Metric tons or Imperial tons
 

Sub displacement is always given in tons, but which "tons" is it...

... the imperial ton = 2000lbs
... or the metric ton = 1000 kg

There's sufficient difference between the two. Anyone know which one is generally used?

This is all I could find:

The short ton is a unit of mass equal to 2,000 lb

The long ton is 2,240 lb
It has some limited use in the US, most commonly in measuring the
displacementof ships, and was the unit prescribed for warships
(e.g., battleships limited to 35,000 tons) in the international
agreements between the World Wars

Here's a funny picture:
http://en.wikipedia.org/wiki/Image:Metric_system.png

edit:
The official US navy website shows both long tons (or short tons) and metric tons for ships.

Sorry, but it's neither.

Ship displacements are always in "long" tons = 2240lbs. This is why early British guns are rated in "hundredweights" (i.e. 12pdr/45 8cwt). One hundredweight is 112pds, so 20 hundredweight (or cwt) is one long ton.

1 metric ton = 1000 kg = 2240 lbs

No. 1000kg = 2204.6 lbs

Note, that's 2204, not 2240.

2240 lbs = 1016kg.