Question 40 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is D **Why option D ("Controllable-pitch propeller") is correct:** When an electricity generating plant features shaft-driven generators (meaning the main engine drives both the propeller shaft and an electrical generator), the prime mover (the engine, typically a medium-speed or slow-speed diesel engine, or a turbine) generally needs to operate at a relatively constant speed or within a narrow, efficient speed band. This constant speed is necessary to maintain the required frequency and voltage for the ship's electrical grid (the generator output). A Controllable-Pitch Propeller (CPP) allows the pitch of the propeller blades to be adjusted without changing the rotational speed (RPM) or direction of rotation of the main engine. This provides full control over the ship's thrust (including ahead, astern, and stopping) while the main engine continues to run at its optimal, constant speed—the speed required for efficient power generation and engine operation. This flexibility makes the CPP the most practicable choice for ships using shaft generators. **Why the other options are incorrect:** * **A) Fixed-pitch propeller (FPP):** With an FPP, thrust and ship speed are directly controlled by changing the engine's speed (RPM). If the engine speed is constantly varied to maneuver the ship, it becomes impossible for the shaft generator to consistently maintain the required electrical frequency and voltage, making the operation of the ship's electrical system highly unstable or impossible without resorting to running auxiliary generators constantly, defeating the purpose of the shaft generator. * **B) Tandem propeller:** While tandem propellers (two propellers mounted one after the other on the same shaft) offer potential benefits in efficiency or power absorption, they are still typically fixed-pitch or controllable-pitch. The tandem configuration itself does not solve the fundamental problem of needing variable thrust at constant engine RPM, which is mandatory when shaft generators are in use. * **C) Detachable-blade (built-up) propeller:** This term refers to a construction method for a fixed-pitch propeller, where the blades are bolted onto a central hub, often used for repairability or large propellers. Functionally, it still operates as a fixed-pitch propeller (FPP) once installed, meaning it suffers the same operational limitations as Option A when paired with a shaft generator.