Question

(a) Explain the effects on stability when a tank is partially filled with liquid. (6)

(b) A ship of 11200 tonne displacement has a double bottom tank containing oil, whose centre of gravity is 16.5m forward and 6.6m below the centre of gravity of the ship. When the oil is used the ship's centre of gravity moves 380mm. Calculate: (10)

(i) The mass of oil used

(ii) The angles which the centre of gravity moves relative to the horizontal.

Answer 1

Part (a)

Slack tanks, or partially filled tanks, significantly impact a ship's stability due to the free surface effect

When tanks are partially filled (slack), the liquid inside moves freely as the ship heels.
This movement shifts the centre of gravity (G) laterally towards the heeling side.
The righting lever (GZ) decreases, resulting in reduced metacentric height (GM) and overall stability.
The virtual loss of GM is proportional to the breadth of the tank and the height of the free surface.

Consequences of Slack Tanks:

Increased angle of heel.
Reduced stability, making the ship tender and more prone to capsizing.

To Minimize the Effect:

Avoid slack tanks where possible; either fill tanks completely or empty them.
Design tanks with longitudinal divisions or swash bulkheads to limit liquid movement.
Use sluice valves in tanks to control liquid transfer.
Prioritize filling smaller tanks at the ship's bottom to lower G and improve stability.

✓ Verified Model Answer (Text Solution)

Slack tanks, or partially filled tanks, significantly impact a ship's stability due to the free surface effect