Watermakers · A buyer's reference

1. How a watermaker works

A marine watermaker is a reverse-osmosis (RO) desalination unit packaged for the small-craft environment. The working principle is simple: seawater is forced, at very high pressure, against a semi-permeable polyamide membrane whose pore size is small enough to pass water molecules but small enough to block sodium and chloride ions, dissolved minerals, bacteria, and most viruses. The water that passes the membrane (the "permeate" or "product") is collected; the concentrated brine (the "reject") is discharged overboard. The pressures involved are 600 to 900 psi for seawater RO; the membrane chemistry is the polyamide thin-film composite (TFC) developed in the 1970s for industrial desalination and miniaturized for marine use in the 1980s.¹

2. Energy recovery: the inflection point

The first-generation marine watermakers (Sea Recovery Aquamatic, the early Village Marine units, the Katadyn Power Survivor) operated as direct-drive high-pressure pumps: every joule of energy used to pressurize the seawater was lost when the brine was discharged. A 30-liter-per-hour unit drew somewhere around 15 amps at 12 V. For a cruising sailboat with a 200-Ah house bank, that was a binding constraint on runtime.

The 1990s and early 2000s saw the emergence of energy-recovery systems — pressure-exchanger and rotary-vane designs that recover the pressure energy of the brine and apply it to the incoming seawater. Spectra's Pearson Pump (patented 1997) and Schenker's Energy Recovery System (patented 2007) are the two dominant ER architectures in the recreational market. Both reduce the energy draw by roughly 80 percent versus a high-pressure system of equivalent output. A Schenker Modular 30 producing 30 liters per hour draws about 4 amps at 12 V; a Spectra Newport 400C at 15 GPH (about 57 LPH) draws about 12 amps; both are about a quarter of the energy budget of the comparable first-generation unit.²

For any cruising boat under 50 feet, the energy-recovery system is now the default choice. We have not installed a non-ER unit on a recreational boat since 2019. High-pressure systems remain the right answer for the high-output industrial and superyacht installations, where the energy budget is unconstrained and the unit's continuous runtime requirement (hundreds of gallons per hour, twenty-four hours a day) favors the simpler hydraulic architecture.

3. Sizing the unit

The sizing question reduces to two numbers: how many gallons (or liters) of fresh water does the boat consume per day, and how many amp-hours per day can the boat's electrical budget afford to spend on water production? Conservative cruising use runs about 4 to 6 gallons per person per day. Charter-grade use (with showers) runs 8 to 12 gallons per person per day. Live-aboard use varies wildly.

The first feature in this issue (Greta's Schenker) is a worked example of sizing on a 225-Ah bank for a two-person crew. The fifth feature (Orcas Lullaby's Spectra) is the same exercise on a 400-Ah lithium bank for a four-person charter party.

4. The membrane

The membrane is the watermaker's working surface. The dominant manufacturers in the marine market are Filmtec (a DuPont subsidiary), Hydranautics, and Toray. The dominant element in the recreational market is the Filmtec SW30 series, available in sizes from 2.5" by 21" (the SW30-2521, used in the Schenker Modular 30) up to 4" by 40" (the SW30-2540, used in most 15-GPH-class units).

Membrane life under normal cruising use, with proper preservation discipline, is six to ten years. Membrane life under abuse — running the unit dry, fouling the membrane biologically, exposing it to chlorinated shore water — is six months to two years. Replacement cost for a Filmtec SW30-2540 is approximately $480 at the time of this writing; for a SW30-2521, approximately $260.

5. What we install and why

Our default recommendation for a cruising sailboat under 40 feet, with a 200–300 Ah lead-acid bank: Schenker Modular 30 (30 LPH, 4 A draw). For a boat with a 400-Ah lithium bank and a charter or live-aboard load: Spectra Newport 400C (15 GPH, 12 A draw). For boats above 50 feet or with continuous high-volume needs, Spectra Cape Horn or Sea Recovery Aquamatic, sized per the load.

We do not install Katadyn Power Survivor units. They are excellent for emergency-rated life-raft survival but their commercial cousins have been superseded by the ER units on every metric that matters in a permanent install. We will service an existing Power Survivor.

6. Membrane preservation (the bi-annual flush)

A reverse-osmosis membrane that is not run in marine water for more than approximately seven days will develop biological fouling. The mitigation is a preservation flush — the membrane is flushed with a propylene-glycol-and-sodium-metabisulfite solution that stops biological activity for up to six months. The preservation solution is sold by every watermaker manufacturer in a 2-quart jug for about $40, and the procedure is sometimes called by its older industry shorthand "pickling," from the food-preservation analogy.

For a private cruising boat: preserve at season's end (typically October on this coast), recommission at season's start (typically April). The shop runs a free preservation clinic at the Northwest Maritime Center on the second Saturday of every October. We bring the chemicals; you bring the boat. We preserve twenty to thirty units in one afternoon.

For a chartering boat or live-aboard: see the Orcas Lullaby feature for the worked example of a two-preservation-per-season schedule with between-charter polish cycles.

7. The four watermaker failure modes worth naming

7.1. Intake fouling

A clogged sediment pre-filter shuts the high-pressure pump down in protect mode within 30 to 60 seconds of startup. The fix is replacement of the pre-filter cartridge (20 micron, typically); the diagnosis is the inlet pressure reading. Owners should carry a minimum of 6 spare cartridges aboard for any passage over 7 days.

7.2. Biological fouling

An unpreserved membrane left more than 7 days in stagnant marine water develops biological growth that reduces output, shortens membrane life, and at high enough levels makes the product water unsafe. Recovered by chemical cleaning if caught early; replaced if not.

7.3. Mineral scaling

Calcium-carbonate scale on the membrane, accumulated over years of high-recovery operation. Reduces output gradually. Mitigated by acid cleaning every 12 to 18 months in heavy use. We do not see scaling failures on Salish Sea installs (the local source water is too cool, too dilute, and too low in carbonate hardness) but the failure is real on tropical installs.

7.4. Chlorine exposure

Polyamide RO membranes are destroyed by free chlorine at concentrations as low as 0.1 ppm over a few hours. The most common exposure is the dockside flush: an owner connects their boat to a chlorinated shore freshwater supply, and the chlorinated water makes its way to the watermaker through a misrouted plumbing path. We install a carbon-block pre-filter in the freshwater line on every install for this reason; the filter costs about $30 and saves about $480 in membrane replacement.

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