by tomodda » Mon Mar 11, 2019 8:46 am
I believe you are looking for "core". A hull with plywood underneath the fiberglass has a "plywood core." That being said, a lot of boats have a balsa-wood core.
In the meantime, my '58 O'day daysailer has stringers - a long, narrow fore-and-aft laid "bump" on the inside of the hull. One on each side, 1.5 inches high and about 7' long. They are there to strengthen the hull, and sit more or less where the boat sits on the trailer bunks. I'm sure that there is a core in it as well.