Exhaust system
75
Exhaust outlet through boat bottom – concept design
Principle sketch, exhaust outlet through bottom
In some installations an exhaust outlet through the
bottom of the boat might be the prefered alternative.
In such an installation a full strength pipe (metal, grp,
or similar) must go from the hull up to a level above
the static water line when the boat is moored in order
to avoid needing a shut-off valve.
Incline the pipe slightly backwards and design the out-
let in the bottom to avoid water being pushed up the
pipe if the boat is towed or running with one engine
only.
Position the outlet in the bottom in a way so the ex-
haust gases will not create negative turbulance flow-
ing into the propeller or trim tabs, not even when the
boat is turning, as this will affect the performance of
the boat.
A by-pass outlet should be installed from the exhaust
pipe, above the water line, to an outlet above the wa-
ter line to avoid high backpressure when starting the
engine and reduce the low idling pressure pulses to
the hull, which create noise.
Often a riser is needed to obtain the correct distance
(350 mm / 14") to water line (WL), see page 74.
1. Exhaust riser
2. Exhaust hose
3. Exhaust pipe
(Full strength pipe)
4. Exhaust outlet
5. By-pass outlet
350 mm
(14")
4
3
2
5
1
WL
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