Around two decades ago, watermakers were still a luxury that only a few boat owners could afford. The systems were considered power guzzlers and very vulnerable. Today, however, many yachts are equipped with watermakers, especially in the Mediterranean. The systems have become affordable and reliable - and standards have risen. What could be better than having plenty of fresh water available in a salty environment? And what's more, as many owners calculate when purchasing a watermaker, the system saves money, as fresh water at the dock is often expensive in southern climes.
However, to see the purchase of a watermaker as an economic advantage is a milquetoast calculation, because for the price of a mid-range product (a good 6,000 euros), a full 60,000 litres of water could be stored even in the Caribbean at water costs of around ten cents per litre. For an average boat, that would be around 150 tankfuls. The undisputed advantage, however: The Watermaker creates independence and the luxury of not having to constantly check the water tank level when planning a route.
Functionality
The principle is similar for all water makers: water from the ocean is sucked in via a sea valve and passes through several increasingly fine pre-filter systems. Firstly, a coarse filter, which filters out larger suspended particles, seaweed and other particles so that they do not enter the pump. Then a sediment filter with five micrometres. Some owners also place a coarser filter with 20 micrometres in front of the filter so that the finer filter does not clog up too quickly. After pre-filtering, the seawater is then pressed into a membrane via a high-pressure pump, which filters the salt out of the water using the reverse osmosis principle.
Osmosis itself is known from biology lessons and boat building; it describes the directed flow of molecular particles through a mostly semi-permeable membrane. In nature, osmosis is important for regulating the water balance of living organisms and their cells. The following chemical law is decisive for the desalination of seawater: if two identical liquids, such as pure water, are in a container separated by a membrane, the osmotic pressure on the membrane is the same on both sides. However, if there is salt water on one side with the same fill level, then the pure water exerts a higher osmotic pressure on the membrane. This is because, as there are not only water molecules but also salt molecules on the side with salt water, the pure water wants to compensate for the disadvantage of water molecules. Over time, the level on the side with salt water rises, while the pure water becomes visually less and less. It looks as if the fresh water is flowing into the salt water.
In reality, however, the quantities of water molecules equalise on both sides of the membrane until the number is the same on both sides. As the salt molecules are too large for the pores of the semi-permeable membrane, they remain on the salt water side and the level is higher there. When testing the salt content, it can actually be determined that the salt water has been diluted by the pure water.
In the experiment, osmosis therefore transports pure water into a salt solution, but retains the salt. For the desalination of seawater, the process is therefore used in reverse, as reverse osmosis. For this purpose, the salt water on one side of the membrane is subjected to a pressure that is higher than the pressure created by the osmotic endeavour to balance the concentration. This allows the molecules of the solvent to migrate against their usual osmotic direction of propagation. The water molecules are pushed through the pores of the membrane to the other side and discharged, while the salt remains behind.
The membrane film for the principle of desalination membranes that is used today was developed in the early 1960s. At the centre of the round membrane is a stable water pipe with many pores through which the desalinated water is drained. Starting from this tube, a membrane (about two square metres in area) is wound outwards in a spiral. The membrane is located in a stable tube that can withstand the pressure required to force the salt water through the membrane against the osmotic pressure. The salt water is pressed along the outside of the membrane through the tube. The water molecules move through the membrane into the central tube and are discharged, while salt molecules and impurities remain on the outside of the membrane.
However, to prevent the pores of the membrane from clogging over time or microorganisms (which were able to pass through the pre-filter) from decomposing on it, a large proportion of the salt water is channelled out of the membrane back into the sea and used to remove these impurities. Only the water molecules pass through the membrane. Dirt, bacteria (E. coli) and other impurities are also filtered out, as the pores in the membrane only have a diameter of approximately 0.0005 micrometres, meaning that bacteria (0.2 to 1 micrometre) and viruses (0.02 to 0.2 micrometre) cannot pass through. Only oil (when operating in a harbour) is harmful to the watermaker because it clogs the pores.
Typology
The membranes used are the same for all watermakers. There are three types of watermaker.
Firstly: the small manual survival watermakers, which are only intended to produce enough water for the crew to survive in an emergency, such as the Katadyn Survivor 06 (approx. 970 euros at SVB) and 35. Operation is manual (small hand pump) and laborious. One hour of pumping produces about 0.9 litres of water with the 06 model, and 4.5 litres with the larger 35.
Secondly, the electrically powered watermakers that maintain the water level in the tank during normal use, such as the Spectra Ventura 200 or the Schenker Smart 30, only need to run the device for two or three hours a day to produce 100 litres of water. Ideally when the engine is running, when electricity is being produced in abundance anyway. There are also smaller products available on the market, such as the Katadyn Power Survivor 40E. As the name suggests, however, this is not a comfort-enhancing watermaker, but merely a model that is designed to meet basic water requirements in an emergency without any problems (electric pump). It produces 5.7 litres per hour at a consumption of 4 amps. Even with a single solar panel, it should therefore be possible to produce water in an emergency.
Thirdly: the large, luxurious watermakers with high output, designed to provide almost lavish comfort on board, to shower, run a dishwasher or even rinse the decks with fresh water. They often supply around 160 litres per hour, but then require 230 volts (generator) or a V-belt pump (main engine) to operate. Examples: Echotec 1000 AML-2S (160 litres/hour, 8.6 amps at 230 volts, approx. 5980 euros), Echotec 900 BML-2 (V-belt pump, 150 litres/hour, approx. 5990 euros). The Echotec 780-DML-2 is special: 120 litres at 80 amps (!) and 12 volts (approx. 7020 euros).
When dimensioning the watermaker, it is important to answer the following questions in advance: How much power is required? What purpose should the watermaker serve? Emergency, daily use or luxury? How much space is available on board? Is only a compact unit (Katadyn 40E) an option? A modular system (Schenker, Spectra, Ecotec) or a complete unit? How do I operate the system? Self-sufficient via its own power generation with solar and wind, via the diesel engine or a generator?
Dimensioning
Normally, the watermaker will only ensure independence and provide a daily shower, so a mid-range model will suffice. As a rule of thumb for the dimensioning of a watermaker for normal use as a bivouac, the following guide value applies: for two people, the production of 30 litres per hour.
Of course, it is possible to supply a boat for two people with a 160-litre model; the watermaker is then only started every few days. However, the intervals between start-ups should not be too long. If the system has not been running for more than three days, it should be rinsed with fresh water to prevent bacteria from forming in the membrane due to the salt water. It is important that no chlorine (often added to the fresh water in marinas) gets into the membrane, as this would quickly destroy it. If the watermaker is not used for more than three weeks, the entire system must be rinsed with a chemical solution and preserved. It is therefore a good idea not to choose a watermaker that is too large and to use it regularly.
Assembly
Most systems today are developed in a modular design so that the elements (pump, filter, membranes) can be mounted individually and fit into any small environment on board. The bilge is a good installation location
The bilge is a good installation location (especially for long membranes), but it is not a safe feeling if the seacock and modules are below the waterline and prone to leaks. The suction point should be so far below the waterline that the seacock does not come out of the water and draw air even in rough seas, because then the system must be vented and the pumps are usually not capable of running dry.
Drive
The pumps on the mid-range models are either powered by direct current from the 12-volt battery bank, 230-volt alternating current from a generator or inverter or alternatively by a V-belt from the engine. If the boat is already equipped with a generator to charge the battery or even operate an air conditioning system, then it makes sense to install a 110 or 230 volt system. This is because if the generator is running anyway to charge the batteries, water can be produced on the side. Alternatively, the electricity produced by the diesel engine when the engine is running can be stepped up using an inverter.
On yachts without a generator, however, 12-volt systems are more popular, as they can often be operated directly from the on-board power supply and solar panels and are not dependent on the main engine or an inverter. In an absolute emergency, it is also possible to switch off all consumers and still produce water using the solar panels alone.
The difference between the systems is really only in how the necessary pressure is built up to force the water through the membrane. With simple 12-volt systems, the current flow will naturally be far greater (80 amps, Echotec) and require thicker cable cross-sections than with a 230-volt system with similar output (8.6 amps, Echotec)
Energy recovery
In order to overcome the osmotic pressure of the salt water and turn it into fresh water, a pressure of around 55 bar is required on the membrane. Pumps require a lot of power (80 amps/12 volts) to build up such a high pressure. Although 12-volt pressurised water pumps (such as a Shurflo model used in the Schenker and Spectra watermaker) are significantly more energy-efficient, they can only build up a maximum pressure of 8.6 bar. For this reason, manufacturers equip their watermakers with an energy-saving energy recovery system, which Spectra also calls the "Clark Pump", but which is based on the same principle.
The concept is simple but ingenious: the 12-volt pump first pressurises a piston. As soon as the required pressure is reached, the piston is set in motion. It advances in its cylinder and at the same time moves a second piston on the other side of the cylinder, which presses water into the membrane at the required pressure. The energy recovery system also utilises the salt water that is still emerging from the membrane at high pressure to harness the power a second time. Instead of simply allowing the residual pressure to escape back into the sea, the water is channelled back into a smaller piston system mounted in parallel. This means that 90 per cent of the working pressure is used again to move the main piston. In this way, the system saves a lot of energy and enables operation with only around 8 to 10 amps.
Advantages and disadvantages
The water-producing capacity of both systems (with and without energy recovery) is very similar, but the energy consumption of systems without energy recovery is of course significantly higher. The Schenker Smart 100, for example, requires up to 60 per cent less energy to produce a similar amount of water as the Echotec 780-DML-2, while small systems such as the Schenker Smart 30 only need 8 to 10 amps of electricity and can be supplied by two 100-watt solar panels when the sun is at its zenith.
The advantage of simpler 12-volt systems without energy recovery lies in the much cheaper and simpler design, which is associated with a lower susceptibility to faults, especially when on board for long periods of time. If the energy recovery pump with its complex pressure and position sensors is exposed to salt water for many years and corrosion occurs, problems can occur time and again. In remote areas without a dealer network, such a pump often has to be sent to the nearest dealer and it can be months before the device is returned repaired.
The simpler systems, which only consist of a pump and a membrane, can often be repaired on site using plumbing tools. If the watermaker is only operated when the engine is running, the higher power consumption is a thing of the past anyway. Simpler is often better on long journeys. Because apart from the filters clogging or a hose leaking or even bursting, such a system can hardly have any other faults if the high-pressure pump is working and the membrane is intact. This is also the reason why long-distance riders prefer to opt for simple solutions or even build their own watermaker. The individual parts such as pressure gauges, valves and filters are available cheaply from industrial suppliers, and boat owners use a simple Kärcher high-pressure cleaner as a pump. This is inexpensive (for less than 100 euros) and easy to replace in an emergency.
High-pressure pumps mounted on the machine and driven by V-belts are another good and problem-free solution. However, it is usually not possible to convert the machine yourself, as, similar to fitting a second alternator, it is necessary to manufacture a suitable bracket for the pump and retrofit the machine with an additional pulley for the V-belt.
"Flying" solution
A fourth principle of water making is mobile watermakers. However, they are currently only offered by the Australian manufacturer Rainman (sold via shipshop.de). The user can choose between different modules, which are mounted in handy boxes and housings and spread out on deck for the duration of the water-making process - practically a makeshift solution for occasional use.
A pump unit (including filter) and a membrane unit are always required for operation. The watermaker can either be connected to the 230-volt on-board power supply, shore power or an existing generator via the RainMan Electric AC pump unit (approx. 5290 euros) or to the boat's 12-volt power supply via the RainMan Electric DC unit (approx. 4990 euros). Alternatively, the RainMan Power model (approx. 6190 euros) contains its own small Honda generator and runs on petrol.
The pump element draws in salt water via a hose laid on deck and presses it under high pressure into a membrane unit. There are three models: the RainMan High Output (approx. 200 to 300 euros extra depending on the model) has two membranes and supplies 100 to 140 litres of fresh water per hour, while the RainMan Compact box produces 50 to 70 litres of fresh water per hour. RainMan Economy (500 to 600 euros less) supplies just as much water, but is delivered unpackaged.
The right choice
It is important that the energy balance is right when making water. For example, a 12-volt watermaker with an energy recovery concept can be a good choice if the boat owner likes to spend several weeks at a time at anchor and completely self-sufficient in the bays of the Mediterranean. If his entire deck is covered with solar panels anyway, then this can also be used to produce water on the side. However, if a generator is responsible for the power supply, then a system powered by 230 volts may make more sense. If the batteries are charged by the main engine while travelling, then a high-pressure V-belt pump mounted on the engine makes the most sense. Water can then only be made during the daily engine runs. In an emergency, this can of course also be done at anchor with the engine running - but it would not be very economical to run the main engine while stationary to produce water. Because then 100 litres of water would quickly cost 20 euros - and that would be significantly more expensive than at the jetty.
There are many pros and cons. For example, a new watermaker with energy recovery can be a good decision for a temporary retirement - because even a vulnerable system will produce few faults in the first three years. However, retirees who have found their dream boat and want to spend most of their time living on it from now on will enjoy a more robust and simpler system more in the long term - but will also have to put up with more power consumption. The bottom line is that the choice of watermaker - as with so many things on boats - is more a question of faith.
