Analyzing the data of vessels that behaved well, and especially the data of vessels that did not survive adverse conditions, various researchers and regulatory authorities defined criteria for deciding if the stability of a vessel is satisfactory. Therefore, it is important to understand that the existing stability regulations are codes of practice that provide reasonable safety margins without giving 100% guaranty that the vessel which meets the requirements can survive all challenges.
According to the International Code on Intact stability, 2008, the following criteria are mandatory for passenger and cargo ships constructed on or after 1st January 2010:
1. The area under the righting lever curve (GZ curve) should not be less than 0.055 metre-radians up to 30° angle of heel.
2. The area under the righting lever curve (GZ curve) should not be less than 0.09 metreradians up to 40° angle of heel or the angle of downflooding if this is less than 40°.
3. The area under the righting curve between the angles of heel of 30° and 40° or between 30° and the angle of downflooding if this angle is less than 40°, should not be less than 0.03 metre-radians.
4. The righting lever GZ should be at least 0.20 m at an angle of heel equal to or greater than 30°.
5. The maximum righting arm should occur at an angle of heel preferably exceeding 30° but not less than 25°.
6. The initial metacentric height GMo should not be less than 0.15 m.
7. Severe wind and rolling criterion (weather criterion)
In addition to the criteria described above, ships covered by the 2008 IS Code should meet a weather criterion that considers the effect of strong beam wind and waves applied when the vessel is in dead ship condition.
Further reading: “Ship Stability in Practice”