My Potter 15 rudder is made of 4 pieces. The top is a solid core about
10"x15", with two pieces of plywood screwed to it on either side measuring
10"x30". The solid blade mounts between the plywood, below the solid top.

One side of the plywood is falling apart, and the other side isn't doing
much better. I went to the lumber yard today to buy some plywood to replace
the bad parts. They have Marine Fir and Marine Okoume. They told me that
neither will be durable; they have to be covered with fiberglass. If that
is true, it certainly explains why the original is falling apart, as it is
not covered in fiberglass.

I bought a piece of fir, which I will cover with spar varnish just to get me
through the end of the year. This winter I will rebuild the whole thing.
(neither the top core nor the rudder blade are looking real good either).

What is the story on plywood? Is nothing suitable for marine use without
being fiberglassed? Would solid wood (3/8") substitute? Any advice on how
I go about doing this so it is durable would be appreciated.