Watermakers for Offshore Cruisers: Sizing, Energy Budget, and the Upgrades That Pay Back
A watermaker stops being a luxury the first time you try to hand-jerry 200 liters of diesel-tainted water from a dock hose in the Tuamotus. For a boat that plans on crossing an ocean, or spending more than two weeks away from a reliable shoreside source, a reverse-osmosis unit is infrastructure, not a gadget. Getting the sizing, installation, and energy budget right is where most cruising owners make the wrong calls, and the cost of the wrong call is a unit that either runs too little to keep up or too much to survive the bank.
Sizing first. The rule of thumb most chandleries repeat is "four liters per person per day," and it's wrong in both directions. A cruising couple in the tropics, with a saltwater deck-wash, careful shower habits, and sensible dish-rinsing routines, is closer to 25–30 liters per person per day total. A family of four with teenagers, a washing machine, and a casual attitude toward the galley tap will hit 70+ liters per person. Size the unit to your actual water profile, not a brochure number.
A 60-liter-per-hour (16 gph) unit run for two hours a day covers a family of four comfortably and leaves headroom. A 30-liter-per-hour unit run four hours a day covers the same load but beats up the membranes faster because the unit is working full out every day. Bigger, run shorter, is the right trade. The reason: RO membranes like steady pressure and consistent feed water, and the more hours a membrane runs in its life, the shorter that life gets. A membrane rated for 5,000 hours runs out at five years at two hours a day; it runs out at three years at four hours a day.
The energy budget is where the decision gets architectural. A typical 16 gph unit pulls around 8–12 amps at 12V (100–140 watts), so two hours is 20-ish amp-hours a day. That's fine on a boat with a modern lithium house bank and 400W+ of solar. On a lead-acid boat with 200W of solar and an engine-run charging profile, that same draw turns into an hour of engine running per day just to keep up, which chews through diesel, engine hours, and noise you didn't sign up for.
High-pressure pump architecture matters more than a lot of owners know. Two designs dominate: the traditional Clark pump with an energy-recovery cylinder (Spectra Watermakers pioneered this), and the plunger-pump / motor design (Katadyn PowerSurvivor, Village Marine, etc.). Clark-pump units sip power at around half the draw of a plunger-design unit of the same output — that's the reason most serious cruising boats end up with some version of a Spectra, Rainman, or Sea Recovery Aqua Whisper.
The AC-vs-DC question comes down to boat architecture. On a boat with a genset or inverter-charger running loads anyway, a 120V AC unit (most Sea Recovery and FCI models) can be simpler and cheaper. On a boat trying to run entirely from the battery bank and solar, a DC-direct unit avoids the inverter loss and lets you run the watermaker as a solar load on sunny days. Rainman makes a nice portable 12V unit that a lot of cruisers are adding as a second system alongside their primary — that's a better plan than running one big unit harder.
Installation. The pre-filter train is where cheap installations fail. Minimum spec: a 20-micron sediment filter followed by a 5-micron, followed by a carbon block filter to pull chlorine and VOCs from marina water. If you're running in clean ocean, the carbon block is overkill; if you're running in the ICW or a harbor with any runoff, it's the filter that saves your membrane. Carry 10 sets of sediment elements; they're cheap and they're what you replace most often.
Feed-water intake needs its own through-hull, below the waterline, forward of the engine raw-water intake so you don't suck exhaust residue. A sea strainer before the pump is mandatory — one jellyfish or piece of floating weed and you've burned out the boost pump. Budget for a separate flow meter on the product side; it's the first instrument that tells you the membrane is tiring.
Commissioning routine that gets neglected. Every membrane ships with a storage solution, and every installation needs a proper fresh-water flush after shutdown. If you're pulling into a marina for two weeks, the right move is a biocide-laced flush to prevent biological fouling; for a short layover, a fresh-water flush from the boat's tank is enough. Not doing either turns a 7-year membrane into a 2-year membrane.
Maintenance that actually matters. Check TDS (total dissolved solids) output at every startup. A healthy membrane produces water at 200–500 ppm TDS depending on inlet salinity; if you see drift past 800 ppm, the membrane is on its way out. A conductivity meter is $30 from a hydroponic supplier and it's the best early-warning instrument on the boat.
The upgrades that actually pay back: a boost pump upgrade if your installation is starving the high-pressure pump, a proper low-salinity pressure gauge (not the Chinese one that came in the kit), and a product-water three-way valve that lets you dump bad water overboard without sending it to your tank. The stock three-way valves on most kits are junk and they fail open after 500 hours.
The boat designer Bob Perry used to say that a cruiser's energy budget tells you what kind of cruising they'll actually do. Watermaker sizing and architecture tell you the same thing: a boat that can make water easily goes places a boat that can't doesn't. Spend the money, plumb it right, and carry enough filters to get to the next port that actually sells them.
