Many cruisers consider watermakers a necessary evil. It is a luxury to have fresh water all the time without having to fill up jerrycans from questionable sources a couple of times per week. But it comes with a prize: energy demands, noise and maintenance. We selected a watermaker with low energy demands and after two years of cruising we have optimized our setup and reduced and simplified the required maintenance. You don't have to reinvent the wheel: read our tips and tricks.
Types and brands
This chapter is not intended to favor a specific brand. We choose for a Schenker model 30 because we could buy a used one in good shape and it matched our criteria. After two years of living entirely on watermaker water, we can say that we are happy with this watermaker, it has low maintenance and has been problem free so far, with one exception: the pump. We found a solution for this problem, more about that later.
When we bought ZwerfCat it already had a watermaker but it took a lot of space, was noisy as hell, and difficult to maintain. The main problem however was that it used a lot of power. Really a lot, about 2000 Watt to produce 90 liters per hour. We had to start the generator to run the watermaker, but since it was our intention to get rid of the generator and operate solely on solar power, we also had to find a more energy efficient watermaker.
To force seawater through a reverse osmosis membrane a pressure of about 60 bars is required. Only a small portion (about 10%) of the water is actually forced through the membrane. The rest, now enriched with the extra salt which was left behind, called brine water, is simply dumped overboard. To pressurize water up to 60 bar takes a lot of energy, and it is a waste that 90% of that water is simply dumped overboard without re-using the pressure. Fortunately, there are some watermakers that re-use this precious energy, and these watermakers are usually marketed as "energy recovery" models. They use a relatively low pressure (7 bar) pump, and a pressure amplifier (Clark pump) that recovers the energy in the brine water to boost the pressure up to 60 bar, somewhat comparable to a turbo in a combustion engine. Spectra was the first brand with an energy recovery model, followed by Schenker, and nowadays there are more brands that use the same concept.
Our Schenker 30 only uses about 9-10 Amps (which equals about 120 Watts) to produce 30 liters of water per hour. This is 4 Watts to produce one liter of fresh water, quite a difference with our previous "brute force" watermaker which needed more than 20 Watts per liter! Well, to be honest, these advertised 30 L/h are really a bit overoptimistic. In reality our production was more around 20 to 25 L/h (which is still less than 6 Watts per liter), until we modified the pump.
A word about capacity. More is not always better! In reality there is only one reason for wanting a high capacity watermaker, and that is when the watermaker uses so much power that you have to run the engine or generator to use it. Only in that situation you want to make as much water as possible in the shortest time. If however the watermaker uses so little power that it can operate on solar power there is no need to hurry. In fact, it is better to have a lower capacity watermaker so you can "spread out" the energy required over multiple "solar hours".
Things they usually don't tell you in the installation manual:
For pure technical reasons, it is often recommended to mount the pump (and assorted filters and valves) below the waterline. We don't like that at all, because there are a lot of hose connections associated with this contraption, each of them increasing the risk of mishaps resulting in a flooded (or sunk) boat. We opted for having just one hose running up from the seacock until clear of the waterline, and to have the prefilter, valves and pumps all above the waterline. The disadvantage is that there is a slight loss of efficiency because the pump has to lift the water above the waterline and sees a negative pressure on its inlet, and tiny leaks cause air to be sucked into the system. Well, we still prefer tiny leaks to result in air inside the watermaker rather than in water leaking into the boat...
Normally the pump draws its input water from the sea. But when cleaning the membrane with chemicals, it is necessary to draw the input water from the output of the watermaker, to keep the chemicals cycling. The manual suggests disconnecting the suction hose and run a hose from a bucket, but we hate to disconnect hoses because it gets harder over time and after reassembly they will often leak. So we Tee'd an auxilliary suction hose with a valve into the main hose and can thus select either input by just opening/closing two valves.
Our Schenker came with a 5" prefilter, but these elements are hard to get. So we replaced it by a 10" filter. Some people prefer to have a dual filter setup, with a 20 micron filter followed by a 5 micron filter, in an attempt to have a longer time between filter replacements. Downside of this strategy is that flushing with fresh water at the end of the production run now requires much more fresh water.
One recurring maintenance issue with watermaker is the replacement and cleaning of the pre-filter(s). We found that we are often reluctant (read: lazy) to perform maintenance if the operation is too cumbersome and/or messy. The old watermaker of ZwerfCat had its filters in the bilge, so we had to take away stuff occupying the floor, take away the floor panel, fumble inside the bilge and spill a lot of water which worked its way over the entire length of the ship, and had to be mopped up afterwards. Which invited an attitude of "nah, let's do the darn filter another day"... So with the new installation we wanted the pre-filter housing as accessible as possible with little concern about spilling any water.
On ZwerfCat we had, as is typical for catamarans of this size, 4 cabins, each with their own bathroom. Since we found 4 bathrooms just a little too much for us two, we converted one bathroom into a "technical wet room". We took out the toilet, put a washing machine in its place, and used the rest for the watermaker installation and beer brewery.
This is just great! In case of any leaks or spillage, we just press the button of the drain pump and the puddle on the bathroom floor is gone. We installed the pre-filter just above the sink, so cleaning/replacing the pre-filter doesn't even wet the floor!
Another advantage is that the brine output is routed simply into the sink. It not only saves an additional hole in the hull, but it is really handy if we want to apply chemicals to the membrane (pickling and/or cleaning). We put the plug in the sink, put some fresh water in it with the chemicals, and open the auxilliary suction hose which is Tee'd into the main suction hose and also ends in the sink, and that's about it. When the procedure is finished, we close the auxilliary suction valve, unplug the sink and we are done. No temporary hoses and we don't even need a bucket!
If you don't have a spare bathroom, you might still be able to install the watermaker against the bathroom wall, concealed by a curtain. Friends did this in their monohull, it looked clean and they were happy with the result.
Reversed filter housing
A common way to clean filters is to apply a reverse flow to them. Of course this water needs to be filtered already otherwise it could contaminate the filter on the clean (output) side. A neat way to clean a pre-filter is to plumb it in reverse into the output (brine) flow of the watermaker. The brine water has already passed through a filter so it is known to be clean, and the residual pressure is enough to overcome the tiny restriction of the to be cleaned filter.
The Schenker watermaker (our 5000 Euro model appears to be a "budget" version) didn't come with a flow gauge. Instead, the manufacturer recommends using buckets to measure the flow. This is acceptable if you only need to do this once to check the installation, but we found that knowing the flow at any moment offers very useful information about the state of the pump and filters, and is very helpful with troubleshooting. We bought a cheap mechanical flow gauge on eBay and plumbed it into the brine output. If we want to know the total flow, we only have to add the production water flow (a quantity you usually know by other means) to it.
With a flow of 300L/h (for a tiny watermaker) the potential for disaster when a hose gets loose is high. Never run the watermaker unattended: The pump will happily pump the boat full of water until either the ocean is empty, the power is gone, or the boat has sunk, whichever comes first. We therefor installed a small float switch in the bathroom where the watermaker is installed, and wired it in series with the pump relais. If somehow water accumulates under the watermaker, the float switch cuts out the pump. This has already happened in our case: one of the many hose connections worked itself loose due to the pulsing high pressure, and while we were on the boat we didn't notice this until the float switch automatically depowered the pump.
Water from the watermaker tastes quite good, but after lingering for a while in the main tank, it always develops some "tank taste". We were also concerned about tiny amounts of aluminium of our tank dissolving into our drinking water: there is some suspicion that aluminium might be a factor in the development of Alzheimer, and anyway it is not by any means contributing to our health. So we made a habit of collecting some water straight out of the watermaker into bottles to use for drinking. After a while however we wanted to make this procedure somewhat easier so we bought a consumption grade 20L "bottle" with tap valve to use as our "day tank". On eBay we found a cheap little float valve which we installed in the lid. The hose which transports the production water to the main tank has an upward loop, and before this loop we installed a T-junction with a hose to the float valve. So when water is being produced, it favors going to the day tank until its float valve closes and then the water has no other choice than to go "over the bump" to the main tank. This way, we always have automatically fresh water from the watermaker in our day bottle, and since the consumption rate is high compared to its volume, the water never stays very long in this tank. If it would ever develop a tank taste (after a year it still hasn't), the tank would be easy to clean out via the lid, with no chemicals required.
Our Schenker watermaker doesn't require much maintenance. This is our experience after two years of living solely on watermaker water.
Watermaker manufacturers tell you to flush the watermaker after each production run with fresh water. This is quite a waste, especially with low capacity watermakers you will end up spending half an hour worth of production for just flushing the membrane. Many people are claiming though that if you run the watermaker within two days again, this flushing isn't required. We never did this flushing, except when we would not use the watermaker within a two days again (which almost never happened), but we had to clean the membrane once a year. It is unsure if this cleaning could be avoided by flushing after each single use. Even then, we rather clean the membrane once a year than spending many hours of waterproduction (and power) just to avoid this simple procedure.
In two years with approximately 1000hrs of usage, we used four Shurflo pump diaphragms. The Shurflo pump is really the weak spot. We might have had accelerated wear on the Shurflo pumps due to the main unit pistons binding in their cylinders. And that appears to be the other weak spot: Although all our seals hold out fine during these 1000hrs, the seals of the pistons hardened or abbraded, which caused the pistons themselves touching the cylinder walls, causing scratches on both the pistons and cylinder walls and greatly increasing the resistance. We would recommend to carry additional sets of piston seals and checking/lubricating/replacing the seals regularly. The problem presented itself by the pressure going up (which we contributed to membrane fouling which was partly true), a lower flow (which we contributed to Shurflo pump wear which was partly true), and by moaning/squeeky sounds coming out of the main unit between the "beats" which we could not identify (and also not the dealers we consulted).
We regularly swap filters (which is easy thanks to our installation) to prevent smells of decaying plankton and to keep as much flow as possible. We clean the filters with a reverse flow of water, and/or by hanging them on a line over the bow when we are anchored. After drying they are good to go again, and this process can be repeated many times.
Both Spectra and Schenker use an ordinary Shurflo pump. This is just a similar pump used for pressurizing the drinking water system, but has been "boosted" by a higher "cam" ratio and the addition of some cooling fins on its motor. It was originally never designed to run for hours at a time at an elevated pressure, so it is not surprising that these pumps wear out pretty quickly. After a while (your milage might vary) the flow starts to drop, which causes the membrane to foul, resulting in the pressure going up while the water production goes down. A high pressure and low production might indicate that the membrane has fouled; a lack of flow is the main suspect. The higher pressure might give the ignorant user the impression that the pump is working fine, but in reality the higher pressure causes the cam diaphragm to wear even faster, resulting in an even lower flow. A better (and earlier) indication that this downward spiral has started is by observing the sea water flow, which is why we recommend installing a flow gauge! We measured the brine flow to be about 270 L/h when the pump head is new, but it quickly drops to 240 L/h, and once it drops below 200 L/h it is really due for an overhaul. With our Schenker the amount of production water is about 10% of the total flow.
To be honest: After our pump modification we discovered that the main pistons were somewhat binding in the cylinders. This additional resistance might have acccelerated our Shurflo pump wear. We asked other cruisers with Schenkers how long their Shurflo pumps last, most of them reported longer life than in our situation.
Apart from buying the pump from your brand distributor, it is much cheaper to buy the pump from a Shurflo dealer. Watermaker manufacturers appear to try to keep the part numbers and their suppliers a secret, but after some research we found them anyway. These are the Shurflo part numbers (which are for our purpose all the same):
- 8000-943-838 Original pump
- 8008-943-839 Alternative
- 8000-543-238 Alternative
In most cases however, you don't have to replace the entire pump, not even the entire pump head. Usually, only the "cam diaphragm" in the pump head needs to be replaced to get the flow up to specs again. Both Spectra and Schenker use the cam diaphragm with 3.0 ratio wich is sold by Shurflo with the following part number:
Unfortunately, this cam diaphragm is "strategically prized" and is quite expensive compared to the pump as a whole. With about 80 USD it is still a considerate saving compared to replacing the entire pump though. If you are sailing in the Caribbean, the shop "Electec" on Sint Maarten has these cam diaphragms on stock.
Apart from wearing out pretty fast, our impression was that the pump, even in good condition, was always marginal at best. The promised 30 L/h was never reached, because various factors were negatively influencing the water flow:
- We installed the pump above the water line, for safety and maintenance reasons. This causes a negative pressure on the pump inlet, so it has to work harder, and the negative pressure often caused some air bubbles to appear (cavitation and micro leaks).
- Despite using very thick cables, we still lost some voltage from the batteries to the pump, and just half a Volt makes quite a difference for these Shurflo pumps. Our "technical wet room" was unfortunately quite a distance from our power house (batteries and solar chargers) so there was no easy solution possible.
- Despite cleaning the filters often, on average the filters were more restricting than brand new filters. We just can't use new filters every day...
It was quite annoying to have to replace the cam diaphragm twice a year, and never reaching a satistying water production for a long time. Beside this maintenance, the pump was very noisy. Because of its design, it was vibrating on all axis and it proved to be impossible to prevent the vibrations to broadcast an audible hum through the entire ship. Even the hoses were vibrating due to the pulsing nature of the pump output. It didn't take long before we really started to hate this Shurflo pump.
At some point, rumour reached us that Spectra had started to use a different kind of pump on its newer high capacity watermaker models. It took a bit of research to find out more about it: The brand of these pumps is Procon.
Procon pumps are high quality rotary vane pumps, available in a myriad of configurations: three different housing materials, three different housing sizes, with or without overpressure valve, with a direct drive or magnetic drive, in any flow and pressure configuration, and a dozen different motors to drive the pump.
The choice of material was easy: only stainless steel is suitable for seawater. For pressure I aimed at about 8 bars, with the overpressure valve set at 12 bars, but the flow was a bit of a puzzle; Schenker doesn't specify this in their documentation. Since the best total flow I could get with the original pump was about 300L/h, with still less production than should be possible, I wanted somewhat more. The next step up was 330L/h. I selected silicium carbide vanes for extra duration. The magnetic drive was more expensive, failure prone, and the advantage wasn't completely clear: we understood that it prevents leakage into the motor in case of a seal failure, but if the pump motor is mounted "upside down" you won't have this problem anyway.
Because of the inevitable voltage loss associated with long cable runs on 12V, I wanted the new motor to run on AC power. We have a powerful but efficient inverter and the loss should be less than by driving the pump directly from our 12V DC net.
It was calculated that a 1/3 HP motor should be sufficient to drive the pump, so that is what I ordered, but for some reason when I received the package (several months late due to Covid19) there was a 1/2HP motor in it. (After consulting with the supplier I was offered a replacement 1/3 HP motor free of charge!) The motor is what is called an induction (asynchronous) type: brushless and the RPM is coupled to the AC frequency, resulting in 1450 RPM on 50 Hz, 1725 RPM on 60 Hz.
Variable frequency drive
The speed of the motor is determined by the AC frequency and practically independent of load and voltage. The pump and motor are dimensioned to operate just about right on our 50Hz inverter output, but I preferred to be able to control the speed by use of a so called variable frequency drive, with direct drive from the inverter output as a fall back option. The VFD also incorporates features like "slow start" and protection of the motor.
We selected a 400W (1/2 HP) version, with a single phase input, and a single phase output. Note that the latter is a rare option, but absoletely necessary to run a single phase motor.
Testing the new setup
The first test was to run the pump directly from our inverter. When I switched the pump on for a few seconds, I thought that the pump was failing to prime, as I heard no other sound than a faint hum. To my surprise I saw a steady flow of water coming out of the brine hose! We couldn't believe how silent that pump is!
We measured the production water output and we saw 35 L/h coming out of our good old Schenker. This was more than it had ever done before! How about the water quality? The TDS with the previous pump was a little over 400 ppm, but with the new pump it was down to 234 ppm. This can be explained by the increased flow which flushes the salt residu much faster away than before, so the membrane sees a lower salinity at its input side.
The only problem was that the current now had climbed to 30 Amps, three times more than before, and the pump motor was expelling a lot of heat, noticably heating up the small bathroom pretty fast. (This problem has now been solved, see below.) This was a bit odd, at least not expected. Anyway, it was time to try the Variable Frequency Drive (VFD). I had some trouble getting it going, but after consulting with the manufacturer and loading a different set of parameters in its configuration, it worked like a champ. To my surprise, at the same 50Hz as our inverter, the current was now down to about 20 Amps and the motor ran a lot cooler. Somehow the motor and our inverter are not a good match. A possible explanation is that the inverter is not built specifically to drive induction motors; it uses a lot of energy trying to make a nice "sinus", but the motor doesn't care too much about wave form and actually tries to distort it quite a bit with its very inductive load. The VFD is built to drive exactly this kind of motor so it can be much more efficient.
Because we could now regulate the flow independend of pressure it was easier to troubleshoot and we discovered that our main Schenker unit had a problem: the piston seals were dried out and causing a lot of resistance. After replacing the seals, polishing and lubricating the pistons and cylinder walls, the pressure dropped and the current immediately went down to 14 Amps! In hindsight, this undetected problem might have accelerated our Shurflo pump wear.
Anyway, the power consumption problem was solved. A current of 14A is for us acceptable: it is a bit higher than with the Shurflo pump, but the output of the watermaker is almost twice as high. Even then, we don't care if the current consumption is a tad higher: the absence of noise and the lower maintenance requirement make it a good deal. (Note: on 60 L/h models the current consumption wouldn't increase this much, see below).
Although we were happy with the 35L/h, the pressure on the watermaker was a bit high with 8 bars. After dialing a frequency of 45Hz, we got the pressure down to 7 bars while the production was still a little bit over 30L/h. With an even lower frequency, the pressure went down even more but so did also the production. 45Hz appeared to be the sweet point for our setup.
We are very happy with the Procon pump:
- It is extremely silent. The only thing we hear outside the bathroom is the "beat" of the pressure amplifier (Clark pump).
- The water production is higher than ever before.
- Water quality improved, TDS got significantly lower due to the improved flow.
- No more problems with voltage loss, drop of RPM when a cloud blocks the sun, etc.
- Very stable flow. The flow is strongly coupled to RPM (each rotation of the rotor delivers a fixed volume) and if the filter would foul, the RPM and thus flow would remain the same, but the pressure (and power consumption) would go up.
- We can adjust the flow with the VFD to maintain the ideal pressure for membrane and seawater temperature condition.
- It is too short to tell, but the Procon pump should last much longer than the Shurflo pump. There are no parts in the motor to wear out, other than the bearings. The vanes in the pump are coated with silicium carbide and should last quite a long time, and can be replaced when worn out.
In hindsight we should have selected a pump with a lower flow rate. Instead of "330 L/h" the 300 L/h option would have been a better match. We now run the pump at 45 Hz, which is 10% less than our inverter frequency. With the 300L/h pump we could run it at exactly 50 Hz to reach the same flow, which means that we could run it straight from the inverter. Still we like to have the VFD in between, so we can adjust the RPM and also benefit from the (not yet conclusively explained) higher efficiency.
Further thoughts, ideas and experiments
The only disadvantage of our current Procon setup is the power requirement, which is a bit higher than the Shurflo pump (disregarding the much higher water output). We could possibly save some power by:
- using a 1/3 Hp motor instead of the 1/2 Hp motor we received.
- using a 3-phase motor instead of a single phase motor. 3-phase motors have a higher efficiency than single phase motors. But in this case you will sacrifice some redundancy because you can not run the motor anymore from just an inverter, you will absolutely need a VFD. Of course you can buy a spare VFD to regain this redundancy.
- upgrading to 60 L/h. When dissecting the Procon datasheets, we noticed that when a pump with a higher flow is selected, the required power doesn't go up with the same ratio. A pump with double the flow would only require 50% more power to drive it. To produce 60 L/h about 21 Amps would be required, which would be comparable to the original Shurflo setup. But to upgrade to 60 L/h we would have to install an additional membrane (like on the Schenker 50 model) or to replace it by a single larger membrane. Each 21" membrane can do about 30 L/h, a 40" membrane would be good for 60 L/h. Interestingly, 40" membranes are only 10% more expensive than the 21" membrane our Schenker is using and you will need only one.
Ordering and configuration notes
As an European, I have a 50 Hz inverter. If you want to run the system at 60 Hz, you need a pump with a lower flow to make up for the higher RPM. Or you use a VFD and just select 50 Hz or whatever frequency gives you the right flow.
Replacing the Shurflo pump of a 30L/h model
Procon pump: The model number of mine is 1W3110A11BA928, but this is for the 330 L/h model. The 300 L/h model (1W3100A11BA928) would actually be a better match if you want to run it straight from a 50 Hz inverter. This pump is stainless steel, has silicium carbide vanes for extra durability, direct drive, and the safety valve set at 175 PSI. I ordered this pump from pacificool.co.nz, they were very helpful.
Where to order a Procon pump
Initially it appeared nearly impossible to order a pump. Because of our Dutch origin, we were forwarded to the EU distributor (Standex) in Ireland. After multiple emails we were finally sent a lenghty questionaire and after returning it we didn't hear anything anymore. We also contacted the NZ dealer pacificool.co.nz and after explaining that we were sailing in the South Pacific they were entitled to accept our order. Pacificool was extremely helpful, answering promptly, forwarding technical questions to the manufacturers, supplying datasheets, but also very helpful in getting the pump as soon as possible to Tahiti for us to pickup.
AC Motor: 1/3 HP would be sufficient. I got mine from pacificool.co.nz together with the pump. There are several motors available, you need one with frame 48YZ (or frame 56C with adapter), I would suggest to get advice from the vendor. I have a Nidec 928, which can run on 115V and 230V, single phase. This motor has a startup winding which will automatically disconnect by a centrifugal switch when the RPM passes about 1150, which equals 40 Hz. I found this a bit too close to my favorite operating frequency of 45 Hz, so I stretched the springs a little to let it open at a lower RPM. It can now go down to 30 Hz without the centrifugal switch closing to the startup setting. You probably don't need to do this if you order the 300 L/h pump which requires 50 Hz.
Variable Frequency Drive: This is optional. With a VFD you can adjust the RPM, you will have soft start, and efficiency might be better. We have model GK3000-1S0004 from www.ato.com. There is some info in the manual about modifications required on a single phase motor to make it run, but you can ignore that. Just connect the motor over the V and W terminals.
The VFD has quite an extensive menu, the following options are changed from the defaults (you might need to change a few things if you run from 115AC instead of 230AC, or use a different motor): P0.06=60, P0.07=60, P0.08=240, P0.09=4, P0.12=6, P0.17=2, P0.18=1, P0.19=60, P0.20=45, P0.22=2.
Replacing the Shurflo pump of a 60L/h model
I haven't done this, but since this model uses two Shurflo pumps, it is a good guess you will just need to double the flow to 600 L/h. A Procon pump model 3 at 70Hz could deliver this, but a model 5 pump can do this at 50 Hz. You will need a 1/2 HP motor. Consult the datasheets or ask Procon for advice.
Upgrading a 30L/h model to 60L/h
This will double your production for only 50% more power consumption. You will have to either add another 21" membrane, or replace it by a single 40" membrane. The latter has the advantage that in the future it is much cheaper to replace the single 40" membrane than to replace two 21" membranes. In either case, you will have to replace some high pressure hoses to make everything fit. For the pump and motor I refer to the previous paragraph.