Class 4 (4th Engineer) SC&S 📅 Mar 2023

Exam Question

(a) Explain why an unstable ship is dangerous (6)

(b) A vessel travelling at 17 knots turns with a radius of 450 m when the rudder is put hard over. The centre of gravity is 7 m above the keel, the transverse metacentre is 7.45 m above the keel and the centre of buoyancy is 4 m above the keel. If the centripetal force is assumed to act at the centre of buoyancy, calculate the angle of heel when turning. The rudder force may be ignored. (10)

Reference Answer

### Part (a): Dangers of an Unstable Ship (6 Marks)
An unstable ship is one in a condition of negative metacentric height (GM), where the centre of gravity (G) is located above the transverse metacenter (M). This condition is exceptionally dangerous and can lead directly to the capsizing of the vessel.
1. **Initial Instability and Capsizing Moment**:
* When a stable ship (positive GM) is heeled by an external force, the centre of buoyancy (B) shifts, creating a righting lever (GZ) that produces a moment to return the ship to the upright position.

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