Ship, Principal Measurements of a

the basic linear dimensions of a ship.

The theoretical principal measurements are the length between perpendiculars (LBP), measured along the transverse axis at the waterline of a loaded ship, from the forward edge of the bow stem to the axis of rotation of the rudder; the breadth, measured along the hull at the midpoint of the length at the waterline of a loaded ship between the outer edges of the frame; the depth, equal to the distance along the vertical at the side between the inner surfaces of the flat keel and the deck; and the draft, measured from the waterline of a loaded ship to the top of the flat keel.

The interrelationships of the principal measurements determine convenience in placing cargoes and passengers and the navigational qualities of a vessel. The length/breadth and breadth/draft ratios determine the resistance of the water to ship motion; the breadth/draft and depth/draft ratios determine the ship’s stability; the length/depth and breadth/draft ratios determine durability and stiffness; the depth/draft ratio determines cargo capacity; and so on.

The dimensions of the principal measurements—length, greatest width, height, and maximum draft (taking into account protruding parts and the vessel’s trim)—determine the vessel’s navigational ability under limiting circumstances (through canals and locks, under bridges, in shallow water), and ability to stand at moorings, in shipways, under repair in docks, and so on.