Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

proven and tested since the early 1970s—

more than thirty years. A U.S. Coast

Guard/ABYC study performed by an inde-

pendent lab found no failures of any kind in

polyethylene tanks. Former technical direc-

tor of ABYC Tom Hale concluded that

“polyethylene fuel tanks pose no risk at all.”

Further, polyethylene tanks have all the

advantages of FRP—lower weight than metal

tanks with zeropossibility of corrosion.

Poly tanks do have two drawbacks. One

is the difficulty of making them strong

enough in sizes over 80 gallons (300 L), be-

cause baffles are nearly impossible to build

in. The second is that poly tanks are available

only in a limited range of sizes and shapes.

Any form, of course, can be fabricated, but

tooling up for a custom poly tank requires an

investment of roughly $3,000 or more. Still,

many off-the-shelf polyethylene tanks are

made and will suit numerous applications at

low cost. Whenever I can find an existing

poly tank that will do the job, it’s my first

choice.

Another thing to keep in mind about

polyethylene tanks is that they expand after

their first fill-up by about 2 percent in all di-

rections. You must allow for this in any tank

installation and follow the manufacturer’s

mounting instructions carefully.

Chapter 5: Fuel Tanks and Fittings

TABLE 5-3. MINIMUM WALL THICKNESSES

FOR FIBERGLASS FUEL TANKS BY

CAPACITY

Capacity—Gallons Thickness—Inches

5–20 0.1

20–80 0.2

80–200 0.3

200–500 0.4

500–800 0.5

Capacity—Liters Thickness—Millimeters

20–75 2.5

75–300 5.1

300–750 7.6

750–1,900 10.2

1,900–3,000 12.7

NOTE: All tanks have to have baffles spaced
no more than 30 inches (76 cm) both
athwartships and fore and aft.
NOTE: Use mat on the inner 50 percent of the
layup, then alternating layers of woven
roving and chopped-strand mat in isopoly-
ester resin. The outermost layer should be
woven roving.

Fiberglass Tanks and Ethanol

Until a few years ago, fiberglass tanks were every bit as good for gasoline as for diesel. The

introduction of ethanol into gasoline has changed this situation.

What Is Ethanol?

Ethanol is the common term for ethyl alcohol, also called “grain alcohol” (C 2 H 6 O). As you

would expect, ethyl alcohol is commonly obtained by processing grain; though, almost any

vegetable matter can be used to produce ethyl alcohol. It’s the sugar or starch in plant matter

that’s distilled into alcohol. This is the same chemical that makes alcoholic beverages alcoholic;

however, alcohol also burns and makes a moderately useful fuel. Since plants are grown as

needed, they are a renewable energy source (unlike petroleum). This makes alcohol a useful

potential alternative to petroleum-based fuels such as gasoline.

The word alcoholis used generically when speaking of ethanol, but keep in mind that

there are other alcohols such as methanol (wood alcohol) and isopropyl alcohol (rubbing

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