How To Preserve Hull Integrity In Timber Hulls
A look at the pros and cons of timber construction in boats, and some useful tips on how to look after these types of hulls.
Timber is my favourite material for permanently moored boats. Providing it's painted or varnished properly it can last far longer than fibreglass, without the risk of osmosis that occurs when water gets behind the gel coat.
There are primarily two types of timber construction — planked and plywood. Planked hulls have been around forever, with traditional caulking between the planks or, since the 1950s, strip planking where planks are edge-glued to one another, creating a low maintenance hull that's less likely to leak when re-floated after long periods of being stored ashore. Double and triple diagonal glued timber strips (cold moulding) also produced light and strong hulls.
Plywood, where thin veneers of timber are laid at 90 degree angles and glued together with phenol-based adhesives that passed a 72-hour test of boiling in water, became popular in the 1950s. On a kilogram for kilogram basis, plywood is actually stronger than steel.
But whichever form of timber construction is preferred, there are a number of don'ts.
One is never, ever fibreglass a timber or plywood hull or deck, because both materials expand and contract according to their moisture levels, whereas fibreglass doesn't. As the wood expands under the fibreglass it stresses the latter, resulting in minute cracks that let water into the wood. The water is then trapped and starts dry rot that even in plywood weakens the structure, leading to ‘springy’ decks and reduced hull integrity. Both my timber yachts had their decks fibre-glassed by previous owners, resulting in me having to remove large sections of decking to replace with fresh plywood sheeting.
Timber and plywood hulls will rot if used regularly in fresh water, and rain water in bilges will start dry rot, particularly in planked hulls. Plywood fares better because usually only the surface layer rots, which can be cured with a two pot epoxy fungicide. However, plywood is susceptible to delamination at the end grain, and when joining sheets, care must be taken to thoroughly protect the joins by ensuring glue completely penetrates the ends.
My second yacht had a cockpit that allowed rain water into the bilge, even when a tarpaulin was attached over the boom and lashed to water-freeing ports in the gunwale toe rail. I found the best way of reducing dry rot in the bilges was to sprinkle rock salt where rain water would accumulate. The salt preserved the plywood, even if it did make the cabin a bit damp. A thorough airing of the cabin on sunny days would remove most of the dampness and all timber and plywood hulls should be ventilated regularly. The yacht was on a swing mooring, and I found that attaching a canvas wind scoop above the open fore hatch while leaving the companionway open created a good air draught through the cabin.
Diesel fuel is particularly effective in rotting planks and I recommend that copper or stainless drip trays be fitted between the engine bearers to catch any drips when fuel filters are replaced. Also, any oil resulting from spillage when replacing the oil filter/s can be trapped in the tray.
Although varnishing a hull doesn't provide the UV protection of a well-applied paint job, it does have the advantage of penetrating the planking or outer plywood veneer, providing better protection. Providing, of course, a one-pot varnish is used, diluted with mineral turps for the first few coats. My first plywood boat, way back in 1965, was painted outside but varnished inside, enabling me to see if water had penetrated the hull sheeting by showing up as staining under the varnish. Left upturned on the waterfront in front of my parent's house, the combination of paint and varnish needed only annual touch ups.
When trailering a plywood hull, always fit cushioned bilge skids to prevent the hull from ‘sagging’ while out of the water. Multi-roller trailers will not provide adequate hull support.
