What is the free surface effect and how does it impact stability?

The free surface effect refers to the phenomenon that occurs when liquids within partially filled tanks on a vessel shift and create an effective rise in the center of gravity, decreasing stability. When a vessel heels (leans to one side), the liquid inside the tank moves toward the low side, creating an additional movement of weight away from the centerline of the vessel. This shift in weight can lead to a reduction in the righting moment, which is the force that helps the vessel return to an upright position.

As the center of gravity increases due to the free surface effect, the vessel becomes more prone to further heeling, potentially leading to instability and capsizing, especially in rough conditions or during maneuvers. Thus, the free surface effect is a critical concept in stability analysis because it can negatively impact a vessel's ability to recover from a heel, making it a crucial factor to consider during vessel design and loading operations.