[Public-list] Laminate/Plywood on boats

[edward schroeder]

It is also my understanding that "marine" plywood differs from regular plywood in that the inner plies are selected so that there are no voids, no "football-shaped" inserts, etc. This is so no moisture can linger within the plies. It is "exterior-type" wothout voids. Exterior-grade has glue that is water-resistant to more than the normal 10 or 12 alternate wettings and dryings of "interior-grade" plywood. Ed. Schroeder; architect.

"Cole, Andrew L" <andrew.cole at llff.com> wrote:John,

My lay understanding is that marine ply is a far superior material to
solid wood for most structural applications on our boats due to its
strength, stability and resistance to splitting cracking and warping.
Plywood has obvious and apparent disadvantages for exterior and finished
surfaces, often the top layer is nothing more than a veneer, i.e. you
cannot refinish it once the veneer starts coming off or wearing off
(which seems to happen if you let the surface become unfinished and
breathe on it wrong). The cockpit seats on our boat were previously
marine ply with a hardwood veneer bordered by 1" teak. Once the seats
were permitted to go unfinished (prior to our purchasing the boat), the
marine ply disintegrated (the top layers at least, the structure seems
fairly solid still) requiring total replacement. If anyone has more
authoritative information on this subject, you should disregard
everything I've said, it is just what I have heard and observed as a
boat user rather than a boat builder.

The hatch slider that is on our boat is not really plywood in the
literal sense, it is similar in that it consists of several thin sheets
arranged together, but it seems more akin to a laminated structural
beam. The adhesives are marine epoxies which are extremely strong, and
the fiberglass layers between the wood layers would seem to add
additional strength by providing a better (more even) bonding surface
between wood layers. Again, these are my lay observations, however I
would not necessarily disregard this method just because you have had
bad experiences with 40-odd year old plywood. There may very well be
plenty of other reasons to disregard it, aesthetic, dislike of
construction process, better methods, etc.

I will send photos of the hatch slider and the laminating jig to George
once I have a chance to take them.

On the subject of marine ply and its advantages/disadvantages, I think I
mentioned in another thread that in order to combine the stability and
strength of marine ply with the surface durability of solid wood, we are
making our new cockpit seat hatches center portions by epoxying 1/2 inch
marine ply to a 1/4 inch solid hardwood top (in lieu of 3/4" ply). I am
hoping this construction will provide a cover that remains directionally
stable while permitting several refinishings before the hatches need
replacing. I have some concern that the hardwood top may
expand/contract differently than the ply, although that issue may be
somewhat alleviated by thorough finishing and sealing of both. I will
send an update to the list with our experiences when the subject comes
up again.

Andrew Cole
Andante - 152 

Cole, Andrew L
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