Regarding John Rogers letter concerning the QM2 in the November 2011 issue of Sea Breezes. The ship’s gross tonnage measurement is 148,528 tons. As Mr Rogers rightly states, this is a measurement of space rather than weight, so the metric ‘tonnes’ for weight does not apply.
A ship’s actual weight is its displacement tonnage, measured by calculating the volume (and hence the weight) of water that the ship displaces. There are two figures, the ‘light displacement’, which is the ship itself when completely empty, and the ‘load displacement’, which is the ship when fully loaded with stores, fuel, passengers and their baggage (cargo in the case of a cargo ship). The difference between the two (ie the total carrying capacity) is the deadweight tonnage. The displacement tonnage is not normally published in the shipping registers, etc so I don’t know the exact figure for the QM2 but I read somewhere at the time of its completion that the light displacement was somewhere around 70,000 tonnes.
I do have to hand, as an illustration, the figures for the P&O cruise ship Oceana, which measures 77,499 gross tons. Her light displacement tonnage is 31,703 tonnes and load displacement 39,007, (giving a deadweight of 7,304 tonnes).
Seaview, Isle of Wight