take a careful look at the diameter of the
exhaust system. A switch from gas to diesel
may up the diameter by a size or more (as
might simply increasing power). Be sure you
have the room to install the bigger piping.
Exhaust Lines Can’t Share
Considering the space problems, it’s tempting
to combine twin-engine exhaust lines at some
point into a single, large exit line. It would
seem that if you just made sure the combined
exhaust-line diameter was sufficient, this
could solve a multitude of space problems.
Unfortunately, you can’t do it. Exhaust lines
can’t be shared, not between main engines
and not between gen sets, compressors, or
any combination of these. Why not? If you
run on one engine with the other off for any
period of time, the exhaust gases from the
running engine will back up into the turbo
and valves of the idle engine. This will cause
unacceptable fouling. In addition, condensa-
tion can build up in the idle engine, which—
combined with the carbon and other by-
products in the engine and exhaust—
can form acids that will damage seals and
gaskets.
The only successful exception to this rule
that I’m aware of is the current Coast Guard
47-foot (14.3 m) motor lifeboats, which do
share the twin-engine exhausts. The design
team did a careful study and calculated that
the two engines would virtually always be
run at the same time. In this case, there
would be minimal exhaust back-fouling prob-
lems. These boats are operated with military
precision and maintained the same way.
Yachts and workboats aren’t. I advise against
sharing any exhaust lines on any vessel.
The Low-Engine Exhaust Problem
We’ve examined exhaust installations for
standard powerboats, with the engine rela-
tively high. Now we’ll take a look at the ex-
haust systems that both powerboats and sail-
boats frequently have in common: exhaust
systems that have to lift spent engine gases
up and overboard from below the waterline.
Powerboats frequently fit generators below
or near the waterline, while sailboats almost
invariably have their main engine low—real
low—in the bilge. In the early days, such
exhaust systems were simply dry pipes ris-
ing as close to vertical as possible—to well
above the waterline—then with cooling wa-
ter injected, sloping down and out a transom
through-hull. There’s still a place for these
standpipe exhaust systems, and we’ll discuss
them shortly.The Waterlift Answer
The usual modern solution to all this is the
waterlift muffler (Figure 7-10). These gizmos
are simplicity itself. The wet exhaust runs
into the top or side of the muffler, which is no
more than a large, empty cylinder. A second
pipe, the outlet or lift pipe, runs up out of the
top of the muffler canister. This pipe contin-
ues down to within a few inches or centime-
ters of the muffler-canister bottom. Since
cooling water initially runs down into the
cylinder (the waterlift muffler), it almost in-
stantly fills about half full with water. The up-
per half of the muffler contains water vapor
and exhaust gases that have nowhere to go.
In a brief instant, gas pressure builds high
enough to blow a mixed slug of water and
exhaust gas down to the bottom of the muf-
fler canister and up out of the exit pipe.Waterlift Pluses
This system has several advantages. First,
wet exhaust is safe and easy to deal with (as
we’ve seen). Second, injecting the water in
the first place quiets the exhaust (as well as
cools it). Third, the process of expansion in
the waterlift canister and forcing the gas/
water mixture out quiets the exhaust even
more. Fourth, the muffler cylinder itself—if
properly configured—forms an excellent baf-
fle against water working its way back up
into the engine. A good case can be made for
waterlift mufflers being the quietest of all
mufflers.
The next step up from the simple water-
lift, incidentally, is a dual-chamber waterlift.
These have their canister divided internally
into two sections (sometimes more). The
process of forcing the air/water mixture
through the dual chambers further reduces
noise.Chapter 7: Wet Exhaust Systems