| Fresh
water accounts for the main cause of rot in wooden boats either by
condensation through lack of ventilation, ingress of rainwater through
leaking decks and openings, or a combination of both.
Antares' decks were a
classic example of what can go wrong when timber decks are laid over a
plywood substrate. Her decks consisted of teak planking, reclaimed from an
old steamer, laid onto a plywood substrate and payed with a polysulphide
rubber.
Regardless of whether the
planking consists of reclaimed or new timber, there are a number of snags
associated with this approach. The main one being that, if there is a
problem it remains undetected for some time and, when the trouble does
show itself it is often too late and the rot has already taken hold. Once
the rot is apparent the only recourse is to lift the entire deck in order
to remove the sodden plywood and any structural members that have been
infected.
A Traditional laid deck of
dried timber, on the other hand, is able to breathe, and therefore dry
out. If a leak does appear it can be dealt with either by caulking or
re-paying. It is rare, on a well ventilated vessel with a traditionally
laid deck using good quality dried timber, to find rot within the deck and
if there is some rot, where fresh water can hang, in way of timber heads
or deck structures, it is generally a relatively simple matter to replace
the offending plank without too much disruption.
Laying timber decks onto a
plywood substrate is really doomed to failure from the outset. It is
almost impossible to guarantee a good seal, even when the substrate is
coated with epoxy, especially where fastenings pass through the timber
planking and plywood. By the nature of its construction, when a hole is
cut in plywood it exposes 360 degrees of end grain, so every layer is at
risk of absorbing water. When moisture is trapped in these mid-layers,
where there is no air circulation, it is impossible for it to dry out.
Instead it festers, creating an ideal climate for any spores to become
active and so the risk of rot is ever present. A further danger is that
the undetected rot does not confine its activities to the plywood and the
underside of laid planking. It is highly likely to invite others to join
the party, including Wharf Borers who feed on decayed timber and, through
their excreta, encourage further decay, munching on deck beams and sending
contaminated water down onto the interior of the hull, behind the beam
shelf and lining, where timbers and planking are soon heading the same way
along with cabin top and deck house structures.
It is recommended that
fastening holes are sealed with epoxy resin but this tends to be a rather
hit and miss affair. With a fastening passing through an inch or two of
planking before it reaches the plywood, there is no guarantee that the
holes have been totally sealed and there is no way of finding out either
- until the damage is done. Further more, when the cost implication is
taken into account you are laying two decks to make one deck, at up to
twice the cost, and there is no guarantee of success. More to the point
the risk of failure is higher and the cost of repair can be greater.
The secret to a good deck
is to use dry timber, so that it has contracted as much as possible, as
thick as practicable to provide plenty of depth for caulking and to allow
for any twisting movement in a seaway, no less than 1 1/8" (according
to the size of boat). The deck should be laid under cover, or at least in
dry conditions so that no expansion takes place until it is caulked up.
When using a Pu or rubber stopping, cut a 10 degree bevel one side and one
end of the planks for two thirds of the depth and cut a rebate on the
other side about 6mm deep and calculate the width to provide a seam of 6
to 8mm, depending upon the width of the plank. Lay the planks using sash
cramps and/or blocks and wedges to ensure that they are close fitted.
Caulk the seams with two lays of cotton, the cotton should finish at the
bottom of the rebate, prime with two coats of primer and pay the seam with
a polyurethane deck caulk. For pitched decks cut a bevel on both sides to
form a full 'V' seam and caulk with oakum.
In Antares' case all of the
above were true. We replaced 12 full deck beams and numerous half beams in
oak, fitted new carlins and a complete centreline structure, stem to
stern, in 2" thick iroko. Two sets of 100mm x 7mm steel diagonal webs
were let into the beams in way of the masts to reduce stress movement
along with steel strap hanging and lodging knees. A new cabin top and dog
house, to replace the rotten structures, were built with a traditionally
laid deck on each. A straight laid deck of 2 3/4" (2 1/4" on the
cabin top and dog house) by 4" wide kilned iroko was
joggled into 9" wide margins fore and aft, all of which were caulked
with two lays of cotton and payed with polyurethane deck caulk from
Marine
Mastics.
If you
want a traditional looking deck then lay a proper traditional deck using
fully dried timber
Otherwise just lay an epoxy/glass sealed plywood deck and paint
it with a two-pack polyurethane sprinkled with glass beads for grip
~
Have one or the other type of deck construction but ... ~
DO NOT MIX THE TWO
and try to avoid anything that allows hidden water
to hang around
~ Finally
~
KEEP YOUR
VESSEL VERY WELL VENTILATED
ESPECIALLY
WHEN LEFT UNATTENDED
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