To make things easier, mass and volume are used interchangeably in Aurora. Changes in mass are not taken into account so an empty freighter has the same speed as one with cargo. While this does make life considerably easier for players, it also means there are some situations where the rules assume mass to be more important and in others volume is more important. The size of a freighter is really based on volume rather than mass which is why there is a different rule for ramming.

Even in the real world, the 'weight' of a ship is not an exact science. Here is a quote from Wiki

**Gross register tonnage (GRT) ** represents the total internal volume of a vessel, where a register ton is equal to a volume of 100 cubic feet (2.83 m³), which volume, if filled with fresh water, would weigh around 2,800 kg or 2.8 tonnes. Calculation of GRT is complex; a ship's hold can, for instance, be assessed for bulk grain (accounting for all the air space in the hold) or for bales (omitting the spaces into which bulk, but not baled cargo would spill). Gross register tonnage was replaced by gross tonnage in 1994 under the Tonnage Measurement convention of 1969, but is still a widely used term in the industry.[1][2]

**Net register tonnage (NRT)** is the volume of cargo the vessel can carry; i.e., the gross register tonnage less the volume of spaces that will not hold cargo (e.g., engine compartment, helm station, crew spaces, etc., again with differences depending on which port or country is doing the calculations). It represents the volume of the ship available for transporting freight or passengers. It was replaced by net tonnage in 1994, under the Tonnage Measurement convention of 1969.

**Gross tonnage (GT) **is a function of the volume of all ship's enclosed spaces (from keel to funnel) measured to the outside of the hull framing. The numerical value for a ship's GT is always smaller than the numerical values for both her gross register tonnage and the GRT value expressed equivalently in cubic meters rather than cubic feet, for example: 0.5919 GT = 1 GRT = 2.8316 m³; 200 GT = 274 GRT = 775,88 m³; 500 GT = 665 GRT = 1,883.07 m³; 3,000 GT = 3,776 GRT = 10,692.44 m³), though by how much depends on the vessel design (volume). There is a sliding scale factor. So GT is a kind of capacity-derived index that is used to rank a ship for purposes of determining manning, safety and other statutory requirements and is expressed simply as GT, which is a unitless entity, even though its derivation is tied to the cubic meter unit of volumetric capacity.

Tonnage measurements are now governed by an IMO Convention (International Convention on Tonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982. In accordance with the Convention, the correct term to use now is GT, which is a function of the moulded volume of all enclosed spaces of the ship.

It is calculated by using the formula : GT = K \cdot V, where V = total volume in m³ and K = a figure from 0.22 up to 0.32, depending on the ship’s size (calculated by : K = 0.2 + 0.02 \cdot\log_{10}V), so that, for a ship of 10,000 m³ total volume, the gross tonnage would be 0.28 x 10,000 = 2,800 GT or 3531.46 GRT. GT is consequently a measure of the overall size of the ship. For a ship of 80,000 m³ total volume the gross tonnage would be 0.2980617 x 80,000 = 23,844.94 GT.

**Net tonnage (NT)** is based on a calculation of the volume of all cargo spaces of the ship. It indicates a vessel’s earning space and is a function of the moulded volume of all cargo spaces of the ship.

A commonly defined measurement system is important; since a ship’s registration fee, harbour dues, safety and manning rules etc, are based on its gross tonnage, GT, or net tonnage, NT.

The Panama Canal/Universal Measurement System (PC/UMS) is based on net tonnage, modified for Panama Canal purposes. PC/UMS is based on a mathematical formula to calculate a vessel's total volume; a PC/UMS net ton is equivalent to 100 cubic feet of capacity.[3]

The Suez Canal Net Tonnage (SCNT) is derived with a number of modifications from the former net register tonnage of the Moorsom System and was established by the International Commission of Constantinople in its Protocol of 18 December 1873. It is still in use, as amended by the Rules of Navigation of the Suez Canal Authority, and is registered in the Suez Canal Tonnage Certificate.

Thames measurement tonnage is another volumetric system, generally used for small vessels such as yachts; it uses a formula based on the vessel's length and beam.

[edit] Weight measurements

While not "tonnage" in the proper sense, the following methods of ship measurement are often incorrectly referred to as such:

Displacement is the actual total weight of the vessel (mostly without pay load). It is often expressed in long tons or in metric tons, and is calculated simply by multiplying the volume of the hull below the waterline (ie. the volume of water it is displacing) by the specific gravity of the water. (Note that the specific gravity will depend on whether the vessel is in fresh or salt water, or is in the tropics, where water is warmer and hence less dense.) For example, in sea water, first determine the volume of the submerged portion of the hull as follows: Multiply its length by its breadth and the draft, all in feet. Then multiply the product thereby obtained by the block coefficient of the hull to get the hull volume in cubic feet. Then multiply this figure by 64 (the weight of one cubic foot of seawater) to get the weight of the ship in pounds; or divide by 35 to calculate the weight in long tons. Using the SI or metric system : displacement (in tonnes) is volume (in m³) multiplied by the specific gravity of sea water (1.025 nominally).

The word "displacement" arises from the basic physical law, discovered by Archimedes, that the weight of a floating object equates exactly to that of the water which would otherwise occupy the "hole in the water" displaced by the ship.

Lightship or Lightweight measures the actual weight of the ship with no fuel, passengers, cargo, water, etc. on board.

Deadweight tonnage (often abbreviated as DWT for deadweight tonnes) is the displacement at any loaded condition minus the lightship weight. It includes the crew, passengers, cargo, fuel, water, and stores. Like Displacement, it is often expressed in long tons or in metric tons.

Steve