Use of Pourable structural foam in older boat of single skin fiberglass

Nothing is as expensive as a free boat. The free boat I received is an older seaswirl P14 of unknown vintage. Its a late 50s early 60s, 14 ft runabout. Construction was single skin fiberglass. The hull has a sharp v forward that rapidly flattens out to an almost flat stern.

The sole and stringers are rotted. Stringers appear to have consisted of 2 less than full length stringers for the rear 2/3rds of the hull and a centerline over the keel stringer of nominally 1x size wood. A small keel runner on the outside of the hull not more than 1 inch deep at any point, appears to cradle this keel stringer. A plywood floor was laid over these stringers.

I have removed as much of the rotted keel stringer as possible. Only "good wood " is left. However the surface is very unfair. Its would be impossible to match the remaining contour without filling the top of the keel area and that would be ablor of filler to make a smooth fair curved upon which to set the new keel stringer.

Alternatively I as considering filling the v of the hull with pourable structural foam of 8lb density and shaping that foam to a fair and flat sole, then bond plywood with epoxy to that foam. This would make the entire under floor a big piece of core, would it not? Or perhaps just enough foam to fair out the keel area and lay a new stringer in top of the foam.

Thanks for any comments or ideas

 

What foam? What are the mechanical properties? Have you priced the foam?

 

No doubt he is talking about PU pour foam, in a higher than normal density. 8lb is certainly regarded as a structural foam, but it would cost something to fill a decent volume like this. The hardest part would be to get it where you want it, and level. you'd have to put your sheet of ply in place, first, somehow, and leave gaps and regularly cut holes for the excess rising foam to squeeze out of, as it expands, and clean up the protrusions later. The underside of the ply could be sealed with epoxy first, cured, then the risen foam would adhere strongly to it. The trouble is that if you get it wrong, it is an expensive mess, and the foam does not give you time to make adjustments, it rises quickly, in minutes. You could conceivably do something like this on a small boat, then glass over the ply. But the cost in foam, and frayed nerves if the rising foam pops the ply up from where you want it, could be a little high. You may even be able to come down to 6 lbs foam. Foam of any specified density, will end up at a higher density than advertised, if constricted in the expansion phase, such as the case of the ply in first.

 

8lb foam as I said in OP. It is an pourable structural PU from Aeromarine.
Boat is only 14 ft. Not a lot of volume. I figure its 12 ft long on deck, 4 ft wide, average depth of the triangle cross section is no more than 6 inches [12 x 4 x 0.5 x 0.5] for about 7 cubic feet of foa
Floor is off so no constraints on upward expansion soon, was going to then shape it flat and then bond the plywood to the foam, perhaps with a layer of cloth in between.
56 lbs of foam would make it pretty stiff, no?

 

What is a case of "Great Stuff" going for these days? lol

Since this boat already has one foot in the dump, I am inclinded to agree with the OP. Just build up enough of the stringers to support a plywood deck screwed down with a few fill/relief holes cut in it and then fill the space underneith with foam. All you need to do is cut off what oozes up and you'll be done. Only problem is that you completely lose the bilge, so the boat will be very wet.

 

The foam is highly adhesive to clean surfaces, no need for bonding, but if you seal the underside of the ply with a coat of epoxy, the adhesion will be very good. I'd hit the bilge with a water blaster to clean it up before pouring the foam, but allow it to dry thoroughly. You could just lay 4' wide ply in there, weight it around the outside, cut a few holes in the ply with a holesaw to allow the excess foam to escape, then cut it back flush when set. Those same holes you could pour the foam in. It is the prospect of the expansion causing a change of shape to your flat ply, that is the worry. Don't use too light ply, maybe 3/8" would be adequate, seeing the foam will be supporting it when set.

 

I should add that it isn't necessary to pour in one go, though that has some advantages, but also potential disadvantages.

 

Does PE foam produce concrete-like "cold joints"?

 

You mean PU foam ? It seems to adhere strongly to various substrates, I can't see why it would not adhere strongly to itself, but not having used poured foam as a structural element, I can't really say to what degree. But I have done subsequent pours into voids for bouyancy purposes, where the first pour was not enough, and have not noticed any detachment.