A bilge keel is a device added to the hull of a ship to aid against rolling in heavy seas. It usually consists of a flat, steel plate welded to the hull at the area where the sides of the hull turn to form the bottom. The bilge keel provides a surface of the hull to resist or act against the water to prevent rolling. By attaching to the hull at an angle to the bottom of the ship, the bilge keel does not add to the depth or the draft of the ship. In some military ships such as a battleship, the device was used as an integral component of the ship's torpedo defense system.
Unlike many anti-roll devices that can be fitted to a ship's hull, the bilge keel does not require any control devices, added pieces or mechanical abilities. The steel strip is simply welded or riveted into place along the hull. While commonly used in various lengths and widths, the average bilge keel is fitted to the hull and runs approximately the full distance from stem to stern. It protrudes out from the turn of the hull to a distance parallel to the sides of the ship. This eliminates any interference with the docking of the ship caused by a protruding keel strip.
In some rare instances, a ship might be fitted with two or more bilge keel strips, however, this is very rare. The downside of installing the strips to the side of a ship is the increased resistance that the bilge keel places on the forward motion of the vessel. The advantage over other types of anti-roll devices such as fins is the absence of interior infringement by the bilge keel resulting in no decrease in cargo space. A fin requires the placement of control and operational components inside of the hull, resulting in a loss of available cargo space.
Some sail-powered ships use a bilge keel to combat roll instead of a longer keel fin. This allows the ship to fight roll while not increasing the draft of the vessel by lengthening the keel fin. Some vessels designed to operate in both deep water, such as a large lake or ocean as well as shallow water such as a river or channel, use this type of keel in place of a deep-running hull. By using the strip affixed to a shallow bottom, a day cruiser or personal yacht can ride smooth in rolling seas and still navigate the shallow waters required to dock in some locations.