MCA CoC Stability & Cargo Safety

Question 1

What is the 'metacentre' in ship stability?

Accepted Answer

The point about which a vessel tends to oscillate when heeled to small angles — its height above the keel determines initial stability

Answer

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?

Accepted Answer

The vessel is initially stable — a righting moment will return it to upright when heeled

Answer

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?

Accepted Answer

Free liquid surfaces in tanks reduce effective GM by creating a virtual rise in G — the larger the tank, the greater the reduction

Answer

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?

Accepted Answer

GM must be positive throughout the voyage and meet the specific standards in the vessel's approved stability booklet

Answer

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?

Accepted Answer

It shows the righting lever at each angle of heel — the area under the curve represents the vessel's dynamic stability reserve

Answer

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.

MCA CoC Practice Test

MCA CoC Practice Test

MCA CoC Stability & Cargo Safety