Passenger Ship - Measures of Size

Measures of Size

By convention and long usage, the size of civilian passenger ships is measured by gross tonnage, which is a dimensionless figure calculated from the total enclosed volume of the vessel. Gross tonnage is not a measure of weight, although the two concepts are often confused. Weight is measured by displacement, which is the conventional means of measuring naval vessels. Often a passenger ship is stated to "weigh" or "displace" a certain "tonnage", but the figure given nearly always refers to gross tonnage, which in this context has nothing to do with weight.

While a high displacement can indicate better sea keeping abilities, gross tonnage is promoted as the most important measure of size for passengers, as the ratio of gross tonnage per passenger – the Passenger/Space Ratio – gives a sense of the spaciousness of a ship, an important consideration in cruise liners where the onboard amenities are of high importance.

Gross tonnage normally is a much higher value than displacement. This was not always the case; as the functions, engineering and architecture of ships have changed, the gross tonnage figures of the largest passenger ships have risen substantially, while the displacements of such ships have not. RMS Titanic, with a gross register tonnage of 46,329 GRT, but a displacement reported at over 52,000 tons, was heavier than contemporary 100,000 – 110,000 GT cruise ships which displace only around 50,000 tons. Similarly, the Cunard Line's RMS Queen Mary and RMS Queen Elizabeth, of approximately 81,000 – 83,000 GT, but displacements of over 80,000 tons, do not differ significantly in displacement from their new 148,528 GT successor, RMS Queen Mary 2, which has been estimated to displace approximately 76,000 tons With the completion in 2009 of Oasis of the Seas, the first of the Oasis Class ships, the Cunard Queens of the 1930s have clearly been passed in displacement, as the Oasis vessels were projected to displace about 100,000 tons.

However, by the conventional and historical measure of gross tonnage, there has been a recent dramatic increase in the size of the largest new ships. The Oasis of the Seas measures over 225,000 GT, over twice as large as the largest cruise ships of the late 1990s.