In shipping, ton is a unit of volume. It is derived from a cask
that held 252 wine gallons which would hold approximately 2100
pounds of water. A register ton, which measures ship volume, is
100 cubic feet; while a freight ton is 40 cubic feet. As a unit
of mass, a long ton is 2240 pounds, a short ton 2000 pounds, and
a metric ton 1000 kg. The only time this is used with shipping
is to describe the ship's displacement or weight, something done
with naval vessels and pleasure craft.