s configuration using something like the Savitsky method, and then estimate the drag of the particular hydrofoil configuration. There’s lots of information on this on the AMV CD’s.

                                                                                      [Date/Time=09-01-2003 – 5:17 PM]

                                                                                        Name:Tom Speer, [Msgid=498274]
                                                                                        Re; Re; Variable geometry foils

                                                                                            • Simple sweep theory says that you can neglect the spanwise component of the velocity for the purpose of calculating the pressure distribution across the chord. So sweeping a subcavitating foil would offer some relief from cavitation, much like making the foil thinner would do.
                                                                                            • Wings are swept to reduce the local Mach number and ensure that the pressure disturbance from the wing’s shape can propagate upstream and avoid forming a shock wave. I don’t see this having a close analog with regard to hydrofoils. There may be some reduction in transverse wave drag from sweep, because the sweep would smooth out the cross sectional area distribution of the foils, like area ruling of a transonic airplane design.
                                                                                            • Spanwise flow may also help inhibit ventilation, especially for surface piercing foils that are swept forward, putting the foil station at the surface aft of the submerged stations. I’ve not seen much information on just how effective a given degree of sweep is in this regard, though.

                                                                                        [Date/Time=09-01-2003 – 5:30 PM]

                                                                                          Name:Tom Speer, [Msgid=498276]
                                                                                          Re; Thin sails vs Airfoils


                                                                                                • You’re right about an aircraft needing to operate over a wide range of lift coefficients, mainly because it can’t change its wing area. So if the cruise speed is, say, more than 3 times that of the landing speed, the lift coefficient will vary over an order of magnitude. It’s critical that the wing produce low drag at high speed. Drag at high lift on landing approach is actually beneficial because it steepens the approach path.
                                                                                                • A sail, on the other hand, needs to operate in a much narrower range of (high) lift coefficients, and produce low drag at high lift. It cannot operate at low lift coefficients at all because it will luff. This accounts for the use of solid wings and wingmast rigs on high-speed sailing craft that do operate at lower lift coefficients (ie, landyachts, C-class catamarans, ice boats). Sail area is reduced (by reefing and sail changes) to match the lift to the available righting moment at high apparent wind speeds.
                                                                                                • At any given deisgn point, a thin section will outperform a thick one from a purely aerodynamic perspective. Thickness in a sail is only desireable from the standpoint of providing the necessary structural support (mast) and for widening the operating range of angles of attack (wider “groove”). But it’s not necessary or desireable to carry the thickness across the whole chord.
                                                                                                • For example, here’s an XFOIL prediction of the flow around a tear-drop shaped wingmast and sail combination:


                                                                                                • . The lee side contour is identical to that of the Clark-Y airfoil. Angle of attack is 8 degrees, the chord Reynolds number is 1.0e6, and natural transition is assumed. Lift coefficient is 1.8 – high but below stall. The white lines show the section contour and inviscid pressure distribution (no boundary layer effects), the yellow line the lee side viscous pressure distribution (with boundary layer effects) and boundary layer displacement thickness, and the blue line the windward side pressures and boundary layer thickness.
                                                                                                • At this angle of attack the lee side is fully attached, and stall will begin at the trailing edge at higher angles of attack. On the windward side, the shape is effectively distorted by the presence of a separation bubble behind the mast. The separated flow acts like a wedge, creating an adverse pressure gradient on the mast earlier than the inviscid prediction. This causes laminar separation, as seen by the short horizontal segment in the pressure distribution, followed by transition to turbulent flow indicated by the resumption of the increase in pressure coefficient. The viscous and inviscid pressure distributions come together again near where the flow reattaches to the windward surface of the sail.
                                                                                                • It’s clear that the sail would benefit from a windward surface that would roughly correspond to the boundary of the separation bubble. This would have little effect on the pressure distribution about the sail but would eliminate the drag of the separation bubble.
                                                                                                • With regard to hydrofoils, a hydrofoil that operated most of the time at high lift coefficients could benefit from using a thin section. This might be a surface-piercing foil designed to lift the boat at low speeds. However, operating at high lift coefficients also means low pressures on the foil, and cavitation at low speed. Plus, narrow displacement hulls are far more efficient than hydrofoils in this speed range so there’s no point to using hydrofoils.
                                                                                                • Hydrofoils are typically operated at high speed, and are necessarily limited by cavitation to low lift coefficients. Their thickness is dictated by the need for structural strength and stiffness, but the thickness ratio must be kept low, again to avoid cavitation. This leads to sections with roof-top pressure distributions, like the NACA 1- and 6-series sections, and small amounts of camber.
                                                                                                • Fully submerged foils have to operate over a range of lift coefficients, just like airplane wings. Surface piercing foils can operate over a narrower range of lift coefficients, and therefore might benefit from slightly more camber and less thickness.

                                                                                            [Date/Time=09-01-2003 – 6:14 PM]

                                                                                              Name:Tom Speer, [Msgid=498303]
                                                                                              Re; Counter intuitive!

                                                                                                  • There are lots of ways to get at this. One way is to consider the superposition of velocities from a thick symmetrical section and a thin cambered section. This is what the NACA did in coming up with their families of airfoil sections.
                                                                                                  • But a more physically grounded approach would be to consider the laws of conservation of mass and conservation of momentum. The law of conservation of momentum says that the net force exerted on a body is equal and opposite to the net change in momentum of the fluid. A hydrofoil creates lift by bending the flow, and this bend is a change in momentum. Just like driving a car around a bend is a change in the car’s momentum and requires a side force on the wheels to make it happen.
                                                                                                  • Where the flow is bent, there must be a radial change in the pressure, so that each blob of fluid has more force pushing in on it from the outside of the turn than from the inside of the turn. So the pressure on the inside turn must be less than the pressure on the outside of the turn. If you start far away from a deeply submerged hydrofoil, the influence of the hydrofoil has died away and pressure is everywhere the same.
                                                                                                  • At the trailing edge of the hydrofoil, the flow comes off in the same direction as the trailing edge (assuming the hydrofoil is not stalled). So the direction the trailing edge is pointed largely determines how much the flow is bent by the hydrofoil. You can see that angle of attack and camber are essentially the same in this regard – both alter the orientation of the trailing edge.
                                                                                                  • As you approach the hydrofoil from above, you are approaching it from the outside of the turn induced by the hydrofoil. So the pressure must be progressively decreasing. If you started from far beneath the hydrofoil, you would be moving from the inside to the outside of the turn, so the pressure is increasing. In both cases, the pressure where you started – far away from the hydrofoil – is the same; the pressure was being subtracted from the freestream as you approached the upper surface and added to the freestream as you approached the bottom surface.
                                                                                                  • Thus, there is high pressure on the bottom and low pressure on the top, and a difference in pressure between the two sides of the hydrofoil. The amount of the pressure difference, averaged over the top and bottom surfaces, is the lift. And the lift is the same as the net change in the fluid’s momentum due to the bending of its direction as it passed the hydrofoil. That’s conservation of momentum.
                                                                                                  • Now consider the effect of thickness. If you keep the upper side contour the same, the flow has to bend the same in order to follow it. But the flow on the bottom side does not have to bend as much, or locally is even bent the other way. So this reduces the pressure on the bottom side. If you kept the top contour fixed and filled in the bottom contour until it was the mirror image of the top, you’d have a symmetrical section and no lift at all at zero angle of attack. Both sides of the section would be toward the inside of the curve as the flow bent around the thickness of the foil.
                                                                                                  • So if you are aiming for high lift, thickness is detrimental. However, high lift is rarely the objective in hydrofoil design. It’s generally far preferable to add area to get the required lift than it is to go to high lift coefficients. Or better yet, to increase the speed without increasing the area.

                                                                                              [Date/Time=09-01-2003 – 6:56 PM]

                                                                                                Name:Tom Speer, [Msgid=498328]

                                                                                                    • From a design perspective, I think you work the problem the other way. The designer chooses the lift coefficient, according to the dictates of the section, and sizes the foil area accordingly. The operator, on the other hand, doesn’t have control of the foil area, and thus has to alter the lift coefficient to fit the conditions.
                                                                                                    • So I think I can hazard a reasonable guess as to the lift coefficient: 0.3 (+- 0.1).

                                                                                                [Date/Time=09-01-2003 – 7:05 PM]

                                                                                                  Name:Tom Speer, [Msgid=498332]
                                                                                                  Re; Foil-Borne Draught

                                                                                                      • No way, dude. Do the math.
                                                                                                      • You want to fly the boat completely out of the water, right? Say with the keel just 2″ above the water. Prop diameter is, I’m guessing, 6″ – 8″. So even with absolutely flat water, the top of the prop is 2″ – 3″ below the surface. Don’t you think it would ventilate like crazy that close to the surface? Remember, there’s no boat in front of it to prevent air from being sucked down in front of the prop when you’re flying.
                                                                                                      • Now consider the effect of waves. Especially without the effect of the hull smoothing things out. Even with no wind and nobody else on the water, a couple of passes by your own boat will ruffle things up so there are waves more than 3″ high. Your prop will be coming completely out of the water.
                                                                                                      • Getting the power to a prop low enough to work while flying is one of the toughest problems in hydrofoil design. A normal short-shaft outboard isn’t going to do it.

                                                                                                  [Date/Time=09-01-2003 – 7:20 PM]

                                                                                                    Name:Tom Speer, [Msgid=498339]
                                                                                                    Re; Re; Foil-Borne Draught


                                                                                                          • Tom,
                                                                                                          • You have a series of interesting posts, but I can’t find what you’re responding to. Almost all speedboat props are intentionally ventilated above 50mph or so. I’m working on such a ‘lowrider’.

                                                                                                      [Date/Time=09-02-2003 – 10:19 AM]

                                                                                                        Name:jim hynes, [Msgid=498668]
                                                                                                        Re; Re; Re; Foil-Borne Draught

                                                                                                            • Jim,
                                                                                                            • To see the messages that Tom was responding to, you should be able to click on the title of the earlier or original message that appears at the top of the screen where you are reading Tom’s reply. That should automatically open the earlier message for you.
                                                                                                            • Propeller ventilation may be intentional at higher speeds on some boats (to avoid cavitation damage to the blades), or in fact the propellers may be designed to operate supercavitating (water changing state to vapour – ie steam) so that the cavitation bubble forms right at the blade leading edge and collapses well downstream of the propeller and so does not erode the blade surface. But if you can make a propeller operate free of ventilation or cavitation at the intended speed of the boat, that is still likely to give a solution that requires less horsepower. In this case Tom describes, sucking in air will simply make the engine race at high revs and low torque while the propeller produces little thrust but lots of air bubbles!

                                                                                                        [Date/Time=09-03-2003 – 11:43 AM]

                                                                                                          Name:Martin Grimm, [Msgid=499571]
                                                                                                          Re; Re; Thin sails vs Airfoils

                                                                                                              • Hello Tom,
                                                                                                              • Thank you for your excellent contribution to this discussion and to my ever increasing understanding.
                                                                                                              • You have now prompted me to raise a long standing observation to which I suspect you may have an answer.
                                                                                                              • I am intrigued that texts on sailing boat aerodynamics extoll the virtues of using full wing masts, compared with normal thin foil sails. The impression given is that wings are always easily superior to conventional sails and more recently Dynawing have promoted asymetrical wings in a similar way.
                                                                                                              • From what I can glean, most of these texts do in fact agree that thin foil “normal” sails are actually superior to symmetrical wings in light wings (low Reynolds numbers). They then go on to compare lift/drag characteristics of both symmetric and asymmetric aerofoils at varying angles of attack and camber to those of thin foils, and conclude that aerofoils have higher lift/drag ratios and are therefore significantly superior in moderate to strong winds to standard sails.
                                                                                                              • What intrigues me is that in practice, the differences between thin sails and aerofoils do not seem that great, and in fact there are no thick full wing aerofoils being used on any racing classes, perhaps with the exception of C-class cats. Some of this is to do with existing rules, costs etc, but even in development classes and sailboards, which have no real limitations, full wing sections are not used today….why? Is this perhaps because the benefits are not really there?
                                                                                                              • I would like to present a perspective, which does not seem to be addressed in the texts and seek your opinion…..
                                                                                                              • My comments are based on the following observations & assumptions.
                                                                                                              • 1) When I talk of “normal” sails, these days all performance dinghies, catamarans and sailboards use fully battened, stiff smooth sail cloth, relatively high aspect sails and mostly use over rotating small wing masts or pocket luff sails to fair the leading edge. They do not luff easily and should not be considered as “soft” sails as on Lasers etc. They are however, all thin foils.
                                                                                                              • 2) When sailed properly, all “normal” sails are trimmed to optimise the angle of attack to roughly match the entry angle of the wind to the surface of the sail. ie: we use telltales and feel to trim the sail so that there is no stall either to windward or leeward at the sail luff. This means that the “optimum” angle of attack is effectively always maintained constant, but it is dependent on the sail camber.
                                                                                                              • 3) As the wind increases, we maintain maximum power until this matches the maximum righting moment available. Thereafter, the mast is tuned to bend so that the top of the sail progressively flattens and twists as the wind strength increases. This maintains constant heeling force, while maximising forward thrust and means that at higher wind speeds, the camber of the sail automatically reduces.
                                                                                                              • 4) Camber in thin sails typically ranges from 15% to 5% and is automatically controlled at the optimum value for the given wind strength.
                                                                                                              • I would like to make the following comments:
                                                                                                              • a) The comparisons in the texts I have reviewed present a lot of data at angles of attack varying from 0-40 degrees, much of which does not seem relevant to the situation I have described.
                                                                                                              • b) Lift values for thin foils actually seem very high, almost twice that of symmetric foils of similar camber, but this fact seems to be neglected in ensuing discussions in the texts.
                                                                                                              • c) Drag values for thin foils are generally higher than for symmetrical foils of the same camber, but are at a minimum near the “optimum” angle of attack.
                                                                                                              • d) What seems to be missing in the comparisons I have seen, is that as wind strength increases, the camber of a thin sail is automatically reduced, which also significantly reduces the drag. This is not necessarily the case for aerofoil sections which are generally, for practical reasons unable to alter thickness with wind strength, hence their camber is not optimised and as a result their drag can in fact be higher than the much flatter thin sails as the wind strength increases. None of these effects seem to be addressed in the discussions and conclusions presented…..or am I missing something?
                                                                                                              • e) I believe, that comparisons should therefore be made at different wind strengths between the lift /drag ratio of say a 15% camber assymetric foil and a thin foil with the lift of a 15% camber section at low speeds and 5% camber at high speeds.
                                                                                                              • f) The net effect is that thin foil “normal” sails actually perform better than aerofoils in light airs and are exceedingly effective at reducing drag at high speeds by automatic camber reduction, which makes them very competitive with thick aerofoils. This may give a more realistic picture and perhaps explain why we do not see big benefits in using wing sails.
                                                                                                              • I have tried to analyse data in the texts with this in mind, but always find the data lacking enough detail to make a proper comparison. Perhaps you have better data available and can make a good comparison.
                                                                                                              • Looking forward to your valued comments,
                                                                                                              • Ian Ward

                                                                                                          [Date/Time=09-05-2003 – 1:23 AM]

                                                                                                            Name:Ian Ward, [Msgid=501163]
                                                                                                            Re; Re; Re; Foil-Borne Draught

                                                                                                                • “Almost all speedboat props are intentionally ventilated above 50mph or so. I’m working on such a ‘lowrider’.” —


                                                                                                                • Jim, would you mind pointing me to any design data on supercavitating/ventilating hydrofoils that’s handy?

                                                                                                            [Date/Time=09-06-2003 – 2:35 AM]

                                                                                                              Name:Mac Stevens, [Msgid=501924]
                                                                                                              re; Thin Sails vs Airfoils
                                                                                                              …even in development classes and sailboards, which have no real limitations, full wing sections are not used today….why? Is this perhaps because the benefits are not really there?

                                                                                                                  • There are tremendous practical advantages to conventional sail rigs, for one thing. A rigid wing has to be “flown” 100% of the time. It can’t be allowed to just sit there and it’s a major production to raise and lower it. For example, when you park a rigid-winged landyacht, dollies are put under the rear wheels that have their axles pointed at the front wheel. This lets the yacht weathervane into the wind as though it were at anchor. It’s also very difficult to transport and store rigid wings.
                                                                                                                  • Another reason is that most boats have a great deal of windage. There are three ways a wing can outperform a conventional rig of the same span – it can produce higher maximum lift, it can have less parasite drag, and it can have less drag due to lift. If you already have the windage of topsides, exposed crew, standing rigging, running rigging, etc., what you save in parasite drag isn’t going to make much difference. In fact, reducing windage is probably the best way to improve sailing performance.
                                                                                                                  • Take a look at this outstanding article by John Shuttleworth:


                                                                                                                  • .

                                                                                                              The air drag of the hull alone is nearly equal to the total water drag of the hull.

                                                                                                                  • And this is for a design that paid extraordinary attention to reducing the windage of the hulls.
                                                                                                                  • The difference in drag due to lift cones in two forms: induced drag and leading edge suction. Minimizing the induced drag depends on the planform shape and the ability to control twist. Modern sails have planforms with square-heads that are not unlike the planforms of rigid wings, and the opitmum planform for minimum induced drag looks like a board sail, anyway (


                                                                                                                  • ). So controlling induced drag comes down to controlling the spanload distribution through twist. This was one of the big breakthroughs for Cogito at the last C-clas competition for the International Catamaran Challenge Trophy (unfortunately it


                                                                                                                  • the


                                                                                                                  • C-class competition for the ICCT, but that’s another story).
                                                                                                                  • On a thick leading edge, the stagnation point lies on the windward side and the flow rapidly accelerates around the leading edge. This low pressure on the leading edge pulls the wing forward. In fact, at high angles of attack, the suction on the leading edge can result in the net load in the plane of the wing being forward. In the early days of aviation, this caused the collapse of several aircraft in flight before designers started adding diagonal wires to brace against loads in the forward direction as well as drag in the aft direction.
                                                                                                                  • With a sharp leading edge, the flow separates and (hopefully) reattaches short distance behind the leading edge, forming a vortex that lies just behind the lee side of the leading edge. This vortex produces low pressures on the surface that are comparable to the leading edge suction of the thick leading edge, but the force is oriented normal to the surface instead of pointing forward. This loss of leading edge thrust shows up as a lift-dependent drag that also scales as lift-squared, making it look a lot like induced drag. This is an area where a wingmast or a rigid wing rig can have an advantage over a wire-luff sail.

                                                                                                              …d) What seems to be missing in the comparisons I have seen, is that as wind strength increases, the camber of a thin sail is automatically reduced, which also significantly reduces the drag. This is not necessarily the case for aerofoil sections which are generally, for practical reasons unable to alter thickness with wind strength, hence their camber is not optimised and as a result their drag can in fact be higher than the much flatter thin sails as the wind strength increases. None of these effects seem to be addressed in the discussions and conclusions presented…..or am I missing something?

                                                                                                                  • You’re probably right. To me, what’s almost universally missing are quantitative comparisons in the context of a systems approach to the design. Shuttleworth’s article is a refreshing exception.
                                                                                                                  • A good question would be why does changing the camber change the drag? To answer that question, you’d have to know what is causing the drag in the first place.
                                                                                                                  • Here is the computed performance of a NACA 65-012 section for three different Reynolds numbers:


                                                                                                                  • . The profile drag is nearly constant with angle of attack near the design lift (zero, in this case). The graph on the right side of the figure shows why. As the lift increases, the point of transition from a laminar boundary layer to a turbulent boundary layer moves forward on the lee side, increasing the drag. But the transition point on the windward side moves aft by almost exactly the same amount, so the total amount of laminar vs turbulent surface area is essentially unchaged. Until the transition suddenly moves all the way to the leading edge on the lee side, producing the jump in drag that marks the edge of the “drag bucket”.
                                                                                                                  • Camber really doesn’t change this – it mainly shifts the behavior to a different lift range. For example, here are a number of sections plotted for comparison:


                                                                                                                  • . Included along with the NACA 65-012 are the venerable NACA 0012, and three sections of my own design. The 0012 does not exhibit the drag bucket behavior of the other sections because its transition point moves smoothly toward the leading edge on the lee side while the windward side soon becomes almost fully laminar. The only difference between the P30012 and the the P30212 sections is the addition of 2% camber. You can see that adding camber simply moves the drag bucket without changing the drag appreciably. But all these sections have essentially fully attached flow.
                                                                                                                  • In a previous post, I referred to this prediction (


                                                                                                                  • ) of a wingmast-sail combination at an angle of attack of 8 degrees. Here’s the same shape at an angle of attack of 4 degrees:


                                                                                                                  • . Notice how much larger the windward side separation bubble is. At a low enough angle of attack, the separation doesn’t reattach and, paradoxically, the windward side is stalled! This is really a negative angle of attack stall, but you can have so much camber that the “inverted” stall actually occurs in the positive lift range. So controlling the windward side separation bubble is the key to optimizing the performance of this section.
                                                                                                                  • This figure,


                                                                                                                  • , shows the effect of changing mast rotation. The previous figures were calculated at the ideal (smooth lee side) mast rotation of 35 degrees. These curves were generated by rotating the mast but keeping the sail shape the same. When under-rotated, there’s a concave corner on the lee side at the mast-sail junction. This causes a separation bubble to be formed there, too. But the windward side separation bubble is smaller. So with the under-rotated mast and low angle of attack, two separation bubbles are formed, and the drag from the two is less than the drag of one big bubble – assuming the flow even reattaches on the windward side. As a result, the optimum mast rotation – the only camber control in this example – is lower for lower lift coefficients. XFOIL couldn’t even compute a solution at low angles of attack if the mast was rotated too much.
                                                                                                                  • The flow is essentially fully turbulent both surfaces, so controlling the amount of laminar flow isn’t the issue that it was with the thick sections. In both cases, changing the camber shifted the characteristics to different lift ranges. But the drag mechanisms were quite different between the sail and solid foil sections, as was the operating lift range. After all, the sail was operating at four times the lift!
                                                                                                                  • This has been a long-winded way of saying, “horses for courses.” I think it’s essential to understand what’s really going on and what the aero- hydro-dynamic mechanisms are if you hope to improve performance in a rational manner.
                                                                                                                  • The conventional rig is the convention for very good reasons. I think it’s worth taking the time to understand why it works so well and where it’s real deficiencies lie. As the cliche goes, “If you’re hunting elephants, you have to go where the elephants are.” I think parasite drag of the entire boat is the elephant to hunt.

                                                                                                              e) I believe, that comparisons should therefore be made at different wind strengths between the lift /drag ratio of say a 15% camber assymetric foil and a thin foil with the lift of a 15% camber section at low speeds and 5% camber at high speeds.

                                                                                                                  • Yes, a comparison under comparable conditions is the way to go. Another factor that’s often missed is to compare the actual lift and drag (in pounds or newtons) instead of just nondimensional coefficients. The coefficients can be misleading when the basis for nondimensionalizing them is changing. For example, if the planform area or span are different (as when reefing) the coefficients can give the wrong picture.

                                                                                                              f) The net effect is that thin foil “normal” sails actually perform better than aerofoils in light airs and are exceedingly effective at reducing drag at high speeds by automatic camber reduction, which makes them very competitive with thick aerofoils.

                                                                                                              This may give a more realistic picture and perhaps explain why we do not see big benefits in using wing sails.

                                                                                                              I have tried to analyse data in the texts with this in mind, but always find the data lacking enough detail to make a proper comparison. Perhaps you have better data available and can make a good comparison.

                                                                                                                  • I think you’re right. Unfortunately, many books (Marchaj’s for example) are long on phenomenology and short on systematic data that a designer can actually use. Books like Larsson & Eliasson’s “Principles of Yacht Design” need to be augmented with more handbook data. Today, computational fluid dynamics is becoming more accessible (like XFOIL). While CFD still has major short-comings, it provides far more understanding into the “why” than does basic test data.
                                                                                                                  • I only have access to the sources that everyone else has. Where I can, I’m trying to generate the kind of systematic data that’s useful for design. The figures I’ve cited are from a rewrite of my wingmast paper that’s in work. I’ve rerun most of the old cases and added the effects of mast rotation, too. Frank Bethwaite has sent me tracings of Tasar mast sections so I can look into the stepped-wingmast approach as well as the teardrop shapes. Unfortunately, XFOIL can’t handle the Tasar sections, so I have to go to a Navier Stokes code, and I haven’t made much progress in that direction lately.

                                                                                                              [Date/Time=09-06-2003 – 2:35 PM]

                                                                                                                Name:Tom Speer, [Msgid=502133]
                                                                                                                Re; Foil-Borne Draught

                                                                                                                    • Ah, I hadn’t caught that. I thought you were talking about an ordinary outboard and wanting to put foils on it. I guess I blew right past the speed you quoted because verybody claims their boats go fast! Surface piercing props are a different animal, of course.

                                                                                                                [Date/Time=09-06-2003 – 2:40 PM]

                                                                                                                  Name:Tom Speer, [Msgid=502138]

                                                                                                                      • I’m sure the society has archived the Johnson/Tulin stuff from the 60’s. Basically you want the section to look like a speedboat prop. These are inherently ‘high speed foils’ have been emperically optimized for decades. A flat pressure face with an arc on the suction side is a pretty good approximation to a NACA 16 series airfoil, so it does a decent job throughout the speed/ cavitation range.

                                                                                                                  [Date/Time=09-06-2003 – 3:08 PM]

                                                                                                                    Name:jim hynes, [Msgid=502153]
                                                                                                                    Re; Paddle Wheel Hydrofoil

                                                                                                                        • First, please excuse the delay in replying to your answers. I live in Pittsburgh PA. Over the years, I’ve traveled down the Ohio River to Marietta, Ohio to watch the annual “Steamboat Races”. Also, Pittsburgh is a departure point for the Delta Queen. Since these craft are relatively flat bottom craft with a paddle-wheel, it has always seemed to me that attacting hydrofoils with a pivoting paddlewheel would allow these craft to achive speeds that would make for an interesting experiment. Please comment. Thank you. Mark Lape

                                                                                                                    [Date/Time=09-07-2003 – 9:37 PM]

                                                                                                                      Name:Mark Lape, [Msgid=502793]
                                                                                                                      Re; Paddle Wheel Hydrofoil

                                                                                                                          • Hi Mark,
                                                                                                                          • My feeling is that the concept you have in mind would be more effective on a light weight low resistance craft, along the lines of a conventional hydrofoils. For a typical paddlesteamer, paddles operating as they currently do probably are the only way of generating the necessary thrust to move the vessel along at any sort of speed with a paddlewheel arrangement.

                                                                                                                      [Date/Time=09-10-2003 – 9:58 AM]

                                                                                                                        Name:Martin Grimm, [Msgid=504503]
                                                                                                                        Re; re; Thin Sails vs Airfoils

                                                                                                                            • Tom, thanks for your rather comprehensive reply. I have noticed you are active in many forums, it is very good to have your contribution.
                                                                                                                            • If I may summarise to confirm my understanding of your comments.
                                                                                                                            • 1) The sheer practicality and low weight of conventional rigs is a clear advantage. Perhaps the real issue is that the efficiency gains of airfoil rigs have not yet been matched by our engineering expertise. After all, current aircraft with fixed wings and massive engines are pretty cumbersome when compared with the sheer elegance of the capabilities of birds.
                                                                                                                            • 2) There is little relative benefit from just fairing the rig to reduce drag as there is so much other drag around from the hull, stays etc.
                                                                                                                            • You refer to a 50ft catamaran, which is probably an extreme example, but definitely an eye opener! I presume a dinghy will have far less relative windage and operates at lower speeds, but still there is a lot that could be done. A sailboard would have least windage, but there is always the skipper of course!
                                                                                                                            • 3) It would appear that spanwise twist is inherent and in fact one of the most important features to control induced drag in thin foil sails. If I read you correctly, Airfoils have only just recently started to introduce similar features, and then only in C-class cats. Any idea how much difference this makes?
                                                                                                                            • 4) Modern dinghy and sailboard rigs have square top high aspect planforms similar to Airfoil rigs, they also have excellent twist control and faired masts. You have already confirmed that parasitic drag of the rig is a minor issue, I teherefore assume that it is the increased lift of an airfoil rig which would give a significant advantage compared to conventional rigs. You have stated that this is mainly due to the effect of accelerating the wind around the forward part of a thick foil.
                                                                                                                            • 5) It would seem therefore that to have a large radiused leading edge on the leeward side is beneficial. In practice we already achieve this by over rotating small wing mast sections, eg: Cats, Tasar etc. The limitation you have described is windward side separation, which I agree occurs often and forces a compromise with rotation angle. Perhaps these rigs really need a pocket luff as well to fair in the windward side. How would this compare with the 50% Wing sections you have proposed?.
                                                                                                                            • 6) Are you aware of any data comparing actual performance of: soft sail, fully battened sail with boltrope track on mast, pocket luff, over rotating small wing mast, large 50% wingmast and full airfoil solid wing? My own observation from dinghies, is that switching from a standard rig with boltrope track on a round mast to a pocket luff sailboard style rig provides about one minute advantage in a 60 minute race, ie: somewhat less than 2% improvement. An over rotated wingmast gives a similar performance increase, but at lower wind strengths, and is a disadvantage in stronger winds. ie it has more power but also more drag and less gust response.
                                                                                                                            • 7) I notice there is a significant difference in how we talk about airfoils and thin sails. As a practical sailor, I see that a thin sail has an entry angle, which is perhaps twice that of the angle of attack of the entire section (depending on the section shape). We try to sail with this entry angle at zero incidence to the wind, and often in moderate to fresh breezes it is the windward side which is stalling, not the lee side, which in fact has flow attached most of the time. Camber may be seen as determining the amount of deflection of the breeze and therefore controls the angle of attack of the section. As both lift and drag increase with angle of attack, this is why I see increased camber leading to increased drag.
                                                                                                                            • 8) I also agree that the situation is worst for headsails with fine leading edges. This is probably why they are so difficult to trim correctly. They also have a severe limitation in getting fuller with wind strength as the luff sags. I believe it is far better to have a flexible mast spar to bend and flatten the sail as the wind increases.
                                                                                                                            • If it weren’t for the notion of “slot effect” I am sure we would find that mono rigs would be faster than jib and main across the wind range.
                                                                                                                            • 9) One possibility that is raised by your comments on the fundamentals is that it would seem an over rotating small wing mast as used on a Tasar etc contained within a pocket luff may provide the best of all worlds, ie: large solid radius on the lee side when fully rotated while maintaining a faired windward side, but still light and practical to handle. Any thoughts on this?
                                                                                                                            • The big question for me is: “Does the improved lift from the airfoil section outweigh the disadvantages? You have not been unequivocal about this, but the conclusion at the moment seems to be that airoil sections only work for special, purpose built craft.

                                                                                                                        [Date/Time=09-12-2003 – 5:57 PM]

                                                                                                                          Name:Ian Ward, [Msgid=506238]
                                                                                                                          Journal of Fluids Engineering Article


                                                                                                                                • Will you please to inform is it possible to read the article
                                                                                                                                • Dorange P., Billard J.-Y., Cid Tomas I., 2000, “Of cavitation inception and development on a two-dimensional Eppler hydrofoil,” March 2000, Journal of Fluids Engineering, Vol. 122, pp. 164-173.

                                                                                                                            [Date/Time=09-12-2003 – 6:27 PM]

                                                                                                                              Name:Andrey Leonov, [Msgid=506251]
                                                                                                                              Source of Pressure Envelope Data Needed

                                                                                                                                  • The information at your website is so useful to get knowledge while studying the hydrodynamics of hydrofoils for fins on windsurfers and sailboats. Will you please advise where is it possible to get minimum pressure envelopes via Internet for thinner than 817 Eppler (like maybe 818 series profiles) as it is suggested to have smaller -Cpmin? Are there any new such calculated profiles in recent years of cavitation studies? Also, it is of interest to know what foils are usually used for windsurfers… Fin brands like Fin’s, Select, etc… and sportsmen intending to challenge speed records as on


                                                                                                                                  • . Were there any attemtps to use anticavitating nonsymmetrical profiles with higher possible effiiciency for speed windsurf runs? Finally, can you advise where to look for the up-to-date materials in reseach of planing boards? I will appreciate any useful information. I have been an amateur windsurfer over the ten years that have passed since I studied high-speed hydrodynamics at MIPT.

                                                                                                                              [Date/Time=09-13-2003 – 9:04 AM]

                                                                                                                                Name:Andrey Leonov, [Msgid=506506]
                                                                                                                                re; JFE and other ASME Articles

                                                                                                                                    • You can buy a copy of the article on line from the Online Journal Publishing Service at


                                                                                                                                    • . I did not check the price, but I expect it will be very expensive. If anyone were to put the full text of this article up on the internet, they would be in violation of copyright law. The best opportunity, I think, is to find a copy of the journal in a university library. Or, perhaps someone will see your posted message and mail you a copy of the article… this is not legal either, but it has been done between colleagues without legal consequences.

                                                                                                                                [Date/Time=09-14-2003 – 3:39 PM]

                                                                                                                                  Name:Barney C Black, [Msgid=507105]
                                                                                                                                  Re; re; JFE and other ASME Articles

                                                                                                                                      • Thank’s a lot .At last I’ve get it

                                                                                                                                  [Date/Time=09-15-2003 – 8:42 AM]

                                                                                                                                    Name:Andrey Leonov, [Msgid=507445]
                                                                                                                                    re; Cavitation Inception

                                                                                                                                        • As a follow-up to my previous reply: the cost to view the article on line is $25, and it will have to be charged to a credit card. As an alternative, why don’t you contact Jacques Astolfi and ask him to send you a copy of the article? His email address is


                                                                                                                                        • . It is worth a try… the worst he can do is ignore you or refuse to send it. You might get lucky and receive a copy free.
                                                                                                                                        • Note J. Astolfi’s cavitation page at:


                                                                                                                                        • For information, here is the abstract of the article (free!)
                                                                                                                                        • An Experimental Investigation of Cavitation Inception and Development on a Two-Dimensional Eppler Hydrofoil
                                                                                                                                        • J.-A. Astolfi, P. Dorange, J.-Y. Billard et al.
                                                                                                                                        • University of Valladolid, Valladolid, Spain
                                                                                                                                        • Received : March 6, 1998
                                                                                                                                        • Abstract
                                                                                                                                        • Cavitation inception and development on a two-dimensional foil with an Eppler E817 cross section issued from an inverse calculus have been experimentally investigated. The foil is theoretically designed to have a wide cavitation-free bucket allowing a large range of cavitation-free angle of incidence (Eppler, R., 1990, Airfoil Design and Data, Springer-Verlag, Berlin). The inception cavitation numbers, the noise level, the velocity distribution, the minimum pressure coefficient, the cavitation patterns (bubble, leading edge “band type” cavitation, attached sheet cavity), together with the sheet cavity length have been experimentally determined. Effects on the velocity field have been studied too with a slightly developed cavitation. For angles of incidence larger than 1 deg, a great difference exists between the inception cavitation number and the theoretical minimum pressure coefficient. However it is in agreement with the measured one obtained from velocity measurements (for 0 deg <alpha<6 deg).=”” discrepancy=”” between=”” theory=”” and=”” experiment=”” on=”” scale=”” models=”” is=”” generally=”” attributed=”” to=”” a=”” flow=”” separation=”” at=”” the=”” leading=”” edge.=”” although=”” there=”” are=”” some=”” indications=”” of=”” separated=”” edge,=”” velocity=”” measurements=”” do=”” not=”” show=”” reverse=”” with=”” clearly=”” detected=”” negative=”” velocities=”” excepted=”” for=”” large=”” angle=”” incidence=”” equal=”” 10=”” deg.=”” concerning=”” sheet=”” cavity=”” development,=”” length=”” found=”” as=”” [sigma=”” 2(alpha?alphai(sigma))]?m=”” m=”” close=”” 2,=”” cavities=”” that=”” exceed=”” half=”” foil=”” chord=”” sigma=”” 2(alpha?alphai(sigma))=”” larger=”” than=”” about=”” 30.=”” <!–=”” noauto=”” –=””>

                                                                                                                                    [Date/Time=09-15-2003 – 9:13 PM]

                                                                                                                                      Name:Barney C Black, [Msgid=507898]

                                                                                                                                          • </alpha<6>

                                                                                                                                      Hydroplane-foil R/C hybrid?


                                                                                                                                            • Iv been running gas rc boats for 8 years now & have been bitten by the speed bug. 🙂 Im building my second rigger now (3 point ellison) to increase my speed from my old 5 point cajun bullet rigger (hydro) which fastest speed was 69 mph gps. Iv been thinking about combining a surface drive rigger with hydrofoils under the sponsons. It would be three point. But the rear would remain at prop shaft depth (no foil) riding on the prop like it always has. But the sponsons would have foils under them.One on each side (2) Instead of the hull rising up & out of the water. The sponsons would need to move up & out of the way of the water to remain at the surface drive level 1/2 the prop above the water level. Or just live with a positive running attitude after foil born.
                                                                                                                                            • My Questions are:
                                                                                                                                            • If hydro foils are so fast. Why wont the world speed record holders use them? The latest design (quicksilver) is a turbine powered canard which is still a hydroplane. Seems to me that running under the waters surface would also be safer avoiding swells & unwanted chop at 320 mph.
                                                                                                                                            • And is there a turn-key flying height controller available? Mr Dixon was kind enough to supply a drawing of a controller. But can anyone recommend specific hardware (sensors, schmitt trigger oscillator, and foil angle controler interface-servo’s & linkage) to use for a R/C project? Where do I find a vertical acceleration sensor & the hardware above? Can Helicopter gyros work for roll control? Can anyone guess on the cost of the controllers needed?
                                                                                                                                            • In my project I would need maybe only a level controller & roll controller since the rear would always ride at the same prop level.
                                                                                                                                            • Thanks for your time.
                                                                                                                                            • Mike Kolder

                                                                                                                                        [Date/Time=09-28-2003 – 2:13 PM]

                                                                                                                                          Name:Mike Kolder, [Msgid=516042]
                                                                                                                                          Re; Hydroplane-foil R/C hybrid?

                                                                                                                                              • Mike,
                                                                                                                                              • AT 70 mph in water, the dynamic pressure is 75 psi. At 100mph where a fast rigger can run on a straightaway, it’s twice that. You couldn’t build a small enough foil (~1/10 The same thing goes for a 300mph full scale ‘boat’. At those speeds, aero lift is almost free, the trick is to trim so the sponsons barely touch.
                                                                                                                                              • Circle racing is another matter. Hydroplanes use a turn fin (bad hydrofoil) on the inside sponson. It pulls down and in around a turn, and usually has a blunt leading edge to help with stability. I’ve been wanting to add a second fin on the outside sponson, but bend them both to about 45 degrees from vertical. This passive, surface piercing setup would have height and roll stability. On the straights both would lift and the sponsons would come up a little, in the turn the angle of attack would change the load sharing so both could pull inward. I haven’t gotten around to trying it (probably take a couple of hours), but I think Ken Cook has.

                                                                                                                                          [Date/Time=09-29-2003 – 10:59 AM]

                                                                                                                                            Name:jim, [Msgid=516453]

                                                                                                                                            FOIL ASSISTED CATAMARAN

                                                                                                                                                • With the assistance of INTERNATIONAL HYDROFOIL SOCIETY users I added a foil to a cat I designed and built. Well have a look and tell use what you think happened….

                                                                                                                                            [Date/Time=12-29-2003 – 5:56 PM]

                                                                                                                                              Name:Michael jaworski, [Msgid=564395]
                                                                                                                                               Image Attached:  “foil1.jpg”   Click Here To View
                                                                                                                                              FOIL ASSISTED CATAMARAN


                                                                                                                                                    • Thanks International Hydrofoil Society users for there help in adding a foil to my catamaran

                                                                                                                                                [Date/Time=12-31-2003 – 10:53 AM]

                                                                                                                                                  Name:Michael jaworski, [Msgid=565051]
                                                                                                                                                   Image Attached:  “2cat.jpg”   Click Here To View
                                                                                                                                                  Re; FOIL ASSISTED CATAMARAN

                                                                                                                                                      • Hi Mike,
                                                                                                                                                      • I would be interested in seeing what your final foil system looks like. Looking at the trim from the photos of your vessel at speed it looks like you have the boat quite well sorted out.
                                                                                                                                                      • best regards
                                                                                                                                                      • Gunther

                                                                                                                                                  [Date/Time=01-02-2004 – 4:46 AM]

                                                                                                                                                    Name:gunther migeotte, [Msgid=565671]
                                                                                                                                                    Surface Ventilated Supercavitating Foils

                                                                                                                                                        • Does anybody know where I can obtain a copy of the paper by Gabor Dobay:
                                                                                                                                                        • “Hydrofoils Designed for Surface Ventilation-An Experimental Analysis”
                                                                                                                                                        • …that he presented at the 1965 Spring Meeting of the Soc. of Naval Architects and Engineers (Seattle, WA)?
                                                                                                                                                        • Any suggestions on other more recent references on the same subject?
                                                                                                                                                        • Thanks.

                                                                                                                                                    [Date/Time=04-23-2004 – 9:13 AM]

                                                                                                                                                      Name:Terry Hendricks, [Msgid=635624]
                                                                                                                                                      building foils

                                                                                                                                                          • hi i have a 8 meter planing boat which weighs in at about 4000 kg
                                                                                                                                                          • it is running a 3 litre 200hp out board
                                                                                                                                                          • i need some imformation on how to lift the hull a bit higher out of
                                                                                                                                                          • the water when it is planning i dont want to lift it clear of the
                                                                                                                                                          • water but just to reduce the drag so as to increase speed and get better fuel economy if anyone can help with any imformation on this subject thanks
                                                                                                                                                          • WAYNE

                                                                                                                                                      [Date/Time=07-10-2004 – 4:15 PM]

                                                                                                                                                        Name:WAYNE, [Msgid=677503]

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