acceptable, but it has a rather high speed and
can produce a high-pitched whine. This is not
a problem in machinery compartments, but it
could be undesirable around cabin areas. Now
that we know we’ll use a 6-inch (150 mm) duct,
we can specify a lower-speed axial blower
6 inches (150 mm) in diameter, as long as it is
rated at least the required 131 cfm (221 m^3 /hr.,
3.7 m^3 /sec.) or somewhat more. This would
provide the quietest powered ventilation for
our below-deck cabin. In general, try to keep
blowers away from passenger areas, though of-
ten practical considerations make this difficult.
LARGEDUCTSMAYTAKEUPTOOMUCHSPACE
Of course, this 6-inch-diameter (150 mm) duct
takes up much more space than the smaller
3-inch (7.6 cm) duct we initially considered.
This can be a problem. The duct needs to run
where it won’t interfere with the accommoda-
tions, structure, or machinery. You don’t want
the duct exposed on the inside of finished
accommodations spaces either. Some refer-
ences allow as much as 1,200 fpm (6 m/sec.)
as acceptable with regard to noise. This is
on the high side, and at 900 fpm (4.7 m/sec.)
noise will be definitely noticeable in quiet
spaces like sleeping cabins. Still, you may have
no choice in some instances but to go with
smaller-diameter ducts and thus higher airflow
speeds than is optimal for reducing noise. Such
real-world trade-offs may be unavoidable given
the limited volume available in some vessels.
PROVIDING ANAIR-INTAKEPATH For most of
the cabin ventilation we’ve been considering,
we’re assuming that some of the vents are on
intake and some are working as exhaust, as
shown in the Nicro ventilation schematics in
Figures 14-3 and 14-4. In addition, there is
free exchange of air between cabins within
the boat and to the outside. This is the perme-
ability(μ) of the structure. Permeability is
generally enough so that cabins can share in-
take and exhaust air between them. In fact,
this permeability to the outside is usually
great enough that provisions for additional
make-up air(in air-conditioning systems)
are unnecessary in most boats under 120 feet
(36 m) or so. (Make-up air is air taken into the
air-conditioning system through a dedicated
outside vent to ensure that sufficient fresh air
gets into the accommodations.)
For a cabin belowdecks requiring a 9-foot
(2.74 m) duct run, intake air due to permeabilityfrom another cabin—from a vent located
somewhere else on deck—can’t be counted on.
For this reason, a second passive intake duct
should be provided with the same diameter or
one size larger than the blower duct. This is as-
suming that the blower is on the exhaust duct.
INTAKEAIRTRAVELS ASTRAIGHTPATH; OUT-
LETAIRDOESNOT Keep in mind that intake
air generally travels a straight path out of the
outlet opening in a compartment until it
strikes something. Outtake air (suction)
tends to be sucked into its inlet from the entire
general area nearby. I once made the mistake
of installing an engine-air intake and outlet
high up on the same aft bulkhead in the engine
compartment, one port and the other star-
board. The vents were large but the engine
room got terribly hot. A quick check using
cigarette smoke during sea trials showed that
the inlet air was blowing straight across the
bulkhead from port to starboard and exiting
immediately. Luckily, this was easily cor-
rected by installing an air-duct hose over the
outlet opening and running it down to the
lower starboard corner of the engine room.
This immediately corrected the problem and
created a nice diagonal circular rotation of
airflow in and around the machinery space.
Apply this principle to any vent system
you install. For our below-deck stateroom,
the intake air louver grille should be, say,
high up and forward on the starboard side,
with the outlet (exhaust) air grille low down
and aft on the port side.
EXHAUST ORINTAKE? An important thing to
keep in mind is that all powered vents for areas
with unpleasant smells should be exhaust
vents. This sucks the bad air out and draws
sweet air in. It’s tempting to try to blow fresh
air into a head or galley, but don’t do it—
you’ll simply blast the foul air into the rest of
the boat. You can blow into sleeping accom-
modations, salons, or other social gathering
areas, however.Mushroom Vents on Hatches
If a boat has plenty of hatches, often one of
the easiest places to install a mushroom vent
(solar powered or passive) is in the middle
of a hatch. The advantage is that you don’t
have to cut any of the deck structure or
worry about hidden wiring or plumbing whenPART FIVE:VENTILATION, AIR-CONDITIONING, AND HEATING