EASE OFTRACKING ANDFIXINGLEAKS. This,
too, is a bit deceptive. The piping in a wet ex-
haust system is straightforward and gener-
ally visible. A drystack system’s keel cooler
is below the waterline and outside the hull.
You can’t get at it easily if it develops a pin-
hole, and it’s hard to find the problem with-
out a haulout. As we’ve seen, some drystack
systems on metal hulls circulate cooling wa-
ter though internal tanks or through chan-
nels of pipe welded to the inside or outside
of the hull shell. Again, leaks in such piping
or tanks can be difficult to locate.
QUIET.This is clearly the strong suit for wet
exhaust systems. Even the best drystack
“hospital” silencers can’t equal the quiet from
the combination of mixing cooling water with
the exhaust, then running it through a muffler
and exiting it through the hull topsides (or
even underwater). Yachts, tour boats, and fer-
ries usually want maximum comfort for the
crew and passengers. A voyaging motor
cruiser is a home. You don’t want to hear the
roar of your diesel endlessly—a soft purr
would be nice.
CORROSION OFEXHAUSTPIPING. Here we
have another strong suit for wet exhaust sys-
tems. If the entire exhaust is hose and FRP
exhaust pipe it can’t corrode at all. Drystack
exhaust systems are all metal and will experi-
ence corrosion over time.
CORROSION OFMANIFOLD AND HEATEX-
CHANGER. This is a clear winner for drystack
systems. There’s no seawater in the manifold
(it’s not water jacketed), so there’s no corro-
sion. Further, there is no heat exchanger with
a raw-water circuit, which also can experi-
ence corrosion and silting and should be
cleaned and inspected regularly. Wet exhaust
manifolds have built-in water jackets. These
will experience at least some corrosion even-
tually. Water-jacketed manifolds are much
cooler and quieter, however.
CANFIT ARETURNOILCOOLER. Only instal-
lations with raw-water intake can use a re-
turn oil cooler, which lowers fuel and engine
room temperature. We’ve seen that you can
have a mixed system with drystack exhaust
and heat-exchanger engine cooling, but this
would not be a pure drystack system.
SPACETAKEN FROMACCOMMODATIONS. It is
amazing how much space drystack installa-
tions can require. On our 60-foot (18.3 m)General, with drystack, the exhaust trunk
fills a column over 2.5 sq. ft. (0.2 m^2 ) in sec-
tion right in the center of the arrangement. It
is a real problem trying to work the accom-
modations around drystack installations.
UNWANTEDINTERIORHEAT INWARMWEATHER.
It’s also surprising how much heat even a well-
insulated drystack exhaust will give off. It can
be quite uncomfortable and force you to run
the air-conditioning more often than you would
with a wet exhaust.
SOOT ONDECK. This is a special problem
with drystack systems and one frequently
overlooked. The fact is that all diesels pro-
duce black soot fairly regularly: at speed,
only in small amounts; during acceleration
and low-speed maneuvering, sometimes quite
a bit. With wet exhaust systems, this soot is
mixed with seawater and goes over the side,
leaving nothing but a streak on the top-
sides—easily washed off. With dry exhaust,
the soot comes out as a fine cloud that de-
scends to the deck, gradually building up an
unpleasant oily film of gray-black grime.
WETEXHAUSTSYSTEMSAREBETTER FOR
YACHTS ANDPASSENGERVESSELS Evaluating
the preceding information, my preference is
for wet exhausts on yachts and most small
passenger vessels. Both wet and dry exhausts
work well when properly configured, but I
think that most boaters will be happier living
with a wet exhaust.Workboats Often Are Better
with Drystack Systems
Particularly without water-jacketing or air-
jacketing, drystack exhausts are somewhat
more reliable for workboats, where more ex-
haust noise and additional soot on deck
aren’t such important considerations. Even
so, many fast workboats—pilot boats, crew-
boats, supply boats, and such—are usually
better off with wet exhausts.
A plus for drystack systems on boats that
operate in very cold weather is that there is
no problem with skim ice in sea strainers and
sea chests. This is really a factor only in boats
running in areas like the Bering Sea or
Antarctica in winter, but in such weather, this
is a notable added benefit for drystack sys-
tems. What’s more, the extra heat from the
dry exhaust will be welcome.PART THREE:EXHAUST SYSTEMS