MCA CoC Ship Stability

Question 1

What is the metacentric height (GM) and why is it important for ship stability?

Accepted Answer

The vertical distance between the centre of gravity (G) and the metacentre (M) — it measures initial stability

Answer

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?

Accepted Answer

6.10 metres

Answer

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?

Accepted Answer

It is the reduction in effective GM caused by liquid in partially filled tanks moving as the vessel heels

Answer

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?

Accepted Answer

The vessel is in unstable equilibrium and will loll to one side

Answer

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?

Accepted Answer

To determine the vessel's lightship displacement and position of the centre of gravity (KG)

Answer

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.

MCA CoC Practice Test

MCA CoC Practice Test

MCA CoC Ship Stability