What is a Ballast Tank?

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  • Written By: Larry Ray Palmer
  • Edited By: Amanda L. Wardle
  • Last Modified Date: 15 October 2019
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A ballast tank is a structure added during the construction of aquatic equipment to provide an adjustable point of equilibrium. Commonly used in ship construction, as well as floating wind turbines, oil platforms, and submarines, a ballast tank can be filled with water to add or remove weight. This weight makes it possible for a sailing vessel or aquatic platform to maintain a specific level in the water despite varying weather conditions.

The function of a ballast tank is controlled by a Kingston valve and a vent, which work together in a system of checks and balances. When the vent and valve are both opened, water flows into the tank, providing ballast. When only the Kingston valve is opened, the air pressure of the closed vent keeps the water out. This bit of physics is used by submarine pilots in times of war. Faster submergence times are possible by traveling in open water with the Kingston valve opened and the vents closed.

In ship construction, a ballast tank is usually centered at the lowest point of the hull. Additional ballast tanks may be installed in the fore and aft positions or on the starboard and port sides of the hull. By positioning multiple ballast tanks around the ship, it can withstand the high winds and waves that are commonly encountered in the ocean environment.


Ships used to transport goods incorporate ballast tanks to account for the weight of their payloads. An empty freighter ship might easily be capsized by high seas. By adding water to a ballast tank, the ship's center of gravity can be lowered, making it more stable. On the other hand, a heavily laden freighter could be swamped by high waves crashing over the bow. By pumping water out of the ballast tanks, the ship sits higher in the water and thus reduces its risk of being swamped.

Submarines use ballast tanks to provide their submerging and surfacing movements. By filling the ballast tanks with water, the craft becomes heavier and sinks below the surface of the water. When it is time to resurface, the ballast tanks are drained of water and the submarine becomes buoyant, rising to the surface.

Floating wind turbines and oil drilling platforms rely on buoyancy and ballast tank technology to maintain equilibrium in the aquatic environment. A buoyancy tank is an air-filled structure that aids in maintaining buoyancy. The buoyancy and ballast tanks used in these platforms help to keep the structure close to the surface of the water through a process called hydrodynamic stability. By positioning the ballast tank under the buoyancy tank, the platform can maintain a specific level of buoyancy and a lowered center of gravity.


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