A few thoughts:
Ply, since it is laminated, is much more resistant to warping than solid
wood. It's also able to take impact and strain without splitting as solid
wood might. Provided you can keep the water out, it's a very stable
material.
Ply is, pound for pound, really strong. Good marine ply is void-free and
uses waterproof glues. I build stitch and glue kayaks, where the 4mm (3/16")
ply is encapsulated in epoxy resin - about 4 coats on each side. Our 'yaks
take a pounding on our rocky beaches, and I've yet to damage the ply itself.
Provided you epoxy seal both the inside and outside surfaces with multiple
coats of resin, or even fiberglass cloth soaked out with epoxy, making sure
you seal the end grain really well, there's no reason the ply won't outlast
any solid wood, and possibly you
:-))
Make sure you use epoxy, and not polyester resin - the poly doesn't stick to
things or hold them together
nearly as well.
BTW - that Potter is one sweet little craft - used to admire them greatly in
our sailing days, before we downsized and simplified things to kayaks and
canoes.

"Toller" wrote in message
...
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.