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Boatbuilding Tips and Tricks

SATURDAY, NOVEMBER 22, 2008

Marine or Construction Plywood for Boatbuilding?

The question always arises on whether to use marine or construction plywood for building boats. Let's first discuss the different types of ply. There used to be a type of plywood called interior ply that was manufactured using natural glues that would let loose when wet. It is not made anymore. Nearly all ply made now, that I've ever come across uses synthetic, waterproof glues. Plies made of softwoods, usually fir but sometimes pine, have knotholes. If these are left open on the surface laminations, it is called "C" grade. If the knotholes are filled with football shaped plugs, it is considered "B" grade, and if it has no knotholes, it is considered "A" grade. So ABX plywood would have no knotholes on one side, filled knotholes on the other and has been laminated with waterproof glue (X = exterior.) The problem lies in the interior laminations. In most plywood the knotholes are left unfilled leaving a void on the inside. In marine plywood these interior knotholes are also filled leaving the ply void free.

For my boats, I only recommend marine plywood in the event the builder elects not to fiberglass and epoxy coat the boat on the outside. If the builder decides to glass cover the boat ABX plywood is sufficient.

As an alternate to marine plywood, hardwood plywood may be used. Mahogany and Birch are the most common. They have no knotholes, so don't have any voids. The only downside to using hardwood plys is that they don't bend easily so sometimes you need to laminate two thinner layers of plywood in an area where there is a lot of bends.

- Jeff


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