Question 64 QMED01 - Junior Engineer

The Correct Answer is B **Explanation of Correctness (Option B):** When a slow-speed, two-stroke diesel engine (common for main propulsion) is directly coupled to a fixed-pitch propeller, the rotation continues even after fuel is cut off due to the propeller being driven by the forward motion (way) of the ship—a phenomenon known as trailing or windmilling. To stop this rotation quickly upon a "stop" order, two primary methods are employed, depending on the engine's operational status, the urgency, and specific company/vessel procedure: 1. **Admitting Start Air in Reverse (Counter-Scavenging):** This is the most common and standard procedure, especially when maneuvering. Start air (compressed air) is admitted to the cylinders at the appropriate timing for reverse rotation. This highly effective torque opposes the propeller's rotation and rapidly brings the engine to a halt. 2. **Applying the Shaft Brake:** The shaft brake is a mechanical system used to hold the shaft still once rotation has stopped, or, less commonly, to help stop light windmilling. However, applying the brake while the engine is still rotating at significant speed due to way can generate excessive heat and wear on the brake pads and potentially damage the thrust block or engine structure. Therefore, the shaft brake is often used only after the reverse air has nearly or completely stopped the rotation, or when the way is very slow. Because both methods (reverse air and shaft brake application) are utilized depending on the specific circumstances (speed of rotation, amount of way, and engine condition), Option B accurately reflects the flexibility and common practice in maritime operations. **Explanation of Incorrect Options:** * **A) In all cases, the engine rotation is stopped by applying the shaft brake:** This is incorrect. Applying the shaft brake while the engine is turning significantly (due to substantial ship's way) is avoided because it causes excessive wear, heat generation, and potential damage. Reverse air is usually the primary stopping method. * **C) In all cases, the engine is allowed to rotate until it eventually stops:** This is incorrect. Allowing the engine to windmill until it stops naturally could take a considerable time, which is unacceptable during maneuvering or emergency stopping procedures. Immediate stopping is required to prepare for starting in the opposite direction. * **D) In all cases, start air is admitted to the engine in the opposite direction of rotation until the engine stops rotating:** This is incorrect. While reverse air is the primary method, the shaft brake is often used in conjunction (either to assist in the final stop or to hold the shaft stationary afterward). Furthermore, the air method may not be feasible if air pressure is critically low, necessitating the use of the brake (if speed is low enough). Therefore, stating that reverse air is used *in all cases* excludes the viable and necessary application of the shaft brake.