Marine Plywood?

External Since: Oct 19, 2006 Posts: 28

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.

External Since: Sep 29, 2007 Posts: 28

Toller wrote:
> 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.
>
> Thinking about it, how about making the top core and one side out of one
> piece of solid wood, and then using a second piece of solid wood for the
> second side? I could partially resaw the core/side to remove most of the
> wood (where the rudder blade goes), and rout out the balance. That way it
> would all expand evenly, since it is all the same wood oriented the same,
> without any pesky plywood.
> Would it be strong enough?

Toller,
One should very much wonder about the individual that told you that
marine fir would not be durable enough.
The difference between the marine PW and others is 2-fold.
1- PW graded as marine has no voids in the core.
2- The bonding adhesive has very low water permeability (it used to be
better than exterior, but common exterior now usually uses the same process.

Time was all small boats that were not clinker plank (even some that
were) were made from marine plywood, and some are still out there.
I have made rudders and centerboards from marine PW.

Fiberglassing PW will just assure that it never has a chance to dry out
(Oh- sure it is completely encapsulated - with ZERO porosity or
permeability - Right - )

You can make PW cheek pieces and replace those on the rudder. What you
make them from is not of much consequence in actual fact.

The plan you outline in that last paragraph sounds like a whole lot of
work.

Matt Colie
Lifelong Waterman, Licensed Mariner, Congenital Sailor
and Third Generation Boatbuilder