meecrafty, my goal was getting single fin drag and three-finesque projection. But there is more to it than that.
BTW, you can fairly accurately measure the angle of attack with a rotating single fin. You simply increase the size of the maximum angle of rotation, and use little side force, and then ride it. At too large of an angle the board will start to draw a tight spiral spin-out with no drive - the fin angle approximated the angle of attack, and you lose it.
Greg Loehr stated he thought (and I tend to agree) that shortboard and fin design was driven to give the rider the best action off the bottom turn. The critical thing is to change from going straight with the wave, to projecting down the line as fast as possible. Optimizing this performance element has resulted in the most competitive success so far.
Also, Bert, vertical fins generate a vortex just fine. They need to be low aspect ratio, though. The leading edge vortex depends on the aspect ratio being low enough. The biggest drawback to the vertical fin is their limited angle of attack. They hit maximum lift:drag at too low an angle for hot-dogging on surfboards at lower speeds. If you make a vertical fin with a low aspect ratio, it will generate gobs of drive at low angles of attack. If the aspect ratio is too small, it will feel gutless. The starfin is a low rake, wide, low aspect ratio, fin (aspect ratio close to 2).
It’s been my contention that leading edge rake was set appropriately for matching the angles of attack. If you observe carefully, you will see that
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longboard fins are 5-10 degrees more upright than shortboard fins
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almost all shortboard fins by all manufacturershave the same leading edge rake
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windsurfing fins, which deal with more limited AOAs, are more upright than longboard fins
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wakeboard fins, and kiteboard fins, are even more raked than surfboard fins.
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the Takayama fins from Red-X are specially designed to go with longboards as rail fins - they are more upright than the shortboard fins
Further, if you make a rake series, and keep chord lengths the same, and fin depth, and just adjust rake, it becomes clear that more rake leads to higher drive at small AOAs, but more limitation in viable angles of attack. In choosing a planshape, the leading edge rake is one of the most important variables. I’'m not suggesting using vertical fins - but take your favorite fin and tilt it 5 degrees on way or the other. The difference in the ride is very easy to notice. That was mainly the point for me in using a straight leading edge - it made studying rake easy to do.
It’s also kind of interesting that surfing planshapes generate the most lift closest to the board, although some think planshapes like the Halcyon Mental, which moves the lift away from the board a little, are better. This is a function of chord lengths close to the base, and close to the tip. One of the things I don’t like about flex fins is their tendency to generate lift closer to the fin tip - thus generating a heeling torque.
Also, Bert, with respect to foils that generate low pressure through a leading edge vortex, lift from the low pressure side is highly concentrated close to the vortex - close to the leading edge. The low pressure would be closer to the wide point of camber is the fin were high aspect ratio and lacked a vortex.
I would be interested to hear about techniques for optimizing foiling. If you make a camber cut-out, it will be specific to one chord length, and difficult to replicate accurately enough from base to tip. If anyone has any tricks, I’m all ears. I generally start with a plate, use a sharpie to draw the wide point, and foil the leading edge rounded, and the trailing edge pointier, but my technique is quite crude.