GM limiting curve for operation in brackish water

Onboard a passenger or dry cargo ship, the statutory compliance related to intact and damage stability criteria is most conveniently determined by using GM limiting curves. The actual GM of the loading condition is compared to a required value, read from a curve for the actual draught/trim commbination. The GM limit values are calculated for the full operating range of draught and trim. However, in practice the GM limiting curves do not cover draughts greater than the deepest subdivision draught or the maximum sea water draught of the ship. When the ship is operated on a sea area of brackish water, like the Baltic Sea, there is no common understanding on how to use the GM limiting curves, and it is not clear whether the ship is allowed to submerge the Plimsoll mark. Furthermore, the definition of the required GM value is not clear since the limit curve does not exceed to that draught. The problem affects many ships built according to different editions of the SOLAS. The authorities and classification societies have different approaches, but a common and well-grounded way is needed. The problems are highlighted by presenting real life practical examples.