On board Wisdom, we have 8 water tanks, totaling up to 160 gallons of fresh water.
Tank 1 is 15 gallons, 120 pounds
Tank 2 is 15 gallons, 120 pounds
Tank 3 is 30 gallons, 240 pounds
Tank 4 is 20 gallons, 160 pounds
Tank 5 is 20 gallons,160 pounds
Tank 6 is 30 gallons, 240 pounds
Tank 7 is 15 gallons, 120 pounds
Tank 8 is 15 gallons, 120 pounds
This may sound confusing to manage, but it works out well. We can easily ration our water by only using one tank at a time. We have an idea of how long we want each tank to last, and if we burn through the water too fast, we are made very aware of it when it runs dry.
The act of opening a new tank also lets us keep a mental picture of how we are consuming our water. When we did our summer trip, we started off with Tank 1 open, and it lasted us almost a week. This made us a bit more relaxed about our water usage (which translates into more frequent showers). We keep Tank 6 (30 gallons) as a backup tank while cruising. If we ran out of water while off shore, Tank 6 would be able to provide us enough drinking water for 28 days (based on 2 people drinking 2L of water per day).
The final advantage of multiple tanks is that the weight can be spread throughout the vessel. 160 gallons weighs 1280 pounds! If we had that all weight in one place, it would seriously affect the trim of the vessel as it ranges between full and empty. By spreading the weight around, the weight is also evenly distributed and we are able to drain the tanks in a controlled fashion, which also evenly distributes the weight decrease.