Note that the fuel-transfer pump must
also deliver fuel at a faster rate than maxi-
mum fuel consumption. For diesels this is the
actual fuel consumed, and—regardless of
manufacturer claims—it’s almost always
close to 0.054 gal./hp/hr., or 0.274 L/kW/hr.
For twin 350 hp (261 kW) engines (700 hp
[522 kW] total), this is 700 hp x 0. 054
gal./hp/hr. = 37 .8 gph. Dividing by 60 gives
0 .63 gpm. Or 522 kW x 0.274 L/kW/hr. = 143
L/hr. Dividing by 60 gives 2.38 L/min. This is
well under the rate of the transfer pump we
just specced, but on large engines with com-
paratively small day tanks (really polishing
tanks), the pump rate should be checked
against actual fuel consumption.
For large vessels with sizable long-
range tanks, you should install an additional
fuel-transfer pump (or a manifold to the
day-tank transfer pump) for shunting fuel
between tanks. This allows moving fuel
from tank to tank to adjust boat trim. Long-
range cruisers may also want to add piping
to allow drawing and returning directly
from the wing tanks. This considerably
complicates the fuel piping, however, and
is gilding the lily for most boats in normal
service. It may also require additional
booster pumps to handle long fuel runs.
MULTIPLE TANK SYSTEMS Though best
practice—even on the largest vessels—is to
use no more than two wing tanks plus a day
tank, the realities of fitting in machinery,
accommodations, and the tankage required
can make this impossible. In this case,
added tanks can be installed. Fuel distribu-
tion to the day tank is controlled at the day-tank-feed manifold. The transfer pump for
an installation like this must be reversible
so you can use it to pump fuel from tank to
tank to adjust trim. Since runs are long and
gravity feed to the day tank probably won’t
be reliable or even achievable, you’re rely-
ing on the transfer pump for operation. A
backup transfer pump in a quick-operation,
switchable, duplex configuration is
mandatory.
Filling multiple tanks with multiple fills is
inconvenient. On large boats, it’s also a chore
to pull the boat off the dock and turn the
vessel around to fuel the opposite side. The
best solution for this is to install identical fuel
fills port and starboard, but manifold them
and all tanks together so either the port or
starboard fill can supply every tank on both
sides. This is big-boat stuff, which is a good
thing because it requires intelligent manage-
ment during fueling. The fuel-fill manifold
should have valves to close off each tank, so
you fill each individually. In addition, there
should be a tank-level gauge panel outside at
each fueling station, port and starboard. This
way one crewmember can monitor tank levels
at the fill, while another shuts off and opens
the appropriate tank valves belowdecks as
instructed.
In laying out or modifying an existing in-
stallation for such manifolded fuel filling,
keep in mind that the pipes are sizable—
2 inches (50 mm) in diameter. It takes care-
ful planning to fit such piping and its required
valves and hangers into the confines of a
boat.
PUMPSSUCK Whenever a pump is used to
transfer fuel toward the engine, it should be
located after the filter—that is, to work in
suction. If the pump is located ahead of the
filter, it mixes or emulsifies water and other
impurities with the fuel so effectively that
the filter/separator won’t function ade-
quately. Follow this principle of locating
pumps for suction rather than for pressure
throughout.Cool Oil
As we’ve seen, the return diesel oil carries
off excess heat from the injectors; the
returned fuel can get quite hot. This will
warm up the fuel in a tank considerably,
even causing potential overflow through thePART TWO:FUEL SYSTEMS
Gasoline Fuel Pumps
Under CFR Yacht and Commercial—with
the exception of fuel-transfer pumps—
gasoline fuel pumps must not operate un-
less the engine is running.Diaphragm
gasoline tanks must not leak even if the
primary diaphragm fails.
Except for fuel-transfer pumps, all
gasoline fuel pumps must be mounted on
the engine or within 12 inches (305 mm) of
the engine.