Some engines (like the Cummins B and
C series) require that the return line extend
down to within 1 inch (25 mm) of the tank
bottom. This is so that air cannot enter the
fuel system on these engines when the tank is
partially full and the vessel is sitting idle for
some time. Other diesels don’t have this
requirement. Be sure to check with the en-
gine manufacturer about this detail before
ordering or building tanks. If in doubt, simply
run the return to the tank bottom, and this
problem won’t occur in any engine.Twin-Tank Diesel Piping
The most common arrangement for diesel
tanks on yachts is simply twin tanks. The
schematic in Figure 4-5 shows a standard
setup with a single gen set. In this system, the
starboard tank feeds the starboard engine
and the port tank feeds the port engine. Dur-
ing normal operation, the valves for both feed
lines and both return lines are open, but the
cross-over valves are closed, as they are in
the gas fuel system earlier. If for some rea-
son it were necessary to run both engines off
one tank you would open the cross-over
valves and close the valves in the manifold
coming from or going to the tank you don’t
want to use (both feed and return).
The generator in this arrangement is
switchable, but in this case, not as an emer-
gency or backup procedure but as a routine
measure. You select either the port or star-
board tank at the manifold. Best practice is to
alternate days: starboard one day and port
the next. If you have more than one gen set,
you can set up a feed and return generator
manifold after the Y-valves. Doing this means
that the piping from the tank to the Y-valve
and the manifold itself must have a cross-
sectional area (at inside diameter) at least
equal to twice the cross-section area of each
feed line to each gen set. The same applies
to the generator-line return piping manifold.
Keep in mind that you must return fuel to
the same tank you take it from. If you don’t—
and the tanks have recently been topped off—
then you will be returning fuel to an already
full tank that isn’t otherwise being used. This
will cause it to overflow and spill through the
vent. Not only is this wasteful and hard on the
local flounder population, but it’s an illegal
fuel spill punishable by a substantial fine.PART TWO:FUEL SYSTEMS
Gasoline Antisiphoning
Gasoline spills into the bilge are far more dangerous than diesel
spills.Accordingly, all gas-engine craft must be equipped with anti-
siphon protection.John Eggers of EVM, Inc., explains that the pur-
pose of the antisiphon protection is to prevent emptying the tank
should someone, say, accidentally step on the fuel line near the en-
gine and break it off.If the broken line were full of fuel and
drooped down below the level of the fuel tank,it would literally
siphon the tank contents into the boat—extremely hazardous!
Antinsiphon protection can be achieved by keeping all portions
of the fuel lines—right up to the carburetor—above the tank top,
using A1 hose, and securing the works so that it can’t fall downward
to create a siphon.Here, clearly,ifthe fuel line ruptures, the gas
will simply run harmlessly back down into the tank.If this approach is
taken, a standard shutoff valve at the tank takeoffis still required.
It’s often difficult or inconvenient to arrange the fuel system this way,
in which case an inexpensive (usually $3 or less) antisiphon device
must be installed in the fuel takeoff line right at the tank.
The most common antisiphon device is a ball check valve that
prevents flow from the tank at low suction (as caused by a leak),
but opens to allow flow at the higher suction created by the fuel
pump.Alternatively, an electronically operated shutoff valve (a so-
lenoid) can be installed at the tank takeoff and wired to the igni-
tion.Since the antisiphon device prevents all flow from the tank
unless the engine is running, no additional manual shutoff valve is
required.Antisiphon fittings are longer than standard hose barb
connectors.Inspect to see that these have in fact been installed.Antisiphon device (left) and plain
hose barb (Courtesy EVM, Inc.)