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A ballast tank is a compartment within a boat or ship, that holds water. A vessel may have a single ballast tank near its center or multiple ballast tanks typically on either side. A large vessel typically will have several ballast tanks including double bottom tanks, wing tanks as well as forepeak and aftpeak tanks. Adding ballast to a vessel lowers its center of gravity, and increases the draft of the vessel. Increase draft may be required for proper propeller immersion.

A ballast tank can be filled or emptied in order to adjust the amount of ballast force. Ships designed for carrying large amounts of cargo must take on ballast water for proper stability when travelling with light loads and discharge water when heavily laden with cargo. Small sailboats designed to be light weight for being pulled behind automobiles on trailers are often designed with ballast tanks that can be emptied when the boat is removed from the water.

In submarines ballast tanks are used to allow the vessel to submerge, water being taken in to alter the vessels 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.

Ballast water taken in to a tank from one body of water and discharged in another body of water can introduce invasive species of aquatic life. The discharge 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.

• Kingston valve
• Vent (submarine)
• Saddle tank (submarine)