View Full Version : who killed the wing sail?

7th April 2009, 10:40 AM
What happened to double camber wingsail development in windsurfing? Every time I see an independent developer or enthusiast develop a working soft wingsail, they seem to disappear without a trace in a year or two. :confused:

Obviously most of these projects are self financed.

But still, were is BIG WINDSURFING when it comes to buying up these good designs?

Camber induced sails have always had very good aerodynamics, but if you're an aero purist then you have to believe in a double surface technology and it's potential to make small powerhouse sails and turn gusty holey winds into plane-able days.

I tried snowkiting this winter for the first time on a lake. (Dedicated ice boarder). The smallest kite they had was a 4m FOIL designed for land only. Everyone was using 7m-8m inflatable water kites. Another guy carved in to the launch area with a foil kite. He had a somewhat tangible fear in his eyes.
As a lightweight, i'd probably be using a 5m Windsurf sail on snow in the 15 knots filtering over the bay, so I said "ok, ill try the 4."
The lesson guy launched the kite for me.
"Woa, it's pulling like a 7m, you'll probably be able to jump with this."
Maybe i''ve bitten off more than I could chew, I thought, but gave it a try anyway.
After a few uncontrolled face plants and body drags downwind and some tedious slow riding upwind I decided I'd mastered the kite's 3rd line release system and exceeded my endurance for another sesh as a kite noob.
The thing was a 4m powerhouse. It wasnt just more powerful than an equivalent ws sail, it was like a force of nature, an example of what a true wing shape can do.

I don't expect that windsurfers would be able to ride 4m on 7m day, but with a true wing who knows what's possible? Who wouldn't want to rig small with a wingsail?

8th April 2009, 04:51 AM
It's an interesting topic that's come up a few times over the years. The closest parallel I can see is Hang Gliders. Not dissimilar to two race sails foot to foot. Similar speeds (at least close enough in aerodynamic Reynold numbers terms). Double surface wiped out single surface in competiton overnight. Although single surface remains for lightweight recreation ; but even then there is some DS at the leading edge. Pretty much like a current race sail.

When the wide luff race sails came out some said it was a gimmick, I thought the sock size and shape would mutate into a wing sail. But intensive development but several brands has proven that
1) They do give a (small) performance improvement
2) There is an optimum DS luff size
Exactly why is debatable, but it does seem to be a fact - at least until someone proves otherwise.

Ten years ago a start up in Scotland got as far as taking orders - but didn't get many. Boards magazine did test an early example and said that as a lightwind sail it did have amazing glide thru' the lulls. But commercially a big name brand would have to sell it to convince a sceptical public. Weight and production costs are huge issues.

It was interesting to see a solid wing in the A class cat championships too, against the best in current sail technology. (Their sail weights make formula look lardy). Competitive in it's first version at world level , but didn't make everything else obsolete.

Actually I've just been playing with an 8m Neo foil kite, they're just some much neater than inflatables to me all round. Jumping with these is more an inevitability than a skill ! Will be interesting to see if foil kites find a niche in the new kite course racing. If so we might need some lightweight high twist DS formula sails to avoid being left behind !

8th April 2009, 05:36 PM
K.lauman (or anybody else) you don't have a picture of an wing sail? Never seen one. Try to googel it but didn't find any except strange looking sails for boats and catamarans.


9th April 2009, 05:03 AM
there is a picture of the powerfoil sail I mentioned on
You have to look carefully to see the battens on both double surfaces.

On the thread there is also some heavweight aerodynamics from the respected Tom Speer. He has run many simulations of various configurations. He says that single surfaces sails work very weel - but only over a narrow apparent wind angle. Writing in 2002 he concluded

"So I think we're seeing a convergence on a sail shape that has a properly formed leading edge, smooth lee side contour, a double luff for 20% to 40% of the chord, and a single surface for the remainder of the mast-sail section. And the purpose of having a double luff is to cut the drag asociated with the windward side separation bubble."

Sounds pretty much like a current race sail no ?

btw Loft sails are pushing an RDM formula setup as a way of extending mast life. I Just wonder if it may also offer an aerodynamic edge having a slimmer leading edge ?

Paul M

13th April 2009, 09:25 PM
Thanks for the thoughtful posts. First of all i'd like to qualify that I sail in the Great Lakes and spend most the summer months on cammed 7.5 or 6.2. and 110 freeride board, or 150 GO. Planing power is a big consideration here, the wind is usually gusty, variable and front based. When your slogging between gusts you have plenty of time to critique your sail's shaping.

Cammed sails are great, and I have one, but I can't help but wonder if everything above the cams is just stalled out and not driving. I just wonder if there's a "softer solution" with fewer mechanical parts and lower rigging tension.

Haven't tried a DS leading edge cammed sail, but forgive me if i'm wrong, most sails like the Pryde RS are high wind blasting sails with low-down shaping, not especially good for improving your planing threshold.

I've been reading about the windward separation bubble/vortex. A big cause of drag i'm assuming. Leeward separation bubbles caused by the mast are worse and can trigger a full on stall.

Really what i'm hoping for in the future is a wing sail with more power for it's size, less mass, adjustable camber, and less torquing effect on the sailor.

I've found some pics of the POWERFOIL which I think was the scottish design from 2001. I'll include them later. Very low aspect with little twist or cord taper towards the top.

14th April 2009, 04:48 PM
"I can't help but wonder if everything above the cams is just stalled out and not driving"

Well, put tell-tales on your sails and answer your own question.

16th April 2009, 05:26 AM
Well there's been a raft of sails designed to lower the planning threshold without adding weight, Plasma et al. Choose the one from your favourite brand.
The Formula sails are more orientated to wide usable wind range (and downwind use), so are more flexible.

21st April 2009, 05:05 PM
I would like to tell you some of the theoric characteristhics of profiles:
1-Concave profile: This is the common one of sails, the single skin profile. Is the one which gets the biggest force for a given intesity of wind. But it is also the one which has the worst Lift/Drag ratio.
2-Symetric profile: The one of fins. (It's most used property is that it can work in both sides, but we are now not speaking about this). Is the one which produces the less force for a given wind, but best L/D ratio.
3-Flat-Convex profile: Flat in one side, cambered in the oher. It has medium characteristhics between 1 and 2, of course.
4-You can also find profiles which are concave-concave, but doubble skin, or convex-convex, but not symetrics. You would get intermedium propertis with them.

For high speed vehicles flying at 800km/h (turbine airplanes), you use asymetric convex-convex profile. For medium speed vehicles flying at 100-200km/h (sportive airplanes), you use flat-convex profiles. For very low speed vehicles, sailing at 20-70km/h (windsurfing boards), you use concave shapes to produce enough power in small sail. Otherwhise you would need bigger sail to compensate the lost in power.
We can consider then that a wide luff sail is an intermediate concept between concave sail and flat-convex. Acording to all i said, now you understand why this design is only used in huge sails. And like this you can get better upwind angles.
A thing I would like to add is that if you want to take more power from a given size of sail, what you can do is to divide it in two sails. So that you have a rig looking like the rig of a boat. Consisting in two sails. Like this you would get more power but worst L/D ratio. Good idea for downwinds. What about putting a zip from up to down in the sail to open it in downwinds and close it in upwinds?

Here i put the link of an interseting web page where you can see wing sails.


21st April 2009, 07:21 PM

that is a very nice link...but there are no details on all the concepts...
It is very interesting, just curious why is not being used anymore...I know that the AA record sail was also a prototype but not sure if it was a wing sail or some asymmetric profile.

I understand though that the development is not that cheap and that getting the money back is nearly impossible.

Post more...it is an inrteresting reading!
ciao Michal.

22nd April 2009, 09:49 AM
Unregistered: I'll try that as soon as the water temps around here are over 5 degrees celcius, till then i'll have to speculate.

andretsin: very interesting about the concave foil's max force yet poor lift to drag. So going to a wing sail wouldn't help downwind speed in this case and you couldn't substitute a smaller wing sail in place of a larger concave sail?

A thought on ergonomics. I'm sure the lrage amount of "heeling force" that the concave foil makes at a high angle of attack is taken into account in windsurfing design and allows windsurfers to hang body weight off the rig, (counterbalancing heeling force) and water start at high angles of attack ( ie sheeted in).

Maybe a wing foil would force the rider into a very upright and on-top-of-the board position that would be very unwindsurf like? ( Though they're always talking about this kind of stance in freestyle equipment design).

It's interesting that you say a concave foil inherently generates more power than a convex/ flat foil, because in my little anecdote a stationary (no apparent wind, true wind only) 4m kite with aforementioned foil shape seemed to have much more static power than a stationary 5.0 windsurf sail. This might have been due to wind gradient factors, ect ect, but at zero degrees angle of attack it still pulled hard in 15 knots of winter wind.

It would make more sense though that this convex/flat foil kite would be optimized to fly in 80 km/hr and above airspeeds, as a kite can reach these speeds power diving even if the sailor is only traveling at 40 km/hr.

24th April 2009, 04:01 AM

Thanks for the summary of the theory, which I'm sure is sound. I think the best L/D would give the best angle upwind ?

I also find it hard to explain several practical competition results, such as why C class catamaran have proven fastest with solid wing sails - despite their weight. Also why double surface hang glidiers wiped out single surface in competition , see

I suspect response to varying angles of attack is as important as coefficent of lift and L/D. Actually our current 'flexy' sails seem more tuned for wind range than max lift coeffiecnt ? Could a wing sail be so user friendly ?

I agree downwind looks like an area where experimentation might still produce new concepts. Back on the phantom 380 thread I posted

"No arguing that drag will increase with slots, which is why I've only really imagined them in the downwind dash scenario. There we are operating at as high as angle of attack as we can , proabaly well past most efficient. I wonder whether by running deeper, if a little slower, with slots (ie closer to the spin equipped boats) it would be possible to make more VMG. For general aviation take landing drag is probably pretty irrelevant to the designer. However if the goal is different then the placement and sizing of the slots would differ to meet the changed target. The only windtunnel work I know of in this area has been done for paragliders, and is shown on
According to independant tests (in the table 2/3 the way down) they got virtually 20% decrease in stall speed (ie high angle of attack) for 2% increase in drag at minimum angle of attack (equivalent to upwind). Certainly those things sell well enough. I've spoken to owners and they say the only disadvantage is a narrow 'poor performance window' at medium angle of attack - might be a problem for a sail on beam reach, but not up/downwind. Just makes me wonder - but we don't see so much experimentation nowdays with rigs, that all seems to be on the kites."

C two four nine
27th April 2009, 05:18 PM
I think the reason that C Class cats use wing masts but other boats don't has been explained by top aerodynamicist Mark Drela, whose foil designs hold things like the human-powered flight and boat records.

Mark notes that with a board or boat, the issue is not the l/D of the rig in isolation BUT THE LIFT DRAG OF THE RIG AND PLATFORM (ie the rig and the hull or hulls). Looked at it that way, everything comes together.

A glider consists of a huge wingspan with a tiny bit of parasitic drag caused by the fuselage, so going for the lowest-drag wing is vital. In contrast, a boat or board consists of a smaller (proportionately) "wingspan" and a massive amount of parasitic drag caused by the hulls. No matter how efficient the L/D of the rig, the L/D of the whole boat is dominated by the huge drag of the hull. Therefore a lot of the time, it's more important to maximise the lift of the rig, even at the expense of maximising the drag of the rig, because the rig drag is a small fraction of the total drag of the entire boat or board.

For example, a C Class cat is an inherently low-drag beast for its size and sail power. Therefore the low drag of the wing sail is very important (and it helps that the boat is so big that it rarely crashes and blows the wing!). So wings work in Cs.

At the other extreme is a cruising yacht; it's got an inherently high-drag hull and therefore maximising lift is vital, and since rig drag is a small amount of overall drag, an inferior L/D in the rig is no problem if L increases.

In boards, we go from longboards in light winds (high drag compared to windspeed) to speed boards in strong winds (low drag compared to windspeed) and the fastest sails follow the same rules; increase lift when your hull drag is high even at the expense of higher aero drag, and vice versa in other conditions.

I'm no expert in this, just passing on theory that matches practice 100%.

28th April 2009, 06:11 AM
On a yacht, where rig size is limited (either by exterior dimensions or via the handicap measurement) what you say is correct. C class cats, for example, have very specific limits.

On a windsurfer, however, you can use whatever rig you want, so it would follow that the best possible L/D would produce the best result. IMO, the 'gray area' of wing viability is the boundary conditions. Windsurfers have slightly more than 1:1 speed to wind speed ratio (for comparison, Yellow Pages just went over 50 kts in less than 25 kts of wind which is in excess of 2:1). So to go 50 knots on a windsurfer, you are looking at nearly 50 knots of wind speed. The impact on the water surface, gust dynamics etc is extreme, and so far it has seemed that the advantages that result from the ability of a properly designed conventional rig to 'autotune' outweigh the disadvantage of the intrinsically lower L/D.

My 2 cents, for what they are worth!

C two four nine
28th April 2009, 09:46 PM
Good poin!

By the way, even very open-minded boat classes with no rig limits have found that wing masts, double luff sails etc are often less efficient than just making a taller, lighter, simpler rig that cuts down on induced drag by its longer span.

There's pics of double-surface windsurfer sails in use (slowly) at speed trials as early as about '82. They still seem to have an attraction because people believe that gliders are the best model for sails, and while I'm no aero expert in any way it's interesting to see that guys like Drela seems to believe that "normal" sails are better in many ways, particularly (as you pointed out) their adjustability; I can recall a post to a glider forum where he seemed very impressed by the way teh America's Cup rigs worked.

Tom Speer (a Boeing aerodynamicist who posts on Boat Design Forum) has noted when writing about solid wing sails that "the notion that because aircraft wings are very efficient and have thick sections, while sails have thin sections and generally lower lift/drag ratios, and therefore a thick sectioned sail will aerodynamically superior to a sail rig with a thin section simply because it is thick, is a mistaken idea. Airplanes have thick sections because they are structurally stronger and because they have to operate efficiently at low lift coefficients in cruise. This is generally not the case for most sailing craft, except for very high-speed craft like landyachts and iceboats."

6th November 2012, 08:36 AM
What happened to double camber wingsail development in windsurfing? Every time I see an independent developer or enthusiast develop a working soft wingsail, they seem to disappear without a trace in a year or two. :confused:

Obviously most of these projects are self financed.

But still, were is BIG WINDSURFING when it comes to buying up these good designs?


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