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

system and the effective net output of air.

Column 3 of Table 16-2 gives the minimum

effective blower system output after allowing

for duct and other system losses.

Example: Assume that After Burner’s

engine vents are fitted with louver grilles at

the outside and with a 90-degree bend in the

water-trap box inside, and then run through

2.5 feet (0.76 m) of duct hose to exhaust from

low in the bilge. Referring back to Table 14-4,

we would find

2.5 feet× 2% per foot= 5%

90-degree bend= 10%

Louver grille= 20%

Total approximate loss= 35%

Effective vent output= 100% – 35%= 65% of

blower’s rated cfm output

If we installed a single 250 cfm blower, the

net blower system output is then

250 cfm× 0.65= 162.5 cfm

or

7 m^3 /min.× 0.65= 4.55 m^3 /min.

Referring to Table 16-2, we find the mini-

mum acceptable blower system output is

Net compartment volume= 290 cu. ft.

(7.89 m^3 )

From column 3 of Table 16-2, we find

Net compartment volume is over 100 cu. ft.

(2.839 m^3 )

Blower system output= Fo=

(0.2× 290 cu. ft.)+ 40 = 98 cfm

or

Blower system output= Fo=

(0.2× 7.89 m^3 )+ 1.13= 2.71 m^3 /min.

Accordingly, the combination of the

250 cfm (7 m^3 /min.) blower we selected with

the louvers and ducting planned should meet

the CFR requirements. The final system out-

put is subject to test measurement on the

actual boat—to prove that the system as in-

stalled really does deliver a minimum of

98 cfm (2.71 m^3 /min.).

Again, if we would install the twin 250 cfm

(7 m^3 /min.) blowers, we would find that un-

der the engine vent sizing rule we would meet

this blower output requirement with ease.

Exhaust Ducts Located

Low Down

Because gasoline vapors are heavier than air

and sink to collect in the bilge, the blower

exhaust ducts must extend down to the

lower third of the compartment volume

(Figure 16-5). In some compartments, this

can pose a problem because the duct can be

so low that the duct intake opening might be

PART FIVE:VENTILATION, AIR-CONDITIONING, AND HEATING

Figure 16-5.
Exhaust ducts for
blowers must
extend down to the
lower third of the
compartment vol-
ume (Courtesy
ABYC)