Free surface correction

A partly filled tank lets liquid shift as the ship heels, moving the center of gravity and reducing effective $GM$ by the free-surface correction $FSC = \rho_l \cdot i / \Delta$, where $i$ is the tank’s free-surface moment of inertia about its own longitudinal centerline. The loss depends on tank breadth cubed, so wide tanks are penalized heavily; longitudinal subdivision and swash bulkheads cut $i$. The corrected value $GM_{fluid} = GM_{solid} - \Sigma FSC$ is what the IS Code criteria are checked against.