During your oral exam you are asked to explain the concept of "free surface effect" and why it is relevant to a yacht with partially filled fuel or water tanks.

Question

Accepted Answer

It refers to the movement of liquid within a partially filled tank as the vessel rolls, which shifts the centre of gravity and reduces the vessel's effective stability (GM) Free surface effect occurs when liquid in a partially filled tank moves towards the lower side as the vessel heels, causing a virtual rise in the centre of gravity and a corresponding reduction in metacentric height (GM), which reduces stability. On yachts this is particularly relevant for fuel and freshwater tanks during heavy weather, and good practice is to keep tanks either full or as empty as practicable, or fitted with longitudinal divisions, to minimise the effect.

Yachtmaster Stability Practice Questions