Question: How is the weight of a ship determined and what is gross tonnage?
(courtesy of daveandzeen)

Answer: It has been said that the measurement of tonnage is as much an art as a science. As soon as a new 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 used to describe a size of wooden cask in which wine was shipped from Iberia (Spain & Portugal) to England, and in 1347 a tax of 3 shillings a 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 A ship's size became known by the number of casks it could carry, and gradually the word tonnage superceded the word burthen which had used previously.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 it by 100 it was close to the number of casks carried. 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 of permanently enclosed spaces in cubic feet of the ship minus certain spaces above the main or tonnage deck, which are called "exemptions".

Net Registered Tonnage - this one the Ship Owners like to keep as small as possible as it is commonly used for for things like harbour 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 the 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 2000lbs, 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 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 can 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.

I have just got back from the Sea Princess so I thought that these figures for her will help illustrate what I said before.

Gross Tonnage 77,499 tons
Net Tonnage 44,193 "

Panama Gross 88,788 "
Panama Net 63,441 "

Suez Gross 83,502 "
Suez Net 72,538 "

From the above it can be seen that Panama and Suez allow almost no Exemptions to the Gross Tonnage and a lot less Deductions to get the Net Tonnage. Dues are usually collected at least partially based on Net Tonnage.

On a maximum draught of 26' 7.5" she has a Displacement Tonnage of 39,997 tons (remember this is in actual weight... Metric tons which = 2016 lbs.)

Her Deadweight Tonnage is 8,293 Metric tons. ( this is the actual weight she can carry - the difference between Empty as built and a Full load of everything...including bodies.
(courtesy of Peter/Gohaze)

Answer: Tonnage is often completely misunderstood and is a constant source of confusion. It all started in ancient Greece when taxes were introduced on shipowners bringing their ships into port, measured against the earning capacity of the ship. They decided the size of the hold by the number of TUNS of wine could fit into the hold and this was called the TUNNAGE. Obviously a measure of volume, this eventually became the mixed in with the weight measurements which confusingly were tons.

If you'll bear with me for a second then, this is the situation as I understand it:

Gross Tonnage - varies slightly to how it is measured according to the rules of the flag state but is the capacity in cubic feet of the spaces in the hull and enclosed spaces above, available for cargo, stores, passengers and crew (i.e. her earning capacity) divided by 100. Therefore 100 cubic feet of capacity is 1 gross ton. Metricization now means it is measured in cubic metres but I'm not sure of the conversion.

Net tonnage - is found by subtracting the working spaces of navigation and machinery, crew and fuel from the Gross tonnage.

Displacement tonnage - is the weight of water displaced by the ship and is effectively the entire weight of the ship and all that is in her.

Deadweight - Is the weight in tonnes of cargo, fuel, stores, fresh water, crew and passengers carried by the ship when she is down to her marks.