General HydroStatics
Ship Stability Software
Command of the Week
(New or interesting aspects of GHS that you may not know about)


As shown in COW 86, GHS provides several ways to account for the effect of liquids with free surfaces. The two most common methods are:
1) update tank waterplanes and centers with inclination changes;
2) freeze liquids and raise the Center of Gravity according to the FSM.
We'll refer to these methods as CG-shifts and Free Surface Adjustment (FSA).

Consider that the two methods are only equivalent across a small range of heel angles. As it says in the Training Manual, "FSM is like GM: it gives you the initial slope but does not tell you anything about what happens at greater angles."

Imagine a vessel and load condition with a non-zero equilibrium angle. It could be due to an off-center TCG, an asymmetric hull form, or an external heeling moment. If the equilibrium angle is more than a few degrees, it should not be expected that SOLVE (CG-shifts) and SOLVE /FSM (FSA) would give the same result. As you can see from the Condition Graphics images below, they truly are different models of the vessel.

The method of accounting for free surfaces of liquids by raising the CG is a two-step process. The first step is to tabulate the total free surface moment, and the second is to freeze the liquids and elevate the vessel's total center of gravity.

GHS uses CG-shifts by default, so no special parameters are needed. When a regulation calls for a Free Surface Adjustment, the correct /FSM parameter must be applied to several commands, notably SOLVE and RAH. It is important to be consistent when applying FSM parameters in a run file since different methods should not be mixed. The SOLVE /FSM:UPRIGHT parameter ensures that no matter what the condition at the time of the command, the free surface tabulation and CG elevation is performed at zero heel and zero trim.

One way to ensure consistency is to define a solve macro as follows. (Be sure to call the solve MACRO instead of the command.)


It's worth pointing out that the sequence:

is nearly the same as the following sequence from Help RAH, except for the missing status report:

 HEEL=0 | TRIM=0

Questions, comments, or requests?
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