What is Free Surface effect and its effect ?

Free surface effects

Provided a tank is completely filled with liquid, no movement of the liquid is possible and the effect on the ship’s stability is precisely the same as if the tank contained solid material.

When a quantity of liquid is drained from the tank, the situation changes completely and the stability of the ship is adversely affected by what is known as the “Free surface effect”.

This adverse effect on the stability is referred to as a “loss in GM” or as a “virtual rise in KG” and is calculated as follows:

Free surface moment = I x p

 where I = transverse inertia moment of tank in m4 and   p = density of tank cargo in t/m3

 The free surface moment is measured in tons x metres  (t x m)

Loss in GM due to Free Surface Effects (in metres)

=  (Sum of Free Surface Moments in tons x metres/ Displacement of Vessel in tons)

The ‘Free Surface Effect’ of all oil, fuel, freshwater, feed water and service tanks should be taken into account in all conditions, when these tanks are not completely filled.

Points to pounder:

In calculating the effect of free surfaces of consumable liquids, it shall be assumed that for each type of liquid at least one transverse pair or a single centre line tank has a free surface, and the tank or combination of tanks to be taken into account shall be those where the effect of free surfaces is the greatest.

It is of great importance to the safety of the vessel that all tanks are included in calculations regarding the corrected GMt.

If the contents of one or more ballast tanks will change during the voyage, this has to be considered in the stability calculations.


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