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The exceptional temperatures seen last summer in Europe led to significant water shortages in numerous regions where access to water usually poses few problems. The constant increase in the number of countries in a situation of water stress highlights the fact that we’ve got to question water reserves and their treatment. While research and development is being focused on “greater autonomy using less energy”, and although sailors always have more than one trick up their sleeve, water needs are significant, and unrestricted water saving solutions remain few and far between. Producing water on board is energy consuming. But with energy consumption being the first obstacle to self-sufficiency at sea, it’s essential to know how to evaluate your needs.


When you put to sea, the question of water reserves, whether for self-sufficiency and/or safety, is an essential point that must be evaluated. To do this, we can establish a daily calculation according to notions of time and distance. To this, we can add the comfort and frequency of use of the vessel in order to determine the equipment to install.

Bottled water – a safety element above all. In 2010, the European Food Safety Authority recommended daily water consumption of 2 liters for women and 2½ liters for men. These figures define the number of water bottles required for survival that need to be carried in the event of having to abandon ship. For coastal sailing, we’ll work on an average of a 24-hour wait for help to arrive and one bottle of water per person. That is, one six-pack of water for every 6 people on board. Far offshore, we can assume that the rescue operation would be more than 48 hours away. You should carry one six-pack per person if you don’t have a hand-held watermaker in your grab bag. If you want to reduce your consumption of plastic as much as possible, you should opt for a 30-liter jerrycan of water stored only for survival in case of abandoning ship. You can also find 5-liter water containers in stores.

The need for water related to comfort. Survival situations apart, this is calculated on the following factors: fresh water ready to drink, water for cooking, water for personal hygiene and finally, water for cleaning the boat. Apart from the water for drinking which we have just defined, these three others amounts will vary according to the desired level of comfort.
For comfortable cruising (and certainly within the framework of commercial charter activity, for example) and without restrictions, we estimate a need for 70-80 liters of water per person per day. In a slightly more restrictive offshore context of a cruising boat, with more water-savvy sailors on board, we could envisage daily consumption of 25 liters per person. For ocean racing or those seeking outright performance, this average goes down to 5l/person.
The differences in consumption are such that not everyone will equip their boat in the same way for leisure boating, whose average trips are estimated at 3 weeks a year here in France, compared with a liveaboard’s boat who is sailing around the world. The same is true if the boat is intended for a commercial activity whether offering high-end cruising services or otherwise.


Water resources come in four main forms. Bottled water, that should be purchased and stored for the safety reasons discussed above. Then, there is tap water from the dockside in ports which, stored in tanks, could be sufficient for coastal boating, provided it is treated. In this case, no watermaker is needed on board, but it requires reasonable storage capacity. Finally, restricted by the elements, sailors have always managed to invent different rainwater recovery systems that can be easily designed on board.

Ask any long-distance cruiser if they collect rainwater. Without much hesitation... the answer is “Yes” in 99.9% of cases. Most often, you see a funnel-shaped tarp suspended in the rigging, with a pipe in its center and connected directly to a collection can. If it is equipped with a filtration system, the water can be stored directly in the tanks.
Among cruising yachties, you’ll find all manner of “do-it-yourself” solutions for collecting rainwater. At anchor, all flat surfaces that are slightly inclined, such as coachroofs or solar panels, can be subject to water recovery. Gutter systems have been designed by many and have even inspired some builders who now integrate gutters directly into the coachroof mold during construction. Under way, water running off a mainsail in a tropical squall and collected by the boom can have a very high yield and is another example of possible savings.
Nevertheless, even if it is collected in the most natural way, the water must be filtered and treated to be stored as pure as possible in the tanks, in order to avoid its contamination and protect the plumbing circuit.


Always with the aim of saving money, these same cruisers will often have a seawater system in parallel with the fresh water circuit, with a faucet typically at the galley sink. This will allow for further savings, such as washing dishes with seawater before rinsing them off with fresh water. There would usually be a second seawater outlet on the deck as well.

Watermakers that work by reverse osmosis using seawater, despite being the highest in terms of energy consumption, are still the best tool for self-sufficiency. Intended for regular use, a watermaker is used for a boat that’s in regular cruising mode, on a long-term basis, spending occasional nights in port.
To summarize this type of equipment, a watermaker can produce, depending on the model, between 50 and 250 liters an hour. The heart of the osmosis process is a high-pressure pump, and it’s this which consumes a lot of energy. Some types of watermakers require an AC power supply, which often means resorting to a generator (thus adding weight and yet another source of consumption).
Nowadays, thanks to the development of energy recovery pumps, there is a piston system that recovers the energy from the reject brine water under pressure to retransmit it to the high-pressure pump before being discharged back overboard. These allow the pressure to be raised from four bars to the sixty or so bars required for pushing the water through the membrane, almost halving the electrical consumption using power supplied from the DC circuit.
Finally, to ensure proper functioning, watermakers require both regular use and careful maintenance. Even though a watermaker may demonstrate fairly convincing performance, it does not really correspond with the quest for self-sufficiency as it dictates a requirement for diesel.



In concrete terms, let’s imagine a one-week cruise with 6 people on a vessel with a water tank capacity of 600 liters. 480 liters of water will be needed per day, that is to say about 3 hours of watermaking per day (assuming production of 150l/h) with a consequent need for energy (3 hours). In such an example, the boat will be equipped with a generator. By restricting the needs to about 40 liters of water per day, consumption will be 240 liters of water. We can then assume that for the same equipment to undertake only 2 hours of watermaking per day (if the power originates from the engine, that can be done in the course of daysailing). With this latter consumption option, reduced to 40 liters, we can also opt for a less energy-consuming watermaker model operating on 12 or 24 volts at a lower amperage. On the other hand, you will need to run the watermaker for 8 hours. Operating on the boat’s direct current, you then need to evaluate the capacity of the battery bank. With four people, the need for water will drop to about 5 hours of watermaker use.

Let's remember that: 
- Depending on the needs of the boat, you’ll consume a minimum of 15 liters of water, which can go up to 80 liters per day and per person on larger boats. 
- The type of boating and the level of comfort desired will define the capacity of the tanks and the existence or not of a watermaker. Depending on the watermaker itself, the installation of a generator may be necessary.  
- Last but not least, there are always ways to save water on board.

What about you? Do you know what your daily consumption is? What solutions have you chosen for your boat? Have you ever collected rainwater? Do you use sea water on board?

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May 26, 2023
Merci Rob et Cédric pour vos retours!
Nous avons navigué dans le Pacifique sud avec ma famille pendant 6 mois à bord d'un catamaran. Et nous avions un dessalinisateur qui s'est révélé précieux! Puisqu'aux Vanuatu et aux îles Salomon, l'accès à l'eau était rare.
Nous avions un dessalinisateur très simple ("agricole") sans électronique, sur batteries, 12V qui produisait 60L/heure, et une capacité de stockage de 600L.
Nous consommions entre 40L et 60L par jour, à 4 (2 adultes et 2 enfants), en moyenne. Avec des filtres qui permettaient de minéraliser l'eau. Effectivement, comme le dit Rob lorsque l'on accueillait des invités, la consommation montait drastiquement, mais nous n'avons jamais manqué d'eau au cours de notre voyage.
J'invite nos lecteurs et visiteurs sur l'Excess Lab à partager leurs retours ainsi que leurs attentes sur ce sujet de production/gestion de l'eau à bord. Hervé et nos équipes du bureau d'étude sont preneurs de vos retours d'expérience sur ce sujet!


Thank you Rob and Cedric for your feedback!

We sailed in the South Pacific with my family for 6 months aboard a catamaran. And we had a watermaker which proved invaluable! Because in Vanuatu and the Solomon Islands, access to water was scarce.
We had a very simple 12V battery-powered watermaker with no electronics, which produced 60L/hour, and a storage capacity of 600L.
On average, 4 of us (2 adults and 2 children) consumed between 40 liters and 60 liters a day. With cartridges to mineralize the water. It's true that, as Rob says, when we welcomed guests, consumption rose drastically, but we never ran out of water during our trip.

I invite our readers and visitors on the Excess Lab to share their feedback and expectations on this topic of onboard water production/management. Hervé and our design office teams welcome your feedback on this subject!

May 19, 2023
Nous sommes 2 adultes et un bébé qui vivons à bord aux Antilles .
On a un dessalinisateur 12/220V 100 L/min et 2x300 L de stockage .

On consomme les 600L sur une semaine sans se restreindre avec petite piscine d eau pour le petit , rinçage du bateau a l eau douce X 1 par semaine ….
Quand on accueil du monde et qu on se retrouve à 6 adultes , c est plus difficile , on consomme alors plutôt 300 L par
Jour minimum …
Avec l ensoleillement des Antilles , on peut faire tourner le dessalinisateur uniquement sur les batteries qui se rechargent via les panneaux sans avoir a recourir aux moteurs: on a donc une eau avec impact écologique excellent !

globalement on la sensation de ne jamais manqué d eau et qu il est facile d en produire à la demande .
April 28, 2023
Nous avons navigué, ma femme, mon fils et moi pendant 10 mois au Antilles. Nous avions un dessalinisateur de 100l heure couplé au moteur et 600 l de réserve d'eau.
A 3 nous consommions en moyenne 60l par jour, soit 1h15 de desal tous les 2 jours.
Dans notre prochain projet je pense que nous opterons pour un desal en 24v avec un gros parc de panneaux solaire et de batteries.
Le plus dur sont les semaines avec la famille et les amis ; les terriens n'ont pas l'habitude de prendre une douche avec 10 litres d'eau ou de faire la vaisselle avec un filet d'eau !