Choosing the Right Watermaker

A watermaker produces high-quality fresh water from salt water. As the machine can be used to produce fresh water at any given moment, it isn’t necessary to store a large quantity of water on the boat. Since water weighs 8.3 pounds per gallon (1 kilogram per liter), it adds considerable weight to your vessel when stored, increasing fuel consumption and taking up space.

Using a watermaker is an excellent solution to the constraint of storage. Today, the majority of watermakers use the reverse osmosis technique. In this guide, we will discuss the criteria to consider when choosing a watermaker.

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How to choose the best watermaker?
People out at sea need fresh water onboard for hydration and hygienic purposes. Storing large amounts of water on a boat can be problematic. The volume and weight of transporting fresh water is a challenge for boats of all different sizes, and it is rare to find boats that can store the fresh water needed by several people for weeks of navigation.
Here are the key points to consider:
- Production capacity
- Energy consumption
- Clutter and noise
How does the reverse osmosis watermaker work?
Watermakers reverse osmosis watermaker

Reverse osmosis is the principle that has been used for forty years in watermakers. It involves pumping seawater and then injecting it under pressure to force it through a very thin semi-permeable membrane. This membrane allows pure water molecules to pass but retains mineral salts and biological impurities (viruses, bacteria, etc.).

The desalination of seawater by reverse osmosis begins when the necessary pressure has been reached in relation to the salinity of the water. The production of fresh water occurs on the side opposite the membrane. The salt and impurities are evacuated in order to avoid clogging the machine.

The machine brings in more water than it produces. Since pressure is needed, a high pressure seawater pump and a valve for the brine discharge into the sea are added.

To increase water production, one or more membranes can be added.
Here are the advantages:
- The most widespread process with the most user feedback
- Good performance
- Reduced maintenance
Here are the disadvantages:
- Electricity production must be calibrated to meet the energy needs of the watermaker
- The drinking water produced by a watermaker does not contain mineral salts, it will be necessary to add them a posteriori to respect the necessary intakes for the human body.
- It will be necessary to adapt the watermaker to the salinity of the pumped water, which is different according to the navigation zones (between 35 and 270 g/l).
How to choose the right production capacity?
The production capacity of fresh water by the machine is given by the manufacturer and expressed in liters per hour. The production you require will depend on the needs of the boat and the estimated consumption in liters per day. The different uses must be taken into account: cooking, drinking, bathing, cleaning, etc. The best way to determine this quantity is to calculate the daily consumption for the number of people onboard on a shorter trip before purchasing a watermaker.

It is important to determine the lean production capacity. Overestimating the requirement risks over-dimensioning the system and therefore infrequent use. Bacteria are then likely to develop and affect the proper functioning of the watermaker. The choice of production capacity will also depend on the number of hours that the watermaker will be running daily.

However, more often than not for larger ships, underestimating how much water it will take to navigate comfortably is more common. It’s important to calculate the amount of water you’ll need for your crew to be comfortable and not only the minimal amount of water it will take them to stay alive. Underestimating can lead to crew tensions.

Marine watermakers can produce from a few dozen liters up to several thousand liters per hour. Your choice will depend on the type of navigation you’ll be doing.

Use simple math to determine the gallons per hour you’ll need the machine to produce during your navigation: Divide the total gallons of water required per day by the number of hours the watermaker will be used.
Why is energy consumption important?
Selmar belt-driven watermaker

Like any electrical device, the watermaker adds to the general electricity consumption onboard. There are three different feeding possibilities.
- Alternating current (AC) 120/220 volt: This is suitable for boats with a generator.
- Direct current (DC) 12 or 24 volt: These versions are well integrated into electrical circuits incorporating alternative energy sources (solar panels, wind turbines, hydro generators).
- Drive on the engine via a belt: This is the system with the best efficiency since the watermaker can run whenever the engine is running. The produced water is then stored in tanks. But this system requires having space available in the engine compartment, which is rarely the case for small units. Larger boats may also consider installation on the engine via a belt.
If you want to be independent of any energy source, there are manually operated watermakers. Due to their very low production of fresh water, they cannot be considered a permanent solution, but they can be a source of drinking water in an emergency situation such as a shipwreck. They do not take up much space and can be stored in a lifeboat or liferaft.

The only difference between 12-volt DC low-output and 120-volt AC high-output watermakers is how the high-pressure pump brings seawater up to the 800 psi needed to drive it through the reverse-osmosis membrane.

Small sailboats, under 45 feet, can use solar panels and often do. These boats operate with 12 volt or 24 volt and thus require a low voltage watermaker.
How can I avoid too much clutter and noise?
The size of a watermaker is an important factor in the choice of a model because the available space is not expandable. Recently, several brands are offering mobile devices. While they are not necessarily less bulky than a fixed installation, they have the advantage of being able to be moved as needed and can be removed from onboard if they aren’t needed for a specific navigation, thus freeing up space. Usually, these watermakers operate with a small internal combustion engine built into the system. Some manufacturers are also producing modular systems designed for separate component placement.

Noise and vibration levels increase as power and output increase. However, since it’s common knowledge that marine enthusiasts prefer peaceful navigation when possible, brands take this into consideration when designing their watermakers. Some models are made specifically to meet minimum noise requirements and let out a small amount of noise that goes unnoticed.