Free surface effect is when liquid in a partially filled tank moves as the ship rolls, shifting the ship’s centre of gravity and reducing stability. What you’re seeing here is free surface effect in action — water sloshing from side to side in a slack fresh water tank. As the ship rolls the water in the tank moves towards the lower side which shifts the center of gravity of the liquid. This results in a movement that acts against the ships natural righting moment. The effect of this fluid moveme