Fresh water, black water, watermakers: onboard plumbing
Tanks, pumps, marine toilets, black water and watermakers: understanding a boat's plumbing to size it, maintain it and avoid breakdowns.
A boat's plumbing does three things: store and distribute fresh water, dispose of waste water, and — on equipped yachts — produce fresh water from the sea. It is an unglamorous domain that directly governs autonomy, comfort and how the boat smells. Understood properly, it takes thirty minutes to size; neglected, it supplies most of a summer's breakdowns.
Fresh water: tanks and consumption
Real cruising consumption runs at 40-60 litres per person per day once showers and dishes are included — half that when sailing frugally. A 12-metre sailboat typically carries 300-500 litres, a 45-foot catamaran 700-1,000, a 20-metre motor yacht 1,000-2,000. Always relate the spec sheet capacity to your crew: four people and 300 litres means three comfortable days of autonomy, not a week. Tanks (stainless, polyethylene or integral) need annual inspection and cleaning — water that "tastes odd" almost always comes from a tank that has never been opened.
Pressure pump and distribution
A pressure pump (the "water system") feeds taps and showers, helped by an accumulator tank that prevents constant cycling. Classic failures: tired pressure switch, clogged filter, leaking check valve. Carry a spare pump or at least a diaphragm kit — this is the part that dies on a mid-August afternoon. The calorifier (engine heat exchanger plus 230 V element) provides hot water under way and dockside.
Marine toilets and black water
The marine toilet, manual or electric, macerates and discharges either directly overboard (forbidden near coasts) or into a black water holding tank emptied offshore or via deck fitting in equipped harbours. Holding tanks are mandatory on boats built since the late 2000s in most of Europe and enforced in a growing number of anchorages — the Mediterranean is progressively closing to direct discharge, as we explain in our clean cruising guide. The golden rule, briefed to every crew on arrival: nothing in the bowl that hasn't been eaten first — on many boats, paper included. Joker valves and seals get replaced every two to three years; persistent smells come from saturated hoses, not the tank.
Grey water and bilge pumps
Sinks and showers drain as "grey water", often straight overboard, sometimes via a dedicated tank on recent builds. Showers empty through a dedicated pump whose strainer needs cleaning more often than you would think (hair). Bilge pumps — automatic electric plus a manual backup — belong to safety more than plumbing: test them at the start of every season, float switch included.
The watermaker: unlimited autonomy, demanding upkeep
A watermaker pushes seawater through a reverse-osmosis membrane at 55-60 bar, producing 30-160 litres of drinking water per hour depending on the model. Count €4,000-12,000 for a 60-100 L/h unit, plus a serious electrical draw that leads straight back to your energy budget. Essential for long-range cruising and charter catamarans, superfluous for coastal sailing where marina taps abound. Its enemy is idleness: a membrane that doesn't run must be flushed and chemically pickled, or it dies silently.
Hoses and seacocks: the quiet critical parts
Every plumbing run ends at a hole in the hull, and those holes deserve more respect than they get. Seacocks should be exercised — opened and closed — several times a season so they never seize, and replaced at the first sign of stiffness or corrosion; a wooden bung tied to each one is old-school wisdom that still saves boats. Hose quality matters just as much: sanitation runs need genuine odour-proof hose, and anything below the waterline deserves double clamping with quality stainless clips. Cheap hose is where good plumbing goes to die.
Where to start
On your boat, or the one you are eyeing: note the real capacities (water, black water), the age of pumps and hoses, and whether the holding tank is compliant. Then set three routines: annual tank cleaning, a pump/toilet spares kit aboard, and systematic winterising of the circuit — full drain plus food-grade antifreeze — before the first frost. Plumbing forgives neither improvisation nor neglect, but it rewards routine.
Frequently asked questions
How much fresh water capacity does a boat need?
Multiply 40-60 litres per person per day by your target autonomy: for 4 people over 3 days, plan 500-700 litres, topped up by a watermaker or marina stops. Frugal sailing (seawater showers, careful dishwashing) halves the figure.
Is a black water holding tank mandatory?
Yes on recent boats (built since the late 2000s in the EU) and in practice in many areas: direct discharge is forbidden in harbours, bathing zones and a growing number of protected anchorages. Check local rules — they tighten every year.
Is a watermaker worth it in the Mediterranean?
For coastal cruising with regular marina stops, rarely: harbour water is everywhere. It becomes valuable once you string together weeks at anchor (Balearics, Greece) or sail with a large crew — provided you accept its maintenance and electrical draw.