What Is Free Surface Effect? How To Reduce Free Surface Effect?

Free Surface Effect

          When a compartment is not completely filled with liquid it is said to have a free surface that tends to remain horizontal . As the ship inclines,the liquid flows to the lower side (in the direction of inclination)thereby increasing the inclining moment.

         Consider a compartment/tank containing a solid weight. when the ship is inclined, the center of gravity remains the same . Only the center of buoyancy shifts towards the direction of the inclination and righting arms (GZ) are formed.

          Now replace the solid weight with liquid of the same weight. when the ship is inclined, the surface remains horizontal but the liquid flows to the lower side, which is equivalent to the transfer of weight to the lower region thereby causing a change in center of gravity, Reducing metacentric height, Reducing righting leverand Increasing angle of heel 

              Must Read:>Change in Center of gravity due to transfer of weights

How To Reduce Free Surface Effect


        Free Surface Effect can be decreased , to a certain magnitude, by inducing pocketing. Pocketing happens when the surface of the liquid comes in contact with the top or bottom of the tank thereby reducing the breadth of the free surface area.

Surface Permeability

      The objects such us engine, pumps, machinery,pipping system etc are prevent the moving water and the “shifting of the wedge” .Thus reducing Free Surface Effect by reducing the flooding water that can fill the space.

Swash Bulkheads (Baffle Plates)

      Swash bulkheads are vertical bulkhead for restricting flow of oil or water due to rolling or pitching of a vessel. Thus reducing the free surface effect. Swash Bulkheads are found in voids, double bottoms, tanks bilges etc.

Sluice Valves

       Free flow (Sluice) valves on tankers allow opposing tanks to be cross-connected. When large, partially filled tanks are connected, Free Surface Effect increases, and the vessel becomes less stable.


Referance: thenauticalsite.com

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