Back to the original question (how light could a 20’ cat be built) and disregarding all the other considerations/arguments IE water, wave, wind, handling characteristics on and off the water, etc, given the available manufacturing equipment, (high pressure die moulding) the availability of the high grades of aramids and aerospace carbons, the latest high modulus high temperature resins, and an unlimited budget (wouldn’t it be nice if it wasn’t just a dream), there is no reason why a “bullet proof” 20’ cat couldn’t be built at a weight of less than 50kgs all up.