One of the most versatile of panel materials,
plywood is used for everything from sheathing houses
to building boats. It’s not a new concept; laminated
wood was used in ancient Egypt and China and
commonly in seventeenth-century European furniture.
By the end of the nineteenth century, plywood panels
were being mass-produced, but it wasn’t until the
1930s, when fully waterproof adhesives were
developed, that plywood became viable for
boat-building uses.
Plywood is made of an odd number of thin layers
of wood (plies) stacked at right angles to each
other. The contact surfaces are coated with glue,
and the plies are then bonded into panels under heat
and pressure. The sum of the parts makes a panel
whose strength is less directional and less prone to
splitting than solid wood; a plywood panel is
dimensionally stable and largely unaffected by
changes in the ambient atmosphere. The most common
panel size is 4 feet by 8 feet, though larger
panels, up to 5 feet wide and 12 feet long, are
available. Complex shapes can be cut that would be
impossible with natural timber.
Plywood varies greatly in price and quality, and
generally you get what you pay for. The cheapest
interior-grade ply is made from poorer-quality wood;
there may be voids in the intermediate layers, and
the exterior surfaces are often of poor quality and
must be painted. Luckily, standards and grade stamps
on panels make choosing the right plywood for DIY
boat projects a little easier.
There are U.S. standards for Douglas fir and
American-hardwood plywoods, while tropical-hardwood
plywoods imported into the U.S. usually conform to
British Standard BS 1088 or comparable European
standards. BS 1088 is the most common and has
stringent codes. For instance: “Plywood shall be
made from untreated tropical hardwood veneers having
a suitable level of resistance to fungal attack,
with a bond of WBP (weather and boil proof) glue
quality between the plies. Bonding shall be with a
phenolic formaldehyde WBP adhesive to BS EN 314-2
class 3. Finally, the veneers may be rotary or slice
cut. The method of cutting is at the option of the
manufacturer unless otherwise specified.
“Face veneers shall present a solid surface, free
from open defects. They shall be free from knots,
other than pin knots, of which there shall be no
more than 6 in any area 30 cm square, and not more
than an average of 2 per 30 cm square. Veneers
showing compression failure shall be excluded.
Occasional discoloration is permissible.”
Plywood that’s claimed to meet a marine standard
has to be suitable for the purpose. These are
minimum standards, however, and some plywood exceeds
them. Still, it pays to be vigilant when buying
plywood. If possible, visit the supplier and check
out the stock before you buy. This is especially
important if you plan to varnish the plywood.
Discoloration is possible (but unlikely) due to
water staining. Color variation is usually not an
issue if all components are to be cut from the same
sheet, but different sheets can have marked color
differences.
Although it may be tempting to use a
non-marine-grade ply for some jobs, this often
proves to be false economy. Boats live in a harsh
environment, and cheaper ply may not be as strong,
it may have interior voids, the glue may not be
waterproof, and delamination may occur,
necessitating costly repairs.
If you do an Internet search, you’ll find a host
of vendors of marine plywood. Prices for a
4-foot-by-8-foot sheet range from about $70 to $150
for 1/4 ( plywood to $140 to $330 for 3/4 ( Okoume
and Sapele plywood; Sapele is the more expensive.
Okoume (Gaboon) is an African hardwood. The
mature trees are tall and straight and free of
knots; they have large-diameter trunks. Okoume
ranges in color from a brown to salmon pink, takes
finishes well, and may be either painted or
varnished. While Okoume is often the first choice
when it comes to marine plywood because of its light
weight, it is not particularly strong and is rated
as nondurable. It must be thoroughly sealed, usually
with epoxy resin, if it will be exposed to water.
Sapele, also from Africa, is moderately resistant
to decay and almost 50 percent heavier than Okoume.
In the U.S., Douglas fir, a moderately rot-resistant
softwood, is used for plywood. Douglas-fir plywood
is usually used for structural applications since it
is difficult to finish. Painting can raise the
grain, and small face checks (surface flaws) can
lead to early failure. When a cosmetic finish is
required, MDO (Medium Density Overlay) plywood—where
the panel is overlaid with resin-treated
paper—provides an excellent surface for paint.
More-decorative veneers than the common plywood
species may be used for the face veneer if the
plywood is to be varnished, with tropical hardwoods
or Douglas fir used for the core plies. Teak and
mahogany are perennial favorites for the surface,
but plywood is also available with other face
veneers; ash and cherry are currently popular.
Plywood plays an important part in today’s
fiberglass boats. It is often used for bulkheads,
doors and drawers, cabin soles, and bunk tops. The
inherent stiffness of plywood panels makes them
ideal for bulkheads that will be subjected to
significant structural loads.
I have built several boats from plywood and find
it an almost perfect material for the amateur
woodworker. A hull can be quickly built from large
panels, so the boat takes shape fast, and it can be
completely encapsulated with epoxy resin. Epoxy is
also a perfect adhesive for joining the panels
together, making the boat very stiff and robust. In
“Delightful Dinghy” (Summer 2005) I wrote about
building a boat with the stitch-and-tape method.
This involves temporarily stitching precut plywood
hull panels together with copper or wire ties and
then glassing over the joints with epoxy and glass
tape. The technique was pioneered by Barry Bucknall,
who used it with great success in designing the
Mirror Dinghy; now boats of up to 40 feet and over
can be built this way.
There are several things you cannot do with
plywood; most important, you cannot bend it in two
directions at once, so making compound curves is
impossible. In addition, plywood will ultimately rot
if left exposed to the weather. More-expensive
grades will hold up longer and generally be more
resilient, but eventually all plywood will fail if
not properly sealed with paint and varnish or other
surface coatings.
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