Everything you need to know about Watermakers

In our new series of guides to boating kit we discuss everything you need to know about Watermakers, vital for making freshwater onboard

The term watermaker is a little misleading as watermakers don’t actually make water, they just desalinate and purify water that’s already there. They do this so effectively that they can turn salt or brackish seawater into fresh water that you can use to drink, cook, do your laundry or wash with.

Often water that has come out of a watermaker is actually cleaner and safer than the fresh water provided by marinas and harbours. The very best ones can even create ultrapure water that dries without leaving residue marks for washing down windows and topsides.

Why might you want a watermaker?

Although it’s usually cheaper and quicker to fill up your boat’s water tanks from a marina, the water quality in some places is questionable and that’s only likely to get worse the longer it sits in your boat’s tanks. It also means lugging around a large amount of weight, taking up valuable space and increasing fuel burn. A watermaker not only frees you up for longer off-grid cruising adventures, it also means you can get away with a smaller water tank safe in the knowledge that you have a limitless supply of fresh water on tap.

Does it taste the same as tap or bottled water?

Not quite as the process also removes some of the minerals or trace elements you find in bottled water and of course it doesn’t contain any of the chemical additives, such as chlorine, usually added to tap water. Technically, it’s purer than both of them but it may taste a little different for that very reason.

Watermakers save you having to carry around a huge tank of fresh water for washing, cooking, drinking and filling the hot tub!

How do watermakers work?

Although there are many different makes and types of watermaker, they all use the same reverse osmosis (RO) principle. This involves forcing water at very high pressure (600-800psi) against a semi-permeable membrane of tightly wound polyester film – hence the tubular shape.

It acts like a very fine filter as the water molecules are small enough to pass through the membrane to the fresh water outlet pipe running through the centre of it but the salt, bacteria and other impurities can’t, and are trapped by the membrane layers wrapped around it. These are then washed away by the flow of saltwater through the system. Only around 10% of the water passing through a watermaker comes out as fresh water, the remaining 90% of concentrated briny water is discharged overboard.

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But that’s not all there is to it, to prevent the membrane getting clogged up or damaged by other contaminants such as oil, suspended sediment and plankton, most watermakers have one or more pre-filters and an oil-water separator to remove them before they reach the membrane. Some watermakers also have ultra-violet light sterilisers added to the product water outlet to kill off any micro-organisms that make it through the membrane and prevent them spreading inside the fresh water tank.

A modular Spectra watermaker requires relatively little space

What different types are there?

The very simplest type are handheld manually powered devices such as the Katadyn Survivor range that often form part of offshore liferaft kits. You immerse the inlet filter into a bucket of seawater, pump away by hand and it will produce enough safe drinking water to keep you alive. Most cruising boats, however, are likely to fit a powered device that does the hard work for you and produces enough for all the boat’s fresh water needs.

Broadly speaking these fall into two main categories: those that produce the necessary pressure by using a traditional high pressure pump, and those that use a dual piston Clark pump known as an energy recovery device (ERD). They both do the same job but go about it in a different way. The other key variables are the size and output of the device, the degree of automation and the extra layers of filtration and purification offered.

How does an ERD watermaker differ from a high-pressure pump one?

Conventional high-pressure pumps typically require a high- voltage AC feed to power them, meaning you need to have the generator running. ERD watermakers use a different method of building the requisite pressure, called a Clark pump. This has twin reciprocating pistons that make use of the high-pressure discharge water already in the system to drive one side of the piston chamber so the electric motor doesn’t have to work as hard to generate the additional pressure.

As a result they only draw about a quarter of the power of traditional high-pressure watermakers and can often be run from 12V or 24V DC systems, making them well suited to sailing yachts and smaller motor boats where electrical capacity is limited.

This, and the fact that the Clark pump tends to be quieter than a traditional design, means ERD watermakers are now becoming increasingly popular on luxury motor yachts too, especially those fitted with a lithium-ion battery bank and solar panels to allow silent running at anchor (with the generator switched off).

A fully automatic all-in-one Idromar system

What are the downsides of ERD systems?

Not a lot. They tend to be more expensive to buy (but cheaper to run) and in the early days the number and complexity of the moving parts meant they were more prone to reliability issues but that’s no longer the case.

In fact, because they run at a near constant pressure rather than fluctuating, some claim they are actually more reliable and that the membranes last longer (6-8 years). It also means they produce a fixed output of fresh water regardless of the salinity and temperature of the water. Manufacturers like Spectra and Schenker, which have been making ERD watermakers for decades, say a lot of their customers are switching to them after years of using high-pressure systems.

What capacity watermaker do I need?

Watermakers are rated according to the maximum number of litres or gallons they produce per hour. As you don’t want to be running your watermaker for 24 hours a day, it’s best to go for one that creates enough fresh water for you and your guests’ daily needs within 4-6 hours. That’s likely to be around 60-120lph for a typical 50-60ft motoryacht.

Bear in mind that the actual output will vary depending on the salinity and temperature of the water. Cold salty water takes longer to desalinate than warm brackish water.

How much does a watermaker cost?

Anything from around £5,000 for a relatively basic powered system with manual controls and limited output to £20,000 for a fully automated system capable of churning out 200lph. Chris Fower, sales director at Fischer Panda (the UK distributor for several big name watermaker brands), says a typical retrofit on a Sunseeker Manhattan 60 would be a semi-automatic Seafari Versatile 700 capable of producing 110lph. This would cost around £12,000 for the unit and £3,000 for the installation.

A typical modular system showing the main membrane tube, pump and a series of pre-filters

What’s the difference between a manual and automatic system?

Although the reverse osmosis principle is relatively straightforward, getting a conventional high-pressure system to produce the correct purity of water takes a certain amount of fine-tuning (ERD systems work at a fixed pressure so do not need fine-tuning). The pressure required and the time it takes to start producing drinkable water depends on a number of factors such as the temperature, salinity and quantity of other impurities that make up the Total Dissolved Solids (TDS) of the raw water intake, as well as the condition of the semi-permeable membrane.

On a manual system this is all done by hand, meaning you have to sample the product water using a handheld TDS meter, vary the pressure accordingly and only divert it into the fresh water tank once it has reached a sufficient level of purity. Seawater typically has 30,000ppm of TDS and only becomes safe to drink below 750ppm, although a well-tuned watermaker should deliver 200-300ppm. An automatic system will do some or all of this for you and back-flush the system with fresh water.

Katadyn make simple electric- and hand-powered watermakers for use on liferafts and offshore cruisers

Why is fresh water flushing important?

Seawater contains lots of natural microbes and bacteria, which as anyone with a sea toilet knows, soon start to smell if left to stagnate for more than a few days. This isn’t a problem if you use the watermaker on a daily basis but if left unused for a week or more the bacterial growth can start to block or even permanently damage the RO membrane.

In the old days owners were advised to flush them though with a special ‘pickling’ solution if they were going to be left unused for more than a couple of days. In practice, few owners did, leading to damaged membranes, costly repairs and a reputation for watermakers being more hassle than they were worth.

These days most watermakers have a built-in flushing system that diverts fresh water back through the membrane to flush out the seawater. This removes most of the microbes and the corrosive effect of salt water. A fully automatic system will do this every time you switch the watermaker off and once a week thereafter until it’s used again. Never use chlorinated tap water to flush the system as chlorine can damage the membrane.

This glass model of a Clark pump, used in ERD watermakers, shows how high-pressure water already in the system helps drive one piston so less energy is required to power the other

Is pickling still needed?

Yes, because even on fully automatic systems the fresh water tank will eventually run dry if left unused for long enough. As a result most owners still need to pickle the membrane during the off season. Each of the manufacturers recommend their own approved pickling and cleaning solutions that use acid or alkaline to neutralise and dissolve deposits, and propylene glycol to preserve the membrane. Be sure to use an approved product as anything else may damage the membrane and/or the pump and void the warranty. Replacement membranes can cost up to £1,000.

What other watermaker maintenance is required?

The cartridges in the pre-filters are likely to need replacing every couple of months while the pumps should have an annual inspection and an oil change. Some systems may also need the o-rings replacing from time to time. A well-maintained membrane should last for 5-7 years and the watermaker itself should be good for around 20 years.

A high spec automatic watermaker, like this Idromar, can make hundreds of litres of water every hour

How much space do they take up?

That depends whether you go for a modular system or an all-in-one watermaker. Modular systems are designed so that the individual components such as the lift pump, pre-filters, high pressure pump and membrane tubes can be installed wherever there is room for them, although it’s best to avoid overly long pipe runs. All-in-one systems are tightly packaged together in a single unit that can be installed in an engineroom or lazarette. The seawater inlet needs to be below the waterline well away from any turbulence that could cause air bubbles, while the discharge pipe needs to be above the waterline.

Which are the best watermakers?

That depends who you talk to and what kind of system you are looking for. Your best bet is to speak to an approved installer and/or distributor who can arrange a proper consultation to assess your needs and work out what is best suited to your boat. As ever you get what you pay for and while cheaper brands and models may seem appealing, you may regret it if they end up causing problems further down the line.

The better known brands are also likely to have stronger aftersales support and more extensive dealer networks and spare parts availability. Well known names include Horizon Reverse Osmosis (HRO), Sea Recovery, Parker, Dessalator, Idromar, FCI, Osmeosea, Spectra, Schenker and Village Marine among others.

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