Boat Plumbing
by Don Casey
Boat plumbing is a lot easier for the do-it yourselfer than plumbing
at home, mainly because it doesn't involve rigid pipes running inside
solid walls. In fact, pipes are rarely used at all on boats, replaced
by easier-to-handle flexible hose.
Here is an overview of a typical on-board water system.
Tanks
Because water is heavy, tanks should be mounted low in the boat. Where
space is available, it is a relatively simple matter to add extra tanks.
Rigid polyethylene tanks are available in hundreds of shapes and sizes,
or you might use a flexible bladder tank-essentially a water bag.
Water tanks typically have three threaded ports, one for the outlet
and one for the vent hose, both 1/2-inch, and one for the fill hose,
usually 1 1/2-inch. Threaded hose barbs allow for hose connections.
The inlet is connected to an on-deck fill. (Be sure the deck-fill has
an O-ring to seal out seawater when it is closed.) The vent line leads
to a vent fitting high in the boat-above the tank at every expected
angle of heel. Be aware that if the vent is not also higher than the
fill, it will overflow when you are filling the tank. The outlet connection
leads directly to a pump or, in a multitank installation, to a manifold
or Y-valve.
Use Teflon tape on all threaded fittings, and don't overtighten fittings
in plastic tanks. Secure hoses with stainless steel hose clamps.
Hose
Supply hose for a boat water system must be non-toxic, non-contaminating,
taste-free, and FDA approved for drinking water. If the system is pressurized
or the hose will carry hot water, you need reinforced hose. The usual
choice for water system hose is clear PVC reinforced with polyester
braid. Use this same type hose for tank fill and vent connections.
Drain hoses connected to through-hull fittings should be stronger than
clear vinyl hose. For this use, select reinforced rubber hose, sometimes
called heater hose. This is the same type of hose used on engine plumbing,
and it typically has about three times the burst strength of reinforced
vinyl hose. Double clamp all hoses connected to through-hull fittings.
Pump
Water pumps on a boat can be either electric or manual. An electric
pump pressurizes the entire water system. Most electric pumps have a
pressure switch that activates when the pressure drops below a set value-usually
around 30 or 40 PSI. Opening any tap on the boat releases pressure and
causes the pump to kick on and run until it rebuilds the pressure to
the cut-out setting. The pump cycles on and off until the tap is closed.
The inlet of an electric pump connects directly to the tank outlet (or
multitank valve), and the outlet supplies water to all faucets and appliances.
Manual pumps-hand or foot operated-supply a single spigot connected
directly to the outlet side of the pump. A regulating valve is not required;
water flow is controlled by the operation of the pump. The primary advantage
of manual pumps is that they dramatically reduce water waste, a major
concern for boats that spend long periods away from water supplies.
Accumulator
Some water systems have an accumulator. Large accumulators have pressurized
bladders in them, but most small ones are just empty tanks teed into
the line downstream of the pump. When the pump runs, it tries to fill
the tank from the bottom, compressing the air trapped inside the tank.
The pressure from the tank allows small amounts of water to be drawn
without the necessity of the pump running, thus reducing pump cycling.
Hot
Water
A marine water heater is simply a small, insulated tank downstream of
the pump. You must have a pressurized water system to operate a water
heater. The pump draws water from the storage tank(s) and fills the
water heater tank. Inside the water heater is an electrical heating
element and usually a coiled tube called a heat exchanger. When AC power
is available, the electrical element (controlled by a thermostat) heats
the water. Away from the dock, the hot engine coolant is routed through
the coiled tube to heat the water in the tank when the engine is running.
Water heaters have four threaded ports. The tank inlet connects via
a tee-connector to the outlet hose from the pump. A check valve is required
in this line or in the heater to prevent hot water from migrating back
toward the pump. The outlet connection supplies heated water to the
hot side of all faucets, also using tee-connectors. The other two ports
are for the heat exchanger connection, which varies depending on engine
installation. Use only metal fittings to plumb a water heater, never
plastic. If a pressure-release valve isn't integral, the heater will
have a fifth port for this essential component.
Faucets
Faucets are the ultimate terminus for water system lines. Manual pumps
require simple spigots, but in a pressure water system, boat faucets
differ from those found ashore only in styling and that they are fitted
with hose barbs. Mixer faucets require two hose connections, one from
the cold side to the supply line from the pump and the other from the
hot side to the water-heater outlet.
Shower
Shower connections are identical to faucet connections. The only difference
is that rather than delivering the water through a spigot, the water
is delivered through a pipe or hose to the shower head.
A nice owner addition to almost any boat is a deck shower, easily installed
by simply teeing into cold- and hot-water supply lines.
Drains
Sink drains typically connect with reinforced rubber hose to a through-hull
fitting. On a sailboat, sinks are best located near the centerline of
the boat so heeling doesn't put them below the waterline. Because head
sinks are often well outboard, they may be plumbed to drain into the
bowl of the toilet to avoid the risk of flooding. There are collateral
benefits of running fresh water through the head.
Shower pans too often drain into the bilge to be pumped overboard by
the bilge pump. However, this arrangement eventually leads to unpleasant
bilge odors, and it risks jamming the bilge pump with hair. Shower pans
should be isolated from the bilge and include a discharge pump, either
automatic or connected to a switch. The through-hull discharge outlet
must always remain above the water.
Washdown
Since few boats carry sufficient fresh water to allow washing the decks
with it, washdown pumps are not connected into the freshwater system.
Nevertheless, a washdown pump is a great convenience for hosing the
deck and knocking mud off the anchor chain.
The inlet fitting of a washdown pump is connected to a submerged through-hull
fitting, and the outlet side is connected to a deck-mounted faucet or
male hose connector. A dedicated through-hull is not required; if you
are installing a deckwash pump; use a Y- or tee-connector to tie into
an existing inlet line. Use heavy-duty rubber suction hose, wire reinforced
to keep the hose from collapsing. Debris will damage or destroy a washdown
pump, so it is essential to have a strainer in the intake line.
Head
Marine toilet plumbing is the subject of another Don Casey Shows
You How... sheet.
For more
information about boat plumbing, consult This Old Boat by Don Casey.
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