Chapter 9:Dry Exhaust Systems
jacket can get fairly hot and will eventually
lead to corrosion in the jacket (as we saw
with water-jacketed exhaust manifolds).
Finally, the water must exit the hull not
through the exhaust (which, of course, is dry),
but through yet another through-hull fitting.
Air-jacketed exhausts don’t have raw-
water pumps, any seawater, sea strainers, or
the related potential for corrosion, but they
require electric blowers to force cool air
into the air jacket. The external air-jacket
pipe must also be configured with flexible
bellows (sometimes called wrinkle-tube) to
allow for its expansion and contraction.
(The air-jacket tube’s OD is about 1.4 times
the exhaust-pipe OD or a bit more.) Further,
the air jacket does lower the surface temper-
ature, but it doesn’t lower it enough to avoid
insulating any area with which the crew may
come in contact. Thus, it still requires
lagging, though this may be somewhat thin-
ner than without air-jacketing. Finally, air-
jacketed dry exhausts rely on the electric
blowers to reduce the exhaust-pipe surface
temperature. If there’s an electric failure or
a breakdown with the blowers, you have a
serious exhaust problem.
Again, air-jacketed drystack exhaust in-
stallations are perfectly sound and accept-
able, but they seem to get away from the
rugged reliability that is a drystack installa-
tion’s principal attraction.
Support Struts on High
Exhaust Pipes
The drawing in Figure 9-30 shows another
way to raise an exhaust pipe higher above the
deck. In this case, it’s not in a stack but is an
individual pipe. When the height of the ex-
haust pipe is more than 7 times its diameter,
it needs support, and you can see the struts
installed for this purpose. The struts are light-
weight pipe or heavy-wall tube with an out-
side diameter^1 /50thof the strut length, pin cen-
ter to pin center (or slightly more). A strut
100 inches (250 cm) long would be approxi-
mately 2-inch (50 mm) OD or a bit more. The
fastening bolts at each end would be one-
quarter the support strut’s OD, or in this case,
(^1) / 2 inch (12.5 mm). Note that the support
struts attach to the exhaust pipe at a sliding
collar. This permits the exhaust pipe to ex-
pand and contract freely.
More on Mixing and Matching Exhaust Types
As we saw previously, you can mix dry and
wet exhaust components to solve installation
problems. In Chapter 7 (page 119) we looked
at a dry exhaust section used to fit an other-
wise wet exhaust into a restricted space. In
this chapter (page 155) we have seen how a
heat-exchanger-cooled engine could use the
raw water for engine cooling to also cool a
water-jacketed drystack exhaust. In fact, you
can have a heat-exchanger-cooled engine
with an all-dry exhaust—without water-
jacketing—and simply dump the cooling sea-
water directly overboard. With a sound un-
derstanding of both wet and dry exhaust
systems, you can mix and match as needed to
solve difficult installation problems.Crewboat Exhaust
A mixed-exhaust installation is often used on
crewboats and offshore supply boats, fre-
quently with triple or quadruple engines
mounted in an engine room aft. The engines are
usually heat-exchanger cooled, but the exhaust
run is very short and almost all dry. As you can
see in Figure 9-31, the dry exhaust exits the en-
gine into an exhaust bellows and dry muffler/
silencer. The exhaust run is mostly horizontal—
usually almost directly athwartships and out
the side of the hull. (The exhaust rises from the
engine to the muffler or horizontal exhaust pipe
and runs athwartships to near the hull side,
then down and out the side of the hull.) The raw
engine-cooling water is injected at a custom fit-
ting located just inside the hull, near the exit
(Figure 9-32).
The advantage of this installation is that
it is short, light, simple, and easy to repair.
The disadvantage is that exhaust gas (and
soot) exit the topsides where they can blow
onboard and dirty the hull side and deck.
This is acceptable on workboats but would
be unacceptable on yachts or passenger
vessels.