Tonnage: Converting Spaceship “Tons” Between Traveller and GURPS

A great benefit of a campaign based on Traveller is that there is such a large amount of Traveller material available to draw from – both fan produced and otherwise. If you want to run a game in a setting with a lot of spaceships, that’s a huge help! However, it is also confusing as hell because Traveller measures everything in displacement, whereas GURPS measures everything in mass, and both involve using the word “tons.” E.g., in both systems you’ll find references to ships as “a 100 ton ship.”

So if a classic Traveller source starts talking about a 100-ton ship like the Type S Scout/Courier, that means “Traveller Tons” – a displacement measure defined as the volume of one metric ton (1000kg) of liquid hydrogen.  So that ship is as big as 100 x the volume of 1000kg of liquid hydrogen. Intuitive, right?  In other words saying the Type S is 100T means the hull’s volume is 1400 cubic meters.

If a GURPS 4e source starts talking about a 100-ton ship you might imagine: Gosh, that’s the same size as the Traveller 100-ton Type S Scout/Courier, right? But GURPS means “short tons” – a corrupted U.S. measure from the Imperial system (that’s the British Empire, not Traveller). A short ton is 2,000 pounds. So the 100-ton GURPS Banshee-class Drop Ship (GURPS SS:4 p. 17) means it’s mass is 100 tons loaded. If it were put on a scale on Earth it would weigh 200,000 lbs. This tells us nothing about volume. (Although obviously mass and volume are related, so maybe “nothing” is too strong.)

And: None of this is to be confused with other kinds of tons, like the metric ton (1,000 kg) or the long ton (2,240 lbs). To make it even worse, when the Traveller rules refer to weight in ship designs they also use the word “ton” and they mean a metric ton (1,000 kg). Note that “tonne” is another word for “metric ton.” GURPS 3e Traveller uses tonnes to describe mass. What a mess!

It is very useful to convert a GURPS ship hull into classic Traveller Tons so that you can roughly compare the ship sizes with the many designs out there on the Web for the Traveller universe.

Converting Traveller Tons to GURPS tons

Here’s how to convert: Each SM of a GURPS spaceship hull has an associated weight in short tons from the GURPS Spaceships hull size table on p.9. Multiply that weight in short tons by 100 to get an estimated volume in cubic feet, then multiply by 0.0283168 to get cubic meters, then divide by 14 to get “Traveller Tons” or the volume of one metric ton (1000kg) of liquid hydrogen.  Or to combine these calulations together, just

multiply the G:SS hull weight in short tons by  0.20226285714 and you’ll get the designation in “Traveller Tons” (T).

That means the GURPS “100-ton” Banshee would be a 20T ship in Traveller.

Converting GURPS tons to Traveller Tons

Or, to do the reverse, just

divide the “Traveller Tons” (T) designation for a Traveller ship by 0.20226285714 and you’ll get the G:SS hull weight in short tons

…as seen on the hull size table on GURPS SS p.9.

That means the Traveller “100T” Type S Scout/Courier would be about a 500-ton ship in GURPS (between SM+7 and SM+8 on the table). To confirm that what we did makes sense, from the hull table in GURPS this ship would be about 36 yards long, pretty darn close to the 33 yards length given in the Traveller specs for that ship. (I converted the Traveller specs from meters.)

Where This Doesn’t Apply

Note that I’m trying to convert between classic Traveller and GURPS in this post. In material produced for “GURPS Traveller” I don’t need to convert because the material has already been converted. Even in GURPS 3e publications Traveller spaceships are already specified so that tons means mass, not displacement, as is the GURPS way. They use the word “tonnage” in GURPS 3e material to help distinguish this from “Traveller Tons.” But it is still pretty confusing.

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