2,000°F (1,090°C), as well as standard engine-
block enamels in an assortment of colors
rated around 550°F (288°C).
Drystack Exhaust Trunks
As we’ve seen, one of the supposed pluses
of drystack exhaust installations is their
simplicity. The basic drystack exhaust we’ve
looked at so far isn’t too complicated, but
most boats have one or more generators, a
cabin heater, and perhaps a water heater and
compressor. If the main engine is drystack,
these items are usually drystack as well. Such
drystack systems run their exhausts up
through a shared exhaust trunk. Even assum-
ing just a single main engine, one gen set, and
one cabin heater, you can see from the draw-
ing in Figure 9-23 that laying such an exhaust
trunk out correctly is not simple, and this isn’t
considering the keel-cooler installations re-
quired as well.
The exhaust trunk must be large enough
to accommodate all the vertical dry exhaust
components with room to spare. To ensure
good airflow through the trunk (both for en-
gine ventilation and cooling of the trunk and
exhaust pipes themselves), about 30 percent
or more of the cross-section area inside the
trunk (after adding insulation) should be
open for airflow (Figure 9-24).
The mounting of each muffler and each
exhaust pipe must be worked out in detail.
The pipe clearances and runs must be
arranged so the pipes don’t interfere with
each other and have a clear run out of the
stack. The walls of the trunk must be care-
fully lined—ideally with both sound and heat
insulation—and there must be large remov-
able access panels for most of the length of
the trunk for inspection, maintenance, and
repair. Simple this isn’t.
Exhaust-Ejector Ventilation:
Exhaust Trunk as a
Ventilator
In many instances, it makes sense to specifi-
cally configure the exhaust trunk to double
as a ventilator. The natural convection result-
ing from the hot air in the trunk can be used
to pull more air out of the engine compart-
ment. This is already the case, as shown in
Figure 9-23, but it can be optimized further.
Figures 9-25 through 9-27 show configura-
tions and proportions for optimizing the ex-
haust trunk’s ventilation capability. These use
the energy in the convective flow of the hot
air around the exhaust pipes to generate ejec-
tor ventilation.
These ejector-ventilator exhaust stacks
work best on single-engine installations, be-
cause with multiple engines there can be un-
predictable cross flow in the air circulation,
particularly if one engine is running and the
other is off.Multiple-Exhaust “Stacks”
Often it’s beneficial to raise the exhaust out-
let high above the deck and superstructure.
There may be a passenger deck too close to
the exhaust, a high pilothouse forward of it,
or similar considerations. In this case, the ex-
haust pipes can be run up into a multiple-
exhaust “stack” left open at the aft end where
the exhaust exits. These stacks can be de-
signed as prominent decorative features, en-
hancing the vessel’s appearance.
Figure 9-28 illustrates a typical example
of multiple-exhaust stacks. The added height
of the stack enhances the convective-
ventilator effect. Note the large access panels
on both sides—a detail too often overlooked.
Note also that this stack sits on top of the ex-
haust trunk below, and the two must be
worked out together.Water-Jacketed and
Air-Jacketed Exhaust Runs
To reduce the load on the insulation and lag-
ging, you can use water jackets or air jackets
to lower the external temperature of the dry
exhaust piping. The drawing in Figure 9-29
depicts a typical installation. The OD of the
water-jacket pipe is usually 1.45 times the OD
of the exhaust pipe, or a bit more.
Water-jacketing of dry exhaust is com-
mon and acceptable, but I don’t see great ben-
efits from it. Yes, it reduces heat in the boat’s
interior, but it counters one of the fundamen-
tal advantages of a drystack exhaust: not hav-
ing a second, separate raw-water pump andChapter 9:Dry Exhaust Systems