There is indeed water all around us but what about fresh water on board? I estimate, as a minimum to survive, we require 2 litres per person per day, so for a 20 day passage (including a bit of contingency) we need to carry a minimum of 80 litres just to survive. But we do, in fact, consume in the region of 50 litres per day (showers, cooking, washing up, odd bit of laundry etc). Our water tanks on board only hold 220 litres - enough for 4 days. To overcome the deficiency, we have installed a water-maker, more accurately known as a desalination plant, on board. This is based on the reverse osmosis technique where sea water is passed through a very fine membrane at very high pressure (up to 900psi/62bar). Our particular unit can produce 42 litres per hour so with a bit of overhead to flush it out at the start and finish we need to run it for about 90 minutes per day. This system is working well, it does however consume a lot of power (30 Amps @ 12V) - see my other posting on power....
In mainland Europe we generally top the tanks up when we are in Marinas but in some off-the-beaten-track places it is often recommended not to drink the local water (Cape Verde for example) and so we don’t top up the tanks there but rely on the water-maker.
Despite the fact the water-maker produces very pure water (in fact the last part of the process is to replace some of the minerals that have been removed) we have an additional filter for drinking water - tastes sweet!
All very well, I hear you say, but what happens when you lose the water in your tank (perhaps through contamination or a leak) AND the water-maker packs up? Well we carry an additional 120 litres of “emergency” water (60 litres in 3 Jerrycans the rest in smaller bottles).
We also carry a hand held water-maker for use in the life-raft should we have to abandon ship for any reason so we could also use that on board if necessary.
So we have plenty of water when all is well and hopefully our contingency plans will cover us in an emergency also.
Note on water-makers: for some reason, these devices are notoriously unreliable especially the more modern units which use a system of “energy recovery”. These systems are designed to be much more efficient (ie it uses the same amount of power to produces a larger quantity of water), the downside is they and much more complicated (and expensive). Also many designs are fully automatic in their operation, again requiring much more complicated systems using sensors and valves. My view, when choosing our water-maker, was to keep it simple and whilst it is less efficient and requires manual operation it is very simple and at least I stand a chance of fixing it if something does go wrong. My point was aptly demonstrated by one of the boats preparing to leave Mindelo on our Rally. Having had no luck, discussing his problem at length with the manufacture, the Skipper of a brand new boat had to completely remove his water-maker and return it to Italy for investigation leaving him no choice but to load up the boat with copious supplies of bottled water. Needless to say it was one of the new bread of fancy units!