There can be little doubt that epoxy has changed the world of wooden boatbuilding, particularly for amateur boatbuilders. But is it really all that it is cracked up to be?
Transom ripped out by impact of the rudder with a whale at high speed, while racing. The stern knee, projecting into the opening at the bottom, is a major strength member to tie the transom to the hull bottom. It has zero epoxy bond of the aft face where it is rebated into the transom and was held only by the bond onto the edges of the rebate.
Epoxy, as an adhesive, is very strong and versatile. Manufacturers have developed many different formulations, with characteristics to suit a host of different applications and surfaces to be bonded. Most are very convenient to use but for wood-to-wood bonding, epoxy is not always the best. Resorcinol has a stronger bond and holds wood surfaces together under water-saturated conditions, i.e. when water has managed to get into the structure of the boat for long enough for it to travel by capillary action to the glue lines. Under those sopping wet conditions epoxy and wood can part company at the most inconvenient time. This is why epoxy is not used in the manufacture of marine plywood but resorcinol is.
Epoxy has the characteristic of being able to stretch well into the curing process, resorcinol doesn’t. That seems like it could be a plus for epoxy but it isn’t. It means that a resorcinol-bonded joint can be unclamped once the squeeze-out feels like hard rubber. At that stage the squeeze-out can be easily trimmed off with a chisel and an unclamped laminated frame will hold its shape. Try to trim epoxy at the same stage and your chisel drags it off and becomes a gummy mess. It is better to remove the squeeze-out immediately after fastening while it is still very soft, or to sand/grind it off later when it is hard. Even when the epoxy feels hard that laminated frame can straighten somewhat because the stretchy epoxy allows the laminations to slip past each other. Once it has straightened a bit you will not be able to pull it back to the intended shape. The laminated epoxy frame must be held by clamps until it has substantially cured, possibly many days in cool weather if your hardener cannot handle the temperature.
How do I know this? From past experience. Experience #1 was a keel floor while building my Didi 38 Black Cat in my garden. I had all of the longitudinals glued in place over the bulkheads and was laminating the first keel floor, with the laminations clamped to the inner surfaces of the longitudinals. The plan was to let the epoxy set then remove the floor from the boat, clean it up and shape it on the workbench, then glue it permanently into the boat before moving on to the next floor. It was a Cape Town winter, cool but well above freezing, at about 10°C. A few days in the clamps, then an hour or two was needed for working it with a plane and belt sander into a beautiful curved and tapered piece of wood, quite elegant. All went well until I tried to put it back into the boat. What a shock when it would not fit. With the ends in contact with the side stringers, it came nowhere near to the bottom stringers and keelson. And there was no way that I was going to be able to pull it back into shape with clamps without damaging the longitudinals. All that I could do was to turf it onto the scrap pile and make a new one.
Experience #2 was the foredeck of the same boat. Although it had a fair amount of camber, I figured that I could pull it in with a single thickness of 12mm plywood rather than laminating from two sheets of 6mm. I scarphed two sheets together to get to the size that I needed and left it for a day or two to cure. I cut it to shape and glued and screwed it to the deck structure with the epoxy scarph running down the centre of the deck. It looked so good I felt satisfaction about another job well done. As I walked into my garage to start on the next piece of deck there was a loud bang; my scarph joint had burst wide open. The glue line in the failed joint showed that it was a perfect scarph. It burst under the bending loads because the epoxy had not cured for long enough before I loaded it beyond what the epoxy was willing to accept at that stage of progressing cure.
Resorcinol is also not a wonder material; it does not bond well to as many other materials as epoxy does and it is not available in the wide range of purpose-designed and very convenient formulations for filling, fairing etc. However, it is the adhesive of choice by boatbuilders who work with traditional boatbuilding methods, gluing large timbers to each other.
Whether you are using epoxy or resorcinol to glue your boat together, you must, must, must do it correctly. That requires making your joints in the way that they will hold together through whatever nature throws at them. Resorcinol joints are strongest when there is wood-to-wood contact in the joint, with only a thin film of glue to hold the surfaces together. It likes well-made joints with minimal play and few spaces to be filled with glue.
In contrast, epoxy joints are strongest when there is a thin coating of epoxy separating the two wood surfaces from each other. It prefers a slightly sloppy joint that can be filled by the excess squeezing out. But you cannot rely on that characteristic to allow you to make bad joints. Your joints should still be as good as you can manage, with the only difference being in a looser joint that slides together effortlessly rather than needing some pressure to close the joint. Any gaps in your joints must be filled solid with epoxy because air gaps contribute nothing to the bond, instead weakening it. The result may be strong enough for general sailing but an impact with something solid and immovable may rip it apart.
The photos show an example of some bad epoxy bonding and the damage when the transom-hung rudder ripped out a big piece of the transom. A surveyor inspecting the damage said that the rudder had simply ripped out in high-speed racing conditions, with no impact involved. I asked for detailed photos and after examining those I requested him to inspect the rudder more closely, suggesting that he may find whale skin on the leading edge. This he did find and it was corroborated by the crew of the boat that retrieved the rudder, having seen blood in the water. If that boat had been glued together with more care the transom should have stayed in place, with the rudder pintles ripping out or the rudder breaking. Epoxy is fairly tolerant of imperfect workmanship but don’t put that tolerance to the test by being careless with your jointing.
Close-up of the stern knee. The aft edge has been coated with low viscosity epoxy but there is no sign of adhesive epoxy applied over the coating epoxy prior to gluing.
It is as a sealer of timber, in the form of coatings, that epoxy really shines. It is about as close as we can come to an impervious coating to keep out water, in both liquid and vapour form. It can make a good job of preserving your wooden boat if you do it right. If you do it wrong then you might just as well save the cost of the epoxy and the time to apply it.
My second big boat build was Concept Won to my CW975 design. It had a long skeg from just aft of the fin keel all the way to the transom. A few years after launch I did some modifications to her to upgrade performance for racing. I fitted a new high aspect bulb keel and I took a skill saw to the long skeg, cutting it off about 50mm proud of the hull, which was as close as the skill saw allowed. The exposed wood looked as pristine as the day that I glued it to the hull, with no moisture nor staining through the epoxy coatings. There was some very localised staining around the bolts where moisture entered through the epoxy/microballoon filler that covered the bolt heads but the wood was not damp.
After five years of hard coastal and trans-ocean racing in my next boat, Black Cat we had her surveyed. The surveyor went all over her hull with a moisture meter. He told me that he had never had such good numbers on a wooden boat before. He measured 12% moisture content both above and below the waterline, except for the saloon area where his meter was reading the water inside the integral water tanks.
Epoxy is also used for new waterproof coatings over fibreglass hulls that have pox problems from water penetrating deep into their polyester/glass laminates.
Here are some pointers about right and wrong.
1. You need at least two full coats of epoxy, preferably three, to protect the timber. The first coat will penetrate the surface, so keep applying it until the surface stops absorbing it, with the entire surface shiny. Apply follow-up coats within 12 hours of the previous coat if possible, to ensure a chemical bond between the coats. If longer, sand lightly by hand to provide a key to maximise the mechanical bond. The 12 hours is a rule-of-thumb but may vary depending on the epoxy being used. Be careful when sanding, a mechanical sander can heat the epoxy to softening point and drag it off, clogging the sandpaper in the process.
Close-up of the rebate in the transom for the stern knee. This shows that the back surface of the rebate was coated with epoxy but there is no sign of adhesion to bond the aft face of the knee. The edge of the transom is grey in colour, indicating that it had weathered for considerable time before the hull skin was fitted. Weathering reduces bond and the photo shows that very little of the hull skin was bonded well enough to even damage the surface veneer on the inside of the hull skin. Weathering like this must be sanded or planed off before applying glue, whether epoxy, resorcinol or any other type.
2. Pay particular attention to any and every area that has exposed end grain. That means anywhere that there is an unprotected plywood edge, as well as ends of frames, trims, knees etc. The epoxy will soak deep into the end grain, which will keep sucking it in until fully sealed. Don’t short-cut this process, you will regret it later. Don’t add filler to this epoxy, the filler will damage the waterproofing qualities of the pure epoxy coating by making it more porous. Leave any filler for a fairing layer over the outside of the waterproofing coats.
3. The target is to get the epoxy to soak as far as possible into the wood and to have an unbroken coating. That means that the resin must be drawn into the wood. If the wood is increasing in temperature it will blow out air, or ‘outgas’. If the wood is cooling it will suck in air from the atmosphere. Use this to your benefit, apply your first coat of epoxy when the wood is cooling down. If outgassing, you will be left with many tiny volcanoes where a hole has been blown through the resin layer as it gels. If you have glass in the epoxy that air will be trapped under the glass as an air bubble. In either case it can be a pain to remedy.
4. Just because you can’t see a piece of timber is no reason to leave it uncoated. The entire inside of a dinghy with self-draining wet deck must receive its three coats, even inside sealed buoyancy compartments that will never be opened again. Don’t let rot start inside a sealed compartment and destroy the wood where you can’t see it, finally showing itself as a fine crack somewhere, which collapses under finger pressure when you investigate it. If it progresses that far you will have considerable reconstruction to do.
5. Do not leave any end grain exposed and accessible to moisture. That means absolutely every penetration for screws, bolts, pipes, wires, hardware, drains or anything else. If the hole is big enough to get a paint brush in, treat it the same as any other end grain and apply epoxy until it will not soak in any more. Then apply another two coats after the saturation coat has hardened. If the hole is too small to treat this way, drill it oversize then fill the hole with epoxy and redrill to the required size after the epoxy has cured, leaving a lining of epoxy around the opening. If the opening is large enough and in a critical location, it may be worthwhile to line it with a fibreglass tube.
6. Don’t use a paste epoxy to fill these holes nor to protect the end grain of plywood or framing. You need the epoxy to penetrate the end grain, which a paste epoxy will not do. Mix your own low viscosity resin and precoat the holes thoroughly with a brush. Then add filler to the resin left in the cup to finish filling the holes.
Another view of the rebate for the stern knee. The smooth surface shows that there was no contact between the knee and the back face of the rebate. Gluing any member into a rebate needs the rebate to be nearly filled with glue, with more applied to the contact surfaces of the member that is being glued. The excess will squeeze out at the sides and ends, to be rounded off into a fillet to further strengthen the bond.
7. Take a lot of care to form holes through double-skinned structure that will take loads, such as drain holes through the bridgedeck of a catamaran. There is a lot of end grain timber in the hole and it is not easy to apply coatings in the hole, so rather add a fibreglass tube to the detail, with glass fabric wrapped into the tube at both ends to prevent cracking around the interface with the surrounding timber.
8. If your hull or deck surface is covered with a layer of fibreglass, don’t assume that this will do the job well. It needs careful work to ensure a sound layer of protection. Voids in the epoxy/glass layer or areas that are not properly wetted-out can trap water and wick it through to the wood surface, where it can start rot.
9. If you are building with fir plywood then your epoxy coatings will likely crack. This is because the resin does not have enough tensile strength to resist the forces at work in the unstable fir surface. Adding a layer of 6oz or 9oz glass fabric will add the strength needed to hold the surface together.
Black Cat, is now nearly 24 years old. Built from okoume marine plywood/epoxy, she has crossed the South Atlantic six times and covered thousands of miles in coastal racing and cruising. She has had one issue with rot, where someone drilled a hole to add a fitting and didn’t seal the end grain. The localised rot had to be repaired. There are many decisions to make in any boatbuilding or maintenance project but whether or not to properly seal the timber should not be one of them.