Reply To: foam hollow spaces.

I have a Mark ll/lll that had its hulls filled with CLOSED CELL foam below the waterline. Cores removed more than 30 years after the foam was put in by the person who built it from a kit, shows absolutely no water in the foam. Its bone dry. But if you use “builder's foam” or open cell foam, the ingress of water is possible, especially over an extended period. The difference between the two is the gas used to form the bubbles remains in the closed cell and water can't get in. The foam is actually a different chemical structure anyway.

Also, and perhaps more important, when the hull hits an object below the waterline, the rigidity of the closed cell foam prevents major damage to the hull (it helps prevent collapsing or ripping inwards) and certainly prevents any ingress of water. The only down side of the foam in the hulls is that there is little room for water to gather and that which does collect can have a go at the wood supports for the flooring. But as there seem to be several HTs with rotting floors beams and no foam – the two aren't necessarily related! But I fear closed cell foam gets a poor press – mainly because its confused with open cell foam.

I am currently stripping out the HT and completely rebuilding it. But I will put foam back into both hulls as well as the fore and aft sections – although it will be closed cell foam (which is about five times the cost of the open cell – or builders foam). Golden rule – if it comes from one bottle or one can its the wrong foam. You need two cans or containers to make closed cell foam.

To keep costs down I've filled 1.5 litre milk containers with polystyrene chips (the type you get in packaging) and then put the caps back on. The hulls are lined with closed cell foam – the bottles set in and then the whole lot surrounded and topped with foam. If you don't fill the containers they collapse. The containers also make it also makes it easier should you need to remove the foam in the future. Apparently this system is quite a common practice by those who use closed cell foam in boats for buoyancy.

Hope this sparks a good debate.