MCA CoC Ship Stability — Questions and Answers
Question 1: What is the metacentric height (GM) and why is it important for ship stability?
- The distance from the waterline to the deck — it determines freeboard
- The vertical distance between the centre of gravity (G) and the metacentre (M) — it measures initial stability (Correct answer)
- The distance between the keel and the centre of buoyancy — it determines draught
- The horizontal distance between the centre of gravity and the centre of buoyancy — it determines trim
Correct answer: The vertical distance between the centre of gravity (G) and the metacentre (M) — it measures initial stability
GM (metacentric height) is the vertical distance between the vessel's centre of gravity (G) and the metacentre (M). A positive GM means the vessel is in stable equilibrium and will return to the upright position after being heeled. A larger GM indicates greater initial stability (stiffer vessel), while a smaller GM means less initial stability (tender vessel).
Question 2: A vessel has a displacement of 8,000 tonnes. A weight of 200 tonnes is loaded 10 metres above the keel. The original KG was 6.0 metres. What is the new KG?
- 6.10 metres (Correct answer)
- 6.20 metres
- 6.00 metres
- 5.90 metres
Correct answer: 6.10 metres
Using the moments about the keel: Original moment = 8000 × 6.0 = 48,000 tm. Loaded weight moment = 200 × 10 = 2,000 tm. New total moment = 50,000 tm. New displacement = 8,200 tonnes. New KG = 50,000 / 8,200 = 6.098 m, approximately 6.10 metres. Loading weight above the original KG raises the overall centre of gravity.
Question 3: What is the 'free surface effect' and how does it affect a vessel's stability?
- It is the effect of waves on the vessel's hull, increasing resistance
- It is the reduction in effective GM caused by liquid in partially filled tanks moving as the vessel heels (Correct answer)
- It is the increase in stability when ballast tanks are completely full
- It is the effect of wind pressure on the vessel's superstructure
Correct answer: It is the reduction in effective GM caused by liquid in partially filled tanks moving as the vessel heels
Free surface effect occurs when a tank is partially filled with liquid. As the vessel heels, the liquid moves to the low side, effectively raising the centre of gravity and reducing the metacentric height (GM). This reduces stability. The effect depends on the tank's breadth (proportional to breadth cubed) and the liquid's density, but is independent of the amount of liquid.
Question 4: What does a negative GM indicate about a vessel's condition?
- The vessel is in stable equilibrium and will return upright
- The vessel has excessive stability and will roll sharply
- The vessel is in unstable equilibrium and will loll to one side (Correct answer)
- The vessel has insufficient freeboard for the cargo loaded
Correct answer: The vessel is in unstable equilibrium and will loll to one side
A negative GM means the centre of gravity (G) is above the metacentre (M), placing the vessel in unstable equilibrium. The vessel will heel to one side (loll) until she reaches an angle where the metacentre rises above G and a new equilibrium is found. This is a dangerous condition requiring immediate corrective action such as filling double-bottom tanks or lowering weights.
Question 5: What is the purpose of the inclining experiment carried out on a vessel?
- To test the watertight integrity of the hull
- To determine the vessel's lightship displacement and position of the centre of gravity (KG) (Correct answer)
- To measure the vessel's speed and engine power
- To calculate the vessel's maximum cargo capacity
Correct answer: To determine the vessel's lightship displacement and position of the centre of gravity (KG)
The inclining experiment (or test) is required by the MCA and classification societies to accurately determine the lightship displacement and the vertical position of the centre of gravity (KG). Known weights are moved transversely across the deck and the resulting angle of heel is measured using a pendulum, allowing calculation of GM and hence KG.
Question 6: A vessel lists 5° to port after loading cargo. The Master suspects asymmetric loading. What is the most appropriate first corrective action?
- Ballast the starboard double-bottom tanks to maximum capacity
- Transfer ballast from port to starboard tanks (Correct answer)
- Discharge cargo from the port side
- Increase speed to reduce the list through dynamic effects
Correct answer: Transfer ballast from port to starboard tanks
Transferring ballast from port to starboard tanks is the safest and most practical first corrective action because it does not change the total displacement and can be done while at sea. Filling starboard tanks only (without emptying port tanks) would add weight and increase draught. Discharging cargo is impractical at sea. The transfer moves the centre of gravity to starboard, correcting the list.
MCA CoC Ship Stability
What is the metacentric height (GM) and why is it important for ship stability?