PBO reader Allen Brunton has a question about plugging moisture traps on his boat. Boatbuilding and repair expert Tony Davies has this advice…

Allen Brunton asks: “Our 1983 Sigma 41 has some spaces which trap water and which would be very difficult to drill limber holes into. They’re awkwardly shaped, often irregular and rough spaces which are difficult to dry out with a sponge.

“Stringers and ribs are formed onto the inside of the hull with glass and resin ‘top hat’ sections filled with rigid foam. They’re usually hidden and trap water until the heel angle is above 45°.

“I’m looking for a long term solution but am concerned about introducing new problems.

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“My idea is to fill the spaces to enable them to drain rather than trap water and I’d appreciate your help in choosing materials.

“The worst spaces are trapped by stringers, ribs, and longitudinal plywood berth sides which have the chainplates attached. These are irregular inverted triangular pyramids 400 x 150 x 100mm high approximately

“A trowelable filler, not too fussy about a bit of paint or damp, like lightweight Polyfilla for house ceilings, then encapsulated with glass cloth and resin is the sort of thing that would solve the geometry, but is there a marine equivalent?”

Tony Davies replies: “I had a very similar problem in the bilge of my own boat: the shape of the hull left a concave section each side that would trap water with no way of draining it.

“I therefore decided to do away with this recess by filling it with expanding foam and sealing it with mat and resin.

“If you are very fussy you can get closed cell foam from GRP suppliers but I used aerosol cans from the DIY shop and made sure it was totally encapsulated.

“A particular advantage with your problem is that this foam uses moisture to cure so not being able to totally dry the areas involved would not be a problem.

“Just ensure there is no actual water slopping about. I think you’ll find this an easy and effective resolution to the problem.”

Buy Soudal Gap Filling Expanding Foam on Screwfix

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