All depends who did the glass work. You can give 100 guys the parts to build in alum and it will always be the same weight. Fiberglass built by historic (chop gun) you'll have a 100 different boat weights in Glass. Someone doing it right by the infusion process (that not many, if any, boat builders use) it will be close each time. With smaller stuff even by that process, to get a minimum workable thickness, it will still weight at least 60 to 70 % more than alum. Once you get into bigger stuff it can be made comparable in weight to alum.