So far, we’ve seen how to determine the
required pump size for bilge systems and fire
mains using ABS and CFR Commercial rules
or Gerr’s Bilge-Pump Rule. Pumps are used on
boats for all sorts of things. In fact, the more
systems there are aboard, the more pumps
there likely will be: pressure-water systems,
deck washdowns, refrigeration and air-
conditioning condensers, bait wells, toilets,
engine-cooling water, fuel transfer, bilge sys-
tems, fire mains, and more. Each pump, with
its associated piping, hoses, and connections,
is a potential source of trouble. They must all
be accessible for maintenance, and they must
all be sized, located, and installed correctly.
We’ll examine the basic types of pumps used
in most boat systems to see which ones are
most suited to specific applications and then
review a more detailed approach to determin-
ing required pump capacity and power.
Types of Pumps
Pumps are one of the oldest machines in the
world. They are available in a bewildering
array: jet pumps, centrifugal pumps, mixed-
flow pumps, diaphragm pumps, ejector
pumps, and many more. Each type has its
subtypes. We’ll deal specifically with the
pumps most commonly found in service
aboard boats. These can be divided into two
fundamental types based on their mode of
operation: positive-displacement pumps and
kinetic pumps.
Positive-Displacement
Pumps
Positive-displacement pumps work by mov-
ing a fixed volume of fluid with each stroke
or revolution. They can be divided again into
two subcategories: reciprocating-action
positive-displacement pumps, and rotary-
action positive-displacement pumps.
RECIPROCATING POSITIVE-DISPLACEMENT
PUMPS Reciprocating positive-displacement
pumps are probably the oldest form of pump
in use. They use pistons or plungers in
concert with flapper valves, or a diaphragm
in concert with flapper valves (check valves).
The latter are diaphragm pumps. Figure 19-1
shows the inlet valve opening on the intake
stroke and then closing on the outlet stroke,
which also opens the outlet valve.
Diaphragm pumps are generally the most
effective manual bilge pumps, though they
have many uses as powered pumps, as well
(Figure 19-2). Reciprocating pumps can be set
up to discharge water flow on one stroke
(with the handle moving in one direction) or
on both strokes (with the handle moving in
both directions). The former is a single-action
pump and the latter a double-action pump.
Large-inlet-diameter (1 in. [25 mm] or
more) diaphragm pumps can handle solid mat-
ter mixed with the fluid. This makes them
excellent for such applications as pumping
bilges (where debris may be mixed in) or
pumping out blackwater (sewage). Some of
CHAPTER 19 Selecting and Sizing Pumps
