Free Surface Effect on Stability

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Free Surface Effect, ship stability, naval architecture, titanic, tankers, ballast, gravity, buoyancy, flooding in Marine Engineer Shipping

Tankers have low freeboard

Free Surface Effect

Have you ever seen a sinking ship? I am sure you have. Nowadays, movies and TV news footage are able to show scenes of ships that have taken in a lot of water. The movie, Titanic, has lots of scenes of a sinking ship. If you have not watched the movie, you might have missed some interesting points.

However, this page is not about sinking ships. In this page, we want to talk about a phenomenon called "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.

Download Naval Architecture Questions & Answers e-book. This e-book contains worked examples in calculations for the Class 1 Marine Engineer Naval Architecture Paper for Competency Exams.

 

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