AB AB Vessel Stability and Trim 1 — Questions and Answers
Question 1: What does metacentric height (GM) measure?
- A vessel's initial transverse stability (Correct answer)
- The vessel's forward draft
- The vessel's speed through water
- The distribution of cargo weight
Correct answer: A vessel's initial transverse stability
Metacentric height (GM) measures a vessel's initial transverse stability; a larger positive GM indicates the ship will resist rolling more strongly.
Question 2: When is a vessel's GM considered 'positive'?
- When the metacenter (M) is above the center of gravity (G) (Correct answer)
- When the center of gravity is above the metacenter
- When the forward and after drafts are equal
- When freeboard is at the minimum allowed
Correct answer: When the metacenter (M) is above the center of gravity (G)
A vessel has positive GM when M is above G, producing a righting moment that returns the vessel toward upright when heeled.
Question 3: What is 'free surface effect'?
- Loss of stability caused by liquid shifting in a partially filled tank (Correct answer)
- The drag effect of waves on the hull
- The buoyancy effect of the waterplane area
- Stability loss caused by topside icing
Correct answer: Loss of stability caused by liquid shifting in a partially filled tank
Free surface effect is the reduction in effective stability caused by liquid in a partially filled tank shifting as the vessel heels, effectively raising G.
Question 4: What is a vessel's 'righting moment'?
- The restoring force that returns a heeled vessel toward upright (Correct answer)
- The force causing a vessel to heel over
- The turning force applied by the rudder
- The holding force of the anchor and chain
Correct answer: The restoring force that returns a heeled vessel toward upright
The righting moment is the couple acting to restore a heeled vessel to upright, equal to the vessel's displacement multiplied by the righting arm (GZ).
Question 5: What happens to a vessel's initial stability when the center of gravity (G) rises?
- Stability decreases as GM is reduced (Correct answer)
- Stability increases as GM grows
- Stability is unaffected by G position
- The vessel trims by the head
Correct answer: Stability decreases as GM is reduced
As G rises, the distance between G and M (GM) decreases, reducing the vessel's initial stability and its resistance to rolling.
Question 6: What does 'list' mean in vessel stability terms?
- A permanent lean to one side caused by off-center weight distribution (Correct answer)
- A temporary rolling motion induced by waves
- The difference in draft between bow and stern
- A sudden change in the vessel's heading
Correct answer: A permanent lean to one side caused by off-center weight distribution
List is a permanent transverse inclination caused by G being off the vessel's centerline due to unequal weight distribution.
AB AB Vessel Stability and Trim 1
What does metacentric height (GM) measure?