I'm working on a small model yacht (~40cm) and I was curious as to why none of the designs I found incorporate ballast inside the vessel and instead use a drop keel. Why can't a model have the ballast inside like tall ships use to, and if so, how would I calculate that? Also, I am in an area where fiberglassing materials are not readily available and I don't even know if I can find balsa for the planking. Any suggestions for waterproofing it? This is just for fun, not for racing or anything like that. As long as it floats in the local pond I will be delighted.

Thank you!