Initial stability

Initial stability is the ship’s resistance to small angles of heel, governed by the metacentric height GM rather than the full righting-arm curve. For heel up to roughly 7 to 10 degrees the righting arm is approximately GM * sin(theta), so a positive GM gives positive initial stability and sets the roll period through T = 2 * pi * k / sqrt(g * GM), where k is the roll radius of gyration. The IMO 2008 IS Code requires a corrected GM of at least 0.15 m for most ships. A large GM stiffens the roll and shortens its period; a small GM makes the ship tender. Initial stability says nothing about large-heel behavior, which the GZ curve covers.