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 hydrostatic stability for a vessel, to reduce or control buoyancy, as in a submarine, to correct trim or list, to provide a more even load distribution along the hull to reduce structural hogging or sagging stresses, or to increase draft, as in a semi-submersible vessel or platform, or a SWATH, to improve seakeeping. Using water in a tank provides easier weight adjustment than the stone or iron ballast used in older vessels, and makes it easy for the crew to reduce a vessel's draft when it enters shallower water, by temporarily pumping out ballast. Airships use ballast tanks mainly to control buoyancy and correct trim.

The concept of ballast tanks, inspired by nature, can be seen in aquatic life forms like blowfish and the argonaut octopus, which regulate their buoyancy to move and survive in water. Humans have adapted and refined this idea for various applications, especially in maritime and underwater engineering.

The first recorded use of a ballast tank in a submarine was in David Bushnell's Turtle in 1776, which played a historic role as the first combat submarine. The ballast system allowed it to submerge and resurface effectively. Later, in 1849, Abraham Lincoln, then a practicing lawyer, patented a ballast tank system designed to help cargo ships navigate shallow waterways by adjusting their buoyancy. This innovation demonstrated how the concept of ballast has been repeatedly reinvented to meet specific challenges in marine technology.

Ballast is used in surface vessels to alter the draft, trim, list and stability. It may also be used to modify structural load distribution, usually the longitudinal load distribution which affects hogging and sagging stresses. It may also be used to change the moments of inertia which affect motion in a seaway. 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. Ballast may be used to compensate for stability losses due to flooding of some compartments.

The ballast is generally the water in which the vessel is floating at the time of ballasting, such as 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 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 main ballast tanks are flooded to achieve approximately neutral buoyancy. Depth control tanks are used for fine control of buoyancy by pumping water in or out to compensate for variations in weight. Ballast water can be transferred between trim tanks to balance the vessel in horizontal trim. The planes are then adjusted together to drive the hull downwards, while still level. For a steeper dive, the stern planes may be reversed and used to pitch the hull downwards.