What is the difference between initial and ultimate stability?

Initial stability and ultimate stability are two important concepts in understanding a vessel's ability to remain upright and recover from heeling. Initial stability is characterized by the vessel's behavior at small angles of heel, which is where the righting lever (or righting arm) is first established as the vessel begins to tilt. The greater the initial stability, the quicker the vessel tends to right itself when it is disturbed at these small angles.

On the other hand, ultimate stability is defined by the maximum righting arm the vessel can achieve. This occurs as the heel increases and the righting arm continues to develop until it eventually reaches its peak. Beyond this point, as the angle of heel further increases, the righting arm begins to decrease, indicating that the vessel is losing its ability to recover.

In summary, the distinction lies in that initial stability pertains to the vessel's performance at small angles of heel, while ultimate stability refers to the point of maximum righting arm before the vessel starts to lose its ability to return to an upright position. This understanding is crucial for ensuring vessel safety and effective maneuvering, especially when operating under various conditions at sea.