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, 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.
