The bilge /bɪldʒ/ of a ship or boat is the part of the hull that would rest on the ground if the vessel were unsupported by water. The "turn of the bilge" is the transition from the bottom of a hull to the sides of a hull.
Internally, the bilges (usually used in the plural in this context) is the lowest compartment on a ship or seaplane, on either side of the keel and (in a traditional wooden vessel) between the floors.
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A bilge pump is a water pump used to remove bilge water. Since fuel can be present in the bilge, electric bilge pumps are designed to not cause sparks. Electric bilge pumps are often fitted with float switches which turn on the pump when the bilge fills to a set level. Since bilge pumps can fail, use of a backup pump is often advised. The primary pump is normally located at the lowest point of the bilge, while the secondary pump would be located somewhat higher. This ensures that the secondary pump activates only when the primary pump is overwhelmed or fails, and keeps the secondary pump free of the debris in the bilge that tends to clog the primary pump.
Ancient bilge force pumps had a number of common uses. Depending on where the pump was located in the hull of the ship, it could be used to suck in sea water into a live fish tank to preserve fish until the ship was docked and the fish ready to be sold. Another use of the force pump was to combat fires. Water would again be sucked in through the bottom of the hull, and then pumped onto the blaze. Yet another suggested use for a force pump was to dispel water from a ship. The pump would be placed near the bottom of the hull so as to suck water out of the ship. Force pumps were used on land as well. They could be used to bring water up from a well or to fill high placed tanks so that water could be pressure pumped from these tanks. These tanks were for household use and/or small-scale irrigation. The force pump was portable and could therefore, as on ships, be used to fight fire.
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.