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On the minimum gasoline engine vent blower capacity.
(This Issue’s Question is on Page
16)
The Know It All questions and correct answers are important design tips for students as well
as other marine professionals. We suggest that you file them away for future reference.
The Question Was:
The builders of MagicFast Boats have retained you to determine the minimum engine-compartment blower capacity for
their new model, the
Rocket X-32
. A 32-foot express cruiser, she’s to be equipped with twin 350-hp V8 gasoline engines
conventional V-bottom boat, her engine compartment is 8.2 feet long and 10.6-foot beam. Height, from the keel to the
underside of the engine-compartment overhead is 5 feet, and height from the chine up to the overhead is 3.9 feet. In a
tion to the two V8 engines, the engine compartment contains five 8D batteries plus a pair of wing tanks, 170 gallons e
Also, there’s a 12-gallon water-heater and a 20-gallon black-water tank. Finally, assorted other small machinery is
mounted in the space, including the forward portion of the exhaust runs, together amounting to about 4 cubic feet of vo
ume. The
Rocket X-32
’s engine vents are fitted with louver grilles at their exits in the hull topsides, and 180-degree ben
in the water-trap boxes inside. They then run through 5 feet of duct hose to exhaust from low in the bilge. What is the m
mum acceptable engine-vent blower capacity?
The Winner Is:
We only received a few answers to the December 2011 Know It All question, and only one of those was correct. This wa
the solution submitted by Westlawn student Allen Leslie Newton. Such a demonstration of sagacity—and perhaps even
perspicacity—clearly shows that Les is too smart for his own good. We are thus duty bound to declare him a Know It All,
with all rights and privileges pertaining thereto. Henceforth, Les should be addressed by the august title of “Know It All,”
and of course, a Know It All certificate, Westlawn T-shirt, and cap are on their way to our distinguished winner.
And the Solution Is:
Gasoline vessels, must meet specific legal requirements under the CFR (Code of Federal Regulations). You need to follo
ABYC standards H-2 “Ventilation Of Boats Using Gasoline.” Keep in mind that the requirements for ventilation regarding
gasoline apply to any blend of gasoline, including ethanol gasoline such as E-10 or E-15, or even E-85 (10%, 15%, or 85
ethyl alcohol by volume).
In the U.S., the law requires that every recreational boat with a permanently installed gasoline engine must comply with
Subpart K – Ventilation, 33 CFR, sections 183.601 through 183.630. There are no exceptions: main engine, generator,
any other gasoline-driven device that is permanently mounted in the boat.
All
gasoline-engine boats must comply with the CFR and be ventilated accordingly, unless—under the CFR—the engine i
installed out in the open, “open to the atmosphere.” This is specifically defined as a compartment or space that has 15
square inches directly exposed to the open outside air for every net cubic foot of compartment space. A small cuddy-ca
boat, with a gas engine under the cuddy would likely qualify as “open to the atmosphere” if the cuddy was wide open al
its after wall—there was no bulkhead closing it off. Critically, if any provision at all is made to seal this open after face o
the cuddy with even a canvass cover, it is no longer “open to the atmosphere” and will have to be ventilated according t
the CFR.
Net Compartment Volume for Gasoline Ventilation
The first step is to calculate the net engine compartment volume.
Tank volume can be found from:
Tank volume, cu.ft. = 0.134 x tank capacity in U.S. gallons
Average-size gasoline engines and small diesels, as well as batteries, can
be approximated from the table to the right.
For larger engines, you should consult the manufacturer.
Based on the overall engine-compartment dimensions, we find the gross
volume:
5 feet height at centerline + 3.9 ft. height chine to overhead ÷ 2 = 4.45 ft. average height
Continued next p
Approximate Machinery Volumes
Volume
Item
cu.ft.
m
3
4-cylinder, in-line engine
2.5
0.0
6-cylinder, in-line engine
3.5
0.1
6-cylinder, V-6
4.0
0.1
Small V-8
4.5
0.1
Large V-8
5.5
0.1
Batteries
0.5
0.0