Building your own watermaker


Modern technology has brought us many new devices that make the cruising life much safer and easier. Since we are not interested in camping out while cruising, obtaining safe and clean fresh water has always been a major priority, especially overseas. The three most common ways of obtaining fresh water on a cruising boat are: 1) an effective rain collection system, 2) from shore via a hose, or 3) a watermaker. Given reasonable storage capacity and a good filtration system, a high-capacity watermaker is the most efficient — especially if you are a boonies cruiser.

A reverse-osmosis watermaker takes clean seawater and, by pushing it through a specialized membrane (special filter) at relatively high pressure, makes fresh water. This means that no longer do we have to carry hundreds of pounds of extra fresh water just to ensure we don’t run out. Also, we no longer must get up in the middle of the night to catch rainwater, or search out places ashore where we can jerry-jug water from sometimes expensive or suspect sources.

Watermakers suitable for use on a cruising boat can be obtained from a number of sources depending on your budget and tolerance for electronics gadgetry. But they all do the same thing: make fresh water from seawater. We think that if you are going to have a watermaker aboard, it should have bulletproof — and mainly mechanical — equipment with minimal electrical and electronic parts; use commonly available (not proprietary) components; make the most water practical in the shortest time (40 gallons per hour is our choice); and be repairable with onboard spares, tools and skills.

As an example, we use about 50 gallons of fresh water a week for daily cooking, drinking, showering and cleaning. A 40-gph watermaker with two 40-inch membranes can make that in one and a half hours with setup and flushing time included. Smaller units can take significantly more time to do the same work, thereby seriously affecting a cruiser’s free time. 



A well-laid-out system like this one will make operation and troubleshooting easier.


A watermaker Hypro pump mounted on a slide under the engine crankshaft pulley.

An engine-driven pump
Because the power needed to run this volume through the membranes is more than 2 hp and a 12V motor won’t handle that, we have chosen to belt our watermaker pump to our main engine on our last two boats. This is similar to an alternator arrangement. You can drive a watermaker pump with a generator, but using only a generator puts another expensive, sometimes cantankerous and complicated piece of mechanical/electrical equipment between you and fresh water. If the generator fails, as they sometimes do, you are back to spending hours per week manually collecting fresh water from rain or shore.

We most often run the watermaker when underway and moving the boat. Infrequently, we run it at anchor and sometimes just flush it with fresh water to satisfy the once-a-week run requirement that prevents biologic buildup on the membranes.

We average less than 10 hours per year in the tropics using the engine just to operate the watermaker.

If you decide to make your own watermaker, there are several sources of information on the Internet, including my guide. Fifteen years ago, it took me quite a bit of time to engineer the specifications, develop a parts list, and then find and purchase the equipment. By that time, I had seen two successful DIY units, so I knew it could be done. I used a mix of some used and some new parts, mainly because I was on a tight budget. Since then, I have refined the unit somewhat, installed two units on our boats and helped several other cruisers do their own using my information.

Extensive engineering details are on our website under “CSY Workshop,” and we have a PowerPoint slideshow in our “Presentations” section. There is also a Seven Seas University webinar I did for SSCA a couple of years ago, with the slideshow narrated. 

The elements of an engine-driven watermaker system are shown in this schematic.

Better than the WHO standard
My first unit on Soggy Paws, our CSY, was still in service after eight years when we sold the boat in 2016. It had the original pumps, membranes and fittings, and was belted to our 60-hp Perkins. It was still producing fresh water that tested to about 250 ppm (parts per million solids content). The World Health Organization standard is 500 ppm.

Because I was careful never to run my watermaker in dirty or silty locations, in addition to having two pre-filters, I never had to clean the membranes in eight years.

The second installation on our catamaran is now belted to a 27-hp Yanmar 3GM30. It has a different mounting configuration, and after four years of use still produces water at less than 200 ppm.
There are many ways to mount a pump to an engine; you just need to be able to tightly tension the belt drive around the crankshaft pulley. You want to use an appropriately sized pulley to run the watermaker at near full rpm from an engine running around 50 percent of its maximum rpm.


Left, membrane tube end fittings. Right, a watermaker high-pressure manifold. Note how Dave McCampbell has labeled the system elements, which allows new crewmembers to more easily learn the system.

Once you have all the parts, any competent cruiser should be able to install a watermaker in a week. It took me two weeks — but then I never claimed to be competent, just thorough.

As with ground tackle, we think bigger is better, within reason. So if you don’t have a watermaker yet, consider easing your cruising workload and making the crew happy by obtaining a watermaker. Then you can wash dishes in fresh water, take freshwater showers every day, and not worry about using too much water — just like normal people.

Dave McCampbell is a retired U.S. Naval diving and salvage officer with over 40 years’ cruising and eight sailboats’ worth of maintenance experience. He and wife Sherry, currently cruising in Southeast Asia, spent eight years crossing the Pacific via Easter Island, Hawaii and Micronesia. Prior to that, they circumnavigated the Caribbean over four years. Their most recent excursion was a 7,000-nm journey from the Philippines to the lower Solomons via eastern Indonesia and Papua New Guinea. Four years ago they sold Soggy Paws, their CSY 44 monohull of 19 years, and moved to the “enlightened side,” purchasing a new Soggy Paws, a St. Francis 44 catamaran. Their extensive website is at

By Ocean Navigator