What direct, negative effect does a large, partially filled tank of liquid have on a ship's stability?

A large, partially filled tank of liquid has a direct, negative effect on a ship's stability primarily through a phenomenon called the free surface effect. Ship stability is its inherent ability to return to an upright position after being tilted or heeled by external forces. When a ship heels, which is its rotational movement about its longitudinal axis, the liquid in a partially filled tank does not remain stationary. Instead, it flows or sloshes to the lower side of the tank, seeking to maintain a level surface parallel to the waterline. This movement of liquid within the tank creates an internal heeling moment that acts against the ship's inherent righting moment, which is the force that tends to restore the ship to an upright position. The practical result of this shifting liquid is equivalent to a virtual rise in the ship's center of gravity (G). The center of gravity (G) is the theoretical point where the entire weight of the ship is considered to act downwards. A higher center of gravity significantly reduces a ship's stability because it shortens the righting arm, which is the horizontal distance between the center of gravity and the line of action of the buoyant force when the ship is heeled. This virtual rise in G also directly reduces the metacentric height (GM). The metacentric height (GM) is the vertical distance between the ship's metacenter (M), which is the point about which the ship effectively oscillates when heeled, and its center of gravity (G). A smaller positive metacentric height indicates less initial stability, meaning the ship will roll more slowly, return to upright more sluggishly, and heel to a larger angle for any given external disturbing force. Consequently, the free surface effect diminishes the ship's overall righting moment, making the vessel more prone to excessive angles of heel and increasing the risk of capsizing, especially in rough seas or during sharp maneuvers. The magnitude of this negative effect is directly proportional to the width of the liquid's free surface area within the tank.