A rather common "feature" of Traveller design systems?
A rather common flaw with Naval Parlance.
A wet navy ship is described by many different tonnages...
Displacement Tonnage - Fueled but not loaded mass, as measured by displacement of water at
STP
Deadweight tonnage - total mass fueled but not laden. In long tons of 2240 lbs.
Gross Registry tonnage - internal volume, at 1 GRT = 100 cu ft.
Net Registry Tonnage - internal volume for passengers and cargo, in 100 cu ft units.
also, Laden versions of all the above...
Cargo Tonnage - internal cargo space - Lower of (1) difference between max allowed draft's displacement and standard displacement, (2) net registry tonnage, (3) max allowed cargo mass by structural limits...
Note that some ships cannot handle more than 1 ton per 150 cu ft of cargo space, because of weak decks, but the ship can remain afloat with far more laden mass. This is actually quite common for fishing vessels... the hull is designed to carry a few dozen tons of ice, but cannot hold nearly as much cargo in the bays, as the bays are designed for the relatively low density water with live fish or crab... (SG ≤1.1)...
A Cargo's tonnage is the higher of (Volume/ 40 cu ft) or mass in long tons, most of the time.
Note that GRT often exceeds DT by 2:1 or more...
NRT is often less than allowed maximum Cargo Tonnage, as the ships are often designed for more massive (higher SG) cargos... and for well-deck holds, like on panamax container ships, only the well to weather deck line counts, but the cargo allowed often stacks twice as tall as the well...
