(molded into and against the hull itself) for
diesel, they aren’t allowed for gas. Steel and
aluminum diesel craft can have integral
metal tanks under CFR Commercial; how-
ever, wood hulls cannot. (You can have inte-
gral wood-epoxy diesel tanks on yachts:
tanks lined with fiberglass in epoxy.) Inte-
gral tanks provide the maximum fuel ca-
pacity at the lowest cost and volume.
They’re very slightly more prone to leak,
though. This is the reason that integral
tanks can’t be used on gas-engine craft.
Cored FRP hulls (under CFR Commercial)
can also have integral tanks for diesel, but
only if PVC foam is used as core in the hull
or any of the tank walls, because there’s
been occasional trouble with sounding
sticks penetrating the inner skin, causing
the most interesting leaks. Regardless, care
is required to ensure that leaks can’t pene-
trate into the core. In addition, if sounding
sticks are used, there must be a rugged
metal strike plate fastened directly under
the sounding hole on the hull inside the
tank to prevent damaging the inner skin.
Pleasure craft can and do use balsa core in
FRP tank walls.
It’s been found that ordinary orthopoly-
ester resin will meet the 2^1 / 2 -minute burn
test requirement. I recommend, though,
that all fiberglass tanks be laid up with fire-
retardant isopolyester resin to Milspec Mil-
R-21607. Iso resins have slightly higher
mechanical properties, greater resistance
to blistering, and higher resistance to
chemical attack than orthos. Using fire-
retardant resins seems sensibly prudent for
fuel tanks. Isos are more expensive than or-
tho resins, but on comparatively small com-
ponents like tanks, the cost difference is
negligible when compared to the cost of
the entire boat.Fiberglass Tank
Construction
FRP tanks have to be laid up over a male
mold to make the inside surface smooth
and finished. All corner radiuses should be
1 inch (25 mm) or larger, and the inside
50 percent of the laminate should be all mat
and slightly resin rich to eliminate any
pinholes. Gelcoat shouldn’t be used on theinside surface, but a heavy layer of resin
20 to 30 mils (0.020 to 0. 030 in. [0.5 to 0.75 mm])
thick is necessary—again, to eliminate any
chance of pinholes. (Though not required,
the best interior finish employs 20 to 30 mils
[0.5 to 0.75 mm] of vinylester resin, which
has still higher resistance to chemical at-
tack.) Outside of the interior mat layup, al-
ternating layers of woven roving and mat
are built up to meet the thickness called for
on the table. Since FRP is bendy, the usual
practice is to use a balsa or PVC core on
the outside of the interior mat laminate
(only PVC is acceptable for commercial
craft)—finishing off the remaining mat-
roving layup over that.Fiberglass Tank-Wall
Thicknesses
Tanks under 20 gallons (75 L) don’t need core
for stiffness. Half-inch core is good for tanks
up to 120 gallons (454 L) or so, while^3 / 4 - inch
(19 mm) should be used over 120 gallons (454 L).
Large tanks, over 400 gallons (1,500 L), need
to be carefully engineered, with baffle spac-
ing, stiffeners (if required), core thickness,
and laminate all calculated. Where fittings
penetrate the tank, the laminate (with core
removed) should be increased to 1.5 times
the scheduled thickness for about twice
the diameter or footprint of the fitting. One
excellent method of attaching fittings—
recommended by Lysle Gray of ABYC—is to
use ordinary marine brass or bronze pipe fit-
tings screwed down tight from inside and out
before the top of the tank is added, and then
glassed over from outside. Baffles and all
other requirements are the same as for metal
tanks. Baffles should be fiberglass, at least
80 percent of the tank-wall thickness, bonded
to the inside of the tank with minimum 2-inch
(50 mm) bonding angles laid on resin-rich
1 .5 oz./sq. ft. (457 g/m^2 ) mat, and glassed over
into place.The Best Tank Material
Is there a “best” material for fuel tanks? I
think so—it’s polyethylene. This may be
counterintuitive, as somehow plastic seems
less robust and less fireproof than metal or
even FRP, but polyethylene tanks have beenPART TWO: FUEL SYSTEMS