A ballast tank is a compartment within a boat, ship or other floating structure that holds water.
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. For example, 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.
In order to provide adequate stability to vessels at sea, ballast is used to weigh the ship down and lower its centre of gravity. International agreements under the Safety Of Life At Sea (SOLAS) Convention require cargo vessels and passenger ships to be constructed so as to withstand certain kinds of damage. The criteria specify the separation of compartments within the vessel and also the subdivision of those compartments. The International agreements rely upon the states which have signed the agreement to implement the regulations within their waters and on vessels which are entitled to fly their flag. The ballast is generally seawater which is pumped into tanks known as 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 which can pump water in or out. These tanks are filled in order to add weight to the ship once cargo has been discharged, and improve its stability. In some extreme conditions, ballast water may be introduced to dedicated cargo spaces in order to add extra weight during heavy weather or to pass under low bridges.
In submarines ballast tanks are used to allow the vessel to submerge, water being taken in to alter the vessel's buoyancy and allow the submarine to dive. When the submarine surfaces, water is blown out from the tanks using compressed air, and the vessel becomes positively buoyant again, allowing it to rise to the surface. A submarine may have several types of ballast tank: the main ballast tanks, which are the main tanks used for diving and surfacing, and trimming tanks, which are used to adjust the submarine's attitude (its 'trim') both on the surface and when underwater.
Floating structures 
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 [it] beneath the [air-filled] buoyancy tank."
|This section requires expansion. (May 2010)|
Wakeboard boats 
Most wakeboard-specific inboard-engine boats have multiple integrated ballast tanks that are filled with ballast pumps controlled from the helm with rocker switches. Typically the configuration is based on a three tank system with a tank in the center of the boat and two more in the rear of the boat on either side of the engine compartment. Just like larger ships when adding water ballast to smaller wakeboard boats the hull has a lower center of gravity, and increases the draft of the boat. Most wakeboard boat factory ballast systems can be upgraded with larger capacities by adding soft structured ballast bags.
Environmental concerns 
Ballast water taken into a tank from one body of water and discharged in another body of water can introduce invasive species of aquatic life. The taking in of water from ballast tanks has been responsible for the introduction of species that cause environmental and economic damage. For example, zebra mussels in the Great Lakes of Canada and the United States.
See also 
- Floating wind turbine
- Floating oil platform
- Kingston valve
- Saddle tank (submarine)
- Sailing ballast
- Vent (submarine)
- Discovery Blog: Scientists solve millennia-old mystery about the argonaut octopus
- Musial, W.; S. Butterfield, A. Boone (2003-11). "Feasibility of Floating Platform Systems for Wind Turbines". NREL preprint (NREL) (NREL/CP-500-34874): pp. 2–3. Retrieved 2010-05-04. "Spar buoys ... have been used in the offshore oil industry for many years. They consist of a single long cylindrical tank and achieve hydrodynamic stability by moving the center-of-mass as low as possible, placing ballast beneath the buoyancy tank."; "to maintain platform stability against overturning, especially for a wind turbine where the weight and horizontal forces act so far above the center of buoyancy. ... significant ballast must be added below the center of buoyancy, or the buoyancy must be widely distributed to provide stability."
- BBC News: Microwaves 'cook ballast aliens'