Ballast tank

A ballast tank is a compartment within a boat, ship or other floating structure that holds water, which is used as ballast to provide stability for a vessel. Using water in a tank provides easier weight adjustment than the stone or iron ballast used in older vessels. It also makes it easy for the crew to reduce a vessel's draft when they enter shallower water, by temporarily pumping out ballast. Airships use ballast tanks for similar advantages. A ballast tank is a compartment within a boat, ship or other floating structure that holds water, which is used as ballast to provide stability for a vessel. Using water in a tank provides easier weight adjustment than the stone or iron ballast used in older vessels. It also makes it easy for the crew to reduce a vessel's draft when they enter shallower water, by temporarily pumping out ballast. Airships use ballast tanks for similar advantages. The basic concept behind the ballast tank can be seen in many forms of aquatic life, such as the blowfish or argonaut octopus, and the concept has been invented and reinvented many times by humans to serve a variety of purposes. The first documented example of a submarine using a ballast tank was in David Bushnell's Turtle, which was the first submarine to ever be used in combat. Also, in 1849 Abraham Lincoln, then an Illinois attorney, patented a ballast-tank system to enable cargo vessels to pass over shoals in North American rivers. To provide adequate stability to vessels at sea, ballast weighs the ship down and lowers its centre of gravity. International agreements under the Safety Of Life At Sea (SOLAS) Convention require that cargo vessels and passenger ships be constructed to withstand certain kinds of damage. The criteria specify the separation of compartments within the vessel, and the subdivision of those compartments. These International agreements rely on the states that signed the agreement to implement the regulations within their waters and on vessels entitled to fly their flag.The ballast is generally seawater, pumped into ballast tanks. Depending on the type of vessel, the tanks can be double bottom (extending across the breadth of the vessel), wing tanks (located on the outboard area from keel to deck) or hopper tanks (occupying the upper corner section between hull and main deck). These ballast tanks are connected to pumps that pump water in or out. Crews fill these tanks to add weight to the ship and improve its stability when it isn't carrying cargo. In extreme conditions, a crew may pump ballast water into dedicated cargo spaces to add extra weight during heavy weather or to pass under low bridges. In submersibles and submarines, ballast tanks are used to control the buoyancy of the vessel. Some submersibles, such as bathyscaphes, dive and re-surface solely by controlling their buoyancy. They flood ballast tanks to submerge, then to re-surface either drop discardable ballast weights, or use stored compressed air to blow their ballast tanks clear of water, becoming buoyant again. Submarines are larger, more sophisticated and have powerful underwater propulsion. They must travel horizontal distances submerged, require precise control of depth, yet do not descend so deeply, nor need to dive vertically on station. Their primary means of controlling depth are thus their diving planes (hydroplanes in UK), in combination with forward motion. At the surface the ballast tanks are emptied to give positive buoyancy. When diving, the tanks are partially flooded to achieve neutral buoyancy. The planes are then adjusted together to drive the hull downwards, whilst still level. For a steeper dive, the stern planes may be reversed and used to pitch the hull downwards. The crew submerges the vessel by opening vents in the top of the ballast tanks and opening valves in the bottom. This lets water flood into the tank as air escapes through the top vents. As air escapes from the tank, the vessel's buoyancy decreases, causing it to sink. For the submarine to surface, the crew shuts the vents in the top of the ballast tanks and releases compressed air into the tanks. The high-pressure air pocket pushes the water out through the bottom valves and increases the vessel's buoyancy, causing it to rise. A submarine may have several types of ballast tank: main ballast tanks for diving and surfacing, and trimming tanks for adjusting the submarine's attitude (its 'trim') both on the surface and when underwater. Ballast tanks are also integral to the stability and operation of deepwater offshore oil platforms and floating wind turbines. The ballast facilitates 'hydrodynamic stability by moving the center-of-mass as low as possible, placing beneath the buoyancy tank.'