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.
Here is an overview of a typical on-board water system.
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 multi-tank installation, to a manifold or Y-valve.
Use Teflon tape or thread sealant on all threaded fittings, and don't overtighten fittings in plastic tanks. Secure hoses with stainless steel hose clamps.
Supply piping 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 will carry hot water, the piping needs to be suitable. The traditional choice for water system plumbing has long been clear PVC reinforced with polyester braid. This same type hose can be used for tank fill and vent connections.
In recent years semi-rigid polyethylene (PE) tubing, long used in RV plumbing, has surged in popularity for boat plumbing. It has much to recommend it. With quick-connect fittings, a PE tubing water delivery system assembles with the simplicity of Tinkertoys. The opaque or at least semitranslucent nature of the PE tubing discourages algae growth that can be a problem with clear hose. PE tubing also comes in colors — typically red for hot water and blue for cold — which looks nice and might make plumbing failures easier to trace. Because the tubing is less flexible than PVC hose and it must be cut to the correct lengths, a PE plumbing system will be slightly more demanding to install. However, the primary negative to PE plumbing is the cost of the fittings, which at this writing run $4 to $8 each. On the positive side, the tubing is actually cheaper than reinforced clear PVC hose.
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.
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.
Some water systems include 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.
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 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 may be fitted with hose barbs. Mixer faucets require two 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 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.
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.
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.