Exam Question

### (a) Methods for Reversing the Motion of a Ship
The principal methods employed for reversing the direction of thrust to achieve astern motion in a ship are as follows:
1. **Direct Reversing Main Engine:** This method is characteristic of large, low-speed, two-stroke marine diesel engines directly coupled to the propeller. The engine itself is stopped and then restarted in the opposite direction of rotation. This is achieved by altering the timing of the starting air admission and fuel injection to suit astern running. The specific mechanisms for this are detailed in part (b).
2. **Controllable Pitch Propeller (CPP):** In this system, the main engine and propeller shaft run continuously in one direction (ahead). To achieve astern motion, the pitch of the propeller blades is hydraulically altered. The blades rotate about their own axis from a positive (ahead) pitch, through zero pitch (no thrust), to a negative (astern) pitch. This provides rapid and precise control of thrust without stopping or reversing the engine.