Free Surface Effect

The free surface effect is the loss of effective metacentric height caused by liquid moving freely in a partly filled tank as the ship heels. The shift of the liquid’s center of gravity raises the virtual center of gravity by the free surface moment divided by displacement, reducing GM and the righting arm. The moment depends on the tank’s transverse second moment of area and the liquid density, so wide tanks are worst; subdivision with longitudinal bulkheads cuts the effect sharply. Intact and damage stability calculations under SOLAS II-1 and the 2008 IS Code apply free surface corrections to slack tanks.