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

Net engine compartment volume is

10.3 m^3 – 2.41 m^3 = 7.89 net m^3

VCR should be between 150 and 250 per hour.
Use, say, 200, so

7.89 m^3 × 200 changes per hour= 1,578 m^3 /hr.
1,578 m^3 /hr. ÷ 60 min./hr.= 26.3 m^3 /min.

The engine vent sizing rule from earlier
recommends 1,697 cfm (48 m^3 /min.), and you
should use the higher number; however, it
can make sense to use the VCR-calculated
flow rate as the minimum size for the pow-
ered blower, so long as this is larger than the
minimum blower size required from the vent-
sizing rule.
The problem with the VCR approach to
engine spaces is that it will give rather high air-
flow rates for large engine compartments with
small engines and little else in the space, and
proportionately lower airflow rates for smaller
engine compartments with large engines and
lots of machinery and tanks crammed in.
For instance, had the fuel and black-
water tanks not been located in the engine
space, the net volume would have been
342 cu. ft. (9.7 m^3 ). In this case, the engine
compartment ventilation would work out to
be as follows:
VCR should be between 150 and 250 per
hour. Use, say, 200, so

342 cu. ft.× 200 changes per hour

= 68,400 cu. ft./hr.

68,400 cu. ft./hr. ÷ 60 min./hr.= 1,140 cfm

or

9.7 m^3 × 200 changes per hour= 1,940 m^3 /hr.

1,940 m^3 /hr. ÷ 60 min./hr.= 32.3 m^3 /min.

The vent airflow found for the less-
crowded engine compartment is 18 percent
greater, even though we are using the same
engines in the same boat.

Venting After Engine

Shutdown

Most boats are run for a few hours, brought
back to the dock or mooring, and immediately

shut down. This is not best practice. Ideally,

the engines should be run at idle for 5 or 10

minutes to allow things to cool down evenly

and gradually before shutdown. This is an op-

erational consideration, not a ventilation is-

sue, but it is a very good idea to run the

engine compartment exhaust blowers any-

where from 15 minutes to an hour after en-

gine shutdown. Naturally, most people don’t

want to sit around waiting for a quarter of an

hour or more just to run an engine blower.

My office has had good results specify-

ing a timer switch at the helm on the blower

circuit. This allows you to turn the blower on

separately from the standard on/off switch

and set the timer for anywhere from 1 to

60 minutes. In hot summer weather, you

would usually choose around 30 minutes, and

in cold weather, 10 minutes would be ade-

quate. You can thus run the blower and leave

the boat, knowing the timer will shut it down

shortly.

Vent Requirements for Inverters, Converters, and Transformers

Inverters, converters, and transformers also

generate considerable heat. On one of my

office’s designs, the inverter would repeatedly

shut down for no apparent reason—rather irri-

tating. The engine room was properly vented

and was usually only around 85°F (30°C)

during routine operation. The problem turned

out to be that the inverter was mounted in a

corner, and with other machinery and battery

boxes nearby, it wasn’t getting enough local

airflow. When it got too hot, it switched itself

off.

We couldn’t really relocate the inverter

easily, but we remounted it with as much all-

around clearance as possible (a few inches),

and we set a small, continuous-rated,

ignition-protected axial fan to blow on the in-

verter whenever it was running. This solved

the problem. Keep the following in mind:

Allow ample clearance all around invert-

ers, converters (battery chargers), and trans-

formers, and make arrangements to get air-

flow to them.

Chapter 16:Ventilation of Machinery Spaces