N40475 Chapter 4 Exhaust systems A range of exhaust components are available for use with all types of exhaust system. The components are designed to connect together, allowing complex systems to be built from stock items, to suit most installations. Caution: In all types of exhaust system the exhaust back pressure must not exceed 15kPa, when measured within 305mm (12 inches) of the exhaust outlet from the engine. Wet systems Figure 1 Wet exhaust systems, where the auxiliary water used to circulate through the heat exchangers on the engine is finally dumped into the exhaust pipe to cool the exhaust gases, are the most common choice for small craft.
Their principal advantage is that a rubber exhaust hose may be used, with a fairly low surface temperature, which presents no risk of fire. The exhaust bore is 125mm (5”). A general arrangement for such a system is shown on Figure 1. In many cases the exhaust outlet passes through the transom, just above the waterline (1). It will be seen that a minimum fall of 5 O (2) is required, and that the point of water injection must be at least 8 inches above the waterline (3), although the actual height necessary for a particular boat can only be decided in the light of the exhaust system design, and the pitch and roll which may be encountered in service. Caution:
It is essential that the exhaust system is designed so that water from the exhaust does not enter the engine under any conceivable operational condition. Page 9