Is fin. Is board. Is lift ... duoh
Hang on. I think I misread your query. You want to know how different fin configurations affect board performance?
OK I'll start.
To start with fins are foils and they follow the same rules as air foils. Hydrodynamics and aerodynamics are the same as far as foils are concerned, just scale differently. There is specific info on the web about foils sections. Google NACCA.
Long span and short chord = most efficient for lift/drag so if speed is the priority then long fins rule.
Short span is better for manoeuvrability. There is some doubt as to whether the traditional curvy wave fins are an optimal configuration, but short is definitely better.
As I understand it fin tips produce induced drag so minimising fin tips is better for efficiency. That's a can of worms that has yet to be exercised. I'm waiting for someone to make a quad fin speed board to sail in very shallow water so we can see what happens.
Generally multifin configurations are going to be draggy so they are limited to wave boards. They could be use for freestyle I guess.
Thickness I think is proportional to drag (not sure) but thin fins definitely have less drag. Thickness is qualified by structural requirement and flex. There is a wide variety in fin stiffness these days. Its particularly important in long span fins. I understand even tiny little freestyle fins need a bit of flex.
As NotWal says, but with windsurf fins there's the added complication of ventilation and cavitation. For most of us cavitation isn't a problem, with a typical fin foil it doesn't happen until the high 40kts.
But ventilation is possible at any time.
Going upwind it's because there's too much angle of attack for the leading edge shape to handle, and the fin stalls, the fuller the entry the greater the angle of attack can be, but at low angles of attack the fin will be slow.
Going off the wind at speed too much negative pressure is caused by the above type of foil and it will ventilate even at low angles of attack, (without stalling) a speed foil has a much finer entry, and the thickest part is usually further back.This sort of fin is more likely to ventilate, (stall) and have higher drag, at high angles of attack, but has less drag at low angles of attack
Thanks for decrepit & NotWal !!!
I try to select suitable fins for my boards - 04 Copello G-cross 95L & 08 Naish FSW 90L. Someone said there's a general rule : big fin for big sail .... small fin for small sail. I find difficult to upwind with Naish FSW screwing up with 30 fin in 15~20 knot condition ....
Should I use freemove / freeride fin or straight fin ???
Or use bigger fin 36~38 ??? 
I'm not familiar with those boards but 30cm sounds like plenty for a 90 litre FSW.
If you use too big a fin you will get tail walking. That is the back of the board will lift out of the water and oscillate from side to side then crash.
A rule of thumb for fin sizing is - for pointers the span of the fin should be about the width of the board where the back foot is. It can be a bit shorter or a bit longer. The qualifiers for longer or shorter are sailor weight, speed and fin style. Wave fins should be about 3/4 to 2/3 of the span of an equivalent pointer I reckon. If you are heavy go a bit longer. If lighter go shorter. The sizes for heavy weights and lightweights overlap quite a bit. As for speed, this is where the sail size correlation comes in. If you will be going fast (small sail) the fin should be from the small end of the scale. If slow (big sail) it should be from the big end.
If you are having trouble getting upwind it could be because the fin is too small (but yours doesn't sound too small) or technique or you are just under powered.
Thanks!
Your illustration let me understand much in fin selection.
Actually, there're many theories in windsurfing sport ....
That the drive to enhance my technique & knowledge in the future....
And this from the man who said we should take it easy on Brucie because he was a reformed character!
Today, I used 34 slalom fin in 20 knot condition ( 04 Copello G-cross 95L & 6.8m )
A little bit overpower but upwind ability is much better.
I guess if there's a limitation of upwind performance in FSW board.
