if you were to fill the hulls with expanding poly urethane foam you would be adding an ENORMOUS amount of weight to the hulls, and the problem with poly urethane foam is that it will absorb and hold water (it will still retain its positive bouancy though) which will add even much more weight. If your hulls ever were "foam core", (and when they were made, about the only cat that was foam cored were Hobies) you can tell if the hulls are delaminated by walking along the hull and tapping it with your knuckles (or the handle of a screw driver), if the glass is sound it shound almost ring if there is delamination or the glass is very "tired" it will have a very dead sound sort of a thugg sound instead of a boing. (its a little hard to write down sounds)