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New: Report on the Collision between USS FITZGERALD (DDG62) and Motor Vessel ACX CRYSTAL


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Water Capacity

Standard estimates of water consumption for yachters assume a certain austere lifestyle. But everyone’sSecond comfort level is different.

Design Considerations

There are many estimates of water consumption available for yachters. But it is better to start with first principles when designing your system. If your boay is intended as a live-aboard in all seasons, while remaining qualified as a passagemaker, getting the heating, air conditioning and water systems right-sized is critical from a comfort and energy-efficiency aspect.

In the water system, this comes down to how much hot water do we need in a day. We don’t need that much for drinking and cooking. The difference is in life-style usage. Everyone’s comfort level will be different. The criteria for a live-aboard are different from a weekend cruiser or racing yacht. Living aboard, we should always have hot water when we need it; not waste water; and conserve energy.

Here's an example operating regime:

  • Two people most of the time
  • Four people for extended multi-week cruises
  • Six to ten overnight

The approach to the water requirements should be conservative, i.e., over-estimate the capacity required. Potentially this could be an energy inefficiency but you will prefer that to an insufficiency of water. It’s a good idea to decide the water system before finalizing the plumbing system.

Calculating Water Capacity

The literature on water consumption is all over the map. Experienced yachtsmen on cruising sail boats can manage on 1.5 galGallon of water per day per person, for all needs excluding bathing and dish washing; although 3-6 gal is more common.

Other yachting estimates are as high as 15 gal a day per person. At the other extreme, one 400-passenger cruise ship, the MVMotor Vessel Millennium Queen (believed broken up in 2001), budgeted 60 gal per day per passenger, which is close to domestic consumption. Domestic consumption is a benchmark for a live-aboard, but estimates still vary widely.

For a standard office, guidelines are 25 gal/day per person but have been shown to be as low as 6.8 with conservation measures [1]. Of course, office estimates don’t include cooking, showering and washing clothes, etc.

For domestic consumption, WaterTech.Online also recommends 25 gal/dGallons per day per person but Natural Resources Canada (NRCan) estimates real usage as 88 gal/d.

NRCan says average daily consumption is 1000 l/dDay (264 gal) for three people with 40%percent for toilets. This puts Canadians among the thirstiest on the planet. Must be all the Beer! More Beer! My toilet flushes with 1.6 gal so NRCan assumes I hit it 22 times a day, or about every 45 minMinute. Putting aside the toilet jibe, I find 88 gal realistic, as shown later.

Environment Canada estimates daily per capita usage as 79 gals. [2]

Vendor specifications also indicate that many marine toilets require 3-5 gal per flush, with the most efficient (VacuFlush) requiring only 0.10 gal/0.39 liter.[3] The more profligate ones can use seawater but VacuFlush requires fresh water.

Other sources estimate that domestic hot-water consumption alone runs 20-35 gal/day per person, with a peak demand of 10 gal. To meet such a peak demand, the hot water heater must deliver 10 gal/person at any time. Additional capacity is needed for appliances, such as dishwashers.

Other capacity recommendations for hot water range from 7 gal per person to as high as 17 gal.

In tests taking showers not baths the observed daily consumption was about 40 gal. Taking a bath (52-in bath) quickly jacked the number up into the NRCan realm. These are the results of the tests:

  • Bathing
    • Bath 21-42 gal, or
    • Shower 20 gal/d (2 @ 10 gal)
  • Toilet 10 gal/d (8 @ 1.6 gal)
  • Dish washing 4 gal/d (4 @ 1 gal)
  • Laundry 2 gal/d (pro-rated from 14 gal/week)
  • Hand washing 2 gal/d (4 @ 0.5 gal)
  • Drinking 1 gal/d (mostly Twinings Earl Grey tea)
  • Cooking 0.25 gal/d
  • Household cleansing 0.25 gal/d

The drinking consumption might surprise you. For years -- well, since 1945! -- the health community has been erroneously fixated on eight 8-ounce glasses of pure water per day.

The Institute of Medicine, which was responsible for the 1945 study, now liberally recommends a total daily water intake of 3.7 liters (0.97 gal), from all sources including water, wine, beer, coffee, tea, soda.

Reduce Consumption

Water consumption can be reduced using (descending order) water-efficient shower heads, toilets, dishwashers, clothes washers, and automatically closing taps. Low-flow shower heads alone can reduce water usage by 30%. Older shower heads deliver 4 to 5 gallons (15.1 to 18.9 litres) of water per minute. The USA Environmental Protetion Agency WaterSense lavatory faucet specification sets the maximum flow rate of lavatory faucets and faucet accessories at 1.5 gallons per minute (gpm) tested at a flowing pressure of 60 pounds per square inch (psiPounds per square inch) (common water pressure in most households).[4]

Tank Overflow

The fresh water tank should have an overflow outlet overboard. Boats have been sunk at the dock when connected to shore water that overflowed the tank in one way or another.


Before doing the above test, I had settled on 25 gal/person/day as a design goal. Afterwards, and because water capacity is so critical for a live-aboard, I increased this to 60 gal/person per day overall. With a 250-gal water tank, this gives a base cruising capacity of two days for two people. AAmpere (amp), SI unit of electrical current 500-gal tank would provide the same for four people. A watermaker would have to supplement this for extended cruises.