What is a reduction gearbox on a ship and why is it used?

A reduction gearbox exists to reconcile two conflicting preferences: engines are generally more compact and efficient running fast, while propellers, especially large-diameter ones on bigger ships, are generally more efficient turning slowly, so something has to bridge the gap between an engine's comfortable running speed and the propeller's ideal speed.

Medium-speed four-stroke diesels typically run somewhere between 400 and 1000 revolutions per minute, and gas turbines and high-speed engines run faster still, all far too fast to drive a large propeller directly without severe cavitation and efficiency losses. A reduction gearbox, precisely machined and hardened gears between the engine output flange and the propeller shaft, steps this down to the low, sometimes double-digit, rpm range at which a large propeller performs efficiently. Large low-speed two-stroke crosshead engines avoid the problem by design, running slowly enough, often below 100 rpm at full power, to couple directly to the shaft, one reason that engine type dominates on large tankers and bulk carriers where simplicity and direct-drive efficiency matter more than compactness.

Beyond simple speed reduction, the gearbox is often where a ship's propulsion arrangement gets its flexibility. Twin or multiple medium-speed engines can be geared onto a single output shaft through clutches, letting a vessel run on one engine at economical speed and bring others online through the gearbox for full power or manoeuvring, common on ferries, offshore vessels and naval auxiliaries needing a wide operating range. The gearbox housing often also carries a power take-off for a shaft generator, and where a fixed-pitch propeller needs reversible thrust, a reversing gear built into the same unit, since some medium-speed engines cannot run in reverse themselves.

Because the gearbox transmits the full power of one or more engines through a relatively small number of highly loaded gear teeth and bearings, it is built to demanding precision, with teeth typically case-hardened and ground, and its lubrication system, a dedicated oil supply with cooling and fine filtration, is treated as seriously as the main engine's own lubrication, since gear pitting or bearing failure inside a gearbox is expensive, slow to repair, and can take the whole propulsion train out of service. Vibration and noise monitoring, oil sampling for wear metals, and periodic borescope inspection of the gear teeth are standard parts of maintaining a gearbox that a ship depends on.