The loss of vessel stability that occurs when liquids in partially filled tanks or flooded spaces slosh from side to side as the ship rolls. Because the liquid's mass shifts toward the lower side, it artificially raises the ship's center of gravity and reduces its ability to right itself. It is a major capsizing risk, mitigated by keeping tanks completely full, completely empty, or divided by internal baffles.
Heard on the job
We have too many slack tanks right now, and the free surface effect is making her roll sluggishly.
Insider tip
Never leave multiple water or fuel tanks half-full in heavy weather; pump them to consolidate the load and eliminate slack water.