The feeling of autonomy that a watermaker gives a yacht is unique. It transforms that weekend-only cruiser into a go-anywhere, life-on-the-hook, adventure craft, breaking the tether to marinas.

Filling your water tanks at the fuel jetty will always be a cheaper alternative to the eye-watering cost of buying and installing a watermaker, but there will be times when your plans are limited by having no access to a tap or if the only source available doesn’t look particularly appetising. And in the event of a burst pipe or accidentally draining your tank mid-passage, it means you’ll be able to keep sailing at the flick of a switch, rather than having to reroute to refill.

Watermaker desalination process

Watermakers – technically known as desalination units – use a process called reverse osmosis (RO) to make drinking water. By forcing salty water at a high pressure against one side of an RO membrane, fresh water will slowly seep through, leaving the salt and bacteria behind. The output is notably devoid of minerals, but the taste can be a little strange at first. Think of the membrane as a filter so fine that even bacteria and salt molecules can’t get through it.

But as well as the pressure, a membrane also needs to have the water flowing over it to flush away all the stuff that didn’t pass through as it would otherwise block the membrane’s pores. In this
way it’s different to a filter because a membrane has a salty inlet, a freshwater output, and a saltier discharge outlet.

High pressure pumps

Creating high-pressure water, which is also flowing at speed, takes a lot of energy, and there are two methods watermakers use to achieve this. One is simple: a high-pressure pump. This can draw a lot of current but it creates the speed of flow needed, and the forceful pressure too. The saltwater is directed to the membranes and the pressure is created by closing down
a tap, called a needle valve, at the discharge to build up pressure on the membrane while still allowing water to flow out through the needle valve at the required speed.

High-pressure pump watermakers are fast, but they aren’t the most energy- efficient way of creating clean water. They are usually tuned to produce 60 litres per hour or more but can draw upwards of 500W and while there are 12V versions, they typically use mains voltage pumps and are better suited to being run from a generator than a battery bank.

They’re designed to fill your tanks quickly so you don’t have to run the generator for long. Mechanically, they’re simple, and apart from the high-pressure pump there are no moving parts to go wrong.

Energy recovery

The alternative method is a Clark pump watermaker, also known as an energy recovery device (ERD). This uses a fast-running but much lower-pressure pump which needs less power. To obtain the high pressure required, the pressure in the discharge water is harnessed by a couple of reciprocating pistons and used to boost the inlet pressure. It takes a few minutes to build up pressure and during that time the output is slow and not very clean.

Watermaker running costs

An ERD unit will typically produce 20-60 litres per hour. The lower demand on the pump means it can be reasonably powered by 12V with as little as 9A, and can produce a litre of water with just 4-5Wh of energy. High-pressure units need between two and three times that energy per litre. If you run your boat from solar panels or don’t have a generator, this is the type of watermaker to go for.

In fact, your boat’s power source is the first thing to look at when considering which type of watermaker to get. But it’s also worth remembering that the extra complexity of ERD units make them expensive to buy and more prone to breakdowns.

When choosing an ERD watermaker it’s therefore really important to consider the availability and costs of spare parts because sooner or later they will need servicing.

Most manufacturers have a dealer network that can provide advice, but don’t count on them stocking parts or offering servicing. These machines are notoriously fiddly to repair, and one dealer told me discreetly it’s not financially viable for him to offer a repair service as the time it takes can spiral into hours. That leaves you with the option to send the machine back to the manufacturer at great expense, or do it yourself.

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After-sales service

Spectra has one of the best reputations for after-sales service. In most places you’re more likely to find Spectra and Schenker dealers, but that doesn’t guarantee they’ll have parts in stock, and all manufacturers will ship parts to you. Spectra also offers rebuild kits which come with step-by-step instructions and special tools to get to those hard-to-reach O-rings. They feel like a lot of money for a bag of O-rings, but are a cheaper option than sending the unit back to the factory.

If you’re buying new, a warranty is highly valuable but once it runs out you’ll find out how pricey the parts really are. Some manufacturers give longer warranties if a certified technician installs the system.

Most ERD units are made out of engineering plastics such as Delrin which can split around fittings if over-tightened or if warm water is used during cleaning.

One leading UK dealer who sells all brands told me that since Spectra changed the type of plastic used a few years ago he’s had no returns, unlike rival brands. Spectra were tight-lipped on the material they use when Yachting Monthly asked them for details.

Watermaker instillation tips

A bad installation can render a good machine useless, so there are a few key points to know. Use dedicated through-hulls for the saltwater intake and brine discharge. Make sure the intake is low enough that it won’t be exposed when heeling or in big waves to prevent air bubbles entering the system. Don’t install the intake where there will be turbulent flow, behind the keel,
for example, or near the props and rudders. The discharge should be above the waterline.

To give the pump the best chance of producing the pressure needed, install it as low as possible in the boat. Follow the manufacturer’s recommendations on hose diameters and avoid long hose runs and tight turns that restrict flow.

Finally, use correctly sized wiring as the 12V pumps will underperform if there are voltage losses in the wiring.

Monitoring quality

A Total Dissolved Solids (TDS) meter measuring impurities in the output water coupled to a diverter valve can automatically protect the purity of your tank. Anything under 500ppm is fine to drink but a well-installed system should deliver 200-300ppm.

You can buy inline TDS meters, or stick meters that you dunk into a sample cup to monitor it manually. You’ll see it slipping over time if there’s a problem developing with the pump or membrane.

Membrane care

The membranes will produce more output and cleaner output in warmer and less salty seas. For the best lifespan, use them every few days. Drying them out, or running chlorinated water through them will do irreparable damage.

According to Dupont, which makes the widely used Filmtec membranes, they should not be left unused for more than 24 hours, but in practice they can be left for around five days before organic growth risks building up on the membrane, which blocks it and reduces performance. This can sometimes be remedied with an alkaline flush if caught early.

The other thing that can block them is carbonate deposits. In this case, an acid flush will restore some of the membrane’s performance.

If idle for a few days, it’s worth flushing the membrane with fresh water. Some manufacturers recommend doing this after every use, but that’s to protect other components in their systems.

If the machine is going to go unused during the off-season, the membrane can be pickled in propylene glycol. Most manufacturers offer branded pickling solutions and restorative solutions which conform to their warranty and don’t react with other materials in the system.

Avoid using the watermaker in anchorages where other boats aren’t using holding tanks. Although the membrane will sift out e.coli, the pre-filters will become a nasty Petri dish of bacteria.

Don’t leave me this way

Different manufacturers have different recommendations, but, as a general guide, here’s what to do if leaving your watermaker unused for any period of time.

1-5 days: In practice, leaving the membrane sitting in saltwater won’t do too much harm, even though the membrane manufacturer warns against more than 24 hours. Check your watermaker manufacturer’s recommendation though. For instance, Spectras suffer from dramatic internal electrolysis if left soaking in sea water.

1-2 weeks: Definitely flush it through with fresh unchlorinated water. Use a carbon filter to remove chlorine if you are flushing with tap water.

More than a month: Pickle it.

They used to recommend using acid (hence the term pickling) but now manufacturers recommend propylene glycol, which is less aggressive on O-rings and other metal components. Buying the branded solution for your machine will give you peace of mind.

If your boat lives in relatively clean water, think about setting a timer so the machine runs automatically every few days for 10-15 minutes. That’s long enough to flush the membrane and prevent organic build-up.

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