What does a negative GM indicate about a vessel?

A negative GM (metacentric height) indicates that the center of gravity (CG) of the vessel is higher than the metacentric height (M). This situation typically suggests that the vessel has a very poor stability profile and is more prone to capsizing.

When GM is negative, it signifies that the righting arm (the distance between the center of gravity and the center of buoyancy) is non-existent or reversed, meaning that when the vessel lists to one side, there is no effective force to return it to an upright position. Instead, the vessel may continue to heel further over, increasing the risk of capsize. Conversely, a vessel with positive GM has a sufficient righting arm that helps it return to an upright position after being heeled by external forces such as wind or waves.

In terms of vessel design and stability, having a negative GM is regarded as extremely dangerous and undesirable, reinforcing the reasoning behind the selection of this answer.