Choosing a Watermaker

By Tom Burden, Last updated: 12/31/2018

The modern watermaker uses a modest amount of energy to turn undrinkable salt water into pure freshwater. A well designed, properly maintained watermaker can significantly increase safety and comfort while cruising. A watermaker is an appliance that requires a bit of maintenance, on a regular basis, for its continued operation (as is noted below). If you become familiar with the operation of the device, use it every day and keep its components in proper working order, a watermaker is truly a miracle machine.

Why you’d want one

  • Reduces weight (your water burden) and increase performance because you don’t have to top off the tanks. It is a worthwhile effort since 50 gallons weigh more than 400lb.
  • Extends your range of travel even when water capacity is limited. Not having to plan your trip around water replenishment amounts to more freedom for choosing your route. Carry enough water for your longest passage, in case the watermaker fails while at sea.
  • Saves money in foreign ports where potable water can be very costly. Many Caribbean ports charge as much as 25¢ per gallon, a big chunk of change for a 300-gallon tank.
  • Provides a safe water supply onboard, especially in parts of the world where the water quality is poor. This helps you avoid health risks that can ruin your trip with parasite-related sicknesses. UV sterilizers can provide additional protection.
  • Emergency water supply if your tanks spring a leak. Water can also become undrinkable if you fill up with crud somewhere and can’t purify it. And when things turn really bad, manual watermakers are indispensable for a ditch bag.

How they work—Reverse osmosis

Reverse osmosis watermakers produce fresh water by pressurizing salt water to around 800psi and forcing it through a semi-permeable membrane. Water molecules can pass through, but practically all contaminants (such as salt, bacteria, viruses, grit, etc.) are left behind. The remaining “brine” (concentrated seawater and contaminants) is discharged overboard, and a new batch of salt water is introduced, in a continuous process that turns 10% of incoming salt water into fresh water, while 90% is discarded.

Katadyn developed an energy-saving method by using the rejected brine to assist in pressurizing the incoming water. This energy recovery principle makes it possible for a manually operated watermaker to produce enough water to hydrate 25 people. The original product, the Survivor 06, led to the introduction of other manual and electric watermakers featuring this technology, which greatly reduces energy use compared to older watermakers.

What type of power will you use?

Beside the desired output, a critical decision concerns how the watermaker will be powered, by 12V or 24V DC, 120V AC or driven by a belt from the engine.

DC watermakers are the easy choice on small boats because of their compact size and manageable current draw (though you may want to run the engine while making water). 120/240V AC and engine-driven systems have higher capacities, and their modular design allows components to be installed separately. But AC systems also have certain drawbacks.

Engine-driven pumps are about the size of a big compressor and may not fit inside the engine compartment of your boat. However, if you can fit an engine-driven pump, you can make water whenever you’re running under engine or at anchor when you’re charging batteries. Because diesel engines don’t like to be underloaded, adding a watermaker pump to the alternator’s load may actually benefit your engine.

120V AC watermakers can be run from either an auxiliary generator or a big inverter with other AC loads to give the genset some additional burden. A better alternative, at least when the generator isn’t required for other tasks, would be to use either a high capacity 12V DC watermaker like the Katadyn PowerSurvivor 80E, or to use a smaller AC watermaker which could be run off of the ship’s inverter. Starting up the generator to make water and adding AC loads to “exercise” it is not ideal and emphasizes the importance of planning and integrating your ship’s systems.

How much output do you need?

The World Health Organization recommends at least one liter (approx. one quart) of fresh water per person per day to maintain hydration. Obviously, that figure is for survival conditions, not summer vacation. A freshwater consumption of two quarts per person per day is more realistic, assuming that this is augmented with other liquids and that normal water conservation methods are used (saltwater showers, rinsing veggies in salt water, etc.).

Cruiser Linda Dashew stated that her family of four used 5 gallons of fresh water per day initially, but graduated to 12 gallons per day as their children grew and their water storage capacity increased. Your water use also depends on the climate and the kind of activities you perform during the day. The level of perspiration when sitting in the shade of the bimini is quite different from grinding the genoa winch all day long.

These relatively modest amounts are based on necessity, as opposed to desirability. On a luxurious vessel with a large watermaker, a crew of five or six might use 80 gallons per day, taking hot showers, running the washer-dryer and dishwasher. The bottom line is that your water consumption can vary dramatically based on your lifestyle and your ability to produce water.

Energy requirements for a DC watermaker

With increased output comes increased current draw, so plan your electrical system accordingly. Using four gallons per person per day requires about 150 minutes of watermaker operation (PowerSurvivor 40E), or half that much using the PowerSurvivor 80E, equaling roughly 9Ah of battery capacity consumption per day per person. Using these examples, small cruising boats can function comfortably with a PowerSurvivor 40E. When consumption exceeds 12gpd per person, we recommend the PowerSurvivor 80E.

Maintaining your watermaker

Ideally, you’d run your watermaker when the engine is on for propulsion or battery charging. Anywhere from two to eight hours of operation should cover your needs. This ensures that the voltage is high, and that the energy goes directly into the watermaking load without being stored in the batteries. Watermakers—unlike household water filters—require TLC to perform reliably.

  • Watermakers perform best when used daily. Daily operation inhibits biological growth and is preferable to running it less frequently but for longer periods. Consider flushing the system with non-chlorinated freshwater after each use.
  • Flush the system with biocide when your watermaker is inactive for a week or more (three days in tropical climate). After each treatment remember to discard the water produced during the first 20 minutes of the next session. Installing two Y-valves on the watermaker intake makes it easy to draw in water from the freshwater tank for daily flushing, or to take in biocide when preparing for longer-term storage.
  • Rinse the pre-filter at least once a week during regular use to prevent silt from accumulating and entering the watermaker.
  • Change the pre-filter once it loses its rigidity. Be sure to replace it with a polyester 30-micron pre-filter instead of a cheap paper filter that breaks down quickly.
  • After every 1000 to 1200 hours of use, replace all O-rings and check valves. (parts come in a Seal Repair Kit.) Consider it a 3000-mile oil change. Cost is around $40 and the entire job takes about an hour.
  • Use acid and alkaline cleaners at least once per year to remove stubborn biological growth and mineral scaling.
  • Add a pre-filter: When operating your watermaker in areas with high silt content, install a more elaborate pre-filtration system, using a 5-micron pre-filter (in addition to the 30-micron pre-filter) and a 2A booster pump to ensure that the watermaker gets enough water flow.
  • TDS Meter: For about $50 you can buy a TDS (total dissolved solids) meter to check water quality before diverting it into your tanks.

Factors affecting maintenance schedules

Watermakers are designed for use in clean seawater, not harbor water or tap water. The maintenance intervals given will drop if your watermaker is used in other environments and the following conditions:

  • High salinity and cold water will decrease output and cause a modest increase in amp draw.
  • Silt or sand can damage components of the high-pressure pump if not removed during pre-filtration.
  • High ambient temperatures foster growth in the reverse osmosis membrane.
  • Pollutants such as oil or chlorine will eventually destroy the membrane. Avoid operating the unit in harbor water. While the membranes are replaceable, they are quite expensive.
  • Air bubbles caused by a leak in the intake line or turbulence around the intake thru-hull can be very destructive to the membrane.

Saving water aboard

  • Plumb your galley, and possibly your head, with salt water. One of our favorite cruiser/advisors recommends installing a diverter-valve on the inlet water to the pressure water pump. When in conservation mode, use the pressure pump to deliver salt water to the galley and head. In non-conservation mode (as when marina-hopping), change back to fresh pressurized water.
  • If there’s no pressurized water, have salt- and freshwater pumps in the galley. Using foot pumps instead of pressurized water can save 50%-75% of the water consumed. And having a dual pump set up with fresh- and saltwater will really stretch the drinking water supply.
  • Cook and clean with salt water. While you can’t substitute salt water for fresh in all applications, you can use it to cook certain types of fish, and mixed with 80-90% fresh water, to boil pasta (10-20% salt). See The Offshore Cruising Encyclopedia for more ideas.
  • Use salt water to rinse vegetables and clean pots and pans and only use fresh water for drinking, sensitive recipes and a final rinse of dishes.

Watermaker Installation Tips

Installation DOs

When choosing a location for the watermaker drive/pump/membrane assembly:

  • Avoid areas with excessive heat. Ambient temperatures above 105°F (40°C) exceed the ratings for the electric motor, and excessive heat can damage or destroy the membrane. Note: Most engine rooms get hotter than 105°F!
  • Choose a dry area. The motor/drive assembly is not waterproof and can corrode.
  • Choose an area free of fuel vapors. The electric motor is not vapor-proof and should not be operated if explosive or flammable materials are present!
  • Find a location that allows comfortable access for routine inspection and servicing.

In addition, you should:

  • Install the pre-filter assembly in an easily accessible location! It needs regular (sometimes daily) inspection and maintenance. For ease of routine maintenance, the choice for this location is probably the single most important decision you will make—plan it carefully!
  • Provide a shutoff valve or seacock in the seawater intake line.
  • Install a coarse strainer in the seawater intake line.
  • We recommend using properly sized ring terminals and a terminal strip near the pump to connect electric power. This allows for easy testing, removal and servicing when required.

Installation DON’Ts

  • Don’t use a thru-hull installed high on your vessel’s hull for your source of seawater intake. This is especially important for sailboats. Even a normal amount of heel when under sail can cause the thru-hull to be out of the water, allowing air into the intake system. A rolling anchorage can do the same.
  • Don’t locate the pump assembly above gear or materials that could be damaged if it leaked.
  • Don’t locate the pump assembly near to sleeping quarters, bunks, or other areas that are normally “quiet” areas for yourself or crew members.

Watermaker troubleshooting

Background: maintaining a healthy watermaker largely involves taking proper care of the pre-filter assembly and seawater intake plumbing. Failure to do so is the most common cause of the two most frequent types of watermaker “failure” we hear about: (1) producing diminished or no freshwater output, or (2) producing “bad-smelling” product freshwater.

Here is what happens:

No freshwater output: The most common cause of diminished or no product freshwater output is air entering the seawater intake system at some point. The pump volume is small and the pressure required to press water through the membrane is rather high (about 800 psi). Since air is highly compressible, a very small amount of air can keep the pump from producing enough pressure to produce product freshwater. Periodically inspect and test the entire seawater intake system to assure that all joints and fittings are airtight, especially the connections at the pre-filter assembly.

Note: Be aware that a stable air gap at the top of the pre-filter housing is not uncommon, and doesn’t necessarily mean that air is getting to the pump itself.

Bad-smelling product freshwater: The purpose of the pre-filter assembly is to trap any particulates in the intake seawater that are larger than 30 microns. A coarse strainer (if installed) performs the same chore for contaminants of larger size. In each case, trapped material remains in the pre-filter housing (or strainer bowl) until removed.

Much of the trapped material is organic: plankton, seaweeds and flotsam of all types. After a watermaker has been turned off, this material soon begins to decompose. As it does, it breaks down into a number of chemicals composed of smaller molecules. Some of these molecules are small enough to pass through the watermaker membrane along with the product freshwater. Perhaps the best-known example of such a chemical is hydrogen sulfide, a gas which (in small concentrations) smells like “rotten eggs.”

Two main factors affect the speed with which these products of organic decomposition will contaminate a watermaker system: (1) the ambient temperature and (2) the quantity of trapped material. We realize that many boaters run their watermakers in near-shore situations while anchored. The amount of trapped material is usually high in such locations, and the pre-filter assembly will require more frequent attention. Moreover, the high ambient temperatures in tropical locations greatly accelerate the rate of such decomposition. Users in temperate climate areas or users processing open-ocean seawater during offshore passages are less likely to require the same diligence.

Watermaker Glossary

Biocide: a chemical used to inhibit biological growth in the reverse osmosis membrane during storage or extended periods of non-use.

Filter: a device that removes suspended solids from a fluid stream. A filter is not the same as a reverse osmosis membrane.

Micron: a metric unit of measurement equal to one thousandth of a millimeter, or one millionth of a meter.

Osmosis: the process of diffusion between two water sources through a semi-permeable membrane, resulting in both water sources striving to reach equilibrium in the concentrations of dissolved substances.

Pickling: a slang term for the process of flushing the membrane with biocide solution (see Biocide).

Potable: suitable for drinking, especially water. With regard to a desalinator, it is generally defined as water with 1,500ppm or lower of dissolved solids that is relatively free of harmful microorganisms.

PPM: parts per million. Used as a measure of the quantities of substances dissolved in water.

Pre-filter: a filter placed in a desalinator system to remove suspended solids from the feed water before it reaches the high-pressure pump and reverse osmosis membrane.

Pressure relief valve: a valve that relieves pressure in a system to prevent damage to system components. In the PowerSurvivor 40E, the relief valve is set to open at 1,000psi, to prevent damage to the pump and membrane.

Product freshwater: potable water produced by the reverse osmosis process.

PSI: pounds per square inch—a unit for measuring pressure.

Reverse Osmosis (RO): a reversal of the natural osmosis process between two fluids that occurs when water is forced under pressure through a semi-permeable membrane. It produces potable water.

Salinity: a measure of the amount of salts, minerals, and other dissolved solids contained in a water source. Salinity (see TDS) is measured in ppms.

Salt: the common name for the chemical sodium chloride. Open-ocean seawater is about 3% dissolved salt, or about 33,000ppm of sodium chloride.

Seal: a device (such as an o-ring) used to prevent fluid leakage between two system components.

Semi-permeable: the characteristic of some materials (e.g., a reverse osmosis membrane) that makes them relatively porous to some substances while blocking the passage of others. Unlike normal filters, semi-permeable membranes usually operate at the molecular or atomic level, allowing much finer “filtering” than simple mechanical filters.

TDS: Total Dissolved Solids—a type of measure commonly used to indicate the purity of water, usually expressed in ppms. A standard TDS meter measures only the electrical conductivity of the water being tested. Therefore, it is only able to measure dissolved substances that are ionized in solution (e.g., sea salt). In particular, they do not indicate the presence of non-ionized substances, such as bacteria, viruses and many other soluble chemicals.