What size reverse osmosis watermaker does my boat need?
There are four of us cruising on a 42ft sailing yacht with 400 litres of freshwater tankage and a 12V battery system. We want to spend longer at anchor and have short showers, but I'm unsure whether a 30 or 60 litre/hour watermaker will keep up.
Answer
Size your watermaker by dividing daily freshwater consumption by the hours you can realistically run it, then add capacity for reduced output and flushing. For four people using about 120 litres a day, a 60 litre/hour unit is a sensible starting point; a 30 litre/hour unit needs longer daily runs.
Measure your water use
Measure what leaves your tanks over several normal cruising days. Crew numbers and shower habits tell you much more than boat length: a careful couple on a 45ft yacht can use less water than a family aboard a 35-footer.
Use these planning allowances until you have your own measurements:
- Careful cruising: 15 to 25 litres per person per day for drinking, cooking, washing-up and limited washing.
- Comfortable cruising with short showers: 30 to 50 litres per person per day.
- Additional demand: add washing-machine consumption, freshwater toilet flushing and equipment rinsing separately.
Four people budgeting 30 litres each need 120 litres daily. A 400-litre tank therefore holds roughly three days' consumption, allowing some reserve. Tank capacity gives you flexibility over when to make water; it does not reduce the daily amount you must replace.
Match output to running time
The calculation is daily litres ÷ daily running hours = required delivered litres/hour. To make 120 litres in three hours, you need 40 litres/hour delivered into the tank.
For initial sizing, use about 75 to 80% of advertised output, then check the manufacturer's figures for your cruising conditions. This allowance is a planning margin, not a guaranteed correction: cold seawater, higher salinity, dirty prefilters and the particular membrane system affect production differently.
- 30 litres/hour: budget roughly 22 to 24 litres/hour for initial planning, giving around five to five-and-a-half hours for 120 litres.
- 60 litres/hour: budget roughly 45 to 48 litres/hour, giving around two-and-a-half to three hours.
- 100 litres/hour: budget roughly 75 to 80 litres/hour, giving around one-and-a-half to two hours.
Allow extra time for starting, checking product-water quality and shutting down. Account for freshwater consumed by flushing, too. If you run a generator for only an hour a day, a larger machine may suit your routine better than an efficient small unit that needs several hours.
Check power and installation space
Examples of efficient units produce 30 litres/hour using 110W, 60 litres/hour using 240W, and 100 litres/hour using 400W. These figures apply to particular machines: pump design and energy recovery make substantial differences between systems.