Watermaker sizing

[Talbot]

..............I've come to the conclusion to go for the biggest I can fit/afford and run it for less time than a smaller one. My key consideration was to have the capacity of the watermaker match the running time of the genny so that I got maximum use of the genny at around 75% load. ...........

I could go much larger if I was relying on a 240v AC pump and the generator, but in reality you are basing your water on a single point of power failure (the generator). Furthermore, if I can make water for an hour without having to run the generator, I would be much happier - thus the sizing of the solar at the max I could fit, and wanting a watermaker that can run at around 50% of the rated watts of the solar (probably a more realistic load for those panels!) Thus I think I am going to decide on the Schenker Smart 60 or the modular 60, depending on which fits best in the available space.

Thanks to all who have contributed.

 

[geem]

I could go much larger if I was relying on a 240v AC pump and the generator, but in reality you are basing your water on a single point of power failure (the generator). Furthermore, if I can make water for an hour without having to run the generator, I would be much happier - thus the sizing of the solar at the max I could fit, and wanting a watermaker that can run at around 50% of the rated watts of the solar (probably a more realistic load for those panels!) Thus I think I am going to decide on the Schenker Smart 60 or the modular 60, depending on which fits best in the available space.

Thanks to all who have contributed.

I dont really think running a watermaker for an hour is practical. You tend to need to run them intially until they start to produce good quality water. This usually takes a few minutes. Some people suggest running them to drain for as long as 20 minutes on start up. Once you have finished making water it is good practise to run some of the clean RO water back into the membrane to keep clean fresh water rather than sea water in the membrane. In reality you would probably want to run the watermaker for at least a couple of hours each time allowing for the time required at the begining and end of the cycle.
Dont forget in addition to the high pressure pump you will have the low pressure feed pump that also uses amps. If you are looking for high output from DC pumps they will tend to hammer the batteries unless you have lots of solar/wind generation and a large battery bank.
With regard to single point of failure, this doesnt apply if you can install a Variable Speed Drive(VSD). My 240v A/C motor is 1500w. my theoretical maximum output of the main engine alternator is 1480w. In reality I will not be supplying this from the engine Alternator as it spends lots of time at low revs, but in a back up situation, if my generator fails I can drop the speed of the RO pump motor using the VSD to match the engine alternator output so we dont see a theoretical discharge. We also have a towed generator, 320w of solar and a wind generator as well as the 4.5KVA genset. There is a limit on how slow you can run the RO pump motor if it is not rated for low speed. This depends on the motor selected but most will turn down to 70-50% of design speed without issue. you need to keep the cables between the VSD and motor short.
drop the pump speed to 50% and you use only 12.5% of the current but you also make lot less water.

 

[Dave100456]

Talbot ... "I could go much larger if I was relying on a 240v AC pump and the generator, but in reality you are basing your water on a single point of power failure (the generator)."

Alternatively an inverter will run RO system and ship's engine will charge batts if generator packs in.

 

[Marsupial]

Running for 1 hour a day is as others have said not a practical solution. Running every day I still need to wait 5 to 20 minutes for the product water to be "acceptable" it depends on what is in the sea, and yes it varies.

Another factor to consider is the rated output of the membrane, usually quoted with a sea temp of 25 deg C, the output drops considerably as the temperature drops by up to 50% (recoverable when sea temp rises) and it drops a further 10% as the membrane ages.

Low volume high pressure 12v systems need more product water to clean the membrane, if you run for short periods you will use more of what you make (as a proportion) to clean the system.

High volume high pressure systems don't have this problem membranes are constantly scoured.

There will always be more than one single point of failure. Lets start with the pressure pump and work back!

 

[AntarcticPilot]

I don't have a watermaker, and have no plans to install one. However, I follow threads like this one as if we ever do set off into the blue, one would undoubtedly be handy. Therefore, I have done some reading on the systems!

The thing I don't understand is, why aren't engine driven systems more popular? They avoid the inefficiencies of using electrical power generated by the engine, have fewer limits on power available and appear to be mechanically simpler (only need a pump, not a pump plus an electric motor). Of course, it means that the high pressure pump must be installed at a location where it can be driven from the engine, but surely on most boats that isn't a problem. Even on my Moody 31, I am sure I could find a suitable location where it could be driven from the propellor shaft by a belt drive. It wouldn't be excessively difficult to run from the engine itself, avoiding the need to have the engine in gear. Operation would be straightforward - engage it when under way, for as long as necessary, without any concerns about electrical generation and storage capacity.

 

[Marsupial]

I don't have a watermaker, and have no plans to install one. However, I follow threads like this one as if we ever do set off into the blue, one would undoubtedly be handy. Therefore, I have done some reading on the systems!

The thing I don't understand is, why aren't engine driven systems more popular? They avoid the inefficiencies of using electrical power generated by the engine, have fewer limits on power available and appear to be mechanically simpler (only need a pump, not a pump plus an electric motor). Of course, it means that the high pressure pump must be installed at a location where it can be driven from the engine, but surely on most boats that isn't a problem. Even on my Moody 31, I am sure I could find a suitable location where it could be driven from the propellor shaft by a belt drive. It wouldn't be excessively difficult to run from the engine itself, avoiding the need to have the engine in gear. Operation would be straightforward - engage it when under way, for as long as necessary, without any concerns about electrical generation and storage capacity.

believe it or not its all about economy, if you have a sail boat then when you are on passage you will probably be under sail - we are. so why run a 56hp engine to do a 6hp job? obviously if you have a mobo its not as critical but most mobos with a water maker also have air con and at least a 6KVA genny.

for us the genny and the water maker make for more comfortable cruising

 

[rivonia]

believe it or not its all about economy, if you have a sail boat then when you are on passage you will probably be under sail - we are. so why run a 56hp engine to do a 6hp job? obviously if you have a mobo its not as critical but most mobos with a water maker also have air con and at least a 6KVA genny.

for us the genny and the water maker make for more comfortable cruising

+1

 

[temptress]

Some approximate numbers which others may add to or challenge:

.....

Running the watermaker for a couple of hours every few days seems reasonable, so watermaking capacity per hour = daily usage could be a good starting point.

Watermakers use about 1Ah at 12V for 1 litre of water, some a bit less (they claim), many a bit more. ....

The last point is that they produce a lot less than the rated amount when the water is colder than 25C, at 15C maybe only 40% of nominal output.

For what it's worth, I think 25 - 30 litres nominal capacity per hour is enough for us, sailing 2 handed.

I've often seen this 1AH for one litre figure stated however I think this is out of date. On Temptress the water maker uses 8 amps at 12v. Measured in Scotland we were getting 22.5 ltrs/hour, Down here in warmer waters we are getting closer to 27 ltrs/hour (Manufactures stated capacity is 30 ltrs/hour). The washing machine will be the killer. we just got rid of our washing machine and went back to hand washing - uses much much less water.

 

[Monique]

Planning to get an Amel Super Maramu as my next boat. They have 1000 liters water plus a 60 ltr/hr water maker. Genset too.

My ex boat had a 60ltr/hr and 600 liter water tank. Needed to run the water maker every day for 3 hours when crew was mostly female. Genset was a god send.

Plan for DC power = choice of batteries or genset.

 

[timchapman]

Fitting a watermaker and trying to size it for my needs - I recognise that that means different things for different people, but some opinions would help.

I will be single or double handed. I have 555 watts of solar, and a generator (but would prefer to use the available solar only for making water) I also have a washing machine.

I am looking at the Schenka smart 30 or 60 (30 or 60 litres per hour) and trying to decide on the right size?

Don't underestimate the noise a watermaker makes, in my case louder than the genny. Go big and run less frequently. Here in the Caribbean a lot of the charter fleets use EchoTec for their reliability, (no electronics), they are also less expensive than other common offerings.
We consume about 30l a day which is comfortably supplied every 3 or 4 days by a 240v Echotec, 70-80l / hour.

 

[geem]

Don't underestimate the noise a watermaker makes, in my case louder than the genny. Go big and run less frequently. Here in the Caribbean a lot of the charter fleets use EchoTec for their reliability, (no electronics), they are also less expensive than other common offerings.
We consume about 30l a day which is comfortably supplied every 3 or 4 days by a 240v Echotec, 70-80l / hour.

+1

 

[Talbot]

.........We consume about 30l a day which is comfortably supplied every 3 or 4 days by a 240v Echotec, 70-80l / hour.

Looked and was quoted on a echotec, but the Schenker 60 meshes better with my available power.

 

[timchapman]

I don't have a watermaker, and have no plans to install one. However, I follow threads like this one as if we ever do set off into the blue, one would undoubtedly be handy. Therefore, I have done some reading on the systems!

The thing I don't understand is, why aren't engine driven systems more popular? They avoid the inefficiencies of using electrical power generated by the engine, have fewer limits on power available and appear to be mechanically simpler (only need a pump, not a pump plus an electric motor). Of course, it means that the high pressure pump must be installed at a location where it can be driven from the engine, but surely on most boats that isn't a problem. Even on my Moody 31, I am sure I could find a suitable location where it could be driven from the propellor shaft by a belt drive. It wouldn't be excessively difficult to run from the engine itself, avoiding the need to have the engine in gear. Operation would be straightforward - engage it when under way, for as long as necessary, without any concerns about electrical generation and storage capacity.

I think in most cases it's hard to install these pumps in yachts due to lack of space, also you are restricted to running the engine at around 2-3l of diesel an hour; a small generator uses a fraction of that. Most watermarking is done at anchor where you wouldn't want to run the engine. Might be useful in a motor boat or a yacht that spends a lot of time under power.

 

[damsis44]

Just to say thanks to all contributors, this thread is really informative. For us, space is an issue, but having a 6-year old on board, we are always watching water consumption and are considering a water-maker in the future. However our immediate focus is water conservation so we're looking at installing a foot pump and disconnecting the electric pump!

 

[Talbot]

Just to say thanks to all contributors, ..................

I am a bit anal about researching problems and analysing the optimum solution (probably cause I have been paid to do this in the past !). The size of watermaker can reasonably be established by number of crew and method of operating, but the selection of model and 240AC versus 12DC needs to be matched to the available power, the intended power charging regime, and the available space.

There is no one size fits all.

The advantage of using a forum like this, is to compare other solutions and compare empirically with your own answers.

Water conservation is a good concept, but can be taken too far, and if you are young and ina much hotter climate than you are used to, dehydration can be a serious problem.

As for running out of ice for the evening drink ................

 

[Jamesuk]

Get the biggest you can afford without having to alter your current set up.

I would say a minimum of 50litres per hour is a golden start point. At a very clean marina I would produce water as the marina water left deposits on the boat at wash down and in short, if it looks awful on deck what will it do in the tanks.

50litre per hour. Imagine carrying that to the boat each day from the shop. 10x 5L bottles. That's a lot of water.

Remember to factor in the price of bottled water for the boat when buying it as you will no longer need to worry about buying bottled water. ;-)

We have a substantial watermaker for the boat I'm on too much but amazing. Especially when we are in a clean marina to run it for no diesel cost.

Just remember that you never need to clean it if you use it all the time ;-) 5years minimum on the membranes

 

[Resolution]

. For us, space is an issue,

I thought it would be for me as well, on my 42 ft yacht. But last year I had a 50litre/hr Parker Racor fitted by the lads down at Landau UK in Swanwick and it is amazing how little storage space has been taken up. The primary filters fitted in at the back of a locker in the aft cabin, under a sink unit, the high pressure pump slotted in the engine bay, and the big tube with the membrane has fitted in behind the saloon seating in an otherwise useless bit of space. Lots of piping of course, and I must try to print up a good service guide this winter, but all in all a wonderful job of hiding it all away.

 

[swansail]

Speak to Jim at Sailfish Marine in Southampton - he rebuilt our Spectra water maker unit and gave us loads of free advice on refitting it. He often has second hand units for sale also.