Marine plywood is a form of plywood constructed using waterproof adhesives to make it suitable for use in wet environments. You'll find this tough material in boats, piers, boat houses and other structures that get exposed to damp conditions. Marine plywood may also serve as the underlay of your boat's vinyl deck cover, the backing for voids below the boat's wall and compartment overhead panels and even as part of the vessel's hull.
Marine plywood is generally made of fir, larch or cedar wood. All layers of wood are at least "B" grade, having knots, but without knotholes. The panels are glued together with a waterproof structural adhesive. The core may have a gap no larger than 3 mm (1/8 inch) throughout.
Marine plywood comes in different sized sheets and thickness grades. These dimensions are suitable for most uses on a boat, ship, or in a residential setting requiring a waterproof engineered wood, such as sub-flooring. Marine plywood is available with veneers in grades "A-A," "A-B," "B-B," Medium Density Overlay (MDO) and High Density Overlay (HDO).
No decay-resistant chemicals are used in the manufacturing of marine plywood, although pressure treated marine plywood is available and is both rot and insect resistant. Some plywoods, such as those made from cedar, are rot and insect resistant due to the nature of the wood.
Marine plywood played a part in the 1944 D-Day Invasion during World War II. Many soldiers arrived on the beaches of Normandy in "Higgins Boats," the brainchild of Frank Higgins, who convinced his father, Andrew Jackson Higgins, president of the Higgins Boat Company, that marine plywood was the best material available from which to build landing craft,
Marine plywood is not waterproof; only the adhesive that binds the layers together is waterproof. If you plan to use marine plywood in a wet environment, it should be painted and any joints caulked just as you would with any other material.