Ship · structure
The hull, piece by piece
Cut a cargo ship across the middle and you see a box girder. It is closed below by a double bottom and above by the deck, and stiffened by ribs. Tap a part, or a name in the list, to see what it is and what it does.
Framing system
LWL: the load waterline.
Tap a part
Parts on the starboard side are numbered; the strakes on the port side are lettered from the keel up.
All the parts
Bottom
Side
Decks
Longitudinal framing
Why a ship is built like this
Waves load a ship like a beam: lifted at the middle by a crest it hogs, with the deck stretched and the bottom squeezed; lifted at the ends it sags, and the stresses reverse. The material far from the middle of the section, the deck and the bottom, does most of the work, just as the flanges of an I-beam do. That is why the stringer plate, the sheerstrake and the keel are made thicker than the plating between them, and why openings in the deck are kept narrow and have rounded corners.
In the transverse system a frame, a floor and a beam at every frame space form a closed ring that keeps the section in shape; the plating alone carries the bending. In the longitudinal system, introduced by Joseph Isherwood (the first ship built on it, the tanker Paul Paix, sailed in 1908), closely spaced fore-and-aft stiffeners add to the hull girder itself, and deep web frames several metres apart hold them up. Long ships, tankers and bulk carriers are built longitudinally, at least in the bottom and deck.
Older ships were riveted: frames were angle bars, plates overlapped in their strakes, and the joints were caulked. Welding, general after the Second World War, lets plates butt edge to edge, and the welded Liberty ships whose hulls cracked in cold seas taught builders about brittle steel and stress concentrations at sharp corners. The names of the parts did not change. The drawing here is a simplified, original diagram, and it is not to scale for plate thickness.