Squeezing out fresh water


The term “watermaker” is a bit of a misnomer, as these units do not really make water but rather turn undrinkable salt water into drinkable fresh water. The correct name is “reverse-osmosis desalination system” or “RO system,” but let’s face it: That simply does not roll off the tongue quite as easily as “watermaker.”

Understanding the correct name helps understand what these systems really do and how they work. Watermakers take salt water and push it through membranes that only allow the smaller water molecules to pass through, blocking and separating the larger salt molecules. The result is fresh water from salt water. Sounds pretty simple, but of course nothing on a boat is that easy.

Membranes are a semipermeable material that, in a sense, filters on a molecular level. In order to do this, salt water is pushed against the membranes at high pressure. The pressure is required to squeeze the water molecules through the membrane material. This is why watermakers use an energy-intensive, high-pressure pump. The resulting water that gets through the membrane is called the product, while what is left over is called brine or reject.

It’s a good idea to make sure the pre-filters are easily accessible. 

Wayne Canning

Almost all watermakers found on smaller boats will consist of a few basic components. To understand these and how they work together, let’s follow the flow of the water through a basic system. First, the water comes into the boat through a thru-hull fitting. This salt or “raw” water is pumped through at least two pre-filters of five and 20 microns by a small low-pressure or lift pump prior to going to the high-pressure pump. These filters are required to remove larger particles that could damage the high-pressure pump or clog the membranes. Some systems will have additional filters along with an oil separator to further protect the system and membranes from contamination. Dirty water can result in frequent filter changes or damage to the membranes, so it is also best to operate a watermaker away from polluted harbors or bays if possible.

Once the filtered raw water gets to the high-pressure pump, the real action begins. The high-pressure pump raises the water pressure to around 800 psi, which is the pressure required to push the water through the membranes. A pressure relief valve is also installed at this point to prevent over-pressurizing of the system. The hoses connecting the high-pressure side of the pump to the membranes will also need to be designed for this high pressure. 

Membranes are key
The key part of the whole system is the membranes themselves that separate the salt from the water. The membranes are housed in special high-pressure fiberglass tubes. The filtered, pressurized raw water enters one end of the tube and comes out the other, while the desalinated water leaves from a separate port in the tube. Larger-output systems will use two or more membrane tubes. The bulk of the raw water that enters the membrane tube does not pass through the membrane. Only about 10 percent actually passes through it as fresh water, with the remainder exiting overboard as salty brine. The pressure in the membranes is maintained with a flow valve on the brine outlet side of the membrane tubes. This valve needs to be adjusted, as the pressure can change due to salinity or temperature of the raw water.

A Spectra Newport 400c semi-modular DC system with digital remote.

Courtesy Spectra

Many modern systems have digitized and automated controls to keep the system functioning correctly. In the past, it was up to the operator to manually adjust flow rates and check salinity levels for the product or fresh water output. Many of today’s systems have automatic controls to do all this with little or no operator input. Remote panels for easy access and monitoring also mean no more crawling into cramped compartments to operate the watermaker.

Selecting the right watermaker can be a bit intimidating with so many types and sizes of units available. For most voyagers, output volume and power supply will have the biggest impact on selection, followed closely by cost. Power options and output are often tied together; the more power available, the higher the output. When it comes to power supplies, the two main options are AC or DC. Engine drive systems are also an option but are not widely used due to installation costs and problems with maintaining steady pressure over varying engine rpms. 

Powering the flow
The AC systems, either 120V or 240V, will generally have the highest output. For boats equipped with a generator, AC can be a good option. For those cruisers without an installed generator, a small portable generator such as the Honda 2000i is a good way to power a midsized watermaker. Trying to run a watermaker from an inverter is not a good idea due to heavy battery loads. A better option for those without a generator would be a watermaker designed for 12 or 24 VDC.

This Watermakers brand unit has a typical simple manual control panel.

Wayne Canning

Improvements have been made to DC pumps, making them more energy efficient. Most DC high-pressure pumps now use an energy recovery system that can increase their efficiency by as much as 75 percent. These systems use some of the high-pressure wastewater to help power the pump, thereby reusing energy normally lost overboard with the brine discharge. Due to this increased efficiency, many of these units have outputs close to that of the AC systems with the advantage of not having to run a generator during use. A vessel equipped with a good amount of solar power will be able to silently and economically produce water during daylight hours.

Once the power supply has been figured out, the output volume needs to be considered. Sales figures can be impressive, but understanding how and when the watermaker will be used is also important to output. Many units are rated in gallons per day output while others will be gallons per hour. Gallons per hour will be generally the most useful number, and those rated for daily output can be converted to hourly simply by dividing by 24.

Before figuring out how much output is needed, think about your daily water usage while aboard. If your boat is simple with a small crew, the needs will be modest —   a smaller unit might do. If you have a larger crew with a clothes washer or dishwasher and you like to do a freshwater wash-down of the decks daily, your needs will be higher. I usually suggest the water tank be filled and the boat used as expected until the tank is empty; from that, you can calculate your daily average water usage. Add a 25 percent margin, along with a bit extra for flushing the watermaker with fresh water. Then, think about how often the watermaker will be run. If you typically run a generator for part of the day, this would be the time for running a watermaker as well. With 12-volt systems, you may want to run it during the best daylight hours for the most solar power. With this information, you can calculate how much output is needed. The biggest mistake most make is buying a unit that is too small for their needs, so bigger is often better.

A Sea Recovery modular unit.

Wayne Canning

Selecting a brand will depend on where you plan to cruise and how hard parts will be to obtain. Many smaller companies use off-the-shelf parts to assemble their systems. This can be an advantage, as parts might be easy to source in more remote locations. Larger companies may use proprietary parts that are harder to source, but they may also have a larger dealer network, meaning parts and service are available worldwide. It can also be useful to check a company’s reputation for customer support.

Cost tied to output volume
Cost can also be a factor for many. As with most things, the more bells and whistles, the higher the cost. The trick is to achieve the cost/option balance that best suits your budget and needs. Generally, cost is going to be tied to volume output; however, control options and installation costs can add to that. Again, it is better to select a system that will have a higher output than you think you may need. A larger system will not have to run as often, or for as long. Should the need for more water arise, there will also be some reserve capacity. Keep in mind that although many systems have automatic controls, it will cost more for these conveniences both in original purchase price and maintenance costs.

Another consideration is whether to purchase a modular unit or a system that consists of separate components. A modular unit has all the pumps, filters and controls in a frame or housing, making installation easier and neater. Typically, these units are about the size of a small generator and are a good way to go if space is available. For those with space issues, a system with all the components separated allows the system to be spread out to fit available space. Noise during operation and accessibility are important should be kept in mind when selecting an installation location. Modern watermakers are quieter than their older cousins, but the high-pressure pumps can still be a bit noisy. This is particularly true with the AC units since they run at a higher rpm. Remember, there are also several filters that will need regular inspection and changing. Pumps, valves and other components will also need service from time to time. The easier it is to get to these parts, the easier the service.

A Matrix modular unit with manual controls.

Wayne Canning

Like most modern appliances, watermakers tend to have computer controls that take much of the guesswork and adjusting out of the operation. Some will even have Bluetooth or Wi-Fi connectivity to allow the units to be operated from a smartphone or tablet. Some units can even be programmed to operate at specific times without user input. This is particularly useful for routine system flushing to keep the membranes from drying out when the system is not being used regularly. For those that are old school, want things simple and like a hands-on approach, many companies still offer basic systems without all the computer controls. This can also help keep the cost down for those on a tight budget, but it will require more time and effort from the operator.

No matter what system is selected, it will require routine maintenance. The pre-filters will need regular inspection and changing. Some systems will alert the operator when pressures get high, indicating filter changes are required, but for most these will have to be checked regularly. Depending on the amount of use and how well the system is cared for, membranes will also need to be changed on occasion. The system and membranes will also need to be treated with chemicals or “pickled” if the system is not going to be used for a period of time, such as during winter layup.

Modern watermakers have come a long way in the last few years, making them almost as easy to operate and maintain as a typical generator. Although there is a price to pay for this convenience in higher purchase cost, the ease of use and maintenance may well be worth the extra expense. After all, we are out using our boats to relax and enjoy the lifestyle — not to do more work. For those on a bit of a budget or who enjoy the hands-on approach, even the simpler systems have improved energy efficiency and performance at an affordable price. Many of my clients ask if it is worth the expense of a watermaker when cruising the islands. My answer is simple: Would you rather be lugging five-gallon jugs of water back to the boat from ashore, or would you rather be enjoying a sunset drink in hand?” Most will take the latter!

Contributing editor Wayne Canning is a marine surveyor, writer and photographer. Visit his website at www.4ABetterBoat.com.

By Ocean Navigator