closed cell foam

I do not understand. You say you went back with marine grade plywood. So, how are you going to pour in the foam if the floor is in place? Am I missing something here? Also, Marine Grade plywood is just a fancy way of saying structural grade plywood in that there are no voids in the laminated layers...and the cost is very high. Marine Grade plywood rots just as fast as regular plywood. So, what I am saying is that ALL sides of the plywood MUST be sealed against moisture.

If you have already installed the floor, then I suggest you forget about the foam. If you try to pour the foam through holes, it will not fill all the voids/spaces and you seriously risk bulging the hull from the created pressure.

The proper method is to pour in the foam BEFORE the floor is installed. The foam is allowed to expand above the floor level and then is cut/sanded to proper elevation.

Thanks for the inputs, but I started pouring foam today. My 82 nautique only had wood below the seats back to the front of the engine box. I traced the old wood and went back exactly like this. My floor was only bad back to the engine box so I took out foam only back to the engine. I tilted the boat on the trailer so the foam would pool at the old foam and expand toward the opening at the front of the plywood between the stringers. I am only pouring a small amount at a time. This worked great on one side, but the other side did not expand as much, I think because of the lower temperatures. There is a gap between the foam and the bottom of the ply. I am going to figure a way to fill this in. I wish I had a little more time to work with the stuff before it starts expanding. In the open areas I guess I will pour and sand away till it is level. By the way I bought my foam and fiberglass at uscomposites found through this website. I have been pleased.

Not bad..here's the way I did it...

I used 1/2" regular plywood...the boat is 57" wide at it widest point (67 Mustang)...that meant plywood had to be laid side to side (>48" wide)...I cut all sections from front to back with about a 1/4" gap at the gunwales...once I was satisfied with the fit, I removed the sections (4 sections) and coated them generously with polyester resin and let it cure...once I was ready to foam, I screwed the back section in and tilted the boat on the trailer to as much as I could get (no motor in boat)...with rear deck panel in, I poured liquid foam (US Composites 2lb density) down each chamber (4) with enough to fill the chambers when expanded...it doesn't take long...once that's in, screw the next deck section in and repeat the process until you're pouring in the bow of the boat to fill the front chambers...the gap at the gunwales allows for expansion and ensures it's full of foam...once it's all cured, go back and trim the excess from the gunwales...I went back and glassed (w/fabric) the deck seams and the deck to the gunwales for watertightness and structural integrity...it's rock solid. I bought 10 gallons (5 gal foam, 5 gal catalyst) and had some left over to set the gas tank.