I agree semi-foiling is probably the way to use foils to go fast on a windsurfer. To help break the surface tension and reduce board drag.

Maybe there's a sweet spot.

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holy guacamole said..
I agree semi-foiling is probably the way to use foils to go fast on a windsurfer. To help break the surface tension and reduce board drag.

Maybe there's a sweet spot.

Agree. Relieve a bit of board drag. The board has to lift 100 kg. And the lift to drag ratio of a planing hull is not as good as a submerged foil.
Maybe arrange for a foil to provide say 80kg of lift and leave the very rear of the hull doing doing 20kg of lift, just really providing the height reference. A pretensioned spring holding the foil's angle of attack could be set to a max of 80kg lift.

Exactly. Maybe there's a sweet spot where the foil additional drag is less than the gravitational mass vectors = more speed.

If it could be made to work on chop ?<5cm it would feel awesome.

I'd reckon there'd be a sweet spot don't discount it. I mean look at Hugo Boss. Who'd have thought a loopy crescent shaped foil to the lee side would improve the performance of an ocean racing mono hull? It's still got a lead keel! One other advantage of that lee foil is that it increases the righting moment, Hugo Boss can carry more sail in a given breeze.

This doesn't work for windsurfers, you can widen the beam but still cannot carry more sail because righting moment is limited by body mass. Gravity shrouds! What you need is a telescoping up wind shroud. Get the lee foil working, get the mast in position apply a handbrake on the boom to lock it in. And sheet in. Still have a degree of freedom in the fore and aft direction for steering. At any instant you could release the handbrake sheet out and be sailing a normal overpowered fully-articulated windsurfer.

Yes the final frontier to be enlightened with foils. Trouble with the current configuration is the mast is vertical, no negative feedback when it comes too far out of the water. There's a drag penalty for each appendage that breaks the surface but given that speed sailing is not a symmetric activity you can get around that. It's not new. The first ever foiling windsurfer used self stabilising inclined foils



and the F50s have a curve to automatically step in if the crew loses control of the joystick.



Here you go. Only one strut breaking the surface.
Tuned to be Inclined at about 45 degrees as it breaks the water surface at 45 knots..
Probably still need a tail elevator.
and probably need a vertical fin somewhere on the fuselage. The centre of lateral lift and centre of vertical lift don't align on a windsurfer.
Balancing that with the loss of lateral lift on the entry end of the foil as it rises might be tricky.

I like your thinking Ian, getting outside the box and considering all the variations. What SailGP and AC have achieved with foils is incredible, I think they might not go much faster than 50 knots until they start using super cav foils like Sailrocket. If we can get a windfoil over 40 knots and up to 50 would be a major achievement.

Looking at Sailrocket I've long wondered whether an "Assfoil", for want of a better term, would do the trick. A hand plane sized submerged foil that doesn't lift but generates downforce through the harness in a straight force line to the harness lines. It would either need a control wand as per a Moth to keep it at a fixed depth or a skilled rider could use hip rotation. I reckon the foil could be very small and still generate 50kg of "virtual sheeting force". The angle of the foil would also have an upwind component. So you'd barely need a fin on the board to keep tracking. You would need some override capability to trip out or adjust the downforce.

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Jetlag said..
Looking at Sailrocket I've long wondered whether an "Assfoil", for want of a better term, would do the trick. A hand plane sized submerged foil that doesn't lift but generates downforce through the harness in a straight force line to the harness lines. It would either need a control wand as per a Moth to keep it at a fixed depth or a skilled rider could use hip rotation. I reckon the foil could be very small and still generate 50kg of "virtual sheeting force". The angle of the foil would also have an upwind component. So you'd barely need a fin on the board to keep tracking. You would need some override capability to trip out or adjust the downforce.

You're onto something Jetlag, it was one of the ideas we were chatting about a few days ago with the engineering/tech crew here.

I think the term is "hapa". Has been tried with a kite. Basically did away with board/water contact altogether with rider a couple of metres above waterlevel. The video I saw was not particularly fast and a bit problematic with control of the foil but that was with lines. A rigid link may have been better. But as a proof of concept it worked.

Now that we know what it's called we can google it. An old idea. Worked for sail rocket.
miriam-english.org/files/ultimate-sailing-rig-without-hull/The%20Ultimate%20in%20Sailing%20is%20a%20Rig%20Without%20a%20Hull.html

We were looking at using a seat with a foil fixed to it.

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Roo said..
We were looking at using a seat with a foil fixed to it.

We need to be careful. Are you intending to still be sailing as a "sailboard"/ "windfoiler"? If that seat gets too sophisticated you'll be coming up against Sail Rocket in the "anything goes" category.