SAMS Vessel Stability and Naval Architecture 1 — Questions and Answers
Question 1: What does metacentric height (GM) measure in vessel stability?
- The vertical distance from keel to waterline
- The vertical distance from the center of gravity to the metacenter (Correct answer)
- The horizontal distance between the center of buoyancy and metacenter
- The distance from the waterline to the main deck
Correct answer: The vertical distance from the center of gravity to the metacenter
GM is the vertical distance from the center of gravity (G) to the metacenter (M) and is the primary indicator of a vessel's initial transverse stability.
Question 2: A vessel with a negative GM will:
- Have excessive initial stability and a very stiff roll
- List permanently to one side and risk capsizing (Correct answer)
- Float higher than its designed waterline
- Develop excessive trim by the stern
Correct answer: List permanently to one side and risk capsizing
A negative GM means G is above M, making the vessel unstable; it will heel to one side (angle of loll) and may capsize if the condition is not corrected.
Question 3: The center of buoyancy (B) is located at:
- The geometric center of the entire vessel hull above and below water
- The centroid of the underwater displaced volume of the vessel (Correct answer)
- The midpoint between the keel and the waterline on the centerline
- The point where the vessel's waterline intersects the hull sides
Correct answer: The centroid of the underwater displaced volume of the vessel
The center of buoyancy is the centroid of the displaced water volume—the geometric center of the submerged portion of the hull—where the buoyant force acts upward.
Question 4: Freeboard is defined as:
- The distance measured from the keel to the waterline
- The vertical distance from the waterline to the upper deck edge (Correct answer)
- The weight of cargo a vessel can carry above its load line
- The reserve buoyancy capacity expressed as a percentage of displacement
Correct answer: The vertical distance from the waterline to the upper deck edge
Freeboard is the vertical distance from the waterline to the freeboard deck edge, and its minimum value is regulated by international load line conventions to ensure reserve buoyancy.
Question 5: The righting lever (GZ) in a stability analysis represents:
- The horizontal distance between vertical lines through G and B when the vessel is heeled (Correct answer)
- The vertical distance from the keel to the center of gravity at any angle of heel
- The angle at which a vessel will capsize under dynamic loading
- The restoring force per degree of heel expressed in foot-tons
Correct answer: The horizontal distance between vertical lines through G and B when the vessel is heeled
GZ is the horizontal (perpendicular) distance between the vertical line through G and the vertical line through B at a given angle of heel, representing the righting arm per unit displacement.
Question 6: Displacement tonnage of a vessel is equal to:
- The total internal cargo volume measured in long tons
- The weight of water displaced by the vessel, equal to the vessel's total weight (Correct answer)
- The deadweight capacity minus the weight of fuel and stores
- The gross tonnage adjusted by a regulatory coefficient
Correct answer: The weight of water displaced by the vessel, equal to the vessel's total weight
By Archimedes' principle, a floating vessel displaces a weight of water exactly equal to its own total weight, making displacement the most fundamental measure of a vessel's mass.
Question 7: Bilge keels are fitted on a vessel primarily to:
- Reduce rolling motion by creating hydrodynamic resistance as the vessel rolls (Correct answer)
- Protect the hull bottom from grounding damage in shallow water
- Improve directional stability and reduce leeway when underway
- Increase the vessel's speed by streamlining water flow past the hull
Correct answer: Reduce rolling motion by creating hydrodynamic resistance as the vessel rolls
Bilge keels are longitudinal fins welded at the turn of the bilge that create hydrodynamic drag as the hull rolls, effectively damping and reducing the rolling motion.
