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

It’s a good idea to add extra engine

compartment vent area for these items as

well. For instance, a 4,000-watt inverter is

roughly working at 5.3 hp.

4,000 watts= 4 kW

4 kW ÷ 0.7457 hp/kW= 5.3 hp

You could simply add this 5.3 hp (4 kW) to the

total engine vent calculation along with any

generator. This isn’t strictly accurate, as only

a modest portion of the 4 kW is lost as heat.

However, it is an easy way to make a gener-

ous estimate for ventilation purposes.

Battery Venting and Considerations for LPG (Propane) and CNG

The Danger of Hydrogen

Standard marine batteries produce hydrogen

gas as a by-product of the charging process.

Hydrogen mixed with air is dangerously ex-

plosive in mixtures anywhere from 4 to 74

percent by volume—quite a large range. It’s

critical that battery compartments or the en-

tire engine compartment be well ventilated

during charging.

A problem here is that gasoline fumes are

heavier than air and sink, while hydrogen gas

is the lightest of gases and rises. The layout

(particularly on gas-engine boats) of the

engine compartment ventilation will often do

little to scavenge hydrogen. It is important

that there be ventilation at the top of the en-

gine or battery compartment to vent hydro-

gen gas overboard. The solar vents described

earlier for general engine compartment vent-

ing will often serve here. Install the vent to

take suction as high up in the engine com-

partment as possible.

In fact—though seldom done—best prac-

tice is to locate the batteries in a separate

compartment or enclosure. This should be

vented directly overboard, with its own dedi-

cated exhaust and intake. The exhaust line

should have a standard 3-inch (7.6 cm), 4.2

m^3 /min., ignition-protected exhaust blower

taking suction from the top of the battery en-

closure, with a passive inlet leading to the

bottom of the battery enclosure. I’ve probably

only seen this done a couple of times in all

the boats I’ve been on, however.

Note that when the boat is left unat-

tended but connected to shore power, the au-

tomatic battery charger will be keeping the

batteries topped up. The powered blowers on

the boat will be shut down and won’t remove

any hydrogen. Again, this is a good reason for

a solar-powered vent—ideally, a solar-

powered day/night vent.

LPG and CNG Venting

LPG (liquefied petroleum gas, or propane)

and CNG (compressed natural gas) are com-

mon stove fuels on many boats. Occasionally,

these fuels are also used for flash-type, “in-

stant” hot-water heaters or sometimes for

cabin heating. These fuels are even approved

for use on passenger vessels. (Be sure to rig-

orously follow the USCG requirements for in-

stallation under the CFR.) As with any such

fuels, the potential for a serious fire or explo-

sion is real. Refer to ABYC standards A-22,

Marine Compressed Natural Gas (CNG)

Systemsand A-14,Gasoline and Propane Gas

Detection Systemsbefore making any such

installation, and be familiar with them when

inspecting or surveying an existing system.

We’re only dealing with ventilation here,

so the fundamental rule is that LPG and CNG

mustbe kept out of the boat. These fuels

must be stored in gas-tight (gasket sealed)

lockers (Figure 16-4), completely sealed from

the interior of the vessel and venting over-

board. The overboard vents must be located

where the vented gases cannot find their way

back into the boat through some other open-

ing such as a hatch, window, or intake vent.

Like gasoline, LPG is heavier than air

and will sink to collect in the bilge. CNG is

lighter than air and rises. When designing

PART FIVE:VENTILATION, AIR-CONDITIONING, AND HEATING

Figure 16-4. LPG lockers (Courtesy Trident)