Your examiner asks: "What is GM, what does a positive GM indicate, and what are the dangers of a very high or very low GM?" Give a complete OOW answer.

A. GM (metacentric height) is the distance between the centre of gravity (G) and the metacentre (M). A positive GM (M above G) indicates stable equilibrium — the vessel will right itself when heeled. A very low GM risks loll, slow period of roll, and potential capsize. A very high GM (stiff ship) causes a rapid, violent rolling motion that can create excessive dynamic loads on cargo, structures, and crew — which can be as dangerous as low stability.

B. GM is always a positive value. A negative GM means the ship is overloaded and must discharge cargo immediately.

C. GM is the distance between the keel and the metacentre. A higher GM always means a safer, more stable vessel regardless of its magnitude.

D. GM only applies to loaded conditions. In ballast, stability is assessed using only the KG value.

A. Key GZ curve features: range of stability (angle at which GZ returns to zero); maximum GZ and angle of maximum GZ; area under the curve (dynamical stability). IMO IS Code 2008 criteria: area 0-30 degrees minimum 0.055 metre-radians; area 0-40 degrees (or to flooding angle if less than 40) minimum 0.090 metre-radians; area 30-40 degrees minimum 0.030 metre-radians; maximum GZ at least 0.200 metres at 30 degrees or greater; initial GM not less than 0.150 metres.

B. The minimum GM under all criteria is 0.5 metres. The GZ curve must reach a maximum of at least 0.5m at any angle between 20 and 60 degrees.

C. The only IMO requirement is that the angle of vanishing stability must exceed 60 degrees. Area under the curve and initial GM are national flag state requirements only.

D. The IMO IS Code applies to new vessels only. Older vessels built before 2010 comply with the 1966 Load Lines convention stability requirements only.

A. New displacement = 5,050 tonnes. New KG = ((5,000 x 8) + (50 x 12)) / 5,050 = (40,000 + 600) / 5,050 = 40,600 / 5,050 = 8.04 metres. Adding weight above the existing centre of gravity RAISES KG, which REDUCES GM (G rises towards M). This reduces initial stability and the vessel becomes more tender.

B. KG does not change when cargo is loaded on deck — only the KM changes because the draught increases.

C. New KG = (8 + 12) / 2 = 10 metres. Adding weight always averages the KG values.

D. Adding 50 tonnes to a 5,000 tonne vessel is negligible. KG and GM remain unchanged for weight additions less than 1% of displacement.