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

to me. We can check the minimum required
pump capacity against the approximate
flooding rate from a rather small hole in the
hull below the waterline. The following is a
reasonable approximate formula for flood-
ing rate.

Formula 18-3. Flooding Rate from Hull
Damage

or

Where

gpm =flooding rate in gallons per

minute

lpm =flooding rate in liters per minute

d =approximate opening diameter, in. or

cm

H =depth of opening below the water-

line (head), in. or cm

Example:Let’s assume a simple puncture
of approximately 2-inches (50 mm) diameter,
located 1.5 ft. (0.46 m) below the waterline.
We would find

or

You can see that even a hole of modest size

(and not all that deep) will quickly exceed the

minimum bilge-pump capacity required by

CFR Commercial or ABS. There are practi-

cal limits to how large a bilge system you can

and should install, but I recommend more

than the minimum requirements of CFR

Commercial or ABS.

Sizing Up a Bilge Pump for

the Real World

Many manufacturers’ bilge-pump capacity

ratings aren’t all they ought to be. Most bilge-

pump manufacturers rate their pump capac-

ity with no hoses attached, with no lift

(head), and with an optimum full-12-volt

power source. (Those that do rate for head

state this clearly, and it’s usually a nominal

10-foot or 3-meter head.) Not a boat in exis-

tence operates a bilge pump like this. The

friction in the 3 to 6 feet (1 to 2 meters) or

more of hose and pipe commonly found in all

bilge systems cuts the flow rate, and the bat-

teries are seldom delivering full oomph (not

to mention the voltage drop in the wiring). In

addition, all bilge systems have some lift

(they have to, obviously) usually between

2 and 5 feet (0.6 and 1.5 m) for smaller boats,

but as much as 12 feet (3.7 m) or more on

larger craft.

Manufacturers rate capacity in gph

(gallons per hour) or gpm (gallons per

2 .08 (5 cm)×× =^2 46 cm 352 lpm minute); or in metric in lph (liters per hour)

5 .67 (2××=in.)18^2 in. 96 .2gpm

lpm=×× 2 .08 d^2 H

gpm=×× 5 .67 d^2 H

Chapter 18: Bilge Systems, Fire Mains, and Fire Extinguishers

ISO Bilge-Pump Requirements

Under ISO 15083-2003—for categories A, B, and C recreational boats—the requirements

for bilge-pump capacity are rather low: a single bilge pump, as shown in the table.

Boat Length Pump Capacity

Less than 6 m (19.6 ft.) 10 lpm (2.6 gpm)

6 m to 12 m (19.6 ft. to 39.4 ft.) 15 lpm (4 gpm)

12 m to 24 m (19.6 ft. to 78.7 ft.) 30 lpm (8 gpm)

Measured with a pressure of 10 kPa (0.1 m of head), or 1.45 psi (0.34 ft. head), fresh water.

Formula 18-3.