Foilboard Design

[Munter]

Foilboards have recently made an impressive showing at the kite racing nationals in the US. Most of the designs I've seen on the web have had a low aspect main lifting foil with a smaller tail foil, both mounted on the end of a single, high aspect vertical foil. This shape is reasonably similar to that which I think I've seen being used by Laird Hamilton for some time now.

Can someone more familiar with foils give me some advice as to why they are using low aspect lifting foils? Is it to make the foils more user friendly and less sensitive to changes in angle of attack? Does it give a more predictable outcome if the foil comes right out of the water? I presume the tail foil is to help with pitching stability which looks like it could be an issue on this type of single foil with no control devices.

A couple of links for reference:

http://www.foilboardkiting.com/foilpgs.html
http://www.ctmfoilboards.com/ecatalo...o4u7mtnaeb4580

I'd say you have it pretty well right. The Nash(Rush Randle-see below) foilboard windsurfers can jump and re-enter with impunity and they use the same basic Delta planform. I've done a little experimental work with a windsurfer system using the same basic setup but instead of a delta for the main foil a
63412 Moth foiler section was used. Took off pretty early but was tough to sail and did not re-enter well-very preliminary results,however.
When a delta system was sailed at Calema against a bunch of boards it was very slightly faster than a normal board in 15knots when on the foils but no significant speed increase. Pretty cool looking though and very,very quiet.
You might also go to www.foils.org and read about the Miller foilboard system-entirely different.
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Rush Randle
Address:http://rushrandle.com/foilboards.htm Changed:6:35 PM on Wednesday, October 12, 2005

[BWD]

Yeh, agree on the rationale.
Heard the problem so far for kite racing has been the foilboards beat convntional ones to windward, but off the wind cannot go deep as they don't make enough drag -if you "catch up" to a kite, it wants to fall.

So the foils sail further and lose. Maybe next version will add a control suface or another appendage for downwind..... funny if someone could win a race by trailing a warp....

[Munter]

Too little drag? That doesn't sound right! Surely its a question of learning the right techniques to use the apparent wind and sail the right angles?

I've never raced a kite - what do the regular boards do downwind? Loop the kite and go straight downwind or sail broad reaching angles?

[BWD]

Munter I think that's what was said from the mouths of the competitors in SF last week, on kiteforum.
If you put a kite up to zenith in light wind and run straight downwind; the kite will try to fall. Similar scenario at 20 knots in 25 knot wind going deep, wind drops, so can the kite, if it has nothing to pull against. If there is a perfect kite flying pattern that protects from this, I don't know what it is.

Normally kiters do go back and forth a bit, to keep to those sailing angles that work for them. Problem in racing is reaching a mark dead downwind. Going straighter towards it gives a VMG advantage not made up for by the speed on higher course.

With a normal board if the kite starts to stall, the tail will just dig in a bit and make enough drag to keep the forces in balance for flight, so you can re-accelerate, more directly toward the mark. Or so it seems to me....

As to your quesiton about low AR foils, you need area for support, but need to be able to steer it with weight shift. If it was really wide and thin, you could not get leverage.
Maybe they should go even lower aspect to give a little more drag when useful in some conditions....

So if you had a foilboard with less drag to windward, and then trailed a warp (so to speak) on the downwind leg, you would do better. It would be funny. Maybe use a small fishing planer. Or more literally, a brake on the strut.
As below: