Question 61 1AE01 - First Assistant Engineer

The Correct Answer is D **Explanation for Option D (Gas turbine) being correct:** Gas turbines operate at extremely high rotational speeds (often exceeding 3,600 RPM and sometimes 10,000 RPM or more) for efficient power generation. To be useful for ship propulsion, this speed must be drastically reduced to the optimal propeller speed (typically 80-150 RPM for large ships). Such a large speed reduction requires significant gear reduction, which is practically achieved using a **double reduction** gearbox. Furthermore, gas turbines require a reduction gear system (geared-drive) because the turbine output shaft is separate from the propeller shaft, unlike some direct-drive systems. **Explanation for the other options being incorrect:** * **A) Slow speed diesel:** These engines are designed to operate at very low speeds (typically 60-120 RPM) and are **directly coupled** to the propeller shaft. They do not use a reduction gearbox (geared-drive) at all, let alone one featuring double reduction. * **B) Medium speed diesel:** These engines operate at higher speeds (typically 300-1,000 RPM) than slow speed diesels and *do* require a gearbox. However, the speed reduction ratio required is relatively modest. A geared-drive system for a medium-speed diesel is usually accomplished with a **single reduction** gear set. * **C) High speed diesel:** Similar to medium-speed diesels, these engines operate at even higher speeds (typically 1,000–2,000 RPM or more) and utilize a reduction gear. While the reduction ratio is greater than for medium-speed engines, it is still often accommodated by specialized, heavy-duty **single reduction** gearing, particularly in smaller applications. Double reduction is generally reserved for the much higher speed ratios necessary to couple a gas turbine to a propeller.