2 ENGINES AND PROPULSION Chapter 2 ENGINES AND PROPULSION


Although their specific details may be vastly different, all marine engines work the same way and their principles are widely understood. So, we can skip through the basics in just a few words. It involves releasing the energy contained within a combustible fuel. The fuel is first filtered to remove any debris and is then passed on to the engine. There it is combusted in cylinders (or cylinder − there may be only one) to produce what is effectively a controlled explosion. Each cylinder contains a piston that’s driven down by the rapidly expanding gases, and that linear action is converted into rotary motion by a crankshaft. This is transmitted through a gearbox and onwards via some form of shaft to the propeller immersed in the water outside the hull. By far the most popular fuels are petrol and diesel − both derived from oil − but a few engines have been converted to burn LPG (liquefied petroleum gas).


Inboard engines


The schematic (Fig 2:1) shows an ‘inboard’ installation. The term is self-explanatory. The fuel tank, engine − usually a diesel − gearbox and most of the shaft is inside the hull with only the propeller and the aftmost shaft bearing outside. This is the most common arrangement for heavy displacement type boats and has the virtue of simplicity and accessibility. Incidentally, the various components that transmit the engine’s power through to the water are often known collectively as the ‘drive train’ and, with traditional inboard installations, the parts are more or less independent of each other, meaning that you can usually work on one unit without dismantling the whole shebang. For example, if there’s a problem with the gearbox you can lift that out for repair without disturbing the other components in the drive train.


Fuel filler Isolation switch


Injection pump


Injectors


Stern gland


Fuel tank Battery Shaft bearing Propeller


Fuel shut-off valve


Fuel pre-filter Fig 2:1


Fine filter


Injection pump


12


RYA Seamanship for Sea Anglers


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