Free surface effect

The free surface effect is a mechanism which can cause a vessel to become unstable and capsize.

When a vessel with full tanks heels over, the contents of the tank do not shift. The tank's center of gravity does not change, so it does not affect the vessel's stability. The center of gravity is the point from which the weight of a ship may be considered to act.

In a partly filled tank, the contents will shift with the movement of the ship. This "free surface" effect increases the danger of capsizing.

When a vessel with partially filled spaces heels over, the contents of the spaces will shift. The center of gravity moves over to the side, making the vessel less stable. To avoid this free surface effect, try to have as few partially filled tanks and compartments as possible.

It is not always possible to avoid partly filled spaces. By dividing a tank into two equal parts with a baffle, the free surface effect is greatly reduced.

Ships carrying liquid cargo, or Tankers, have to be designed so as to minimize the effects of the free liquid surface. Water ballast, fuel oil, fresh water, lubricating oil, and other liquids carried in the ship can also contribute to the free surface effect.