Today's rules may be found in 46, Code of Federal Regulations, Subchapter G. This is about 110 pages of rules for measuring, lists of exemptions and exceptions, etc. In addition the admeasurer's office has cabinets full of files listing decisions affecting the above, most of which are aimed at reducing the tonnage.
Tonnages include: deadweight tonnage, gross registered tonnage, net registered tonnage, under deck tonnage, between deck tonnage, light ship tonnage and others. I quote from the Society of Naval Architect and Marine Engineers book, PRINCIPLES OF NAVAL ARCHITECTURE. "They are archaic rules written in antiquated language, requiring unnecessarily complicated calculations which result in tonnages not truly representative of either the size or the earning capacity of the ship. There is extreme difficulty of interpretation leading to inconsistent results within and between countries. The lack of logic in the rules leads at times to tortured interpretations, made more to fit the words of the regulations than the intent of the rules or the circumstances of the particular case."
Now, Hank, tonnage is a meaningless description of any ship or boat. Those of us who had to deal with it cheated to the extent we could. The examiners who checked our work did so with tongue in cheek, knowing we were cheating. Everybody involved, owner, architect and examiner knew all of this, had another drink, thanked eah other profusely after the admeasurement, and then had a farewell drink at the owner's expense.