I think that's why nobody has figured out how to use supercavitation with wind powered craft yet.
Fascinated by these tecnical topics.
Good article on apparent wind at www.deniswee.net/ApparentWind/trueapp.htm
and vector diags 
Soooo.....
the landyachties can do 120kph in 20-25kph of wind...on land...low friction
the limitation for windsurfers is fin & board surface contact with a much higher friction factor/interface.
I've been thinking for a while (similar to Nebs) about ventilating one side of the fin - but only ventilate when up to 30, 40 or whatever knots
. You get the inertia that Notwal was referring to, instead of spin-out.
cast a tube into front of fin, drill fine pinholes, connect to tube out thru fin box
All great theory...who's gonna try it
Hey Wineman, it's really piqued my curiosity...
I reckon all you'd need would be a wedge-shaped fin (sharp at the front, squared off at the back) and then drill a hole through to the top of the base right at the trailing edge. This is to let air into the low pressure gap behind the fin.
You'd make sure to use some form of washer arrangement to hold the fin in place that allowed air to be sucked down past the fin screw.
So then you'd have a fin with no low-pressure side -- it's all high pressure, hence no cavitation. And spinout wouldn't be a sudden loss of lift, it would be a gradual increase of board slew. A ventilating foil like this would be more efficient than a normal fin in spinout mode, due to the better shape of the high-pressure side (concave instead of convex). You could make it the right size as well.
It would probably be a lot less efficient than a normal foil shape though 
One of the tricks is to put no side pressure on the fin, maybe?
Seems to be a spun out fin regains a bit of support at 30 degrees off straight ahead, which lets one survive at low speeds.
I will try grinding the back half of the low pressure side concave, but I don't think fun will be the result.
Got to decide which is the most futile fin around here to sacrifice...
in simple words you could say that the more rake a fin has the more angle of attack you need for the same amount of power.
at very high speeds this is hardly noticeable, but at lower speeds it feels like the board is creeping with the tail out. but at the same time the stal angle is increased as well, which makes the fins raked fins feel quite secure.
but there are a few problems to overcome to make heavily raked fins perform well.
the center of effort is moved back quite a bit which upsets the trim of the sail/board combo.
if i would sail in a spot exclusively on weed fins i would move the fin box further forward to re-balance the CoE of the fin to the standard fin balance.
to get fins with rake working nicely it is easiest to use fins that are very stiff, as any flex in raked fin causes the tip to twist off which transfers the power closer to the base which can lead to spinouts.
you also need to consider the taper ratio when changing the rake a lot.http://picasaweb.google.com/C3carbon/Design/photo#5008932566699128018
picasaweb.google.com/C3carbon/Design/photo#5226722255199320226
i don't know yet if C3 will brig out a dedicated weed fin in the future, but it's certainly not rled out.
Boogie
Very informative boogie, thanks a lot, I hadn't realised the taper ratio changed like that. hope you don't mind another question another question.
inside the bubble i would presume it's all bubbly and density is low..so then if this was true to make a board go faster it would be safe to make some sort of smaller fin just infront of the leading edge of the board's fin and make this cavity to lessen frictionif am missing the whole point here then just disregard my comment
