CMI Vessel Stability & Structural Integrity 1 — Questions and Answers
Question 1: What does metacentric height (GM) measure in vessel stability analysis?
- The distance between the keel and the waterline at midship
- The righting moment acting on a stationary vessel
- The distance between the center of gravity (G) and the metacenter (M) (Correct answer)
- The height of the center of buoyancy above the keel
Correct answer: The distance between the center of gravity (G) and the metacenter (M)
GM is the vertical distance between the center of gravity (G) and the metacenter (M); a positive GM indicates the vessel will return upright after heeling, making it the primary measure of initial stability.
Question 2: How does a vessel in a 'loll' condition differ from one with a 'list'?
- Loll results from off-center weight placement; list results from negative GM
- Loll results from negative GM; list results from off-center weight or asymmetric loading (Correct answer)
- Both loll and list are caused by negative metacentric height
- List results from free surface effect; loll results exclusively from cargo shift
Correct answer: Loll results from negative GM; list results from off-center weight or asymmetric loading
Loll arises when GM is negative (the vessel is unstable upright and seeks equilibrium at a heel angle), whereas list is caused by a transverse shift in the center of gravity from off-center weight distribution.
Question 3: What does the GZ curve (righting lever curve) represent in a vessel's stability diagram?
- The angle of heel under wind loading at various sea states
- The metacentric height variation across different drafts
- The righting lever (arm) distance at various angles of heel (Correct answer)
- The buoyancy force distribution along the hull length
Correct answer: The righting lever (arm) distance at various angles of heel
The GZ curve plots the righting lever—the horizontal distance between the lines of action of buoyancy and gravity—at successive angles of heel, providing a complete picture of the vessel's stability across its range of inclination.
Question 4: Under the IMO Intact Stability Code (IS Code 2008), what is the minimum required area under the GZ curve from 0° to 40° for passenger vessels?
- 0.030 m·rad
- 0.055 m·rad
- 0.090 m·rad (Correct answer)
- 0.120 m·rad
Correct answer: 0.090 m·rad
The IS Code (Part A, mandatory under SOLAS) requires the area under the GZ curve from 0° to 40° (or to the angle of flooding if less) to be not less than 0.090 m·rad, ensuring adequate dynamic stability reserves.
Question 5: What is the 'free surface effect' in vessel stability?
- The stabilizing contribution of ballast water in fully loaded tanks
- The reduction in effective GM caused by transverse movement of liquid in partially filled (slack) tanks (Correct answer)
- The effect of wave action on a vessel's external waterplane area
- The stability benefit derived from increased waterplane area at larger heel angles
Correct answer: The reduction in effective GM caused by transverse movement of liquid in partially filled (slack) tanks
Free surface effect reduces effective GM because liquid in partially filled tanks shifts transversely when the vessel heels, raising the virtual center of gravity and diminishing the righting moment.
Question 6: When investigating a capsizing incident, what should a CMI investigator assess as the first priority?
- Engine room maintenance logs and fuel consumption records
- Navigation charts, GPS track, and voyage plan
- Stability booklet and the actual loading condition at the time of the incident (Correct answer)
- Crew certification records and watch schedule
Correct answer: Stability booklet and the actual loading condition at the time of the incident
The stability booklet and the reconstructed loading condition at the time of the incident are primary evidence for determining whether the vessel had adequate stability margins, which is the central question in any capsizing investigation.
Question 7: The International Load Line Convention (Plimsoll marks) primarily regulates what aspect of vessel safety?
- Maximum allowable crew complement by vessel tonnage class
- Minimum freeboard to ensure adequate buoyancy and stability reserves in different zones and seasons (Correct answer)
- Required liferaft capacity relative to persons on board
- Minimum hull plating thickness by sea area
Correct answer: Minimum freeboard to ensure adequate buoyancy and stability reserves in different zones and seasons
Load line marks indicate the maximum allowable loading depths in various geographical zones and seasonal conditions, ensuring vessels maintain sufficient freeboard to preserve buoyancy and stability reserves against sea conditions.
CMI Vessel Stability & Structural Integrity 1
What does metacentric height (GM) measure in vessel stability analysis?