RMS Titanic - Dimensions

Titanic's dimensions are frequently given incorrectly in books and websites. To some extent, this is understandable, as methods of measuring a ship vary somewhat and the terminology can be confusing. At first sight, the measurements of length and beam should be simple, but in fact they are complicated by the fact that the shipbuilders and the Board of Trade defined measurements in different ways. These methods are explained with the aid of a profile of Titanic.

Length Overall. (LOA)

This means exactly what it says. It is the length of the ship measured between the extreme ends. It is much-loved by shipowners, as it may make a ship appear to be larger than it effectively is. This was especially true in the days of sail, when a ship might sport 40 feet of jib-boom and bowsprit that added nothing to the passengers' comfort, but gave the owners bragging rights.

Length Between Perpendiculars (LBP). Builder's Style.

Shipbuilders measured a ship between the point where the stem cut the designed waterline and the after edge of the rudder post. This was believed to give a reasonable idea of the ship's carrying capacity, as it excluded the small, often unusable volume contained in her overhanging ends.

Length Between Perpendiculars. (LBP) Board of Trade Style.

For reasons known only to the bureaucracy, the Board of Trade measured LBP from the extreme end of the bow beneath the bowsprit (if fitted) to the after edge of the rudder post. The method was also used by Lloyd's Register and this has led to numerous books quoting the lengths of ships incorrectly. For instance, Carpathia was somewhat longer overall than the 540 feet often given.

Beam.

Shipbuilders measured beam between the extremes of the outer surfaces of the ship's plating. This was logical, as the designer used this dimension for hydrodynamic calculations. The Board of Trade and Lloyd's Register measured the extreme beam, including rivet heads, overhanging railings and the like. This measurement was useful to ships that used drydocks or locks.

Tonnage.

In the shipping world, a ton is not always a measurement of weight, though lubberly travel agents frequently assert that a 110,000 ton cruise ship is heavier than the 95,000 ton USS Nimitz. The confusion is caused by the old custom of measuring merchant ships' capacity in 'tons' of 100 cubic feet. (Originally the 'tons' or 'tuns' were large barrels used to carry cargo). In fact, the cruise ship mentioned would weigh considerably less than Nimitz, which really weighs, or more correctly, displaces, about 95,000 tons.

Two tonnages are calculated for merchant ships. Gross Registered Tonnage (GRT) measures the interior volume of the ship, with a few minor exceptions. Much of this volume is unprofitable to the owners, but it is the measurement generally quoted, as it gives a general idea of a ship's size. Net Registered Tonnage is also calculated. This includes only the volume capable of being used for profitable purposes and excludes spaces unavailable to passengers or cargo, such as the engineroom, fuel tanks and the crew's quarters. Taxes and charges are often based on NRT.

The real weight of a merchant ship is generally only of interest to her designers. It is known as displacement, as it is equal to the weight of the water displaced by the ship when floating at her designed waterline. It may be divided into 'lightweight', which is the displacement of the empty ship, and 'deadweight', which is the displacement of her cargo, crew, fuel stores and all necessary equipment. Deadweight varies widely, according to the ship's function. A large cruise ship may have a deadweight of as little as 7,000 tons, while a very large oil tanker may have a deadweight of more than 300,000 tons. It may be noted that that deadweight is generally given in British tons of 2,240 pounds, or in tonnes of 1,000 kilograms.

Moulded Depth.

Moulded depth is the distance from the bottom of the keel to the upper surface of the weather deck, (in Titanic's case, deck C) measured vertically.

Block Coefficient.

The block coefficient is the ratio of the immersed hull volume at a particular draught to that of a rectangular prism of the same length, breadth, and draught as the ship. ('Length' being the ship's LBP, builder's style).

Prismatic Coefficient.

The longitudinal prismatic coefficient is the ratio of the immersed volume to the volume of a prism with length equal to the ship’s and cross-section area identical to the midship section. ('Length' being the ship's LBP, builder's style).

The following dimensions are taken from Harland & Wolff sources and are authoritative. Titanic's LOA is sometimes given as 882' 8". According to Bruce Beveridge, one of the soundest authorities, this results from the casual use of approximate decimal fractions of a foot by Harland & Wolff, notably in the Drawing Office Notebook.