Do I fill a GRP hull with foam?

srm

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My boat is a small Prout catamaran about 30 years old. It has buoyancy tanks in the ends of each hull.

My surveyor suggested filling the tanks with closed cell foam, as this would be more effective protection in the case of a collision. However, on stripping the antifoul the boat has had a few blisters treated in the past, and all are on the outside of the buoyancy areas.

Do GRP hulls actually need to be ventilated on the inside to prevent a build up of moisture that some sources imply occurs due to GRP being semi-porus? Other sources imply that it is vapour pressure on the inside that is the main cause of osmosis, due to moist air being heated (by sun on the hull) in an enclosed space. In the latter case closed cell foam would seal the inside of the hull internal air.

I have coated the outside of the underwater area with West epoxy and copper/epoxy antifoul which should reduce any water migration into the laminate.

Basically the question is; can the inside of a GRP hull be sealed from the air or does it need to be ventilated? I know most builders don't bother to provide ventilation, but there is also a thriving business treating these same hulls a few years later for osmosis.

Sean
 

Sans Bateau

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ETAP yachts have had all the voids in the hull filled with close cell foam since the early 70's, no problem. It is very rare for an ETAP to have moisture problems in the gel coat, however the GRP layup and gel coat is of a very high quality (so my surveyor told me).
 
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