View Full Version : What about hydrofoil and windsurfing?

6th April 2007, 02:43 AM
Hi chaps!!
I would like any information about real and serious test with hydrofoils mounted on windsurf...as I can see on the net, there are several very-efficiency machines with hydrofoils..but few windsurf with this piece.....which is the problem???...if there is any?

10th December 2008, 05:08 PM
Hi, it seems that today I'm rediscovring all the old posts...
Are you still there, LukasV?
I'm not from the starboard team (althought i would like to), but i will try to answer you anyway.
Main problem with hydrofoils is that they are too complex compared to the simplicity of windsurfing.
When sailing there is an easy balance to stay in the surface: If the board goes up you are too heavy in the air an fall down to surface again. And if you try to go down in the water the buoyancy (dinamic from planing, or static from pressure) increases so you go up to surface again.
When flying in airplane or sailing in submarine you move in only one fluid (you are not in any surface between two fluids). In this conditions there is no automatic balance to stay in one determined height. So you need controls to stay in the height you want. This controls are composed by some intelligent central (person, computer or mechanism) and the rudders which act in the fluid.
To sail in hydrofoil is like to fly inside the water (althought your body is out of it). So you have to control your height, and in this case you have to be very precise, because you have to stay in very strict range. i.e. if the fin is 70cm your range should be +-10cm. If you try to control this height with your balance, like hydrofoils surfers does, you spend too much power and concentration in it, so you can't aply it to drive the sail at 100%. It is like trying to make bycicle competition in monocyle. You would have less friction, but is not possible to pedal well while you are trying to keep the balance.
If you put intelligent controls to do the work for you, then is what i told you: is too complex for the windsurfing concept.
A part of this practical reasons, there are also phisical reasons.
If you compare the 3 curves of resistance vs speed of displacement, hydrofoil and planning boards you would find that, even if you manage to make a hydrofoil board able to fly in 1knot wind and also a planning board able to plan in same wind, displacement board would be better in low speeds, hydrofoil in medium and planing in hight. And it is like this, and you can't change it. Of course, you can make a hydrofoil faster than planing board in hight winds. But this would mean that the planing board was not well enought designed.
Why is it like this?:
-Low wind (<4knots):
Hydrofoil needs such a big foil to keep flying that it's induced drag and surface friction is bigger than the one of a simple dispacement board.
Planning have exactly the same problem. Instead of induced drag we would talk about waves.
-Medium wind (4 to 12knts):
Displacement hull makes too big waves and has much more wetted surface than necessary.
Planning board starts to work well, but maybe hydrofoil has less wetted surface (because upper part of the foil produces much more lift than the bottom, or bottom of the planning board, so you need less surface). So maybe hydrofoil is better.
-Hight wind (>12knots):
If the foil was designed for lower speeds now it will have more surface than necessary, so it's not going to be efficient enought. While planing boards, when they increase the speed they go more and more out of the water reducing their surface (is perfect autoregulation). If the foil was designed for high speeds it will need lot of wind to take off, and i guess this is not what people is looking for in hydrofoils. And even more, in very high speeds the foil would have cavitation problems (like the spinout of the fin). The solution would be a supercavitating foil (foil which has the upper surface covered of air) but this is what a normal planning board is, so you don't have to complicate your self with hydrofoils. And also, with the fin piercing the surface it would have ventilation problems, inducing it to frecuent spinouts. This would be solved with some (i don't remember how are they called, maybe strings?) horitzontal small surface just below the surface to stop air going down to the fin. But this would increase again the wetted surface and induced drag.
Conclusion: Hydrofoils would be interesting for the range from 4 to 12knots but we (humans) still didn't discover the perfect autoregulation simple system to control the height, so by the moment is too complex for windsurfing (althought it would be great to try the feeling for one day).

11th December 2008, 12:01 AM
Andretsin makes some very good points.
The Windrider Rave ,perhaps most successsfull (easiest to sail?) small (ish) foiler(Trimaran) looks to have serious pitch adjustment problems in anything like rough sees. (Its foils are controlled mechanically(semi-automatic) by a sort of trailing rope) On smooth water in constant wind things look better (apparenly the control mechanisms are only automatic over a limited wind range;human input (pulling bungees ??) is needed upper and lower limits.On top of controling overall pitch of vessel (via rudder/jostick) and steering via pedals/rudder and then trimming sails and then keeping ride height constant it does sound a handfull.(Gybing looks fantastic on it though;if you get it all right)
Hydroptere (current record hoder for nautical mile and its done 48 knots over 500 metres) actually looks easier to sail than the rave but I believe pilot still controls ride height to an extent but by arranging foils at an angle it does have some level of autoregulation.
Years ago some free thinker attached 2 sets of foils arranged in a V to a Mistral.As board went fater it lifted further out of water reducing amount of V in water. (auto regulation?)
To be honest that old attempt looks more functional than the single foil attachd to skeg we`ve seen more recently.(Which looks a nightmare in pitch)

I`ve often wondered if some arrangement with foil/skeg could be used to maybe not increase top speed , but to make small board behave more like a bigger one.(ie use foil to lower minimum windspeed board will work in ??) (or carry a bigger rig?)

There seems to be renewed interest in foils been used on smaller craft which WS seems to be playing no part in ? (Theres a video of a K1 using foils which baltantly out performs its non-foiled competitor. Top performing Moth Dinghies all utilise foils but Moths are difficult/complicated to sail even without them.

I do think any adoption of foils to lift board free of water would produce a new sport. It would just never feel like a board ???? (Its not??)

Anybody sailed (Piloted) Rave or Hydroptere ??? Or foiled Moths ???

11th December 2008, 03:41 AM
I think hydropter don't have any kind of height control. It uses the piercing foils which work for lift and daggerboard at the same time. This ones are autoregulating (when the speed grows the foil goes more out of the water). But i guess these piercing foils have the problem that when the speed increases and they go up, they are reducing also the daggerboard surface, and this is not good.
And for that, you would need a "v" foil, which means two foils piercing the surface, having the spray friction, and the foil from windward i guess it wouldn't be working well, so it would contribute only to drag. Even that, it is maybe the most simple system which works more or less well, so maybe it would be good to put in a board.

11th December 2008, 03:24 PM
Hydroptere has ballast intake/control for pitch.( I think) and the self regulating foils lifting in some ways might be a bonus.Power of any fin increases to square of speed;reduction of dagger board area but with more speed will keep good lift to windward.(Board sailors for years have been looking at ways to reuce fin size at faster speeds.( if there`s enough lift to pick boat up there`s suffiient to maintain angle to wind)
Besides Hydropere must have something going for it ! 47 + knots 500 metres and outright record holder 1 mile ! (And set in 28 knots wind)(outright holder in Cat D too !)
Believe it was gusting 45 when Antoine set his record !

WS needs to do something.We`ve lost outright record to kiters.(So they claim) (50.5 knots) and Hydroptere has the mile !!!

Where`s LeeD when you need him !!!

11th December 2008, 04:20 PM
The problem is that power to leeward from the sail is also increased to square of the speed. I tryed to calculate once if it's true that you can reduce the size of the fin if you increase your speed (without changing the sail), but it was not so easy. I don't remember very well how it was. I guess it was depending on the shape of the resistance curve of the board. And in some cases i found that you have to increase the fin while increasing the speed, and in other cases was other way round.

11th December 2008, 11:18 PM
Have a look at

Looks great !!! Loads going on though. (And the Moth is only 3.5m long shorter than raceboards !(oldstyle)

12th December 2008, 03:09 AM
Interesting discussion going on here !

Fundamental problems for a foiled windsurfing hull: how to control for roll, bank, pitch, lifting height and steering ?

Pitch and/or lifting height self-control mechanism: http://fr.youtube.com/watch?v=yuFwDm8t3IM

Both hands are already busy at holding the boom (and sail angle to apparent wind) and feet are busy to keep your body upright and balanced on the hull while controling the banking, pitch and steering of that same hull. How to do that when adding 1 or 2 foils in that equation ?

Cheers !


12th December 2008, 03:00 PM
I guess i wrote in discussion about serenity with foils, that what i would do is to put a simple invert "T" foil (in the fin, of course) because i think is the most simple and efective one, and some other one in the nose. Both of them fixed. While getting speed the foil of the front will rise the nose until it gives the proper angle to the reear one (angle to all the board). Then all the board would get out of the watter. The front one would go completly up until it's on the surface, working like a ski, withstanding arround 3-5% of the total weight of person and board. Then you don't need moveable parts. The design fo both foils has to be so that when you roll the board to one side, the front ski goes to that side faster than the rear one, so then you can turn like you use to do in planning boards.
It is great for serenity shapes, because you can sail ok in displacement mode when low wind, and in higher winds you don't care how is the shape of the hull because you are already in foils. So good overall board and new feelings. And you don't put extra variables in the equation :-)

12th December 2008, 04:18 PM
Would be a good starting point but 2 problems would immediately show .
a) At some point (depending on size of rear foil) the rear foil would also become surface bound.Infact at some depth from surface foil would loose its upper surface lift.(It would suck air down to it rather than lift board.(aeration not cavitation)) The Moth uses a trim tab controlled via rotating tiller.(Sailor uses tiller both to steer and control available lift.The rave uses mechanism as in earlier post. Hydroptere is self regulating as mentioned arlier)

b) I dont think the 2 foil arrangement would footsteer.(not with both fixed)
As board angled over front and back foils foils would be on competely different "arcs". To steer effecively arcs would have to be arranged to co-incide somehow.But if either foil is allowed to "rotate" to align itself for turning during "straight" sailing it would twist off to leeward.(Ie our foiling system has to also provide lift to windward)
The moth just sails straight without rudder input.(irrespective of lean)(look on youtube)

We need a simplified ride height system.And probably a foot operated rudder.(Defaults to straight? rear footstrap twist to operate ??) Two foil system would take care of pitch.

12th December 2008, 04:20 PM
Using either foil on surface is defeating object.Our boards alreay do that.
Lift to drag ratio of fin on surface would be same as a planing surface.

12th December 2008, 04:30 PM
I don't know about point b). I think it would be possible to work as I said, but maybe I'm wrong.
But point a) is not like you said. Front foil has shorter leg, so while it is in the surface and the board is almost horitzontal, the rear foil is deep inside the water. If you increase speed front foil can't go even upper because it is already in the surface. Maybe rear one would do. But only a little bit, until the pitch of the board is that so that the rear foil has zero attack angle, so it can't go even upper. Don't think about other boats. It's a radical different system, mixing planning in the front ski and foiling in the rear hydrofoil.
PS: I know the front ski works like normal boards. The advantage is that the overall wetted surface of ski+hydrofoil would be less than the one of normal board (i hope) and also drag comming from waves making. And, the best property is to have an allround board.

12th December 2008, 06:24 PM
Sorry I disagree.
Rear foil would have to be large enough to to "fly" at at most 10-12 knots.??? Without something to reduce its lift by 20 knots (or there abouts) it would break surface and start porpoising. There must be a system to "steer" rear foil in a vertical movement.(or reduce its lift).If your system worked it would have en empoyed on Moths years ago.
Good example is Airchair.Imagine keeping pitch same on that and then increasing speed. to 20k.If rider didnt lean forward (lowering ride height/lift by reducing angle of attack) foil would simply rise to surface lose lift ; sink and start again.
Keeping board up at front (or down) (ie maintaining pitch) without reducing lift or "steering" rear foil would create extremely high drag and in practice just wouldn`t happen.(ie system would not be balanced; rear foil generating loads more lift than needed but held in place by a smaller non foiling foil a few feet away ????.
If angle of attack is good for 10knots (to get flying) how is it changed to maintain "equilibrium" at 20k + ???
It would only work at a very specific speed when all forces are just balanced.If that speed (in theory) was 35knots it would never get there.If it was 18 knots it would do little more.
Your idea is very similar to Aquaskipper.Try towing one at 20 knots and see what happens.! Great at 8knots !
Our system would have to work form just under non flying speed of craft to 40knots + ?? Its a tall order and why its not been done YET.

12th December 2008, 06:47 PM
Do you know how does Trifoiler works?
It's almost the same.
Front two foils controlled by respective skimmers, are keeping constant height in the front (this would be replaced by the front foil of my idea). Rear foil (wich is in the rudder), is fixed. When it goes at low speeds, the boat has trim to the back so the rudder foil has bigger angle of atack, giving enough lift. At higher speeds the boat is more horitzontal (tail up) and angle of atack of rudder foil is lower, so it produces the same lift althought the speed is higher. You must know that changes of 2degrees in the angle of attack mean huge changes in lift. Look at curves of lift coeficient in front of angle of attack.
I have to say that rudder foil in trifoiler is not fixed at all. You can trim it deppending on the wind and if there is one or two persons in the boat. But you are not changing it during the day, only before going to the water, so we can consider it like fixed.

12th December 2008, 07:24 PM
Some time since I saw a Tri-foiler (had a quick go in one around 1995;as a passenger) but if I remember rightly there were ome kind of sensors dragging in water;but things could have changed. Have a read of http://www.hobiecat.com/sailing/TriFoiler%20History%20Original/Magazine%20Articles/Multihulls%201990.htm

Not sure we`d actually see any performance improvements; afterall speedboards are about same wetted area as a ski. (on of foilers front "skimmers")

12th December 2008, 08:15 PM

12th December 2008, 08:43 PM
Now i don't have time to read. I'm at work. I just wanted to ad that foils work at 4-8degrees of atack angle. So with only few change on the board pitch you would put the foil to angle 0 of atack (or -2 if you prefer, that use to be the angle they say there is zero lift) so no more lift will be produced and it won't go out of the water.

15th December 2008, 10:07 AM
Using either foil on surface is defeating object.Our boards alreay do that.
Lift to drag ratio of fin on surface would be same as a planing surface.

The new approach that might cry out for windsurfing development is http://www.sailrocket.com/

It has obviously its problems (I never seen before a boat doing a back flip at 50 knots!!!) but it it is incredibly fast and efficient. The boat is 45-52 knots in 18-22 knots of wind. Amazing.

The boat is very different from a conventional hydrofoil and ...I am no engineer but it might suggest a different kind of windsurf (much wider and very short, no rear fin, with the joint sliding across the deck to allow manouvers?)

17th December 2008, 10:29 PM
Iworked some years ago a Formula Weedfin Duo 40cm with two wings, both are 20cm
long and 10 to 7cm wide. Both are 40cm. 8cm under the bottum. Angle for lift is
5°. A Starboard Formulaboard and 12,5mē sail with a dutch windsurfer made with
5-7knots wind 34km/h . He said that it was 4-5km/h faster than with a 70cm fin.
My fin is in the normal box. I tried it later with more wind. I made 43 km/h,but there made the wings so much lift at the end of the board, that I feeled a brake in front. I must find the point of balance, it should be 30 or 40cm more in front. No time to try it. Wolfgang

17th December 2008, 11:53 PM
Hi Wolfgang
I`d wondered if the fitting of tiny fins(Arranged horizontally) (2cm length 1cm depth at most) to speed boards might reduce overall drag.Not by totally lifting rear of board out but by offering more efficient overall lift drag ratio .ie board would rise out slightly towards design Vmax but not to extent to raise board or seriously change angle of attack of board.(enough support to reduce drag of board?) Same as your idea on Formula but vastly reduced in area in for faster speeds.

Many slalom water ski`s have similar "foils" (on skeg)to adjust angle of attack of ski.(Think skiers call them "brakes".) They are used to increase drag in this instance so skier can brake better for turn. We`d be doing opposit. ??? Wondered if you`d tried it ?

Not explained it well but think you get idea ?

18th December 2008, 04:03 AM
I'm sorry, I'm spanish and i didn't understand last two comments. What is Weedfin? And when floyd said "fitting of tiny fins(Arranged horizontally) (2cm length 1cm depth at most)" do you mean to put this fins and any normal fin?
I would like you to explain everything again if possible with other words.
Anyway, i think that fins which are wider than deeper (very low aspect ratio) have much more drag for the same lift as an other of high aspect ratio. That is why fins, wings, sails, and all this stuff has that shape! I don't think there is any other better solution to produce lift with low drag than a fin which is more or less like the ones that already exists. (Boats with the keel extended from bow to stern disapeared!!)

18th December 2008, 01:21 PM
A Weedfin is a fin with rake 45°. She is vertical 40cm and diagonal 56cm.I used the fin
in Strand Horst Netherland. The wings are convex and concave. I tried the wings
higher and deeper. 8cm was the best result. I am 90kg, it was no problem to start faster, than a boy with 50-60 kg. Look:www.designlessacher
There can you see weedfins