Q: Why does GM appear on the RA plots sometimes but not at other times?

A: The GM should appear on the RA plot whenever the RA curve includes an angle of equilibrium. Since GM is the slope of the RA curve at equilibrium it is normally plotted tangent to the curve at such an angle.

Another case where GM does not appear is when the displacement varies due to certain tank modes such as spilling. In this case the righting moment curve is plotted and it would be unnatural to plot GM on the same graph.

See the sample run file RA-GM.RF for a demonstration of various cases where GM is plotted and not plotted.