It is an
effect occurs when the liquid moves across the tank in the direction of heel,
making the centre of gravity moves away from the centerline, reducing the
righting lever and increasing the angle of heel.
movement of the centre of gravity has been caused by the transfer of a wedge of
liquid across the tank, so the righting lever was reduced. While the righting
lever is reduced, it is useful to assume the effect of free surface is to raise
the centre of gravity, so reducing the metacentric height of gravity.
design point of view, the tanks with a longitudinal centerline division can
reduce the free surface effect. Also, in practical view, pressing up tight
tanks can prevent the liquid from moving across inside tanks.