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

primary filter, feeling it will ensure cleaner

fuel to the engine. This is acceptable, but it

will clog up more quickly unless the boat is

using very clean fuel.

The best place for a 10-micron primary

fuel filter would be in the duplex filter installed

after a day tank (polishing tank). The filter

from the wing fuel tanks to the day tank would

have a 30-micron element, and the duplex filter

after the day tank would have the 10-micron

filter elements. This way, the coarser grit is re-

moved before the day tank and then somewhat

finer grit after the day tank. One drawback is

the necessity of purchasing and keeping track

of two different filter types. Using all 30-micron

primary filters is fairly standard.

The secondary filter is the fine filter on

the engine itself. The secondary filter re-

moves the finest of remaining impurities.

Secondary filters generally range from 2 to

7microns. If in doubt, use 2 microns, but

always refer to and follow the engine manu-

facturer’s recommendations.

Gas-Engine Piping

Figure 4-4 shows a straightforward gas-

engine fuel-piping arrangement for a twin

gas-engine boat with twin tanks. In normal

operation, the port tank feeds the port en-

gine and the starboard tank the starboard

engine. The cross-over valve is closed. If,

however, you want to run off one tank only,

you can shut the takeoff valves on the tank

you don’t wish to use and open the cross-

over valve.

The generator feeds off either tank

through a Y-valve, though you could eliminate

the Y-valve and simply open and close the

takeoff valves for the generator at each tank.

Diesel Return-Line

Complications

Gas engines (with the exception of some

fuel-injected gas engines, as noted above)

simply run their fuel one way: from the tank

to the carburetor. However, diesel engines

use their fuel not only as fuel but also to

lubricate and cool their injectors. The excess

diesel—carrying off considerable unwanted

heat—has to be piped back to the fuel tank.

This makes diesel fuel runs twice as compli-

cated as those for gasoline. It’s straightfor-

ward enough, however, if the piping is set up

as shown in Figure 4-5 or 4-7. The return line

(sometimes termed the “spill”) should enter

the tank as far from the takeoff line as possi-

ble to prevent the drawing of foaming fuel

into the pickup. The minimum workable dis-

tance is 15 inches (38 cm), but more is always

better.

PART TWO:FUEL SYSTEMS

Principal Differences Between ISO 10088 and ABYC

Standards

What we’ve been covering here is for compliance with ABYC guidelines.ISO 10088 is gener-

ally quite similar but has a few differences, most importantly,

1 .ISO 10088 requires access not just to the fuel-line and tank fittings but to the entire fuel

line as well.

2 .ABYC requires 15 inches (381 mm) clearance between fill openings and any vents.ISO

10088 adds two additional clearance requirements:

a. The clearance between a gasoline tank and any part of the engine must be a

minimum of 100 mm (4 in.)

b.The clearance between a dry exhaust and a gasoline fuel tank must be a minimum of

250 mm (10 in.)

NOTE:I agree with both ofthese as bare minimum clearances—more is better.

3 .ABYC requires the minimum inside diameter of a fill pipe to be 1^1 / 8 in.(28.5 mm).ISO

10088 requires a minimum inside diameter of 31.5 mm (1^1 / 4 in.).