Marine Plywood is slightly heavier than raw western red cedar. 6mm Okoume,
5ply marine ply is 20 pounds a sheet or 27 pounds a cubic foot. Western red
cedar is variously between 22 and 25 pounds a cubic foot, depending on who
you believe. (And, as Bram noted, individual boards vary substantially) If
you figure in the epoxy joint every 3/4's of an inch, there is little doubt
the joined cedar would actually be heavier prior to the application of
glass. The plywood holds the added advantage of a controlled production
environment and the none of the variables of individual placement of strips
one to another.
All in all, I'd vote for plywood bulkheads being stronger, lighter and a lot
faster to put together.
Note that Meranti 6mm, 5 ply marine ply is 35 pounds a sheet vs the 20 of
Okoume. Not all marine plywood is equal. Meranti is less susceptible to
decay than Okoume. With careful application, and installation, the Okoume
should be fine.
Keith
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