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What is Tonnage?

by Ken Pick

In Traveller, a "ton" of starship size is defined as 14 cubic meters, the volume of a ton (1000kg) of liquid hydrogen. This is bit abstract for most Traveller players, who have no quick way to compare this "Traveller Ton" to everyday experience. This essay will remedy that.

[Editor's note: The "Traveller Ton" is routinely called the "displacement ton" in some Traveller materials and in conversation between Traveller fans. This is not the same as the displacement ton discussed below.]

Visualizing a Traveller Ton

A Traveller Ton is 14 cubic meters, which is about 500 cubic feet; the standard 10x10x10 foot (3x3x3 meter) cube used in most FRP game mapping is about 2 Tons Traveller.

Capacity

Examples

The standard Type A Free Trader has a cargo capacity of 82 Tons Traveller. This translates into 20 full-size American ground cars, or 20-24 bobtail truck loads, or10 "big-rig" truck loads, or 8-10 typical American railroad boxcars.

The standard Type R Subsidized Merchant (capacity 200 Tons Trav) holds 50 full-size cars/bobtail truck loads, or 20 "big-rig" loads, or 20-25 boxcar loads.

Starship-to-surface ship comparisons

How big is a Traveller starship compared to a "wet" surface ship? This is complicated by the use of several different tonnage measurements for surface-ship measurements.

1. Civilian Ship Tonnage

1.1 Gross Tonnage:

The actual volume of the ship's interior, in "tons" of 100 cubic feet (about 1/5 of a Traveller Ton). This is the closest methodology to Traveller tonnage, and converts easily at a rate of 5 Tons Gross = 1 Ton Traveller. Examples are:

Going the other way:

The Commercial Efficiency Ratio (CER) is a relative cost/benefit ratio indicating how well a ship will perform in commercial service. The higher the CER, the more profitable the ship will be to operate.

To calculate the CER for a ship:

Total the Net Tonnage ("revenue space") of the ship in tons, counting each passenger stateroom as 4 tons, each passenger low berth as 1/2 ton, and cargo tonnage as straight tonnage. (Do not count crew staterooms, sickbay/ emergency low berths, or vehicle bays unless the vehicles are part of the cargo; Net Tonnage is only the part of the ship that can be used to carry passengers and cargo.) Multiply this net tonnage by the Jump number, then divide by the ship's cost in MCr.

CER = NetTonnage * JumpNumber / MCr

  • A CER of 3 is considered minimum for practical commercial service; below this, the ship cannot pay its own way.
  • If the ship is primarily a passenger ship (passengers pay more per ton), the CER can get down to around 2.5.
  • A CER of 5 or more can turn a profit even at the common carrier rate of Cr 1000/ton.
  • The CER assumes that the ship will operate at or near its maximum jump number. Generally, it is not practical to operate a ship much below its designed jump speed.

1.2 Net Tonnage

The "revenue space" of the ship, i.e. the volume of the cargo holds and passenger accomodations only, measured in "tons" of 100 cubic feet. This is a bit harder to calculate; in Traveller terms, net tonnage counts only the cargo hold, passenger staterooms, and passenger low berths. Net tonnage is used in Traveller primarily to calculate the Commercial Efficiency Ratio (CER), a measure of a ship's relative profitability in service. Examples (all from the Traveller end) are:

1.3 Deadweight Tonnage

This is the actual weight of the ship's full-load cargo capacity, the maximum weight of cargo the ship can carry. As this is a weight measurement and Traveller tonnage is a volume measurement, there is no way to convert between the two.

2. Military Ship Tonnage

Warships are always measured by "displacement tonnage", the actual weight of the ship, which translates directly into the hull volume in cubic meters below the waterline. This varies according to how fully the ship is loaded; fortunately, various arms-control treaties of the battleship era (TL5-6) defined specific types of loading for tonnage determination. The most useful of these are:

Full Load Displacement
Fully loaded -- crew, supplies, ammunition, fuel all topped off. This is the closest approximation to Traveller measurement. The volume of the ship below the waterline translates into Traveller tonnage on a 14-to-1 basis. (However, most of the ship is above the waterline.)
Standard Displacement
As Full Load, but no fuel. This is the default size measurement of a warship; if size is given in "tons" with no other qualifier, it is standard displacement tonnage.
Submerged Displacement
Used only for submarines, this is the weight of the sub at neutral buoyancy, which translates directly into hull volume in cubic meters. This translates directly into Traveller tons at a ratio of 14 tons submerged = 1 Ton Traveller.

Since displacement tonnage is a measure of weight and Traveller tonnage of volume, any conversion between the two will be approximate.

To convert Surface Ships from wet-navy to Traveller:

2.1 Always start with full-load displacement tonnage.

2.2 Divide the full-load tonnage by 14 to get the Traveller tonnage for the amount of the ship below the waterline.

2.3 Estimate the proportion of the ship below the waterline (about 1/3 for most ships, down to 1/2 for heavily armored battleships) and multiply by the reciprocal of this fraction.

2.4 A rule-of-thumb for 2.3 is:

2.4.1 Unarmored ship (Destroyers, TL4-5 light cruisers, most TL7-9 construction) -- 5 tons full-load = 1 Ton Traveller (coincidentally, this is the same ratio as a civilian ship's Gross Tonnage).

2.4.2 Moderately-armored ship (TL4-5 armored cruisers, TL5-6 battlecruisers, most TL6 cruisers) -- 6 tons full-load = 1 Ton Traveller.

2.4.3 Heavily-armored ship (TL4-6 battleships only) -- 7 tons full-load = 1 Ton Traveller.

Examples

(Unarmored)

Destroyer USS Fletcher (TL6): 2000 tons std, 3000 full-load = approx. 600 Tons Traveller

Destroyer USS Cole (TL9): 8400 tons full-load = approx 1700 Tons Traveller

Carrier USS Enterprise (TL7): 75000 tons std, 90000 full-load = approx. 18000 Tons Traveller

Light Carrier HMS Invincible (TL8): 16000 tons std, 20000 full-load = approx. 4000 Tons Traveller

Carrier USS Nimitz (TL8-9): 80000 tons std, 92000 full-load = approx. 18000 Tons Traveller

(Moderately-armored)

Battlecruiser HMS Hood (TL5): 42000 tons std, 45000 full-load = approx. 7500 Tons Traveller

Typical "Treaty Cruiser" (TL5-6): 10000 tons std, 13000 full-load = approx. 2000 Tons Traveller

Armored Cruiser KMS Graf Spee (TL6): 12000 tons std, 16000full-load = approx. 2600 Tons Traveller

Battlecruiser KMS Scharnhorst (TL6): 32000 tons std, 38000 full-load = approx. 6300 Tons Traveller

Carrier HMS Ark Royal (TL6): 22000 tons std, 28000 full-load = approx. 4500 Tons Traveller

Carrier USS Enterprise (TL6): 20000 tons std, 26000 full-load = approx. 4300 Tons Traveller

(Heavily-armored)

Battleship USS Oregon (TL4): 10000 tons std, 12000 full-load = approx. 1700 Tons Traveller

Battleship HMS Majestic (TL4): 15000 tons std, 16000 full-load = approx. 2300 Tons Traveller

Battleship HMS Dreadnaught (TL5): 18000 tons std, 22000 full-load = approx. 3000 Tons Traveller

Battleship USS Arizona (TL5): 25000 tons std, 33000 full-load = approx. 5000 Tons Traveller

Battleship KMS Bismarck (TL6): 42000 tons std, 50000 full-load = approx. 7000 Tons Traveller

Battleship USS New Jersey (TL6): 45000 tons std, 58000 full-load = approx. 8000 Tons Traveller

Battleship HIJMS Yamato (TL6): 60000 tons std, 72000 full-load = approx. 10000 Tons Traveller

To convert Submarines to Traveller tonnage:

2.5 Start with submerged tonnage.

2.6 Divide the submerged tonnage by 14 to get the Traveller tonnage.

Examples

Type VII U-boat (TL6 -- Das Boot): 900 tons submerged = approx. 65 Tons Traveller

USS Nautilus (TL7): 4000 tons submerged = approx. 300 Tons Traveller

USS Los Angeles (TL8): 6900 tons submerged = approx. 500 Tons Traveller

Kursk (TL8): 18000 tons submerged = approx. 1300 Tons Traveller

"Red October" (TL8): 29000 tons submerged = approx. 2000 Tons Traveller

Notes

The use of 100 cubic feet as a "ton" for civilian shipping dates back to the 19th Century (TL4), when the British Empire was Earth's maritime superpower. (Longitude on Earth is still measured in degrees East or West from the Royal Observatory in Greenwich, a suburb of the British capital.) The "foot" of 30.5cm was a standard measurement of the British Empire of the time; the familiar metric system was just becoming established after being originated by the Brits' bitter rivals, the French.

Standard Displacement was defined by the Washington Treaty of 1922, a TL5 arms limitation agreement drawn up to prevent a bankrupting naval arms race after the end of the First World War. This treaty defined maximum sizes and weapons for various types of warship, limited each navy to a maximum total tonnage for "capital ships" (the largest combatants), and provided for replacements to be built at the end of a defined service life. The treaty spawned other treaties further limiting navies, but all such treaties were allowed to expire or were withdrawn during the buildup for the Second World War.

This page last updated on 06 March 2023 by Jeff Zeitlin.  Text ©2000-2023 Ken Pick. Page design ©1998-2023 Jeff Zeitlin
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