Concave sections were mentioned (in relation to the NACA 63 and 64-series).
I have heard someone say that Marstöm's Tornado rudders have concave surfaces in front of the trailing edge.

So I would like to know:
Are concave surfaces on symmetric foils of any advantage for high-perfomance 16-20-foot cat?
Which symmetric NACA-profiles have concave sections?
Or are concave sections/surfaces only useful in asymmetric profiles?

Stein