One of the most frequently asked questions in sailing forums is how much water you need to carry on board a boat. The problem when answering is that this amount varies substantially depending on the distance to be covered, the type of sailing – in a competition the water needs of the crew will be greater -, the weather, the efficiency of water use and even the desired level of comfort. Some people only use drinking water to drink, and they shower, cook and clean with salt water, while others take long showers and even have a washing machine on board (two opposite ends of consumption).
However, even without going to extremes, needs continue to vary. We explain how to calculate the amount of fresh water you need on board each day based on different factors.
How much water does a person need to drink per day?
First of all, we must assess our hydration needs as it is the most important use of fresh water on board a boat.
In 2010 EFSA (European Food Safety Authority) recommended a daily intake of 2l of liquid for women and 2.5l for men. The amount includes water, any other beverage, and hydration from solid foods (usually 20%-30% of the total).
Although there is no scientific evidence of the benefits of drinking 2l of water per day, as water requirement varies depending on, for example, the weather and physical activity, the NSH recommends drinking 6 to 8 glasses of water a day.
However, when we sail we usually lose more water. If due to the physical effort of sailing we have lost more than two kilos of weight, the American Dietetic Association recommends drinking between 450 and 700 ml of fresh water for every 500 g of weight loss, and we should drink even if we are not thirsty. Environmental conditions also play a role: The hotter we get, the more we sweat and lose hydration through the skin.
Variables to calculate the amount of water on board
Once the requirements for fresh drinking water for consumption have been established, other variables must be taken into account:
- Personal hygiene: Water needed for showering, shaving, brushing teeth… Here too our water efficiency will be important, i.e. our ability to use the minimum amount for each task without wasting it. For example, do we leave the water running while we brush our teeth or lather our hair, or bathe in the sea and rinse with fresh water, instead of washing completely with fresh water.
- Laundry: If we wash our clothes by hand or machine, how often will we do it, and if we wash with salt water and rinse with fresh water, or do all the laundry with fresh water.
- Cooking: What kind of water and how much we use for cooking, and for washing food, dishes… We apply the same consideration to the shower or the laundry: Do we wash with salt water and rinse with fresh water? Do we rinse dishes with the water running?
- Boat cleaning: We must also assess whether and how often we will need to clean the deck.
- Emergencies: We should always have some extra for unforeseen circumstances, for example if we need an extra shower or to clean the boat’s deck of some substance that has fouled it unexpectedly.
In addition, external conditions must be taken into account: For example, in more saline waters we will need more fresh water for our personal hygiene (we will need to eliminate more salt) and to clean the boat (so that the salt does not spoil the deck as much). In cold climates we will tend to shower less, as we will not sweat excessively.
We recommend that you thoroughly review a complete daily routine and note all uses and approximate amounts of water we have used. We must calculate the requirement per person, and then add the possible extras.
Practical cases
As we have just seen, there are many variables to take into account, related not only to the use of drinking water itself, but also to our efficiency in using it. In any case, we are going to see some examples that will help us to specify our needs.
A standard case would be sailing through the Mediterranean in summer, with average temperatures between 32º C and 38º C during the hottest hours. The boat has a crew of four on board, and they want comfortable sailing, without excessive water consumption but without going to minimums. The calculation for this case is 70-80 litres per person per day, i.e. an approximate total of 270 litres per day.
This means having the desalination plant running for 3 hours a day and producing 90 litres per hour, in order to have the necessary water available for the following day.
Another case would be a long sea crossing with a small boat and a crew accustomed to having the minimum comfort. The estimated cost of water would be 25 litres per person per day.
In a more extreme case, in regattas and in high competition the crew goes to survival minimums, and use only 5 to 10 litres of water per day per person for consumption. It should be noted that these crews are physically and mentally trained for these extreme situations.
Thus, once you have determined the amount of water you will need per person per day, we recommend that you have a water treatment plant on board. There are those with a production capacity from 25 l/h to 4000 l/h, so you can find one that fits your needs perfectly, although we suggest that you do not choose one that is too tight. According to our experience, it is very important that the desalination plant we install has sufficient power to half fill the boat’s tank in three hours.
Based on the data and experience of our customers, we have calculated the amount of water needed and the drinking water plant that is recommended for each estimate:
| People on board | 2 | 4 | 6 | 8 |
| Litres of daily consumption | 123 | 241 | 364 | 492 |
| Litres/hour to produce | 25 | 48 | 73 | 98 |
| Recommended drinking water treatment plant | Splash25 | Water-Pro 60 | Water-Pro 90 | Water-Pro 120 |
Sources: BBC Good Food, Zero to Cruising, Eco-Sistems.
