Safety, Fire & Survival · Damage Control — Free Surface Effect
Why is uncontrolled free water on a deck or in a partly flooded compartment dangerous to a vessel's stability?
- AIt only matters when the vessel is at anchor
- BIt increases the metacentric height and makes the vessel too stiff
- CIt has no effect as long as the water is fresh
- DThe free surface effect shifts the liquid to the low side as the vessel heels, reducing effective metacentric height (GM) and stability✓ Correct
Explanation
Liquid with a free surface moves toward the lower side as the ship heels, shifting the center of gravity and causing a virtual rise in G — a loss of effective GM (free surface effect). Large free surfaces (e.g., firefighting water building up) can erode stability and lead to capsize, so dewatering is urgent.
Authority: Bowditch (Pub. No. 9); SOLAS II-1; principles of stability
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