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Watermaker Operating Tips

Operating your watermaker is easy and safe, as long as you obey some simple rules.

  1. Never run the high pressure pump without positive pressure at its inlet.
  2. Never pressure-shock the membrane
  3. Never let chlorine touch the membrane

With that said, here are the basics of starting up your system to make water:

  1. Make sure the seacock is open and the strainer is clean
  2. Start the low pressure (LP) pump, and allow it to purge air and stabilize feed pressure
  3. Start the high pressure (HP) pump, and verify there is still positive, steady pressure at the HP pump inlet
  4. Slowly increase operating pressure until the rated product flow is reached, not more than 800-900 PSI
  5. Water quality will improve after a few minutes. After it stabilizes, test the product water by taste or TDS meter
  6. If product water is good, divert it to your tank.

Shutting down is pretty much the reverse of steps 1 thru 4 above:

  1. Slowly decrease operating pressure
  2. Turn off HP pump
  3. Turn off LP pump
  4. Close the seacock

Fresh water flush your system regularly to prevent organic growth inside the system. This is accomplished similarly to operating the watermaker, except using fresh tank water as the feed source:

  1. Make sure you have a good carbon filter in the fresh water flush line to prevent chlorine from destroying the membrane
  2. Open the fresh water flush valve, and allow it to purge air and stabilize feed pressure
  3. If your fresh water flush is teed into the plumbing prior to the LP pump (best practice), turn on the LP pump.
  4. Turn on the HP pump, and verify there is still positive, steady pressure at the HP pump inlet
  5. Allow system to flush for about 2 minutes
  6. Turn off HP pump
  7. Turn off LP pump
  8. Close the fresh water flush valve

Some systems use electronic controls to automate some or all of these steps.

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Water – The Most Important Resource

After spending many months aboard “Unbound,” we had come up with a series of lists: Must-have; Nice-to-have; and Really-cool-to-have. Top on the Must-have list was water independence.

While cruising in the Virgin Islands (mostly US and British) we had no troubles finding decent water. For $0.10-0.15 USD per gallon, we couldn’t complain about a thing. Obtaining water from several different marinas was fairly simple, and a routine occurrence all over the islands. We would pull the big Cat up to the fuel dock and buy fuel and water at the same time, as well as pump our holding tank (another story). Of course, we didn’t need much fuel, but it made the fuel attendant happy to not just be selling water. With a family of five on board, however, we found that we needed to tank up fairly frequently. After implementing some conservation practices and educating everyone about careful showering and dish-washing our water consumption was reduced to a reasonable compromise.

Then we sailed for the Spanish Virgin Islands. Here, the marinas were too small to accommodate our wide beam, so we were forced to haul water in jerry jugs. We found a fish market/gas station in Culebra that would allow us to fill our jugs for free, so we hauled water jugs every time we went ashore. Hauling water is a lot of work, so we increased water restrictions to conserve.

We topped off our tanks and stowed full jerry jugs of water in preparation for our long voyage to George Town, Great Exuma, Bahamas. While en route, we were successful at using very little water. Once in George Town, we found that we could get free reverse-osmosis water at the dinghy dock behind the Exuma Market. This is a fantastic service to cruisers, and there is often a line of people waiting to fill up. We spent a few weeks in George Town making friends and seeing the area. Meanwhile, my back started to hurt from hauling jugs everywhere I went.

We started our island hop up the Exuma chain to see some of the fantastic sights The Bahamas has to offer. To conserve water, our rule for everyone was “one fresh water shower per day.” Now, being somewhat obsessive about salt water in the upholstery, we required a fresh water rinse after swimming. This became a problem. Now that we were away from easy access to water, but where there is spectacular snorkeling, we had to restrict our fun! This was a total BUMMER! Also, it is absolutely necessary to dive on the anchor after setting it, to be sure it is safely secured to the bottom. Chalk up one more shower. We ended up loosening our restrictions somewhat on showering, but soon put our water supply in jeopardy.

Once we arrived in the US, a reverse osmosis (RO) watermaker, also known as a desalinator, became a top priority for continued cruising. We found that while there is no purely economic justification for a watermaker based on the low cost per gallon we paid for water, the restriction on our freedom was unbearable. We had to have a watermaker or the Captain was getting off!

We chose to install an engine-driven modular unit for its output and space flexibility. Being equipped with two small diesel engines, a cruising catamaran is an ideal candidate for engine-driven accessories. We did not have a generator on board, and it made no sense to install a 12 volt watermaker and then run the engines anyway to charge the batteries.

Once the desalinator was installed, we celebrated by drinking the most expensive water we would ever see!

First Glass of RO Water

First Glass of Water: $6219.27
No more Jerry Jugs: PRICELESS!

We felt that if we associated the cost of the RO watermaker to the first glass of water, we wouldn’t concern ourselves with the cost per gallon. From now on, the water we used would only cost us a little diesel fuel.

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Basic Troubleshooting Guide for Watermakers

Your Watermaker is not making good product water anymore. What do you do?

If your water maker (or desalinator) is no longer making good water, there are several possible causes. I will try to break it down into logical troubleshooting steps for you, so you don’t spend a lot of money replacing parts or membranes hunting for the solution.

Every desalinator needs to have an operating log of some sort. You don’t have to record every time you use it, but it best practice to document the operating conditions when newly installed, at every seasonal startup, after cleaning chemicals are used, and whenever major components are replaced. This information is critical to understanding whether your system is performing as it should.

First of all, every seawater desalinator needs the following in order to function:

  • Filtered feed water, supplied under pressure
  • High-pressure pump
  • Reverse Osmosis (RO) membrane in a pressure vessel
  • Pressure regulator (back-pressure type or needle valve)
  • Pressure gauge to measure RO system pressure

Optional features that make operation and troubleshooting easier:

  • Flow meter for product water
  • Flow meter for feed water or brine (I prefer to measure feed water)
  • Pressure/vacuum gauge to measure feed water pressure
  • TDS meter to measure product water dissolved solids (mostly salt)
  • A method to separate the product flow in multi-membrane systems
  • Helpful, but rarely seen, is a pressure gauge on the feed inlet side of the pressure vessels
  • Thermometer to measure feed water temperature
  • Refractometer or high-concentration TDS meter to measure salinity of feed water

Check Your Calibrations

Before you assume that something is seriously wrong with your reverse osmosis system, make sure you check the calibration of your TDS meter, and inspect all pressure gauges and flow meters.

As the battery in a TDS meter fails, or the electrodes become fouled, the readings can change dramatically. Make sure the electrodes are clean, and the battery has plenty of juice in it. Borrow a handheld TDS meter from a neighbor and compare the reading you get with their’s.

Inspect your pressure gauges for damage or bent pointers. Has the damping fluid drained out? Does the pointer move when you tap the gauge? Is there any leakage or rust around the fittings?

Inspect your flow meters for fouling and damage. A dirty flow meter will not be accurate. Most can be taken apart and cleaned. Be careful not to scratch the inside of the float chamber or bend the guide rod. Note that all flow meters that use a “floating” ball or slug need to be installed vertically, and they are read at the widest point of the float.

Check Your Product Water from Each Pressure Vessel

If you have more than one pressure vessel in your system, check the product water flow rate and TDS from each vessel. Typically, the first vessel in the set will yield more product flow at lower TDS than then next vessel. This is because the feed water for the second vessel is brine from the first vessel – lower flow and higher salinity.

Check Your Strainer, Pre-Filters and Boost Pump

High-pressure pumps require the feed water to be supplied under pressure to prevent cavitation. Make sure your sea strainer (you do have a dedicated thru-hull and strainer for your watermaker, right?) is clear, and the sediment filters are clean enough to allow good flow to the high-pressure pump.  Make sure your boost pump is working properly. This is why we strongly recommend a pressure/vacuum gauge between the last pre-filter and the high-pressure pump. Don’t make your high-pressure pump “suck” the feed water, or it will soon “suck” at making pressure!

Check Your Feed (Source) Water

Feed Water Temperature

Before you send an email or make a phone call, you need to know what your feed water temperature and salinity are. A simple thermometer will tell you the feed water temperature.

Effect of Feed Water Temperature on Product Water:

  • Increasing Temperature:
    • Higher TDS
    • Higher product flow rate
  • Decreasing Temperature:
    • Lower TDS
    • Lower product flow rate

Feed water temperature can make a significant  impact on your product water flow rate. Reverse osmosis systems are rated based on feed water conditions of 32,000 ppm, 77° F, and operating at 800 psi. At 90° F, you can expect to be making approximately 25% more water, and at 48° F, you should expect 50% LESS!

Feed Water Salinity

Measuring the salinity of the feed water is quite simple with the right tool. The Boundless Outfitters Yacht Services department uses a refractometer to measure feed water salinity. A refractometer is a  simple-to-use device that uses light to measure the salinity. You just dip the end into the feed water source and hold it to the light. Look into the viewer and read the salinity directly. The reading will not be as precise as a high-concentration digital TDS meter, but at one tenth the cost, it is close enough for our purposes.

Effect of Feed Water Salinity on Product Water:

  • Increasing Salinity:
    • Higher TDS
    • Lower product flow rate
  • Decreasing Salinity:
    • Lower TDS
    • Higher product flow rate

Feed water salinity can make a significant impact on your product water flow rate. At 35,000 ppm, you can expect to be making approximately 10% less water, and at 25,000 ppm, you should expect 25% MORE!

If you are cruising and don’t have access to a refractometer or high-concentration TDS meter, you can still get an approximate value for feed water TDS. It just requires a some simple math, and some careful measuring. You do need to have a handheld TDS meter for this.

Carefully measure 1/2 cup of feed water. Use a good measuring cup – the Pyrex types used for cooking are not very good, as the thick glass and printed markings make for inaccurate measurements. Molded plastic measuring cups have the markings molded in, so they are more accurate than the glass ones.

In a clean container (a 6 quart pan works well) add the 1/2 cup of feed water to one gallon of distilled water. Mix it well and let it sit for about five minutes. Then measure the salinity with your handheld TDS meter. Multiply the reading by 33 to get the approximate salinity of the feed water.

Check Your High-Pressure Pump

There are components inside your high-pressure pump that are susceptible to wear and tear. Valves and seals are critical elements for ensuring consistent flow and pressure. You should keep a manifold rebuild kit in your spare parts locker. Make sure there pump oil is at the proper level.

Your high-pressure pump should move the same amount of water regardless of the pressure it is producing, as long as it is within its design limits. Measure the feed flow rate with the system pressure at Zero (open your bypass valve, or back off the regulator fully). A feed or brine flow meter is great for this, but if you don’t have one,  you can use the empty jug from the distilled water you just used for the last test. You will also need a stopwatch. Measure the time it takes to fill the jug from the overboard (brine) discharge of your watermaker.

Now, increase the pressure of your system to 800 psi (or whatever pressure is appropriate for your conditions). Measure the feed flow rate again. This is why I prefer to have a flow meter measuring feed water before it gets to the high-pressure pump. You just read it directly. This meter should never vary during operation. If you have a flow meter measuring brine discharge, you need to add this flow rate to the product water flow rate to get the total feed flow. I have seen many systems with brine flow meters, and most use different units from the product flow meters. Make sure you convert to consistent units before you add the product flow to the brine flow.

If you don’t have a flow meter for feed or brine, you will need to get your jug and stopwatch out. Make sure the product water is being rejected back into the brine stream so you measure total flow.

If there is any difference between the feed flow rate at Zero pressure and the total flow rate at operating pressure, you will need to carefully inspect the high-pressure pump, motor and wiring, and repair as needed. Some small DC-powered watermakers may run slightly slower under load due to resistance in the wiring, and battery draw-down. Please keep this in mind while troubleshooting.

Check the Voltage at the High Pressure Pump

Over time, vibration and salt air can cause increased resistance in electrical connections. The results are reduced voltage for equipment, and temperature rise in electrical components. A further complication for motor circuits is that the current draw increases as well. This is because most electric motors use a constant amount of power (Watts). Since power consumption is the product of voltage and current (P = V x A), if the voltage goes down, the current goes up. Current is what causes heat to build up in areas of high resistance.

Ideally, the voltage measurement at the connections nearest the high-pressure pump motor should be at least 97% of the voltage at the power source in all running conditions. Worst case, the motor voltage should be no less than 90% of the source voltage. Reduced voltage causes motors to run slower, which reduces the flow rate of your high-pressure pump.

Effect of Feed Water Flow Rate on Product Water:

  • Increasing Flow Rate:
    • Lower TDS
    • Higher product flow rate
  • Decreasing Flow Rate:
    • Higher TDS
    • Lower product flow rate

Check Your Pressure Regulator

If your filters, pumps and all of your measuring equipment is in good working order, you can check your pressure regulator. Does the system pressure vary? Some systems use a “set it and forget it” pressure regulator arrangement. While running your system at operating pressure, adjust the regulator up and down (do not exceed 950 psi!), and watch the pressure gauge. Does it increase and decrease smoothly? Can you set it to any pressure and have it hold steady? Most regulators can be taken apart and inspected/cleaned.

Effect of Feed Pressure on Product Water:

  • Increasing Pressure:
    • Lower TDS
    • Higher product flow rate
  • Decreasing Pressure:
    • Higher TDS
    • Lower product flow rate

Check Your Pressure Vessels and Membrane Elements

Make sure all the plumbing is in good condition and free of leaks. A cracked or corroded end cap in your pressure vessel can cause leaks, and on rare occasions, brine contamination of the product water.

If you are having trouble with a new membrane, open up the pressure vessel and check the installation.

  • Verify that the brine seal is installed correctly
    • The brine seal prevents the feed water from bypassing the membrane
    • The sharp edge of the brine seal faces upstream (toward the feed inlet)
    • There should only be one brine seal
    • The brine seal can be at either end of the pressure vessel as long as it is facing the correct direction
  • Inspect the membrane product tubes for scratches or nicks
  • Inspect the o-rings, and replace as needed
  • Use proper o-ring lubricant for assembly

If the brine seal is not oriented correctly, feed water can bypass the membrane. This will reduce product flow rate and increase TDS.

Be very careful when changing the product water o-rings (the ones on the inside of the end plug, where the membrane product tube enters). If you scratch or nick the grove where the o-ring sits, you can create a path for brine to contaminate the product water.

Effects of Failures Inside Pressure Vessel:

  • End Plug Crack or Corrosion
    • Higher TDS
    • Higher product flow rate
    • Leaks
  • Brine Seal Damaged or Installed Backwards
    • Higher TDS
    • Lower product flow rate
  • Product Water O-ring or O-ring Groove Damage
    • Higher TDS
    • Higher product flow rate
  • Membrane Product Tube Damage
    • Higher TDS
    • Higher product flow rate
  • Membrane Failure
    • Higher TDS
    • Higher product flow rate


Reverse Osmosis Watermakers are complicated systems that require careful maintenance. Many cruising boaters are quick to blame the membranes if the product water flow is reduced, or TDS increases. As you can see from the above troubleshooting tips, there are several possible causes for reduced (or increased) product flow and increased TDS.

Happy Cruising!
Tim Allen

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How to Preserve “Pickle” Your Watermaker Membranes

Preserving your reverse osmosis membranes for long-term storage is quite easy, and we strongly recommend this procedure if you will be unable to flush your watermaker with fresh water for more than three weeks. This procedure will protect your membranes for up to six months. If you wish to keep your system preserved for longer than six months, you will have to thoroughly flush, then re-preserve your system every six months. Extended storage can cause some rubber and plastic parts to decay, so don’t over do it. Membrane preservative (or storage chemical) is available in two forms:

  • Cartridge Form
  • Powder Form (bulk powder)

The type of preservative you use depends on your system design. I will discuss how to use both types of pickling chemical.

Cartridge Form Preservative

First, lets discuss how to use the cartridge form of storage chemical. This type requires your desalinator system to have a recirculating loop to cycle the brine discharge back into the water inlet before the low pressure boost pump. Some watermakers (like those made by Village Marine) include a valve that creates a recirculation loop. Village Marine calls this valve the “cleaning valve.” Here is a sample plumbing diagram for Village Marine Little Wonder Modular watermaker:

Little Wonder Diagram

Note the cleaning valve, and how it returns the discharge to the boost pump inlet. Before preserving your membranes, first thoroughly flush your watermaker with fresh water. Then remove your prefilter from the housing and discard it. If you have two prefilters, remove the 20 micron filter. Install the preserving cartridge in place of the prefilter. Note that the cartridges we sell fit both standard and “Big Blue” 10″ housings. Switch the cleaning valve to the cleaning (recirculate) position. If you do not have a cleaning valve, disconnect the brine discharge hose and connect it to the boost pump inlet.

Close the seacock. Make sure the product water will not go into your tanks. Also, make sure the system pressure is reduced to ZERO before running with chemicals in the system. Use your fresh water flush valve to purge any trapped air from installing the preserving cartridge. Run your desalinator for 30 minutes to thoroughly disperse the preserving chemical. Shut it off, and leave it for up to six months!

Powder Form Preservative

Now let’s talk about how to use the powder form of the preserving chemical. Use this method for larger desalinators and systems that do not have a recirculating loop. After thoroughly flushing your watermaker with fresh water, disconnect the brine discharge hose and run it to a clean bucket filled with fresh, dechlorinated water. You can use RO product water or tap water that has been run through a carbon filter.

Disconnect your boost pump inlet hose (make sure you close the seacock) and run it to the same bucket. Make sure you don’t allow the boost pump to lose its prime. Add the preserving chemical to the bucket so that you end up with a 2% solution overall. The preservative needs to be 2% by weight of the total water in the system, not just the bucket! Here is a quick way to estimate the total water in your system:

  • Start with the water in your pressure vessels:
    • 2.5 x 21″ Membranes – About 3 Lbs each
    • 2.5 x 40″ or 4.0 x 21″ Membranes – About 6 Lbs each
    • 4.0 x 40″ Membranes – About 15 Lbs each
  • Then add the water in your prefilters:
    • 2.5 x 10″ Standard Housings – About 2 Lbs each
    • 2.5 x 20″ Standard Housings – About 4 Lbs each
    • 4.5 x 10″ Big Blue Housings – About 5 Lbs each
    • 4.5 x 20″ Big Blue Housings – About 10 Lbs each
  • Then add the water in the plumbing system and your bucket:
    • Plumbing – About 1-2 Lbs
    • Bucket – 8 Lbs per gallon

For example, a system with two 2.5 x 40″ membranes and two 2.5 x 10″ prefilter housings will have about 17 pounds of water, not including the bucket. If you add two gallons for the bucket, the total water in your system will be about 33 pounds. Calculate the amount of preservative powder to use:
Total Water (Lbs) x 0.02 = Preservative (Lbs) In our example, 33 Lbs x 0.02 = 0.66 Lbs, or 2/3 of a 1 Lb Jar Carefully mix the preservative chemical in the bucket of fresh water. Wear safety goggles and a mask, and work in a well ventilated area. This stuff is pretty harsh. Set the system pressure to ZERO, and make sure the product water will not go into your tanks. Run your watermaker boost pump and feed pump to recirculate the preservative for 30 minutes.

Freeze Protection

To preserve your watermaker for cold climates, it is best to use Food-Grade Propylene Glycol (Non-Toxic Antifreeze will work) to protect your system. For best freeze protection, a 60% propylene glycol to 40% water ratio should be used. Calculate how much PG to use by the method above. Please note that PG alone will not prevent organic growth. Non-toxic antifreeze contains some growth inhibitors, and will provide protection of your desalinator for up to six months.

Recommissioning After Storage

After long-term storage, you will need to flush the chemicals out of your watermaker thoroughly before making any product water. Switch your cleaning valve to “Run,” or reconnect your brine discharge and boost pump inlet hoses. Replace your prefilter(s) with new, and replace your fresh water carbon filter. This is a good time to consider changing your high-pressure pump oil, also.

Open the seacock, and run your desalinator at ZERO pressure, making sure the product water (if any) gets dumped overboard. Let the system run at ZERO pressure for 30 minutes to flush the storage chemicals out of the nooks and crannies. Slowly increase the operating pressure to 800 PSI (or to the maximum rated product flow if you are floating in brackish water). Make sure the product water is dumped overboard, and let the system run for another 30 minutes.

Compare your product water quality and flow rate with your benchmark values from when the membranes were new. (You did write that down, didn’t you?) Write the new values in your watermaker log. After adjusting for temperature and salinity differences in the raw water, if your product flow rate has reduced by more than 10% you should run a cleaning cycle – first with organic detergent, then with scale remover. The procedure is almost exactly the same as for preserving your membranes.

Happy cruising,

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Crevice Corrosion Can Ruin Your Watermaker!

You may have heard of Crevice Corrosion. Perhaps you  have confused it with galvanic corrosion (electrolysis), but the chemical process is quite different.

Without explaining all the chemistry (as some of it is beyond my Chemistry 201 studies!), I will explain what happens. Essentially, Crevice Corrosion occurs to Stainless Steels when in the presence of seawater that has been depleted of oxygen. How does this happen?

Seawater that is trapped against stainless steel equipment loses its oxygen over time by causing oxidation of the materials it is in contact with. Crevice corrosion does not require dissimilar metals to occur. In fact, it can even occur where a plastic part is clamped to a stainless steel part (or even painted stainless steel!), if seawater is allowed to become trapped between the parts.

The result looks a lot like galvanic corrosion, but is not caused by the same problems.

Common boating equipment that frequently suffers from crevice corrosion is trim tabs. I have seen trim tabs that have been properly maintained, with zincs replaced regularly, fall prey to crevice corrosion. This usually occurs between the hydraulic ram base and the tab itself. You should inspect yours, and always make sure that the components of your trim tabs are well bedded in a waterproof compound (3M 4200 or 5200 or similar adhesive). Don’t trust silicone for this critical task.

Another piece of equipment that we see suffering from crevice corrosion is your watermaker high-pressure pump and high-pressure fittings. This is due to inadequate fresh water flushing of your desalinator. Would you believe that some watermaker manufacturers still build systems with no easy method for fresh water flushing?

The potential for crevice corrosion is why I personally prefer Titanium Alloy or Nickel Aluminum Bronze (NAB) for high pressure watermaker pumps.

To learn more about crevice corrosion, please see this Wikipedia article on crevice corrosion.

Happy cruising!