Question 67 OSE01 - Chief Engineer - OSV

The Correct Answer is C ### 2. Why Option C is Correct **C) This type of reduction gear is used with a fixed pitch propeller and a non-reversing engine.** This statement describes a setup where the prime mover (the engine, such as a medium-speed diesel or gas turbine) only rotates in one direction (non-reversing). When paired with a **Fixed Pitch Propeller (FPP)**, the only way to achieve astern thrust is to reverse the rotation of the propeller shaft. Therefore, the reduction gear unit must be designed to not only reduce the engine speed to the desired propeller speed but also to incorporate a mechanism (typically clutches and idler gears) that allows the operator to select ahead or astern operation, changing the direction of the shaft rotation while the engine continues to run unidirectionally. ### 3. Why Other Options Are Incorrect **A) This type of reduction gear is used with a controllable pitch propeller and a reversing engine.** * **Incorrect.** If the engine is reversible, the reduction gear does not need to include reversing mechanisms. If a Controllable Pitch Propeller (CPP) is used, reversing thrust is primarily achieved by changing the propeller pitch, making the reversing capability of the engine/gear largely redundant for maneuvering. **B) This type of reduction gear is used with a fixed pitch propeller and a reversing engine.** * **Incorrect.** If the engine is reversing (e.g., a slow-speed main engine), it handles the change in shaft direction. The reduction gear in this scenario only needs to reduce speed and does not require complex integrated reversing clutches or gears. **D) This type of reduction gear is used with a controllable pitch propeller and a non-reversing engine.** * **Incorrect.** This is a very common arrangement, but the reduction gear required is simpler. Since the propeller pitch controls forward/astern thrust (CPP) and the engine rotation is constant (non-reversing), the reduction gear only needs to perform speed reduction. It does not need to incorporate complex reversing mechanisms itself. The reduction gear required for C is mechanically more complex than the one required for D.