Watertight bulkhead

Floodability is the susceptibility of a ship's construction to flooding. It also refers to the ability to intentionally flood certain areas of the hull for damage control purposes, or to increase stability, which is particularly important in combat vessels, which often face the possibility of serious hull breach due to enemy action, and which rely on well-trained Damage Controlmen to equalize and then stop flooding of the hull.

Floodability is reduced by dividing the volume of the hull into watertight compartments with decks and bulkheads (which also increase the strength of ships), use of double bottom (or double hull), and by other means. If a ship's hull is divided into watertight compartments, any flooding resulting from a breach of the hull can be contained in the compartments where the flooding occurs. In most case, the watertight compartments are fitted with a system of automatic doors, which can be triggered either remotely or locally as soon as flooding is detected (and early example of such as system was used on the RMS Titanic, which sank in spite of its watertight bulkheads). Smaller vessels and submarines generally feature watertight hatches between compartments, which are closed manually to block water from escaping the flooded compartment. As long as the flooding is localised, this can allow a ship to retain sufficient buoyancy to remain afloat, but if numerous compartments are opened to the sea, the ship can sink regardless. If a ship is fitted with longitudinal bulkheads (running fore and aft) as well as transverse bulkheads, flooding along one side of the ship can cause a serious list, which can threaten to capsize the vessel. In such cases, damage control parties can intentionally flood the corresponding compartment on the other side, equalizing the list (although this can happen in ships without longitudinal bulkheads, as well). Such techniques can work fore-and-aft as well, for example, if a flooded bow is holding the rudder and propellers out of the water.

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Wikipedia