All About Engines

The Petrol Motor 223 equally well in either direction, and to reverse it is only necessary so to advance the spark that a back fire occurs. Generally, this sets the engine running in the opposite direction, but it is not absolutely cer- tain, and, in any case, it entails rather rough usage. For very small boats there are several very in- genious arrangements whereby a small motor is fixed to the stern, and a screw propeller driven from it by means of a vertical shaft and bevel wheels. The cylinder is fixed horizontally with a flywheel above and the shaft projecting below. They work on the two-stroke principle because the absence of valves and valve gear simplifies the engine, enables them to be produced at a low cost, and facilitates revers- ing. Lubrication is effected generally by mixing lubricating oil with the petrol in the proportion of i to 20. One of the best-known “ outboard ” motors is the Evinrude motor, illustrated in Fig. 127. With a single cylinder this can be made to give from ij to 4 horse-power. No rudder is required, the boat being steered by turning the engine about the ver- tical shaft by means of a short tiller. When the engine is running this gives a very powerful effect ; but, of course, it has no influence when the engine is at rest. Another method which has been developed for the propulsion of barges, and of small boats required to navigate very shallow or weed-choked waters, is the use of an aerial propeller. That is to say, the boat is propelled by a screw working in the air and driven