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

and flexible hose is used to connect to the en-
gine and any other vibrating or moving
components.

ENGLISH-UNITCOMMON-RAILSIZINGRefer-
ring to the pipe tables (Appendix B), we see
that the inside diameter of the Sched. 40,^3 / 4 in.
branch pipes is 0. 824 in. ID (inside diameter)
each. Use the following formulas to find their
cross-section areas.

Formula 4-1a. Cross-Section Area in
English Units

π(0. 824 in. ID ÷2)^2 = 0 .53 sq. in. π≈ 3. 14

0 .53 sq. in. ×4 branch pipes × 1. 1 = 2 .33 sq. in.

minimum common-rail section area

Referring to the pipe tables (Appendix B),
we see that a 1^1 / 2 in. Schedule 40 pipe has an
ID of 1. 610 in.—a bit too small. So we use the
next size up: 2 in. Schedule 40, OD 2. 375 in.,
ID 2. 067 in., as pictured in Figure 4-9.

METRICCOMMON-RAILSIZING The inside di-
ameter of the 26.9 mm OD × 2 .9 mm wall
branch pipes is 26.9 mm – (2 × 2 .9 mm wall) =
21 .1 mm ID. Their cross-section area is then

Formula 4-1b. Cross-Section Area in
Metric Units

π(21.1 mm ID ÷2)^2 = 349.7 mm^2 π≈ 3. 14
349 .7 mm^2 ×4 branch pipes × 1 .1 = 1,538 mm^2
minimum common-rail section area

We know that the outside diameter must
be somewhat larger than 44.2 mm. Referring to
the pipe tables (Appendix B), we calculate that
a 51 x 3.2 pipe has an ID of 44.6, which will do
nicely [51 mm – (2 x 3.2 mm) = 44 .6 mm].

COMMON-RAIL MANIFOLDS HAV E MANY

APPLICATIONS Note that common-rail mani-

folds are useful in many applications. You can

employ them for freshwater systems, seawa-

ter intake (sea suction, see Chapter 17)

either attached to a sea chest or to individ-

ual seacocks, or any other application with

multiple pipes. Keep the common-rail mani-

fold approach in mind for general use.

Fuel-Transfer Pumps

The fuel-transfer pump should be self-priming

and have the flow capacity to fill the day tank

in a reasonable period of time, say under 15 or

20 minutes. In real-world installations, pumps

seldom deliver more than 70 percent of their

rated flow. Thus for, say, an 80-gallon (303 L)

day tank, you want about an 8 gpm-rated

(30 L/min.) pump (80 gal. ÷ 0 .70 = 114 gal., and

114 gal. ÷15 min. = 7.6, say, 8 gpm; or 303 L ÷

0 .70 = 433 L, and 433 L ÷15 min. = 28.8 L/min.,

say, 30 L/min.). Rotary sliding-vane pumps

meet the criteria for self-priming (at least

within 2 feet [60 cm] or so of lift, adequate for

most boats), and they’re available, rated for

continuous use with diesel fuel. Though sel-

dom critical, it’s worth considering a reversible

vane pump, which will enable you to empty

the day tank back into one of the main tanks

for cleaning or repair. Gear pumps are another

good option, particularly for larger vessels

requiring high transfer rates.

High-volume transfer pumps can gener-

ate high pressures should a blockage occur.

In the worst case, this can lead to a fuel-line

rupture and a horrendous fuel spill. Such

pumps should be protected with a relief

valve. Since you can’t have the relief blowby

spraying into the boat, the blowby should be

plumbed through a bypass line into the near-

est tank, usually the day tank. An alarm and

an automatic pump shutoff on the relief valve

complete the safety picture.

If the crew monitors the fuel transfer

carefully, they can shut off the fuel transfer

before overfilling. It is, however, too easy to

forget. I recommend that a second tank-level

sensor be installed in the day tank and dedi-

cated to the transfer pump shutoff. It should

be set up to automatically switch off the

transfer pump at about 97 percent full. A one-

time audible alarm (a few beeps, a momen-

tary ring) alerts the crew.

required common-railID 2

1,538 mm

44 .2m

2

=×

=

π

mm

required common-railID 2

section area

=×

π

required common-railID 2

section area

=×

π

Chapter 4: Fuel Piping and Fuel System Bonding

required common-railID 2

2 .33 sq.in.

1.

=×

=

π

7722 in.

Formula 4-1b.

Formula 4-1a.