floatation foam structural component?

[halfblindrabbet]

I have a 1968 18' mako that I picked up as a project boat. I knew three things that needed immediate attention. 1) front deck is soft, needs replacing 2) the keel just under the bow is soft, needs serious attention 3) the transom has some movement, needs replacing

Underneath the deck is floatation foam. I am removing the foam to rebuild the front deck and it accured to me that there are no stringers here. The deck is supported by the foam. Is the foam for structural purposes as well as floatation. If the foam is removed to access the keel/hull does it need to be replaced?

 

[Mark42]

Re: floatation foam structural component?

Some boats use floatation foam as structural part of boat.

I think Chrysler boats from the 70's are a good example: No stringers, and solid foam under the plywood floor.

From what I have read on foam manufacturers web sites, the "new" foams are much more impervious to water intrusion, and will probably outlive who ever uses it. Even completely submerged under pressure conditions, newer foams don't absorb water well at all. So I wouldn't worry about some water that leaks in and lays around. But I would still provide a tube or sorts to let any water thay may work its way to the bottom of your foamed boat to drain out the back.

I would have no problem at all removing rotten stringers and pouring foam only as replacement on a smaller craft, say runabouts under 17'. Would ensure the floor is fiberglassed plywood and well attached to hull with pour holes drilled. Then do the pour, let it expand and squeeze out the pour holes, then cut clean, epoxy plugs in holes and let it go at that.

Remember that foam glues everything it touches together. Very sticky stuff.

Bigger longer boats get a little more complicated with twisting forces and I think stringers and cross members are needed. But I feel comfortable with just foam for smaller craft.

 

[Chris1956]

Re: floatation foam structural component?

Rabbett, Supposedly Boston Whalers are made of foam between fiberglass sheets, and no stringers. I never heard that about Mako's. Clearly if you have no stringers, you cannot just engineer them from scratch, or the boat will have "hard points" which cause more trouble. I guess I would just look close to assure there are no wooden structures (frames, stringers) that just disolved.