Free Surface Effect occurs when liquid moves in what scenario?

• A. In a partially filled tank or space
• B. In a fully filled ballast tank
• C. In the engine room
• D. When water leaks into the hull

Answer: (A)

Free Surface Effect happens when liquid inside a compartment is free to move as the boat tilts or moves. In a partially filled tank or space, the liquid surface can slosh toward the low side, shifting the vessel’s center of gravity and effectively reducing initial stability (metacentric height). That’s why keeping ballast tanks either full or empty minimizes this movement. A fully filled ballast tank has little to no free surface to shift, so it doesn’t produce this stability loss. Water leaks into the hull or issues in the engine room aren’t about free surface movement, so they don’t illustrate this effect.