Choosing the Appropriate Wood for a Marine Build or Repair

There are different grades of wood, specifically plywood, that are graded depending on the intended use, environment/exposure and certification/testing. Broadly there are 4 grades of wood that categorize the different applications, A-grade to D-grade that have multiple applications in many different industries from structural work and wood airplanes to crates and boxes for shipping goods. To begin we must explain how the grades are intended to inform the builder of the uses and limits of the plywood and compare that to suggestions from our technician and other credible resources to inform a potential build or repair. Full un-abridged sources are cited below, and are available on our website under Marine Catalogs and Build manuals or can be found in their original sources online.

A-grade (BS 1088 Marine Grade Plywood, engineered plywoods, etc.), B-grade (6566 Marine, Fir Marine or exterior grade home), C-grade (home interior wood, pressure treated outdoor) and D-grade (large knot holes, non structural wood, crate material, etc.). It is important to understand the capacity of certain kinds of wood and the limits they have when exposed to humidity, water, temperature change and adhering to fiberglass. Not all plywood can me reinforced externally to stabilize internal weakness. This weakness can be caused by voids in the core laminations, low adhesive strength of the glues and limits of pine and fir in the cores. Due to the amount of fiberglass required to do this reinforcement we will increase the overall cost and weight, and at a certain point this will become more expensive than moving to a superior grade plywood.

A commonly occurring question that we are asked at Timber Boatworks is how to reinforce c-grade (home grade plywood or pressure treated plywood) for a transom, stringers, floors, outdoor decks, hot tubs, holiday trailers, gazebo and sunroom applications. It may seem like a simple answer should follow this inquiry, but there are two factors to resolve home grade materials in an outdoor environment, which are temperature expansion or contraction and waterproofing. We are asked how to fiberglass with polyester resin or epoxy resin the plywood so that it is stable enough to not warp, weather check, crack or leak in the outdoor environment. The way that we attempt to increase the durability of the exterior grade plywood this is several layers (2 or more layers on both the top and bottom surface of the plywood depending on the resin chosen and the type of structure and waterproofing required) hoping that the material does not internally delaminate with temperature change. Pressure treated plywoods offer one more issue on top of the internal voids, non-boil tested adhesive and low layer count laminations, which is the material has already been oiled with the pressure treating chemicals. The EpoxyWorks magazine article “Choosing the Right Wood for Your Boat Repair,” they say that “the chemicals (used in pressure treating use) the chemicals-using a water or oil-based carrier are injected into the chamber… while others use wax in the treatment.” This means that it is creating the water-resistant barrier with oils and waxes, but we then have to adhere polyester or epoxy resin to a waxed surface. This surface cannot be sanded or decontaminated with a solvent to prepare the surface for fiberglassing, since the material is impregnated with the wax or oil chemicals. Without proper preparation and even with several layers of fiberglass external reinforcement the plywood is not guaranteed to perform in the marine or outdoor environment, not including the extra resin and fiberglass cost. The plywood material cost is low, but the resin and fiberglass will make it comparable in price to b-grade or even A-grade materials.

Exterior grade plywood is generally considered b-grade or C-grade, “with C-grade plies, voids and caps are common and water may sit in them and start to rot the wood. With fewer, thicker plies, the flaws have more significant effect on overall strength and stiffness. The b-grade materials would still require waterproofing and fiberglass depending on the application, in marine we do require fiberglass for new plywood to be bonded to the hull or transom.” (Choosing the Right Wood for Your Boat Repair) In the EpoxyWorks article “Stringer Repairs in Fiberglass Boats,” the typical stringers “[i]n fiberglass boats, you will find that most often, stringers are composed of a core material overlaid with a fiberglass skin. The skin usually externs a few inches on either side of the stringer. ..[This] ties the stringer to the hull or bulkheads and spreads the load of the stringer over a larger area.. Some cores are structural, or active, and some are inactive, used primarily to provide a form for structural fiberglass skin.” The plywood is a crucial component of the structures in the hull and that fiberglass is required for the lamination of new materials to the hull or transom. Some marine grade plywood is also in the B-grade category, being Marine Fir where there are 7 or more laminations with water proof glue. These plywoods are more expensive than the C-grade but will require less layers of fiberglass, and in some cases just barrier coating with epoxy to waterproof. This grade of material can be used for temperature-controlled applications for marine construction, certain floors and stringers if the boat is not wintered outside can use this material. This grade is also commonly found in RV walls, holiday trailers, hot tubs, outdoor decks and sunrooms and certain inflatable boats.

The A-grade plywood has the strictest testing and parameters for it to be considered British Standard 1088, BS 6566 or WBP/ ENG 314-2 class 3 Water Boil Proof glue plywood. These materials are more expensive than B-grade marine plywood, but in certain constructions they are required for transom, inboard motor, trailer wall and floors and wood airplane constructions.  From the Technical guide for B.S.1088:1966 (abriged guide) the specifications are “Okume (Mahogany), with the quality of veneer… There shall be no end joints. Gaps in faces (fills on top veneer) shall not be permitted… Bonded with WBP adhesive. Number of Ply’s, multi-ply constructions may be used in any thickness, three-constructions shall not be used in assemblies exceeding 8mm (any plywood over 8mm thick must have more than 3 ply). Bonding between veneers shall be bonded using WBP resin, continuous over the entire surface area of the board.” For example, the BS 1088 mahogany 18mm plywood (3/4 inch imperial) is 9 ply with the strict manufacturing above. Unlike B-grade and C-grade plywoods that can have 3 or 5 ply the A-grade has 9 ply, which is more layers of strength and less risk of delamination with the WBP (water and boil proof glue.) For the plywood to receive the WBP certification it must be “[immersed] for 72 hours in boiling water, followed by cooling in water at 20 degrees Celcius for at least 1 hour” before strength testing the laminations.

There is many composite boards and materials like Coosa, FRP, honeycomb core and foam core composites that combine many manufactured woven materials and resins to make ultra light durable waterproof products. These materials are what all builders would like to use, however, the are cost prohibitive and difficult to source. Coosa panel is currently $565.95 for a 3/4 x 4 x 8 sheet, and is retailed in Ontario so shipping may be required. These are the top end of the construction materials and there is no downside to building with composites other than cost and availability, most boat builds are satisfied and standardized to marine B-grade or A-grade plywood and planking.

There are certain structural repairs, like transoms and stringers that a builder has to select appropriate materials for both the expected lifespan of the repair and purpose of that component structurally for the function of the boat. The transom in particular is a structural component to the hull but also may be used as a motor mount or inboard outboard leg support. We do want these areas to be build strong enough to handle vibration, stress, flex and torque from the hull and motor pushing through the water. In the EpoxyWorks magazine the West System technician fixes a “Chris-Craft Transom Repair,” by laminating “new plywood [that consists] of two layers of 1/2” and one layer of ¾” Okume marine grade plywood BS1088. We chose marine-grade plywood instead of exterior-grade plywood because marine grade is free of voids and high in quality.” This guide continues with “[He] coated both sides of the ¾” layer of plywood with neat epoxy at the same time I applied neat epoxy to the transom. Then I thickened the epoxy to a mayonnaise consistency.. and applied it to the transom with a notched spreader.. to achieve a consistent thickness. [He] handed me the first layer (of plywood) of panels to bond to place.” “The laminate required two layers of 17 oz. biaxial fiberglass fabric with mat.. The first layer of fabric extended onto the hull sides and bottom by 3” and the second layer extended 2”, creating a gradual transition. The fiberglass fabric was overlapped on the vertical seams by 2” and the overlaps for each layer were staggered.” This demonstrates the appropriate and materials when laminating marine grade BS1088 Okume Mahogany plywood with 1708 fiberglass in a transom repair. This Chris-Craft has an inboard motor with a thru hull leg for the propellor. Once again, structure allowing for the motor leg function required in boat construction.

If a builder is considering using a C-grade or B-grade plywood we highly recommend adding to the build budget for a larger volume of polyester or epoxy resin to strengthen and waterproof the wood. Let’s compare plywoods plus expected volume of resins in a boat transom lamination, with enough fiberglass for strength. Let’s assume that one full sheet of plywood can be cut and laminated to have the 1 ½ inch thick layer common for a transom, and that it is fiberglassed in with enough volume of resin and fiberglass to make the plywood structural and install it to the hull with fiberglass.

N/C= Not compatible or not advisable

This chart only demonstrates current pricing (July 2 2024) from marine retailers and home stores for the materials listed, this is also doing some calculation on volumes required for epoxy to saturate 6-ounce e-glass and polyester saturating 1.5 oz chopped strand mat. The numbers for saturating e-glass with West System Epoxy on a non porous surface is 1.61 fluid ounces per square foot. The estimated saturation of polyester resin through 1/8 inch mat is 12.8 fluid ounces per square foot, with an expected waste of material of 10-15% per batch. These numbers may be a bit confusing, and certainly will change depending on the quality of resin, air temperature, saturation and quality of fiberglass and the hands-on experience of the builder, however, this gives us a starting point on what is the bare minimum required for epoxy and general purpose polyester resin to build a budget.

The expected life span of the marine plywoods in an outdoor environment will be significantly longer than an exterior grade or a pressure treated, so the calculations above do not account for how often this repair will have to be redone. As mentioned through out this post, simply using WBP (water boil proof) glue in the plywood construction will greatly increase its lifespan and strength. We have heard the complaint that marine grade materials are simply more expensive than home grade because they get a rubber stamp that says marine, this is a reductionist view because marine grade materials legally must be tested and certified to be marine grade, and there is several different grades and certifications within marine as well. This is the same for marine electrical, navigation, paint and engine parts, they have higher levels of certification and this does lead to more cost, however, these products do not fail in high humidity and drastic temperature changing environments. Choosing the correct material for your build or repair will directly affect its durability and lifespan and save money and reduce risk of harm, so let’s use the proper material in the correct application for the best boating experience.

Sources Cited:

https://www.epoxyworks.com/index.php/chris-craft-transom/

https://www.epoxyworks.com/index.php/choosing-the-right-wood-for-your-boat-repair/

B.S. 1088:1966 (abridged) Standard Specification for Marine Plywood Manufactured From Selected Untreated Tropical Hardwoods

https://coosacomposites.com/where-to-buy/

https://www.westsystem.com/instruction/user-manual-product-guide/