MCA CoC Stability & Cargo Safety 1 — Questions and Answers
Question 1: What is the 'metacentre' in ship stability?
- The point about which a vessel tends to oscillate when heeled to small angles — its height above the keel determines initial stability (Correct answer)
- The lowest point of the vessel's hull
- The centre of gravity of the vessel
- The mid-point of the waterplane area
Correct answer: The point about which a vessel tends to oscillate when heeled to small angles — its height above the keel determines initial stability
The metacentre (M) is the point about which a ship rotates at small angles of heel. When M is above G (centre of gravity), the ship is stable (positive GM). When G rises above M, the ship is unstable and will capsize.
Question 2: What does a positive GM (metacentric height) indicate?
- The vessel is initially stable — a righting moment will return it to upright when heeled (Correct answer)
- The vessel is unstable and likely to capsize
- The vessel has too much ballast loaded
- The vessel's stability is at the minimum legal requirement
Correct answer: The vessel is initially stable — a righting moment will return it to upright when heeled
A positive GM (M above G) means the vessel is in stable equilibrium. When heeled, the buoyancy force acts through B1 to create a righting lever (GZ) which returns the vessel upright. Negative GM (G above M) creates a capsizing moment.
Question 3: What is the effect of free surface on stability?
- Free liquid surfaces in tanks reduce effective GM by creating a virtual rise in G — the larger the tank, the greater the reduction (Correct answer)
- Free surface increases stability by moving ballast to the low side
- Free surface has no effect on vessels above 500 GRT
- Free surface increases initial stability but reduces range of stability
Correct answer: Free liquid surfaces in tanks reduce effective GM by creating a virtual rise in G — the larger the tank, the greater the reduction
Free surfaces of liquid in partially-filled tanks shift when the vessel heels, effectively raising the centre of gravity and reducing GM. The free surface effect depends on the breadth of the tank (cubed) — wide slack tanks have a disproportionately large effect.
Question 4: What is the minimum GM requirement under MCA stability regulations for most UK commercial vessels?
- GM must be positive throughout the voyage and meet the specific standards in the vessel's approved stability booklet (Correct answer)
- Any positive GM is acceptable regardless of magnitude
- GM of at least 1.0 metres is always required
- GM is a guideline only — MCA does not set minimum values
Correct answer: GM must be positive throughout the voyage and meet the specific standards in the vessel's approved stability booklet
The MCA requires vessels to comply with the stability criteria in their approved Stability Booklet, which sets minimum values for GM, maximum angle of heel under wind, GZ curve area, and range of stability. These vary by vessel type and service.
Question 5: What is the GZ curve (righting lever curve) used for in stability assessment?
- It shows the righting lever at each angle of heel — the area under the curve represents the vessel's dynamic stability reserve (Correct answer)
- It plots the vessel's draft against displacement
- It shows the trim of the vessel at different loading conditions
- It shows the effect of ballasting on the vessel's speed
Correct answer: It shows the righting lever at each angle of heel — the area under the curve represents the vessel's dynamic stability reserve
The GZ curve plots the righting arm (distance between the buoyancy and gravity force lines) against the angle of heel. The area under the curve up to the angle of vanishing stability represents the vessel's dynamic stability — its resistance to capsizing.
Question 6: What precautions should be taken when loading a large heavy item on deck?
- Calculate the effect on GM, lashing and securing the item properly, and ensure the weight does not exceed deck loading capacity (Correct answer)
- No calculation is needed if the crew can carry it on board
- Place the item as high as possible for easy access
- Secure with at least two ropes from the same side
Correct answer: Calculate the effect on GM, lashing and securing the item properly, and ensure the weight does not exceed deck loading capacity
Deck cargo raises the vessel's centre of gravity, reducing GM and potentially creating negative GM if excessive. The stability effect must be calculated, the item properly secured with appropriate lashings, and the deck loading capacity (tonnes/m²) must not be exceeded.
