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Watermakers for Long-term Cruising

It’s easy to spend a lot of time and money on sails, engines, generators and electronics when gearing up for a long-term cruise. However, one vital element (literally) that is often overlooked or left until the last minute is freshwater.

Granted, you may be planning to cruise in areas where clean drinking water is easily obtained. But there’s still the hassle of having to come into the dock to fill tanks. And for those considering an ocean crossing, will those tanks be large enough? Finally, those sailing into less-frequented cruising grounds, in particular, need to ensure they have a safe supply of clean water that they can rely on, and the best way to do this is to produce your own.

The good news is that watermakers, like many other technologies, have advanced by leaps and bounds over the past decade and are now significantly more efficient and reliable than their forebears. Prices have dropped too, so there’s no longer any excuse for not at least considering one when fitting out for a long cruise.

Click image to enlarge

Click image to enlarge

How they work

All modern marine watermakers use the reverse-osmosis (RO) principle to produce pure, potable water from seawater: an approach whereby seawater is forced at high-pressure through a semi-permeable membrane that allows the passage of water molecules, but not salt, organic material or bacteria. The resulting freshwater is piped to the vessel’s water tanks and the remaining “brine” discharged overboard.

One of the main ways in which watermakers vary is in the type of pump they employ, which can either be electrically driven or run directly off the engine. The watermakers installed on cruising yachts up to 60ft LOA typically run on either DC or AC. If you have an AC generator or alternator on board, for example, it only makes sense to use its AC output to drive the watermaker directly. Alternatively, those with large domestic battery banks and plentiful natural energy resources such as wind or solar might prefer to run an AC-powered watermaker in concert with an inverter or choose a DC-powered model.

Modern DC-driven systems are considerably more energy efficient than their predecessors, as most incorporate some sort of power-saving energy recovery system (ERS). A decade or so ago, a 12V DC unit producing around 10gph would have drawn 40A or more at startup. Now that has been reduced to roughly 15A, with a constant draw of just 10A. Of course, this current is halved with a 24-volt DC unit, but the overall energy consumption remains the same.

With an engine-driven watermaker, the high-pressure pump is mounted on the engine and is belt-driven, much like an alternator, usually with an automatic clutch. This kind of setup can produce large quantities of freshwater, even with a relatively small engine and is noticeably more productive than using an alternator for generating pumping power. An automatic regulator provides a constant pumping pressure regardless of engine revs, so engine-driven units produce between 20-60gph, similar to AC models, while DC units are generally limited to around 10-30gph.

The key to the aforementioned ERS approach lies in that fact that when the discard water leaves the membrane in a watermaker it is still under pressure, possibly up to 1,000 psi. An ERS, therefore, works by using a set of valves to channel the energy of this pressurized waste to help drive the pump forcing yet more water into the membrane before it is discharged over the side. Using this method, manufacturers can reduce energy consumption by as much as 80 percent over conventional systems.


Modular or self-contained?

Watermakers can be purchased in two distinct ways: modular or self-contained. The former comes as a number of separate components that can be mounted in different places around the boat. This approach provides a great deal of flexibility in the inevitably constricted space available. It also enables you to position regularly serviced items, such as filters, in easy-to-reach locations, although some items will need to be kept close to one another. (It also makes good sense to keep all the plumbed-in modules on one side so that hoses don’t need to be taken all the way across the boat.)

Self-contained designs, on the other hand, come pre-assembled and integrated, making them much quicker to install. They are also, however, bulkier, often coming boxed inside a fairly large metal casing, making them much better suited to larger yachts with larger engine spaces.

Installation, maintenance

When installing a watermaker, it is important to bear in mind that you’ll need to access parts of it for regular maintenance if you hope to keep it working correctly. This is where modular designs score over the all-in-one units, as you can place frequently serviced items such as raw water filters (usually two) close to hand. That said, if you buy a microprocessor-controlled (MPC) unit with a remote-control panel and gauges, the pre-filters are probably the only items you’ll need to access regularly as system flushing will be done automatically.

Without question, the majority of problems with RO watermakers are caused by lack of use. Leaving them for a few weeks with stagnant water inside almost always results in the membranes and filters becoming clogged with debris and bacteria. For this reason, it is important to take care when calculating the required capacity of the watermaker, so that you’re not forced to use freshwater unnecessarily just to keep it running.

Apart from regular use, the best way to keep a watermaker clean and productive is to flush it out frequently with freshwater. Most modern units now have automatic flushing systems, which purge the saltwater side with freshwater at pre-determined intervals, even when the boat is left unattended. Note that chlorinated water must never be used, as chlorine will destroy the membranes. Either your own tank water must be used, or a carbon filter should be fitted in-line when using dockside water for flushing.

Occasionally, it becomes necessary to chemically cleanse the membranes to remove any oil, fuel, grease or bacterial growth. Though required far less frequently than freshwater flushing, this is a critically important process for ensuring smooth operation.


Two other factors you might want to take into consideration when choosing a watermaker are the average water temperature and its salinity in the areas you intend to cruise. The colder the seawater and the higher its salt content, the slower the purification rate will be. Therefore, if you plan to cruise the higher latitudes, you’ll need to bump up the specs for your chosen watermaker. Although some units incorporate Automatic Pressure Regulation (APR) to compensate for fluctuations in these two elements, it is done more to protect the device than to increase production. Some units also incorporate a means of adjusting the pump speed, which can be set to suit the environmental conditions.

Something else that will affect the output rate of a DC-driven unit is voltage. If your batteries are low, for example (less than 12.5V), production will drop noticeably, which results in many skippers having to run the engine in order to top off their tanks. If this is a cause for possible concern, you can either increase the capacity of your service bank or choose an engine-driven pump, which will be considerably more productive and require less engine run time.

Along these same lines, if your actual production rates end up being lower than stated for a particular model, it could be because your electrical cables are too small, creating a voltage drop between the batteries and the pump. To find out if this is the case, put a meter over the pump terminals when it’s running to check the actual voltage reaching the unit. If the difference between the battery volts and the pump reading is greater than 5 percent, you will need to increase your cable sizes.

Finally, there can be problems if air gets into the system, which is usually caused by the seawater inlet being incorrectly positioned. If there’s any doubt as to whether the inlet might occasionally be above water level or in aerated water, then it is advisable to fit a primary, low-pressure pump with an integral pressure switch somewhere along the inlet pipe.




The Sea Xchange portion of Dometic’s product line is the one sailors will want to look at. These high-quality systems are available in either modular or self-contained configurations with a plethora of options with respect to everything from membranes to sources of power. Highlights of the line include a user-friendly interface and the option for remote operation. The company’s units are especially compact for use aboard smaller vessels with cramped engine spaces.


The 12-volt and 24-volt DC DML-series of modular WMs from Echotec utilize low-speed, permanent magnet stainless steel pumps and triple plungers for improved reliability and energy efficiency, rather than a more complex, and possibly more vulnerable, computerized energy recovery system. However, this results in them needing almost twice the electrical energy per gallon of freshwater produced. The company also supplies a compact Pro model range, producing between 20-150gph for those that have space, as well as higher capacity AC and engine-driven units.


Parker Hannifin’s HRO produces the Seafari Quest range of DC watermakers for small to medium-sized cruising yachts, with a booster-pump ERS to help reduce power consumption. They are available with MPC electronics, flow rates of between 8-25gph, in either self-contained or modular formats, with or without automatic flushing. For those preferring to use an AC generator for power, the company’s Seafari Mini series offers 7-31gph with similar dimensions and flexibility to the Quest.


Made in Italy, the lightweight Osmosea WMs use the very latest ERS technology to produce more water with less power. The DC-powered N12 range automatically adjusts for sea temperature and salinity, while its modular construction simplifies installation within confined spaces. Features include remote control, “re-mineralization” and auto-flush. Its rugged construction, which includes the latest resins, carbon-fiber and 316 stainless steel, should ensure a long and reliable lifespan. The 15.8gph model is available with either single or dual pumps—the latter model being switchable from two- to single-pump operation to save energy, though reducing flow to 8gph.


Made by the Swiss company Katadyn, which also owns Spectra, the Survivor range for small yachts starts with a small hand-operated unit and goes up to the DC-powered PowerSurvivor models, the 40E and 80E. They are small, light and very energy-efficient, thanks to their simplicity and patented ERS. The pre-assembled 40E draws a mere 4 amps to produce 1.5gph, whereas the modular 80E will give you around 4gph for just 8 amps. With these minuscule power demands, these systems could be left to run all day on natural power alone. The PowerSurvivor-40E also converts to manual operation in an emergency.


An interesting concept, this modular Australian-produced “portable” desalinator is comprised of a pressure supply unit (PSU) and a variety of membrane options. The pump unit comes in three guises: DC, AC and gasoline-driven. The first is designed to run off a typical 12V battery bank, the second off a 2Kw portable generator and the third off its own portable gasoline generator/pump unit. Rainman offers three standard RO membrane options, which are compatible with either the electric or gasoline PSUs. Custom configurations are available upon request.


This well-respected brand of watermakers boasts an 80 percent power saving, thanks to the inclusion of ERS throughout the range, which includes a variety of different compact, modular or self-contained units. The Smart series (30/60/80/100) is small and lightweight, with outputs from 8-26gph and manual or automatic flush options. The larger modular and self-contained ready lines are more sophisticated, with full MPC options if required and output up to 80gph. All feature automatic pressure regulation, with components made from carbon for corrosion resistance.

Sea Recovery

Another Parker Hannifin subsidiary, Sea Recovery offers a wide range of watermakers catering to everything from smaller cruisers to offshore drilling platforms. The company’s Aqua Whisper line is aimed directly at the sailing market and is available in both modular and self-contained configurations, and running off either AC or DC. The Aqua Whisper DX offers comprehensive remote monitoring, while the Aqua Whisper Mini is especially compact for fitting into the tight spaces found on most sailboats.


Katadyn-owned Spectra produces a wide range of watermakers, from the hand-operated Survivor 06 to a commercial AC model capable of producing 10,000 gallons per day. The most suitable units for cruising yachts are its DC/AC-powered Ventura, Catalina and Newport models, offering production rates of between 6.3 and 41gph. The simpler Ventura design comes in an analog or automatic version, the latter including such features as MPC control and auto-flush. Although the MPC models are fully automatic to simplify operation, they can also be run manually in the event of MPC failure. The majority of Spectra WMs are extremely energy-efficient, as they utilize the Clark Pump ERS.


One of Tecnicomar’s most popular systems designed for mid-range cruising yachts is the modular Oasi device, which is available in two sizes: 30 and 60. The company also makes a self-contained AC model called the Sailor Compact, available in a “slim” version for installation into extra tight spaces. 

MHS Summer 2018



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