duplex fuel filters (see, for example, Fig. 4-3).
Fuel tanks and fuel supplies are seldom as
clean as they ought to be. A bad batch of
fuel or some sludge getting into the fuel
lines can clog a filter and stop an engine
quickly. Invariably, this happens when a
menacing squall is bearing down or during
close-quarters maneuvering in harbor. The
duplex system shown in Figure 4-3 permits
switching to the second filter; removing,
cleaning, and replacing the clogged filter;
and continuing on without even slowing
down. (On a gas-engine craft you wouldn’t
want to change the filter with the engine
running, but you can still switch over and
then clean or change the filter later.) You
can make such a system using over-the-
counter filters, valves, and piping compo-
nents, or—much better—you can purchase
a ready-made duplex system from compa-
nies like Racor and Separ. With a single-
engine vessel, the duplex filters and fuel
lines need only be large enough to handle
the one engine.
Of course, duplex filters are optional.
Most boats get along well enough with single
filters. You can simply substitute a single fil-
ter for a duplex in the schematic if that’s your
preference. Considering the relatively small
expense compared with the cost of an entire
boat, however, I think it’s penny-wise and
pound-foolish not to install duplex filters on
all but the smallest of day boats and
runabouts.Fuel-Filter Mounting
and Location
Each fuel filter or fuel strainer must be
mounted securely on the engine or the boat
structure, independent from the fuel-line con-
nection unless the fuel filter or strainer is in-
side a fuel tank. You must be able to access
the filter easily and regularly to inspect the
filter bowl and to drain when needed. This
means having a clear space of at least
8 inches (20 cm) under the filter so you can
put a cup or can under the filter drain valve to
collect the discharge. You also need clear-
ance above the filter to remove the top and
pull the filter out vertically. This is usually at
least 85 percent of the filter height above the
top of the filter, but check the filter-element
dimensions and the manufacturer’s drawings
for the filter. Then during installation, do a
dry run. Remove the filter top, and draw out
the element to make sure there’s adequate
clearance.Sizing the Filters
You must be certain that each filter (whether
operating solo or in a duplex) and all associ-
ated fuel lines are sized to handle maximum
possible flow. For diesel, total fuel flow rate
through the filter in gallons per minute
(gpm) can be generously estimated as gpm
= max. engine hp ÷360, or L /min. = max.
engine kW ÷ 70. 9. Note that this flow rate is
considerably higher than actual engine fuel
consumption; it includes the extra flow for
injector cooling and lubrication—the extra
taken off by the return line. The rule of
thumb is that one-third of the fuel delivered
will be burned and the remaining two-thirds
is returned to the tank. (We can expect this
ratio to increase to 20 percent: 80 percent in
some new, very-high-power engines cur-
rently in development.) A vessel with twin
350 hp (261 kW) diesels would require a
minimum of two 1.0 gpm (3.68 L/min.) filters
(350 hp ÷360 = 0.97, say, 1.0 gpm; or 261 kW
÷ 70 .9 = 3.68 L/min. Multiplying by 60 min-
utes—60 gph or 227 L/hr.). Greater capacity
would do no harm. In the twin-duplex filter
arrangement shown in this chapter’s
schematics, there would actually be four
water filters/separators, a total of two for
each engine.PART TWO:FUEL SYSTEMS
Figure 4-3. Separ
duplex fuel filter