Wood is structurally one of the strongest of materials on a strength/weight basis, not only for boatbuilding but also for many other types of construction. As for most materials, one has to work with the limitations and advantages of the material to get the most out of it. It used to be that boats needed to be built from timber species that were well-proven over time to be durable against rotting of the material, both immersed in water and in the damp environment inside a boat.
In the 1970's the Gougeon Brothers started to experiment with using epoxy resins to coat the surfaces of the boats that they built, as well as to bond them together. They started WEST Epoxies as a commercial supplier and I was one of the first to import their epoxy materials into South Africa for a 36ft boat that I was building at the time.
Used as coatings, these epoxies proved to be pretty effective for waterproofing the wood surfaces if used correctly. They proved most effective on laminated wood, strip cedar and plywood, which provided very stable surfaces for the epoxy to coat without excessive surface movement. It can't be counted on to hold together and waterproof surfaces of broad planking nor low-grade softwood plywood, which are unstable and subject to surface cracks that the epoxy cannot span.
The thickness of the epoxy, i.e. the number of coats that are applied, affects how waterproof it is. The greater the thickness the more waterproof it will be. And this is where builders can fall into the trap that results in moisture penetration and rot problems in the future. That said, epoxy coatings are not perfectly waterproof, moisture will permeate through them over a long time. The thinner the epoxy coating the sooner that will happen.
I launched the Didi 38 "Black Cat" in 1995 for the 1996 Cape to Rio Race. In 1999, after four years continuously in the ocean, I had her surveyed. The marine surveyor did moisture testing over the entire hull, above and below waterline. He reported back to me that both above and below waterline his readings were 12% moisture content in the skin, i.e. ideal moisture content for an engineered wooden structure. The exceptions were an area on each side of the hull where his moisture meter read elevated levels because it was reading the contents of the integral freshwater tanks. That boat is now 30 years old and is still going strong.
I applied three coats of 100% solids low viscosity epoxy over all internal and external surfaces of hull and deck. Over that, on the outside, I applied two coats of high build epoxy that served primarily as a fairing and sanding layer before the finishing coats of marine paint. Most currently available boatbuilding epoxies are of similar specification. Don't consider using any epoxy that has a solvent in it because moisture will enter through the pores left when the solvent evaporates out. Three coats are the minimum to consider if you want a durable and effective waterproof coating.
Voids in the structure must be treated in the same way. Just because they are sealed voids is not reason to leave them uncoated on the inside, all surfaces inside must also have a minimum of three coats of epoxy to seal them. Those voids may become pretty warm and any moisture that gets into them will make for a steamy environment, ideal for the growth of mould and rot.
There are unavoidably places where the hull or deck skin must be penetrated for hardware, bolts, chainplates, electrical wiring etc. and they must not be forgotten. Any penetration must be properly waterproofed to prevent water penetration into the timber. This is particularly important anywhere that end grain of timber will be exposed, whether plywood or planking end grain.
These are the very basic principles within which builders should work. But some builders, who should know better, will sometimes ignore or flaunt these principles. When that happens one can almost guarantee that there will be problems. As example, I will use one of the boats built to one of my large plywood catamaran designs, a boat on which these basic principles were forgotten and the boat suffered further when bad decisions were taken during remedial work.
The boat was built by a competent builder, who had been involved in a previous build of the same design. The problems started when the owner wanted to have laid teak decks fitted to both cockpits on top of the epoxy-coated plywood cockpit soles. This is not normally a problem because the teak can be glued down onto the sole, with no mechanical fasteners. But the sub-contractor said that he would screw the teak down. The builder objected but was overruled by the owner. So the teak was screwed to the plywood against the advice of the builder.
Right there they broke the two cardinal rules of wood/epoxy boatbuilding. First, don't puncture or penetrate the epoxy coating unless you absolutely have to. Second, if you have punctured or penetrated the epoxy coating, seal the timber thoroughly where you have done so.
The builder (remember that he objected to the screw fasteners but was overruled) was aboard this boat a few months after launch and saw that there was a serious issue with water leaking through the cockpit sole into the void of the double-skin bridgedeck structure. He told the owner to return the boat to the yard so that the problem could be fixed but the owner never did bring it back.
While the owner didn't return the boat to the builder's yard, after I don't know how long he took it to a different yard, where the teak deck was removed and replaced with artificial teak in sheet form, glued down onto the original plywood cockpit sole. This didn't fix the problem and eventually there was major rot throughout the cockpit soles.
I was called in at this stage by the new owner, the first time that I was consulted about waterproofing issues with this boat. I visited the yard and saw extensive rot in both layers of the bridgedeck structure. And I was able to see that, in laying the imitation teak, the repair yard had not remedied the root problem of water entering the timber but had actually aggravated it. The sheet material was laid onto adhesive that was applied with a toothed spreader, possibly the method recommended by the manufacturer. The trouble with this is that it formed channels under the imitation teak material to duct any water leakage to screw holes that had not been effectively sealed. And water did get under the sheeting and it did get into and through the plywood through those screw holes.
Once into the space between the two skins of the bridgedeck, the water was able to enter the lower skin because the original builder had made a separate construction error. The detail drawing shows doublers over the joints of that lower plywood layer. The doublers serve two purposes, they strengthen the joint and they seal it against water penetration into the plywood end grain. Without the doublers those butt joints were exposed to water getting into the end grain and wicking throughout the plywood.
The same mistakes that were made in the aft cockpit were repeated in the forward cockpit as well.
There is no excuse for this chain of errors in any quality boat construction project. The damage that resulted was started by a misguided sub-contractor insisting that he must use mechanical fasteners to secure the teak deck covering to the structure. An owner who lacked the knowledge needed to make the decision overruled the main builder, who knew better. Quality adhesives have been available for more than three decades that allow teak to be glued down onto the deck, whether wood, GRP or metal. Whoever applies the teak deck can use weights, clamps, wedges and other methods to hold the teak strips in place while the adhesive cures. With modern materials and methods there should not be any mechanical fasteners used in this process.
And if you want to use sheet decking materials, be aware that if you bond the sheets onto adhesive that is applied with a toothed spreader there will be voids and channels that you can't see, that can trap water in places where it will lie and, over time, cause corrosion of metals and rot in wood. Laid onto a balsa-cored deck this method can also possibly trap water that can permeate through the GRP skin into the core.
There are also other precautions that can be taken when building or repairing a wooden boat. Borate salt (sodium borate) is very effective in killing off existing rot and also preventing rot from starting. It is available as a powder that can be dissolved in warm water and painted onto wood surfaces, where it will penetrate into the surface and lie in wait to fight rot that tries to start up as well as poison wood-eating insects. After it has dried, epoxy coatings can be applied over it. This same solution can also be painted onto areas where rot damage has been removed, before rebuilding with new structure. Borate is also available in rod form, which can be placed into holes drilled into timber structure, once again lying in wait for rot to attack by dissolving into any moisture that penetrates the timber.
Cuprinol is another readily available option. This is a highly-effective copper-based liquid that is painted onto timber as a preservative. Once dried, it can be over-coated with resins, paints, stains and varnishes for long-term durability.
Follow the proven guidelines for boatbuilding with wood and you will have a successful project that will give you many years of joy on the water. Break those rules and you may be in for years of headaches and repairs.
I have drawn many designs for wooden boats of various construction methods. See them on my main website at https://dixdesign.com/ or my mobile site at https://dixdesign.com/mobile.
