covered at times with bilge water. The CFR
makes allowances for this.
You can see in Figure 16-5 that a low,
shallow bilge compartment area, in particu-
lar, can end up with the exhaust duct very
low in the bilge to meet this one-third-height
requirement. In such instances where normal
bilge-water accumulation could cover or be
sucked into the exhaust vent intake, you can
and should raise the exhaust intake high
enough to avoid this problem.
INLET-ANDOUTLET-DUCTSEPARATION The
air intake and outlet ducts in a compartment
must be separated by a minimum of 24 inches
(61 cm) unless the compartment is physically
too small to permit this.
Warning Placard Required
It is also a legal requirement that the engine
blower (often called a bilge bloweron gas
boats) must be run for a minimum of 4 minutes
beforestarting the engine. A warning placard
has to be placed near the helm and the engine-
ignition switch (with the blower switch clearly
labeled and visible nearby) saying
This label must be clearly visible to the oper-
ator when he or she is in normal position to
turn on the engine or engines. This also
applies to the ignition switches of any other
gasoline-operated equipment such as gener-
ators or compressors.
Natural or Passive
Ventilation Required
for Gasoline Boats
In addition to the powered blower, all gasoline
engine compartments must also be fitted with
natural ventilation. The net volume of the
compartment and adjoining compartments are
determined exactly as done previously for the
powered blowers. The 2 percent rule also
applies for determining whether an adjoining
compartment is considered part of the total
net compartment area, and compartments
open to the atmosphere aren’t included.
NOTE: Natural ventilation is not required
for a compartment that contains metal
gasoline fuel tanks but doesn’t have gasoline
engines. The exception is when there is
electrical or mechanical equipment in the
compartment that is not ignition protected. In
that case, such a fuel-tank-only compartment
requires the same passive or natural ventila-
tion as a gasoline engine compartment.
Many plastic gasoline fuel tanks have
sufficient permeability to require natural ven-
tilation. Check with the tank manufacturer
about the tank’s permeability with regard to
USCG requirements for natural ventilation of
the tank compartment. It’s generally a good
idea to provide natural ventilation for plastic-
fuel-tank areas. (Note: As of 2009 or 2010,
new EPA regulations may require a vapor
barrier built into plastic fuel tanks.)
Natural ventilation is also required for
compartments that stow portable gasoline
engines (like stowed outboards or portable
compressors) or portable fuel tanks (like the
standard portable 6-gallon [23 L] outboard
fuel tank).
Under the CFR, you can supply fresh air
from outside the boat or from another venti-
lated compartment. You can also exhaust to
outside the boat or into another ventilated
compartment not connected with the engine
compartment. In general, I recommend that
all supply and exhaust air be from outside the
boat to outside the boat. However, if you have
a boat with, say, a bow compartment, a mid-
ships bilge compartment, and a stern com-
partment, it can be both acceptable and sensi-
ble to take air in through the bow, direct it aft
through the midships bilge, and exhaust it out
the stern compartment, as in Figure 16-6.
Also, the exhaust duct or opening for the
powered blower can double as the exhaust
duct for the natural or passive ventilation sys-
tem if it has sufficient area.
MINIMUMNATURAL-VENTILATIONOPENING OR
DUCTCROSS-SECTIONAREA Formula 16-5
determines minimum allowable cross-sectionChapter 16:Ventilation of Machinery Spaces
WARNING
Gasoline Vapors Can Explode, resulting
in injury or death.
Before Starting Engine, Operate Blower
for 4 Minutes
and Check Engine-Compartment Bilge
for Gasoline Vapors.
Verify Blower Operation.