>>So while that figure might be very close to the actual weight, we don't have a giant bathroom scale.<<

While life itself does have variables, there are certain values that are known, such as the volumn of a hull, the weight of water of a given level of salinity, the weight of fresh water, but also the weight of the materials that go into a hull.

>>So sure you can figure out those things from mathematics. Why? because it makes sense, it figures out, so while that figure might work out on paper, how does it equate to the real world?<<

Quite well, thank you very much. Well enough that the weight of the Titanic's hull was accurately calculated for launching, as was the weight of the materials that were used for same. Launching the hull would have disasterous results were it otherwise. Mathamatics is not just untested hypothosis, it's a way of working things out in the real world with extreme precision from calculating weights, to navigating a ship well enough that she can make port on time without using landmarks of any kind for reference. It's also good for making extremely accurate predictions of how much cargo a ship can carry, how far it will settle into the water with a given set of weights put on board, etc. You can thank Archimedes for that.

If you can find a better system that works, have at it. Just be mindful of the fact that mathamatics deals with the real world as does every other scientific dicipline in a way that's both measurable and testable. Precious little else can make the same claim.