I made a 4x16' jonboat using the same process as Carolina Skiff uses
which they call a 'box-beam'.You can go to the U.S.patent office site
and pull up the Car.Skf. patent (#4,495,884) to see how it's
done.Basically it's 2 fiberglass skins connected by perpindicular(?)
fiberglass ribs which are formed against foam 'beams'.The foam itself
doesn't contribute much, if any, to the strength of the sandwich. In
my jonboat I used 2x3" beams and ended up with a bottom 2" thick with
ribs every 3".I used polyester resin so I couldn't use styrofoam type
stuff.I went to a commercial insulation peddler and he sold me a
urethane type foam that was used on commercial roofs either under tin
or as a base for 'hot-mopping' flat roofs.It was about 1/2 the price
of the equilevent amount of urethane foam from Home Despot.It could be
had in any thickness up to 6" or more.It was 'structural' in the sense
that it could be walked upon a reasonable amount in the course of
roofing without crushing.I sawed it up on a hot day wearing just
cutoffs and discovered that evening that it also had fiberglass hairs
laced all through it for strength.The mold for the boat I made from
'tile board',a 1/8" masonite type board with a white smooth surface
used as a cheap shower or bath liner,also from Home Despot for about
$10 a 4x8 sheet.