Free surface effect

Free surface effect is the loss of a vessel’s stability when liquid in a partly filled tank moves as the vessel heels, shifting its effective center of gravity.

Metacentric height: The distance between a vessel’s center of gravity and metacenter, used as a measure of initial stability. Free surface effect reduces effective metacentric height, weakening the vessel’s initial resistance to heel.
Ship stability: A ship’s ability to resist capsizing and return toward an upright condition after disturbance. Free surface effect is one factor assessed in a ship’s overall stability.
Ship heeling: A vessel’s rotation about its longitudinal axis, causing one side to lower relative to the other. Heel creates the slope that drives liquid toward the tank’s lower side.
Solid cargo shift: The movement of secured or unsecured solid cargo from one position in a vessel to another. A solid shift changes weight location, while free surface effect arises from liquid moving within a tank.
Righting moment: The restoring torque that tends to return a displaced vessel to an upright position. Shifting liquid reduces the restoring moment available at a given heel angle.
Tank baffles: Internal partitions that restrict liquid movement within a tank. Baffles can limit transverse liquid flow and reduce free-surface movement.
Fluid statics: The study of fluids at rest and the forces they exert on immersed surfaces. The effect depends on how a liquid’s pressure and mass distribution change as its container tilts.
Trim: A vessel’s inclination in the longitudinal direction, measured by the difference between forward and aft drafts. Trim is a fore-and-aft attitude change, unlike the transverse stability loss associated with free surface effect.
Free surface correction: A calculated deduction from a vessel’s metacentric height to account for liquid movement in partly filled spaces. This correction quantifies the stability penalty caused by slack tanks.
Ballast water: Water carried by ships to control draft, trim, and stability. Partly filled ballast tanks can create free-surface losses during loading or transfer.

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Roll-on/roll-off shipRelated: Floodwater spreading across a vehicle deck can sharply reduce stability.