Wood in boats is bad, right? That’s been the consistent message to boat shoppers and owners for a couple of decades now. It seems like almost a requirement that boat ads announce “100% wood free construction!” or “No Wood - No Rot!” or some similar claim.
Frankly, there’s a lot of truth to the idea that wood is a bad material for building boats. But the fact is that things are a lot more nuanced than that. Wood — even much maligned plywood — can actually be a great choice for certain applications. Let’s look at some of the strengths and weaknesses of marine plywood as a boat building material.
The Problem with Plywood
Plywood has a lot going for it as a material for boat building — excellent compressive strength and screw retention, relatively light weight, great workability and low cost. Its main weakness is that — like all wood — it absorbs water and eventually rots.
Plywood, though, is especially susceptible because water spreads readily through the panel from the point of contact. Additionally, water can break down the glue that holds the plies together, weakening the panel.
Solid wood and end-grain balsa coring also get wet and rot, of course, but moisture doesn’t spread as quickly or as far as it does through plywood, and there’s no glue to break down.
The bottom line is that if plywood gets wet, it’s a problem.
How Plywood Got a Bad Rap
Plywood has been used in boat building for roughly a century now, but the perception of it changed starting around the early 1970s, with the beginning of large-scale manufacturing of fiberglass boats.
Prior to that, builders had been careful to protect plywood from water by encapsulating it in various waterproof coatings. But production builders cranking out hundreds of boats a year weren’t so careful. Plywood decks, stringers and transoms were often left inadequately encapsulated, and screws were frequently driven into plywood without sealant.
Encapsulated properly, plywood won’t rot; likewise, even poorly built boats won’t rot when built with composite materials.
As a result, waterlogged and rotten decks, transoms, stringers, bulkheads, hatches, and more were commonplace. Plywood was blamed, which was understandable. But the real problem was the combination of plywood with poor craftsmanship. Encapsulated properly, plywood won’t rot; likewise, even poorly built boats won’t rot when built with composite materials.
Today, some of the most beautiful, capable, and finely crafted boats in the world — Carolina-style custom sportfishers like those made by Bayliss, Jarrett Bay, Spencer, Jim Smith, Buddy Davis, and other legendary boatworks — are built with fiberglass-encapsulated plywood.
Plywood’s Strengths as a Boat Building Material
There’s a lot to like about plywood for boat building. It’s relatively lightweight, extremely resistant to compression (crushing), rigid, holds screws well, and bonds well. It also happens to be easy to source and quite inexpensive compared to composite alternatives.
For smaller projects, the price difference might not be a major factor, but for larger projects — replacing the deck on a 24-foot pontoon, for example — the difference adds up.
Plywood vs. Coosa Board
Coosa board is the non-wood boat building material most similar to plywood. Both are rigid structural sheets, meaning that they’re inherently strong enough to work as decking, bulkheads, etc. without being reinforced with fiberglass.
Other panels used in boat building, like closed-cell foam and honeycomb, get their strength only when laminated with fiberglass. (In practice, both Coosa and plywood are almost always fiberglassed, which makes them even stronger and stiffer than they are alone.)
Coosa Bluewater 26 is lighter than the same-thickness plywood by roughly 30 to 40%, depending, of course, on the particular plywood product you’re comparing it to. Plywood is more rigid but less impact-resistant.
Plywood also holds screws better than Coosa, and least when they’re driven into the face as opposed to the edge, where Coosa has the advantage in screw-holding.
Plywood is less expensive and easier to source outside coastal areas. Plywood and Coosa are both relatively easy to cut and shape with standard woodworking tools, but cutting and sanding Coosa creates a fine, irritating dust that necessitates a mask, eye protection, and gloves.
What is Marine Plywood?
Not all plywood is created equal. In fact, not even all plywood marketed as “marine” is created equal. Basically, marine plywood is better made than non-marine and uses better wood.
Marine ply has more and thinner plies for a given thickness, fewer or no defects like knots and core gaps, and uses waterproof glue to bond the plies. It’s also heavier than non-marine plywood of the same thickness because it contains less gaps and more glue.
Marine plywood is made from a variety of different species of wood.
Exotic tropical hardwoods like okuome, ipe and meranti are favored for their water resistance and relatively light weight, but Douglas fir also performs well and costs considerably less.
It’s the plywood of choice for many top Carolina builders.
Fabricating with Marine Plywood
Obviously, the key to successful marine fabrication with plywood is making sure the wood is completely protected against water intrusion. The simplest way to do this is to coat the wood carefully with two or more coats of epoxy.
Prep work is key; epoxy bonds very well to plywood, but the surface must be sanded with coarse-grit sandpaper to create a profile and then thoroughly cleaned to remove dust and debris.
Better yet, encapsulate the plywood fully in fiberglass, using an epoxy resin. Adding fiber reinforcement not only stiffens the plywood but also makes for a tougher, more impact-resistant “shell.” Epoxy alone can be brittle.
Minimizing water penetration through fastener holes is also very important. Some experts say there’s simply no way to keep water from seeping into screw holes and that using screws or bolts in plywood is just asking for trouble. That may or may not be strictly true, but the fact is that fasteners are used in plywood panels extensively on boats of all kinds. Just take extra care to seal fasteners.
One approach is to use a high quality marine sealant in holes and under fastener heads to protect the exposed wood in the hole as much as possible. Another is to install the screw, then remove it, saturate the inner walls of the hole with epoxy, and then reinstall the screw before the epoxy fully cures.
Perhaps the most waterproof option is to drill an oversized hole, fill it with a “plug” of thickened epoxy, and then drill the fastener hole through the epoxy. This can be impractical for every single screw hole, but for large holes or holes below the waterline — engine mounting bolts, for example, or trim tab mounting screws — an epoxy plug is a good idea.
Applications for Marine Plywood
Marine plywood lends itself well to almost any application where a rigid flat or nearly flat panel is needed. Main decks, raised decks, pontoon decks, transoms, stringers, bulkheads, consoles, and fiberglass boxes can all be readily built with sheets of fiberglass-encapsulated plywood.
The real question is when to use plywood as opposed to Coosa, the primary composite alternative. That’s open to debate. If money is no object, there frankly aren’t many arguments for plywood. It’s heavier and can absorb water and eventually rot if not completely sealed.
True, plywood doesn’t require as much protective equipment and will make less of a mess than Coosa, but if money is truly no object you probably aren’t doing your own work in the garage anyway.
Marine plywood lends itself well to almost any application where a rigid flat or nearly flat panel is needed.
Beyond that, it becomes a question of priorities. For pontoon decks — which require a lot of material and are relatively easy to re-do 15 years down the road — plywood makes a lot of sense.
Quality marine plywood is as rigid as Coosa and actually offers better compressive strength, while costing between a third and half as much. Of course, the cost of epoxy coating or fiberglassing the plywood has to be taken into account, but it’s still considerably cheaper.
On the other hand, for major structural components like stringers, bulkheads, and transoms, the longevity of Coosa is highly desirable.
Replacing a plywood casting deck if it gets soft after 15 years isn’t fun, but it’s do-able. But cutting your deck out and replacing stringers after 15 years is major surgery.
Knowing your coring won’t rot offers real peace of mind.
On the other hand, for smaller aluminum boats where replacing the transom is relatively easy, plywood may be a good option.
Also consider how much the area is exposed to water. A forward casting deck or console, for example, will get splashed and rained on but will rarely, if ever, be exposed continuously to standing water. Stringers, on the other hand, can spend the majority of their time sitting in a few inches of bilge water. That gives water a much better chance to find its way through the fiberglass and into the core.
Plywood Is an Option
The point here is not to sing the praises of plywood nor to run it down. Instead, the point is simply that — despite its bad rap — plywood remains a viable material for building and repairing boats. In fact, in certain situations, it’s a better all-around choice than modern composite alternatives. So don’t rule plywood out automatically for your next project; keep it in mind as an option and weigh the costs and benefits.
Despite its bad rap marine plywood remains a viable material for building and repairing boats.