Marine Grade Plywood vs Basic Plywood

 02 Jul 23:

Many times we read this on our building forums... "I'm building a boat and my local home improvement store only has cheap plywood. Does the plywood have to be marine grade?"

The answer is no, it doesn't have to be marine grade... but if you'd like for the boat to last more than one season...

Three things to look for with plywood are the number of plies, voids and the type of adhesive used. Marine grade plywood will have more plies per thickness than construction grade plywood, as shown below. The sapele marine grade plywood has five plies per quarter inch compared to the fir ply that only has three plies.

You might have noticed that the basic grade of plywood above has some gaps between the plies. This is not where problems start for boatbuilders but it is where they end. The basic grade of plywood does not use waterproof adhesive, so if water gets between the plies then it is all over for a boat, the wood will rot faster than you can get it home from the home improvement store. 

So how does water get inside the plies? Either through the edge grain void or though a face void. Marine grade plywood either has no voids on the faces or they have been repaired with patches, using waterproof glue. This marine grade sapele plywood has A graded faces on both sides and is made with waterproof glue. 

This fir plywood has multiple face voids, and there are large voids inside the plies. It will start peeling apart if it is exposed to moisture for a small period of time. 

Can we use it to make some Christmas yard art? Sure. We seal up voids as they present themselves, to slow down the deconstruction cycle. And the voids and rough surfaces can be faired, primed and painted to make the plywood more visually acceptable, but we'll discuss how spending a little more up front when building a boat will save many dollars in prep time and extend the life of a boat.

Video: https://youtu.be/BWRkrK6jY9E

Using marine grade plywood and marine grade fasteners allows us to build the "100 Year Boat." 

Where to find marine grade? 

1) We've ordered it through local cabinet and trim shops, the good folks at these places can add a few sheets to the orders from their suppliers. This may also be good spot to pick up some solid white oak, mahogany or other boatbuilding hardwoods. 

2) Marine Construction supply houses usually stock a few sizes of marine grade fir plywood. They may have one good face with no voids and a back side with a few patches, graded A/B. Or B/C. This ply requires a lot of prep for a smooth finish, plus priming and painting with good marine paint. 

3) Visit or order from supply houses. We picked up wood for several projects from World Panel in North Carolina, they also have a Florida warehouse and ship. There are many other businesses that ship. 

Fair Winds!

Clark and Skipper