Propeller Design Research Paper

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,

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.

Simple gearboxes

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.

The bevel gear on the input shaft turns two slightly larger bevel gears that spin freely on the vertical shaft. One is driven by the top of the input gear and the other by the bottom of it, so they rotate in opposite directions, but they are the same size as each other so they turn at the same

A more widespread arrangement is the layshaft gearbox.

In this case, a small gear on the input shaft turns a larger gear on the layshaft. Like the gears on the vertical shaft of the MS2, this is free to spin around the layshaft, but can be locked on to it by a clutch. If the input shaft is turning clockwise and the clutch is engaged, the layshaft turns anticlockwise and more slowly. At the other end of the layshaft a smaller gear meshes with a large gear on the output shaft, driving the output shaft clockwise and even more slowly.

Astern gear is achieved by a second layshaft. Its very similar to the first, with two gearwheels, one of which has a clutch that can leave it free-spinning or lock it on to the shaft, while the other is constantly in mesh with the output shaft. The key feature about the second layshaft is that it is not driven directly by the input shaft, but

by the first layshaft. This means that it 's turning in the opposite direction - so if one clutch is released and the other engaged, the direction of the output shaft will be