Is coating with epoxy always necessary when boatbuilding in wood?
I read the latest article by naval architect Michael Waters the other day and it prompted me to ask him if he thought using epoxy on wooden hulls was always necessary for the hulls of a small trimaran — if the boat was going to primarily be used for daysailing (or as a weekender)?
In other words, would it be possible to build a good wooden sailing tri without using epoxy and sheathing on the hulls? Could a small trimaran builder put hulls together and just paint them?
What really got me to thinking about this was Mike’s observation that many early small tris, such as the Crowther Buccaneer pictured in Mike’s article, were never “epoxy sealed” … yet some of those hulls are still in good shape after 30 years. By contrast, other wooden hulls (sealed with epoxy) have rotted away over the years because moisture got trapped inside the wood that the epoxy was supposed to be protecting, and the wood never had a chance to dry out and ended up rotting.
Mike replied with the following …
“Joe, many of the early Buccaneers were indeed originally built without epoxy, and so were a lot of Pivers and some Searunners too. Personally, I am of the opinion that epoxy has sometimes been over specified without much thought – based on heavy marketing and some misguided hype that ‘epoxy can fix and seal everything.’
As you can see from the picture in my article, it can also cause problems too. With wood, you either seal it totally or let it breath and that’s not always an easy choice. If the epoxy cannot be applied in a layer without breaks, then there’s more than a fair chance the wood will fail if it gets real wet at all (as often happens in an ama).
Epoxy still makes sense below the waterline though and up to 4″ above it, though one could use a thick epoxy paint like coal-tar epoxy that’s now commonly used on the inside of a ship’s ballast tanks.
For the W22 small trimaran model I’m designing, I will be recommending an epoxy exterior for some areas only that are subject to abrasion and impact – (otherwise the Awlgrip polyurethane) – but alum paint inside much of it, except below the waterline … where it will be glass for the main hull and epoxy in the interior of each ama. Under the deck will be treated with epoxy too, but in a quite unique way that I will detail later.
This will give a good compromise between cost, weight and service life. Strengthwise, epoxy and cloth should be on the INSIDE anyway – in order to resist wave pressure and impact, through a surface in tension.
I have some different ideas on prefinishing the plywood too, but you’ll have to read about these later ;-)
Later this year, I hope to post an article reviewing the old argument of epoxy vs. polyester, despite the fact that many designers think that was all over and settled years ago.
While I have no argument with many of the superior qualities of epoxy, the one factor that is often not considered is, which resin gives you the most ‘bang for your buck’ and that I contend, should depend on where and how it is used. A well done epoxy base is tougher than paint though and the latter will need a little more attention and regular maintenance – but yes, good paint well applied, can still work fine. Read more later.”