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.

Cheers,

Paul

Thanks for the informative posts.

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.

Cheers,

Paul