TPD 1399E Chapter 3
Page 7
Exhaust systems
A wide range of exhaust components are available for
use with all types of exhaust systems. Some of 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 75mm (3 inches) of
mercury when measured within 300mm (12 inches)
of the engine exhaust manifold outlet.
Wet Systems
Wet exhaust systems, where the raw water which
circulates through the engine-mounted heat exchanger is
nally injected by means of a special exhaust elbow (A)
into the exhaust pipe, 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 re.
The exhaust elbow incorporates a water feed connection
(A1) which may be used to cool a shaft stufng box,
and a sampling point (A2) for temperature or pressure
measurements.
Warning! Use a siphon break where a water lift
exhaust system is specied, see page 10.
A general arrangement for such a system is shown in (B).
It is usual for the exhaust outlet to pass through the
transom just above the waterline. The minimum average
fall (B1) should be 5
O
, and the point of water injection
(B2) must be at least 200mm (8 inches) above the
waterline, although the actual height necessary for a
particular boat can only be decided in the light of the pitch
and roll which may be encountered in service.
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