yep, better live happy and short than long and sad

Rolf,

basicaly board area depends on two factors:
* avoid stalling at low speeds (e.g. being manouvrable at the start line)
* avoid cavitation at high speed
Both depends on the force which the rudder or board has to produce. The force depends on the boat balance and leeward force from the sails. The leeward (or better sideslip) force of the sails depends on their efficiency and the righting moment of the boat. Since a A-cat has a higher mast and a more narrow platform the sideforces are considerable less (especially compared to a double handed F16). I know this from my boat: The board size is just fine double handed, single handed up-wind it is a bit too much so I can drop the traveller a bit to get a more reasonable sail angle of attack. It may be a bit cryptic what I say, so don't hesitate to ask if you want to know more details.
The foil which I posted here some time ago was for the Tornado (which is now history , like us human beings, boats don't get fitter with age...), where we had high speed cavitation problems (I am really not wondering, even todays Tornado rudder look like F104 sections for me, you know the aircraft they called a rocket with wings). Maybe you want to go with a bit more low speed focused section. Again, don't hesitate to ask. I have started a small report about foil design for boats, which I want to finish a publish hopefully soon.

Foil planform is as important as the section, but much simpler. You can go with all those fancy shaped boards, we have seen, but you can achieve the same with a simple trapez. You can achieve it even with a rectangular shape with some forward sweep, if you dare to do something others don't do. The basic idea is to have an elliptic shape. You can do this with an elliptic planform or with a trapez with a 0.4 chord ratio and zero sweep at 25% chord. If possible from a structural point of view, the rudder should have the same depth as the board. Don't stick too much about aspect ratio, absolut depth (draught) is more important than aspect ratio and a larger rudder can have less drag than a small one.
I personally think that a homebuilder can make a good foil only with a simple trapez planform, because the surface of the foil is only single curved (unless you have a buddy who has a CNC machine). Already a small change in thickness distribution will ruine the hydrodynamics (1% percent change in thickness has lot of effects, 1% is equal to 1mm on a board with 100mm chord).

Cheers,

Klaus