What stability parameter is reduced by the free surface effect?

• A. Displacement.
• B. Metacentric height (GM).
• C. Waterline length.
• D. Hull shear force.

Answer: (B)

Metacentric height represents initial stability: the vertical distance between the center of gravity and the metacenter. When the ship heels, the buoyant force shifts and a new metacenter is defined by the hull geometry, and the restoring righting moment is roughly proportional to GM. The free surface effect comes from liquid sloshing inside partially filled tanks. That moving liquid doesn’t stay fixed with the hull, which changes how buoyancy acts and effectively lowers the metacenter relative to the center of gravity. In consequence, GM decreases, so the ship’s initial stability is reduced. The other options—displacement, waterline length, and hull shear—are not directly altered by this internal liquid motion in the same way.