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.
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