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

of fuel consumption. Even better, you can

install a filter between the main tanks and

the day tank as shown in Figure 4-7. In out-

of-the-way places where fuel quality is of-

ten very poor, this gives you a chance to

double filter and to monitor fuel quality as

it feeds the day tank. (This practice is

called “polishing” the fuel.) For this rea-

son, I don’t like to fit the day tank with a

deck fill, though a vent, of course, is still

necessary. I prefer to pump the day tank

full only from the main onboard tanks.

With this fuel polishing, you can safely use

dirtier fuel—if you have to—than you

would be able to with a single pass through

a single filter. What’s more, dirt and sludge

have a second chance to settle out in the

day tank. Obviously, in such situations, all

the filters have to be checked, drained, and

cleaned frequently.

Bypass Feed Lines

Should the fuel-transfer pump fail, the

schematic shown in Figure 4-7 includes

bypass feed lines direct from the wing

tanks to the engines. The bypass valves are

normally closed—opened only in emergen-

cies. Note that this emergency configura-

tion violates the always-take-and-return-

from-the-same-tank rule. There are no

return lines to the wing tanks. If you draw

directly from the port tank to feed both

engines, the return fuel will still be going

into the day tank. At 60 gallons per hour

(227 L/hr.) gross fuel flow, roughly 20 gallons

PART TWO:FUEL SYSTEMS

Figure 4-7. Diesel
day-tank fuel
piping