Food is a "Nice to Have", but you don't need it to survive for a short period of time. Fresh water is mandatory! On average, the human body can survive about three weeks without food, but only a few days without water!
Fresh water in your possession is mandatory, hypothetical and potential fresh water is not a replacement. Water in your tanks that you have on board is what counts, not water that your watermaker could produce or water that your rain collector could collect.
Think about it this way, would you go on a hike through the dessert with a bottle of water, or a rain collecting system with a straw on one end? What if it doesn't rain? Water that you could have is water that you could also not have.
A good rule of thumb for calculating water needs is to plan on 2L per person per day. If you are a crew of two, it would then be 4L of water per day. If you prefer gallons, 3.8L is roughly one gallon, so you can just plan 0.5 gallons per person per day, or for a crew of 2, 1 gallon per day.
But how many days will you be at sea? A good rule of thumb is to plan on sailing 100 miles per day, so the distance you want to cover should be divided by 100 to tell you roughly how many days to plan for. If you are sailing 1000 miles, it would be 10 days, 1800 miles would be 18 days, and so on.
Now it's time for reality to show its face. You won't always sail 100 miles per day, and you won't always sail the shortest and most direct route. When we sailed from Bermuda to the Azores, our route was 1800 miles, which should take 18 days. We ended up sailing 2,200 miles and it took 24 days.
It is always good to carry enough water for the journey and half as much again. This means that the number of days you calculate should be multiplied by 1.5 to give you a safer option.
in our example of 1800 miles taking 18 days, 18 x 1.5 = 27 days. As you can see, we came in under that mark, but still close to it. It is wise to not carry the minimum water you need, but rather make sure that the water you carry is enough for the journey and more.
What I mean is, say you can hold 100 gallons of water, don't fill them up only to 18 or 27 gallons for the journey. Instead, fill them full and know you will be arriving at your destination with plenty of water to spare.
Water is the most valuable item you carry on your boat, and should be treated as such. It should never be wasted, and it should never be lost. The condition of the tanks needs to be monitored and the use of this resource needs to be closely observed.
You might be thinking "Why not just carry an infinite amount of water?" The answer is water is heavy. Water weighs about 8 pounds per gallon, so every 100 gallons of water you add to your boat adds 800 pounds to it as well. The more water you carry, the heavier your weight penalty will be.
On Wisdom, we carry 200 gallons of water which tallies up to be 1,600 pounds or 727 kilograms. This is quite the weight penalty, and this is only the water in the tanks. We then carry another 40 days supply of water in plastic bottles, adding another 42 gallons of water and 336 pounds to the boat. This brings our total water weight up to 1,936 pounds when full!
Why carry so much extra weight? Why not just have a watermaker or a rain collector? The answer is simple, rain doesn't always come and watermakers break. When we sailed from the Bahamas to the Azores, we were unable to collect any water. In the first three weeks from the Bahamas to Bermuda, we had no wind and the 700 mile journey took 20 days. When it rained, the waves would pick up as well and the deck remained salty. Opening up the rain collector to let in rain water would also let in sea water. When we left Bermuda for the Azores, we had no rain for the entire 24 day journey.
Rain collectors are convenient when you are anchored in a remote location and protected from the seas brought up by the storm. In these conditions, we were able to stay away from civilization for longer, but not as a reliable enough method to depend on it to survive an ocean crossing.
