Why are windfoils so slow? The overall 500m speed record is held by a foil, Sailrocket, at 65 knots. Kitefoils can go 45 knots...so why are windfoils so slow at 35 knots when a conventional windsurfer with a fin is averaging 53 knots over 500m.

Windfoils are great in light winds and very efficient but once the wind is above 15 knots windsurfing is quicker. What's holding the windfoil back, can it be overcome and how fast can they ultimately go.

Small front wings, maybe sub 450 sq.cm., and short mast long fuz, and a lot of balls.

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LeeD said..
Small front wings, maybe sub 450 sq.cm., and short mast long fuz, and a lot of balls.

Already been tried, that's pretty much the setup that maxes at 36 knots.

Stability one major problem. Who likes crashing at 40 mph from 18" above the water.
Old Miller foiling was 2 struts, 2 wings nose and tail.
Cartwheels start to happen around 27 mph depending on several factors.
I like the stability of windsurfing.

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Roo said..
Why are windfoils so slow? The overall 500m speed record is held by a foil, Sailrocket, at 65 knots. Kitefoils can go 45 knots...so why are windfoils so slow at 35 knots when a conventional windsurfer with a fin is averaging 53 knots over 500m.

Windfoils are great in light winds and very efficient but once the wind is above 15 knots windsurfing is quicker. What's holding the windfoil back, can it be overcome and how fast can they ultimately go.

You are comparing a bit of apple and oranges.

Sailrocket is not touchable by kites or windsurfs unless you come out with a way to support a foil that actually pushes down, not up. It is a completely different beast.

If you go back just 6 years Kite foil speeds over 500m were sub 30 knots. 45 knots for kites keeps getting cited but do you now on what time distance? The above forties (42 knots) I have seen in the Bay Area have always been over a 2" (max speed).

For windsurfing: already fins go faster than foil ONLY in very flat conditions. Fins top out very quickly if you add any kind of chop or swell. Recently reported 1 Km speed (33 knots and change) indicate that foils have already an advantage in open water. (Sailrocket, by the way. can only operate in ultra flat water and on one side.)

So ... give it time.

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duzzi said..

Roo said..
Why are windfoils so slow? The overall 500m speed record is held by a foil, Sailrocket, at 65 knots. Kitefoils can go 45 knots...so why are windfoils so slow at 35 knots when a conventional windsurfer with a fin is averaging 53 knots over 500m.

Windfoils are great in light winds and very efficient but once the wind is above 15 knots windsurfing is quicker. What's holding the windfoil back, can it be overcome and how fast can they ultimately go.

You are comparing a bit of apple and oranges.

Sailrocket is not touchable by kites or windsurfs unless you come out with a way to support a foil that actually pushes down, not up. It is a completely different beast.

If you go back just 6 years Kite foil speeds over 500m were sub 30 knots. 45 knots for kites keeps getting cited but do you now on what time distance? The above forties (42 knots) I have seen in the Bay Area have always been over a 2" (max speed).

For windsurfing: already fins go faster than foil ONLY in very flat conditions. Fins top out very quickly if you add any kind of chop or swell. Recently reported 1 Km speed (33 knots and change) indicate that foils have already an advantage in open water. (Sailrocket, by the way. can only operate in ultra flat water and on one side.)

So ... give it time.

Those windfoil speeds of 33 knots are in flat water with a 30 knot offshore breeze in the south of France. The windfoil has maxed out. Many windsurfers go faster than that in the same conditions, add in flat water and they go even faster. The question is what's holding the windfoil back?

Roo, perhaps if YOU windfoil, you might see the solution all by yourself?

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LeeD said..
Roo, perhaps if YOU windfoil, you might see the solution all by yourself?

I do, www.seabreeze.com.au/forums/Windsurfing/Foiling/WindFoil-flight-schoolRoo-style but because it is so slow I prefer to windsurf until windfoiling becomes faster.
Still don't have an answer why it is slower.

So currently a formula board and 11m sail is faster and less dangerous

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Roo said..

duzzi said..

Roo said..
Why are windfoils so slow? The overall 500m speed record is held by a foil, Sailrocket, at 65 knots. Kitefoils can go 45 knots...so why are windfoils so slow at 35 knots when a conventional windsurfer with a fin is averaging 53 knots over 500m.

Windfoils are great in light winds and very efficient but once the wind is above 15 knots windsurfing is quicker. What's holding the windfoil back, can it be overcome and how fast can they ultimately go.

You are comparing a bit of apple and oranges.

Sailrocket is not touchable by kites or windsurfs unless you come out with a way to support a foil that actually pushes down, not up. It is a completely different beast.

If you go back just 6 years Kite foil speeds over 500m were sub 30 knots. 45 knots for kites keeps getting cited but do you now on what time distance? The above forties (42 knots) I have seen in the Bay Area have always been over a 2" (max speed).

For windsurfing: already fins go faster than foil ONLY in very flat conditions. Fins top out very quickly if you add any kind of chop or swell. Recently reported 1 Km speed (33 knots and change) indicate that foils have already an advantage in open water. (Sailrocket, by the way. can only operate in ultra flat water and on one side.)

So ... give it time.

Those windfoil speeds of 33 knots are in flat water with a 30 knot offshore breeze in the south of France. The windfoil has maxed out. Many windsurfers go faster than that in the same conditions, add in flat water and they go even faster. The question is what's holding the windfoil back?

In open water Nicola Goyard did 33+ knots average over 1 kilometer and 36 and change 2" vmax. If you add you distance and variable conditions foils are already ahead of slalom boards (see the recent Japan Demi, with winds way above 15 knots). But hey ... if 25-35 knots of speed in open water is too little for you, sure wait it out until they reach 50 knots.

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duzzi said..

In open water Nicola Goyard did 33+ knots average over 1 kilometer and 36 and change 2" vmax. If you add you distance and variable conditions foils are already ahead of slalom boards (see the recent Japan Demi, with winds way above 15 knots). But hey ... if 25-35 knots of speed in open water is too little for you, sure wait it out until they reach 50 knots.

Duzzi, sad to say, but a windfoil will never hit 50 knots with the current configuration. Nic hit 36 in flat water offshore winds, in true open water with swells and chop a windfoil won't get close to those speeds. Question is still why are they slower than kitefoils, windsurfers and sailrocket? What speed do you think they were doing in the round island race in Japan in the open water? It may surprise you that it was only 25 knots for 2 sec in those conditions, the windsurfers were much faster but couldn't sail as deep off the wind as the windfoilers around the back of the island.

Falling fron 18" high at 17 mph is fine.
Falling from 18" high at 32 mph is a equipment and body part wrecking nightmare.

My 2 cents


windfoils are nearly always tuned to generate lift. Our foils aren't just supporting a rider and a small foot platform., Theyre supporting a big board and rig. I've heard plenty of people say gravity isn't a factor, in reality it is. Our foils are larger than kite foils as a result of having to fight gravity harder.


I've not seen it, but i doubt the speed foil that did 35knts is smaller than an average kite foil wing. When you add up all that foil sitting in the water, its a lot more immersed area than a correctly trimmed speed board. As you'd expect kite foils are somewhere in between.


then you've got the big boats like sail rocket/gp50s and the Americas cup cats. Capable of higher speeds again, because they can carry much larger and more efficient rigs on what is a small foil relative to their size. With the exception of the gp50s, theyve also got a much better handle on controlling stable foiling height.


i guess in summary:

-our foils are (somewhat by necessity) too big relative to the platform we have.

-our whole platform and how we interact with it is too unstable for higher speeds.

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Roo said..

duzzi said..

In open water Nicola Goyard did 33+ knots average over 1 kilometer and 36 and change 2" vmax. If you add you distance and variable conditions foils are already ahead of slalom boards (see the recent Japan Demi, with winds way above 15 knots). But hey ... if 25-35 knots of speed in open water is too little for you, sure wait it out until they reach 50 knots.

Duzzi, sad to say, but a windfoil will never hit 50 knots with the current configuration. Nic hit 36 in flat water offshore winds, in true open water with swells and chop a windfoil won't get close to those speeds. Question is still why are they slower than kitefoils, windsurfers and sailrocket? What speed do you think they were doing in the round island race in Japan in the open water? It may surprise you that it was only 25 knots for 2 sec in those conditions, the windsurfers were much faster but couldn't sail as deep off the wind as the windfoilers around the back of the island.

"Max Speed" only matters if you are going for a speed record. If you are racing in open ocean who cares if you can go faster in some direction that has nothing to do with the race? The fins in Japan were not faster than the foils, they were slower. From the video that went around the first windsurfer arrived something like30' after the first foil ...

So, let's just stick with speed. When it comes to "Max speed" in ultra-flat flat conditions yes, obviously Sailrocket is much faster an efficient than anything else. It is a completely different design from anything else too , and one that cannot be really used for a kite or a windsurf ... or for anything really other than a speed record in one direction.

Than there are regular kites and close behind regular Windsurf. Kite foils are much slower than regular kites, and windfoil are slower yet. But I would not be surprised if just a couple of years from now windfoils will be as fast as kitefoils. A friend of mine recently watched some training in Baja california and commented on how close the speed of race windfoil and kitefoil were.

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duzzi said..
"Max Speed" only matters if you are going for a speed record.

rubbish!

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duzzi said..
So, let's just stick with speed. When it comes to "Max speed" in ultra-flat flat conditions yes, obviously Sailrocket is much faster an efficient than anything else. It is a completely different design from anything else too , and one that cannot be really used for a kite or a windsurf ... or for anything really other than a speed record in one direction.

so what? Irrelevant to the OP's question

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duzzi said..
.....But I would not be surprised if just a couple of years from now windfoils will be as fast as kitefoils. .......

Well I would be AMAZED!!!

Back to the Question.... How will this miracle happen? What makes them so slow now? (I think Subsonic is on to it )

Thats a bit like saying Carpets will fly in a few years , because........ well.....just because.....

Windfoiling replaced Formula.
Slalom and speed still belongs to windsurfing...for now.
Light wind sailing is all about foiling.
Sub 6 mph windspeed is the realm of longboards.

Just to be clear this is not a foil versus fin argument but a realistic look at all the different types of craft used to set a speed over a 500m distance. Essentially they are all mechanical systems that balance forces to achieve equilibrium which results in speed. How they achieve this equilibrium is what makes them interesting and holds the key to going fast.

Over on the Windsurfing33 forum in France they have just started up a similar discussion. Here's a few quotes that do give an insight into each of the the setups.

"The difference with the kite comes from the fact that they have no sail on the board.
In fact, from their symmetrical wing, there is no torque to have. The only forces transmitted to the Foil are those passing through the rider and therefore it can be concerned with transmitting the forces of the wing in the axis of the mast.

In the case of a board, it is more complex: on the one hand, the sail being attached to the deck, it tends to want to tip downwind which requires a couple of return to hold the sail. This recall torque is obtained by moving the sail off-center. With a small wave sail you can get it by keeping your feet in the center of the board and just stretching your arms. The distance between the axis of the sail and the center of mass of the man is already sufficient. With a larger sail, by also eccentricating the feet, this distance between the axis of the sail and the center of mass is increased, which makes it easier to hold the sail. However, even if the distance between axis and center of mass is small, the rider-sail assembly generates a lateral component which tends to capsize the Foil. The width therefore makes it possible to create a righting torque to counterbalance the lateral force.

So to conclude, the control of the sail requires a minimum of eccentricity in board and not in kite as the wing is symmetrical and that it therefore does not require a torque. The presence of a return torque on the board requires moving away from the sail laterally. The whole sail-rider, asks to capsize the Foil which is compensated by the distance to the Foil from the rider. And overall, the greater the distance to the sail, the more control you have over the sail, and the more control you have over the roll of the Foil.

In addition, the kite is in a 'more' virtuous circle which allows to put almost in opposition the foil and the kite (in windfoil when we tilt the sail we also tilt the board sideways ), which is close to perfect balance, hence the superior control of the kite compared to windsurfing. Besides, the V max in kites are of the order of 45nds (Nico Parlier, Th?o de Ramecourt etc ...), but especially with stuff made for banana courses (upwind and downhill), so if we put a kitefoiler with material developed for speed, I do not see any problem to exceed 50nds see more with the current configuration.

The only one to have achieved the perfect balance is the sail rocket, which puts its foil and its wing almost exactly in the same axis. In this case, it only accelerates the result: it passes the 60nds. Will one day the windfoil be as efficient as the kite? I do not know, but in the current configuration and geometries the kite has more advantages."

It's a pretty good summation of the way each system is balanced and the efficiency of each design. This diagram of sailrocket sums it up visually in a simplistic manner, of course there are force vectors in other directions involved. The question then becomes how do we modify the windfoil setup to better balance the force of the sail against the foil to go faster?

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sailquik said..

duzzi said..
"Max Speed" only matters if you are going for a speed record.

rubbish!

duzzi said..
So, let's just stick with speed. When it comes to "Max speed" in ultra-flat flat conditions yes, obviously Sailrocket is much faster an efficient than anything else. It is a completely different design from anything else too , and one that cannot be really used for a kite or a windsurf ... or for anything really other than a speed record in one direction.

so what? Irrelevant to the OP's question

duzzi said..
.....But I would not be surprised if just a couple of years from now windfoils will be as fast as kitefoils. .......

Well I would be AMAZED!!!

Back to the Question.... How will this miracle happen? What makes them so slow now? (I think Subsonic is on to it )

Thats a bit like saying Carpets will fly in a few years , because........ well.....just because.....

Such a pleasant reply! Fine example of how not to discuss a topic on the Web.

Anyway: the difference ... is the kite. Kite, rider and foil are pretty much on the same vector. For a windsurf the sail /board connection generates a component that is not aligned with the foil.

Solutions? anything that reduces that component. We are already seeing that with foil specific sails. That is the reason why windfoils gained around 5 knots in max speed in a bit more than a year. Other solutions ... no idea, I am not a naval engineer but they will come. I might be wrong but I see little reason for windfoils and kitefoils not to be within a few knots from each other, as their fins version are. My crystal ball also sees windfoils with an overall advantage in open water.

Having said that ... 25-30 knots of max speed is more than enough already for most of us! Including foil-kiters! (who have a dismal rate of injuries and timpani raptures already!)

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duzzi said..

Solutions? anything that reduces that component. We are already seeing that with foil specific sails. That is the reason why windfoils gained around 5 knots in max speed in a bit more than a year. Other solutions ... no idea, I am not a naval engineer but they will come. I might be wrong but I see little reason for windfoils and kitefoils not to be within a few knots from each other, as their fins version are. My crystal ball also sees windfoils with an overall advantage in open water.

Having said that ... 25-30 knots of max speed is more than enough already for most of us! Including foil-kiters! (who have a dismal rate of injuries and timpani raptures already!)

Windfoils went faster because they used smaller foils, Nico's was 350cm on the front. He was using standard race/slalom sails when he his 35 knots originally.

Sorry but 25-30 knots is not enough at all, we are doing 35-37 knot peaks out in the middle of the Columbia River on windsurfers, I want a windfoil that does 40 knots in the same conditions!

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Roo said..

duzzi said..

Solutions? anything that reduces that component. We are already seeing that with foil specific sails. That is the reason why windfoils gained around 5 knots in max speed in a bit more than a year. Other solutions ... no idea, I am not a naval engineer but they will come. I might be wrong but I see little reason for windfoils and kitefoils not to be within a few knots from each other, as their fins version are. My crystal ball also sees windfoils with an overall advantage in open water.

Having said that ... 25-30 knots of max speed is more than enough already for most of us! Including foil-kiters! (who have a dismal rate of injuries and timpani raptures already!)

Windfoils went faster because they used smaller foils, Nico's was 350cm on the front. He was using standard race/slalom sails when he his 35 knots originally.

Sorry but 25-30 knots is not enough at all, we are doing 35-37 knot peaks out in the middle of the Columbia River on windsurfers, I want a windfoil that does 40 knots in the same conditions!

Well, too many variables in this discussion I am out. Sorry you have to wait for 40 knots!

The next crop of foils might make things more interesting. With foils doing slalom, the bigger players are going to have to focus on fast a bit more. Up till now its been about course racing upwind and downwind.

End of the day it's just a drag vs "how much power you can harness" equation; Sailrocket is the king, GP50's second (AC boats in there too), kites a close 3rd and windsurfers a distant last. There's not much that can be done to change that equation on a windsurfer, I expect we will probably see a windfoil get into the 40's just, but I don't see them going much faster, the righting moment just isn't there. They will probably win some races on the PWA slalom tour when they are raced against fins this year due to conditions, IE water state, wind etc.

The power required to overcome the drag on the foil increases with the cube of velocity which is why you hit such a 'wall' on the foil. We hit 22-23kts on the race foils in 10kts of wind, in 20kts we're only going a few knots faster, because of that drag function. The only way to realistically improve that is to fit tiny foils, like Nico did....but that only works to a point. You still have to carry, lets say a 300cm^2 front wing, at least 300mm of mast in the water, a speed sailor probably has 1/4 of that foil drag and a little bit of hull drag.

Look at sailing Moths, they have been highly developed by the best minds in the world outside of the AC cycles for the past 10 years and while they are a lot faster around a course than 10 years ago in terms of outright top speed I don't think the gains are as large. Most of their recent gains are Aero drag wise. They also have more righting moment than a windsurfer. I could well be wrong but I don't see anything radically changing, the laws of hydrodynamics are pretty well known at this point and gains are in the small percents and declining, it is what it is.

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CJW said..
End of the day it's just a drag vs "how much power you can harness" equation; Sailrocket is the king, GP50's second (AC boats in there too), kites a close 3rd and windsurfers a distant last. There's not much that can be done to change that equation on a windsurfer, I expect we will probably see a windfoil get into the 40's just, but I don't see them going much faster, the righting moment just isn't there. They will probably win some races on the PWA slalom tour when they are raced against fins this year due to conditions, IE water state, wind etc.

The power required to overcome the drag on the foil increases with the cube of velocity which is why you hit such a 'wall' on the foil. We hit 22-23kts on the race foils in 10kts of wind, in 20kts we're only going a few knots faster, because of that drag function. The only way to realistically improve that is to fit tiny foils, like Nico did....but that only works to a point. You still have to carry, lets say a 300cm^2 front wing, at least 300mm of mast in the water, a speed sailor probably has 1/4 of that foil drag and a little bit of hull drag.

Look at sailing Moths, they have been highly developed by the best minds in the world outside of the AC cycles for the past 10 years and while they are a lot faster around a course than 10 years ago in terms of outright top speed I don't think the gains are as large. Most of their recent gains are Aero drag wise. They also have more righting moment than a windsurfer. I could well be wrong but I don't see anything radically changing, the laws of hydrodynamics are pretty well known at this point and gains are in the small percents and declining, it is what it is.

Excellent CJW now we're getting somewhere. Righting moment is the key and how much loading can be put into the foil. As you say Sailrocket, the GP50 and AC boats are master of it. How do we change the dynamics on a windfoil to load the foil more. Our current setup is too elastic. Forget about jibing for a moment and just think of a setup for speed.

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CJW said..

The power required to overcome the drag on the foil increases with the cube of velocity which is why you hit such a 'wall' on the foil.

The power in the wind also goes up as the cube of windspeed so it's not that simple.

Was chatting to a kite foiler the other day. He was envisaging the problems outlined by Roo and wondering how we did it. Once up on the foil he said it was pretty easy to keep height with a little fore and aft weighting. The lift from the kite would vary more slowly, the eddies are larger up there and even though the kite will have some moment about the foil it has to at least come from the direction of the strings and that won't change rapidly.

The fast foilers like the F50 (50 knots max). which also have deck-mounted sails, are a lot longer which slows the unwanted pitch response. The main foil angles are controlled by the crew with joysticks and the foil on the rudder has "active control" to maintain pitch . Not sure what that means, possibly the fine tuning is done electronically taking the output from a gyro?

sailgp.com/about/f50-catamaran/

The kite foilers are the ones to try and match speed wise. Youngsters getting into cheap performance sailing are going to do read up on this. Was out on a slapper on the same stretch of water as the young kiters warming up for the Lighthouse to Leighton race. A humbling experience
. The winner averaged close to 32 knots ! And they go upwind! They were a little slower on the most recent race, too much wind and weed.

I'm thinking control is the issue. If someone can get a small foil to sail down a trench under control in 45 knots of wind it'll be faster than a slapper. Would anybody be inspired to attempt this?

I suspect unless the wind foil and water foils are in perfect aligned as in Sailrocket, these dreams will remain dreams.

Until then, 40-50 knots is more than enough fun for the average Joe.

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holy guacamole said..
I suspect unless the wind foil and water foils are in perfect aligned as in Sailrocket, these dreams will remain dreams.

Until then, 40-50 knots is more than enough fun for the average Joe.

Well you are probably right. But sail rocket has a configuration all of its own. The F50s are a lifting foil with gravity counterbalance, the same as windfoilers. So is it a fundamental scaling problem a counterbalancing problem or a control problem? Probably scaling favours an F50 being able to do 50 knots in 20 knots of breeze, but the original question implies speeding off the wind in a lot more breeze than that. For this there is no requirement that the craft can even get going in 20 knots, that it can sail in open water or that it can even get back upwind.

The main foil can thus be made really small if you assume you'll have at least 30 knots to get going. The mast is currently constructed so it doesn't break under the asymmetrical loading when one wing broaches before the other. With reduced foil span these max loads are down so the mast can be significantly thinner and less draggy. It can also be shorter due to the flat water and the yet-to-be invented control system. You can probably taper the mast a bit more where it meets the fuselage. Especially if you lay it up in one piece and don't need 3 nicely spaced bolts to hold it together. You might find with less drag the optimum angle is further off the wind so you'll have to dig another trench or wait for a wind shift.

I predict nobody will bother, as someone recently said the Zen of foiling is completely different to the Zen of speed sailing.

Yep. The zen of foiling is effortless smoothness in light winds. The idea of imminent serious injury is kind of a turn off for the average foiler.

I guess there's always someone crazy enough to push the limits and try something far too dangerous for the average foiler. Happens in all sports. Here's to the reckless folk.

IanK some of us will bother, speed is speed however you achieve it. I still remember the first time I saw a hydrofoil on Sydney Harbour and was amazed with how effortless it all seemed. Riding on one was incredible as a kid, I was intrigued by the small wing supporting such a large craft.

Like any problem there is always a solution, sometimes a totally unexpected one. I've been running my ideas by an engineer and set the wheels in motion to start testing a speedfoil setup. I stopped foiling as it always felt so slow compared with windsurfing, I think it's time to get back into the air! IanK the control system is the key as you say and also the ability to load the foil using sail force rather than weight. It took 50 years to get Sailrocket to work, hopefully this won't take as long.

I'm one of the reckless folk Holy G. It may work but I won't know until I try....but it's a challenge worth taking.

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Roo said..


Like any problem there is always a solution, sometimes a totally unexpected one. I've been running my ideas by an engineer and set the wheels in motion to start testing a speedfoil setup.

The surface piercing semi foilers might have merit. Not as slippery but self stabilising height wise. Aim to be always bouncing off a hull. Something like Hugo Boss for instance. It's sailing without foils on this tack but you can see the port foil ready to go.


What about vents on the mast to upper foil thick points. To progressively diffuse lift as it gets too high?

And let's face it. Everything has gone to foils. Speed sailors are the only ones who haven't. (The F50s weren't designed with speed as the primary objective, they're course racers, 50 knots was just a by product.) Get with it.

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holy guacamole said..
. Here's to the reckless folk.

You need to get Guy Martin on the case. If anyone can break a speed record he can.