When the foil is really thin, it acts as a flat plate. There isn't much you can do to affect its performance. Analysis of even 6 per cent thickness shows much less sensitivity to shape than thicker foils. A limiting situation is model boats with sheet metal keels. Just round off the edges enough that they are safe to handle.
All shapes ideally have an infinitely thin trailing edge.
Practical considerations, like nicks and dings, require a minimum thickness. All of the people who have studied this agree that the best thing is to compute the shape as if the foil were to be slightly longer. Then cut it off at the desired thickness with the edges square. PLAN FORM Plan form is the aerodynamicist's term for the side view of the foil. The preceding analysis of cross section only used a 2 dimensional view, but the real world is 3D. Near the bottom of a foil, the water may turn toward the bottom edge instead of going straight back and as a result produce less lift.