Watermakers for Bluewater Cruising: Selection, Operation, and Maintenance

A watermaker changes the fundamental equation of cruising. Without one, your range is defined by your tank capacity — typically 5-10 days for a couple practicing conservation. With one, you have unlimited freshwater autonomy. You anchor where the sailing takes you, not where the next dock tap is. You shower without guilt. You rinse the salt off the deck, the dinghy, the fishing gear. It sounds like a luxury until you've spent three weeks rationing water in the Tuamotus, and then it sounds like the most important piece of equipment aboard.

Here's how to choose, install, operate, and maintain a reverse osmosis watermaker for offshore cruising.

How They Work

Every marine watermaker uses the same core process: reverse osmosis. A high-pressure pump (typically 800-1,000 PSI) forces seawater through a semi-permeable membrane that allows water molecules to pass while blocking salt, bacteria, and contaminants. The purified water — called permeate — goes to your tanks. The concentrated brine goes overboard.

The system's components are straightforward: a raw water intake and strainer, one or more pre-filters (sediment and carbon), the high-pressure pump, the RO membrane housing, a product water line with a TDS (Total Dissolved Solids) meter, and a brine discharge. What varies between systems is the pump technology, power source, automation level, and build quality.

Choosing a System

Output capacity. Budget a minimum of 20 liters (5 gallons) per person per day for comfortable cruising — that covers drinking, cooking, dishes, and modest showers. A couple on a 42-footer should look at systems producing 25-60 liters per hour (6-16 gallons per hour). Run the unit for 2-3 hours daily and you'll fill your tanks while managing power consumption. Always size one step up from what the math says you need — when you take on crew, host dinners, or anchor in a hot climate, consumption rises fast.

Power source. This is the critical decision. DC systems (12V or 24V) draw 7-25 amps and integrate directly with your house bank and solar array. They're ideal for boats without generators and for running during peak solar hours. AC systems produce more water faster but require an inverter or generator, which means more complexity and fuel consumption. Engine-driven systems mount a pump directly on the diesel and produce large volumes while motoring — excellent for passage-making but useless at anchor unless you run the engine.

For most cruising sailboats under 50 feet with a robust solar installation and lithium house bank, a DC system is the most practical choice. The Spectra line has long been the benchmark for energy efficiency in this segment, drawing as little as 7-13 amps. ECHOTec and Dessalator are strong contenders for reliability and serviceability in remote locations.

Modular vs. self-contained. Modular systems ship as separate components — pump, membrane housing, filters, control panel — that you install in different locations around the boat. This provides maximum flexibility in tight engine compartments and allows you to position serviceable items (filters, membrane) in accessible locations. Self-contained units come pre-assembled in a single frame, which simplifies installation but requires a larger contiguous space.

Budget. Expect to spend $3,500-7,000 for a DC system suitable for a cruising sailboat, with installation adding $500-2,000 if you hire it out. High-output AC systems and premium brands like ECHOTec range from $6,000-16,000+. A manual hand-pump unit (like the Katadyn PowerSurvivor) costs a few hundred dollars and serves as an emergency backup but won't sustain daily freshwater needs.

Installation Essentials

The raw water intake should draw from a dedicated through-hull, not share with the engine cooling system. Locate it well below the waterline and away from discharge points (heads, grey water, engine exhaust). A quality raw water strainer upstream of the pre-filters catches debris before it reaches the system.

Pre-filters are your first line of defense for the membrane. A 20-micron sediment filter followed by a 5-micron filter is standard. Some installations add a carbon block filter. These are consumables — carry plenty of spares.

The product water line should include an inline TDS meter and a three-way valve that allows you to divert product water overboard until TDS readings confirm potable quality (under 500 ppm; most well-functioning systems produce under 200 ppm). Never send untested water to your tanks.

Mount the membrane housing where you can access it for inspection and replacement. The membrane is the heart of the system and it's a wear item — plan on replacing it every 3-5 years depending on usage and water quality.

Operation

Run the watermaker in clean seawater. Avoid harbors, river mouths, anchorages near agricultural runoff, and areas with visible algae blooms. Sediment-laden water clogs pre-filters rapidly and can damage the membrane. Most cruisers run their systems offshore or in clean open-water anchorages.

Time your runs to coincide with peak solar production. A 2-3 hour run during midday sun on a well-paneled boat can produce a full day's water without touching the battery bank. This is the synergy between solar and watermaker that makes modern cruising energy management work.

Always check TDS before directing product water to your tanks. A spike in TDS indicates membrane degradation or a seal failure. Divert to waste, diagnose, and fix before continuing.

Maintenance

The cardinal rule of watermaker maintenance: flush with fresh water after every use, and pickle the system for any layup longer than a week. Saltwater left sitting in the membrane promotes biological growth that destroys performance and creates off-tastes.

Most modern systems have automatic freshwater flush cycles — the system runs fresh water through the membrane for 60-90 seconds after shutdown. If yours doesn't, do it manually. For extended layups (off-season storage, haul-outs), flush the system and then fill it with a preservative solution (sodium metabisulfite is standard) per the manufacturer's instructions.

Replace pre-filters on a schedule tied to water quality and running hours — monthly in clean tropical waters, more frequently in marginal conditions. Carry a minimum of 12 pre-filter elements for a year of cruising.

The membrane itself needs periodic chemical cleaning to remove scale and biological buildup. The frequency depends on usage and water quality, but every 6-12 months is a reasonable interval. Cleaning kits are available from watermaker manufacturers and typically involve circulating an acid wash followed by an alkaline wash.

Monitor pump pressure and output over time. Declining output at normal pressure indicates membrane fouling or aging. Increasing pressure needed for normal output indicates a pump issue. Keep a log of pressure, flow rate, and TDS readings — the trends tell you what's happening before it becomes a failure.

The Spare Parts Kit

For bluewater cruising, carry: pre-filter elements (12-24), pump seal kit, high-pressure pump oil (if applicable), O-ring kit for all connections, a spare TDS meter, chemical cleaning kit, preservative solution, and — if you're going truly remote — a spare membrane. Membranes are the most expensive consumable (typically $300-500) but they're the single point of failure that can't be improvised.

Is It Worth It?

A watermaker adds cost, complexity, maintenance, and power consumption. For coastal cruising with regular marina access, it may not be justified. For bluewater cruising, extended liveaboard life, or any passage plan that includes remote anchorages far from reliable freshwater sources, a watermaker isn't a luxury. It's infrastructure — the system that enables everything else.

The cruisers who love their watermakers run them regularly, maintain them religiously, and carry the spares to keep them running. The cruisers who hate theirs bought the wrong system, neglected the maintenance, or let the membrane die during a six-month layup. The technology is mature and reliable. The variable is the operator.

References: Cruising World, SAIL Magazine, Travel Sketch Sailing, Two Get Lost, Fisheries Supply, ECHOTec