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

PART FIVE:VENTILATION, AIR-CONDITIONING, AND HEATING

As installed power increases, the blowers

become more important:

• Above 500 hp (373 kW), blowers are
  required at a minimum of 33 percent of
  recommended airflow.


• From 750 to 1,000 hp (560 to 745 kW),
  blowers are required at a minimum of
  50 percent the recommended airflow.


• Over 1,000 hp (745 kW), blowers are
  required at a minimum of 100 percent
  the recommended airflow.

NOTE: The power is the power of allthe

machinery in the engine space: all engines,

all generators, compressors, heaters, and so

on.

AsAfter Burnerwas fitted with twin

325 hp (242 kW) diesels (with the gen set in

another compartment), this boat requires a

minimum of 197 sq. in. (1,260 cm^2 ) of vent

area.

325 hp× 2 = 650 hp

650 hp ÷ 3.3= 197 sq. in.

or

242 kW× 2 = 484 kW

484 kW× 2.6= 1,260 cm^2

The real After Burnerwas fitted with four

4-inch by 5-inch (10 cm by 13 cm) louvered

vents, which may seem like plenty but works

out to a bare 80 sq. in. (520 cm^2 )—less than

half the required minimum. What’s more, the

air had to travel down long, narrow, twisted

ducts to reach the engines. No wonder this

vessel ran so hot. In fact, it’s a wonder it kept

running at all.

Due to its complex ducting—the proper

venting for After Burner’s engines would

be 20 percent greater than the minimum of

197 sq. in. (1,260 cm^2 ), indicated by Formula

16-2, or around 237 sq. in. (1,525 cm^2 ), plus

a pair of 250 cfm (7 m^3 /min.) exhaust blowers.

(The minimum airflow formula recommends

1,697 cfm, and 1,697 cfm× 0.33= 560 cu.

ft./min., which about equals two 250 cfm

blowers—or, minimum flow recommenda-

tion is 48 m^3 /min., and 48 m^3 /min.× 0.33= 15.8

m^3 /min., which about equals two 7 m^3 /min.

blowers.) These blowers should be wired

to run whenever the ignition is turned on

for diesel boats, and on gasoline craft

should be turned on at least4 minutes be-

fore engines start, and left on thereafter.

This means the blowers (gasoline or diesel)

mustbe rated for continuous duty. It also

means that you need to check the electric

system to ensure it can handle this modest

but continuous additional load—a potential

problem on some low-powered sailboats,

but seldom a difficulty on powerboats.

Engine compartment blowers should

usually be set to exhaust, not as intake. If air

is forced into the engine room, it raises the

pressure, slightly driving unpleasant engine

odors into the rest of the boat.

Two hundred and fifty cfm (7 m^3 /min.)

blowers are standard 4-inch (10.2 cm) diam-

eter blowers. The exhaust outlets can be led

through baffle boxes on the side of the hull.

A clamshell vent is installed on the hull exte-

rior over the opening. If the vent runs

through the hull side, the clamshell opening

should face aft and be angled down about 15

degrees. If the vent is through the transom,

the clamshell opening should be angled

down and inboard, toward the boat center-

line, at about 30 degrees.

Remember that this vent area rule gives a

minimum number. You can’t have too much

engine compartment ventilation! Check the

vent area on any boat or design you’re deal-

ing with. You’ll often find that you have less

than the recommended minimum. Even

though the boat has the minimum, if at all

possible you should add more vent area or

powered ventilation. The engine will last

longer, fuel consumption will improve, and

what’s more, the engine compartment will

smell sweeter too.

Traditional Half-Cowl

Side Vents

Many traditional motorboats have a pair of

half-cowl side vents (two each port and star-

board, four total) providing most or all of the

engine ventilation. These half-cowl side vents

(and also clamshell vents) can be useful. You

will see, however, that even four of these in

any size that will reasonably fit on the side

deck will not provide adequate engine vent

area on their own.