casting with the exhaust running inside chan-
nels that are surrounded by cooling water.
Because the channels are usually of cast iron,
corrosion will occur sooner or later in the
water jacket of the manifold. Inspecting
water-jacketed exhaust manifolds should be
considered routine long-term maintenance.
Table 7-1 presents tabulations by BoatU.S. for
the probability of failure due to corrosion in
exhaust manifolds.
TABLE 7-1. ESTIMATED EXHAUST
MANIFOLD SERVICE LIFE
Years in Service Probability of Failure
3 0.5 percent
4 25 percent
5 45 percent
6 65 percent
7 85 percent
8 90 percent
9+ 100 percentI think this is rather pessimistic. Also, it
primarily applies to gasoline engines with
raw-water cooling. Diesels with closed-
circuit (heat-exchanger) cooling will experi-
ence far fewer problems with corrosion. Our
office has had several such boats in service
over a dozen years, with no sign of problems.
Nevertheless, there will be some problems
here eventually.
Injecting Low
Again, to keep water out of the engine, the
water injection point must be at least
4 inches (102 mm) below the bottom inside
surface of the highest bend in the exhaust
riser—6 inches (150 mm) is better. Unless
you’re assembling your own exhaust piping
from scratch, the injection location is already
built into the riser. If, however, you have a
choice during refits, during repairs, or for
new construction, go for a riser with the
larger 6-inch (150 mm) drop.
Surge Chambers
and Flappers
You don’t want some—or any—ocean water
forced back up the line when backing down.
Sportfishermen in particular jockey around,
going back and forth in rough conditions
while fighting fish. These boats need special
protection from back surge. The surge cham-
ber shown in Figure 7-1 absorbs a great deal
of the force of a slug of water in what’s essen-
tially a compressed-air cushion. Fitting an
external check-valve flap on the transom or
an internal check valve is also a wise precau-
tion. Indeed, if the bottom of your exhaust
exits the transom lower than 6 inches (150 mm)
above the static waterline, you should consider
the combination of an external flapper or in-
line check valve and a surge chamber. (Note
that some engine manufacturers disapprove
of transom flappers because they can block off
the exhaust line completely during extended
backing maneuvers.)
RUMRUNNERCOVERSAn interesting variation
on the common external flapper valve was
used on some rumrunners during Prohibi-
tion. Powered with twin World War I surplus
aircraft engines—often producing in excess
of 500 hp (373 kW) each—these craft fitted
rubber clamshells over their transom ex-
hausts. The rumrunners fabricated these
gizmos from old car tires and attached them
so the only opening was straight down, with
just about an inch (25 mm) clearance above
the waterline at rest. Needless to say, keeping
away from the neighborhood Coast Guard
cutter required stealth. Not only did these
rubber clamshell covers help prevent back
flooding, but the rubber itself appears to have
flexed enough to absorb some engine noise.
Much of the remaining racket seems to have
been deflected down into the water.The Best Exhaust Piping
Because it doesn’t corrode, fiberglass ex-
haust tube and exhaust hose, meeting SAE
J2006 standards, are the best for a trouble-
free installation. The hose must withstand a
minimum temperature of 280°F (138°C) and
have no tendency to collapse. In the old days,
this was called “steam hose.” Today, there are
hoses specifically manufactured for marine
wet exhaust application. This is one of those
rare cases where what’s best is actually less
expensive and lighter than metal.
There is, though, always the chance of a
water pump failure. If this happens, your
engine will seize up lickety-split. But in thatChapter 7: Wet Exhaust Systems