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A bilge keel is a nautical device used to reduce a ship's tendency to roll. Bilge keels are employed in pairs (one for each side of the ship). A ship may have more than one bilge keel per side, but this is rare. Bilge keels increase hydrodynamic resistance to rolling, making the ship roll less. Bilge keels are passive stability systems.
On commercial shipping the bilge keel is in the form of a strake, or small keel or blister, running along much of the length of the hull. They are typically fitted one on each side, low down on the side of the hull, so as not to increase the draft of the vessel. In battleships they were often quite large and used as part of the torpedo protection system.
A bilge keel is often in a "V" shape, welded along the length of the ship at the turn of the bilge. Although not as effective as stabilizing fins, bilge keels have a major advantage in their low impact on internal ship arrangements. Unlike fins, bilge keels do not have any components inside the hull that would adversely affect cargo or mission spaces. Like fins, bilge keels have the disadvantage of increasing the hydrodynamic resistance of the vessel, thus hindering forward motion.