Water Supply Considerations

With conservation and use of clean sea water when ever possible, I expect to require 1 to 2 gallons of fresh potable water per person per day. This requirement will increase if I am sailing in areas (such as the Mississippi River) where the water is too dirty to use for washing purposes without some form of prior filtration and treatment. Many of the vessels that I am considering have built in storage tanks for up to 20 gallons of potable water so I would anticipate needing to refill the tank every 10 to 20 days. If I am to be self sufficient for longer periods in a salt water environment, I need the capability to store larger quantities of water, re-supply with fresh water from a land based source frequently, or manufacture fresh water from the environment as needed.

The massive storage solution (50 to 100 gallons of fresh water) is a triple problem on a small boat since there is not enough space to store the water supply, the water will grow stale being stored for long periods and must be filtered, and the extra weight would represent a considerable fraction of the total displacement. Also, refilling a large tank is a major issue if you have to use two containers of five gallons each to fill at a land based source, transport to the boat via dinghy, and then transfer into the storage tank. At about 8 pounds per gallon, this would not be an easy task.

The frequent re-supply solution is the most practical and cost effective in the continental US since I plan to be no more than three days off the coast. However, purchase of high quality and contaminant free water in a strange port may be awkward. Also, storage of water presents it’s own problems with making sure that the storage and transfer containers remain clean and sanitary. In most cases, this will require additional filtration at the point of use.

This leaves manufacture of water on an as needed basis as the most desirable (and costliest) choice. If I produce the water on demand and store only enough for one to three day’s supply. I do not need to worry about additional filtration or the accumulation of contaminates in the storage containers. Unfortunately, the production of fresh water from salt water is an energy intensive and costly operation.

The first choice is solar heated distillation. While the energy for this system would be free, the production rate is very low since the temperature difference between the heated water and cold surface for condensing is low. This system is probably not capable of meeting the 2 to 4 gallons per day that I would require for myself and for those occasions when my wife joins me.

The second choice is a small Reverse Osmosis unit. Electrically operated, the smallest commercial unit will produce about 1.2 gallons per hour at an energy cost of about 4 amps (and about $2,200 out of pocket). The same unit is available in a manually operated format (at about $1,300). These units are known to operate well only if they receive constant use. The membranes are also extremely sensitive to hydrocarbons so if the input water is fouled by residual fuel or oil in the harbor, the membranes will rapidly fail.

Neither of these water production choices are attractive so I will keep looking. Of course, a reverse osmosis unit at a lower initial cost would make this picture more attractive. Sailorman occasionally gets the older PUR model 35 units and rebuilds them for sale as reconditioned devices. The model 35 does have a poor maintenance record and has been discontinued in favor of a more reliable design by the manufacturer. A request to Sailorman for a manual PUR Survivor 35 reconditioned unit has not produced results so far [Update - Sailorman has come up with a manual 35!! I have purchased this used unit at about 2/3 the cost of new and will report on how successful it's operation is at a later time].

For the special case of sailing in environments where the water must be filtered and treated even for washing purposes, it will be much more cost and space effective to stop and fill water tanks from shore based facilities as needed. This is also because I will need to purchase fuel and use pump out facilities at the same time in these waters.

If I do not find an alternative for fresh water production, the fall back position of filling up the onboard storage tank with water as needed will have to do. This will include installing a bacterial and taste filtration system at the point of use. There is no question that this is the most cost effective solution when sailing in continental US waters. Outside of the continental US, this will be more of an issue.

This page was last updated on September 14, 2000 05:08 PM

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