Whether you’re crossing an ocean or day-tripping through the islands, the crew will need water to drink and, hopefully, for more than the occasional shower. With a fuel-efficient passagemaker and a large fuel tank, it is more likely that running out of fresh water, not fuel, will bring a boat back to the dock—unless you can make your own fresh water with a watermaker.

How It Works

In its most basic sense, a watermaker uses a high-pressure pump to push seawater through a filter called a membrane. All watermakers have membranes, which are installed in sealed, pressure-rated tubes. The membranes have pores small enough to keep out salt and other contaminants, while still allowing fresh water through. This sheeted material is wound spirally around a permeate tube. As salt water is pushed through each layer of the membrane, it is progressively filtered until it is pure, fresh water.

Watermakers typically utilize a low-pressure pump to prime the high-pressure pump that then pushes the water into the membrane at around 800 psi. This process is called reverse osmosis. Osmosis is how plants and trees move liquid and nutrients up their stalks. In this process, a high concentrate moves through a semi-permeable membrane to a lower concentration area in a plant’s cells.

The opposite occurs with reverse osmosis, since we want the highly concentrated (salty) side to move to the low concentrate (fresh) side of the membrane. This can be accomplished with high pressure. The extra-salty water being created on the outside of the membrane (called brine) is pumped overboard and is about 90 percent of the salt water being used. The fresh water making its way to the center permeate tube is pushed into the boat’s freshwater tank.

What Are the Parts?

Elementally, to make fresh water you need a thru-hull, a seacock, a strainer, pre-filters, a low-pressure pump, a high-pressure pump and discharge plumbing to another thru-hull at the waterline. On the output side, freshwater plumbing and test valving, pressure and water quality monitoring, electrical controls and wiring round out the required components. All electrically powered systems will have these basic components.

Like most things on a boat, there are a range of options for additional pre-filtration, ultraviolet sanitizing, water quality analysis, and a host of automatic features and remote panels—all with additional cost and ease of use in mind. 

How Large a System is Necessary?

This is often the first big question, and different factors must be considered depending on the boat and its use. A general rule is 5 to 20 gallons per person per day. This can be influenced by many factors. The crew on a small boat crossing an ocean may just want a backup to the tanks to ensure enough water to drink. A bigger boat may have a crew of eight, all showering daily, with dishwashers and laundry—and they may want to wash the decks with fresh water every day. This boat will need larger tanks, meaning the water will have to be replaced at a larger volume.

Watermakers are typically rated by how many gallons they can produce in a day. A 200-gallon-per-day unit produces about 8 gallons an hour. Theoretically, if the boat has a 100-gallon tank, it would take 12.5 hours of running the watermaker to fill the tank. You may not be using that much water each day, but depending on the length of the voyage, full tanks can be mission critical, especially if a problem occurs with the watermaker.

Another advantage of a larger watermaker is that the quicker you make water, the less wear you put on the equipment, and the fewer pre-filters you’ll need. Assuming there is plenty of power for the equipment and space for the installation, an argument can be made that bigger is better.

However, like every installation on a boat, there are always compromises. Not only does the equipment need to fit, but it will also require room for servicing. You will be dealing with plumbing, and with changing filters and membranes, which means you’ll occasionally be spilling water. It’s better that this spilled water can easily drain into the bilge versus pouring onto a varnished sole or into a locker full of gear.

You’ll want to consider the equipment located around the watermaker as well, and provide a barrier between electronic equipment and engines in particular. It is important that the watermaker remain clean, and that contaminants (such as diesel and oil) do not spray it. The water inside needs to remain sterile, and you don’t want a dirty hose to permeate and contaminate it.

Watermakers can be sold in multiple configurations. Some are palletized, meaning the whole unit is contained on a mountable platform with the exception of thru-hulls, strainers and any remote monitoring or control panels. The alternative is a modular system of individual parts that can be installed where they best fit. This can be useful if there isn’t space for a palletized system. It will also test your ability to make well-supported, leak-free plumbing connections.

You may need to fabricate and install shelving sealed with epoxy and paint prior to unit installation. The units can be heavy when they’re full of water, and they need to be installed securely so they do not come adrift underway. One solution is to fiberglass portions of the shelves to the hull or bulkhead. Larger units may also have multiple pressure vessel tubes that can take up a large amount of space.

The units can be somewhat loud when running. Different units have different decibel ratings. Depending on where the unit will be installed, this may be a consideration. For instance, a lower-output unit may need to run into the night. You wouldn’t want a loud unit to be mounted under or abaft the bulkhead adjacent to your bunk.

Watts to Water

Energy use is another factor. How will the unit be powered? Depending on the size of the unit, there can be substantial wattage use. Bigger units are typically AC and are powered by the boat’s generators. Smaller DC units can be powered by the boat’s battery bank via the alternator or solar panels. A load survey may be in order to make sure there is enough battery and charging capacity to run the unit.

Consider that a Spectra Ventura VT-150 is rated at 6.3 gallons per hour (150 gallons per day) and uses 9 amps at 12 volts DC, which is 17 watts per gallon. Spectra uses pumps with clever energy recovery systems via the Clark pump or Pearson pump that are particularly efficient and provide for lower energy use.

A much larger 3,600-gallon-per-day FCI unit, by contrast, uses 2.24 kW, which is 18.6 amps at 120 volts AC. If you are already running your genset for climate control and adding the watermaker load does not exceed your available genset-produced wattage, a powerful watermaker like this can likely run for a much shorter period of time to replenish the tanks.

Installation

Once you’ve chosen a unit, you’ll typically need to install two thru-hulls. These should be completed to American Boat and Yacht Council specifications with proper backing blocks and seacocks. The exit thru-hull should be located just above the waterline. The intake thru-hull needs to be located away from other thru-hulls (especially head or macerator discharges). A strainer and all hoses containing seawater need to be properly rated. Depending on the design, pre-filters should be installed so that there is room to take them apart for changing the filters without causing damage from spilled water.

Securely mount the components and connect the plumbing. The output plumbing should be installed with a T-valve so product water can be sampled, or the unit can be pickled before the water runs directly to the boat’s tanks. There are different ways to plumb product water into the tanks. The most common way is to split the fill or vent hose and install a T with a reducer to accept the smaller watermaker output hose. Alternatively, if the tank has a removable inspection cover on top, this can often be removed and tapped to accept a fitting. Multiple tanks may benefit from a manifold.

Additional options for watermakers are plentiful. Many units have remote controls and display panels, and many can monitor the total dissolved solids in the output fresh water. Other features can include carbon filters for membrane-damaging chlorine; and ultraviolet light disinfection systems, which use more electricity but can kill additional viruses, microorganisms and protozoa.

Note that using the watermaker in a harbor or in brackish water can be problematic. Silt is a watermaker killer, plugging pre-filters and membranes, and scoring pump walls. Don’t use the watermaker if there is any oil in the water. If you see a sheen, the membrane can be destroyed.

Care and Feeding

Once seawater enters a watermaker, all the little critters in the seawater start growing. This means that a watermaker should be considered a living thing that has to be maintained to keep it alive. Allowing stagnant seawater to remain inside your unit will eventually result in the system failing because the filters will be plugged with growth. The interior piping will also need disinfection.

Each manufacturer has instructions for how often to run the system to keep it fresh. If a unit is to be left unused for a period of time, then a pickling solution must be run through it. Units must also be winterized to prevent them from freezing. More sophisticated units may have additional capabilities with timed freshwater flushes.

The pressure required to make fresh water depends on the temperature and salinity of the water. Early watermakers (and some simple units still sold today) felt a little like a science experiment to use. The unit would be started with a pressure dial turned down. After a set period of flushing, the operator would slowly turn the knob to build pressure while carefully watching the gauge. The product water valve would be set to flow into the bilge. Once the pressure built to a set point, samples would be drawn from the product water to test the quality for total dissolved solids (particularly salt). If the test was acceptable, the product water would be diverted into the freshwater tank.

More modern systems automatically set pressures and test the product water. If the product water meets a set standard, it is diverted to the tanks. It is still a good idea to test the water yourself before diverting, since poor output could contaminate your entire tank.

It is also a good idea to make sure your tank and plumbing are clean prior to using the watermaker and periodically thereafter. It doesn’t make any sense to create clean water and then run it through a filthy tank or hoses.

You’ll also want an accurate way to measure the levels in your water tanks. Watermakers are not designed to have much resistance on the output and can be damaged by trying to overfill a water tank.

After reading far too many gruesome stories about seafarers running out of water far from shore, it is a relief that there is modern technology to greatly reduce that possibility. Long showers, fresh water to spare for deck washing, and having plenty to drink can vastly improve the passagemaking experience and the disposition of the crew.

This article was originally published in the July/August 2024 issue.