Watermakers A-Go-Go

22 Nov

Peros2 008
Here I will try to distill some basic information common to all watermakers. I won’t hide the fact that I’m affiliated with Spectra Watermakers (I’m a consultant there), but I cruised for ten years and became intimate, oh-so-intimate, with a Katadyne, nee PUR, nee Recovery Engineering PowerSurvivor 35 then 40:
boat water makers 12v 24v 22212 392745
First of all, if you’re not planning to do some serious cruising, don’t get a watermaker! A watermaker will be the most maintenance intensive device you have aboard. They take constant vigilance and care, and the second you install one and first expose it to sea water, so begins the long (or short) decline of the membrane, the specialized “filter” that separates fresh water from salt. And they ain’t cheap: The smallest models run $4000-$5000. More output, more money. And it’s a fairly involved installation, which will take up some real estate, and require electrical connections, plumbing connections, and several thru-hulls. If you’re just a weekend/week-here-or-there cruiser, sticking a hose in your tank from time to time will be a simpler, cheaper option.

If you are going cruising, a watermaker is one of several key pieces of technology that can really improve your quality of life. You’ll never have to worry about the quality of dock water, or how to get it in the first place. You’ll never have to break your back carrying jerry jug after jerry jug out to your boat from some remote location. Your tanks will runneth over, more or less, as you regularly observe the small technological miracle of turning sea water into fresh. And not just any fresh water, but the best damn glass of water you’ve ever tasted.

Watermaker maintenance isn’t too onerous, but it must be regimented and regular, otherwise you’ll destroy your watermaker, or at least the membrane.

Tip: Reverse osmosis water has very few dissolved solids, so if you’ve got flooded batteries aboard you don’t need to go searching the supermarket aisles for distilled water to top up your batteries. You’re pretty much making it every day.

Reverse osmosis technology has been around since the fifties, but didn’t really become viable for small yachts until the eighties. The PowerSurvivor 35, so-called because it could be powered electrically or manually, was the first affordable shipboard watermaker for a small yacht, and the first that most cruisers remember. (Larger systems for larger yachts have been around a bit longer.) At about $2000 the PowerSurvivor 35 would reliably put out 35-40 gallons of water per day. Cruisers got to know and love them so much that if they wanted more capacity they just installed two of them.

Many cruisers, myself included, will have the sound of the PowerSurvivor etched into our brains for life. Two of my old shipmates and I could do a three part harmony, one imitating the grr-err-grr-err of the electric motor, the next doing the shhhh-chah of the piston, and the third doing the chunk-chunk of the reversing valves. They were fairly quiet, but when you listen to something for hours every day, for years on end, it sticks with you.

I am biased toward sailboats with simple DC systems, and I don’t like the sound of generators spoiling my cocktail hour, so for me a watermaker must have two qualities, and these are non-negotiable: They must be quiet, and they mustn’t use a lot of electricity.

To give an example to the contrary, on my trip to Clipperton Island last year, the owner of the expedition boat installed a watermaker that he got from…I don’t know. It didn’t have a name. To backtrack a bit, there’s nothing technologically unique about desalinating water using reverse osmosis. If a reasonably mechanical person set out to build a watermaker they could do it with off-the-shelf parts: a high pressure pump, a membrane, a pressure vessel, a constricting valve, various high pressure hoses, and voila. The owner of my Clipperton boat did pretty much just that, and we couldn’t carry on a conversation anywhere on the whole boat while this thing was running. It’s electric pump used 30-40 amps, so one of the main engines had to be running. The high pressure lines whipped so violently that we had to screw them to the bulkhead with cushion clamps, but they still strained like angry pythons, and we were afraid they were going to rip off the bulkhead and attack us. But the contraption made about eight gallons per hour of fresh drinking water, and it made our whole trip possible:
Clipperton, an island without a lot of potable water

Compared to the contraption on my Clipperton trip, all of the manufacturers of yacht watermakers have done it with considerably more finesse, that is, they’re fairly quiet and energy efficient. You’ll often read comments from cruisers saying they could have just built their own watermaker at a fraction of the cost, using generic parts. Oh really? Again, a motivated mechanic could assemble the parts and desalinate water, but all the years of R&D and refinement by the major manufacturers add up considerably. The home grown models, while simple, are usually loud and aren’t very efficient.

Here is an excellent video tutorial on reverse osmosis desalination, and how a simple, homegrown system would work:

The efficiency comes from somehow recovering the energy consumed in bringing the water up to pressure. Reverse osmosis starts happening at about 430 PSI, but the sweet spot seems to be considerably higher, usually around 800 PSI. Energy is consumed bringing sea water up to 800 PSI, so to let it go squirting off in a big fountain after it’s passed the membrane would be a big waste. This pressurized water, and the energy it contains, is carried back into the pump and recycled. This requires a sophisticated, specialized pump. This diagram shows the connections to a Pearson Pump, used on larger Spectra models. You’ll see it has a high pressure outlet to the membranes, and a high pressure inlet coming right back:
pearson diagram2

By the way, that 800 PSI, while a very high pressure, isn’t the highest hydraulic pressure on your boat. That honor goes to the high pressure side of your diesel injection system, which can be as high as 160,000 PSI. If you’ve got a hydraulic backstay adjuster, it can run up to 5000-6000 PSI.

To give one example of such a specialized pump (again, I’m biased) the proprietary Clark Pump (no connection with me, although I wish I could take credit) used in Spectra watermakers up to 1000 gallons per day, is a triumph of elegance and ingenuity. It isn’t technically a pump, but a pressure intensifier. There are no wires or motors in the Clark Pump: It just takes the pressure from a Sureflow feed pump, of 80-120 PSI, and intensifies it to osmotic pressure. I’ve even built a few Clark Pumps, and I’m still not sure I understand them, but they work, and they’re quiet:
clark pump tn

Reality Check: Watermakers are all named and quantified in gallons of output per day. On a small yacht you’ll seldom run a watermaker all day. Even the smaller units (Katadyne PowerSurvivor 40, Spectra Ventura, Village Marine Little Wonder) will consume 5-10 Amps at 12 Volts DC, meaning they will drain 120-250 Amp hours from your battery banks in 24 hours. This is a fairly large drain, and this energy will need to be replaced somehow. If you’re motoring for days on end, run the watermaker 24/7 and take lots of showers. But if you’re sitting at anchor, as cruisers tend to do, you’ll probably just run your watermaker part of the time. I got into a groove a running my watermaker for 4-5 hours per day, while the sun was shining on my solar panels, then flushing with product water until the next day. Whenever I ran my engine, I ran my watermaker.

Pre-filters, pre-filters, pre-filters!

I’ll say it again, pre-filters! Pre-filter changes are the most important, and most neglected tasks in watermaker upkeep. All systems employ pre-filters before the seawater gets to the pump or the membrane. Different systems employ different levels of pre-filtration, but at a minimum they filter out seaweed, plankton, whale poop, sticks, stones, etc. They must be changed regularly. Here’s why: Have you ever smelled stagnant seawater, like when you flush your head for the first time in two weeks? It’s the foulest smell on earth, worse than raw sewage, in my book, but actually containing some of the same ingredients as raw sewage, namely hydrogen sulphide, which gives off that rotten egg smell. When all that plankton, squid eggs, and whale poop gets filtered out by the pre-filter(s), it doesn’t go away, it just sits there in the pre-filter, and soon begins to rot.

Imagine a scale of filtration: At one end of the scale is a coffee filter; at the other end is reverse osmosis. In between, at various levels of filtration, which are measured in microns (millionths of a meter) and fractions of microns, we’ll see increasingly small things getting filtered out: dust, bacteria, dyes, viruses. Wow, by the time we get to reverse osmosis almost nothing gets through but water molecules. It’s some damn pure water. Almost. Almost nothing else gets through. When that crap in the pre-filter decomposes, the hydrogen sulfide molecules are a small as water molecules, and don’t get filtered out. They end up in the water, and let me tell you, my friends, it’s a rude morning wake-up to have your coffee, and your whole tank, smell and taste of raw sewage. And there’s no way to guard against this: Many manufacturers include a salinity probe with a diversion valve, so that if the water is too salty I won’t be allowed into your tanks. There’s no such thing as a stink-o-meter, so changing the pre-filters is the only preventative.

Many watermaker manufacturers have been blamed for the stink, when it’s not their fault. Pre-filter changes are the owner’s responsibility. How often to change them depends on the sea water. Strangely, in the island tropics, where you’ve got a nice warm environment for bacterial growth and decomposition, the water is usually very clear and contains few microorganisms to decompose. Cruising somewhere like Hawaii or Tahiti you can probably get away with going a week or so without changing the pre-filters. Somewhere warm where the water’s murky, like a coastal tropical area with a river outflow, you should probably change the pre-filters every day or two. How will you know the difference? Because you’ll be checking all the time. If they’re black you’ll know you’ve got decomposing ocean muck in your pre-filters; if they’re white you’ll know they’ll still clean. You’ll be intimate with your pre-filter situation, because if you aren’t you might contaminate your whole water supply and be very unpopular with your guests.


With all this in mind, the pre-filter location is paramount. It’s worth routing hoses all over creation to get those pre-filter housings to a convenient place. If you have to climb into a scalding hot engine room to burn yourself while changing a pre-filter you’ll be less likely to do it.

Pre-filters can be cleaned and recycled, at least a few times. I got in the habit of dragging dirty ones behind the boat, if underway, or hanging them overboard at anchor, baking them in the sun for a day or two, then recycling.

Beyond regular pre-filter changes, watermaker care becomes a little more varied. It’s a big, sophisticated pump, with parts that will wear over time, so pump re-builds every year or two, or every 1000-2000 hours of use. In this regard, watermaker maintenance can be compared to diesel engine maintenance: Not terribly difficult once we’ve got it figured out, but we’ve got to stay on top of it.

What’s stickier is the general discussion of membrane care. The membrane is a very specialized, high tech thing, not really a filter, by definition. While pressurized water flows past the membrane, some of it, called permeate, goes through the membrane and goes into your tanks as fresh water. The rest goes overboard as brine. As long as this process goes on, the membrane is generally happy. If you shut your watermaker down and leave it full of seawater, stuff will start to grow on the membrane and foul it.

The first line of defense is to flush your membrane with product water, which is part of the shut-down process. It hurts to watch all this hard-won water flush the membrane and go overboard. On some systems this flushing happens automatically; on others it’s a manual operation, some more manual than others.

Back to pre-filters: In some systems the fresh water flushing includes the pre-filters; in others it just flushes the membrane. In neither case does it clean the pre-filters. The only way to get the soon-to-rot crap out of the pre-filters is to change them.

If you’re going to let it sit for more than a few days, fresh water flushing isn’t enough and you should pickle the membrane with a biocide solution.

Pickling is hard on the membrane. Fouling will clog the membrane. Chlorine (as from a municipal water supply) will damage the membrane. Oil or petroleum products will damage the membrane. Time will damage the membrane.

How much damage? Nobody seems to know, but all of these things are bad. It turns out a little chlorine from a municipal water supply won’t really hurt a membrane that much. Oil usually floats on the surface of the water, while the intakes are below the water, and a little oil won’t be a complete disaster.

There are cleaning solutions for membranes, alkaline for biological growth, acid for mineral deposits. Cleaning is also detrimental to the membrane.

What’s a sailor to do when all these things damage the membrane? We do our best, follow the directions, stay away from the damaging practices when possible, and choose the lesser of the evils: Obviously pickling is preferable to leaving the membrane unprotected for two months. If we do our best our membranes will probably lead reasonably long lives. In the worst case, a replacement membrane for a small unit runs $400-$600, less if you shop around.

I cleaned, pickled, and flushed my membrane, forgot those things and left it steeping in sea water for days on end several times, and my membrane lasted ten years. It might still be okay, but the rest of the watermaker, or at least the electric motor, has definitely given up the ghost. I have no intention of installing another one until the far horizons beckon again.

This article was syndicated from The Adventures of the Vessel Condesa


  1. Grant McDonald

    Hello Clark,good article.You might be able to help me here.I have a Pur/Katadyn E80.It is the compact unit with 2 small membranes both of which are buggered.I have no idea where to get some more.I am cruising around Malaysia at the moment,not much chance here me thinks.I am hoping you might know of some where in the states that sells and posts them.No doubt Katadyn products are expensive but I’ve heard there are places that sell off the shelf membranes at a better price.Hope you can advise.Regards Grant,Obsession11,Malaysia

Leave a Reply

Your email address will not be published. Required fields are marked*. Comments are moderated and generally will be posted if they are on-topic and not abusive. For more information, please see our Comments Policy.

More from the AIM Marine Group