Free surface effect. A tank which is completely filled with liquid is said to be pressed up”, while one which is not is called a “slack tank”.
The liquid cannot move in a pressed up tank when the vessel is inclined by external (or internal) forces. The position of the centre of mass (or “centre of gravity”) of the liquid will not change. In a slack tank, the liquid can move and the position of the centre of gravity will change, moving to the lower side. This will cause the position of the centre of gravity of the entire ship to change.
The vertical line of force of the ship’s weight acting vertically downwards through the new position of the centre of gravity can be extended upwards in a reverse direction to cut the original centre line of the vessel at a new “virtual centre of gravity”. This will cause a virtual reduction of the metacentric height and have an adverse effect on the vessel’s stability. (See Metacentric height.)
If the tank is subdivided longitudinally the virtual rise in the position of the centre of gravity is reduced and the reduction in metacentric height is not so severe as if the tank was not subdivided. The cargo tanks in oil tankers are subdivided for this reason and the structural components within the tanks also reduce the free surface effect.
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