Question:

When we issue the command 'GHS', the hydrostatic properties shown in the result has a note below stating, "True Free Surface Included". From what we understand, when "True Free Surface Included" is shown, the VCG value in the hydrostatic properties is already corrected with the free surface adjustment. However, in the Righting Arm result, the VCG values in the hydrostatic properties is again added with free a surface adjustment. Please clarify on this.

Answer:

The VCG value in the hydrostatic properties is NOT "corrected" with the free surface adjustment. But the GM and moment-to-trim values are.

In the Righting Arm report, the CG is elevated with the free surface adjustment.

In reality, free surface does not change the VCG at equilibrium.

The "VCG correction" for free surface is a technique for simulating the adverse effect that free surface has on righting arms over a range of heel angles, without actually calculating the new CG at each angle. This technique raises the CG temporarily, during the RA curve calculations to simulate the CG shifts expected.

The GHS command report says "True Free Surface Included" to let the reader know that free surface is included in the GM and moment-to-trim values. GHS views free surface as a waterplane phenomenon and so it appears in the metacenter and total waterplane moments of inertia.

The RA report does not show the total CG because it changes at every heel angle. The RA /FSM report shows the total CG and also the "CG correction" that is being used to simulate the CG movement.