Keep your watermaker healthy and you won’t go thirsty.
Look in the engine room of any true bluewater yacht and you’ll find a reverse-osmosis (R/O) watermaker, one of the “must-have” accessories for the ocean cruiser. While a watermaker doesn’t replace ample freshwater tanks, the ability to desalinate makes life at sea more pleasant: Everyone can take daily showers, for instance, a big plus in a relatively small interior of passagemaker in the tropics. And when you reach your island paradise, and discover they’re charging 50 cents a gallon for fresh water of dubious quality, a watermaker lets you desalinate your own. It’ll taste better, too.
Fundamentally, a reverse-osmosis desalinator goes against nature: Osmosis is the tendency of molecules to pass through a semi-permeable membrane from a less concentrated solution, e.g., fresh water, toward a more concentrated one, e.g., salt water, to equalize the two sides. An R/O watermaker pushes salt water (“supply water”) against the membrane at high pressure, which reverses the osmotic flow. Some of the high-concentration, saltwater molecules pass through the membrane’s microscopic pores into the low-concentration, freshwater side, and then they head off into the ship’s freshwater tanks; the salt and other dissolved contaminants are left behind. The rest of the supply water flows past the membrane, flushing the osmosed-out minerals back into the ocean.
Like every mechanical system, reverse-osmosis watermakers need routine maintenance, and unless you treat them right you’ll have more downtime and spend more than necessary on repairs.
The semi-permeable polymer membrane, for example, is the heart of a watermaker, and without proper care its life will be short. Some membranes carry a $1,000-plus price tag, and higher-capacity watermakers have more than one, so keeping yours going as long as possible pays off. (Most manufacturers say a membrane should last five years on average, longer with scrupulous care.)
Common sense plays a big part in all this very necessary feeding and care. Indeed, when it comes to extending the life of your watermaker’s membranes, there’s a very basic, very reasonable rule you can follow, according to Berkeley Andrews, the marketing rep for Sea Recovery (premium), HRO Systems (mid-range), and Village Marine (economical) watermakers, three manufacturers that are all owned by corporate giant Parker-Hannifin. “Simply try to operate your watermaker where the water looks somewhat clear or blue,” he says. “So you won’t have to change your prefilters (installed between the watermaker itself and its raw-water intake) quite as often. If you need to make water in adverse, dirty, or silty conditions, that is okay, but just plan to bring a few spare prefilter replacements so you can keep making water.”
Keep the Salt Water Flowing
There is a wide range of watermakers on the market (see “Resources,” right) from a wide range of manufacturers, starting from basic low-output models with minimal bells and whistles to electronically controlled setups that require little more than pressing a button. And sometimes not even that: When accessorized with high- and low-level sensors in the water tanks, a sophisticated desalinator with programmable controls can start and stop itself without human intervention, and keep itself clean with regular freshwater flushes. But no matter how plain or fancy, a watermaker needs an ample flow of supply water, which means, as Andrews says, keeping the prefilters working properly.
A watermaker’s supply side, between the seawater pickup and the membranes, consists of a low-pressure pump, the prefilters we’ve already mentioned, and a high-pressure pump. Most manufacturers recommend adding a sea strainer at the through-hull, too. Joe Pinto, National Account Manager for Reverse Osmosis Systems at Dometic, makers of Dometic Sea XChange watermakers, says the strainer should filter out anything bigger than about 50 microns. Clean the strainer regularly, like you’d clean any seawater strainer.
The low-pressure pump pulls water from the strainer and pumps it at about 30 psi into a set of prefilters, typically a 20-micron followed by a 5-micron. As the filters clog up, supply pressure drops; there is a gauge on the desalinator to monitor this. Some operators, according to the experts, watch the pressure daily and change the filters when it falls below a certain point, while others ignore the gauges and wait until the desalinator’s low-pressure cutoff stops the system (usually at around 6 psi). Both ways work apparently, but the latter results in lower freshwater output during the last stages of filter clogging. Of course, in keeping with the aformentioned advice from Andrews of Sea Recovery et.al., if you happen to be boating in dirty water, or desalinating in a polluted harbor, you’ll have to change the filters more often.
When changing filters resist the temptation to use less-expensive pool or spa filters, although they might fit. Such filters are most likely going to be made of paper and will fall apart under hard use and high pressure. “It’s certainly true,” states Andrews, “that prefilters need to be changed periodically. But we suggest to most of our customers that they go with a no-cost option on their units that will allow them to use a commercial variety of prefilter. Yes, they the prefilters are slightly more expensive but they last about five times longer than the standard kind.”
Downstream of the prefilters there’s a high-pressure pump that forces the supply water through the membrane, typically at around 800 psi. If you use a paper filter and it disintegrates, shreds of paper will be drawn into the high-pressure pump and damage it, usually beyond repair. These are expensive items—some pumps can cost between $1,500 and $2,000 to replace, so think again about saving money on filters.
Every 500 hours, by the way, some pumps also need an oil change—not a big deal, since their crankcases often hold only a couple of ounces. There are also easy-to-replace seals that need service periodically. Recommended service for various pumps may be different, and some watermakers use water-lubed pumps. Read your owner’s manual to find out which you’ve got.
The high-pressure pump moves a lot more water across the membranes than will make it through the membranes, the overflow serving to wash away salt and other solids left behind by the desalinated water. But this isn’t enough to keep the membranes clean; after each use, the system should be flushed with fresh water from the ship’s tanks. Most watermakers do this automatically, by the way, but a few require operator attention.
Don’t Forget to Flush
Flushing with fresh water is huge, though. If you just run sea water through the watermaker, bacteria and other biologicals can begin to thrive. Add heat, like you’ll find in most engine rooms, and the biologicals replicate and attach themselves to the surface of the membranes. They’ll degrade the performance of the membranes, and are difficult to remove if you don’t do it right away, as soon as you finish with the watermaker. Flushing runs fresh water through the entire system and dumps it overboard, along with salt, the biologicals, and any other contaminants.
But how often should you really flush your system? Actually, the answer depends on where you are. In cold New England waters you don’t have to flush as often as in the tropics. Some manufacturers , like Sea Recovery for example, set their auto-flush default values at every seven days, but in some warmer, more tropical areas that’s not enough. The operator can change the interval at the control panel. You can leave the boat for long periods with auto-flush activated—there’s no need to “pickle” the system with chemicals—but it’s a good idea to have a boat-watcher keep an eye on things. If dockside power goes out, the system might not reset itself and auto-flush could stop flushing. If your watermaker doesn’t have auto-flush, you’ll have to take a hands-on approach and run the system every week or so, or “pickle” it by adding a chemical to preserve the membranes. Yet again, follow the manufacturer’s instructions.
And don’t forget. A charcoal filter in the flush line keeps chlorine that might be in the ship’s water tanks from damaging the membrane; that filter will need attention periodically, too. Membranes don’t like chlorine or petroleum—some manufacturers install an oil/water separator alongside the prefilters to keep oil and gas from polluted water from getting into the supply water. Since oil floats and the seawater pickup is deep, this may be overkill, but if your watermaker has a separator, don’t forget to change its filter periodically as well.
Finally, if you’re storing the boat for the winter, inject the membrane vessels (the tubes where the membranes live) with food-grade glycol to prevent freezing and keep the membranes wet. Even better, the experts recommend, remove the membrane vessels completely and store them ashore in a warm place. Then come spring you’ll be ready to desalinate again.
How Much Water Do You Need?
There are plenty of watermakers on the market (a unit from Blue Water Desalinators is shown below), but what sort of capacity is best for you? “We figure 25 to 50 gallons of water per person per day,” says Chris Rollins, president of Blue Water and a 30-yearveteran of the watermaker biz. “You’re not going to drink that much water, certainly, or use nearly that much for showers and cooking. What we find is lots of people these days use an awful lot of watermaker water to wash down their boats--afterall, nothing beats pure water for a good washdown!”
Of course, whatever you use your watermaker for, you’ve still got to figure out how often, and how long, you want to operate it. If you use, say, 30 gallons per day and you want to desalinate for three hours every day, you’ll need a 10 gallon-per-hour watermaker, or 240 gallons per day, to keep the tanks topped up. But a 400-gallons-per-day model, making 16 gallons per hour, will let you desalinate every other day for just under four hours—maybe that’s a better deal. It’s a balancing act.
And the extent that you typically cruise matters, too: If you’re spending every other night in a marina, you can get away with a smaller desalinator, or none at all. But if you’re cruising far afield for weeks, or in areas where the water ashore is suspect, you’ll want a watermaker that can handle 100 percent of your needs every day. You have to make the call.
Or you can do what most of us do: Figure what model will fit in the available space you’ve got onboard, won’t tax your electrical system, and won’t break the bank, and buy it. Unless you’re running a cruise ship, even a watermaker with minimal production will do the trick, although you might end up operating it more hours each day. But it beats being thirsty.
This article originally appeared in the August 2014 issue of Power & Motoryacht magazine.