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This page updated:
October 2009

Reverse-Osmosis Watermaker
Condensation Watermaker / Solar Still

Reverse-Osmosis Watermaker

[Reverse-osmosis] Watermakers work by filtering salt water. The filter is so fine that even minerals (salt) are taken out. High pressure (800 PSI) is needed, so they consume a lot of power. Usually there is a strainer, a couple of pre-filters and a supply pump, before getting to the high-pressure pump and filter.

West Marine's "Choosing a Watermaker"
"Reverse-Osmosis Watermakers" article by Patrick Childress in 10/2001 issue of Sail magazine
Long section on watermakers in "All In The Same Boat" by Tom Neale.
Article about basic principles by Chuck Husick in 10/2006 issue of Cruising World magazine.

From Tropicbird on Cruising World message board:
We carried a watermaker for 10,000 miles in the summer of 1996, and I cannot imagine life without it.

... article about watermakers says: Never run them in harbors, clean the filters every few days, and run the units daily.

We ran the watermaker every day and also any time we were under power. The electric draw was about 15 amp hours, and we made about 10 gallons a day; this was enough for two good showers each day (Tam has hair to her waist), the galley, rinsing out the swimsuits, and keeping the salt wiped out of most of the interior.

You will be cleaner, smell better, be more pleasant to sleep with, and your skin will appreciate it. (In my pre-watermaker life -- a Singlehanded Transpac and a South Pacific trip -- I would get salt water sores, even doing the alcohol wipes; after 33 days with a watermaker in 1996, Galapagos to Hawaii, none.)

The watermaker also frees you from dubious shore water sources. You can always arrive in port with full tanks you can drink from. (Conserve your drinking water in harbor by putting a fitting in downstream of your pressure pump to attach a dock hose to that feeds into the water heater for showers; drink out of the foot pump from the tank.) And you can fill your tanks again after you go to sea. People who sail out of US ports get used to good, safe water out of the dock hose, a situation that is not so reliable elsewhere.

It does not take a very expensive or complicated system. We used a Powersurvivor 35 [manually-operated].

"PUR 180 water maker is both a power hog and unreliable. Village Marine Tech seems to give good service. All watermakers should have multiple filters. ... Watermakers are a pain, but one you will not want to live without."

"Village 'Little Wonder' ... very pleased. Actual output is about 5.5 GPH at 17 A. Get 20 micron primary filter; this keeps the 5-micron pre-filter changes down to a minimum. The 20s can be washed and re-used indefinitely. We also got the 'fresh water flush' kit, which allows for easy lay-up for up to 3 weeks without 'pickling' the membrane."

From Jim and Diane:
All watermakers are high maintenance. ...

We absolutely did not run the unit in oily water. This will ruin the membrane. I remember sailing along with the watermaker running and seeing an oil slick ahead. One of us would run below to turn the unit off. We didn't worry about polluted water. It is my understanding that an R/O unit will filter bacteria but not viruses. Maybe we should have been more careful, but we never had a problem. The big AC units offer UV lights to sanitize the output.

All R/O units like to be run every day. If you are going to go more than two or three days (like you are at a dock) without running the unit you have to pickle it. This involves running a chemical solution through the membrane to keep things from growing in there. When you put the unit back in service you have to flush all this stuff out.

Don't run the watermaker in any dirty (especially, oil-polluted) anchorage.

From GeorgeB on the Cruising World message board:
I'm beginning to lean away from an RO watermaker:
  1. They are expensive to buy and to maintain. They are finicky to keep functioning. They are big energy users.
  2. You still need to keep sufficient water available on board in the event that your watermaker fails half way across whatever ocean you're crossing.
  3. Wherever there are human beings there will be fresh (potable) water. The problem is not so much salinity as chemical/biological contaminants.
  4. Purifying otherwise potable water to acceptable (U.S.) drinking standards is a lot easier, cheaper and more reliable than desalinating sea water.

From Jerald King on the Cruising World message board:
I talked to a lot of cruisers who have spent years in Baja and points south. Their conclusions were always the same:
  1. A watermaker is the single most liberating device you can add to the boat.
  2. Finding good water is a big hassle.
  3. Loading water into the boat from five gallon containers is a real hassle. Most figure on about 1/2 a day every week to find and load the water.
  4. There is always water available if you are willing to work for it. That means: find the source, verify the quality, somehow get it to the boat at anchorage, get it into the water tanks on the boat.
  5. Watermakers do require a lot of attention. They must be flushed regularly, the incoming seawater must be clean, the raw water filters must be good. The installation is critical for ease of operation and maintenance.
  6. Well-maintained watermakers seldom break down.
  7. Watermakers do use a lot of WATTS.
  8. The smaller Spectra watermaker really does produce a gallon of water per amp hour of 12V power. The Clark pumps in the Spectra have proven to be very reliable, at least for the two years of experience cruisers have with them.
  9. In order to minimize maintenance you should backflush the watermaker with about 3 gallons of clean, non-chlorinated water each time you finish using it. That means you should probably make 30 or so gallons at a time so that the backflush doesn't consume too high a percentage of the product.
  10. Watermakers are expensive to install.
  11. MOST watermaker users would not consider living on a boat without their watermaker. Almost all watermaker users are very positive about the benefits of the device - even considering the cost and maintenance issues.

From McRory's Logbook:
[Bahamas, 1996:] The water from our watermaker has been excellent, turning out 50 gallons since it was installed. If not for our system, we'd be forced to buy drinking water, which costs between $2 and $8 a gallon here in the Bahamas. More than diesel fuel! Tap water stinks, is salty, and not safe to drink unless you add bleach.
[I went to Bahamas in early 2001, and again in 2004. There was free city water available at or near the dinghy docks in Nassau, Black Point and Georgetown (none in Abacoes and not drinkable in Eleuthra). Quality seemed fine to me, but I added a little bleach anyway. I was told the Georgetown water can be salty if there is not much rain for a while.]

From Peter Hendrick:
Among things that we specifically avoided based upon 1st year experience:
Watermaker (we believe they tend to be expensive to maintain and drain the batteries; we manage to collect our own or jerry jug from shore if absolutely necessary).

From Colin / SVMandalay on Latitudes and Attitudes Cruisers Forum:
We installed a Village Marine Little Wonder prior to our recent cruise to the Bahamas. It is rated at 6 - 8 gallons per hour and it seemed to perform on spec. It draws 14 amps at 12 volts and our usage averaged one hour per day. I recommend getting the largest capacity unit you can since you run it for less time to get the water you need. We usually ran it with the engine on, but we also ran it from the batteries, especially when the wind generator was putting out well.

We only took on water in Nassau while at a marina where we were paying for water anyway. Otherwise, we made our own. Water in the Bahamas varies in price from free to 60 cents per gallon for R.O. water in George Town. In any case, you have to haul it in jugs in your dinghy in most places, since the water is not dockside. There was very little rain in the Bahamas in the winter and catching rainwater is not realistic.

I am very glad we installed the watermaker. I recommend the unit we bought.

From Mac on Latitudes and Attitudes Cruisers Forum:
A friend of mine cruises the Bahamas every winter with his wife, and relies about 90% on catching rain for potable water. The remaining 10% is purchased ashore in jerry jugs.

Something to consider: If you are planning to spend $2,000 on a water maker, plus spares and the equipment to make it run, how much water could you purchase at the Georgetown rate of 60 cents per gallon for the same amount of money? The last time I purchased water in Marathon, it was 10 cents per gallon delivered. Seems to me that you could purchase a lot of water and avoid the possible ownership hassles.

If the water that you buy has a few bugs in it, a little cheap scotch poured into the water tank should clean it right up.

From Andraecium:
... No matter what system you have they: Require a fair amount of power to run (either battery or engine), demand regular maintenance, are noisy, and (for safety's sake) require your presence while running. After a few afternoons of "hangin-out" with your loud, high maintenance, energy-draining friend you will soon learn the value of the "Jug Shower".

From Brian Sawyer on Cruising World message board:
You can't hardly go to Mexico, Belize or Guatemala without one. There is no water on shore available that I would want to put in my tanks. Water in the Exumas sells for as much as $.50/gallon. Mine is 110v and runs off the generator. It makes 20/gph (rated at 480/gpd). Watermakers are noisy. You don't want them to run all the time. I only have 135 gallons of tankage but I can fill them in about 6 hours. It is nice to have all the water you want. You can shower after taking a swim and keep the boat hosed off.

My experience:
So far (5 years in USA and NE Caribbean): free water is available in many places, such as Key West, Nassau, Black Point Exumas, Georgetown Exumas, Luperon DR, La Parguera PR. And most fuel docks, when you buy some fuel. And you can catch rainwater and use it for showering and dishwashing at least. If you have enough tankage and jugs to carry you through the places where water is not free (such as Marathon, the Abacoes, the Turks and Caicos, all of the Virgin Islands), you'll never have to pay for water. I did pay 18 cents/gallon in St Thomas. I probably use 1000 to 1500 gallons per year.


After almost 10 years aboard, now in the Leeward and Windward islands, there is enough rainfall on the leeward sides of the bigger islands that I can catch all of the rainwater I need. Helps that I have a big pilothouse roof to catch the water. Haven't bought water since St Thomas several years ago.

From Rick on Cruising World message board:
Is a watermaker worth it ? It is all relative.

If you cruise where water is at a premium, yes ... or you haul it, and sometimes get it from wells where you gotta move the tadpoles out of the way. If water is readily available and free it is not needed. I cruised the Bahamas both way and would never I repeat never do it again without a watermaker. GPH is relative a crappy pur 40 will give you the easiest current draw. But you will *at very best* get 1 1/5 gallon per hour with low current draw, but the damn thing gotta run for ten hours for 15 gallons and boy is it noisy. Then there is higher output where you get 15 gal per hour while engine or genset running. It's all relative.

From Rick Kennerly on The Live-Aboard List:
Watermakers do work extremely well when away from land, on passages and such. The trouble is that only about 5-10% of your time as a cruiser is spent on open water, but your water consumption is steady. You can see the problem.

We've never had a problem finding good water, Caribbean or Pacific -- except in South Carolina once. And from the experience of others, watermakers tend not to be as useful as one might think. Cruisers tend to spend a lot of time in one place, usually in city harbors or around villages where there is considerable runoff from land and usually sanitation outflow in the harbor -- silt and waterborne bacteria, a watermaker's nightmare. Even in pristine anchorages, you'll be sharing the water with a lot of people who have thru-the-hull heads. Sure, you can add UV and all that stuff to your watermaking set up, but it does increase the initial buy-in as well as operating overhead.

I have nothing against installing a watermaker. If you're planning to spend a few years in really remote areas, they make a lot of sense. And we actually carry one of the little PUR 06s for emergencies. ...

From Robert Reib on Great-loop mailing list:
The first question you should ask yourself before you spend too much time evaluating watermakers is "Do I need a Watermaker?". For most coastal cruisers, a watermaker is a real waste of money. There are few places along the East Coast, Canada and the Bahamas where fresh water is not readily available. The one area I know of where you are expected to pay for water is the Bahamas. Even there the $0.05 to $0.10/gallon is such a low figure you can't justify the cost of a watermaker. If you spent 4 months a year in the Bahamas and used 300 gallons a month, it would only cost you $30 a month or a total of $120 for water. Many of the marinas charge less than that per month if you are a monthly customer.

On top of that, you will find you can't use your watermaker in the harbors in the Bahamas, because of the condition of the harbor waters. So the one place where you think you might use your watermaker (Bahamas) they cannot be used most of the time.

For those boaters that plan to cruise beyond the East Coast and Bahamas, a watermaker is a good idea. For the rest of us it is a waste of money. After cruising 8 years and over 40,000 miles we never found a need for a watermaker at any time. During that period we have not spent a total of more than $100 for water. Pretty hard to justify a watermaker at about $5,000 on that basis.

You should invest ($40) in a good charcoal-based water filter on board your boat. Water from a boat's water tank just doesn't taste very good unless it is properly filtered.

Summarized from "The Voyager's Handbook" by Beth Leonard:
  • A watermaker is not a substitute for adequate water tankage.

  • Watermaker efficiency is reduced by colder water, saltier water, older membrane.

  • You want to run a watermaker about an hour a day, to keep it clean but not wear it out.

  • Water intake should be as far below waterline as possible (to avoid surface pollution), and have a prefilter.

From article in 11/2003 issue of Latitudes and Attitudes magazine:
RO water makes drinks and food taste better, and washing your rigging with it will make the rigging last longer than if you washed with city water.

From Dave Skolnick on World-Cruising mailing list 6/2008:
I'm still collecting prices and have yet to decide if I will really buy and install a watermaker. Based on the prices I have so far, it is difficult to justify a watermaker on economics: making lots of assumptions about water use and lifestyle, water from the watermaker comes to 30 to 50 cents per gallon (numbers change significantly with usage (conservation makes the watermaker MORE expensive) and time frame (I used 3 years)). I did not account for maintenance yet, which would also make a watermaker more expensive. Justification of a watermaker has to be on the basis of lifestyle (no or less hauling of water jugs) and health (more control over water quality in your tanks).

I do keep looking at watermakers, but I suspect in the end I'll find other things I'd rather spend the money on.

From Dick and Ginger Stevenson in 2008 Ocean Voyager from Ocean Navigator magazine:
We have a Katadyn 160E and for five years now it has worked without a hiccup. It has no bells and whistles and does not have the best amp to gallon ratio, but is drop-dead easy to use and maintain, and since it is small it was easy to install. To use it, we just turn one valve to get flow into a hose and, after a short period, we use our taste buds (sophisticated gear) rather than instruments to tell us when the water is ready for the tank. When not needed or wanted, putting the unit to bed for up to a year takes 5 to 10 minutes. ... It makes six gallons of water an hour ...

From Dave Richardson on the WorldCruising mailing list:
IMHO the Spectra and the Village Marine are the best of the 12V watermakers. The Clark pump on the Spectra is the most efficient, and using a standard 40 PSI source pump is brilliant, but the Village Marine is the most common and therefore can be repaired just about anywhere. However, all 12V systems have limited output and must run several hours a day, gobbling amps from the battery bank to output just a few gallons of water. Don't get me wrong, I too have had one (a PUR 180 which I would have gladly thrown overboard would it not be polluting). Their chief advantage is their compact size and the common availability of 12V on board.

On our new boat, with a dedicated engine/mechanical space where I can walk around (this was one of 3 key criteria in our boat selection) we have selected a 120V AC-powered unit that runs off the genset. Clearly it is larger with a 1 HP motor and 2 large membranes but it also puts out about 3200 liters per day depending on salinity and temp. The pump is the industry standard "CAT" pump which over 50% of the makers use. Likewise membranes, sensors and valves are all standard so I believe I will be able to obtain parts just about anywhere in the world. We have a 8.5 KW 120V 60 Hz generator going on board which runs about 1 hour per day anyway so making water is a side benefit. One hour will produce about 125 liters or 30 US gallons per day again depending on salinity. We run it daily and often jerry the extra output to friends around us. Our consumption has been something around 6 gallons per day but I am sure this will increase over time with increased capacity and availability. Read this as we will waste more water.

Water makers like to be run. Daily is best. Green beasties grow in the residual salt water left in the system virtually overnight. If you are leaving the system idle for more than a week you will have to pickle it. In other words add a preservative to the system. Formaldahyde (yes the same stuff that you embalm a human body with) is the common preservative. Really ugly stuff to work with and a task watermaker owners hate. ... If you don't pickle the system look forward to buying new membranes which seem to be about the same price as replacing the system (close anyway). Also, another warning, when installing put a large charcoal filter with a 99% chlorine removal spec in line with the flush water source. Chlorine degrades those expensive membranes and as the flush water source is your tanks they often have chlorine in the water either from shoreside or what you added.

Efficiency: with a DC-driven unit you will go from generator or alternator or battery charger, to batteries, to DC pump each dropping something like 50% of output along the way. Well, battery chargers are about 80%, alternators 60% but batteries are horrible at about 50%. With AC-driven you are going from alternator directly to the AC motor. Direct engine shaft drive is even better. Balmar makes a combination genset and watermaker but the design looks clugey to me.

So in my opinion to answer your question, if there is a convenient location and you have a generator, and you plan on using lots of water or run it for only short periods the AC version (either 110V or 230V) is the way to go. Simply put it is more efficient. Space is however usually the driving criterion.

From Norm on The Live-Aboard List:
Membranes are quite fragile. I have damaged two sets (at $500/set) simply by leaving the cleaning valve in the "clean" position causing the seawater to re-circulate and overheating the membranes. If membranes are not used every few days they must be preserved or microorganisms will plug up the membrane and make it useless.


Watermakers are not really labor intensive, especially if you use them often, say every other day. Perhaps the "labor" you heard of is the cleaning and preserving of the membranes needed when the units are not used often. The only time I did any cleaning, aside from the times I accidentally damaged the membranes by overheating them, was when I made water in the brackish sounds and rivers of North Carolina and fouled the system. Don't make water in these areas (and the shrimp caught there is not worth eating either).

Most of the "labor" I do with the watermaker here at anchor in St. Augustine is the washing and changing of the 20 micron and 5 micron seawater filters because these waters carry a lot of silt. (In contrast, in Key West there is very little silt, the filters take a long time to foul, but the seawater strainer fills with grass every few hours of operation.) The filter elements can be washed out with a garden hose and re-used, but each time the element is washed out the usable time decreases somewhat so eventually the element must be changed. When the HP pump feed water pressure falls a bit below zero due to fouling of either of these two filters I go down into the engine room and with a glance at the gauges see which filter needs attention. These filter housings and the 20 micron elements are for sale in Home Depot. I buy the 5 micron element from the watermaker seller. (They are about $14 each. If anyone knows where I can get them cheaper please let me know.) I am in the process of tracking down a reusable element for the 20 micron element.


My Village Marine 25 GPH rapidly was reduced to 17 to 13 GPH, and my Spectra 9 GPH to 6 to 4 GPH. The published specs are under ideal conditions, new membranes in clear water.


Do not buy a Village Marine. They ripped me off $350 for their "Lifetime Guaranteed" titanium high-pressure pump when it failed because they put a brass plug on an unused high-pressure port which destroyed the stainless internal parts. In addition, they use non-standard sized membranes, at least on my machine, which you can buy only from them at elevated prices.


Get the machine as separate parts. My Village Marine prepackaged unit is difficult to get into to fix things like leaks, and dripping seawater tends to damage metal. It is much nicer if drips find their way to the bilge quickly and the whole area can sprayed down with fresh water occasionally.

Be sure to have all your filters and strainers installed where they are easy to get to to change filter elements or clean strainers and all the spillage and drippage (which will be considerable) will drain down to the bilge. Have a fresh water hose there to rinse the prefilters and to spray down the area when you are finished servicing them.

Install a muffin fan ("computer fan") to blow on your electric motors when they are operating, they will last MUCH longer that way.

I am told that if you rinse the RO membranes with fresh water (absolutely NO chlorine please) they will work better longer. Chlorine or oil will do serious damage to the RO membranes.


[9/2004:] While I am on the subject of watermakers, I thought I would update the list on my battle with Village Marine.

Several years ago I bought a 25 gph watermaker from Village Marine Tech in Ft Lauderdale.

It has been one problem after another ever since.

I have already written about many of them, and at first VM sent me new parts to fix the problems, which included:

Failure of both electronic boards (various parts on the boards).

No auto shutdown for low feed water pressure to high pressure pump to prevent cavitation damage. This low pressure happens normally, when the filters plug up with silt from the seawater. The sensing pressure switch was in the machine and wired up but the software ignored it. Instead of simply sending me the proper EPROM, they sent me an entire PCB and a new sensing switch!

Failure of all three membrane housings. There were actually two failures, illustrating both lousy engineering and lousy craftsmanship. During the changing of these housings I discovered several "316" fittings in the high pressure seawater circuit ("all fittings are 316 stainless", VMT brochure) were actually plated brass. They were replaced with 316 by VMT.

Failure of salinity/temperature sensor. Bad materials causing cracks in the plastic body.

And so it went, one thing after another until finally the "Lifetime Guaranteed" titanium high pressure pump failed.

This pump consists of a three-plunger high pressure pump very much like a pressure washer pump. The pump head is a titanium block with various holes drilled in it. There are basically three cylinder bores with an intake gallery and a discharger gallery, six check valves (two for each cylinder) and cylinder seals for the ceramic pistons driven by a gearbox and 240 vac electric motor.


> Now that you've had it for a while, what are your
> impressions of the Spectra watermaker?

Spectra is very proud of their gear so I bought only the low pressure pump, the high pressure Clarke Pump and the pulsation damper from a Spectra dealer in New York. I got the membrane and housing from SK Watermakers, the prefilter housings from Home Depot, the prefilters from Farm Tek Supply, stainless gauges from a local auto supply and a high powered low pressure pump from Depco in St Pete FL.

The membrane and housing are a standard 40" type. These membranes are standard, common, and available from any RO dealer. This keeps the price down. This is the opposite from my Village Marine Tech unit that takes an odd size membrane available only from Village Marine Tech at "you pay our price or throw away your watermaker" prices.

The high pressure "Clarke Pump" is unique. It is actually a pressure booster that takes the low pressure water at around 100 psi and boosts it to several hundred psi for the membranes. I have had no problems with it yet but seawater dribbling from its control valve is corroding the base plate, a job on list #2.

I hooked up both small and large low pressure pumps in parallel. The circuit was: sea valve, sea strainer, both low pressure pumps in parallel, 20 micron filter, 5 micron filter, pulsation damper, Clarke Pump, membrane, overboard sea valve. There are some valves and gauges also.

I could choose which low pressure pump to be used with switches. They both had check valves on their outputs.

At first the system performed as advertised, but within a few days the output fell rapidly to less than 5 gph.

The small low pressure pump, which came with the Clarke Pump, failed early on. First the high pressure limit switch failed so I bypassed it. Then it started leaking, so I removed it. Then the pump output fell to unusable levels. The dealer I got it from in NY refused to sell me any parts and told me (apparently I was talking to the "boss") that he would not have sold me the gear in the first place. He did not say why and I did not ask but my guess is that he would realize a much greater profit if he had sold me a complete system. I have not used that pump since.

The large low pressure pump I bought from Depco after carefully examining the one offered by Spectra, and I believe I have substantially the same pump for $250 less than Spectra's price. This pump is an Italian rotary vane type pump with graphite interior and built-in relief valve. Both pumps have 12 VDC motors. It worked well at first producing 18 gph, but it too gradually failed until I was getting less than 5 gph of product.

After examining the entire situation I concluded that the high silt level of seawater in St Augustine was the culprit that destroyed both pumps by abrasion.

The Depco pump was sent to Depco for rebuild, thence to the distributor in CT and they replaced the interior parts "gratis". They agreed that the abrasion of the silt probably killed the pump. However, only the soft interior parts needed replacement and they were relatively inexpensive. I could order the parts and replace them easily myself.

When I got the pump back I rebuilt the system but this time with the two filters upstream of the Depco pump to protect it from silt. I had to add yet another pump upstream of the filters, between the sea strainer and the filters, to push the water through the filters to feed the Depco pump. This is a Flo-Jet type general purpose pump.

This setup works well now, putting out 14 gph of product.

But I am in the relatively clean water of New England and do not know how it will stand up to the heavy silt of St Augustine.

As things stand now, this system is not safe for unattended operation because there is no auto shutdown for the system if the feed water pressure to the Depco pump should fall below zero due to fouling of the filters and cause cavitation of the Depco pump. Cavitation of this pump is not a Good Thing and I would need to rig some sort of auto shut down. When I am on board now, I can hear it complain and shut it down. I also would like another gauge showing the Clarke Pump's feed water pressure and maybe the RO membrane feed water pressure.

I would also like to rig (I have all the parts, just haven't had the time ...) a diverter valve and timer to dump the first three minutes of product (it is salty at first) so that, with the anti cavitation shutdown system, the rig is capable of fully automatic operation. I could then hook it to the Trace inverter, which has three programmable voltage (12 VDC) sensitive relays I could set to turn on the RO when the batteries are fully charged by the solar panels and wind generators.

So far the system works, producing 14 gph of good fresh water but at a cost of 30 amps.

Perhaps with more reengineering I could get the efficiency up. Spectra claims one amp hour per gallon, and this is probably possible with a small low pressure pump in clean seawater in the original configuration. But like many advertised specs, it has not worked out this way in the real world for me.

From David on The Live-Aboard List:
Spectra gear is good but contains some CRAP engineering - the fittings onto the Clark pump and the membrane end cap have a tapered thread to seal them on the early versions - later versions have a straight thread and an O-ring seal - then the hole on the end cap of the membrane is off centre and near the edge - seen and experienced cracks on both the clark pump and the membrane end caps. Yes, the clark pump has a lifetime warrantee BUT Spectra expect you to place a $18K deposit before they will send you a new one. After much shouting and squealing the Spectra dealer in St John US VI's 'Proper Yachts' got our cracked pump replaced but he had to lodge the deposit mmmmmmmm!!!! In the VI's 1 Amp per gallon is achievable with some care, 'sea grass blocking the inlet' being the big concern.

From "Reverse-Osmosis Watermakers" article by Patrick Childress in 10/2001 issue of Sail magazine:
  • Engine-driven can be unreliable because vibration gets transmitted to high-pressure pump (bad).

  • Want check-valve between watermaker output and fresh water tank, to prevent chlorinated water from coming back into the watermaker.

  • Install a TDS (Total Dissolved Solids) meter to monitor output water quality. 220 PPM is acceptable, 540 may taste bad, 700+ means replaces the membrane.

From "Waterworks" article by Aussie Bray in 3/2004 issue of Sail magazine:
  • Usually several prefilters to protect high-pressure pump and membrane: "plankton" 100-micron cleanable, then oil-separator, then 25-micron, then 5-micron.

  • May get 2-3 liters of water per AH at 12 volts.

  • Can leave membrane in seawater for about 3 days, but should flush and leave in fresh if leaving idle for longer than that.

  • If leaving idle for 3 weeks or more, should "pickle" with biocide to keep bacteria from growing on membrane.

  • Siting: noisy pump might go in engine compartment, but membrane shouldn't be heated beyond 120 F.

From "Secondary Engine Loads" article by Don Casey in Nov/Dec 2001 issue of Blue Water Sailing magazine:
A watermaker pump wants a fairly narrow RPM range, which means an engine-mounted pump probably can be run at anchor or underway, but not both.

From Colin Foster on The Live-Aboard List:
... they DO work in fresh or brackish water and the output does increase substantially. BUT the handbook that came with our watermaker indicated that under these conditions you MUST decrease the membrane pressure to avoid permanent damage. This is because there is a required ratio of product water to total water flow. They rely on this total flow of water over the dirty side of the membrane to wash out and dissolve salt and contaminants and flush them overboard. In conditions where your output increases either due to low salt content, OR temperature it is important to reduce the output product to normal saltwater operating specifications. If your watermaker does not have an adjustable pressure regulator and you cannot reduce the flow to normal levels, you risk premature deterioration of the membrane. We found that when running the watermaker while negotiating the ICW, it required a constant vigil due to rapid changes in salinity.

From Derrick Aubin on The Live-Aboard List:
The membrane is specific for saltwater, brackish, or freshwater. If you change water, you should get a different membrane from the manufacturer.
From Andina Marie Foster on The Live-Aboard List:
You can stick with a salt water membrane but you have to have an adjustable pressure regulator so you can drop the pressure as the salinity goes down. The aim is to keep the product water rate equivalent to the membrane design. For example if the manufacturer recommends 5% product water and 95% water flowing over the membrane then although it will put out much more than this in brackish water you must drop the pressure to get back to the same ratio. This makes sure there is enough flow and turbulence over the dirty side of the membrane to keep it clean. Too high a production rate will clog up the membrane.

This can take some vigilance in places like the ICW where the salinity can change rapidly and frequently in some areas.

From article by Chris Fletcher in 6/2009 issue of All At Sea magazine:
Factors involved when deciding whether to buy a watermaker or buy water from docks:
  • Costs.
  • Health and environmental factors.
  • Convenience.
Cost of a watermaker includes watermaker purchase price, additional fuel and maintenance for generator (or whatever the power source will be), supplies for watermaker (count on 8 new prefilters per year, and one new membrane every 2 years, plus pickling solution).

Health and environmental factors: bottled or purchased water may not be pure, and disposable bottles are bad for the environment.

I might assume consumption at 20 gallons/week/person. Cents/gallon is much harder to generalize; could be close to zero in Puerto Rico, 20 cents in Virgin Islands, 30 cents in other islands ? If only one person aboard, and catching some rain, that is a factor too. If have large water tanks, you can drive the cost lower by buying in cheapest places.

The "watermaker cost" includes purchase price, fuel, and maintenance. It is derived from the "Gallons/Week consumed": 0-100 consumed requires $4500 cost, 101-300 requires $6500, 301+ requires $11000. The article does not give these directly, and I don't know how they're derived.

The article included provision for buying some bottled water, at $1/gallon; I have omitted that.

From Norm on the IRBS live-aboard mailing list:
I have a Village Marine 25 GPH, 240 VAC unit I bought from their store in Ft. Lauderdale. It is a unitized type, not the modular type with separate components. I got a good deal on it as it was a close-out, but would have preferred the separate parts for ease of service. The titanium high pressure pump and membrane pressure vessels are warranted for life. I accidentally salt damaged the membranes by leaving the seawater valve in the recirculating (cleaning) position early on. Village Marine sold me a new set of membranes at cost and gave me a fresh set of cleaning chemicals gratis (I had used mine up trying to resurrect the membranes). It has over three hundred hours and I am completely satisfied with it. Village Marine was recommended to me by a fellow cruiser, Flintstone, and I second that recommendation.

As suggested by Village Marine, I added a valve and filter to allow fresh water rinsing of the membranes which I do every time I shut the unit down. The filter is a carbon type to eliminate the possibility of ingesting chlorine from dock water taken aboard, a deadly enemy of the membranes (the other is oil, for which there is an oil ejector in the unit).

Also on recommendation of Village Marine, I added a 20 micron filter, in addition to the 5 micron filter that comes with the unit, to filter the seawater. This is imperative because the 20 micron becomes plugged quickly in silty water. It is much like the primary and secondary fuel filters. The 5 microns from Village Marine are expensive but you can buy 20 micron filters cheaply in Home Depot.

I added compound gauges, just like the ones on my fuel system, to see when the filters need changing.

The 5 micron filter is attached to the unit. Changing the filter usually spills a bit of seawater onto the unit. Now I just put down some paper towels as a dam (paper towels from our living quarters that are merely wet are recycled to the engine room), and rinse off the unit with fresh water when finished. I have considered moving the 5 micron filter to alongside the 20 micron filter which is remote from the unit where any spillage goes directly into the bilge, but the 5 micron filter lasts about 10 times longer than the 20 micron filters do so it's not enough of a hassle to move me to do it yet.

All three filters housings and filter elements are the common, standard, filters available in Home Depot with the exception of the 5 micron, which Village Marine says is a specially good one that will not shed anything that could harm the membranes. Since I bought a bunch of them when I bought the unit, and I have not used many, so far I have taken them at their word.

The only improvement I would like at this point is for a re-useable 20 micron filter system.
From Norm on the IRBS live-aboard mailing list:
... If it is an all-in-one unit, remove the 5 micron seawater from the unit. You do not want the seawater that spills when you change the filter to get into the machinery. Mount the 5 and a 20 side-by-side where the seawater can spill with impunity. Use the same compound gauges on the input and output of the 20 micron as I have described for my fuel system for the same reasons. There is already a compound gauge on the output of the 5 micron. Buy the fresh water rinsing kit, consisting of a filter housing with charcoal filter to filter out any chlorine that may be in the water, and a three-way valve to select FW rinse (connect the filter to your domestic pressure supply) or seawater. Chlorine and oil are the membrane's worst enemy.

Buy the 5 micron filters from Village, they are about $15 each, but get the 20 microns from Home Depot. The only ones we have found that work are GE model FXUSC. They last about 20+ hours here in silty St Augustine. For comparison the Ace Hardware brand lasted ONE hour.

From Norm on The Live-Aboard List:
I have a Village Marine watermaker and although the pumps and membranes are working fine so far, have had trouble with the salinity PC board and sensor. Now it is out of warranty and I have a $700 salinity board that has failed. Yesterday I found the feed water pressure gauge has gone bad.

If I could do it over I might use their major components, the titanium HP pump and the guaranteed-for-life membrane housings. But the control system is much too complicated and failure prone. They put the 5 micron filter on the main chassis so I get salt water on the chassis every time I change the filter element. It should be separately mounted where saltwater spilled from it can be well tolerated. It needs a shutdown trip if the feedwater pressure gets to low due to filter fouling. A Murphy (has adjustable electrical contact on pointer) gauge would work well here. I had to add compound gauges on the 20 and 5 micron filters to show when they should be changed. For the money they want, they could do much better.

I have a friend who has been living from water made with a Spectra watermaker for years and is full of praise for it. Notice too, that over the years Spectra ads have become bigger (more expensive), a sure sign of the success of their product. I will buy one too, but set it up using separate components and in ways I have learned from my experience with the Village Marine unit.
From John / Truelove on The Live-Aboard List:
Sounds like you have an "automagic" unit. I have the modular unit: no controls, just an on-off switch and a knuckle-buster. I look at the pressure once or twice during the 3 hrs/day I run it, but doesn't change much; if it did, the change in sound would alert me. I toss the 20 micron filter overside occasionally to clean it - haven't changed it out for about a year. Don't really need gauges - drop in output and change in color is a good indication that it needs changing.

From Norm on The Live-Aboard List:
My recommendation is to get a Spectra watermaker. You can buy the unit complete or buy the bits and pieces and build it as you like it. One gallon per amp/hour can't be beat and you can run it with a solar panel or two. I wish I had done that instead of buying the 240 vac unit I have now.

From Norm on The Live-Aboard List 7/2007:
My Village Marine Tech RO watermaker had a "Lifetime Guarantee" on the titanium high pressure pump. Some idiot at the company used a brass plug to close an unused high pressure port in the pump head. This, in time, destroyed the 316 check valve parts in the pump head. Upon returning the pump head to Village Marine Tech they sent me a new (looked rebuilt to me) pumphead but then charged my Schwab Visa account $350. I complained to Schwab and sent a letter explaining the situation and they removed the $350 charge, but later allowed it. I called Schwab several times but the account reps declined to return my calls. I call it conspiracy to commit larceny.

Later I also discovered, when they began to leak, that the "316 stainless fittings" used in the high pressure seawater circuit were actually nickel-plated brass. It cost me a further $300 to correct that additional larceny.

From Brian Woloshin on Cruising World message board:
I had a Pur 35 and it is so slow it is like watching grass grow to get water. Having owned a Pur and from other sailors experiences I would not recommend them at all - they wear out very quickly. I replaced mine with a Village Marine Little Wonder 150 which is a better/higher output unit. Now there is the Spectra which is very energy efficient but much higher priced. For 12v units I think it's between these two.

From Stuart James:
I have a Pur Powersurvivor 35 desalinator. It takes about four amps to run it. The solar array generates five plus amps on a typical day. Thus, no problem running the desalinator. It produces about 12 gallons during an 8 hour run, which would last me 5 or 6 days of careless use. This unit can also be operated by hand if one should end up in the liferaft.

From Rich West on rec.boats.cruising newsgroup:
I have a PUR 35 and it's really a piece of junk. It looked more like a lawn sprinkler than a watermaker with all its leaks. I sent it to PUR to upgrade to a PUR 40 ($$$$$), now it doesn't leak, but it doesn't make water much either. When it gets the slightest air bubble (and it's impossible not to get some air when the water is at all rough), it stops making water until you bleed it.

I have also had terrible problems with the company. Several times now they have told me one thing just to go back on their word once they had my money.

I can't wait to get rid of my PUR and get a real watermaker.

From Rick on Cruising World message board:
SK Watermaker (FT Pearce Fl): Could not be happier, we averaged seven gallons an hour and the darned thing is so quiet that had to touch the motor to make sure it was on. We have the DC version and the motor and pump is very husky ... comes in component form ($2.3K) and we installed it going down the ditch. ... very very happy with the performance!

Forget the Pur 40-E. They got problems. I had two Pur watermakers and took em both back for full refund. ... (Pur 35 better record than the Pur 40-e.) ...

From Greg Walsh on Cruising World message board:
[I recommend] bigger watermakers.

If you have a genset I would recommend going the high-capacity AC-driven route rather than the 12V route. Having owned both kinds and also having talked with other owners of both kinds, it appears that this is a case of bigger is better (or at least less trouble). I think that the reason is twofold:

1) You must run a small watermaker for a lot longer to make the same amount of water. Longer runtime equals more wear and tear and more trouble.

2) 12V watermakers use more complex and trouble-prone pumps in order to gain the required efficiency. That being said, 12V watermakers do get you more water/amp.

We happen to currently have an MT watermaker which is a European brand not often seen in the US but it (like many other AC-powered watermakers) uses a US-built CatPump for which parts are easily available.

From "Second Thoughts" article by Tim Murphy in 6/2004 issue of Cruising World magazine:
From Roy Romine: if you're powering it with a genset or engine, get a larger, high-capacity watermaker rather than a small one. The larger watermaker gives you a decent amount of output for the amount of time you'll be willing to run the genset or engine.

From Andina Foster on The Live-Aboard List:
We've used our watermaker in all situations and water quality has never suffered. The water is so pure that we take mineral supplements when on it for long periods.

I counted a total of 14 processes and four pumps before we drink it:
1. Incoming leaf and fish strainer.
2. Fine sand and grit strainer.
3. An adapted swimming pool sand filter that is back-washable - can save many $$$$ on primary filters when in dirty water.
4. Primary 30 micron cartridge filter.
5. Secondary 5 micron cartridge filter.
6. Hydrocarbon filter - the membrane is sensitive to oil and fuel in the water so this is necessary in most busy harbors. Replacement cartridges were $80 each but they last for a long time.
7. The RO membrane itself.
8. An ultraviolet sterilizer to kill any bacteria that got this far.
9. A charcoal filter.
At this stage the product water enters our on-board (cement-lined steel) tank(s). Total capacity about 600 gallons.
10 An inlet strainer on the potable water pump.
11. A 30 micron dirt and rust cartridge filter.
12. A 5 micron cartridge filter.
This is now the fresh water used on the boat.
13 A charcoal filter on a drinking water spigot in the galley for water used directly in consumption.
14 There is also an extra strainer downline from the charcoal filter to catch any particles of charcoal that wash out of the filter.

We've used the watermaker in brackish and fresh dirty water situations. The important thing to watch is that the produced water never exceeds about 5% of the total water passing over the membrane. This means adjusting pressures up as high as 850 psi in cold salt water but down to 200 psi in warm fresh water. This can be tricky in the ICW where salinity and temperature can change over the full range in half a mile in some places.

The back-washable swimming-pool-style filter was a real money saver. While the primary cartridge filters could last for months in the crystal waters away from harbors in the Bahamas, they could last less than half an hour in muddy ICW water. The major operating expense was cartridge filters but by adding the sand filter I extended their life by a factor of 10 or more in dirty water. I had it set up with a couple of valves so I could backwash frequently and the waste water ran through the galley drain so I could see the clarity and know when it was clean.

Waterlog International makes a tow-behind-the-boat water-powered watermaker. Waterlog article

From Norm on The Live-Aboard List 8/2006:
If I were to buy a new watermaker I would call SK Watermakers at 800 489-0852. I met this fellow a few years ago and he impressed me with his knowledge. (But company has been sold to new owner since then.)

I probably would not buy a compact unit, that is a bolt-in-connect-it-up unit, but there may be acceptable unit machines available that are properly designed out there.

I would tend to get individual components and fit them into my boat.

For example: my boxed RO had a seawater filter, the 5 micron, installed right beside the 240 vac motor that drives the LP pump so every time I opened the filter to service the element, a task I must do every 20 minutes in St Augustine FL, seawater always splashed onto said 240 vac electric motor which still has a rust problem even though I moved the filter early on. The membrane housing HP fittings are also above the motor and the plated brass fittings (that the manufacturer claimed were 316 and charged accordingly) eventually de-zincified and leaked seawater down onto this motor. Likewise, seawater that leaked out elsewhere, wet the base of the box, and now the foot that supports the heavy HP pump is rusted badly. I will have to replace that piece with a stainless custom-made mounting foot. I saw a RO in a magazine ad yesterday with FOUR such filters in the same area as electric pumps and controls! The prefilters should be completely separate from the box to avoid getting seawater into the machinery as happened on my machine.

The box RO looks good in a photo or sitting in the showroom to the inexperienced, as it did to me. Buy it, bolt it on, hook it up and there you go. And that works until the first time you need to service something. In order to tighten 80% of my hose clamps I have to remove the membrane housings on my machine. I can't tell you the number of times I cursed Village Marine for their lousy engineering and rip-off sales practices. I could of course, take it all apart and do it right, most of the components are high-quality, but that would be a lot of work.

However, there may be properly designed box RO units that keep the electrical parts separated from the seawater and open up to make service easy, but beware of swallowing the fantasy of bolt-it-on easy.

If I were to install a 12-volt machine now, I would first realize that it will require plenty of service, mostly due to running saltwater through many devices mixed in with electrical wiring close by.

The prefilters will be rinsed off countless times, more so in silty water, less so in clear, with a water hose, reinstalled, and the local area hosed off with sweet water. I would mount all the pre-filters in a place where seawater can splash around without harm to anything and drip down into the bilge.

Mount all electric pumps with the pump housings down and dripping onto a safe path to the bilge, say just above the prefilters, and the motor ends up. The electrical controls would be above the pump motors.

A rack-style configuration might work well instead of a box. Put the pre-filters on the bottom, the pumps go above them with the motor ends up, and the electrical controls at the top. Power supply wires should be on the overhead and drop down to the unit. ALL seawater leakage should drip down ONLY onto parts that are unaffected by seawater! ALL seawater connections will leak someday!

The RO circuit is this: seawater in through the hull, through a seawater strainer. These strainers are set up typically to have the debris inside the strainer basket so it is easy to clear it by removing and cleaning out the basket, but this way you can't see how much debris is in the strainer to decide if it needs to be cleared. I prefer to set them up backwards so the debris is outside the strainer where I can see how much is in the strainer. Then the seawater is pumped up to about 15 psi by the LP pump and pushed through the pre-filters to the input of the HP pump. The pre-filters will require your attention when running the RO, so place them where it is easy to service the elements. If you have a big enough boat, a swimming-pool sand filter is said by Andina to be very effective and easy to flush. The HP pump feeds the membrane housings where a pressure relief valve at the output of the housings keeps the pressure in the housings at about 800 psi. This pump is very much like a pressure-washing pump you might rent to clean a building. The seawater then goes back overboard. Fresh water squeezes out of the membranes and into your tanks.

It would be nice to have an auto-shutdown system, so you can leave the unit on unattended without damaging the HP pump when the pre-filters foul and cause the feed water pressure to go negative cavitating the HP pump (that's a bad thing). You can put a pressure switch on the feedwater line holding the HP pump relay closed to do this shutdown inexpensively. On my machine the Low Feedwater Pressure Shutdown was originally not enabled in the software chip on the PCB but Village Marine did send me a new PCB with the correct program and another pressure switch that I had to install myself

When the machine is started up, for the first few minutes it will produce salty water out the fresh water line. This salty water must be discarded. My big machine has a salinity sensor feeding a PCB and solenoid valve to do this. This hardware is only a convenience, not needed if you simply let the machine run a few minutes before you send the output to the tanks. A handheld salinity meter is available for about $50.

I would also realize that I will not get the advertised rated flow in actual use. My 25 GPH machine is 25 GPH with new membranes, but after a few weeks it settles to 17-18 GPH in normal seawater and stays that way for years. These days I don't do any cleaning routines; when I have in the past they did increase output for a while, but not back to the original output, and then it settled back to what it was before the cleaning. Rain or operation in brackish water, like the NC sounds, makes the salinity lower letting the RO produce more, up to 35 GPH on my big machine.

I wish I knew when I started what I know now; my RO would be much easier to keep working properly. I would have it much more accessible to service. Realize that you will need to get at each movable part of the entire machine at some time or another and saltwater will ooze or squirt out of every connection eventually.

Use only 316, bronze, or plastic in contact with seawater. My Village Marine $6000 RO came with plated brass HP seawater fittings. Rip-offs are everywhere. "Lifetime Guarantees" are phony, at least that has been my experience with Village Marine Tech.

I do have a Spectra 12 VDC watermaker. Right now it is down because the LP pump is inop and there is no strong pressure pushing me to fix it.

I too, thought that it was too expensive for what I got, too much "ain't it wonderful it's so efficient so send us lots of money!!!". I managed to get the Clarke pump, an LP pump and pressure pulse smoother (a little tank with membrane, just like the one in a drinking water system but smaller and higher pressure). The LP pump feeds seawater to the Clarke pump at about 100 psi with the pressure pulse damper on an offsticker. The Clarke pump boosts the pressure up and feeds it to a standard 40" membrane housing mounted on the bulkhead, which I bought, along with the HP hoses, from SK Watermakers. It needs to be set up like the 240 vac RO. It is the same game, no matter what the size of the RO. Add some gauges, ss 0-150 psi to see what is happening with the feed water, and a flowmeter to see the product flow. A low feedwater pressure auto shutdown would be nice to shut down the unit when the prefilters foul. In this system it would be good to have three pumps. The first LP to push water through the prefilters at about 15 psi. Without this pump you are relying on suction only to draw seawater through the filters; pushing it through will let the prefilters work longer before you have to pull and rinse them off. The second LP pump, a coke machine vane water pump, feeds the Clarke pump at ideally 125 psi. Likewise, the first few minutes of fresh water output will be salty and must be discarded. When I called back to the outfit in NY where I bought the Clarke pump and diaphragm LP pump to get a new pressure switch for the LP pump, the owner of the company was angry at me and declared that he would not have sold me the gear. I bought it from one of his sales staff. I did not ask him why, I just bypassed the switch.

If I were do it again, I would not buy a Spectra machine. They are overpriced and have to be reengineered to work well in less-than-ideal conditions. I would contact SK and discuss the situation with him. The Spectra might work well in clean southern waters as some have reported to me, but in my world it has had problems.

Make your own watermaker: Leo Litchfield's "How to build your own watermaker"

A water-purifier that purifies fresh water (not saltwater): MSR Miox Water Purifier ($130, plus salt and purifier and test-strip supplies; uses camera batteries at $10 per 50 gallons).

From article by Michael Specter in 10/23/2006 issue of The New Yorker magazine:
  • Best guess at minimum water a person needs each day: about 50 liters.
    Most people drink 2 or 3 liters; rest is used for cooking, bathing, sanitation.
    Americans consume 400-600 liters per person per day, the highest in the world.

  • Irrigation for agriculture accounts for more than 2/3 of water use worldwide.

  • Half of the hospital beds on earth are occupied by people with an easily preventable waterborne disease.
    Simply providing access to clean water would save two million lives each year.

  • It takes a thousand tons of water to grow a ton of grain, and fifteen thousand to grow a ton of cow.
    Thirteen hundred gallons of water go into the production of a single hamburger.

  • There are more than 75,000 dams in the USA. That is a new dam built almost every day since signing the Declaration of Independence.
    The USA stores about 5000 cubic meters of water per person.

  • The USA today uses far less water per person, and less water in total, than we did 25 years ago (mainly because of changes in the economy, but also improvements in efficiency).

  • There are nearly 20,000 desalination plants in operation today, worldwide.

Condensation Watermaker / Solar Still

Wikipedia's "Solar still"
SolAqua's "Solar Still Basics"

Watercone (no way to buy one ?)
Aquamate Solar Still
ClearDome Solar Thermal

Some cautions:
from wholesalewaterdistillers.com

From Mother Earth News's "Water Wiser Solar Stills":
"The total amount of surface area determines the yield of the solar distiller. Dennis Lemon, author of Pure Water Nature's Way, says on a sunny day, for every 1,000 square inches of cover surface, the solar distiller will produce about 1 gallon of pure water. An efficient 4-by-8 foot distiller should yield 2 to 4 gallons." SEA Panel:
From the maker:
Not made for boats, but for land. And not large volumes, but about a gallon per day per panel.

They're definitely not made for a liferaft, there is an inflatable version from another manufacturer for that, about $200 and it doesn't make a lot of water. A better solution for liferafts is a forward-osmosis system, which is kind of a little bladder, the inside is filled with an electrolyte like a sports drink concentrate, which gives a concentration gradient from outside of the bag to inside. So the bag fills with water and leaves the salt behind. But the key here is that if you have 30,000 ppm of salt outside the bag, you need to have something like 35,000 ppm of sugar/salt inside the bag, that's the rub of F.W.. But it works for survival.

I'm going to eventually make a roll-tolerant solar desalinator for boats. But so far, all of our stuff is for land only.

My main market is Developing Nations, because they don't need filters or renewable components, and they run off the sun. But they'll work for hurricane preparedness and just generally turning seawater into fresh.


Neither of our standard stills will work on a boat, the salt water will get into the collection channels when the boat rocks. And a tub with a tilted plexiglass will work, but you would need to baffle the water somehow.

I did have one idea ... maybe I can use our existing [Rapid Deploy] still, but put absorbent foam in it. That would baffle the water, keep it from sloshing out and contaminating the collection channels, and increase the efficiency of the still by increasing the surface area of the feed water.

As long as you get reasonable sun, you should manage about a gallon a day, and it's good water since it's "warm distilled" rather than steam (hot) distilled. I think it's as tasty as any decent bottled water, it doesn't have that flat taste from boil distillers.

About making your own still on a boat, from Mike Wofsey, the owner of SEA Panel:
Before you get too involved in it, you should probably know what kind of yields to expect. Rule of thumb for the Passive Solar Still is that in sunny weather, they can produce an average of 0.1 gallons per day for every square foot of basin area. So a 4 foot x 2 foot basin area will produce a bit less than one gallon (3 liters) in typical weather ... sometimes more in the height of summer, less in winter. Of course you can set it up to collect all the rain that falls on it too, which helps.

A solar still on a boat can be difficult because of the rolling.

One of the ways to improve the efficiency for boats is to use a wick-type still, where the water is wicked up a cotton sheet. This dramatically increases the surface ares, which increases the yield, sometimes to twice the rule-of-thumb I mentioned. The other thing is to insulate the basin somehow, either with foam or fiberglass. Are you planning to make the condensing surface out of glass or plastic?

The ultimate efficiency of these things is determined by the thermodynamic efficiency, which is defined as:

efficiency = 100 * ( 1- ( condensing surface temp / basin temp))

So if your basin is 120-degrees F, and your surface is 100-degrees F, your maximum efficiency would be 16.7%. The goal to keep efficiency high then is to try to seal the unit as well as possible to make a greenhouse, and keep that basin temp high, and then try to keep the condensing surface cool. On a boat, you have an endless supply of cooling liquid, the ocean. So if you use the ocean water to run over the top surface to cool it, then you can increase the efficiency, for instance, the temperature in the basin is 120-degrees, but the surface is now 80-degrees (since the evaporation of the seawater from the surface lowers the temperature below the temp of the water, you would now be looking at 33%, about double. If you can divert your engine cooling water to go into your basin, then you can improve efficiency even more, say up to 40%.

It all depends on how complex a project you want. But they are fun. Even if it's cheaper to buy water than get it from a still, I like doing the still. If you look up plans for your boat, consider Googling a "tilted wick" unit. This one might be very practical: still.