CHAPTER 5 Fuel Tanks and Fittings
Hold That Tank
The fuel tank installation in Figure 5-1
shows the basic requirements for a stan-
dard inboard tank installation. Clearly, you
don’t want your tanks sliding around—not
ever! They have to be extremely well
secured. Wood or wood-cored-FRP chocks
and blocks that are laminated, screwed,
glued, or bolted in place or metal straps fas-
tened with turnbuckles do the trick. Riser
chocks—on which the tanks rest—should
be 2 inches (50 mm) wide for tanks under
150 gallons (570 L) and 3 inches (75 mm)
wide for tanks over 150 gallons (570 L).
They should be about 1^1 / 2 to 2 inches (38 to
50 mm) high (or higher as necessary to sup-
port the tank properly), spaced roughly on
15-inch (38 cm) centers and arranged to
permit drainage of water and airflow for
ventilation. Band-It bands and buckles of
316 stainless steel do a nice hold-down job.
The breaking strength of the chocks, secur-
ing bands, fastening bolts, and screws must
be 4 times the total combined weight of the
tank and its contents, or more.
Aluminum-Strap
Hold-Downs
Perhaps the simplest, least expensive, and
most common tank hold-downs are made
from aluminum flat bar^1 / 8 inch (3 mm) thick
and 1^1 / 2 inches (38 mm) wide (Figure 5-2).
These straps are thin enough to bend to any
required shape but wide enough to take fas-
tenings and provide sufficient bearing on
relatively thin tank walls. The effective
strength of these hold-down straps is limited
by the strength of the fasteners at either
end. In wood chocks or wood or plywood-
cored FRP chocks, use three No. 14 (6 mm)
self-tapping stainless screws. (G-10 is an
even better core material than wood for this
application.) With a minimum bury of 1^1 / 4
inch (32 mm), this gives a breaking strength
of around 1,400 pounds (635 kg) at each side
of the strap—2,800 pounds (1,270 kg) total.
If you were securing a 300-gallon (1,136 L)
diesel tank, a look at Table 5-1 shows it
would come in at 2,335 pounds (1,059 kg).
Four times this is 9,340 pounds (4,236 kg),
and 9,340 divided by 2,800 pounds per strap
is 3.33 (4,236 kg ÷1,270 kg per strap= 3 .33).
Accordingly, you would use four straps—
more wouldn’t hurt.
NOTE: Some high-speed planing craft
can expect to experience g-forces (acceler-
ations from slamming) of 6 to 8 g’s. Such
boats need to give additional consideration
to tank fastening. Tanks midships may not
experience such high g’s, but tanks and
their hold-downs located farther forward
may have to be designed to withstand the
total expected acceleration, plus a safety
factor of 3.
