Fran, in the normal variable conditions how would you choose where to put the fin? The rails are low aspect and therefore inherently not efficient in terms of preventing leeway, aren't they?
Cats can have very deep, narrow shapes and a cat without boards cannot compete with a cat with boards.
You may be right C249, what about a "sabre" daggerboard with a simple dagerboard hole: light and much more easy to built than the large ones...
...Maybe dangerous to have the daggerboard half out on the deck, or on the arm when speeding more than 30 knots!