PRACTICAL
Fixing foam
-cored decks
Structural problems with cored decks can
affect many older boats. Rupert Holmes
looks at how to restore their original stiffness
D
egradation of the foam or
balsa in cored decks is a
common problem, especially
in boats built before the
mid-1990s. Given that the alternative is
a huge amount of extra weight and
expense, almost every glassfibre boat,
irrespective of its age, has a cored
deck. Failure is most often noticed as
movement or a spongy feel in the top
layer of the laminate when you walk on it.
It’s important to recognise this as a
structural issue – a thicker laminate is
enormously stiffer than a thin one, even if
the core material doesn’t inherently
possess a great deal of strength. However,
the core must remain in good condition
and stay bonded to the glassfibre. If not,
the two sides of the laminate can move
and flex independently. This dramatically
reduces the stiffness of the boat’s
structure – there is less resistance to stop
the loads on the shrouds collapsing the
sides of the hull inwards, or forestay andbackstay tension bending the boat into a
banana shape.
Water ingress around poorly sealed
fittings is often the culprit, especially on
balsa-cored decks where the timber can
turn to mush when it’s wet. Alternatively,
particularly on boats built before the
mid-1990s, the problem may simply arise
through the bond to the glassfibre on one
or both sides of the core material failing.
Improved building methods mean it’s a
problem less frequently encountered
among newer boats.
Most boats are built with relatively small
and discrete areas of core, with glassfibre
between each piece of core material. If
you remove headlinings you can often see
where each individual panel is.
The most common repair method is to
work from the inside, removing a window
in the laminate below each panel of core
material with an oscillating saw, then
replacing the core and relaminating. This
works really well for a professional repair,but laminating cored structures overhead
is a skilled task that takes time to learn. On
the other hand, working above the deck
means a DIY repairer will have gravity
working in their favour.Tackling the job
In any case with this example, a 30ft
Dufour Arpege, a repair from underneath
the deck is not a feasible method. The
deck moulding was innovative at the time,
with inner and outer mouldings bonded
together with core material between them.
It has the advantage of creating a good
cosmetic finish on both sides and is a
similar process to that used by many
boatbuilders today, but in the late 1960s
and early 1970s manufacturers didn’t have
the advantage of resin infusion and
vacuum bagging to ensure optimal resin
distribution. As a result, a lot of Arpeges
suffer from delamination of the deck.
This was the case for Andy Lewis’s
La Rochelle, which he bought after she’d
been languishing in a Greek boatyard for
several years. As a former chief engineer
and manager of a charter base with 70
yachts he wasn’t afraid to get stuck in.
His solution was to cut out the affected
areas from the top, dig out the old core
material, glue new core in and then
replace the original laminate back on top
of the new foam.Time 4 days
Cost £250
Saving
£1,500+