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By Stephen Sommer


Watermakers for Small Cruising Boats Steve, are watermakers a practical option for those of us with 30-foot cruising boats? Your article on “shoestring air-conditioning” for small boats makes me wonder whether we can have watermakers, too.


nce again, the answer is YES, if you want. Just like the air-conditioning, it’s a bit of a force-fit on a small boat, especially if you believe in the KISS principle. A watermaker fits more easily in a large boat that has a genset that runs a lot. There are a number of very different alternatives that all use the same basic technology: reverse osmosis. Each one requires a high-pressure saltwater pump to force water molecules through a membrane that will not allow salt to pass through. Because of the high pressure and the fact that you need to pump about four times as much saltwater as you actually desalinate, this is a power-intensive process. All that extra water is required to flush the removed salt out of the membranes. In round numbers, it takes about 60 watt-hours (5 amp-hours) to make a gallon of water, unless special means are used to recover power from the flush water. Strangely enough, this kind of energy recovery is only found on the largest and smallest capacity systems. The mid-size systems are the most power hungry.

A FEW GENERAL RULES OF THUMB: Match your production rate over the period that you have power available for watermaking, on a daily basis. For example: If your power source is the main engine, which you run two hours per day, and you use 50 gallons per day, then a 25 gallonper-hour watermaker (600 gallons per day, GPD) would be a good choice. A bigger watermaker would cost more, be harder to install and would not last any longer! Watermakers are more often harmed by disuse than by heavy use. Plan on making water every day; it’s best for the longevity of the watermaker. Re-evaluate your consumption. An ordinary cruiser may get by with a couple of gallons per day per person, but once you have a watermaker, you won’t be ordinary. Expect to take freshwater showers often. Plan on rinsing fishing and diving gear and maybe even decks with freshwater. You could become very popular among “ordinary” cruisers. A couple of jugs of freshly made water that hasn’t been marinating in old water tanks for weeks can be your contribution to a nice meal on a buddy’s boat. Plan on 20 gallons of water per person per day.

work for you. Otherwise, continue on to the next two options. Belt-coupled, Engine Drive: These systems range from 400 gallons per day, GPD, to 1200 GPD. A magnetic clutch and Vbelt, like those found on automotive air-conditioner compressors, is used to drive a high-pressure piston pump from your main engine. Large capacity systems make sense in this configuration because there is a lot of power available, and you’d like to get your watermaking done as quickly as your batterycharging. Small Capacity, Energy Recovery: I have only recently starting recommending these systems because the technology is new. They use clever techniques to harvest power from the super-salty brine discharge water, to drastically reduce the power required. These systems draw so little power that it is practical to run them from batteries, wind power or solar power. They are the lowest capacity systems, but that’s okay because you can run them many hours per day. Unfortunately, low capacity does not mean low cost. The energy recovery devices are the most expensive components, so you might get five gallons per hour from a system that costs the same as a 50-gallonper-hour conventional system. That’s not as bad as it sounds; just run it ten times as many hours. A word of caution: The energy recovery devices are complex, expensive and relatively unproven. The makers of these systems realize this, and some offer lifetime warranties on the energy recovery device. Don’t buy one without such an assurance and make sure that it’s in writing and that the company has been in business long enough to make it likely that they will be around when you need help. Stephen Sommer is a degreed electrical engineer with extensive experience in electrical, mechanical, refrigeration and air-conditioning systems and holds a USCG Masters license. He consults in all areas of yacht systems, which include all the equipment on board yachts beyond a basic hull and motor or sails. Have a systems problem or question? Ask Stephen Sommer. Email:

SOME COMMONLY AVAILABLE CONFIGURATIONS: Conventional AC Power Drive: This is essentially the only configuration found on modern yachts with gensets. AC power from the genset is used to drive two pumps: a fractional horsepower feed water pump that forces the water through a series of conventional pre-filters, and a multi-horsepower pump that drives a high pressure piston, diaphragm or vane pump, which does the real work of driving water through a semi-permeable membrane that will not allow salt to pass. Hybrid, AC/Battery Conventional: Just like with air-conditioning, it is possible to run the smallest of the conventional watermakers (about 400 GPD) on power from a large enginedriven alternator, via a large inverter. If you happen to get a great deal on a small watermaker and already have a very large alternator and a large modified-sine-wave inverter, this could NEWS & VIEWS FOR SOUTHERN SAILORS


May 2004



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