But windfoilers have gone wide with not so much lean? All that weight on the wide rail generates a moment about the longitudinal axis that can only be counteracted by the mast generating lateral pressure.

The kite foil configuration has the mast generating a lot less lift, the greater inclination of the wing provides more counteraction to the lift of the sail.

This appeals to my sense of elegance. Generating your total hydrodynamic lift with one foil in the ideal direction has got to be associated with less drag than splitting the lift between two foils at right angles. (Pythagorus and all that - you're generating more total hydrodynamic lift to get the same resultant lift). As well as that the mast is a symmetric foil and won't have the lift to drag of an asymmetric one. ( With the mast generating less lift it can then be made thinner, as most of the forces will now be compression, i.e. less bending )

The only reason I can think of is that a large sail with a wide boom gets a little cramped on a narrow board. Why not have a travelling mast foot? Slides fully leeward to give you max leverage on the sail but not the width to inhibit the ability to crank the foil windward.

The dynamics of having a much bigger airborne power source which sits well away from you and isn't attached to the board is what allows for a reduced board and foil platform. As you say, having the sail on the board, right there with you means you need some width to provide distance there to begin with.

Wider foil wings and longer foil masts also create a need for more width, to give the rider more leverage on the foil when everything is loaded up

Subsonic said..


Wider foil wings and longer foil masts also create a need for more width, to give the rider more leverage on the foil when everything is loaded up

In my thought experiment that's where the argument becomes circular.

.The width is needed to counter the leverage on the mast.
.The reason you have to have hydrodynamic leverage on the mast is because of the width of the board. You can't bank it far enough to windward without dipping a rail.

But then again, the top windfoilers are mostly ex Formula racers. They're used to sailing off a fin. They did it for decades. Of course they will want to sail off the mast, They're comfortable just using the foil to replace the vertical lift that used to be provided by a planing hull.

That's all a bit speculative of course, just thinking out loud. I'm still a beginner on a free ride foil with an old-timers flat learning curve.

(The foils, even if wider are still balanced, the resultant foil lift goes straight up the mast and doesn't contribute to moments about the longitudinal axis down the centre of the board.)

Yep been there done that was a few interesting ideas I tried around that time last year
Made 2 prototypes, first one proved the concept was possible and the second one was actually very functional however in the end had no positive impact on performance
But yes I was working under the thinking of bringing all the mast foot pressure to the windward side of the board may be an advantage and the first prototype made it apparent that when sailing upwind this is where the track would slide anyway, did try sailing with it locked to the leeward side with the first proto but proved to be rather uncomfortable and detrimental to performance. Anyway was satisfied after playing around with it for a while that there were negligible to no performance advantages and just made the sailing even more complicated

Too bad Jim Drake is dead. I'd love to see a forces diagram like the ones he made for windsurfing. Seems that the kite has an "up" force component that easily adjustable by changing the kite angle. I'd guess that the kite can be used to "carry" most of the kiter's weight. Windsurfers have relatively more fixed angles, and the forward and sideways force components are larger than the "up" component.

For kite foiling, balancing the forces seems straightforward, with the foil's COE being between the feet, and the foil angle compensating for sideways forces. Windfoiling if more complicated. A substantial part of the force from a windsurf sail has to be transmitted through the mast base. Relative to the foils COE, the mast base is "off balance". This must be balanced with other "misaligned" forces, like feet being far off-center and a large mast. Moving the mast track to windward only helps with respect to the sideways imbalance, but not with respect to the lengthwise imbalance.

In theory, a sail design that allows for an attachment much further back, aligned with the front wind COE, should allow for a more kite-like stance and board tilt. That's quite a large design change relative to current windsurfing sails, though. But the changes in boards and sails between the earliest wind foils and now certainly go in that direction, with shorter clew lengths and mast bases further back. Even so, the wider boards are needed to allow maximum "counterbalancing".

..which is why the all-out racing foils have REALLY long masts, such as 110 cm. You can tilt them over at steep angles and still keep the long-span wings in the water.

Ian K said..

PaulUK said..
It's an interesting topic. One point that A Albeau mentioned is that the kite foil angle allows them to use the kite to keep the foil in the water as it generates more lift at higher speed. Windfoil sails don't generally seem to do that - though some do claim to generate downforce.

Geometry suggests our larger width foils would also break to the surface sooner if heavily tilted,

That's a good point. The vertical lift varies with the Cosine of the lean angle. But the Cosine curve is pretty flat around 90 degrees. At 45 degrees a bit of angle tweak is becoming useful. Tweaking it down to 43 degrees a kiter would lose more lift than from between 80 to 78. Of course they'd accelerate upwind if they didn't sheet the kite on. Aeronautics 101, you lose height if you bank too hard.

If the stance perth kite foilers use going upwind is anything to go by, they'll all Be making regular trips to the chiropractor by age 40/50.

And that while PWA riders start riding their foilboards flatter and flatter. If you look at the top WF1 guns they're really not banking the board towindward much anymore!

The more you bank, the more the wind pushes down the nose and the lower you fly / less front foot power you get. For maximum performance you want the wind to lift the nose, resulting in more power! This is still very possible while still having the board a few deg towindward with the correct nose rocker and a wide nose. Its all a balance.

People forget to factor in the wind, but when going 20-25kts against 15kts of wind the big nose of a formula or WF91 board catches a lot of wind, really changing the distribution of forces.

Gonzalo Costa Hoevel explained it all really well in a video, I'm trying to find it..

EDIT: Think its this one but dont have the time to go through 50min of video:

EDIT: its at 23:20 ;)

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