GZ curve

The $GZ$ curve plots the righting arm against heel angle and is the central diagram of intact stability. Its slope at the origin equals $GM$, the area beneath it measures dynamic stability (energy to heel the ship), and it crosses zero at the angle of vanishing stability. The IS Code 2008 sets quantitative criteria on the curve: area to 30 degrees at least 0.055 m-rad, area to 40 degrees at least 0.090 m-rad, and maximum $GZ$ of at least 0.20 m occurring at 25 degrees or more.