Question 28 QMED01 - Junior Engineer

The Correct Answer is D **Explanation of Option D (Correct Answer):** The main reduction gears (MRG) are a crucial component in the propulsion system of ships powered by steam or gas turbines (or sometimes medium-speed diesel engines coupled to a single propeller shaft). Turbines are inherently high-speed, low-torque machines, operating most efficiently at very high revolutions per minute (RPM), often exceeding 5,000 to 10,000 RPM. However, marine propellers are most efficient at much lower speeds, typically between 80 and 200 RPM, depending on the ship design. The primary function of the MRG is to mechanically **reduce this high turbine RPM to the much lower, efficient propeller RPM** while simultaneously increasing the torque delivered to the propeller shaft. This speed reduction is essential for efficient hydrodynamic propulsion. **Why the Other Options are Incorrect:** * **A) provide a means of reversing the main engines in an emergency:** Reversing the direction of rotation is typically handled either by a separate astern turbine stage built into the main turbine casing (in steam turbine plants), or by using a controllable pitch propeller (CPP), or by coupling the engine to the shaft via a reversing gearbox/clutch system. The main reduction gears themselves are fixed-ratio, speed-reducing devices and do not provide the mechanism for directional reversal. * **B) transmit vibration and thrust to the ship's hull:** While the reduction gears are structurally mounted and transmit torque, their purpose is not to transmit vibration. In fact, they are designed to operate smoothly and minimize vibration. More importantly, the **thrust** generated by the propeller is transmitted to the hull through a separate component called the **thrust bearing** (which is often, but not always, housed within the reduction gear casing, but is functionally distinct from the gears themselves). The purpose of the gears is speed reduction, not thrust transmission. * **C) reduce engine room noise levels during high-speed operations:** Although modern gears are designed for quiet operation, noise reduction is a secondary design consideration (or benefit) stemming from precise manufacturing. The **purpose** of the gears is fundamentally mechanical speed conversion and torque multiplication, not acoustics.