MCA Certificate of Competency Ship Stability and Trim Questions and Answers — Questions and Answers
Question 1: A vessel has a negative initial metacentric height (GM) when upright. What is the resulting condition of the vessel called?
- Stiff
- Tender
- Angle of loll (Correct answer)
- Angle of list
Correct answer: Angle of loll
An angle of loll occurs when a vessel has a negative initial metacentric height (GM), making it unstable when upright. The vessel will heel to one side until the centre of buoyancy (B) moves outboard, increasing the KM and establishing a new, stable equilibrium at a certain angle of heel. This is different from an angle of list, which is caused by an unequal distribution of weight across the vessel.
Question 2: According to the MCA's interpretation of the International Load Line Convention, which of the following is a key intact stability requirement for a general cargo ship?
- The initial metacentric height (GM) must not be less than 0.30m.
- The area under the GZ curve up to 40° must be at least 0.09 metre-radians. (Correct answer)
- The maximum righting arm (GZ) should occur at an angle of heel not less than 45°.
- The angle of heel due to turning must not exceed 15°.
Correct answer: The area under the GZ curve up to 40° must be at least 0.09 metre-radians.
The UK's Maritime and Coastguard Agency (MCA) adheres to the Intact Stability (IS) Code, which specifies several criteria for intact stability. One of these is that the area under the righting lever (GZ) curve shall not be less than 0.09 metre-radians up to a 40° angle of heel or the angle of downflooding if this is less. The minimum initial GM for a general cargo ship is 0.15m.
Question 3: A ship is loading a timber deck cargo and intends to sail with a timber load line. Which of the following stability criteria is specifically required under UK regulations for this condition?
- The initial metacentric height (GM) must not be less than 0.25m.
- The angle of heel due to grain shift must not exceed 12°.
- The maximum righting arm (GZ) must be at least 0.25m. (Correct answer)
- The area under the GZ curve up to 30° must be at least 0.08 metre-radians.
Correct answer: The maximum righting arm (GZ) must be at least 0.25m.
For ships loaded with timber deck cargoes making use of their timber load line, the MCA requires specific stability criteria to be met. One of these is that the maximum value of the righting lever (GZ) should be at least 0.25m. Another key requirement is that the metacentric height (GM) should not be less than 0.10m at all times during the voyage after corrections.
Question 4: A vessel has several slack double-bottom tanks. What is the primary effect of the free surface of the liquid in these tanks on the vessel's stability?
- An increase in the effective metacentric height (GM).
- A reduction in the righting arm (GZ) at all angles of heel. (Correct answer)
- A virtual reduction in the height of the centre of gravity (KG).
- A decrease in the vessel's displacement.
Correct answer: A reduction in the righting arm (GZ) at all angles of heel.
The free surface effect, caused by liquid moving in a partially filled tank, results in a virtual rise of the vessel's centre of gravity (G). This leads to a reduction in the effective metacentric height (GM) and consequently reduces the righting arm (GZ) at all angles of heel, thereby impairing the vessel's stability.
Question 5: When calculating a ship's trim, the Moment to Change Trim by one Centimetre (MCTC) is a critical value. Which of the following factors primarily determines the MCTC?
- The vessel's KG and KM.
- The longitudinal position of the centre of gravity (LCG).
- The vessel's displacement and longitudinal metacentric height (GML). (Correct answer)
- The transverse moment of inertia of the waterplane.
Correct answer: The vessel's displacement and longitudinal metacentric height (GML).
The Moment to Change Trim by one Centimetre (MCTC) is calculated using the formula: MCTC = (Displacement x GML) / (100 x L), where L is the length between perpendiculars. Therefore, it is primarily determined by the vessel's displacement and its longitudinal metacentric height (GML).
Question 6: A UK-flagged vessel is loading bulk grain. According to the International Grain Code as enforced by the MCA, what is the maximum allowable angle of heel due to grain shift?
- 10 degrees
- 15 degrees
- The angle at which the deck edge is immersed
- 12 degrees or the angle of deck edge immersion, whichever is less (Correct answer)
Correct answer: 12 degrees or the angle of deck edge immersion, whichever is less
The International Code for the Safe Carriage of Grain in Bulk (International Grain Code), which is mandatory under SOLAS and enforced by the MCA, states that the angle of heel due to the assumed shift of grain shall not be greater than 12 degrees, or for ships built on or after 1 January 1994, the angle at which the deck edge is immersed, whichever is the lesser value.
MCA Certificate of Competency Ship Stability and Trim Questions and Answers
A vessel has a negative initial metacentric height (GM) when upright.
What is the resulting condition of the vessel called?