Ship measurements consist of a multitude of terms and definitions specifically related to ships and measuring or defining their characteristics.
Beam - A measure of the width of the ship. There are two types:
- Beam, Overall (BOA), commonly referred to simply as Beam- The overall width of the ship measured at the widest point of the nominal waterline.
- Beam on Centerline (BOC) - Used for multihull vessels. The BOC for vessels is measured as follows: For a catamaran: the perpendicular distance from the centerline of one hull to the centerline of the other hull, measured at deck level. For a trimaran: the perpendicular distance between the centerline of the main hull and the centerline of either ama, measured at deck level. This term in typically used in conjunction with LOA (Length overall; see below). The ratio of LOA/BOC is used to estimate the stability of multihull vessels. The lower the ratio the greater the boat's stability.
Carlin - similar to a beam, except running in a fore and aft direction.
Complement - The full number of people required to operate a ship. Includes officers and crewmembers; does not include passengers. For warships, the number of people assigned to a ship in peacetime may be considerably less than her full complement.
Cube - The cargo carrying capacity of a ship, measured in cubic feet. There are two common types:
- Bale Cube (or Bale Capacity)- The space available for cargo measured in cubic feet to the inside of the cargo battens, on the frames, and to the underside of the beams. It is a measurement of capacity for cargo in bales, on pallets, etc., where the cargo does not conform to the shape of the ship.
- Grain Cube (or Grain Capacity)- The maximum space available for cargo measured in cubic feet, the measurement being taken to the inside of the shell plating of the ship or to the outside of the frames and to the top of the beam or underside of the deck plating. It is a measurement of capacity for cargo like grain, where the cargo flows to conform to the shape of the ship.
Displacement - A measurement of the weight of the vessel, usually used for warships. (Merchant ships are usually measured based on the volume of cargo space; see tonnage). Displacement is expressed either in long tons of 2,240 pounds or metric tonnes of 1,000 kg. Since the two units are very close in size (2,240 pounds = 1,016 kg and 1,000 kg = 2,205 pounds), it is common not to distinguish between them. To preserve secrecy, nations sometimes misstate a warship's displacement.
- Displacement, Light - The weight of the ship excluding cargo, fuel, ballast, stores, passengers, and crew, but with water in the boilers to steaming level.
- Displacement, Loaded - The weight of the ship including cargo, passengers, fuel, water, stores, dunnage and such other items necessary for use on a voyage, which brings the vessel down to her load draft.
- Deadweight Tons (DWT) - The difference between displacement, light and displacement, loaded. A measure of the ship's total carrying capacity.
- Cargo Deadweight Tons - The weight remaining after deducting fuel, water, stores, dunnage and such other items necessary for use on a voyage from the deadweight of the vessel.
Draft (d) or (T) - The vertical distance from the bottom of the keel to the waterline. Used mainly to determine the minimum water depth for safe passage of a vessel.
Draft, Air - Air Draft is the distance from the water line to the highest point on a ship (including antennas) while it is loaded. Technically, standard draft is the distance a ship can pass over, air draft is the distance a ship can pass under.
Length - The distance between the forwardmost and aftermost parts of the ship.
- Length Overall (LOA) - The maximum length of the ship
- Length at Waterline (LWL) - The ship's length measured at the waterline
Shaft Horsepower (SHP) - The amount of mechanical power delivered by the engine to a propeller shaft. One horsepower is equivalent to 746 watts in the SI system of units.
Summer Water Line - The waterline when ship has standard load. Sometimes called Design Waterline.
Ton - The unit of measure often used in specifying the size of a ship. There are three completely unrelated definitions for the word. One of them refers to weight, while the others refer to volume.
- Measurement Ton (M/T) or Ship Ton Calculated as 40 cubic feet of cargo space. See Bale Cubic - example, a vessel having capacity of 10,000 M/T has a bale cubic of 400,000 cubic ft.
- Register Ton - A measurement of cargo carrying capacity in cubic feet. One register ton is equivalent to 100 cubic feet of cargo space.
- Weight Ton (W/T) - Calculated as a long ton (2,240 pounds)
Tonnage - A measurement of the cargo-carrying capacity of merchant vessels. It depends not on weight, but on the volume available for carrying cargo. The basic units of measure are the Register Ton, equivalent to 100 cubic feet, and the Measurement Ton, equivalent to 40 cubic feet. The calculation of tonnage is complicated by many technical factors.
- Gross Tons - The entire internal cubic capacity of the ship expressed in tons of 100 cubic feet to the ton, except certain spaces which are exempted such as: peak and other tanks for water ballast, open forecastle bridge and poop, access of hatchways, certain light and air spaces, domes of skylights, condenser, anchor gear, steering gear, wheel house, galley and cabin for passengers.
- Net Tons - Obtained from the gross tonnage by deducting crew and navigating spaces and allowances for propulsion machinery.