Watermakers for Boats

Watermakers for boats looks at the various options for boaters. Watermakers onboard allow you to make your own fresh water and become less independent on shore life. Creating clean drinking water on your boat and while cruising is a necessity. Water tanks can only hold so much water and restricted space onboard may preclude you from carrying enough water for your needs. Whether you have enough capacity for all your water needs or not, having a marine watermaker can make planning a long distance cruise easier.

Aqua wisper
Aqua wisper

Wouldn’t it be great if you could add more drinking water to your tanks at the flip of a switch? Well you can but there are many things you should know about yacht and offshore marine watermakers. They are complex in installation and maintenance. However if you learn the basics you can easily keep your marine water maker producing your water requirements

How Boat watermakers work

Watermakers use Reverse Osmosis to make water. This is the opposite process of osmosis that plants use. Wiki definition of Reverse Osmosis and description below;

Reverse osmosis (RO) is a separation process that uses pressure to force a solvent through a membrane that retains the solute on one side and allows the pure solvent to pass to the other side.

The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions).

This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2 to17 bar 30 to 250 psi, for fresh and brackish water, and 40 to 70 bar 600 to 1000 psi, for seawater, which has around 24 bar 350 psi, natural osmotic pressure which must be overcome.

Although Reverse Osmosis seems like a complex system it is really a simple and straightforward water filtration process. High-pressure (pump driven) reverse osmosis systems have been used for years to desalinate water to convert brackish or sea water to drinking water


Types of Watermakers for Boats

SK watermaker
SK watermaker

Self-Contained watermakers

There are watermakers for any size boat. They come in modular setups, Self-Contained or Portable and handheld units.

Most manufactures produce self-contained units that house all the parts of the watermaker between the thru hull and the water tanks. These setups are easily controlled but require a large amount of space. This is SK watermaker SC unit which has sizes 8 to 10 GPH up to 83 to 85 GPH.





Modular watermaker systems

Spectra Catalina 300 watermaker
Spectra Catalina 300 watermaker


Modular systems can be mounted in individual parts and do not require one large space but several much smaller spaces. The images of the Spectra Catalina 300 watermaker installation are thanks to MadDogVoyager.

MadDog are one of those boat owners who like to share what they do and have onboard. Notice the membrane is fitted vertically inside a hanging locker while the pumps and filters are underneath a v berth.





Portable and handheld Watermakers

Power survivor

Katadyn Power Survivor 06

Katadyn Swiss portable watermakers, this one is recommended for life rafts. Also look at their low power consumption and 12V power watermakers.



Solar Still

Solar Still

Energy Free!

Aquamate Inflatable Solar Stills use solar radiation to distill and make pure drinking water from seawater. The solar watermaker is light and folds into a 26 x 23 x 7cm pack. The solar still watermaker is an ideal addition to the grab bag for any ocean going yachtsman.



Watermaker Installation

Spectra Newport

  • Thru hull
  • Strainer
  • LP pump
  • Particle Filters
  • HP pump
  • Membrane unit
  • Discharge overboard for brine
  • New fresh water supply to tank
  • Membrane Flushing system


Watermaker Components


Components for a typical Marine Watermaker

A typical marine watermaker installation involves a seawater supply, preliminary filtration to remove weeds and large contaminants, a low pressure pump to push the water through the particle filter which remove particles down to 5 microns, a high pressure pump to supply the RO membrane. Fresh water from the membrane is then sent to the water tank.

For every gallon of fresh water produced roughly 10 gallons is required to pass through the membrane, the remainder goes overboard in the form of brine.

Hose runs should be minimal especially in low energy systems.

Thru hull It is important to have good water flow through the thru hull fitting, by having a forward facing scoop type 3/4 inch thru hull as low as possible in the boat. Any air getting in the system will need bleeding. The intake should not be near heads or grey water outlets.

Strainer A typical strainer will remove large to medium size debris that comes in with the sea water. The strainer needs to be inspected so it needs to be accessible. Do not install near electrical equipment as water can spill during strainer cleaning.

Low Pressure Pump The low-pressure feed pump for water is to ensure that the high-pressure pump is always supplied with an adequate flow of water. It should also be mounted low in the boat.

Particle Filters The raw water-supply system must remove any particulate matter that could damage the high-pressure pump. Most systems use two filters, which are fitted with progressively finer filter elements. 30 micron and 5 micron filters are typical. The performance of the RO membrane can be degraded by oil, so many systems also include a separate oil-elimination filter.

High Pressure Pump This pump needs to build up the pressure to around 800 psi to push the water through the membranes. The pump requires a lot of power to produce the 800 plus psi.

Two different types of pumps are used in sailboat RO systems: electric-motor-driven plunger pumps similar to those used in pressure washers, but marinized, and hydraulic amplifiers, pumps that amplify relatively low-pressure seawater and obtain the 800 plus psi pressure required for successful RO operation. It appears plunger pumps are popular for watermakers. Spectra and SeaRecovery use these type of pumps. Spectra uses a CAT pump.

Membrane units The key element of the RO system, the salt-separation membrane, is semi-permeable, its small pores removing anything other than water molecules. The membranes used in seawater RO systems are usually made of a TFC (thin film composite) membrane consisting of three layers, of spiral wound sheets of PA polyamide.

Brine discharge The discharge should be above the waterline so you can monitor the discharge.

New fresh water to tank The produced water is plumbed to the vessels storage water tanks

Membrane Flush The membrane needs to be flushed regularly to remove the particles and salt that have been built up on the supply side of the membrane during the RO process.

Other features include, Automatic pressure adjustment which allows for variation in salinity and temperature of the sea water.


Power Supply

sk watermaker

Power to the pumps, is supplied by battery, generator or engine driven pumps. Large capacity watermakers generally need an AC power source. Smaller capacity and low energy watermakers can use DC AC or engine driven pumps.

AC It takes energy to pump water at high pressure. Most of the power consumed in an RO system is used to power the high-pressure pump. Large-capacity systems are typically powered with 120- or 220-volt-AC motors and can require generators capable of delivering at least 3 to 4 kilowatts (to handle the motor starting load).

On a boat with a genset which is run often or continuously a relatively small watermaker can produce the required water. If the genset is run only occasionally a larger capacity watermaker is needed.

DC Smaller low energy systems can be driven with 12- or 24-volt-DC motors, making them more suitable for typical cruising boats. Selecting the optimum system requires consideration of the vessel’s power capability and the number of hours per day the system will have to be operated to provide the desired quantity of water.

Running the DC watermaker at the same time as the batteries are being charged will help output of the watermaker. Make sure when you are researching a DC watermaker that the DC system on your boat can handle the loads. Battery capacity may need to be increased

Engine driven pumps It’s possible to power an RO system’s high-pressure pump directly from the propulsion or genset engine, bypassing the need for electricity; however, this type of installation will require custom engineering.

Manual Power Survivor supplies manual hand pump for water making


Size Matters

How much water are you going to need, what size watermaker do you need?

Calculate how much water you need by estimating your daily usage/person/day. Older estimates allowed for a minimum of 1/2 to 1 gallon/person/day. This rate does not include showers, dishes and other extras. Nowadays with modern conveniences its more like 2 1/2 to 3 galls/person/day.

Work out how much you want to run the watermaker. You don’t want to run it all day, but you may want to run it while the generator is on, while the engine is on and you are charging your batteries. So maybe 2-3 hour per day run time is reasonable.

Tank size is also an issue. Its no good making 400 gallons if the tanks are not big enough.

Normally watermakers give their estimated outputs based on 70 degree F. Colder water temps may produce less water.



Operational Consideration

aka spares

Power Survivor

One example I can tell you about watermakers and maintenance involves Chessie Racing during the 1997-1998 Whitbread round the world Yacht Race. Due to saving weight and space requirement there is a lot of emphasis on watermakers for essential water supplies on these 60 ft race boats.

The race starts in England and the boats sail around the world anticlockwise sailing into the southern ocean before sailing back into the Atlantic and then back to the UK. It was in the southern ocean that Chessie ran into watermaker issues.

They had many spare parts but still the watermaker failed. They even had backup PUR hand pumps and for a week of sailing that’s all the water they had. They eventually had to plan a pit stop to get more spare parts and the nearest point of land was Ushuaia Argentina the most southern city in the world. they had their shore crew meet them with more spare parts and they were quickly back in business and on their way. It was an important lesson in having enough spare parts but most importantly the fact they had hand pump backups as this fact saved their lives as they were over 1,000 miles from land when the issue surfaced.

This picture is of New Zealand Atlantic rowers who used the Katadyn Power Survivor as their water supply plus backup air guitar


Watermakers Maintenance Procedures

First check your manufacturer’s maintenance requirements. The diagram above shows the parts in the system. All these need to be checked but the filters in particular are most important. Clogged filters and a dirty membrane will make for poor and low production.


Regularly check the sea water strainer and the two particle filters


Watermakers like to run often even daily for a couple of hours. Running the watermaker like this coupled with automatic back flush, which takes the minerals salt etc. off the membrane surface, keeps the membrane clean and in good working order.

A membrane not cleaned in a week can get bacterial fouling. If you do not use the watermaker in a week you should flush the system or put it into storage mode which involves pickling. Membranes should not be stored dry.

Membrane Flushing

Some systems have an automatic flush system built in, other smaller units require manual flushing. Some systems may have a separate tank for water for flushing.

Manual flushing of the membrane requires switching the valves on either side of the membrane, the inlet side and the discharge side, and then turning the unit on to circulate clean water.

Many larger systems have an auto flush mode. This can save lots of time and convenience.

Flushing water needs to be free of chlorine and so a carbon filter is recommended between the tank containing the flushing water and the membrane. Chlorine can react with some cleaning chemicals.

Membrane Pickling The membranes should be permanently immersed in liquid: either sea water before treatment, fresh water provisionally stored or sterilizing liquid if the water maker is not used for extended periods of time Pickling involves special biocides which are mixed with the product water and pumped into the system leaving the membrane saturated. Use the manufacturers recommended biocide.

Spectra Z-brane note, Spectras Z-brane which claims to reduce the need for picking of the membrane. It may be sensible not to totally rely on this. Z-brane will reduce the number of times you will need to pickle the system but probably best to pickle the system if the watermaker is not used for extended periods. Temperature

The membranes should not be exposed to temperatures freezing. Overpressure due to expansion caused by freezing can rupture the membranes and prevent the salt from being filtered out. The membranes must not be exposed to temperatures above 60°C as high temperatures may also prevent salt from being removed.

Brackish water Watermakers are not recommended for use in muddy or polluted water as the pre filters get clogged and damage the membranes. If you do use the watermaker in these waters only run it for very short periods: as soon as clean sea water becomes available clean the membranes and run the system without pressure for 30 minutes with the pressure regulator open. Spare parts

Carry plenty of spare filters. Filters last 6 months after first use. Also carry spare parts for pumps etc especially if you are going to be away from land or good stores.


Marine watermakers Manufacturers

Aquamarine , Dessalator, ECHOtec, Horizon Reverse Osmosis, Pur watermaker, RO watermaker, SeaRecovery, Schenker, SK watermakers, Spectra Watermakers, Village marine,