Stability & Cargo Safety Flashcards

What is the 'metacentre' in ship stability?

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.

What does a positive GM (metacentric height) indicate?

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.

What is the effect of free surface on stability?

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.

What is the minimum GM requirement under MCA stability regulations for most UK commercial vessels?

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.

What is the GZ curve (righting lever curve) used for in stability assessment?

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.

What precautions should be taken when loading a large heavy item on deck?

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.