Sizing a watermaker

The first step is to determine how much water you will use – not need; that’s survival – we’re talking about comfortable voyaging. A person can survive on as little as a couple of pints of water a day to avoid dehydration in temperate climates, while in the tropics it can get up to a couple of quarts or more, depending on body build and level of activity. As far as water usage is concerned, that depends on the individual(s) and suggested amounts vary widely. One-half gallon per person per day used to be considered adequate for voyaging sailboats, doing the dishes in saltwater and taking saltwater showers or brief swims for personal hygiene. Since then we’ve become addicted to the creature comforts, and the general rule of thumb nowadays is between a half and one gallon per person per day for drinking, and hand and face washing only. This doesn’t include dishwashing, cooking, showering, making ice, hosing off the deck or anchor/chain, etc. Coffee, tea, soft drinks and alcoholic beverages are not acceptable substitutes due to their diuretic effects. Actual water usage varies widely among cruisers, but today’s voyaging couples are more likely to use between five and seven gallons per day or two and a half to three gallons per person per day. Thus, your first task is to make an estimate of your total daily usage – including crew and guests. The accompanying tables may be of help. (Tables 1 and 2.)

Manufacturers generally rate their watermakers in gallons per day (GPD) based on 24-hour-a-day operation; however, you should start to think in terms of gallons per hour (GPH). Few watermakers are designed to operate around the clock and, in any case, you’ll only want your unit – and consequently, your engine, generator and/or battery charger – to operate a few hours per day. Reverse osmosis watermakers like to be run every day, to keep it clean, but not so much as to encourage biofouling of the membrane. The goal will be to match your watermaker’s run time to your normal engine or generator run time for battery charging, refrigeration, etc.

The tanks
With a properly sized watermaker, you can make all the water you’ll need. Now, where are you going to put it? In addition, what happens if your wonderful watermaking machine stops working? A watermaker is not a substitute for adequate tankage. But what’s adequate? Adequate is, as one waterfront wit put it, “enough to get you across whatever ocean you’re crossing when the watermaker fails.” I would add parenthetically, €˜or to get you to a port where good water is available.’ The ideal configuration will consist of a watermaker and enough water for the kind of passages you expect to make, stored in multiple tanks together with a backup stored in jerry jugs for unexpected guests and emergencies.

For example, two people on a 25-day passage, say San Francisco to Hilo, Hawaii, consuming 3 gallons per person per day with 100 gallons of fresh water tankage and a 150 GPD (6.25 GPH) watermaker would do just fine, even if the watermaker stopped working 10 days into the trip. However, if the water is in a single tank and the tank should develop a leak or become contaminated with diesel fuel, they would have a serious problem. But if the water was in two 50-gallon tanks and just one tank was lost, our passagemakers could go into conservation mode, cut back to one-and-a-half gallons per person per day and easily make it to their destination. They could also supplement their tankage with an emergency store of 10 five-gallon jerry jugs, just in case problems should develop with the remaining tank.

Watermakers like to be run frequently, even daily for a couple of hours. This, along with an automatic backflush after each use, will go a long way toward keeping the membrane clean and extending its life. Also, it keeps the tanks topped off in case the unit should fail. Another very worthwhile feature is automatic pressure adjustment for temperature and salinity variations. These two features will greatly minimize the required maintenance.

Don’t use the watermaker in harbors or marinas unless it is equipped with a good seawater strainer, a multi-stage prefilter system including an oil separator and an ultraviolet (UV) sterilizer. The proper place for the UV sterilizer is after the freshwater storage tanks and before the boat’s pressure water distribution system; this can remove as much as 99.9 percent of biological contaminants. A charcoal filter is also required to prevent chlorine in water from dockside sources getting to the membrane. It should be placed in the system before the watermaker when tank water is used during backflushing.

Finally, the watermaker is a critical element for distance cruisers and it’s important for the mariner to understand how it is supposed to work and how to troubleshoot it. This knowledge, together with a complete spares kit, a good supply of filter elements and the chemicals recommended by the manufacturer could make the difference between an enjoyable passage and a survival experience.      

Ev Collier is an electrical engineer, freelance writer and the author of The Boatowner’s Guide to Corrosion.

By Ocean Navigator