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

or lpm (liters per minute). Unfortunately, that

shiny new pump rated at, say, 1,500 gph

(5,700 lph) in the catalog will seldom deliver

more than half that in a real-service installa-

tion. Indeed, 40 percent of rated capacity is

about the norm. The real flow out the

through-hull outlet from this 1,500-gph

(5,700 lph) pump thus will be about 600 gph

(2,280 lph).

Gerr’s Bilge-Pump Rule

Allowing for this loss in flow rate, what’s the

minimum pump capacity that should be

installed in a given boat? The following for-

mula will give sensible results.

Formula 18-4. Gerr’s Bilge-Pump Rule:

Minimum Pump Capacity

Pump capacity (gph) =1,000 +(100 ×

displacement in long tons)

or

Pump capacity (lph) =3,785 +(372×

displacement in metric tons)

Where

gph =gallons per hour

lph =liters per hour

Returning to Fluky Flooder, we would find

1,000 +(100× 25 .2 long tons) =3,520 gph

3,520 gph ÷60 min./hr. = 58 .7 gpm

or

3,785 +(372× 25 .6 metric tons) =13,308 lph

13,308 lph ÷60 min./hr. =222 lpm

This is almost 6 times more than CFR

Commercial for Fluky Flooderand more

than twice the ABS requirement. This is the

minimum power-driven pump capacity

(manufacturer’s rated capacity; we’ve al-

lowed for only 40 percent efficiency) that

Flukyshould be fitted with. If you have less

capacity installed, go out and get a bigger

pump or an additional one. Remember,

however, the “minimum”—you can never

have too much pump capacity. It wouldn’t

be unreasonable to install 3 or 4 times this

flow rate if you had the space and available

power. In addition, every boat should be

fitted with at least one manual bilge pump

of at least one-third the recommended

power-driven pump capacity (up to the

maximum that a manual pump can be ex-

pected to deliver).

To maximize pump performance, you

should do the following:

• Use the smoothest hose you can find.


• Avoid or replace the common corru-
  gated bilge-pump hose completely.


• Minimize the number of kinks and
  bends.


• Keep the hose run as short as practical.

You also need to make certain all electric

connections are tight, sound, and corrosion-

free. A loose connection or corrosion buildup

will cause a voltage drop that will greatly

reduce the pump’s discharge rate.

The Vented Loop on

Bilge Systems

Keep in mind that when heeled, sailboats can

immerse a bilge-pump opening in the top-

sides that’s well above the waterline when

the boat is vertical. You must be careful to

work out the location of the bilge-pump exit

in the worst likely heeled condition. If it will

be below the heeled waterline, you must in-

stall a vented loop as described in Chapter 7,

page 112.

It’s best to locate the vented loop as

close to the centerline as possible. You can

see in Figure 18-3 that moving the vented

loop inboard allows you to lower it, while

still being above the heeled waterline. Un-

fortunately, accommodations, machinery,

or tanks often take up the middle of the ves-

sel, so you’ll frequently have no choice but

to locate the vented loop outboard. But see

how much you can reduce the static head—

if you can move the loop inboard and thus

lower it. This either increases bilge-pump

output or allows the same output with less

power.

PART SIX: PLUMBING SYSTEMS WITH NOTES ON FIRE SUPPRESSION

Formula 18-4.