Rebecca Hayter sets a list of questions to consider when buying a watermaker and keeps it as simple as H2O + NaCl.
Washing machines, showers of five minutes not 30 seconds, toilets that flush with fresh water: the luxuries of life on land are increasingly available on boats, and that old maxim – just add water – applies to many of them. That’s giving watermakers a boost as a retro-fit item on boats, from 10m and up. For some boats, watermakers are a new commodity for the reasons noted above. Other boats are replacing their original watermaker and its complex maintenance regime for a new technology watermaker that is more efficient, has a higher output and largely takes care of itself. So if you’re considering a new watermaker, load up on big words like desalination, semi-permeable membrane and reverse osmosis, and go shopping with the following questions.
WHY DO YOU WANT A WATERMAKER?
Most likely, you want to run water-hungry equipment on board, to know that the water in your tanks is safe to drink, and to make extended cruises without regular detours to fill your water tanks.
IS YOUR BOAT DC OR AC-POWERED AND DOES IT HAVE THE ELECTRICAL CAPACITY TO RUN A WATERMAKER?
A DC-powered watermaker with a low current draw generally makes around 20-30 litres per hour, although some energy efficient, DCpowered units such as OceanFlo make up to 70 litres an hour and even 150 litres an hour. An AC-powered watermaker generally produces higher volumes; most can easily produce 100 litres or more an hour. However, watermakers can be spec’d to produce much higher volumes, with more membranes and more powerful pumps. Some distributors deal only in AC-powered watermakers; others, only in DC-powered. If your boat runs on DC, ie, 12 Volt or 24 Volt, you need to know how many amps per hour your watermaker will use and whether your boat can supply sufficient power. If it can’t, you might consider upgrading the boat’s battery capacity or simply run the watermaker when the boat is underway so that the engine is charging the batteries which are powering your watermaker.
Grant Matthew of Lusty & Blundell which distributes HP (high pressure) watermakers says that if your boat runs on AC, current draw is less of a concern. A generator of, say, 5kW or more will generate sufficient power to run a watermaker, along with equipment such as a fridge, toaster, coffee maker. These days, bigger boats of 15m or more generally run lithium battery banks and inverters, with smaller generators, and easily have enough AC power for domestic fridges, induction cookers and watermakers.Â
Boats in the mid-range, say 12-15m powerboats, are where electrical capacity can be an issue, so it pays to figure out your power options before installing a watermaker.Â
Chris Dent of OceanFlo has developed an energy efficient, DCpowered watermaker to be paired with lithium ion batteries and, possibly, solar power. It’s an option for mid-range AC-equipped boats with low-output generators. Instead of using the generator, they run a DC-powered watermaker off the battery while the boat is underway. ‘A DC watermaker is always easier to fit,’ Dent says.
HOW MUCH WATER DO YOU WANT TO MAKE?
The general rule is about 50 litres per person per day, then double it because with more fresh water onboard, you will find more ways to use it, eg, adding a washing machine or washing salt off the boat’s windows. Desalinated water is tasteless and not to everyone’s taste, but, unlike town supply, it has no minerals so it doesn’t leave residue on decks and windows as it dries. For the same reason, water from a watermaker doesn’t result in calcification in the heating elements of the hot water cylinder. And, if it is used to flush the boat’s head, there will be less build-up of calcium in the sanitation hoses. So, it is likely you will quickly find more uses for desalinated water, assuming you have enough tankage to store it.
WHEN DO YOU WANT TO MAKE WATER AND HOW QUICKLY?
This will depend on your power system, your cruising habits and the capacity of your water tanks. For example, if you run a generator for an hour or so every morning and evening to power major appliances, including the watermaker, and you want to make 200 litres of water within those time frames, you will need a watermaker that can produce 200-300 litres an hour. But, when making water at anchor, ensure the sea water is clear and clean. Avoid making water in areas where, for example, the water is muddy, heavily populated with other boats, affected by run-off from farms or near an industrial wharf or marina.
HOW DO WATERMAKERS WORK?
Watermakers do not make water. Clouds do. Watermakers filter salt and other suspended solids from sea water to achieve drinkable, desalinated water. Simply, a low-pressure pump brings a high volume of sea water into the boat via a through-hull fitting and sea strainer. The sea water travels in a dedicated hose to a pre-filter of 20 microns, followed by another pre-filter of 5 microns, to capture remaining debris such as fragments of seaweed.
From the pre-filter, a high pressure pump forces the sea water at around 60 bar (800 psi) into the heart of the desalination process: the semi-permeable membrane inside a cylinder. The more membranes on a watermaker, the higher its litres-per-hour output.
 The semi-permeable membrane is a hi-tech fabric with tiny holes, tightly rolled like a roll of paper towels around a central tube, also with tiny holes, all of which is encased in the cylinder. ‘Semipermeable’ relates to the membrane’s role as a customs officer which allows movement of the solution in only one direction. The sea water is forced into one end of the cylinder at high pressure so that it permeates through the tightly rolled membrane. But, the molecules of salt, minerals, bacteria and contaminants in the sea water are too big to get through the membrane’s fine holes. Only 10- 30% of the water molecules filter through the membrane and into the tube running through the centre. This desalinated water, now known as the product water, heads to the testing station.Â
The water that is left behind is highly concentrated salt water known as brine water and is directed overboard. The process of separating the fresh water from the salt under high pressure is known as reverse osmosis.
HOW DO YOU WANT TO TEST THE WATER?
According to the World Health Organisation (WHO), water that is safe to drink is defined as having no more than 500 dissolved solids parts per million.
The most basic test system, popular with racing sailors, is the human tongue; it adds no extra electronics, is reliable and saves on cost. A crew member may taste the product water direct from the watermaker in the engine room or have it routed to a tap in the galley, where it can be tongue-tested or tested with a TDS (total dissolved solids) meter. If the water is still salty, it is directed overboard. If it tastes drinkable, it is directed by a valve to the boat’s tanks. The human tongue tests only for salt, not for other contaminants, but this doesn’t usually matter offshore.
The electronic option runs the product water through a salinity meter to test it. It may have an automatic diversion valve to direct the water overboard, if it is still too salty, or to the boat’s tank if it meets the watermaker’s programmed PPM setting, generally 500ppm or lower.
WHERE WILL YOU MOUNT YOUR WATERMAKER?
To meet a wide-range of installation options, watermakers may be configured as horizontal, vertical or even modular with components such as pre-filters and membranes mounted separately and linked by plumbing. If retro-fitting, you will need to add and re-route plumbing and wiring, and perhaps install a partial bulkhead for mounting the watermaker’s components. Ensure the initial through-hull fitting for the intake water is low in the hull so that, if making water while underway in a seaway, the intake is always submerged. Watermakers don’t like sucking air. The low volume pump also needs to be sited low so that water naturally enters the system due to pressure from outside the hull.
When planning your installation, make sure the pre-filters are easy to access as they need to be replaced when they register a pressure of less than five psi on the intake side. Some brands have recyclable filters that can be flushed and re-used instead of being discarded. Feed the product water line into the breather line at the top of the boat’s water tank. Don’t feed product water into the bottom of the water tank because it could back-fill into the semi-permeable membrane. This would be a problem if your tank contained town-supply water which contains chlorine, which could destroy the membrane. As a precaution, a charcoal filter on the flushing side of the membrane will remove the chlorine.
Even if you install the watermaker yourself, it will probably be commissioned by the distributor as a condition of warranty. AC watermakers need to be fitted by a certified electrician for insurance purposes.
HOW WILL YOU MAINTAIN THE WATERMAKER?
The semi-permeable membrane is where it gets interesting and where watermaker maintenance has become a lot simpler, thanks to electronics. Once a watermaker has been used, the membrane will retain moisture which will allow the growth of bacteria, which will degrade the membrane. For this reason, and to avoid build-up of salt which can clog the membrane, the membrane needs to be flushed out after use, or at least every three days. The flushing can be done manually or, if fitted, by an automatic flush system. If the boat is left for an extended period, the membrane needs to be flushed at least once a week to minimise bacterial growth. If the boat is connected to shore power, flushing can be managed automatically. Alternatively, the membrane can be ‘pickled’ in a solution of sodium metabisulphate to stop bacterial growth. Some watermakers, such as the HP brand, have a membrane automatic conserving system that adds sodium metabisulphate into the system every time it’s flushed.
If cruising in an area of low salinity, such as a fiord where the water is mostly melted ice, the watermaker will work in doublequick time, but this rapid rate of production can expand the tiny holes in the membrane which will allow salt molecules to slip through when the boat returns to normal sea water. Like most boat equipment, the best way to maintain your watermaker is to use it. Even if the membranes are being flushed regularly, the pumps can deteriorate through lack of use.
The semi-permeable membranes should last five to seven years, but will eventually need replacing. Each membrane is about 1100mm long, so plan your installation with sufficient space to remove it completely. If you have two or more membranes, they will lay in opposite directions, so you need to allow space for removal either side. If you service the membranes yourself, make sure you replace them in the correct direction, otherwise the brine seal will prevent the water going through the membrane to be filtered. Which leads to another point: when comparing brands, compare their how-to guides for taking you through the various processes.
EXTRA FEATURES
All watermakers are rated to filter salt water, defined as 35,000 parts per million, at 25 degrees Celsius. In cooler sea water, watermakers may under-produce. In warm water, say the tropics, or in brackish water, they will overproduce. Neither is good because the pressure at which the sea water is forced into the semi permeable membrane needs to be fairly consistent at around 60 bar (800 psi) to achieve reverse osmosis. If it isn’t, you or the electronics can make an adjustment, usually to a needle valve, to achieve the correct pressure. Bacteria grows faster in the tropics so regular flushing is essential. High-spec watermakers will do most of the thinking for you, at a cost, and can be connected to the boat’s multi-functional displays at the helmstation to keep you in the loop. It will monitor the condition of the filters, litres per hour produced, time required to full tanks, test water quality, and even isolate and bypass an inoperable membrane to maintain water production with the remaining membranes. But whether you select a watermaker from DC or AC, whether it’s mostly manual or has all the tricks, it’s important to understand how it works. Oh, and to be totally au fait with the concepts of desalination, semi-permeable membrane and reverse osmosis.
