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.

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 ???

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....

Rubby said...

Long span and short chord = most efficient for lift/drag so if speed is the priority then long fins rule.

There are holes in NotWal's explanation. I don't want to go into this because it has been covered extensively on the various pertinent windsurfing URL's. However, Long span short cord isn't always most efficient for lift/drag. In fact at low getting-up-on-a plane, wide cord is most efficient. Racing fins are long span/short cord because once a board is fully on a plane, the long span keeps the windward rail down while the short span promotes higher speeds. A high aspect fin where the cord is wide at the top gives better lift at high speed. However, this type of fin is not as stable at speed as one that has a low aspect.

Somehow Scratchynuts, I find it difficult to take someone's aerodynamic explanations seriously if they can't even spell "chord" correctly. Notwal even did it right, just above what you wrote!

It's a bit like a mechanic who says "That long wobbly thing" when he means the crankshaft.

Or maybe it's just me.

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.