Nice one Bender

Thickness of the profile affects how low the pressure is on the windward side of the fin- even when no lift is being produced. As you go thicker, the pressure drops. As you load the fin, the pressure drops. At a point where the pressure is low enough to pull dissolved gases to come out of the water and/or vapourise the water (close to vacuum), thats where cavitation starts.

The "section" refers to the shape of the fin from front to back. Its 2D. The section determines how well controlled the pressure is over the surface. Some shapes are better at reducing cavitation than others. There is also the possibility of getting laminar flow, which gives a reduction in drag. The section design determines a lot of other things critical to a good fin.

Rake does a few things. Firstly, it shifts the lifting centre further back. It also reduces the lift slope (ie need more angle for same increase in lift compared to upright fin). The main reason for using it speed fins is that it reduces the low pressure magnitude, so that you can run a thicker fin for the same cavitation inception speed. It also has the nice advantage of being more tolerant to weed!

Article from deepfried:
Boogie from C3 talks about his new creations.
23.02.05

C3 fin desiner Boogie has been seen recently testing a revolutionary new Speed fin that offers blistering speeds while giving incredibly good control. The following is and excerpt of a conversion with Boggie about is new fin range...
"When i started the X series speed fins I had a few ideas and parameters in mind that I thought would be necessary in order to achive very high speeds [>50kn].

I spent quite a while analysing and developing foilsections with a CFD program until i found the one which I thought would be the best compromise for low drag, cavitation inception at high speeds and an acceptable L/D ratio and stall charactereistics at lower speeds.

I wanted to stick within a certain range of reynolds numbers for the foil and further optimise the cavitation inception which determined the outline geometry. I was after a very unusual lift distribution for this fin as well. The final result is a fin that the testers are very excited about.

I was amazed and puzzled about the results when I tried the first few fins from the CNC machined mould.... [there is no handfoiling prototypes with my building process, so I made two moulds with the same outline but slightly different thickness. X1 is about 1mm thicker at the base than X2] a few tweaks of lay-ups and rake angle later it was obvious that something special is going on...

I can't remember any fin since I began my involvment in windsurfing, that at a length [or rather depth] of 24cm @ 30deg, and a thickness of less than 6mm at the base and around 2mm close to the tip, has plenty enough power to be sailed with a bagged out 6.2 on a 53cm wide slalom board upwind without the slightest spin out and when you turn the same combination downwind is able to go over 40kn with superior control and smoothness....

A picture of the fin does not do it justice to how it feels when you sail it, but here you go anyway.

I wouldn't really call the new C3-Xseries a revolution, but its definitely a very large step forward in fin development. And that after only a few months R&D on this particular model, i'm just warming up on speed fins... watch out, there is more in the pipeline."

In the October edition of Windsurf (UK) there is quite a long interview with Dunkerbeck, in his comments about breaking 50 knts he said he will use a 9cm fin, surely this must be a misprint, maybe it should be 19

Maybe mals supercavitating wheels weren't so much of a joke?