Last season when Sophie and I did a few longer anchoring stops, having to refill the water tanks was our single most limiting factor. So this year, we started discussing the merits of buying a water maker ahead of our season in the Mediterranean.

We looked at many different watermakers, and while they all looked exciting, all of them were out of our price range. But one of you guys a tipped us off on building our own watermaker, and that made us think: could we really build our own desalination device and spare us a juicy expense?

How to make water on a sailboat?

So this is what this article (and video above!) is about. We talk about how a watermaker works, what are the different components of a watermaker and what their role is. We also look at different types of watermakers, compare branded watermakers to DIY watermakers and talk about the pros and cons of each.

On Polar Seal, we are going the DIY route with the help of a kit we acquired from SeaWater Pro. In a coming video, we will talk about how to install such a watermaker. But for now, let’s dive into the fascinating world of making your own water!

What does a watermaker do?

A watermaker is essentially a fancy name for a filter system. We are trying to filter out a bunch of particulate forms the ocean, including salt, which we really do not want to drink. The watermaker does with using two systems.

1: pre-filters: The first is made of general filters, the type you would find under a skink or in a Brita water filter.

2: reverse osmosis membrane: The second part is a reverse osmosis filter.

Reverse osmosis what!?

To understand reverse osmosis, we need to talk about the process of osmosis.

A solution that is less concentrated will have a natural tendency to migrate to a solution with a higher concentration. For example, if you had a container full of water with a low salt concentration on one side, and water with a high salt concentration on the other side, and they were separated by a semi-permeable membrane, then the water with the lower salt concentration would begin to migrate towards the water container with the higher salt concentration. That’s osmosis.

Reverse Osmosis is the process of Osmosis in reverse. A reverse osmosis membrane is a semi-permeable membrane that allows the passage of water molecules but not the majority of dissolved salts, organics, bacteria and pyrogens. While osmosis naturally occurs without the energy required. To reverse the process of osmosis, you need to apply energy to the more saline solution.

In this case, the water will be forced through a reverse osmosis membrane at high pressure. While doing that, the salt is extracted from the water… cool eh?!

What are the different parts of a watermaker and how do they work?

Now that we understand what a watermaker is, let’s have a look at how to make water with a watermaker.

1. Water comes from the sea through the through hull.

Through hull is as it names indicate, a hole in your hull. Onboard Polar Seal, we have a few through hulls for things such as our sink water, toilet, engine…

We need both a way for water to get from the sea to our boat inside the water maker, and one to get the discharge out. Some manufacturers require dedicated through hulls, others don’t care if you share from an existing one.

2. The sea water is strained through the strainer:

This is part looks a lot like the strainer on your engine. Its purpose is to filter out all the big particulate from the sea prior to the water going through the low-pressure pump.

3. The system is kept pressurized with a low-pressure pump.

Since out watermaker will be installed above the waterline, the system needs a pump to keep it pressurized so the water flows through it. The low pressure keeps the system from getting air bubbles in the system, which we really do not want and we will talk about it later.

And now that we have water coming in and pressure, it’s time to start filtering.

4. The water gets filtered through a series

Next, the water flows through a series of filters. 20 microns and 5 microns represent the size of particles that these filters filter. These filters can be bought anywhere! We are not removing the salt yet, but more impurities such as… fish poop?

After the filter, the water runs through a gauge which helps us monitor if there is any type of blockage in the system. This can be a good indication of the filter needing to be changed or maybe a problem with the low-pressure pump.

5. The water goes through a high-pressure pump

So the water is still salty but we’ve made it a little cleaner. Still not good to drink, but getting closer! Now we are getting in the process of desalinating the water. The water will go through a membrane to work, we need to have high pressure flowing through it, and this is what the high-pressure pump does.

6. Water flows through a high-pressure pump and into the reverse osmosis membrane.

The water flows into the high-pressure pump to the high-pressure vessel and the reverse osmosis membrane. The pressure in the membrane is gradually taken up to 400 to 800psi, , depending on how cold and/or salty the water is (and the limits set by the manufacturer).

7. The crossroads

This is where the salt and particulates say goodbye to the water. Well, the whole thing happens via a control valve, which helps you set the pressure in the membrane too. The control valve comes with two ends for pipes: one for the fresh water, one for the salty discharge.

The salty discharge (also called brine) exits the boat via a through hull, while the fresh water stays in.

9. The test

The water needs to be tested before being put in the tanks. Imagine if we’d start filling our tanks of precious fresh water with bad one? So before we’ve made sure it’s potable, we let it drain in the sink and test it with a TDS monitor (yes, a TDS monitor, and not a TSD monitor as Ryan like to call them ;))

The TDS monitor tests any dissolved solids in the water such as minerals. Minerals are not always bad, but aren’t always desirable in high concentration. The world health organization has standards, and recommend nothing above 1000mg/l. Anything below 600mg/l is considered good and anything below 300mg/l is considered excellent.

10. The water goes in the tanks

Once we’re happy with the water, we switch the valve, and it fills the tanks!

11.Thought we were done? Nope. Gotta flush that system!

The reverse osmosis membrane a fragile piece of equipment that needs constant care. Care that starts right after we are done making water. After every use of the watermaker and at least once a week, the watermaker needs to be flushed with fresh water.

But we can’t flush it with whatever water. We need to do it with water from our freshwater tanks, but because our tanks can be filled with water from the marina, it can contain some chlorine, a chemical that the membrane is particularly sensitive to.

12. The water gets filtered in a charcoal filter, and back in the system!

This filter removes primarily chlorine before the water runs through a one-way valve back into the system to flush it!

So, you want to buy a watermaker?

For that, you have two options:

Option number 1: buy an off-the-shelf watermaker

Watermakers typically cost between 5000 and 1000$ upwards. A few makers of watermakers are Spectra, Rainman, Eco-tech, Schenker. And although they all come with different functionalities, they all make water the way that we described.

Those systems are generally optimized to use less power. They can come fully assembled an all you need to do is to mount them anywhere you want and connect them to your water system. They also may come with systems to automatically start and stop the system or advanced sensors, as well as recovery, pumps to use less power while making water.

Off-the-shelf watermakers may come with their own filter systems that can be different sizes to fit in smaller spaces.

Advantages of an off-the-shelf watermaker:

They typically use less power,
They have optimized efficiency (less power, more water)
They could be easier to install
They can come with customer support.

Cons of an off-the-shelf watermaker:

Expensive systems
Can be hard to get replacement parts
Can be more expensive to replace parts

So if you don’t want to spend between 5000 and 10000 USD, there is another way to get a watermaker onboard, and it is to build your own. That is the route we are taking on Polar Seal!

Option 2: build you own watermaker

All the parts named above can be bought wherever. It is easy to source those parts at a hardware store or on internet.

Obviously, this makes a watermaker substantially cheaper. Depending on the motor or pump, a DIY system can cost around a couple of 1000 bucks. Most of the cost lies in the membrane, the vessel and the high-pressure pump.

Advantages of a DIY watermaker

A lot cheaper than an off-the shelf watermaker
Easy to source replacement parts
Cheaper to replace parts

Cons of a DIY watermaker

Takes more time to install
Basic system, not optimized for efficiency
Can be time-consuming to source the parts… BUT

On Polar Seal, we ordered a kit of parts to build our own watermaker from SeaWater Pro.

SeaWater Pro

SeaWater Pro is the IKEA of watermakers: a lot cheaper, great value for the function. and you build it yourself (though you don’t have to deal with the unpronounceable Swedish names)

SeaWater pro gives you options for different types of motors and pumps. For example, there are DC and AC versions available, as well as different type of pumps that will impact the cost and efficiency of your DIY watermaker.

SeaWater Pro evens offer kits already mounted and ready to install! But on Polar Seal, we really wanted to build our own watermaker and got a 12 volts kit with a brass pump and a single membrane.

At some point, we may get a second membrane (that’s a big advantage of a modular system, we can change it at any time!), but for now, one should make a big difference already.

In a coming video, we will show how to install a DIY watermaker. In the meantime, if you have questions, feel free to drop them in the comment section below!

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Apr 10, 2019Anonymous

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