I think that's why nobody has figured out how to use supercavitation with wind powered craft yet.

nebbian said...

The canard on the Miller hydrofoil is supercavitating, and it appears to work just fine:

http://www.foils.org/miller.htm

no, it's not a supercavitating hydrofoil. it's a foil shape that was originally designed to work as a supercav foil, but which is used as a ventilated surface planer...

and yes, the shape works fine for the purpose.

Boogie

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

nebbian said...

So then you'd have a fin with no low-pressure side -- it's all high pressure, hence no cavitation.

and no lift either... and that is what a fin is for. if you don't need the lift from the fin then the fastest non cavitating solution would sure be to use NO fin...

And spinout wouldn't be a sudden loss of lift, it would be a gradual increase of board slew.

why? if you would try to put any angle of attack on the wedge fin to create some lift, you would get a sudden spin out from the separation at the leading edge. you could probably delay that somewhat by increasing the thickness/angle of the wedge, but projected frontal area is a major factor for drag.

It would probably be a lot less efficient than a normal foil shape though

very true... and because it is possible to delay cavitation with some simple tricks up to around 60kn... what's the point?

Boogie

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...

NotWal said...

C3 said...

... and because it is possible to delay cavitation with some simple tricks up to around 60kn... what's the point?

Boogie

What tricks, in principle at least if you are (understandably) reluctant to give away your secrets?

Is it true that cavitation starts as low as 20~30 knots and gets out of hand at 50?

here you can see pictures of cavitation at 16kn [8m/s]....
http://www.fluidlab.naoe.t.u-tokyo.ac.jp/Research/CavPictures/

with most current windsurfing fin designs you get cavitation inception around 42 to 44kn in my estimation, but by adjusting the sectional shape to a section which is designed to delay the onset of cavitation, thinning out the section and using rake you can increase the cavitation inception speed.
but of course it's all about finding the best compromise. most measures you can take to delay the cavitation have negative effect on other parts of the performance envelope.
the higher you try to push the cavitation inception speed the smaller the usable range becomes.

Boogie

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.

C3 said...

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.

Is that for the same thickness to chord ratio?

What happens if you keep the thickness to flow ratio the same?