Yacht engines have a lot in common with every other kind of engine, indeed in many cases the same engines used in yachts are also used in various forms of equipment that have nothing to do with the sea, including agricultural, automotive and industrial applications. However, whereas the vast majority of these are cooled by air passing through a radiator, most yacht engines are cooled by the water they float in, seawater for the purposes of this article. This can be achieved either directly, where the seawater pumped into the boat passes through the engine (figure 1), or indirectly, where the engine is cooled by a coolant that is itself cooled by the seawater via a heat exchanger (figure 2).
In each case the seawater passes out of the boat via a water lock or trap, often made of plastic material, and flexible rubber hose. The water flow prevents these from being overheated by the hot exhaust gases, (figure 3). The upper bend is there to prevent any water driven from astern by waves from entering the exhaust hose and ultimately, the engine. In some installations a silencer is also added in the downward leg above the skin fitting in figure 3, adding to the height of the top bend.
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