When it comes to onboard projects, there are few things more useful than plywood. It's not always pretty and it's usually not cheap, but plywood almost always gets the job done if you select the proper type.
Cheap solutions, such as particle board or luan plywood, are ill suited for marine projects.  Though it is common, there is more to understanding plywood than you might think. Sheets are made from a variety of domestic and imported woods, come in different ply configurations and are manufactured in several different ways. Choosing the right combination will help insure that your projects come out right and that you don't overpay, because marine-grade plywood – even fir – is pricey stuff.
Choosing plywood begins with understanding how it is constructed. Plywood is composed of several layers of wood veneers glued together to form a panel. The layers of veneers are placed at ninety degree angles 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 holes. Those defects result in voids in the finished panels, which can form weak spots and areas for water to accumulate and promote rot and delamination. The adhesive used to join the veneer layers should be an exterior, waterproof glue that will hold up in the marine environment.
One common method for testing a piece of plywood is the boil test. Place a sample of the plywood in boiling water for at least five minutes and see if the glue holds. Another method is to run it through several cycles in the dishwasher. If the glue holds up, you have a winner.
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, and they are tempting because of the price (about $9 for a 4-foot by 8-foot sheet).
Luan plywood does have its uses. If I need to make a precise pattern, I will often use luan rather than a more expensive woods. One of my boats has a panelized overhead, and the removable panels are made of exterior luan covered with white marine vinyl. But I wouldn't use it for any critical application.
PAUL ESTERLEPlywood derives strength from laying the grain of each ply at a 90-degree angle to its mates.
High-quality plywood will have five to seven thin layers of veneer. This forms a stronger, more balanced panel that is less likely to warp or crack.
MANUFACTURING
Plywood 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, then placed on a machine like a round cylinder. A long blade is then placed against the surface of the log while the log is rotated. The veneer comes off the log, much 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.
Once off the log, the veneers are cut to size, forming the alternating layers of the plywood panel. Glue is applied to the veneers as they are stacked, and the assembled panels are then placed in a press and the glue is cured by heat and pressure. Once the panels are cooled, they are sent to trim saws to be cut to the final panel dimensions, most commonly 4 feet by 8 feet. Longer sheets – up to 10 feet – are also commonly available.
Another method of producing veneers for plywood is to move the log against a blade in an up-and-down motion, producing sheets that are the width of the log being sliced. This method is used to produce very high quality veneers, often for the face of plywood panels. These veneers are kept together in the order they were sliced, then trimmed and matched together to produce a panel with a uniform surface grain.
TYPES OF PLYWOOD
There are many, many different types of plywood, and they vary widely in cost and quality. If it grows into a tree, it has probably been made into plywood. Common species for marine panels are fir, okoume, sapele, mahogany, meranti and teak – and it matters which you choose.
