Rainman watermaker review – Rainman naked 12V system (economy)
What is a watermaker, and why do you need one?
A watermaker is a piece of equipment you can install in your boat, or any off-grid situation, to make crystal-clear drinking water from undrinkable, salty sea water and electricity.
A lot of people want a sailboat so they can travel independently and visit remote locations. A watermaker essentially removes your reliance on shore support for fresh water; drastically increasing your range and independence while making life on board a lot more comfortable to boot.
Watermakers can seem complicated at first, but they’re actually remarkably easy to install and operate – although some more than others!
In this article we’re going to explain why we decided to buy watermaker in the first place; how we went about choosing a make and model; how we installed it in our cruising sailboat, and why we ultimately decided on a Rainman watermaker as the best watermaker for full-time liveaboards on a sailboat.
How does a Rainman watermaker work?
There are a lot of different watermakers to choose between, but they mostly work in a very similar way – they’re basically pressure-washers configured to force salt water through a very fine filter at high pressure.
The filter, or “membrane”, is specially designed to remove all of the things you don’t want in your drinking water – like bacteria, viruses and salt – and only allow clean water to pass. You can drink the resulting water (it’s usually cleaner than tap water on land!), and naturally you can also use it for all your other domestic needs like cooking and cleaning.
The watermaker needs two things to work: a raw water source, like the sea, and a power source, which on a sailboat is typically your batteries and solar panels. You can get watermakers that run directly off the 12VDC / 24VDC your batteries provide, or from your inverter at 120VAC or 240VAC depending on where you are in the world.
You can also run a watermaker from a generator if you have one, which is a popular approach for boats that consume a lot of water (e.g. charters). A select few manufacturers, Rainman being one of them, even offer units that run directly off petrol – a sort of generator-and-watermaker in one. But which is the best watermaker for a cruising sailboat, and how can you choose between them?
How to choose the best sailboat watermaker
Like anything in sailing, the absolute best sailboat watermaker for your boat depends on your needs and intended cruising pattern – but we’re going to talk you through the criteria we decided were the most important, and how that process ultimately led us to buy a Rainman watermaker.
There are a few thought-exercises we ran through before committing, such as “how much water do we currently use”, but also “do we want to use more than that after we install the watermaker”?
We thought about what we really wanted to achieve by installing our Rainman (read more on this in the Rainman watermaker review section!) – were we looking to cover our existing needs and remove our reliance on shore support, or should this represent a significant quality-of-life upgrade at the same time? And having calculated all that, did we have enough power to back it up? Here’s the thought process we went through.
Step 1: calculate your water needs
Before our Rainman watermaker review – how much water do you actually need?
If you’re reading this article, it’s likely that you don’t currently have a watermaker… and if you don’t, it’s equally likely that you have learnt how to eke out the water in your tanks for weeks or even months at a time!
Prior to installing our Rainman, Emily had perfected the art of taking a full shower in less than one litre of water (0.2 gallons). She would draw it into an empty bottle and often come out with water left over! I however am not known for my restraint and delicacy, and as such was relegated to washing in the sea. As a result of her aqueous austerity, Emily also became the underwater ceramics technician onboard Hot Chocolate (i.e. she does all our washing up, because I waste too much water). All of the above came from our tanks, while water for drinking and cooking came from shop-bought bottles.
And for us, that was the #1 thing – we wanted to remove our reliance on bottled water, because it’s horrible for the environment. It’s also super inconvenient – water was the main thing limiting our range and tying us to shore support. Water weighs 1KG per litre (8.3lb / gallon for our friends across the pond). We drink about 3L each, per day, in the heat of the summer; so we’re looking at hauling 42KG of water per week (6.6 stone) from the supermarket just to cover our basic hydration.
We often cooked in bottled water, too, because a lot of countries have heavy metals like lead and mercury in the tap water and you don’t really want your pasta with a side of poison. We calculated that we were actually getting through about 8L per day combined in food and drink alone, which meant hauling 56KG of bottled water a week under the blistering Greek sun (nearly 9 stone)… and no doubt further inflating our water requirements as a result!!
Then there’s the cost of the bottled water, which is small but does add up, and the added inconvenience of having to make regular bin runs to deal with all that plastic (~38 bottles a week). Bear in mind we’re just two people – if you have a family or regular guests, you’ll need to increase those accordingly.
A close second was the ability to shower whenever we wanted (run away to sea, they said… it’ll be romantic, they said…) – so that’s an extra litre every day or two for Emily, and an extra 10 or 20 for me! Truth be told I was quite happy washing in the sea, but it’s very nice to be able to shower in fresh water for special occasions, or be able to rinse equipment in fresh water after it’s been in the sea (e.g. knives, spearguns…).
A stretch-goal was to have enough water on tap (pun intended!) for me to become assistant-second-in-command underwater ceramics technician, and take my fair share of the washing up. We also quite liked the idea of being able to soak lines, clean the salt out of deck hardware, etc. because that’s something we typically only get to do at the start and end of the season, but that was very much viewed as a bonus. Washing the salt off your gear extends its life dramatically which can be a good way to offset the cost of a watermaker if you’re on a budget.
So in summary, we didn’t just want to cover our existing needs – we wanted a little more to play with as well – but we didn’t go as far as treating it like water on land (showers every day, washing clothes, etc). If you have a washing machine, heads that flush with fresh water, teenagers that seem to be waging war on their microbiomes with endless soapy showers… you’ll need to account for those as well.
We estimated our water needs to be:
Drinking: 3L / person / day;
Cooking: 0.5L / person / day;
Personal hygiene: 5L / person / day (e.g. a 10L shower every other day);
General needs, e.g. cleaning: 1.5L / person / day;
Total: approx. 10L / 2.2 gallons per person, per day – meaning 20L total per day.
Step 2: Calculate your power needs (and how you’re going to meet them)
Some brands of watermakers are more efficient than others, but from our own experience and from talking to others, it seems like it takes around 10 Watt-hours to make a litre of water – i.e. about 0.8 Amp-hours from a 12 volt battery. Your mileage will vary based on things like the chemistry of your battery bank, the length of your cable runs, and even the temperature of the sea, but that seems to be a pretty usable yardstick.
Our Rainman watermaker draws about 28A when it’s running, and makes about 34 litres per hour.
Because we’re full-time liveaboards, and working remotely from the boat as well, we already have quite a lot of solar – for a monohull, at least – 360 Watts of mono panels. In theory they should push a good 30 Amps into the batteries in full sun, but in practice it peaks at more like 20A and we make a total of 2.2kWh (2200Wh) in an average day.
We need to make 20L of water per day to keep up; that means we need to allocate 200 Watt-hours from the 2200 we make in a day; i.e., installing our Rainman increased our overall power needs by about 10%.
For comparison, Emily’s laptop draws about 90W an hour and mine draws 120W, so running our Rainman watermaker for an hour a day is about the same as running both our laptops for the same period. A 12V fridge often draws about 3 amps, so about 36W – meaning keeping the fridge running consumes ~850Wh per day, about 4 times more than our watermaker.
Honestly, I imagined it would take a lot more!
Of course, our solar output drops drastically as the Autumn / Fall rolls in, but then so do our water needs… so it’ll be interesting to see how that pans out and whether we’ll need to augment it with fossils fuels, e.g. making water after motoring.
Step 3: Your cruising pattern
Everyone’s needs are different, and it’s important to factor them in when choosing a watermaker.
One of the main reasons we were drawn to Rainman watermakers is that they use all off-the-shelf, standardised parts. If your Rainman breaks down, it almost doesn’t matter where in the world you are – you’ll likely be able to get standardised spares in the nearest major city. We won’t name any names, but in our quest to identify the best watermaker for our sailboat we discovered that a lot of manufacturers use proprietary parts – ones you can only get from the manufacturer.
We considered this to be extremely undesirable, because
(i) as a cruising sailboat, we want to be able to service our watermaker anywhere in the world
(ii) it can lead to price-gouging, i.e. they can charge whatever they want for spares because you can’t get them anywhere else; and
(iii) – it demonstrates an astonishing lack of awareness on the part of the manufacturer.
Your mileage may vary, but we bought a watermaker for the independence. In fact, we more or less bought the whole boat for independence! Trading a reliance on bottled water for a reliance on obscure, single-source, non-standard parts completely defeats the object.
I have worked in technology all of my life, and for every instance where proprietary parts were truly necessary I could show you 20 more where it was simply an attempt to create a “walled garden” and force you to buy overpriced spares from the manufacturer themselves. In a further handful of cases, it’s to force planned obsolescence and make you throw away an old unit rather than repairing it – which again, is the opposite of how we try to live.
In a non-watermaker example, this is exactly the reason we were forced to replace our windlass in our first season. A simple, $5 helical gear stripped, but because it was proprietary and the manufacturer had long since gone bust, we had no option but to replace it in its entirety – at a cost of about $1500, and all the needless waste accompanying it. We found a machine shop who could copy the stripped gear, but it would have cost nearly $1000 anyway after set-up costs. Not everyone will agree with us but we found that a horrible waste and just an awful design philosophy, and we cursed that manufacturer in a way that would make Blackbeard blush as we hauled up 80M of muddy chain by hand…!
So try to think about your intended cruising patterns and how awkward it would be if you needed spares. If you typically cruise the same grounds, it’s unlikely to affect you that much. If you’re planning an expedition or a circumnavigation however, you’re likely to want to look for manufacturers with a practical design philosophy like Rainman.
Ask around in forums and social media groups and try to gauge what it’s like to deal with the manufacturer. How self-serviceable are the units? How reliable are they? Again, this is something that led us straight to Rainman watermakers. The units have a reputation for being absolutely bulletproof; the pre-sale communication was outstanding, you can get spares anywhere in the world and (nerd alert) the documentation is outstanding too.
Although we eventually went for a fully-installed unit, we also loved the fact you could get a portable version that stows away in a locker or can even been taken in a bail-out situation if you have a 12V source to run it from. You can use the same Rainman watermaker in salt water or brackish (mixed salt and fresh) just by adjusting the pressure level, where some manufacturers require you to purchase a specific unit for each. While we didn’t actually need a lot of these features we appreciated that Rainman was grounded in real-world use cases and had given us that flexibility.
You should also consider if you really need a watermaker. Yes, they are wonderful. Yes, they are basically magic. Yes, it would be nice to be able to stand downwind of your significant other without a peg on your nose. But they also cost about $5,000, and you do have to keep using them every 2-3 days or fill them with pickling solution, else they will foul up and ruin the membranes (Rainman has an “autoflush” unit that makes this a lot easier, see below). If you don’t live aboard full time, you might actually find it an inconvenience!
Rainman Watermaker Review: Rainman 12V Naked
Having considered all of this, let’s talk about the Rainman Naked 12V unit we eventually bought, and why – although the reasons should already be becoming apparent! Here is our honest Rainman watermaker review – warts and all.
Rainman Watermaker Review – The Parts
A Rainman system comes in a few separate parts, which you can configure to suit your needs.
The pressure supply unit, aka. PSU. This is the pump that draws water from the sea, and then pressurises it ready to be forced through the membrane.
The RO unit. RO stands for reverse osmosis, which is the process a watermaker uses to filter the water. The RO unit therefore is a long tube containing the specialist filters that let it separate clean drinking water from salty sea water. The PSU pressurises sea water and sends it into the RO unit. Out of the other end, you get a small amount of pure, clean, drinking water, and the waste product – brine.
Control panel. Rainman make an optional control panel that lets you start and control the unit remotely from somewhere else in the boat. We installed our Rainmain in a pretty awkward location (in a lazarette under the cockpit), so we were very grateful to be able to mount the control panel somewhere rather more accessible! The control panel has instruments that tell you the pressure the system is operating at, how much water you’re making, and even the quality of the water (via a traffic-light system). You can also use it to control the autoflush function (see below).
Autoflush. Another optional extra, but one we would highly recommend. As detailed earlier in the article, you have to run a watermaker very regularly else marine life will grow on the filters and ruin them. The best antidote is regular, even daily, use, but you can also fill the unit with “pickling” solution that stops the fouling from growing. Generally speaking, that means you run the watermaker 2 – 3 times a week while living aboard, and then pickle it in the off-season when the boat is on the hard standing or home berth. It’s a bit of a pain, but not nearly as annoying as lugging 50KG of water from the nearest supermarket and / or chronic dehydration.
Enter the autoflush. Once a week, or at another interval of your choosing, the autoflush will open a valve and take fresh water from your tanks to flush the membrane and prevent fouling. This extends the life of the membranes significantly and removes human error from the equation. Considering an average sailboat has north of 10,000 moving parts to worry about, we’re big fans of anything that essentially maintains itself! You’ll still need to pickle if you don’t run the unit for 30 days, but if you’re like us and can stretch a tank of water to 14 days or more it’s really convenient.
Rainman Watermaker Review – Your Power Options
When specifying a Rainman system, you have three basic choices to make:
- Power source
- Portable (cased) or permanently installed (naked!)
Power source. When buying a Rainman, you can choose between three different power sources:
- AC electric. An AC electric unit runs off mains power, i.e. 120VAC or 240VAC depending on where you are in the world. That mains power in turn can come from a few sources: if you have an inverter, like many cruising sailboats, you’re already making mains power from the 12VDC / 24VDC supplied by your battery bank. You’ll need an inverter that can supply 1250W, although 1500W is recommended. You can also get mains power from the shore, via your shore power connector – although if you spend a lot of time on shore power you might not need a watermaker at all! If you have a generator on board, there’s a high probability your generator is also outputting mains power as well. AC electric units are much faster than DC, and can produce about 4X as much water per hour – while consuming about 3X the power, so they’re more efficient as well. The best option for boats that meet the power requirements, particularly larger boats, boats with power to spare, boats with high water consumption (e.g. charters).
- DC electric. This is the option we chose, for reasons we’ll explain. DC electric means the unit runs directly off your 12VDC batteries. At the time of writing, I don’t believe Rainman make a 24VDC unit, but 24VDC boats can just use the AC version above via an inverter.
The DC version makes a lot less water per hour than the AC version, because of nerdy reasons related to DC and current. A DC system tops out at 34 litres per hour, where an AC can make up to 140. They’re also somewhat less efficient, although if you’re using an inverter to make AC you should bear in mind your inverter is typically wasting 10 – 20% in the process so the numbers aren’t quite as clear-cut as they seem.
It’s marginally quieter than the AC system, but you need to be able to position it fairly close to your battery bank because again, nerdy reasons to do with DC.
We chose the DC unit because:
- Our inverter is only 1200W, and we didn’t want to have to upgrade it to meet the requirement.
- Our generator is also only 1000W, and we prefer not to use it unless we have to.
- Our water needs are only ~20L a day, so we didn’t really care about making water slowly. One hour a day would easily cover out needs.
- We could easily install the Rainman 3ft from the battery bank, so the cable run wasn’t an issue.
- Petrol. Rainman also make a unit that runs directly off dead dinosaurs. Essentially, it’s a suitcase generator like the popular Honda EU-series with a pressure supply unit bolted on (the genset inside is actually a Honda GXH50 4 stroke). It’s a great idea, particularly if you’re using a Rainman in a non-boat scenario (bugout cabin, disaster relief, etc). You can burn fossil fuels directly to make water without the need for a separate generator. I can definitely see the applications, and although it wasn’t relevant for us, it’s another example of how Rainman’s design philosophy seems firmly grounded in the real world. If you liked the sound of the control panel or the autoflush unit, bear in mind you can’t use them with a petrol unit. Rainman says the unit burns about 600ml of petrol per hour, which at today’s gas prices is about EUR 0.70 per hour, or one Euro of fuel for 200L of water.
Rainman Watermaker Review – Size and Quantity Options
Once you’ve decided on a power source, you can then choose the size and quantity of RO membranes to go with it:
- The AC and petrol units can both take two full-size membranes for maximum output – and you’re encouraged to go down that route, because the increase in water output far outstrips the increase in power consumption. Alternatively, you can fit one full-size membrane if you’re really on a budget, or there’s a cute compact unit with two half-size membranes if you’re seriously short on space and/or weight! For 90+% of boats the first option is the best and there isn’t much reason to consider the others. With two full-size membranes you can expect to make 100 – 140 litres of water per hour. With a single membrane, or the two half-size units, you’ll make 50 – 70.
- The DC unit only supports a single full-size membrane – there are no other options. You’ll also make less water than an AC or petrol unit using the same membrane – about 30 litres per hour. If you’re a family, or you have high water needs for some other reason, you’ll likely want to try to get up to an AC unit with two membranes as you’ll make up to 5X as much water per hour. For us, that wasn’t really relevant – it was the difference between making water for 20-30 minutes a day, or an hour. It didn’t seem worth upgrading our inverter or buying a larger genset.
Rainman Watermaker Review – Portable or Permanently Installed
And finally, decide whether you prefer a portable (cased) or permanently-installed (naked!) system:
- If you want petrol, it only comes in the portable, cased form factor. Note that you can’t use the control panel or the autoflush with portable form factors.
- AC or DC electric units can either be portable or permanently installed. The same caveats apply regarding the autoflush and control panel. The cased form-factors come in a rugged, blue plastic case that seems like it will stand up to a lot of abuse.
Portable systems are good if you simply don’t have space to permanently install the unit, and they offer the flexibility of being able to stow your Rainman in a locker when not in use. The PSU needs to stay quite cool while in use, and operating the unit in the open air is a pretty solid guarantee that it will. They’re also great if you don’t have a spare through-hull and you don’t fancy cutting a hole in your boat – you just dangle the hose over the side, making sure it’s good and deep but not amongst any fouling.
However, you add a lot of man-hours to your watermaking operation (setting up, packing down, manually flushing…), which should definitely be factored in. You can’t autoflush, and you’re more likely to make a mistake – sucking up air or fouling can damage the unit, and the more times you manually dangle a hose over the side the greater your chances of doing so.
On Hot Chocolate, we already had two perfect, spare through-hulls from where the engine-driven fridge compressor used to be. One of them is deep down in the keel, allowing for a clean bite of water with near-zero chance of fouling or air bubbles. We can make water while underway without needing to watch the hose. The latter was perfectly positioned above the waterline so we can observe the brine output and check it for bubbles (part of the procedure for making water). This made choosing a naked system a no-brainer for us, but I think even if we didn’t have the spare holes in the boat we would probably have made the same decision.
And that’s it – that’s your three basic choices. If you went for a naked system you have a further two optional extras in the form of the control panel and autoflush, both of which represent pretty significant quality-of-life upgrades if you can afford them – but it’s still a fantastic piece of kit if you can’t!
Rainman watermaker review of the installation documentation
I know we’ve already mentioned this, but the documentation is really excellent and it’s clear there has been a lot of thought put into the product. It’s evident Rainman expect people to self-install, whereas when we spoke to one of their competitors (who we won’t name!) they stated “if you need to ask us any questions you shouldn’t be installing it”! Again, this shows a level of pragmatism, forward thinking and just common sense on Rainman’s part that we sometimes found lacking in competitors.
If you’re a manufacturer reading this, please understand that cruising yachts absolutely are going to install and maintain their own electronics, and we’ll desert you in droves if you try to lock us out!
Reviewing the Rainman watermaker installation process
We were pleasantly surprised to find that the unit came with almost everything we needed to install it. We were expecting to supply our own hoses, sundries, etc. but Rainman supplies everything you need – hoses, jubilee clips, hose splitters, T-valves… while we were installing it our local chandlery told us a horror story about a boat who had been waiting eight weeks for a replacement hose for their non-Rainman watermaker, so we greatly appreciated the completeness of the kit! I believe the only items we had to add was a splitter to tee the fresh water output into our tank, because we have the flexible plastic variety and the supplied fittings assumed a conventional solid tank. We happened to have one on board, so our additional material overheads were exactly $0.
Installation was really very straightforward. End-to-end it took me two days to install and test the unit, plus an extra day to get the autoflush set up.
We had a tiny teething problem in that one of the hoses was supplied with an adaptor, and we needed to remove it to screw it in correctly. We raised a ticket with Rainman, they replied in about 2 hours, and had already followed up to see if we were sorted by the time we read their first email!
Only basic hand tools were required to complete the installation, and in fact most of the connections are toolless (e.g. push-fit connectors). We’re now about three weeks into our ownership, and so far, I can honestly say we don’t have a bad word to say about either the company or the unit itself. Our intention is to update this in 6 – 12 months once we’ve had our Rainman for a season and let you know how it shapes up!
Our Rainman arrived from Australia to Greece in a little under 3 weeks, although Greek customs took a further 3 weeks to clear! We ended up having to pay a total of EUR 1100 in VAT and import duties, which was about what we expected. We’ve heard of people not getting charged anything, so it seems to be a bit of a lottery. The biggest hurdle we faced importing it was getting an EORI number (a Greek tax code for imports worth more than EUR 1000). We ended up using our boatyard’s EORI number, meaning they paid customs and we paid them back.
If you have any questions about Rainman or watermakers in general, feel free to use the comments section below and we’ll do our best to help. We’ve also tried to compile some answers to the questions we had when we started this journey and some of the most common ones you see online (find this section below). If this review was useful to you, please consider sharing it or joining our newsletter for more quality content and awful puns!
Check out Rainman products here.
Can I run a watermaker off an alternator?
Yes, you can use the power from your alternator while motoring to run your watermaker. Just be aware that if you’re underway, you might be creating bubbles… and if you suck up a bubble it can damage your membranes. Make sure the intake hose is good and deep and your wake is free of bubbles, or just bank the power and make water when you get to the other end.
Can you drink water from a watermaker? Is it good / bad for you?
Absolutely. When your watermaker is functioning correctly it should make water with around 300PPM (parts per million) of contaminants. Safe drinking water is usually 500 – 800, so in most cases, it’s actually cleaner than the water you usually drink!
How to take care of a sailboat watermaker
The number one enemy of watermakers is fouling. When you suck up sea water, it contains billions of microorganisms like plants and algae. Your watermaker filters them out, but some of them get caught in the membrane and can start to grow. The best way to take care of a watermaker is to run it very regularly to flush the microorganisms out before they get a chance to take hold. Run your watermaker every 2 – 3 days minimum, pickle it when not in use, and strongly consider getting a system with an autoflush function like a Rainman – it will make your life a lot easier.
How much water does a Rainman watermaker produce?
It varies, but between 30 litres an hour for a smaller unit or up to 140 litres an hour for a larger unit.
How much electricity does a sailboat watermaker need?
It depends on the size of the watermaker, but basic units consume around 400W, and bigger units around 1.2kW.
Can I run a watermaker off solar power? Do I need a generator if I have a watermaker?
Absolutely, you can run a watermaker off solar power. We have a 400W unit that we run from our 360W panels, via our 12V battery bank. We rarely need to generate in order to make water.
12V vs mains-powered watermakers
Mains-powered watermakers are generally more efficient, producing more water both per Watt and per hour. However, you need a fairly large inverter or generator to use a mains-powered unit. We opted for a 12V system because we didn’t need the speed of a mains-powered system, and we didn’t want to have to upgrade our inverter to accommodate one. You can find more information about why we chose a DC system in the body of this review.
Can you use a watermaker to make water from a river?
Some watermakers can make water from brackish water as well as salt. Some manufacturers require you to specify at the point of purchase, whereas others (like the Rainman) can handle it with a simple pressure adjustment. Just be sure to read and follow the manual carefully or you can damage your membrane.
Solar still vs boat watermaker?
A solar still is a device that makes clean water by evaporating sea water. It’s a great piece of kit to have in a grab-bag, particularly if you’re attempting a longer passage. You can get inflatable solar stills that fold down to the size of a pack of cards. Solar stills are excellent emergency gear but not a replacement for a watermaker, because they typically only make 1-2 litres per 24 hours – vs 2,000+ litres per 24 hours for a watermaker. Their principal advantages are portability and the fact they don’t need any electricity to operate. Both have a home on an ocean-going sailboat!
A number of plans circulate online for DIY watermakers. Watermakers are essentially pressure washers that force sea water through a membrane or filter to produce clean drinking water. It’s reportedly possible to build your own using off-the-shelf components (like pressure washers!), and save perhaps $1000 vs buying a pre-configured watermaker. While we’ve heard a lot about the concept over the years, we’re yet to meet a sailor who has built one themselves. Let us know in the comments if you’ve built one and how it’s working out for you!
Watermakers VS Brita filters, filter jugs, lifestraw, etc.
What’s the difference between a water filter jug and a watermaker, and why does one cost $10 while the other costs ~$5000?
Filter jugs use filtration and something called “ion exchange”, meaning the water trickles through a pad made of something like cotton to remove large particles, and activated charcoal or ion exchange resin which reacts with and “grabs” semi-volatile organic compounds out of the water. Sometimes they also use silver, which is antimicrobial, or UV light to kill out bugs. Filter jugs typically can’t remove heavy metals like lead and mercury, and they certainly can’t desalinate water (remove the salt and make it safe to drink). For that level of filtration you need to pressurise the water and use a much more sophisticated filter called a “reverse osmosis” membrane, hence the cost and complexity of a watermaker. The closest cheap equivalent is a solar still (see above).