Chapter 20:Freshwater Systems
down and the bladder will hold pressure—
unavoidably leaking air very gradually. Even-
tually pressure will drop below, say, 10 psi
(1.4 kPa), and the pump will automatically
kick back on again. In this way, the pressure-
water system is always pumped up. In addi-
tion, the accumulator smoothes out pressure
pulses and knocks to achieve a continuous,
even flow. Of course, when you have water
running for a bit, the pump will switch on.
Frequent cycling with all water taps off is an
indication of a leak or (much less likely) a
faulty pressure sensor.
Variable-Speed Water
Pumps
Some newer pressure-water pumps are strictly
on-demand pumps (as for the very simple
system) without an accumulator, but they man-
age the flow with a variable-speed pump motor.
The motor automatically speeds up to deliver
more water as more faucets are opened. When
designed specifically for larger systems, these
can work quite well, though so far, I’ve most
frequently used accumulator systems.
Pressure-Water-Pump
Capacity
It’s almost as bad an idea to install a pressure-
water system that has too much capacity as
it is to install one with too little. Water pumps
for freshwater systems typically range from
1 to 12 gpm (3.8 to 45 lpm). Installing a 6 gpm
(23 lpm) pump on a small boat with two
faucets not only adds weight and cost, but
won’t match the plumbing-service demands.
Also, don’t size the pump based on maximum
demand with every faucet and shower open at
full at the same time. This will never happen
in real life. Follow Table 20-1 as a good guide.
Water-Pump Location
You would think you could locate the
pressure-water unit—pump, accumulator,
and so on—pretty much anywhere that didn’t
exceed the pump’s suction-lift limitations.
True, but don’t forget these pumps make
noise. On one 50-footer (15.2 m) I designed,
the builder installed the pressure-water pump
under the master-stateroom berth, just about
dead under the owner’s pillow. To say the
owner was ticked off at being woken up five
or six times a night as the pump cycled on
would be an understatement! I arranged to
move the pressure-water pump into the
engine room, where it had been originally
drawn. Keep all such noisemaking machinery
as far away from sleeping accommodations
as possible.Avoiding Overpressure
At the dock, you want to be able to screw
right into the town or city water supply. Stan-
dard fittings are available, and you can see
where this is located in Figure 20-2. The
absolutely vital thing is that this attachment
be fitted with a pressure regulator. Compo-
nents of your boat’s water system may be
rated to only 3 or 4 psi (21 to 27 kPa) maxi-
mum. City water systems can have much
higher pressures. More than one boat has
ruptured some part of its water piping and
sunk at the dock because a regulator wasn’t
installed. Even if nothing is ruptured, without
either a check valve or a regulator in place,
the water tanks will fill up and spray water
out their vents. What an interesting sight! If
the vents can’t keep up, a tank or some other
fitting can rupture.Tee to Heat and Standard
Water Heaters
At this point, we’re ready to branch off
and make heat (literally). The cold water sim-
ply continues on to a distribution manifold,
but to get hot, the water is teed off into the
heater. Most marine water heaters are
insulated tanks. They heat the water from a
120-volt (or 240-volt) AC electric element
at dockside (or when the generator is
running). Underway, the engine-cooling waterTABLE 20-1.FRESHWATER-PUMP
CAPACITY
Max.Pump CapacityNumber of Fixtures gpm lpm
1–2 3 11
2–3 4 15
4–6 7 26
7 or more 11 42