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The Free Surface Effect

The Free Surface Effect

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.


bow view of a vessel with a full tank.
When a vessel with full tanks heels over, the contents of the tank do not shift. The tank's centre of gravity does not change, so it does not affect the vessel's stability.  
bow view of a vessel with a partially filled tank.
In a partly filled tank or fish hold, the contents will shift with the movement of the boat. This "free surface" effect increases the danger of capsizing.  
bow view of a vessel with two partially filled compartments.
When a vessel with partially filled spaces heels over, the contents of the spaces will shift. The centre 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.  
a compartmentalized vessel.
You cannot always avoid partly filled spaces. By dividing a tank into two equal parts with a baffle, the free surface effect is greatly reduced. Using boards to divide fish wells into compartments will also help.
Free Surface Effect It has a lot to do with the stability of a ship. A ship that has taken in a lot of water will also experience this kind of phenomenon that will make it unstable. Part of the study of naval architecture deals with ship stability.

Ships carrying liquid cargo, or Tankers, have to be designed so as to minimize the effects of free liquid surface. Water ballast, fuel oil, fresh water, lubrication oil, and other liquid carried in the ship can also contribute to the free surface effect.
The drawing shows a cross section through the midship of a tanker ship. If there is some dynamic force that makes a ship tilt to one side, notice how the oil in the tank finds its own level and tends to shift more towards the tilting side.

The center of gravity of the oil in the tank will also shift. If the ship has enough buoyancy, it is able to right itself. 

However, if the tilt is too big, the shift in the center of gravity of the oil may become too big. Instead of righting the ship, the buoyancy force on the ship may even turn the ship in the same direction of tilt, and the ship rotates and overturns.

What can be done to minimize the free surface effect?

  The ship is fitted with compartments so that there are several tanks instead of one big tank. Even though the same quantity of oil is carried, notice how the oil behaves. The center of gravity of individual oil tanks will also shift, but the summation of all the centers of gravities does not shift the center of gravity of the ship that significantly as before.

Another way to minimize the free surface effect is to fill the tanks nearly full. In this case there is less room for the liquid to move about freely. This method may be a bit difficult to control for tanks carrying consumables like fuel oil, domestic water, and potable water.

The shape of the tanks can also be built to ensure stability, but in most cases, ships are built for maximum storage capacity and the rectangular cross sectional shape is most feasible.

The tanks in a Tanker are built in compartments for this purpose. The sides of the tanks also serve to protect the ship from complete flooding should some damage to its hull occur.

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