Building Logs

Power to burn

Posted by Paul

One of the most important systems on our boat will be our electricity supply. Because we intend to live aboard and we intend to try to maintain some of the lifestyle we like now except on a more mobile platform. We spend a lot of time watching tv and on the internet. We earn our living on our laptops often while watching tv. TV is going to be an issue on the boat because of reception in many places, but we are not too concerned about that. We often watch tv via a download or dvd of the entire season of the shows we like. We rarely if ever watch commercial free to air tv any more. We hate the ads, hate the fact that the tv stations run off their schedules, hate the fact that often tv channels delay episodes or show them edited or out of order, so basically we just hate mainstream media. So we watch what we watch at our times. Most modern tv’s now accept a usb stick to play media files, so via our computers we have ample opportunity to watch what we want when we want it, regardless of where we are, even in the middle of the Pacific Ocean. And I am a bit like a gold fish with some of the stuff I like, such as Law and Order or CSI. I watch repeats and have mostly forgot the plot line or outcome so its like watching a new episode. In fact I prefer the older episodes. After a while, writers run out of reasonable or plausible plots, so earlier episodes are more believable. But I digress.

Whilst being on a boat in beautiful locations will probably mean we have other interesting things to do (I will have to replace all the time I have spent building with other pass times!!) we will still love watching tv and surfing the net especially in the evenings. And that, on top of the other requirements of life aboard a boat such as the fridge, the lights (LED’s throughout so they run at a fraction of the power of old incandescents) will require electricity. And we will need to generate and store our own most of the time. We are not trying to escape civilisation and from time to time we think we will like being in marinas, so we will have shore power. And who knows, we may even find a marina we like so much we want to make it our base and “live” there. But most of the time we will probably be mobile and on mooring buoys or the anchor. And for that we need to have our own power.

Cats are very weight sensitive, and traditional power generating and storage is very heavy. We intend on a very big solar panel array on our massive roof (10 x 100 watt monocrystaline panels) connected to (to start with) 400 amp hours of LifePo4 batteries. For those that dont understand this new technology let me explain what I have found.

Battery Bank

Lead acid battery technology is about 150 years old now. And apart from some different ways to manufacture them, not much has changed with them. Without going into too much detail (which I dont know) the car battery you know has lead cells and an sulphuric acid catalyst that originates as water and is turned into acid and back again in the process, that reacts with the lead plates to take a positive ion from one plate to another in the form of an electric current. Each cell in the battery is about 2 volts per cell hence 6 cells (often marked by 6 points where the cell needs to be topped up with water) = 12v battery. The size of the cell denotes how much charge (life in time to discharge) the battery is. So a battery could be 100 amp hours which theoretically could run a 1 Amp item for 100 hours or a 100 amp item for an hour. I say theoretically because a battery is rarely if ever at full charge after its manufacture. About 20 years ago the electrolyte (water to acid) changed from liquid to gel in some batteries to improve their ability to work somewhat inverted (often needed on a yacht) without leaking and also self venting batteries that released the gasses created (hydrogen) but not the electrolyte meaning at least you did not have to keep them topped up with water, making them “maintenance free”. (Another technology is the Absorbed glass mat battery but again they are still basically lead acid technology, the glass mat just holds the electrolyte so it cannot spill). But whilst you may not have needed to physically maintain them you still needed to manage them in order to prolong their life. The biggest maintenance issue with Lead Acid batteries is that they cannot be continually used past half their storage capacity. They can of course but each time you do it you shorten their ability to keep doing it, so you shorten their life cycle.

And the final nail in the lead acid battery coffin is they are huge in both size and weight. A 100amp hour deep cycle (ability to be deeply discharged on a regular basis, as opposed to a starting battery that only ever uses a fraction of its storage before being recharged but has the ability to output a lot of power very fast, needed to turn over a car to start it) gel battery is about 35kgs and is about 300mm x 200mm x 200mm meaning 4 of them would weigh in at 140 kilograms, 300mm x 200mm x 800mm (0.05 of a cubic meter). Its the 140kgs that is the killer on a cat. And because of the inability to drain them past 50% you effectively have a 200 amp hour usable bank.

One final point before going on to the advantages of Lithium chemistry. Lead acid batteries are resistive to charge. So you need high amp chargers to push the power into them, and once you get to about 90% charge they resist even harder meaning that the effort required to top them up past 90% gets harder and almost impossible beyond 95%. You expend more energy than the last 5% trying. So in effect, each 100 amp hours really only has 45 amp hours of regularly usable power. Another 50% is there for emergencies, but you ought not be using it on a regular basis if you want to prolong their life. Lead acid battery manufacturers dont like discussing life cycles and site the multitude of variables that can effect the outcome as their reason, which is fair enough, but the average seems to be about 500-700 full cycles. Which usually means about 5-7 years on a boat, assuming two cycles per week. There are other minor disadvantages with lead acid, such as shelf life (they self drain if not recharged within a certain time) but this ought not be a disadvantage if you are living aboard or have solar panels and a decent solar regulator, unless you take a break from the boat, which happens. Its not all doom and gloom for the lead acid battery. The one area they have it over other battery forms is initial purchase price. But even that is a false economy as I will show. I guess the one redeeming feature they have is that being very old technology people trust it and they distrust new technology, but new is only new for a while, then it becomes old or mature so lead acid batteries days are numbered.

LifePo4 is a Lithium iron phosphate battery, invented in the mid 90’s at MIT (the iron is the cathode). Other forms use cobalt, nickel or magnesium as the cathodes and they have some advantages over iron but also some disastrous instability issues (they explode!!!). Lithium iron chemistry, despite being slightly less energy dense than other lithium chemistries, is much more stable and safe and are now found in electric cars such as the Prius or the Volt as examples and is a far lighter and smaller chemistry than lead acid.


The most striking advantage for use in a vehicle is weight. Unlike lead acid (2.1v per cell for a total of 12.6v) a cell of Lithium is 3.6v. So 4 of them equates to 14.4v or slightly higher than an equivalent  fully charged 12v battery. So in layman terms, it means you start at a slightly higher voltage, and with lead acid batteries, as the power of a battery drops so too does the voltage and below a certain voltage the power is unusable (many items wont work below a certain voltage). Not only do Lithium retain full voltage all the way down to 5% of their storage capacity, the higher voltage you start with the more power you have before the voltage drops (through too long a wire travel) to unusable levels (within the tolerance of the appliance, you cannot pump 24v into some items for example).

While we are on the subject of 24v, the reason some people still use it is because over a given length the wire gauge required is much less to keep the voltage drop in check and that drop is disproportionately less on higher voltage, for example if you lost 1.2v over the length of wire, it represents 10% of your voltage at 12v but only 5% at 24v. Heavier gauge wire is not only heavier but more expensive. Long travels of wire reduce voltage through resistance, again the higher the voltage you start with the higher it emerges at the end of its wire, meaning you dont end up with unusable voltages at the appliance end that can happen with lead acid over long wire runs with inappropriate wiring (the thicker the wire gauge the better it handles or minimises voltage drop). Batteries compensate for voltage drop with raised amps, meaning the same appliance draws on more of the reserve of the battery at lower voltages which in turn means the battery is drained even faster. Its a viscous circle.

Back to the size/weight issue. The unit in the pic above (left) is 3.6v at 200 amp hours. It weighs just 5.5kgs and is 275mm high, 182mm wide and 70m deep. 4 of them wired in series gives you 14.4 volts at 200 amp hours and 2 lots of that (pic on the right) wired in parallel gives 400 amp hours. The total size of this bank is 275mm x 182mm x 560mm (or .028 of a cubic meter compared to .05 for lead acid) at 44kgs. So about half the physical size of a lead acid bank but at about 1/3 the weight.

But it gets much better than that. Because of the draw down maintenance issues of lead acid, you only really get full use of half the amp hours. With Lithium it is not only applicable but recommended you periodically drain them to about 5% and recharging them to about 95% is not only easy (less resistance) but much faster. It would not be an exaggeration to say that compared to the resistance of lead acid, lithium’s practically suck the power into them. On shore (or generator) with an appropriate charger you could go from 5% to 95% in a couple of hours compared to all day and night for the equivalent lead acid. So in daily use terms 400ah of lithium is about the same as 700ah of lead acid, which would easily add up to 245kgs compared to 45kgs. 200 kilos saved for the same effective bank size. The boat is about 4000kilos unladen and the payload is about 2000kilos. To give up 10% of it to lead acid seems pretty dumb these days.

Its hard to argue with those facts, but one area that puts people off is the initial purchase price. 400AH of Lithium would usually cost about $3000 at retail compared to about $1600 ($400 per 100AH) for reasonable quality lead acid. So about double the cost. But because you get about triple the charge cycles with Lithium they ought to last 3 times as long making them cheaper in the long run. And given you need nearly double the AH in lead acid that halves the cost of lithium again. And just to add no brainer to the equation, a bunch of us had the importer import us a special order which got us close to wholesale on them and we only paid $500 per 100AH. So I paid $2000 for 400AH only slightly more than the cost of a decent quality lead acid battery. And I get all of the benefits. I imagine, by the time I need to replace these, the volume in production will have dramatically reduced the prices even further to the point that they are as cheap as lead acid at retail. I am usually the person importing stuff for other builders to particpate in the deal, but in the case of the batteries, Mike in Perth, also building the same boat as me so we are getting the masts together as well and probably sails too, has organised a great deal from a local (Perth) importer. It saved me having to import them, the prices were as good as I could find direct from the factories, so a big thanks to Mike for putting the deal together for us all. About 6 of us took advantage of it. 3 building the bi-rig  1230 all with 400ah banks. One guy bought 600ah, in all I think Mike bought about 2000ah between us all. Thanks Mike.

About the only real downside with Lithium, is that you need a slightly more sophisticated solar regulator which makes them a bit more expensive but that aside, its hard to find a compelling argument for sticking with lead acid batteries and only fear of change would keep people from Lithium in my opinion. One other downside is that as much as they are easy to charge (little resistance) they are also more giving of their power so you could melt a spanner if it shorted the battery terminals as an example. They give out a massive load if called upon, big enough to kill you, so you need to take extra precautions when working on them such as rubber gloves to insulate you from them, you have to be super careful with metal tools around them so as not to short them, or where and how you store them so nothing can fall onto the terminals and short them and you require very good circuit breakers to ensure that should an unwanted high load be generated the circuit breaker kicks in. Other than that, there really isnt any known downside.

Power generation

All of that power storage needs to be collected. Our original plan was for about 400 watts of solar and a 400 watt wind gen and a 2kw petrol generator (2000 watts at 12v = 167 amps x 2 hours =  334ah recharged for extreme emergency power. Of course that is theoretical, in reality you lose power at every turn, from the generator output at 240v down to 12v at the battery charger, the reality is you probably get about 80 amps an hour charging at best).

When we first researched the build, solar was at about $10 per watt. So 400 watts was about $4000. It has come down considerably since then, but still not as much in Australia as it has for the rest of the world. In the US, it is about $2 per watt. In Australia it is still at about $3 per watt for standard monocrystaline panels. More on cost in my next blog post. Here is the low down on the current technology. Again, I am no expert, this is just what I have learned from my hours of research.

There are 3 types of solar panel, Amorphous, Monocrystaline and Polycrystalline (also known as multi crystaline). Amorphous panels use a method of printing to create the panel. Cheap to produce but not very effective. Fine if you have unlimited space I guess but not of much use to boat owners due to their poor performance numbers. These cells typically have less than 10% efficiency and 7% is the most common.

So it comes down to mono and poly silicone panels. Mono are made by slicing thin single crystals, poly are made from crystal fragments glued together. The difference in efficiency is minimal and condition dependant. Mono panels are more efficient in general terms, the best panels being 22%, but of course this is also the most expensive at as much as $15 per watt. Most average about 18%. Poly panels are rated about 1% less efficient in normal conditions (so about 17% instead of 18% for a given physical panel size, a difference of about 4-5% less efficient for the same footprint) but they handle high temperatures better than mono and they also handle partial shade and extreme angles of light (they start to work earlier on rising sun and longer into setting sun) than mono panels but again by very fine margins. All panels lose about 2% in the diodes and transfer of the energy to the batteries. So there are 2 measures, cell efficiency and module efficiency. Most mono panels claim about 18% cell efficiency and 16% panel efficiency. So the slight advantages of poly in certain conditions are usually not so advantageous that they counter the overall efficiency advantage of mono. Poly is usually 5% cheaper than mono but because space is such a problem on a boat, I think mono is the best option but its a close run thing.

Because I have found panels at a fantastic price (by importing them myself, more on that in the next post) I have decided to not bother with a wind gen and to put a much bigger solar array on the massive roof we have. A lot of people dont like wind gens because of the noise they make and I dont want to pull into a quiet anchorage and be the one making noise that the other boats anchored there instantly notice. So to compensate for the lost wind generation I will put 10 x 100 watt panels on the roof. 1000 watts. 100 watt panels are rated at 5.8amps per hour. So 10 of them gives just under 60 amps per hour in ideal conditions. Lets say not so ideal conditions are say 20% less then the test conditions the factory used to quote their numbers. And say 6 hours of usable sunlight hours per day. Thats just under 300 amps per day. There is no way we could ever use that much power per day.

Its not very hard to use 100 amp hours per day on a boat though. The fridge would probably use 60AH’s per day alone. 100 watts of LED lights on in the boat for 2 hours uses about 15ah. And while its unlikely we would leave lights on for so long, an anchor light might run at 10 watts but run all night. So with just the fridge and some lighting we are up to 75AH per day. Most LCD (backlit LED) TV’s run at about 20 watts per hour and are already on 12v and use a transformer in home use, so you use a direct power cable to the 12v power on board, and save a little power loss that usually takes place in the transformer lost to heat. But even at 20 watts, that is about 2amps per hour, and say we watch for 4 hours a night that’s another 8 amps and we are 83AH per day. Make toast or boil an electric kettle, turn the radio on during the day, flush the toilet or have a  shower which operates 2 pumps, one to get the water flowing the other to pump the waste water into the holding tank and of course keep the laptops charged and you can see how easy it is to bump up to 100ah per day.

On days where we are sailing there will be various equipment running on power, the chartplotter, the autopilot, the depth sounder, the radar etc. This will require further management and understanding of usage, but under sail there are other power generation methods, such as motor sailing (the alternators run at about 12amps each so 24 amps per hour charging if both are running) or running the generator (if you already have the noise of the motors, running the generator to top up is not a problem. You can even get water powered generators that you can tow behind the boat when sailing. Often cats sail faster than you would like them too, so slowing them down by towing a hydrogen is not always a problem. Buying one is, they are not cheap! But with so much power generation in the panels and so much storage I doubt even on the long passages where the instruments and auto pilot are on non stop that we would have power issues. Wind gens are now quite cheap (you can get a 400 watt model for around $500) so one day, if our power use seems to exceed our ability to generate and store, we could buy one and only deploy it when needed under sail for example, where the noise it makes is not so noticeable, and the other purchase one day might be another 200 amp hours of batteries. But as I say, I doubt I will ever need more than we are setting out with.

Using about a third of our generating solar power per day and only a quarter of the storage power, it means we could have 2 or 3 really poor days (raining and no sun) in a row and still not completely drain our power storage. If this occurred we would run the generator to top up the batteries. As I said, in one hour we would put about a third of our storage back in. But I dont anticipate that will happen often. Even on poor days you are still generating some power. So even if we were only getting 20% generation on poor days, that is still 60amps per day and if we use say 100 amps per day, that means a 40 amps per day shortfall, and that would take 9 days before we ran out of power and needed to run the generator. Those sort of numbers make me feel very comfortable about our requirements.

The solar panels arrive next week. Once they do, I will post some pictures and more information on them. The panels are not the usual solar panels (glass tops and aluminium frames, they are semi flexible so they can curve to match the curve of our roof. That makes them a little more expensive than regular panels but as I am importing them, they still come out cheaper than retail for normal panels. Its all very exciting now. So hopefully I will post again next week on the solar panels.

The website seems to be back up and running normally and posting will become more regular again. Dont hesitate to make a comment or drop me an email. A lot of this tech stuff is new to me, so if I have any of my sums wrong, let me know.

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4 thoughts on “Power to burn

  1. phil

    whilst your calculations for solar panels has been remarkably well researched you have missed one very key item were the greatest loss of efficiency is to be found ,, When ever you plug one panel to another even using the best gold terminals the loss per panel is around 5% of panel output , that is assuming THE BEST OF CONNECTORS , now multiply that by the ten panels you are running the loss through this alone is huge, interesting that you choose mono when in your own for and against you found poly to handle heat better with better sun angle property’s , then to compound this problem you go to a flex panel that is getting the sun angle at various angles due to the nature of how you will mount them , a mono will only operate at the least best cell so a cell around the angle slightly away from good light will drag the WHOLE system down to the ability of that worst cell to operate at . Having been in this industry both here and in Germany for 20 years , i find your thinking very flawed , however it is flawed in the same way as the panels ect are sold as these facts are never disclosed , if you saw the REAL figures for this you would just burn your monie and get some warmth
    As you have already ordered your panels , the best advise i will give you is , get the best gold connectors you can source and please try to mount these as flat and on A single plane as is possible , as they are mono try also to get some air flow under the panel as mono is much more susceptible to overheating and takes much longer to reset , think of your bare feet on the deck in midsummer and put the panels in the same situation as far as conductive heat from underneath them . also have a good look at vertical twist wind generators now coming out of Germany , more efficient and more importantly very quite in operation
    All the best
    phil wells

  2. Paul

    Hi Phil,

    thanks for the feedback. I have considered most of the issues you raise and bear in mind I have done so from the perspective of just what seems common sense but without an insiders background information. As with all things on a boat, every decision is a compromise of competing features.

    For example, poly vs mono. The numbers for the manufacturers I researched had a difference in efficiency of mono over poly of about 5%-6%. That is, the poly panel of the same output has a 5% larger footprint. But the advantage in efficiency of poly over mono in the areas of reduction in efficiency at raised temperatures or the advantage of poly over mono at increased angles of light were around 1% each. Even combined (which would not happen because the incidence of increased angle of light happens at dawn or dusk before or after the heat of the sun reaches full effect, but lets agree that in the afternoon although the hottest of the day is past, the heat may be retained in the panel and take longer to dissipate), the advantage of mono at +5% less the advantage of poly at -2% meant the mono still had a relative advantage of +3%. Of course if space was not an issue then the poly might have been the way to go. But space is a super issue on a boat roof or targa bar or davits or bimini etc in fact just 10mm wider might be the difference between fitting or not fitting an extra panel, so say for example you missed on getting the last panel to fit by 50mm (10mm x 5 panels side by side = 50mm more space required) and you can only fit 9 x 100watts instead of 10 x100watts then the loss is much greater than the efficiency gained if you see what I mean. So on that basis it seemed the best choice by a very small and seemingly insignificant margin. In fact the poly panels are about 5% cheaper. But it seems, reading many cruising blogs that everyone chooses mono. I decided that on the basis of popularity that there may be other factors that point to mono as being the better choice. The market leading flexible panels at 10x what we are paying are also mono.

    To mitigate the heat issue I have already decided to mount the panels on ply pads that lift the panel off the cabin roof by about 10mm and open top and bottom so air and water can circulate under the panel. Only the sides of each panel will be in contact with the roof, the solar cells do not extend to the edges by about 30mm each side so this is the area that will contact the pads and hopefully the air flow under the celled area keeps the heat from building in the panel to the same extent that a fully surface mount would create. I want the panel to be as flush to the roof as possible for windage reasons and 10mm is close enough that I can still step on the panel in an emergency (fixing the boom, the panels should flex underfoot by the 10mm and the roof would stop further flex and stop any damage). The curve of the roof is not great under the area that I want to mount the panels, it only slopes about 100mm over 1.3 meters (length of panel) each side (a longer panel would have gone further around the curve so that formed part of the decision process also), but flat panels (with 35mm high frames) would be difficult to fit to the curved roof, be unsightly and possibly dangerous given that the sharp edges of the panels would overhang the curved cabin sides or I would need to build a platform under the panels to fair this overhang back into the roof shape and also perhaps cause problems in super high winds and be able to lift free as a result. All of this for flat panels results in more work, difficult to achieve and changing the profile of the boat roof, all of which also contributed to the choice process. So curved panels solve these problems. At 10am through 2pm when the sun goes directly overhead in the tropics (more so the closer you get to the equator) the full panel will still be receiving the suns light but as the sun lowers onto the horizon, assuming the boat is pointing in the right direction (side on to the setting or rising sun) then the curved part of the panel is actually at a better angle than a flat panel would be, its true at this point that the poly would have the slight advantage but this time is less efficient anyway, so I felt it better to take better advantage of the superior generating time of 10-2 rather than have the slight advantage at dawn and dusk.

    I should also add that I am splitting my solar array into 2 separate and independent banks, one to port one to starboard of the centre line. I have 2 reasons for this. At dawn and dusk assuming I have the boat facing in the right way, one side will be advantaged while the other not so, so rather than drag it all down to the least, I will have one side generating via its own wiring down to its own 40 amp mppt solar regulator (rather than to one 80amp model). Another reason for this is to keep the wiring size down to a manageable size and voltage loss as it is a 10 meter journey to the regulators from the panels and also for redundancy so that should one regulator fail I still have another working well. I would appreciate your comments on this idea.

    Its also true that poly handle partial shading better than mono, but again by very small margin and this was part of the reason for having multiple smaller panels rather than less of larger output panels so that if one panel is shaded by the mast the others and more of them, will continue to enjoy unshaded light.

    Also on the issue of more small panels vs larger panels. One of the manufacturers we evaluated made a 180w panel and the factory we eventually settled on made a 120w panel but the overall dimensions of those panels had some disadvantages. For example, the 180w panel is wider and longer, of course, because the higher wattage is a function of sq meter-age. But the longer length of the panel sent it too far around the curved cabin sides. And alternative was to mount them fore and aft side by side, but again this would have meant that you could only fit 4 x 180w in the space available as the rest is too curved in different directions. The panels can only curve one way, they cannot handle a compound curve (curving left to right as well as top to bottom). So having evaluated all of the options of size and how they could best be mounted on a curved surface the 100 watt panels had the best dimensions to maximise the space available.

    I had not considered the connectors. I had assumed that the connectors that the panels came with would be sufficient. I have instructed the factory to keep the cables that would join each panel to each other as short as possible, but this was more so we did not have to hide or stow excess length of cable. I guess I can change the connectors to better quality.

    I have also looked at vertical axis wind gens, but they seem to have poorer performance than the traditional windmill bladed gens by a factor of about 30%. I have not found one that generates anywhere near the same output (given the same conditions) but if you have a link or a company I would be most interested. Cost of course is also a factor. As is physical size. The vert axis wind gens I have looked at all had drum diameters of 500mm and heights as much as a meter. This is a very difficult size to place on a boat and would usually need to be on a mast, again too big to be easily mounted. Smaller than this would be great, but could their wind cups have the torque to turn an adequately sized turbine to generate worthwhile output, anything less than 200w is hardly worth the effort I would have thought but am happy to stand corrected. As it was with the panels we settled on, we could have bought more efficient panels (ours are rated at 18% but there is a brand here quoting 22%) but they are 10x the cost. I could afford one of their 100watt panels for the same cost of 10 that I have ordered. So it seems a better equation to get 1000w at 18% than 100 watts at 22% wouldnt you agree? The same would be true of a good vert axis wind gen, if it costs thousands of dollars, it may be efficient and it may be quiet but I couldnt afford it.

    I hope it does not sound like I am debating your input, just letting you (and other readers know) that the decisions made were as a result of trading off one advantage to another including cost. Please keep the comments coming as I really do appreciate them. I had hoped I had covered over every possible contingency and changing out the connectors is one I am now going to research.



  3. Brian Timpe

    My other power need will be water generation, as in creating water.

    Thanks for the informative posts.

  4. Paul

    Hi Brian, Thanks for reading the blog. I am by no means an expert, most of what I post I have researched via the internet and mostly other forums.

    Regarding water making, I will eventually install a RO watermaker and have been discussing whether I will make my own. A friend also building the same boat as I am is doing so and has sent me instructions on how he is doing it. It seems that many things are becoming demystified and the internet is filled with instructions on how to make things ourselves that were hitherto the province of experts. And of course at a considerable price discount to the experts. I am told you can make a system capable of at least 50 litres per hour for well less than $2000 (the 40 inch membranes should be able to handle about 80 litres per hour production each if I understand correctly).

    It seems that a reverse osmosis desalinator is quite simple really. You pass water under high pressure (actually quite low as high pressures go, only about 800kpi which I am informed is not really that high) past (not through as I originally thought) a membrane designed to trap salt but let water pass through and these are readily available and interchangeable for any brand or design of desalinator, and on the other side of the membrane fresh water is passing at normal pressure, and apparently this is a well know principle in nature that the high pressure wants to equalise with the normal pressure and it is this “normalisation of pressure” that causes the salt water to want to migrate through the membrane, not being forced through it under the pressure by the pump itself. As a result, some water (only about 10%) passes through the membrane and comes out the other side as fresh water, the rest carries on past the membrane and out of the boat, taking with it the salt molecules that were left behind by the water that did pass through. The fresh “product” water carries on through the middle of the membrane and usually on to your holding tank or to a vessel to store it. So you need to run 2 pumps for a watermaker to work, one to get the salt water to go through the system the other to pressurise the system to the 800kpi.

    I am informed that whilst a 12v pressure pump will work, higher voltage pressure pumps work better or more efficiently so I may end up with (as my friend has) a 240v system and use my inverter to power it, apparently the losses through the inverter are less than the efficiency advantage of the higher voltage pumps. It is also why many people have direct drive (a belt from their diesel engine) watermakers, because the 12v is less efficient. I guess it becomes an issue of torque, I dont know. I do know that there are many commercial 12v systems so 12v is quite acceptable and works but that higher voltage or direct drives work more efficiently and you would assume that to mean use less power, at least thats what I take it to mean. It could also mean the pump lasts longer and that 12v pumps are pushed to their limit so their lives are short. Dont know with any authority on that one but I have noticed that most 12v systems are at the lower end of output volumes, not that that might be a problem if there are just 2 of you aboard that I will cover in a moment, for example the highest output Katadyn 160 only produces 160 liters per hour whereas other systems start as their lowest output models at 250 liters per hour. My understanding is that some off the shelf made systems that are 12v such as Katadyn use complex recirculation to repower the system to make better use of the otherwise inefficient 12v pumps but these add complexity which adds both cost and things to break, so perhaps keeping it to bare bones simple is the way to go, and as with the boat itself, if you build it and it breaks you should be able to fix it yourself.

    The number of membranes you have will dictate the output levels you can achieve moreso than the water you can pump past the membranes as they are the theoretical limiting factor not the pumps, so in other words, if you could pump more water past the limited membranes you have, it would just flow on past and out again, not increase the production through it.

    On top of the membranes and some high pressure pipes and joins, and the pump to bring the salt water up to the system, you will need a couple of filters to get rid of any imperfections, marine life, sand or grit suspended in the water etc, as the membranes are quite delicate and whilst not prohibitive, they are not cheap, so you want to be sure to prolong their lives. Some systems also have a pressure guage and a salinity guage but these are not essentials. Other than that, I dont think there is much more to them, maybe a relief valve to ensure a blockage somewhere does not result in a build up of pressure to the point you blow your system up, and a series of taps and diversion pipes so you can reject water until the system is working correctly and then direct the water on to you desired location. I am reliably informed that you wont need a pump to get the water from the the watermaker to your tanks even if they are a couple of meters higher than your watermaker and a few meters away, the system pressure will do this for you.

    The other thing I have learned is that desalinators either need to be cycled (started up and used) on a fairly regular and short basis, that is no longer than about every other day, otherwise they start to get algal or other nasties growing in them that fouls the water product and ruins the membrane. If you cant cycle them or plan on longer term leaving the boat, you need to “pickle” the system in a biocide that stops this growth, which would then need to be flushed from it once you recommence use. With this in mind, it is often not a good idea to get the biggest machine you can, because if for example there are only 2 of you on board, and you also have plenty of water storage (for when more people are aboard) you may find if there are only 2 of you aboard and your tanks are full you dont need to run the bigger machine so often and will need to go through the pickling process more often. So it might be more convenient to run a smaller machine every second day for a couple of hours, than a big machine once a week then have to go through all the other functions to keep a rarely used machine in working order. Just a thought to consider.

    Thats about the limit to my current knowledge, of course that will grow some if and when I eventually attempt to build one and of course once I do I will be sure to share the knowledge.

    Hopefully, this chat will encourage other readers that know more about it to post further and we will both learn more.



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