Boat Building Plywood

Plywood selection is becoming difficult. Only 10 years ago, inexpensive exterior plywood with no voids was still available but this is not the case anymore.

There are three elements to consider when selecting plywood: the glue, the type of wood and the quality standards.

Glue is the easy part: exterior and marine ply use the same glue or in all cases, glues that are always sufficient for marine use.

The wood type is another story. Most US domestic marine ply use fir or a specie close to fir while plywood imported from Asia or Africa use Meranti or Okoume.
All have a good resistance to rot, Meranti somewhat more than the others. Okoume is lighter and bends more easily but is more expensive.
There is also a difference in the way the plywood veneers are cut: rotary or sliced.
Marine Meranti or Okoume are rotary cut and have smooth surfaces. Fir marine ply is sliced and shows the grain. That grain will show through paint or resin. The only way to hide it is to fiberglass the surface and this ads weight and expense making the final cost of cheap fir higher than Meranti or Okoume.

The last item to consider is quality. Quality means a large number of thin plies, consistent wood species used throughout the plywood (no cheap core), consistent thickness (no thin outside veneer hiding cheap wood inside), no voids and good aspect.
The best quality marine plywood are Meranti and Okoume to BS1088 (British Standard 1088). Next comes the same species to BS6566 followed by Fir Marine to the APA (US) standard.
The exterior plywood available today has too many voids. Not only do those voids weaken the plywood but in case water find its way through the fiberglass and resin, moisture will be trapped in the voids and rot will follow. Scratches in the bottom or holed drilled for accessories provide a way for moisture to penetrate the core of your hull.

Currently (2006), the situation is as follows:

- for small boats that do not live in the water and are not subject to heavy loads like dinghies and canoes, good exterior plywood will work. It is understood that all plywood surfaces are completely coated with epoxy resin. Marine plywood is always better.

- for larger boats, marine plywood is a must.

The hulls of planing power boats or offshore sailboats must be built from quality marine plywood like Meranti or Okoume BS 1088 if only for mechanical reasons. Exterior and fir marine ply is just not strong enough: too many voids, not enough veneer plies.
If the builder wants to save on plywood cost, he can use fir marine or BS6566 for the interior parts.

Small boats can be built from almost any plywood if economy is a priority. It is a valid choice to use cheap plywood for a first small canoe as long as the builder understands the compromise. If you value your labor or want to keep the boat, use marine ply, the difference in cost is not significant if you take all the supplies in account. You are investing not only in your labor but in epoxy resin, fiberglass, paint and accessories.

Compared to the total cost of the boat, the difference in cost between quality marine plywood and cheap plywood is small: less than 10%.

Marine plywood like Okoume or Meranti are the best choices: they have superior mechanical characteristics, bend easily and are easy to work with. Build with marine plywood to save many hours of work and obtain a much better looking boat with a higher resale value.