Which watermaker for your multihull?

More than any other resource, water on board is a necessity. We use it for drinking, but also (and especially in quantity) for washing ourselves, washing dishes and cleaning the boat. As a reminder, for drinking, 2 liters (just over 2 quarts) per person per day are required, but you’ll see below that freshwater consumption on board, again per person and per day, is 30 times higher than that! To avoid having to stop off too often or, worse still, to ration your water, the best solution is to install a watermaker, a tried-and-tested system that is constantly being improved.

What defines drinking water?

According to the WHO (World Health Organization), water is considered drinkable when it contains no more than 1,000 ppm salt (1,000 particles per million). In the case of a watermaker, the result is generally between 100 and 600 ppm, depending on the temperature of the water pumped, with a good average of around 400 ppm. In addition to these satisfactory values, the other positive aspect of a watermaker is that it produces water of excellent quality and pleasant taste, thanks to the absence of chemical treatments.

How a watermaker works

The main reason why seawater is unfit for human consumption is its salt content, which is around 1,000 times higher than in the fresh water we consume. The basic idea is therefore quite simple: take seawater, filter it to remove salt and bacteria, and transform it into fresh, drinkable water.
At this stage, we need to distinguish between two desalination methods: evaporation and reverse osmosis. The first solution is mainly used by commercial boats and certain large yachts, and is designed to produce service water, i.e. non-drinking water. To produce drinking water, the second method, reverse osmosis, is preferred.

What is reverse osmosis?

Reverse osmosis is a natural phenomenon enhanced by technology. Simply put, osmosis occurs when two liquids of different densities are separated by a membrane. This membrane can be natural, like our skin, or artificial, as in the case of a watermaker. The phenomenon of osmosis means that the less dense liquid seeks to cross the membrane to join the denser liquid. Applying pressure to the denser liquid (salt water) reverses this process, filtering out salt and bacteria. The watermaker thus consists of a pump, a series of membranes and usually a few filters.
The pump draws in seawater, which then passes through the various membranes to lose its salinity and be filtered. Since water molecules are generally smaller than salt and bacteria, the process is highly efficient.
The membranes, made mainly of polyamide films, expand or contract slightly with temperature, letting water through but blocking salt and bacteria. This phenomenon of expansion explains why the performance of a watermaker varies according to sea temperature. In concrete terms, a watermaker will have a higher output in warm water, but on the other hand, filtration will be less fine, which can let bacteria ...