Critical KG or GM Analysis

One of the most important tools for addressing ship stability is the derivation of Critical KG or GM curves for different values of draft and trim. These critical values provide a convenient and accurate means of assessing compliance with particular stability regulations.

For a large number of stability criteria it can be extremely time consuming to compute the critical KG or GM values, however Calc is very efficient at this analysis process. In order to perform the analyzes, simply set the relevant Criticals buttons on the Data Input dialog. The program performs the analysis on the last deterministic run carried out in the design. you can choose between either critical KG or critical GM values, determined for intact and/or damaged criteria. The process adopted by the program is to take the GZ curve from the deterministic run and apply a GG'sinq shift until the limiting value for the individual criterion is achieved. The maximum allowable KG is then derived by determining the minimum of the limiting KG values found for the different criteria. Similarly, the minimum allowable GM is derived by determining the maximum of the limiting GM values found for the different criteria. The method of applying a GG'sinq shift, although far faster than working from first principles, does cause a slight discrepancy in that the trim of the vessel with the critical KG would in practice be different from that with the original KG from the deterministic run. To minimize such a problem, you are advised to use a KG value in the deterministic run which is as close as possible to the critical value to be subsequently achieved.

It should be noted that the "shift moment" option as described in Producing Compartment Load Subsets can only be used for loading conditions. When a compartment designated to use the shift option is used in deterministic stability, the shift option is ignored. This is because two different fluid distributions, for the same draft and trim and having the same KGf or GMf, would actually produce different GZ curves if the shift moment were used.

Using the Critical Stability dialog, a tabular matrix of critical KG or GM values can be produced for a range of drafts and trims against a range of criteria, both intact and damaged, although some regulations may only require intact curves.