MCA CoC Ship Stability 2 — Questions and Answers
Question 1: What information does the GZ curve (curve of statical stability) provide to the Master?
- The vessel's speed at different draughts
- The vessel's fuel consumption at varying displacements
- The righting lever at different angles of heel, showing range of stability, maximum GZ, and angle of vanishing stability (Correct answer)
- The vessel's trim at different loading conditions
Correct answer: The righting lever at different angles of heel, showing range of stability, maximum GZ, and angle of vanishing stability
The GZ curve plots the righting lever (GZ) against the angle of heel. It shows: the initial GM (slope at origin), the maximum righting lever and the angle at which it occurs, the range of stability (from 0° to the angle of vanishing stability where GZ returns to zero), and the dynamical stability (area under the curve). This is essential for assessing the vessel's ability to resist capsizing.
Question 2: Under the MCA's stability criteria (based on the IMO Intact Stability Code), what is the minimum required GM for a cargo vessel in the departure condition?
- 0.05 metres
- 0.15 metres (Correct answer)
- 0.50 metres
- 1.00 metre
Correct answer: 0.15 metres
The IMO Intact Stability Code (incorporated by the MCA) requires a minimum initial GM of 0.15 metres in all loading conditions. Additionally, the area under the GZ curve up to 30° must be at least 0.055 metre-radians, and the maximum GZ must occur at an angle of heel preferably exceeding 25° but not less than 25° for most vessel types.
Question 3: What is 'angle of loll' and how does it differ from a list caused by asymmetric loading?
- Angle of loll is a permanent structural deformation; list is temporary
- Both are the same condition with different names
- Angle of loll occurs when GM is negative and the vessel heels to find a new equilibrium; list is caused by G being off-centreline due to weight distribution (Correct answer)
- Angle of loll only occurs in beam seas; list occurs in calm water
Correct answer: Angle of loll occurs when GM is negative and the vessel heels to find a new equilibrium; list is caused by G being off-centreline due to weight distribution
A list is caused by the centre of gravity being off the vessel's centreline (asymmetric loading), and can be corrected by redistributing weight. An angle of loll occurs when GM is negative — the vessel is initially unstable and heels until the waterplane area increases sufficiently for the metacentre to rise above G, creating a new, precarious equilibrium. Loll is corrected by lowering G (adding low weight, not shifting weight).
Question 4: When a vessel grounds at low water on an even keel, what is the primary stability concern?
- The vessel's draught will increase
- The upward ground reaction force reduces displacement, effectively raising G above M and potentially causing the vessel to heel dangerously (Correct answer)
- The hull will immediately flood through bottom damage
- The cargo will shift due to the impact
Correct answer: The upward ground reaction force reduces displacement, effectively raising G above M and potentially causing the vessel to heel dangerously
When a vessel grounds, the seabed exerts an upward force (ground reaction) that reduces the effective displacement. This is equivalent to removing buoyancy, which raises the effective KG relative to the metacentre. If the ground reaction is large enough, the effective GM can become negative, causing the vessel to become unstable and heel to one side (loll), potentially with catastrophic consequences.
Question 5: What is the 'moment to change trim by one centimetre' (MCTC or MCT1cm) and how is it used?
- The amount of fuel needed to alter the vessel's speed by one knot
- The moment required to change the vessel's total trim by one centimetre — used to calculate the effect of loading, discharging, or shifting weights on trim (Correct answer)
- The force needed to turn the rudder by one degree
- The weight needed to increase draught by one centimetre
Correct answer: The moment required to change the vessel's total trim by one centimetre — used to calculate the effect of loading, discharging, or shifting weights on trim
MCTC (moment to change trim by 1 cm) is the longitudinal moment (in tonne-metres) required to change the vessel's total trim by one centimetre. It is used to calculate the change in trim when weights are loaded, discharged, or shifted longitudinally. The formula is: Change in trim (cm) = (w × d) / MCTC, where w is the weight and d is the distance from the centre of flotation.
Question 6: A vessel displacing 10,000 tonnes has KM = 8.5 m and KG = 7.2 m. A weight of 500 tonnes is shifted vertically upward by 4 metres. What is the new GM?
- 1.10 metres (Correct answer)
- 0.80 metres
- 1.30 metres
- 1.11 metres
Correct answer: 1.10 metres
Original GM = KM - KG = 8.5 - 7.2 = 1.30 m. Rise of G due to shifting weight upward: GG1 = (w × d) / W = (500 × 4) / 10,000 = 0.20 m. New KG = 7.2 + 0.20 = 7.40 m. New GM = 8.5 - 7.40 = 1.10 m. The metacentric height has decreased because the centre of gravity has risen, reducing initial stability.
MCA CoC Ship Stability 2
What information does the GZ curve (curve of statical stability) provide to the Master?