Tank Openings and Penetrations
On gasoline tanks, all openings or penetra-
tions (e.g., vents, fills, takeoffs, level gauges,
cleanouts)mustbe on the top of the tank—
no exceptions. Diesel tanks may have open-
ings and penetrations on the tank sides, ends,
or tops, but it’s best to put most openings on
the top to minimize chances of leaks. The
exceptions are large cleanout manholes.
Such cleanouts are highly recommended on
tanks over 300 gallons (1,150 L) or so. For
smaller craft and smaller tanks, it’s difficult
to justify the added expense of cleanout
openings, and it’s frequently impossible to
locate them where they would be accessible,
in any case.
Tank Drains
Diesel tanks can also fit tank drains at the
lowest point of the tank. These are a good
idea for cleaning convenience, but it’s fre-
quently not worth the effort, as access to
these drains is usually very restricted in the
boat. If you do fit tank drains, make sure
that—in addition to the drain valve—there is
a screw cap closure at the very end of the
drainpipe. This way, even accidentally open-
ing the drain valve won’t empty the tank into
the bilge. It also becomes possible to fit a
screw-on hose fitting to pump out directly
into a storage tank or barrel, which makes
these drains much more practical.
Vent-a-Tank
Tank vents allow air to exit the tank so you
can add liquid (which is our goal, after all).
Vents (termed breathersin Europe) also pro-
tect against developing excessive pressure
in the tank. It is best to recommend not top-
ping off tanks 100 percent. On a hot summer
day, the temperature in the underground stor-
age tank can be 65°F (18°C), whereas the
temperature in the engine compartment may
be over 100°F (38°C). If the tank is topped off
with cool fuel, the fuel can heat up and ex-
pand enough to spill 2 or 3 gallons (7 to 11 L)
overboard through the vent. Yes, another
illegal oil spill.
ABYC requires that the vent line should
be at least 25 percent of the cross-section
area of the fill pipe. For standard 1^1 / 4 - inch
(DN 32 mm) ID fills, this calls for a minimum
of^9 / 16 - inch-diameter (15 mm) hose to match a(^7) / 16 - inch-diameter (DN 11 mm) vent fitting.
This is a requirement that I think could use
some revision. Modern fuel docks pump at
such a tremendous rate that these small vents
are often inadequate. Further—though they
are frequently configured in just this way—
the vent opening on the tank shouldn’t be
near the fill. Instead, it should be at the op-
posite end of the tank. On one of my designs,
a relatively long, narrow tank was fitted with
the standard minimum vent and fill sizes pre-
viously specified and with the vent near the
fill. The result was that even though we could
feel air whooshing out of the vent, the pres-
sure sensor on the fill nozzle would shut it off
regularly. Then a moment later, a large bub-
ble of air would burst out the fill pipe, spew-
ing fuel over the deck. Installing a second
vent near the fill helped, but it didn’t fully
eliminate the problem. Adding a vent at the
tank end away from the fill was the cure.
Unfortunately, it’s hard to find off-the-
shelf vent fittings larger than^5 / 8 - inch diame-
ter (18 mm). One solution is to install a
(^3) / 4 - inch NPT (DN 20 mm) pipe fitting in the
tank (0.82-in. or 20.8 mm ID±) with a Y tee
forking up to two standard^5 / 8 - inch (DN 18
mm) vents. On large or long tanks, you would
install one of these at each end, running to
(^5) / 8 - inch (DN 18 mm) vent fittings—four vent
fittings total for each tank.
CFR Commercial has a number of vent-
size variables, but basically it requires a min-
imum vent cross-section area equal to^3 / 4 - inch
(20 mm) OD tube with a 0.035-inch wall
(20 gauge or 0.889 mm). This is 0.36 sq. in.
(232 mm^2 ). The^3 / 4 - inch NPT (DN 20 mm)
pipe fitting Y’ed to two^5 / 8 - inch (DN 18 mm)
vent fittings meets this requirement.
Vent-Run Considerations
Fuel tanks must never vent into the hull; they
must vent overboard as shown in Figures 4-17
and 5-1. Any other arrangement will dump va-
pors and spill into the bilge—a sure harbinger
of a real blast! To keep water out, the vent
should be as high as possible and equipped
with the 4-inch (100 mm) or somewhat higher
gooseneck or riser shown. Vent openings
must be at least 15 inches (38 cm) away from
Chapter 5: Fuel Tanks and Fittings