Class 4 (4th Engineer) SC&S 📅 Oct 2022

Exam Question

(a) Show by means of a diagram how the centre of gravity of the liquid in a partly filled tank moves during rolling (6)

(b) The centre of gravity of a ship of 5000 tonnes displacement is 6m above the keel and 1.5m forward of midships. Calculate the new position of the centre of gravity if 500 tonnes of cargo are placed in the tween decks 10m above the keel and 36m aft of midships (10)

Reference Answer

### Part (a): Movement of Centre of Gravity in a Partly Filled Tank
#### Concept: The Free Surface Effect
When a vessel with a slack (partly filled) tank rolls, the liquid within the tank moves towards the low side. This movement of a mass of liquid causes the centre of gravity of that liquid (denoted as 'g') to shift to a new position ('g1') in the direction of the heel.
This shift of the liquid's centre of gravity (g to g1) causes the ship's overall centre of gravity (G) to also shift in the same direction, parallel to gg1, to a new position G1. From the perspective of stability, this horizontal shift of G has the same effect as if the centre of gravity had been raised to a higher point, known as the 'virtual centre of gravity' (Gv).

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