Vessel Stability and Construction Flashcards

Which of the following best describes the 'free surface effect' on a vessel?

Answer: The tendency of liquids in partially filled tanks or compartments to move as the vessel rolls, reducing stability.

The free surface effect occurs when liquids in slack (partially filled) tanks or compartments move from side to side as the vessel rolls. This movement of weight reduces the vessel's righting moment and overall stability, making it more susceptible to capsizing.

A vessel's operator notices that after loading new equipment on the top deck, the vessel rolls more quickly and feels 'stiff'. What has most likely happened to the vessel's stability?

Answer: The metacentric height (GM) has been excessively increased.

Placing heavy weight high up on a vessel raises its center of gravity (G). A large distance between the center of gravity and the metacenter (M) results in a large metacentric height (GM), which creates a powerful righting arm. This leads to a quick, snapping roll, often described as being 'stiff' or 'crank'. While stable, this can be uncomfortable and place stress on the vessel and its cargo.

What is the primary purpose of watertight bulkheads in a vessel's construction?

Answer: To divide the hull into watertight compartments to limit the extent of flooding in case of damage.

Watertight bulkheads are vertical partitions that divide the vessel's hull into separate watertight compartments. Their main function is to prevent progressive flooding throughout the vessel if the hull is breached, thereby containing the water and preserving buoyancy and stability.

If a weight of 10 tonnes is shifted from the main deck (Vertical Center of Gravity = 5m) to the lower hold (VCG = 2m), how will this affect the vessel's overall Vertical Center of Gravity (KG)?

Answer: The KG will lower.

Shifting a weight downwards will always lower the vessel's overall Vertical Center of Gravity (KG). The center of gravity of the entire vessel moves in the same direction as the shift of the individual weight. Lowering the KG generally increases the vessel's stability.

Which term refers to the theoretical point above the center of buoyancy through which the force of buoyancy acts when the vessel is heeled?

Answer: Metacenter (M)

The metacenter (M) is the point of intersection of the vertical lines passing through the center of buoyancy in the upright and slightly heeled conditions. For initial stability, the metacenter must be above the center of gravity (G). The distance between G and M is the metacentric height (GM).

According to Transport Canada's requirements for small commercial vessels built after March 31, 2005, which standard must be used for assessing their stability?

Answer: International Standards Organization (ISO) 12217-1.

Transport Canada mandates that new small commercial vessels (built after March 31, 2005) must be assessed for stability using the ISO 12217-1 standard. This standard provides a consistent methodology for evaluating the stability and buoyancy of small craft.