Question 48 OSE02 - Assistant Engineer - OSV

The Correct Answer is B **Explanation for Option B (Correct Answer):** The illustration (MO-0142, commonly representing a traditional reduction gear system in marine engineering) depicts a propulsion reduction gear set designed to transfer power from a high-speed engine (like a diesel engine or gas turbine) to a low-speed propeller shaft. When this type of gear system (which typically includes an **astern element** or a **reversing mechanism** within the gearbox itself) is used, it allows the engine to always rotate in the same direction (a **non-reversing engine**). The reduction gear handles the speed reduction and also the direction change necessary for reversing the vessel's thrust. This setup is inherently required when driving a **fixed pitch propeller (FPP)**, as an FPP can only generate astern thrust if the shaft direction is reversed. Therefore, this type of reduction gear is used with a fixed pitch propeller and a non-reversing engine. **Explanation of Incorrect Options:** * **A) This type of reduction gear is used with a fixed pitch propeller and a reversing engine.** If the engine itself is a reversing engine (meaning the engine physically stops and starts rotation in the opposite direction for astern movement), the reduction gear set would typically not require a built-in reversing mechanism, as the engine handles the change in direction. While theoretically possible, it is redundant and not the typical configuration for complex reduction gears shown. * **C) This type of reduction gear is used with a controllable pitch propeller and a non-reversing engine.** When a **controllable pitch propeller (CPP)** is used, the pitch of the blades is changed to reverse thrust or adjust speed. The engine and propeller shaft always rotate in the same direction. Consequently, the reduction gear set does not require a reversing mechanism; it is a simple reduction gear (forward only). * **D) This type of reduction gear is used with a controllable pitch propeller and a reversing engine.** This combination is highly impractical and redundant. A CPP eliminates the need for shaft reversal, and a reversing engine is unnecessary with a CPP.