| The MAXVCG Process |
| The purpose
of the MAXVCG command is to produce curves of maximum VCG vs. displacement
based on a given stability criterion. The assumption behind this is
that all loadings which produce a given displacement and LCG have
the same maximum VCG as well as having zero TCG. Therefore, when free
surface is involved in a particular load, the VCG must be adjusted
for the free surface effect before comparing it with the maximum VCG.
The use of maximum VCG data should be restricted to loads where the
TCG is zero. The MAXVCG command takes a series of weights and LCGs as inputs and finds the corresponding maximum VCG for each combination of weight and LCG, assuming that the TCG is zero. Alternatively it accepts drafts and trims as inputs which it converts to weights and LCGs using the intact waterplane at zero heel. When trim, rather than LCG, is an input, the program must adjust the LCG in concert with the VCG in order to maintain the given initial trim. This requires additional steps and takes additional time. Since the details of any particular loading are irrelevant during the generation of maximum VCG curves, the present weight and tank load settings are ignored during the process. The present fixed-weight VCG, however, is taken as the "floor" value. If the MAXVCG process finds that a maximum VCG is below the floor, it does not attempt to find its value but merely reports that it is less than the floor value. In addition to displaying a table and graph of the maximum VCG curves, the MAXVCG command causes a "data base" of the maximum VCG information is retained in memory. This data base is also saved on a disk file by means of the SAVE command. Although the data held in memory is lost when the GHS session ends, it may be restored by RUNning the file which was written by the SAVE command. This data base of maximum VCG data is accessible through two commands: 1) a special short form of the MAXVCG command; and 2) the LOAD EDIT command, which is available only when the Load Editor module is included in the GHS installation. When a particular condition has been defined (weights and tank loads) and equilibrium has been found and the displacement, LCG and TCG are within the limits required by the maxvcg data base, then the command MAXVCG (without parameters) will access the data base and report the maximum VCG which corresponds to the present condition. The maximum VCG curves and the data base which result from a MAXVCG command are based on certain factors which must be identically present whenever the curves are applied. These factors include the stability criterion, the heeling moment and the configuration of damage. When any of these factors are changed, the data base becomes invalid and can no longer be accessed. It is possible to produce a composite data base consisting of the lowest maximum VCG values from more than one execution of the MAXVCG command. Details can be found in the MAXVCG command section of the Command Dictionary or by typing HELP MAXVCG at the GHS prompt. When damage is present (tanks with type = FLOODED) the MAXVCG command keeps the tanks in the flooded state but it still assumes that the TCG is zero. This means that the resulting maximum VCG data does not apply to load conditions where the flooded tanks carry any load before damage, since the loss of the load would change the condition. When the assumptions inherent in the maximum VCG curves cannot be satisfied, it is necessary to analyze each case of loading separately. This can be done by beans of the SOLVE MAXVCG command. After setting a load condition (weights and tank loads) the SOLVE MAXVCG command will find the maximum value of the lightship VCG (or the VCG of a particular added weight) which satisfies the present stability criterion. There are no restrictions on the TCG or the tank loading prior to damage. |
| |
