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 gearbox.
Two gearwheels, whose teeth mesh together so that as one turns, the other has to turn as well. The smaller gear has 9 teeth, so if it is turning at 1000 rpm, its teeth are moving at 9000 teeth per minute. The larger wheel is twice the size, and has 18 teeth, so although its teeth must also be moving at 9000 teeth per minute, that means versnellingsbak revisie schiedam only 500 rpm. Note, too, that if the smaller wheel is turning clockwise, the larger wheel must be turning anticlockwise.
Now imagine that you’re using a spanner a foot long to turn the smaller wheel. If you apply an effort of 10 lb to the end of the spanner, you’re applying a torque of 10 lb/ft (10 lb at a radius of 1 ft). The gear-wheel is much smaller – let’s say it has an effective radius of 1 in. That means its teeth must be pushing on the teeth of the other wheel with a force equivalent to 120 lb (120 lb at a radius of 1/12 ft = 120 x 1/12 = 10 lb/ft). The larger wheel has an effective radius of 2 in, so a force of 1201b to its teeth corresponds to a torque of 20 lb/ft (120 x 2/12 = 20 lb/ft).
In other words, by using a 9-tooth wheel to drive an 18-tooth wheel, we’ve halved the speed doubled the torque, and reversed the direction of rotation.
Real boat gearboxes look more complicated, but depend on exactly this principle. In fact, the main difference between the simple gear train and the Volvo MS2 is that the MS2 uses coneshaped bevel gears, so that although the input shaft is horizontal, the central driven shaft is vertical.
The motive for this is that it provides a simple way of achieving a choice of ahead or astern gear.