Google strikes again ...
Background Discussion - Peter Stocker; firstname.lastname@example.org
It has been said that the measurement of tonnage is as much an art as it is a science. As soon as a tonnage regulation is brought out myriad Naval Architects, Engineers, Designers, et al immediately see how they can get around it to their benefit.
The word 'tun' was originally a size of cask in which wine was shipped from Iberia (Spain & Portugal) to England, and 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 A ship's size became known by the number of casks it could carry, and gradually the word tonnage superceded the word burthen which had been used previously. It was found that with the design of ships in those days that if one 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. Hence we have the "Measurement ton" of 100 cubic feet per ton still in use today.
Gross Tonnage - the one most commonly quoted in the cruise industry because it makes ships seem bigger - is the internal volume in cubic feet of the ship minus certain spaces above the main or tonnage, deck which are called "exemptions" .
Net Registered Tonnage - this one Ship Owners like to keep as small as possible as it is commonly used for things like harbor dues and other taxes - is the Gross Tonnage less certain non earning spaces and allowances which are called "deductions"
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 - this is when the ship is built with nothing in it.
Loaded Displacement Tonnage - this is when a ship is fully loaded 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 to the international ones. There used to be a lot of variations between countries and they thought they were being conned, so they came up with their own for everyone.
Other tons in common use are the "Short Ton" of 2000 lbs, particularly in the US, but not much at sea. The "Metric Ton" or tonne which equals 2204 lbs. and this is becoming more used.
So, we have different "tonnages" for different uses. Passenger ship owners like a large Gross to make their ships appear bigger than the competition, and a small Net to save money. Tanker owners use Deadweight tonnage to show how many actual tons of oil the ship will carry, and the World's Navies use Displacement as their ships weigh heavy for their volume, and to impress or lie to their friends and enemies.
Peter Stocker; email@example.com