How Ballast Works
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Background
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Ballast is a nautical term that refers to additional weight, in a solid or water form, that is taken onto a ship. Ballast is used to help keep a ship stabilized, sufficiently submerged in the water or to the stresses on the ship properly balanced. Historically, before the advent of pumping technologies, ballast was typically a solid in the form of rocks or sand. The rocks or sand would have to be moved onto or off the ship manually depending on the amount of cargo on board. On many modern cargo ships, water is the main form of ballast. According to the Canadian Ballast Water Program, ships are usually capable of taking on around 25%-30% of their dead weight tonnage (total weight a ship can carry safely).
Moving Ballast
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When water ballast is brought on board a ship, it is moved into ballast tanks. This is generally done with a pump that brings the water in through vents. To prevent the accidental pumping in of large pieces of foreign material, the vents are typically covered with heavy grates to help filter out large detritus or animals. Water may be pumped into the ballast tanks for a number of reasons. The off-loading of a large amount of cargo is one major reason. As the cargo is removed from the cargo bay, the ship displaces less water and rises. If a ship rises too high in the water, it can become unstable and topple over. Ballast water may also be taken onto the ship en route as fuel is consumed, which has a similar effect as cargo removal. In the event of severe weather, a ship may take on additional ballast to ride somewhat lower in the water as a safety measure.
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Likewise, there are also situations in which a ship needs to remove ballast. Any time a significant addition of weight occurs, such as loading cargo, taking on a large number of passengers or a major refueling, the ship displaces more water. In these situations, the water in the ballast tanks may be partially or completely pumped out to restore the proper degree of displacement. The process of pumping ballast water out can have negative consequences. In recent years, there has been concerns in areas with heavy, ocean shipping traffic over the effects that pumping ballast water from the open sea into coastal waters has on the local water-based ecosystems. Concerns tend to focus particularly on the introduction of non-native species and the potential displacement or destruction of native populations (fish species, for example).
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