The overwhelming majority of engine-driven craft use propellers to convert the power of the engine to useful work. There are plenty of alternatives which may have advantages for specific applications, but propeller systems are good all-rounders that are reasonably cheap, simple, reliable, efficient and easy to use.

They suffer, however, from one potentially significant drawback, which is that a large slow turning propeller is generally more efficient than a small fast-spinning one. Even on small pleasure craft, where the maximum size of propeller is often limited by hull shape, the optimum shaft speed is usually in the order of 1000 rpm - only a half or a quarter of the speed of the engine. Running the engine more slowly isn't the answer, because you'd just lose most of the power you've paid for by buying it in the first place. What you need is something that will reduce the shaft speed but increase its torque (turning effort).

It's also very useful to be able to reverse the direction of rotation, to provide astern power to stop the boat or make it go backwards, or to fit counter-rotating propellers on a twin-screw boat. Again, there are alternatives such as variable-pitch propellers whose blades swivel on the hub, scoop-like deflectors, or even boat engines that can be stopped and restarted in the opposite direction, but by far the most popular arrangement is a reversing marine gearbox.

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