While the main structure of most of our boats is fiberglass, they still contain an amazing amount of wood. That wood is very often some version of plywood. One of the questions I get asked most often online is, “Where can I find marine plywood?” If you are reading this near where boats are built, suitable plywood is usually readily available. More so than say, Arizona for example.
There is much more to the plywood question than where to find it, though. For one thing, marine ply, even fir, is pricey stuff. It also comes in a wide variety of types and species as well as domestic and imported.
Plywood is composed of several layers of wood veneers glued together to form a panel. The layers of veneers are placed at ninety degrees to each other to provide strength in all directions. The quality and performance of plywood sheets depends on the quality of those veneers. Plywood used in marine applications should be solid, with no loose knots or other holes. Those defects would result in voids in the finished panels. Those voids form weak spots as well as locations for water to accumulate and promote rot and delamination.
The adhesive used to join the veneer layers together needs to be exterior type glue that will hold up in a wet or marine environment. One common method for testing a piece of plywood not otherwise marked as marine is the boil test. Place a sample of the plywood in boiling water for at least five minutes and see if the glue holds.
The veneers are produced in one of two ways. The first is called rotary slicing. A log is soaked in hot water for an extended period to soften up the wood. The log is placed on a machine and the exterior is turned so the log is a round cylinder. Then a long blade is placed against the surface of the log while the log is rotated. The veneer comes off the log like a sheet of toilet paper. The center of the log that can’t be sliced is sent to home centers and sold as landscaping timbers. Most have two sides cut to form flats so they can be stacked.
Once off the log, the veneers are cut to size, some four foot by eight foot and others eight foot by four foot. These then form the alternating layers of the plywood panel. Glue is applied to the veneers as they are stacked to form the plywood panels. Stacks of these assembled panels are placed in a press and the glues cured by heat and pressure.
Once the panels are cooled, they are set to trim saws to be cut to the final panel dimensions, most commonly four feet wide by eight feet long. Longer sheets, up to ten foot are also commonly available. Sheets longer than ten feet are scarfed together. After sanding, the panels are bundled for shipping to their final destination.
Another method of producing veneers for plywood is to move the log against a blade in an up-and-down motion. This produces sheets of veneer the width of the log being sliced. This method is used to produce very high quality veneers for the faces of plywood panels. These veneers are kept together in the order sliced. In use, they are trimmed and matched together to produce a panel with a uniform or decorative surface grain.
The number of veneer layers is also an indicator of plywood quality. The next time you are in a home improvement store, take a look at their cheap luan plywood panels. There is one thick center veneer and two thin surface veneers. These sheets are usually warped and/or cracked. Higher quality plywood will have more, thinner, layers of veneer, usually in the five to seven ranges. This forms a stronger, more balanced panel less likely to warp or crack.
Common species for marine panels are fir, okoume, sapele, mahogany, meranti and teak. Solid veneers with no voids, exterior glues and multiple layers of veneers alone do not produce excellent marine plywood. For example, okoume is considered to have poor rot resistance. In fact, an okoume bulkhead on one of my boats rotted out from the inside of the panel from water wicking down the end grain from an undetected leak. Teak has very good rot resistance, but is so expensive that it is only used for face grain on expensive plywood.
For best results, any plywood going into a boat should be sealed with several coats of epoxy. This is especially important on the end grain, where water is most likely to enter the plywood and cause trouble. Fir plywood, exposed to sun and weather, will soon develop surface cracks known as “checking”. The only successful way to deal with this is to cover the surface with lightweight fiberglass cloth and epoxy resin.
So what marine plywood do I use in my projects? Well, none of the above. I use a plywood product called MDO (Medium Density Overlay). This plywood was developed for use in outdoor signs. Most of it is made from fir, with no voids and exterior glue. One of the best parts about it is the phenolic coating bonded to both surfaces of the panel. This phenolic surface is ready for epoxy or paint, needs no sanding and doesn’t check. Oh yes, it is also much more reasonably priced than marine plywood.
You won’t find it at your local home improvement store, though. You’ll have to go to a real lumber yard, sign shop or plywood distributor to find it. But it is readily available in our area.
Some current price comparisons are:
¾” Fir Marine plywood panels, 4’x8’ - $ 93.81
18mm Okoume plywood panels, 4’ x 8’ - $235.94
18mm Sapele plywood panels, 4’ x 8’ - $277.91
18mm Teak and holly plywood panels - $225.94
18mm Teak plywood panels, 4’ x 8’ - $394.70
¾” MDO plywood panels, 4’ x 8’ - $ 62.84
Many people think pressure treated plywood (or any pressure treated wood, for that matter) is a great product for use aboard boats. I disagree and don’t recommend it. First of all, the wood is low quality southern yellow pine. Voids and patches abound. Most of this stuff sites outside at many lumber yards and is wet and warped. I also think that the pressure treating chemicals don’t play well with marine epoxy adhesives. In fact, one of the major suppliers of marine boatbuilding epoxies, Gougeon Brothers (WEST System Epoxy) recommends not using pressure treated plywood. Not only that, breathing the sawdust from working it is bad for your health, stay away from it.
So there you are. The next time you need plywood for use aboard your boat, you can make an educated choice.