How to figure out max cargo weight on a sailboat?

Question

I'm interested in 41 foot gaff cutter that's gutted out on the inside. It looks like the waterline is low on the hull. if while building the inside could I keep it simple and haul something like a motorcycle? what about the weight of the lumber used to make a berth, head or kitchen?

Answer 1

25 per cent of the displacement I think...It really depends on the conditions