A bilge

Bilge

The bilge is the lowest compartment on a ship where the two sides meet at the keel. The word was coined in 1513.-Bilge water:The word is sometimes also used to describe the water that collects in this compartment. Water that does not drain off the side of the deck drains down through the ship into...

 keel

Keel

In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...

 is used to reduce the hull's tendency to roll

Ship motions

Ship motions are defined by the six degrees of freedom that a ship, boat or any other craft can experience.- Translation :HeaveSwaySurge-Vertical axis:Vertical axis, or yaw axis — an axis drawn from top to bottom, and perpendicular to the other two axes...

. 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

Ship stability

Ship stability is an area of naval architecture and ship design that deals with how a ship behaves at sea, both in still water and in waves. Stability calculations focus on the center of gravity and center of buoyancy of vessels and on how these interact....

.

On commercial shipping the bilge keel is the form of a strake, or small keel or blister, running along much of the length of the hull

Hull (watercraft)

A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...

. They are typically fitted one on each side, low down on the side of the hull, so as not to increase the draft

Draft (hull)

The draft of a ship's hull is the vertical distance between the waterline and the bottom of the hull , with the thickness of the hull included; in the case of not being included the draft outline would be obtained...

 of the vessel. In battleship

Battleship

A battleship is a large armored warship with a main battery consisting of heavy caliber guns. Battleships were larger, better armed and armored than cruisers and destroyers. As the largest armed ships in a fleet, battleships were used to attain command of the sea and represented the apex of a...

s they were often quite large and used as part of the torpedo

Torpedo

The modern torpedo is a self-propelled missile weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with it or in proximity to it.The term torpedo was originally employed for...

 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

Stabilizer (ship)

Ship stabilizers are fins mounted beneath the waterline and emerging laterally. In contemporary vessels, they may be gyroscopically controlled active fins, which have the capacity to change their angle of attack to counteract roll caused by wind or waves acting on the ship.The bilge keel is an...

, 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.

Design considerations

When designing a bilge keel, there are important decisions to consider. To minimize hydrodynamic drag, the bilge keel should be placed in way of a flowline where it does not oppose crossflow. For such a usage the ends of the bilge keel should be tapered and properly faired into the hull. Also, a bilge keel should not protrude from the hull so far that the device could be damaged when the vessel is alongside a pier

Pier

A pier is a raised structure, including bridge and building supports and walkways, over water, typically supported by widely spread piles or pillars...

, even with a few degrees of adverse heel. Bilge keels on commercial vessels should not protrude below the baseline either, where they could be damaged or fouled by grounding. Note that small bilge keels are often fitted to smaller fishing boats precisely to protect the hull on drying moorings and to help keep the vessel upright

Effectiveness

It is constructed from flat plate so as to present a sharp obstruction to roll motion. The roll damping provided by bilge keels is more than that of barehull ships, but falls short of other roll damping devices. Nevertheless it is considered prudent Naval Architecture to install a bilge keel whenever possible as it is the only device effective in the severest of seas. Bilge keels can also be used in conjunction with other roll damping devices.

On sailing yachts

Bilge keels are often fitted to smaller sailing yachts. Bilge keels minimise the draft of the vessel compared to a single fin keel thus enabling it to negotiate shallower water. Bilge keels on sailing yachts extend below the lowest point of the hull extending slightly outwards. Such an arrangement also enables the vessel to stand upright on firm sand or mud at drying moorings without the need for detachable legs, and is simpler than retractable fin keels while giving the hull greater protection. Bilge keels are not as effective as central fin keels in preventing leeway

Leeway

Leeway is the motion of an object that is floating in the water to leeward due to the component of the wind vector perpendicular to the object’s. The National Search and Rescue Supplement to the International Aeronautical and Maritime Search and Rescue Manual defines leeway as "the movement of a...

(sideways slippage) caused by crosswinds but are preferred by many small craft owners due to their other advantages.