Chapter 9:Dry Exhaust Systems
Drystack Versus Wet Exhaust
Though wet exhaust systems are most com-
mon on boats, there appears to be an inclina-
tion to favor drystack exhaust systems for
serious ocean cruising yachts as being some-
how, well, “tougher.” For almost all yachts and
for most passenger vessels, however, I gener-
ally think a wet exhaust system is superior. It’s
important to keep in mind that this is most def-
initely not a question of right and wrong.
Rather, it’s a question of trade-offs. Both wet
and dry exhaust systems have advantages and
drawbacks. We can compare wet and drystack
exhaust systems in Table 9-1.
We can examine these values item by item:
SEAWATER INENGINECOMPONENTS. Here the
drystack system is the clear winner. There is
simply no seawater in any engine component.
Still, with a properly designed wet exhaust
system, you can eliminate seawater in every-
thing except the exhaust manifold itself.
HOLESBELOW THEWATERLINE. This is a bit
deceptive. You would think that there would
be no holes below the waterline with a dry-
stack system, but as we’ve seen, drystack sys-
tems require external keel coolers. These
usually necessitate two holes through the
hull (though some are arranged to have just
one). Alternatively, there’s a complex set of
internal or external metal channels for
coolant flow, and these may develop pinholes
or larger leaks. Thus there is only a slight im-
provement for drystack systems.
FIRESAFETY. This is the drystack system’s
Achilles’ heel. These things are really hot—
dangerously so.
SIMPLICITY. We’ve seen that the drystack sys-
tem’s components—metal expansion bellows,
expansion slip joints, safety cages, insulation/
lagging, keel cooling, and so on—add up to a
surprising amount of complexity on a well-
designed system. The wet exhaust system is
no more complex and possibly less so.
RELIABILITY.This is pretty close between the
two systems. The only thing likely to go
wrong with a properly designed wet exhaust
system is a broken raw-water pump or (over
the long haul) corrosion in the exhaust man-
ifold. Drystack systems can have problems
with expansion bellows, mounts, insulation,
keel coolers, and so on.
PARTS TOINSPECT ANDREPLACE. Wet ex-
haust systems have the raw-water impeller,
quick and easy to take care of, and the heat-
exchanger zinc, also fairly easy. Drystack
systems require inspection of the insulation,
bellows, mounts, and keel coolers.TABLE 9-1. DRYSTACK AND WET EXHAUST SYSTEM COMPARISON
Drystack Wet ExhaustSeawater in engine components 5 1
Holes below waterline 3 2
Fire safety 1 3
Simplicity 3 3
Reliability 4 3
Parts to inspect and replace 2 3
Ease of tracking and fixing leaks 2 3
Quiet 2 4
Corrosion of exhaust piping 3 5
Corrosion of manifold and heat exchanger 5 1
Can fit a return oil cooler 1 5
Space taken from accommodations 2 5
Unwanted interior heat in warm weather 2 4
Soot on deck 1 5
TOTAL 36 47
NOTE:Ratings are from 1 to 5, with 5 being best.