In not-too-distant past replacing a single membrane on a small watermaker was a $600-$800 hit. Now, as with so many other things, you can go online and buy a membrane for $150-$220. And it doesn’t matter whether you’re replacing a 20-inch membrane or a 40-inch – the two most common sizes – the price is the same.
Before I go any farther, when a watermaker is performing poorly it is seldom the membrane, but the membrane is the first thing people want to blame. You must first ensure that everything else is within spec before you condemn a membrane. This means that the system must be doing exactly what it’s supposed to do with regard to flows and pressures, and still making crappy water (low quantity or high salinity). Pumps must be pumping the quantity of water they’re supposed to pump, at the right pressures, or water quality and quantity will suffer, even with a perfect membrane.
Membranes don’t just up and fail, or rather, when they do it’s a one in a thousand thing. When they fail they usually decline slowly, over a period of 5-10 years, sometimes longer, or they fail because they were abused (chemical damage, lack of flushing, or lack of pickling…tsk tsk).
Cautionary tale over. $150-$220 for a membrane still isn’t free, especially if you’ve got a system with multiple membranes, but it changes the game somewhat. Say you left your boat in a hurry last season in the Caribbean, and you’re not 100% exactly, positively sure you stored the watermaker properly. You could fly back to the Caribbean armed with various cleaning chemicals, your fingers crossed, and the prospect of buying a membrane anyway, at Caribbean prices, or you could just buy a membrane online, stick it in your baggage, and replace it as a matter of course. Guess work averted.
Likewise with the long term view: At this price you might just replace your membrane(s) after 4-5 years when you suspect they’re fading, and be done with it. An older or fouled membrane can often be brought back among the living by chemical cleaning, but the chemicals can be expensive and the cleaning process can take hours of hands-on time, and soaking overnight, with various buckets and hoses strung about in awkward places.
I don’t mean to encourage gratuitous membrane replacement, filling the worlds landfills with used membranes, but you get the idea. And it’s no sure bet a new membrane will make better water than an older one. There is a lot of variation in membranes, even the exact same part number from the exact same production run, so if you’ve got an older membrane that is still performing well, stick with it. I’ve seen them perform within spec for up to 15 years.
Final caution: new membranes are shipped stored in nasty storage chemicals. The membrane must be flushed for at least 20 minutes to remove the chemicals, or it will be damaged when the system is pressurized. With a new membrane installed, run the system unpressurized for at least 20 minutes before making water.
Final final caution: Membranes don’t have long shelf lives in their packaging. If you’re thinking you’ll just buy a spare membrane to have on hand for a few months or years down the road, this is a bad idea. The membrane will undoubtedly be dead after, say, six months.
Replacing a membrane is quick and straightforward, as long as you’ve got access to the pressure vessel end cap, and room to slide the membrane out. Here is a video on how to do it on a Spectra. The process is similar or identical on other types of pressure vessels. The only thing you really have to remember is to keep the brine seal on the correct side:
Photos courtesy of Jolyn and Ken Zielesch, aboard cruise ship Rhapsody of the Seas
Bill Edinger, founder and President of Spectra Watermakers (and my boss), set out earlier this year on a five-month sailing sabbatical to French Polynesia aboard his Norm Cross-designed 45-foot trimaran, Defiance. He, family, and guests sailed to the Marquesas, Tuamotus, Society Islands, then up to Hawaii, all in a very seamanlike manner.
On the final stretch from Hawaii to San Francisco they hit the edge of the Pacific High, as returning sailors are wont to do, and motored for a day or two. This is when disaster struck. In Bill’s own words:
“Sunday the seventh we were motorsailing along in light wind. I was just coming off my 12:00 to 3:00 AM watch and was down below starting coffee for Kevin who was next up when we heard a loud bang. We both ran on deck to see the mast canting aft by about 30 degrees. I yelled to Kevin that we needed to get a halyard forward to keep the mast from coming down. He ran forward but before either of us could do anything the mast came all the way down. It seemed slow as the boom and vang collapsed on the top of the dodger and the mainsail sort of cushioned the fall. The mast of course was hanging out over the end of the port ama (float). The good thing of course is that no one was hurt.”
“The toggle on the forestay had failed. Unfortunately this is the only stay going forward. When the boat was re-rigged a while back this stay was upped a size to 1/2″ dyform wire which should have been bulletproof. I can only think it failed from shock load fatigue as the mast pumps a little fore and aft in a seaway.”
The offending toggle:
“We retrieved the mast by cranking it onboard forward with line and winches. Every few inches we had to stop to see what lines and rigging were hanging up and clear them before moving on. The mast appears in good shape. The main was a total wreck as we needed to cut it away, and the jib furler was over the side and dragging behind. We tried to save it but in the end had to cut it loose. Boom looks salvageable. The dodger was wrecked on one side and the port rails bent. Once we got the mast onboard we started lashing everything down and dismantling what rigging we could to clean things up. Over the next day we refined things by supporting each end of the mast with some milk crates and shims to keep the mast from rocking back and forth on the cabin top.”
“The Radar seemed undamaged so we rigged up a mast using our dinghy floorboards, mounted the antenna on deck just above the maststep and managed to get it working fine. I also retrieved the VHF antenna and jury rigged the VHF radio and AIS which is working so so.”
“I have to say the teamwork was fantastic. The whole crew was focused and calm throughout the whole ordeal.”
“By the time we got everything cleared up it was late morning and we re-started the engine and got going again. We figured we had enough fuel for about 500 miles but had over 1100 to go. I called the coast guard and reported that we had a non-emergency situation but would be needing fuel to get all the way back to San Francisco. They had us checking with them every four hours or so and by the middle of the second day called us to report that the cruise ship Rhapsody of the Seas would divert from its course to deliver us fuel by late afternoon.”
“Around 3:00 PM the cruise ship delivered 100 plus gallons of fuel (as well as a bunch of fresh fruit and other goodies thrown in!). We were definitely the show of the day as 1000 or so passengers lined up to watch the fuel transfer by three guys in a RIB-type boat. As soon as the third and last trip was made the whole crowd broke out in cheers!”
A happy ending and a story to tell for those aboard the cruise ship. Bill and crew motored through the Golden Gate a week later.
And a happier ending that they saved the mast! Dismasting stories always seem to involve “cutting away the rig,” and I always think that was at least $10,000, more like $30,000 with a mast like Bill’s, sinking to the bottom of the ocean. In rough seas you’d have no choice, but I always figured I’d give it the old college try to get that mast aboard somehow. Still, I thought 60-foot mast on a 45-foot trimaran for 1000 miles of motoring through a nasty part of the Pacific…this should be interesting. It wasn’t until I saw the pictures that I understood the diagonal approach to seagoing mast storage on a trimaran.
Terminology Moment: When your mast falls down on the open sea, by accident, this is called dismasting. When you take it down on purpose, say by a crane in a boatyard, this is called demasting. Please make a note of it.
Once I heard about this I immediately heard three more stories about masts that came down while just motoring along, from the shock loads of the seas. I wonder what the percentage is of masts lost in this manner compared to masts lost in full combat mode?
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:
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 water, 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 that need topping up you don’t need to go searching the supermarket aisles for distilled water. You’re making it every day.
Reverse osmosis technology has been around since the fifties, but didn’t 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 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.
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. The PowerSurvivor was 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: It must be quiet, and it 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, he 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 just that, and we couldn’t carry on a conversation anywhere aboard while it ran. Its 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 screwed them to the bulkhead with cushion clamps, but they still strained like angry pythons, and we feared they’d 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 the watermaker manufacturers 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 inefficient.
Here is an excellent video tutorial on reverse osmosis desalination, and how a simple, homegrown system might work:
A watermaker’s 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 this pressure, 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:
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 may 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:
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 must 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. The pre-filters 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 roe, and whale poop gets filtered out by the pre-filter it doesn’t go away, it just sits there, 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 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 go right through the membrane.
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 salty water is diverted away from 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.
Pre-filters:
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 and burn yourself to change 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 if at anchor, then baking them in the sun for a day or two.
Beyond regular pre-filter changes, watermaker care is 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, but it’s a necessary evil. On some systems this flushing happens automatically; on others it’s a manual operation, more manual on some 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 clogs the membrane. Chlorine (as from a municipal water supply) is detrimental to the membrane. Oil damages the membrane. Time damages the membrane.
There are cleaning solutions for membranes, alkaline for biological growth, acid for mineral deposits. Cleaning also damages the membrane.
How much damage? Nobody seems to know, but all of these things are bad.
What’s a sailor to do when all these practices 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. It turns out a little chlorine from a municipal water supply won’t be the end of the world. Oil usually floats on the surface of the water, while the intakes are below the water. A few days steeping in seawater will probably pass unnoticed. 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, 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.
