Marine Plywood: The Boatbuilder's Friend
Why every boatowner should learn to love plywood
By Mark Corke
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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.
Information about plywood:
APA—The Engineered Wood
Boulter Plywood Corp
Maine Coast Lumber