Class 4 (4th Engineer) SC&S 📅 May 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 7.45 m above the keel and the centre of buoyancy 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

### (a) Dangers of an Unstable Ship (6 Marks)
An unstable ship is one in a condition of negative metacentric height (GM), where the transverse metacenter (M) is below the center of gravity (G). This condition is extremely dangerous and can lead to the capsize and total loss of the vessel. The principal dangers are:
1. **Creation of a Capsizing Moment:**
* In a stable ship (positive GM), when an external force causes a heel, the forces of buoyancy and gravity create a 'righting moment' that restores the vessel to the upright position.

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