old glass hull v. new glass hull
I see statements like "they didn''t always know how thick to make them so they were conservative." From talking to early fiberglass boat designers and from reading early fiberglass design texts, I think that early fiberglass boat designers understood the limitations of the fiberglass that they were working with quite well. While the udertood the material they did not understand how to over come its short comings. Dispite their heavy layups, I would not say that early fiberglass boats were all that conservative from an engineering standpoint.
To explain, even early fiberglass was quite strong but fiberglass is a very flexible materal. Because fiberglass is so much more dense than wood and so much less stiff than wood on a pound for pound basis, they could not achieve the stiffness of a wooden boat at anywhere near the weight of a wooden boat. Early designers also understood that fiberglass is a very fatigue prone material and so flexure would ultimately rob the laminate of much of its initial strength. In order to come close to the stiffness of a wooden boat, they built these early boats heavier than they needed to be strickly for bending strength. The irony is that these heavier hulls actually increased loadings and thereby stress on the hulls and were not all that effective in reducing flexure.
It is only in the 1970''s that designers began to realize that they could achieve stiffer lighter boats by using more sophisticated internal framing and through the more extensive use of cored components.