It was easier to find than I expected...y'all are gonna love it:
Tonnage has nothing to do with the weight of your boat. It is a measure of how much wine a vessel can carry.
The word "tun" was originally a size of a cask used to ship wine from Spain & Portugal to England. In 1347 a tax of 3 shillings per tun was imposed and this was called "tonnage." A ship's size became known by the number of casks it could carry, and the word tonnage started being used to describe a ship's size.
It was found that if you took the length x the breadth x the depth of the hold under the deck and divided by 100 it was close to the number of casks. That is where we get the "Measurement ton" of 100 cubic feet per ton.
There are several kinds of tonnage: The first two are used by the tax collector. The next two are used by designers. The fifth and sixth are used by freight salesmen and canal operators and the last one is used by the USCG for documenting boats.
Gross Tonnage - is the internal volume in cubic feet of the vessel minus certain spaces above the main or "tonnage" deck, like stacks and ventilators, which are called "exemptions" .
Net Registered Tonnage - is obtained by deducting from the gross tonnage the volume of space that can't be used for paying cargo or passengers, that is to say the space occupied by the engines, the crew's quarter, the stores, etc.
Displacement Tonnage - is the actual weight of the water "displaced" by the ship and is usually quoted in long tons of 2240 lbs.
Light Displacement Tonnage - is the weight with nothing in it.
Loaded Displacement Tonnage - is the fully loaded weight to the maximum and is on her summer draft in salt water.
Deadweight Tonnage - is the difference between Light and Loaded Displacement Tonnage....the actual carrying capacity of the vessel.
Panama & Suez Canal Tonnages - these are different from the internationally accepted definitions. There used to be a lot of variations between countries and the canal owners thought they were being conned, so they came up with their own definitions.
Simplified Measurement System - The USCG decided that all this was way too much for bureaucrats to deal with for yachts so they came up with their own formula:
Take the horizontal distance between the outboard ends of the boat not including rudders and bow sprits. Multiply that by the maximum beam outside to outside.
Multiply that by the distance from the sheer line not including bulwarks or cap rails to the outside bottom of the hull not including the keel.
Add the volume of the deck house/cabin top. Multiply by .5 for sailboats and .67 for power boats.
Divide by 100.
This will give you the "Gross Tonnage". Net tonnage is 90% of gross for sailboats and 80% for power boats.
It should be obvious to anyone who's managed to get this far that your boat's "tonnage" no longer has anything to do with anything real; it only exists in the mind of some government bureaucrat.
And a bit of fun trivia:
Rummage was the manner in which the wine casks were stored in the hold of the ship and came to refer to the whole ship's cargo. after a voyage any unclaimed and damaged cargo was stacked on the dock beside the boat and offered for sale - a rummage sale. another word of French maritime origin.
