"My question is how to calculate the area of a leeboard that should be in the water. I can't decide how large or small to make them.
At the moment I use leeboards on my 17 foot long Folboat folding kayak and on my 14 foot long homemade cedar round bottom double ended rowing skiff. The aluminum road signs used by our county seem to be about the right stiffness. But I do not steal road signs. I use some surplus aluminum I found in a second hand store. I drill a hole in the top of the sheet of aluminum and run a cord through the hole and tie the board to the inwale of the skiff or to the coaming of the kayak. Pressure of the boat keeps the lee board pressed against the hull.
When changing tack the weather board floats out and is grabbed and put in the boat.
Since the thin metal is too stiff to bend I haven't tried to create a foil shape that might help the boat go to windward. If I want to go to windward I paddle or row. This is much faster than tacking upwind. Besides, my sails are so flat they are probably only useful on a reach or downwind.
Any idea of how to calculate what size board I should use?"
Disclaimer: Always wear life preservers, drink and party responsibly, sail at your own risk, and please pass the word that leeboard sailing is now a blast from the past that will soon be in the news as being the latest sailing craze.