A fuel tank is being topped up while underway and is now roughly half full, with the vessel rolling in a moderate sea. What stability effect should the OOW be aware of?

A. A partially filled ("slack") tank creates a free surface effect — as the vessel rolls, the liquid shifts, creating a virtual rise in the centre of gravity that reduces the vessel's effective stability (GM); where practicable, tanks should be kept either full (pressed up) or empty rather than slack, especially in poor weather

B. A half-full tank has no effect on stability compared to a full or empty tank

C. Free surface effect only applies to water ballast tanks, not fuel tanks

D. The effect is eliminated automatically by the vessel's stabilisers

A. Only the Chief Engineer needs to consider this, never the deck watchkeeper

B. None — stability is fixed regardless of loading

C. Increased displacement generally raises the centre of gravity if weight is added high up, and free surface effects in partially-filled tanks can reduce the effective metacentric height (GM)

D. Additional weight always improves stability regardless of where it is placed

A. This only matters when the vessel is at anchor

B. Negligible — watertight doors below the waterline are rarely closed anyway

C. Only relevant if the vessel carries passengers

D. Significant — a failed watertight closure compromises the subdivision the vessel's damage stability calculations rely on, potentially allowing progressive flooding between compartments in the event of damage