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Ship stability is the performance of a boat in the water in a variety of conditions, including fully laden and in rough seas. Careful thought goes into ship design to maintain stability in adverse conditions, and personnel are also aware of this as an area of concern so they can load their craft appropriately. Engineering programs provide an opportunity for making adjustments to ship design to meet stability needs, because the operator can simulate different designs and conditions and see what happens, to determine whether a ship needs additional features.
The center of gravity and center of buoyancy are both important factors in ship stability, as is the ship's overall height above the water. These determine the tendency to tip and how the ship behaves when it starts to heel, leaning to one side. A low center of gravity is generally desirable because it makes the ship harder to tilt and will encourage it to return to an upright position in all weather conditions. Ships with a high center of gravity can turn turtle, flipping and becoming impossible to upright.
Laden ships will behave differently in the water, an important consideration for cargo ships. A fully laden oil tanker, for example, performs differently than an empty one, and the engineer needs to factor this into the design. The conditions are also a consideration, as high winds, driving rain, and rough seas can have a negative impact on ship stability. The goal is to build a ship that naturally tries to right itself, and will always attempt to return to a neutral position when it is pushed by the elements.
The physical design of the hull and the contents of the ship is not the only consideration for ship stability. An engineer can also add passive and active stabilizers. An example of a passive system is outriggers, seen on a number of oceangoing vessels to make them more stable in rough seas. Outriggers make it extremely hard to capsize a ship. Fins are an example of an active system, adjustable to compensate for changing conditions and keep the ship as stable as possible.
Engineers designing ships work on a number of measures to improve ship stability, and ships are carefully tested in sea trials before being released to customers. During these trials, personnel will determine top speed, cornering, and other characteristics of handling. They can also confirm seaworthiness and check for performance fully laden, empty, and in adverse weather conditions. If there are doubts about ship stability, retrofitting is necessary to correct them.