How do free surfaces impact vessel stability?

Free surfaces within a vessel, such as those found in partially filled tanks or holds, can significantly influence the vessel's stability. When liquid freely moves within a tank, it can shift the center of gravity and effectively lower the metacentric height (GM) of the vessel. This reduction in GM diminishes the vessel's ability to right itself when tilted, which compromises overall stability.

When the ship heels over, the liquid moves to the low side, increasing the righting lever but reducing the effective righting moment. Consequently, the vessel may become less stable as it is more prone to further heel under external forces such as wind, waves, or weight distribution changes. Therefore, the impact of free surfaces is critical in stability assessments, particularly in ensuring that the vessel maintains a safe and steady position in varying conditions at sea.

It's essential to account for free surface effects during stability calculations, particularly when loading or unloading cargo, to maintain the vessel's safety and performance.