Question 62 1AE01 - First Assistant Engineer

The Correct Answer is A ### Explanation of Why Option A is Correct Option A accurately describes the function and purpose of a main propulsion reduction gear (also known as a gearbox) in a marine power train: 1. **Prime Mover (Engine/Turbine) Characteristics:** Engines and turbines (the prime movers) are most powerful and efficient when operating at high rotational speeds (RPM). However, increasing speed generally results in a decrease in the required torque input for a given power output ($P = T \times \omega$, where $P$ is power, $T$ is torque, and $\omega$ is angular velocity/speed). Therefore, the prime mover operates most efficiently at **relatively high speed and low torque**. 2. **Propeller Characteristics:** Propellers, especially large marine propellers, are designed to operate efficiently by moving a large volume of water slowly. High rotational speeds (RPM) on a propeller cause cavitation (formation of vapor bubbles), noise, vibration, and significant energy losses. Efficiency is maximized at **relatively low speed**. Furthermore, to deliver the immense pushing force required to move a ship, the propeller must operate at **high torque**. 3. **Function of the Reduction Gear:** The reduction gear bridges this gap. It takes the high-speed, low-torque rotation from the prime mover and converts it into the low-speed, high-torque rotation required by the propeller, ensuring both components operate within their respective optimum efficiency ranges. ### Explanation of Why Other Options Are Incorrect **B) The reduction gear allows the prime mover to rotate efficiently at relatively high speed and high torque and allows the propeller to rotate efficiently at relatively low speed and low torque.** * **Incorrect:** While the prime mover operates at high speed and the propeller operates at low speed, the torque relationships are wrong. A high-speed engine usually produces lower torque (for a given power), and the propeller *requires* very high torque, not low torque, to move the vessel. **C) The reduction gear allows the prime mover to rotate efficiently at relatively low speed and low torque and allows the propeller to rotate efficiently at relatively high speed and high torque.** * **Incorrect:** This reverses the speed requirements. Prime movers operate best at high speed, and propellers operate best at low speed. Furthermore, operating a large marine propeller at high speed and high torque is highly inefficient and damaging due to cavitation. **D) The reduction gear allows the prime mover to rotate efficiently at relatively low speed and high torque and allows the propeller to rotate efficiently at relatively high speed and low torque.** * **Incorrect:** This configuration completely reverses both the speed and torque requirements for efficient operation of both the prime mover and the propeller.