thought this an interesting read. can't confirm or deny the more tecnacal stuff.

http://wavegrinder.com/surf_fin_science.htm

Well this bit is obviously wrong

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Hydrodynamics teaches that underwater foils should have appropriate thickness, and should not be too thin, or cavitation will occur, a condition that is aggravated by turning. Underwater foils should be between a 9 percent and a 15 percent thickness, comparing fin width to the local chord length.

Think he really means ventilation, not cavitation.
In speed windsurfing anyway, it's foils that are too thick that cause cavitation.

If he gets a simple thing like this wrong, I wouldn't rely on the rest of it too much

probably is complete rubbish,

but interesting anyways. i wonder if the points on fin tip are right? never really understood to much about tips other than thickness.

Aerodynamics is a black art, rarely understood, and if you think you've got a handle on it someone throws you a curve ball

I doubt that surfboards go fast enough for tip shape to have any effect whatsoever. My understanding was that an elliptical planform is the most efficient (like the spitfire's wings) but there are loads more factors in a windsurfing fin than just pure efficiency. Twist, for example, airplane designers put in a bit of 'washout' into their wings but that's for a completely different reason than why we put in flex and twist.

I think the only way anyone will get a handle on it is to build loads and loads of fins, test them all, and see which ones work. Slowboat, Decrepit and Le Sam seem to have done this, I'd listen to them over me any day

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decrepit said...

Well this bit is obviously wrong

Hydrodynamics teaches that underwater foils should have appropriate thickness, and should not be too thin, or cavitation will occur, a condition that is aggravated by turning. Underwater foils should be between a 9 percent and a 15 percent thickness, comparing fin width to the local chord length.

Think he really means ventilation, not cavitation.
In speed windsurfing anyway, it's foils that are too thick that cause cavitation.

If he gets a simple thing like this wrong, I wouldn't rely on the rest of it too much

cavitation is the creation of a vacuum on the low pressure side due to overloading or poor design. On poorly designed propellers will chew them away in a very short time, as with pump impellors also.

Interesting read,but you carn,t compare sonic and sub sonic with surfboard fin design 0012 is not a bad start for a low speed fin but the winglets will produce lift or stall when the angle of attack changes. As to improving board speed when paddling .....There is so much surf board in or under the water no fin will improve its performance that much untill you reduce the wetted area of the board. But you added winglets to the fin therefore increasing it area and drag. Maybe he,ll sell a few ???? I,ll stick with the fins i,ve got thanks