Why a giant cruise ship weighs far less than its tonnage suggests
Titanic had a gross register tonnage of 46,328 and weighed a little over 52,000 tons, so the two figures roughly matched. Oasis of the Seas, completed in 2009, measures over 225,000 gross tonnage yet displaces only about 100,000 tons. Tonnage counts enclosed volume, not weight, and modern cruise ships are mostly light, boxy space.
The old gross register tonnage treated each 100 cubic feet of certain enclosed areas as one ton. In 1982 it gave way to gross tonnage, a formula based on the moulded volume of every enclosed space, whether it holds engines or cabins. That number decides crew rules, safety requirements, registration fees and port dues, and dividing it by the number of passengers gives a rough sense of how roomy a ship feels.
Cunard's Queen Mary and Queen Elizabeth of the mid-1930s both came in around 81,000 to 83,000 tons by either measure. Their descendant Queen Mary 2, in service from 2004, rates 148,528 gross tonnage but is estimated to displace only about 76,000 tons. The gap keeps widening because cruise ships have grown upward with slab sides, stacking light superstructure on a heavy hull full of machinery. Top tonnage more than doubled after the late 1990s.
The big cruise ship is a fairly recent idea. For decades cruise vessels were smaller than the great liners, until Knut Kloster of Norwegian Caribbean Lines bought the liner SS France in the 1980s and turned her into the SS Norway. Her success prompted ever larger orders, finally ending the Queen Elizabeth's 56-year run as the biggest passenger ship ever built. Liners were designed for speed on long crossings, while cruise ships put pools and theatres first, and the main market moved from the North Atlantic to the Caribbean. Queen Mary 2 still holds the record for the largest ocean liner.
Safety rules have followed. The International Ice Patrol was set up in 1914 after Titanic sank. Under International Maritime Organization rules, passengers must be mustered within 24 hours of boarding and a ship must be able to evacuate everyone within 30 minutes of the abandon-ship signal. Since 1 October 2010, ships in international waters have had to be built or refitted without combustible materials.
Source: Passenger ship
