[sailingdog]

Some of the newer GRP boats are built ridiculously light, some are built reasonably heavy, with proper respect to design constraints and laminate thicknesses.

Older boats often didn't use stringers, as the heavier layup was stiff enough to not require their use. The lighter laminates used by some modern designs often requires the use of stringers or framing to make the hull stiff enough to be safe. However, the laminate itself is often not thick enough to really be safe, even though the engineering analysis of it says that it has enough strength.

Dave Gerr, in his book The Nature of Boats,
has a great example, where a boat manufacturer talks about how stiff and strong a laminate being used is and demonstrates by hitting it with a hammer, which bounces off. A naval architect takes the hammer and reverses it and easily drives the claws through the laminate.

There are good boats, both old and new, and there are bad boats of all ages. A badly designed boat, even using the newest, high-tech materials and techniques, is still not going to be seaworthy or durable. Some of the older designs were made without the full understanding of the material limitations—and may have more resin and chopped strand mat than would make a durable, strong boat.