the vertical pipe or hose exiting it) should be
less than 42 inches (106 cm, see Figure 7-12)
to keep back pressure within acceptable lev-
els. The transverse exhaust pipe (and most of
the rigid wet exhaust piping) can be ordinary
fiberglass exhaust tube. Don’t forget, how-
ever, to install flexible connectors between
the pipe and anything rigid: the side outlets,
the engine, and so on. This should be top-
quality wet exhaust hose. If you connect to
some dry exhaust section, however, you have
to use flexible metal exhaust bellows.An Exhaust Lift Solution
Another useful and these days often forgotten
approach for getting the exhaust from a low en-
gine installation high enough to run out the
transom is the standpipe exhaust system
(Figure 7-16). (This can be used in place of the
waterlift muffler in a North Sea exhaust.)
In this configuration a dry exhaust line
conducts the exhaust vertically upward from
the exhaust manifold. This first portion of the
run is dry and quite hot; it has to be very well
and securely lagged (insulated) to prevent
burns or fire. Don’t forget a flexible metal bel-
lows at the joint between the manifold and
exhaust piping. Without the bellows, you’ll
get dangerous cracks surprisingly soon
(see Chapter 9). The engine cooling water isinjected into a large standpipe that caps the
dry exhaust line. Here the water and gases
mix, exiting the bottom of the standpipe to
run out the transom.
STANDPIPES: OLD AND NEW Figure 7-17
shows the old and new versions of the stand-
pipe. The old version works fine with two
caveats, one being that the top of the stand-
pipe gets very hot. Over several years’ use,
this arrangement has been known to cook the
underside of a wooden deck until it looked as
black as charcoal briquettes. The other
caveat is that water lies in the bottom of the
standpipe until you drain it manually—a cor-
rosion headache. The new version injects
water into the top of the standpipe. With a
protective deflector cap to distribute the
flow, the entire standpipe is kept cool and the
water and gas mix more completely than in
the old version. There’s also little standing
water in the bottom.
If you’re repowering or designing a deep-
bodied displacement hull, the dimensions given
will allow you to fabricate a sound standpipe
system. Again, you should use only the very
best grade stainless, specifically 316L. Anything
less is asking for corrosion problems down the
line—a false economy. If you really want to do
it perfectly, hunt around for Hastelloy C or
Inconel. (This can be some hunt, but it is worth
it even if quite expensive.) Whatever the alloy,PART THREE: EXHAUST SYSTEMS
Figure 7-16.
Standpipe exhaust
system