Free Surface

GHS can calculate the effects on stability of the liquid contents of tanks in a variety of ways. It automatically finds the centers of gravity of all liquids as they respond to the heel and trim of the ship. This is known as "true C.G. shifts" or sometimes as the "moment of transference method". It is the most realistic way to account for free surface, since it models what truly happens as the ship is heels and trims.

GHS can also use various "free-surface moment" techniques to account for stability degradation due to free surfaces of liquids in tanks. While these methods are not as realistic as true C.G. shifts, they have much tradition behind them and are still often preferred by regulatory agencies.

For the purpose of assigning free surface moments, GHS has several mechanisms which give the user the ability to meet the requirements of various regulations. Often these regulations require not the true free surface moment in a given condition but a "formal" value derived by other means.

GHS allows one or two free-surface moment "methods" to be assigned to each tank. The method may return the true free surface moment in a given condition or a predetermined value such as the maximum free surface moment from a range of tank loadings. If two methods are assigned, the first applies to tank loads from zero up to 95% load while the second method applies at 95% and above.

GHS also provides for "floor" (minimum) values of free surface moment, both overall and with respect to each type of liquid being carried.