Negative GM and angle of loll

Negative GM and angle of loll are two related concepts that can occur in a ship's stability.

Negative GM:

GM, or metacentric height, is a measure of a ship's stability. If the GM is negative, it means that the ship's center of gravity (CG) is above its metacenter (M). This condition is unstable because the restoring force that acts on the ship when it is heeled is less than the destabilizing force. As a result, the ship will tend to capsize if it is heeled even slightly.

Angle of Loll:

Angle of loll refers to a situation where a ship has a negative GM and is heeled to one side until it becomes stable again. In this state, the ship's hull is inclined to one side and the waterline is no longer parallel to the deck. The angle of loll occurs because the buoyancy force acting on the submerged portion of the hull shifts to the side of the center of gravity, creating a torque that keeps the ship in a stable position.

While the angle of loll can stabilize a ship with a negative GM, it is not a desirable state because it can impair the ship's maneuverability and increase its resistance to forward motion, causing it to use more fuel.

In summary, a negative GM indicates an unstable condition for a ship's stability, and if the ship is heeled, it can lead to capsize. The angle of loll is a stable condition that can occur in a ship with a negative GM, but it is not ideal because it can affect the ship's performance.

By capt. NJS

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