Unfortunately Martin cut him off on answering the 3 factors that are important in a fast speed fin, he got to talk about a very thin profile as no 1 (Which reduced the acceleration of the water across the foil reducing the cavitation effect, what were 2 and 3. He never got to describe the rake? Does anyone know?
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!
"He never got to describe the rake? Does anyone know? "
Boogies X2 and X1 fins have 30 degrees rake but they were not designed that way. They were originally upright but didn't work as intended 'cause they were so thin they developed reverse twist. That is the tip twisted to windward. This is because on a symmetrical foil the load is at the 1/4 chord point/line ie 25% back from the leading edge while the foil shape has it's maximum thickness at 40.3%.
This caused the upright versions to develop amazing lift as the tip increased angle of attack up to the point where the tip stalled ( causing loss of lift) due to too much angle then the tip would spring back and start the cycle all over again. Sailing it was weird. It was slippery but I was all over the place , couldn't sail a straight line. So then Boogie , no doubt having spent considerable time and money having the alu molds made, started playing around with rake.
As Slowie says , raking it shifts the lifting centre back. I got an early version at ~21 degrees but it still wasn't right. When he raked them to 30 degrees ( he rebased mine..beautiful work) the loading was far enough back to reduce the excess twisting and produce a foil that is slippery but just seems to develop more lift the harder you push at speed... basically a great speedfoil. I don't think I've ever spun out on it. I don't think it is as slippery as Mal's or Chris' fins but good enough. While Boogie may disagree, I feel it's unique properties was a serendipitous combination rather than intentional one.
Raking reduces lift and drag and as Slowie says flattens the L/D curve. Basically it makes the foil think it is going slower than it actually is so it also delays cavitation but as it reduces lift you have to increase the area so the surface drag is proportionally greater. So if you know how to design a low cav profile an upright fin will be more efficient. But on the practical side rake is safer (weed sand banks etc).
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."
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."
3 years later it seems Martin V.M still prefers it over the new Prolimits ones that Boogies has designed... though I guess those are still a "work in progress"
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 
he used a 16cm one to win in Namibia... maybe he is taking the low aspect route so he can sail almost on the beach............
