Do I fill a GRP hull with foam?

[srm]

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 from 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

 

[wooslehunter]

Can't see how adding foam would adversly affect osmosis. It's there because water is being drawn from the outside into a layer close to the surface. Anyone know anything else?

Also there are some boats that have double skin with a foam filling in between anyway.

 

[bruce]

just be sure that you don't put too much foam in at first, could cause reshapping of the hull and pop a deck seam during expansion...

 

[Swagman]

Adding such internal foam will not reduce or increase any osmosis risk - if you have any voids in the glass then one day osmosis will appear whatever is backing the internal surface.

But agree with previous poster that when injecting foam it is very easy to forget expansion rate between liquid and rigid form - and I've seen bulkheads cracked by the resultant pressures created.

May sound daft as I know foam is cheap - but so are inflated foil / plastic bladders from (used) wine casks.
Blown up by mouth you can cram multiples into a void, and if you ever need access removal - is much easier than foam.

 

[RivalRedwing]

Just how accident prone are you ? do you have a history of collisions? If, as I suspect the answer to the above are 'I'm not' & 'no' then I would leave the foam in the chandlers. It is too easy to put in, a right SOB to remove and slowly degrades over time - just think what a surveyor might say when you try selling the boat.

 

[anubis]

I would contact Etap yachting via David or Ron at Woodrolfe. Etap specialists. Re foam chemistry.
There was an article in PBO, I think, concerning the volume increase in foam if applied DIY and the concern over expansion pressure damage.
Good luck

 

[simonfraser]

it's stayed afloat for 30 yrs . . . . the foam is b***** to get out

my scamper has similar voids up front, an access hole would be nice

 

[snowleopard]

many owners or boats with foam-filled compartments have complained of chipping out huge quantities of waterlogged foam from so-called watertight compartments.

the commonest type of foam-in-place material is REP (rigid expanded polyurethane) which comes as two liquids to be mixed and poured in. it works very well for insulating fridges but when used below the waterline it will definitely soak up water. it has very little structural strength and can be crushed by as little as 2 ft head of water.

i haven't tried the aerosol type that you buy from diy/builder's stores for buoyancy but it does seem a lot tougher. it would be pricey for large volumes.

 

[William_H]

Agree don't put foam into your bouyancy tanks. You need to have access to dry the tank and keep it clean. If you are really worried about collision and sinking (cats are hard to sink), and you shouldn't be, put in some plastic bottles or wine cask bladders. will

 

[srm]

Thanks Guys,

Someone else suggested filling the tanks with ping pong (table tennis) balls.

Sean

 

[claymore]

No - Don't do it.
We used to have a Dell Quay Dory at the sailing club and over time the foam absorbed water which made the boat really heavy. Bouyancy bags perhaps or see what is available in the canoe market.

 

[joe17]

No

No! I have an old boat that a previous owner had made unsinkable by sealing the whole of the bilge, under bunks etc. and filling with foam. Of course there were leaks and water has got into the whole lot and now I am spending time trying to get all the wet foam out. Its a real mess.

 

[TheoSr]

Just make sure that you only make small holes in your boat or the ping pong balls will escape when you need them most.
Keep smiling, Theo.

 

[oldharry]

Re: No

Bad luck - he obviously used open cell foam as supplied cheap from the builders merchants!

 

[aitchw]

Re: No

Not necessarily. I don't know of any foam that will not take up water if given sufficient time and contact. In the enclosed confines of a buoyancy compartment it's a certainty.