Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

Chapter 16:Ventilation of Machinery Spaces

Ventilation of Engine Spaces

Engine Vent Requirements

Almost like a living, breathing creature, the
engine needs care and consideration.
Throttle it with a clogged fuel line or choke
it with insufficient air, and performance
will falter. Internal combustion engines are
much like people doing hard exercise. The
perfect conditions for jogging, for instance,
are a cool, dry shady day at sea level. This
is the environment you should strive for in
the engine compartment: low humidity,
50°F or 10°C, and oodles of clean, fresh air.
In reality, such engine room conditions are
impossible to achieve, but it’s the ideal to
strive for.

GIVEITAIR, GIVEITAIR I’m frequently
amazed by the tiny engine vents I see fitted
in high-powered craft. On one real 35-foot
(10.6 m) twin-diesel sportfisherman—we’ll
call her After Burner—lifting the engine
hatch after a hard day’s run releases a blast of
air almost too hot to bear. In fact, a ther-
mometer placed in After Burner’s engine
compartment topped out at 135°F (57°C). No
wonder this vessel didn’t need to turn on its
cabin heater until late fall!
The reality is, though, that engine com-
partment temperatures this high are hot
enough to reduce the strength of a wood
or fiberglass hull. Further, engines aren’t
designed to work in this kind of heat any
more than you are. Gaskets and hoses soften
or crack in prolonged high temperatures,
and your engine can’t develop its full power.
This is because hot air is less dense than
cold air. Less weight (mass) of air can be
forced into the same space when it’s hot
than when it’s cold. Since the amount of
power your engine produces depends on the
quantity of air crammed into the combustion
chamber during each intake stroke, rarefied,
lightweight hot air delivers less oomph per
gallon or liter of fuel burned than dense cool
air—no matter how much you turbocharge
or intercool.

ENGINE VENT SIZING RULE Some of the
information I’ve come across on the subject
of engine compartment vents is contradic-
tory and unnecessarily complicated. Some

systems base vent area on engine compart-

ment volume, others on boat length or

beam, and various other engine manufactur-

ers have specific recommendations for their

engines that, needless to say, seldom agree

with the engine-compartment or boat-length

methods.

Regardless of the method used to size the

vents, the goal is not only to supply adequate

combustion air but also to keep engine-

compartment temperature ideally to no more

than 30°F (17°C) above the outside air tem-

perature. I’ve worked up a pair of simple

rules that will allow you to determine proper

engine vent area.

Formula 16-2. Minimum Engine Venting

for Heat-Exchanger or Raw-Water

Cooled Engines with Wet Exhaust

Minimum Vent Area (sq. in.)= hp ÷ 3.3

Minimum Vent Area (cm^2 )= kW× 2.6

Minimum Airflow (cu. ft./min. or cfm) =

(2.75× hp)− 90

Minimum Airflow (m^3 /min.) =

(kW ÷ 9.5) −2.5

Formula 16-3. Minimum Engine Venting

for Keel-Cooled Engines with Drystack

Exhaust

Multiply the minimum wet-exhaust engine

vent area and airflow by 2.2.

The minimum vent area is the minimum

area for natural ventilation without blowers.

It makes allowance for standard louvers or

grilles over the vent openings and for some

short, simple form of baffle box or vent pip-

ing. If more extensive baffling or piping is

employed, you’ll need to increase the mini-

mum area by 20 percent or more. Such more

complex configurations are the case with

most boats.

Vents sized per the preceding informa-

tion should provide the airflows indicated by

the minimum airflow formulas. However,

since you can never have too much air in an

engine room, I like to install vents equal to at

least 10 percent greater area than recom-

mended, if possible, plusforced-air exhaust

blowers equal to about one-third the recom-

mended capacity per minute.

Formula 16-2.

Formula 16-3.