Lots of the old boats (like my 1965 Bertram 20) used very heavy layups for a couple of reasons; they really did not know how strong the layups were and, in those days, they used a lot of mat and woven roving - both of which need a whole lot of resin. Now we have stitched fabrics like biaxial, triaxial, etc which are substantially stronger than woven roving or cloth because the fibers lay straight and do not have kinks in them plus these newer fabrics take a lot less resin for a good layup. The net result is that it probably takes 1/3 to 1/2 of the 'old style' layup thickness and weight to get the same or greater strength with the new materials. That said, it is easy to have a strong but much thinner layup with the new stuff that still gets damaged by the idiot slinging an anchor on a thin part of the deck (strong but no impact tolerant layer).
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