stringer pieces in place.

[nestorpr]

Actually the foam will stiffen the whole assembly up and will act like a solid stringer. This is what DIAB cored panels are like and recently they came out on Shipshape TV explaining how a foam filled hollow fiberglass core could be stiffened by filling it with foam, the thicker, the stiffer. This will give the stringer a lot of strength, like using wood but keeping the weight and maintenance down. BTW, if you use CLOSED CELL foam it will not absorb water, more expensive but worth the money.