Hull plating.

The outer surfaces of the hull is built of steel plates that are welded together and to the main internal structural members of the hull. The plates are welded end to end to form a panel of plating which runs forward and aft and is known as a strake. The most important and heaviest strakes are the garboard strakes which run each side of the keel, and the sheer strakes which are the uppermost strakes of side plating. The strakes are lettered consecutively from the keel outward and upwards. The main members of the hull internally are:

• the keel (backbone of the vessel),
• the stem post (the upward continuation of the keel to the upper deck forward),
• the stern post (the upward continuation of the keel to the upper deck aft),
• the frames (the “ribs” of the hull to which the hull plating is attached);
• the longitudinals (the fore and aft steel beams joining the frames and together with them, provide the support for the hull plating),
• the beams (the transverse steel beams which connect the port and starboard frames and provide the lateral support of the deck plating),
• the girders (the steel beams running fore and aft joining the athwartship beams and together with them provide the support for the deck plating),
• the decks (horizontal planes of steel strakes running fore and aft),
• the bulkheads (these are the internal steel “walls” separating compartments from one another),
• the pillars (these are vertical steel beams which help the beams and girders to support the decks over large compartments).

Basic example of ship construction (Admiralty manual of seamanship).

Double bottom.

The keel, the floors of the frames and the bottom longitudinal form the cellular structure extending two thirds of the length of the hull known as the double bottom. It is enclosed by a layer of watertight plating called the inner bottom or tank top.

Its purpose is the prevent the ship from flooding should the outer bottom plating be holed. Besides strengthening the hull, the double bottom is also provides storage space for fuel or fresh water. In some merchant ships it also encloses ballast tanks in which sea water can be pumped to provide greater stability.