by Doug Thompson
When your cruising plans call for long days at sea and anchorages far from civilized services, the conservation of ship’s stores is top of mind. Managing food and fuel is crucial—when they run out, your body and boat shut down. Potable water is also a concern. When the freshwater holding tank is empty, the vast, salty ocean cannot slake your thirst—water, water everywhere, nor any drop to drink.
Onboard watermakers have made the ocean your reservoir as reverse-osmosis systems turn raw seawater into drinkable freshwater. Processes have improved, equipment has become smaller and prices have dropped since these devices entered the marine market in the late 1980s. Whether or not you need a watermaker depends on your boating activities. If you enjoy coastal U.S. cruising and rarely stray from ports where freshwater is available, then the need for a watermaker is small. But if you have a sportfishing boat and are out for days at a time or cruising to foreign ports, then a watermaker is a necessity. At 8.35 pounds per gallon, water is heavy. A sportfishing boat embarking on a 50-mile run can save a lot of fuel by starting with minimal water in the holding tanks. Once you start trolling, the generator is turned on and the watermaker starts making freshwater while you fish.
Watermakers use a series of pumps, filters and membranes to turn raw seawater into freshwater. The current Environmental Protection Agency (EPA) standard for potable freshwater is less than 500 parts per million, and all the manufacturers’ products in this article meet that standard. The process is reverse osmosis, the same technique used on a large scale at desalination plants. At the heart of the process is the membrane, a filter with pores the size of a red blood cell. After the larger particles in seawater are filtered out, the high-pressure pump forces water through the membrane, which removes almost all of the dissolved solids. This method rejects up to 99 percent of salts, contaminants and pollutants from seawater until the water purity meets the EPA standard.