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Net compartment volume = 319.7 cu.ft. (assuming openings on the bulkheads are less than 2% of their area.)
From column 2 of the table “Gasoline Blower Requirements” (previous page) we find:
Net compartment volume is over 100 cu.ft.
Blower capacity = Fr = (319.7 cu.ft. ÷ 2) + 100 = 259.8 CFM
In this case, the powered-blower requirements would not be met with a single standard 4-inch dia. 250 CFM squirrel-ca
exhaust blower. Any combination of blowers that will deliver 260 CFM would be acceptable. One off-the-shelf blower th
would meet the requirements would be Delta “T” Systems, 12-volt ignition-protected centrifugal blower, rated at 350 CF
Gasoline Blower System Output
Ducting, grilles, and such reduce the efficiency of the ventilation system and the effective net output of air. Column 3 of
the “Gasoline Blower Requirement Table,” gives the minimum effective blower-system output after allowing for duct an
other system losses.
We know that the
Rocket X-32
’s engine vents are fitted with louver grilles at the outside, and a 180-degree bend in the
water-trap box inside and then run through 5 feet of duct hose to exhaust from low in the bilge. Referring to the table
“ABYC Estimated Effect of Blower-System Components,” we find:
A 180-degree bend equals two 90-degree bends. So:
5 feet x 2% per foot
= 10%
(2) 90 degree bends
= 20%
Louver grille
= 20%
Total approximate loss = 50%
Effective vent output = 100% - 50% loss = 50% of blower’s rated CFM output
If we installed our single Delta “T” 350 CFM blower, the net blower system output is then:
350 CFM x 0.50 = 175 CFM
Referring to column 3 of the table “Gasoline Blower Requirements” we find the minimum acceptable blower system o
put is:
Net compartment volume = 319.7 cu.ft.
From column 3 of the table “Gasoline Blower Requirements” we find:
Net compartment volume is over 100 cu.ft.
Blower system output = Fo = (0.2 x 319.7 cu.ft.) + 40 = 103.9 CFM
Accordingly, the 350 CFM blower we selected, with the louvers and ducting planned, should meet the CFR requirements
The final system output is subject to test measurement on the actual boat to prove that the system, as installed, really
does deliver a minimum of 103.9 CFM.
Note that there are two vent runs. One has the engine blower exhausting the engine space, with its duct running to low i
the bilge. The other is the passive air intake.
In closing it’s important to note that the above is the
U.S. legal requirement for the gasoline blowers; however
more blower capacity is generally recommended. More fresh air means more efficient combustion, a cooler engine co
partment, and less mildew rust and decay.
ABYC Estimated Effect of Blower-System Components
Percent Loss of Blower Rated Capacity
2% per ft. of length
Ducting Bends – 90˚
10% each bend
Clamshell Vent Intake
Screen – 1/4-in. mesh