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Looking after the vessel's fresh water

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What is the issue?
Freshwater is a necessity for life aboard a vessel. Humans need approximately a ½ litre of water per day to survive and two litres to avoid thirst. But we also like to wash with it and clean things with it so we need to obtain and store a large quantity of water aboard.

Why address this?
Since water comes to the boat tanks from any number of sources it susceptible to being contaminated. Disease-causing organisms, such as bacteria, viruses, parasites could potentially be present in a water supply as well as nitrates, herbicides or pesticides runoff from the farming community. None of this is good to consume.

Bad water can typically be detected by its aesthetics; it either looks bad, tastes bad, smells bad or any combination of the three. But many times the contamination is odourless, colourless, and tasteless. Keeping the water safe and drinkable requires some common sense, some equipment and some dedication to a procedure, and time and effort on an ongoing basis. It is an area worth some consideration for the health of all aboard.

How to address this?
During our circumnavigation, we largely operated from the shore side resources. Apart from doing some due diligence, taking word of mouth advice and checking the aesthetics, we never encountered a problem. A lot of this was down to having a relatively modest tank that kept our reserves flowing in and out of the water system on a regular basis. This helped to flush out the system and move the water out before it has a chance to become stale. So, there is no need for alarm, you can tank it in and drink away without any issue and many boats do just that.

But there may also have been an element of just pure good luck to this for us, and the need for good fortune can be vastly minimised by a few modest steps and a few effortless procedures. These would only serve to make the water safer and more pleasant to drink.

The broad areas to improve water are to get the tank and system right to start off with. Gain as much water independence as possible and then handle shoreside water supplies as best as you can.


The best first step is a good clean water tank. Your boat will come with a water tank, or tanks, of some sort and these vary widely in capacity, construction material and security. A good tank will be well secured, sturdy, corrosion proof, preferably baffled to prevent water battering at its walls and with an inspection port that allows it to be cleaned occasionally. If you are very lucky you may have a second tank so that, if the single tank leaks or is contaminated, there is a fallback reserve.

But this is often a given with a yacht and you have to deal with what you have and make the best of it. Unless that is, the tank is entirely dubious or unreliable that it warrants replacement. Apart from that you have to get off to a clean start with the tank(s):

  • • Check that the O-ring in the deck water fill cap is in good condition so that no deck wash is washing down into it. Replace if required before the tank is cleaned.

  • • Check the tank air vent has a protective mesh screen to keep bugs out and it is operating freely. Having an inadequate or a blocked vent will deform the tank as water is pumped out and cause it to fail sooner rather than later.

  • • Remove any carbon canisters or micron rated filters and empty the tank(s).

  • • Work out what is required to fill the vessel's water tank(s) with a bleach solution at about 125 grams of plain household bleach to 25 litres water, or 1/8 cup of bleach per 10 gallons.

  • • Mix the calculated amount of bleach into 5 litres, or a gallon, container of water before adding to the tank to reduce any spot corrosion on aluminium tanks. Use only pure bleach without any extra additives or scents.

  • • Pour the mixed water/bleach solution into the tank and then fill the tank with good quality water.

  • • If possible, fill until some of the solution is safely ejected through the vent to sanitise the breather pipe.

  • • Turn on each tap, hot and cold, by running foot/hand pump, until the water coming out smells strongly of bleach.

  • • Let it stand at least 4 hours but no more than 24 hours or, even better, go sailing and let it actively slosh around in the tank.

  • • Drain the water tanks through each and every tap aboard. It is essential to purge the system no later than 24 hours, because the bleach can damage the tank and piping.

  • • Fill water tanks with fresh water and drain again through each tap of the boat at least 5 times until the smell of bleach is no longer present.

  • • If the smell of bleach persists add a teaspoon of hydrogen peroxide per 100 litres, premix in 5 litres and add as above, to neutralise the bleach.

This process is safe for aluminium tanks as long as it is done for the suggested time and not more than once or twice a year. If there is some residue left on the walls of the tank after this, it will not contain pathogens. Most likely it is just an algae, which is produced over time in almost any situation. If you are anyway unsure of the mixture and uncomfortable with it in anyway an alternative is to just visit your local pub or bar and ask what they use to clear their lines. Buy the product and flush the lines with the same solution as recommended on the product.
Please note

Never use vinegar for the purpose or removing bleach from a tank. It ferments and undoes all the hard cleaning work.

25 Litre Stackable Plastic Jerry Can
Photo: Courtesy of Wavian
If your tanking capacity is light and you need to add storage for a long trip or store an abundance of water, the occasional jerry can or water bladder can be called upon. A couple of jerry cans should always be held for an isolated lifesaving backup and should be considered essential. They will also be required for moving water from shore sources via the tender.

Bladders may be called upon if there is an abundance of water to be stored. Unlike their rigid counterparts, small flexible bladder tanks make adding stores of drinking water aboard relatively easy. They are available in all kinds of shapes to fit in the uniquely available locations available in a sailing vessel. They may be hard to place, and the weight may affect a boat's trim, but they do work.
However, they are best thought of for temporary applications and not a replacement for a fixed tank as they are cumbersome, can tear, puncture, flop around and occasionally leak.


Being independent of a shore supply is always best as you simply do not have to worry about a contaminated water supply. There are two broad approaches to this, running a watermaker and/or harvesting rainwater.

Watermakers: The most reliable and abundant solution of all is to install a watermaker which provides the ultimate in water security and volume. Perhaps better described as a desalinator, a watermaker uses reverse osmosis to turn seawater into fresh by removing the salt. It has an infinite source of saltwater to desalinate and an undamaged water-maker membrane will strain out all bacteria and almost all viruses.

Schenker Smart 30 basic watermaker targeted at yachts
Photo: Courtesy of Schenker

Having a watermaker aboard entirely frees a vessel from relying on dubious water found ashore as it produces water that is infinitely purer than anything that can be acquired from most municipal water supplies, it is as good as any bottled variety. Provided sufficient backup drinking water is carried to support the crew for the longest passage, a vessel with a watermaker needs to carry less water making it lighter and faster which of itself shortens passages. The abundance of clean water they can provide makes showers less of a concern which helps make cruising much more comfortable.

A watermaker is very nice to have but it comes with its downsides too. They are very expensive to purchase, require a large amount of maintenance and adds an extra level of complexity to the boat. You need to use it regularly to keep the membranes in good condition and if it is left from three days to a week, depending on the environment, they can go badly wrong which is very expensive to resolve and stops water production. That means you cannot leave the boat for long periods or you must have someone run the water system whilst you are away, so they can be very restrictive. Manufacturers also only recommend operating watermakers whilst underway. So they should not be run in marinas, ports or harbours as these areas may contain gas, oil, sewage, sediment and other debris that can plug up inlet filters and damage the equipment.

Most importantly watermakers require a large amount of energy to make water and are typically driven off the engine. Small to medium units can be run to a limited extent off the batteries without an engine, how often and how long will depend on the onboard charging and storage system. However, most users will likely struggle to keep the batteries adequately charged without running the engine. So in essence, it is best to think of them as turning diesel into H2O. At the end of a season watermakers also require additional flushing and treating the membrane with chemicals, called pickling, to keep it healthy when not being used.

However, they are particularly useful for vessels that do not have a huge carrying capacity. And with a machine producing pure water on demand, it provides the option to stay in isolated areas as long as you wish. Likewise, if you are using the engine to travel you probably have the spare horsepower that can be put to work. The same applies when bulk charging, if running the watermaker directly off the engine, or whilst float charging when running it on electricity, the watermaker can leverage the power at a marginal or zero extra cost.

With a watermaker, a cruiser may cut off the shoreline hose and not worry about the next fill-up, and no need to feel guilty about rinsing after a swim or hosing off the boat after a day of cruising in sloppy water. That all makes for much more pleasant lifestyle.

Mother Nature is about to replenish the vessel's tanks
Photo: CC0

Water Harvesting: As already mentioned a watermaker is very nice to have but obviously expensive and adds an extra level of complexity and maintenance. The obvious next best independent means of provisioning water is to collect what Mother Nature provides. Once you get set up to catch water you will always have that option available if it does rain and you could try it first before investing in a watermaker.

Water collection system varies according to specifics depend on the boat. Tanking rainwater Experience provides an overview of the process and options available. Many more ideas will emerge if you make enquiries and of course look at what other people are doing around you. You must, of course, take into consideration any local air pollution that may exist in the cruising area and allow a runoff to take out what is in the atmosphere and the deck, or tarp, before putting it into your tank.


If you are tanking an outside water supply, that is not proven safe, from an uncertain water catchment system, it is advisable to take steps to ensure water quality. This can be done with filters, bleach and/or sterilizers. However, being careful with the water you take aboard, and always using a sediment filter on the intake with a granulated charcoal near the outlet, will to the largest part keep the water safe.

Hose Pipe: Examine the provided hose pipe carefully to make sure it is in good condition. Let the water run through the hose for enough time to flush out the hose itself. A hose filled with water, sitting in the sun, can provide a breathing ground for all kinds of growth and one of the greatest sources of sludge. If possible use your own hose and make sure it is emptied out after each use.

Aesthetic Check: Whilst the hose is running, look at the water and check if there is any odour. If it looks dirty or discoloured or smells bad, just do not use it. You cannot uncover contaminants like bacteria, metals or cysts by aesthetics but many of the problems can be detected by a simple inspection.

Camco TastePURE Water Filter with Flexible Hose Protector
Photo: Camco
Prefilter:Prefilter water to remove dirt before it goes into the tank. There are various filters that will prevent solids from getting into the tank. Many make their own from coffee filters in funnels and they can slow down filling but are very worthwhile. A neat solution for their sheer convenience is a hose-end filter. They cost very little and can apply filtration ratings from 100 microns to 5-10-micron; the smaller the micron the bulkier the housing. Mount the filter on the boat end of the hose and you can eliminate a lot of the problems at source.

Point-of-use charcoal filters: Installing a granulated charcoal filter is normally the first step people take because this type of water filtration as it is a well-established domestic practice these days. These filters screen out dirt and rust, as well as chlorine and the recipe of chemicals that are created when chlorine is injected into drinking water. There are a wide variety of filters developed for business and homes that deal with naturally-occurring elements that find their way into public water supplies and that may either taste bad or pose some threat to health. Many are easily adapted for boating use.

Granulated charcoal filters should be installed near the point of use, so that clean tasting water has only a short run to the tap and drinking utensil. There is nothing left in drinking water flowing through a fine charcoal or ceramic filter that tastes or smells bad or will leave one with a rumbling gut. These filters can also be used with a watermaker as they also will remove the taste and odour of sulphur that sometimes comes with water maker-produced water. It is, however, an important to keep an eye on as the filters need to be changed when necessary. If not maintained, they can be a breeding ground for bacteria.

Some boaters use both sediment and granulated charcoal filters as prefilters when they fill the water tanks. This is ill-advised because it strips out the chlorine from the water entering the tank and it is the chlorine that keeps the bacteria under control when it is stored. If you don’t chlorinate the water in your tanks, all sorts of bacteria will grow in it, particularly if you’re in a warm climate.

Ultraviolet Light Additional health protection can be provided with UV sterilizers. Water passing through these units is sterilized by a UV lamp and consider safe, effective method of bacteria control. These also benefit from another particulate filter in the system to remove the remainder of the contaminants. UV-C radiation eliminates all microorganisms but it is a big step beyond filtration and one for those that want to be ultrasafe.


We could not stand the taste of bleach in the water but some people religiously add it because it will kill most common bacteria. If you wish to take this precaution the dosage is five to eight drops of general unscented liquid household bleach, containing 5.25 – 8.25 percent chlorine, to treat 4 litres (one gallon) of water. Add a little more if it is cloudy. That’s approx ½ – 1 teaspoon per 20 litres or about five gallons. It is best to let it stand for a couple of hours first before drinking.
Please note

Bleach will not kill some disease-causing organisms commonly found in surface water but it will not remove chemical pollutants.

Any filter that has activated carbon, will remove the chlorine at the tap. This may make this approach slightly more palatable.

Finally, if there are notices posted recommending that water should be boiled, usually because of E.coli, faecal coliform bacteria or other bacteriological contamination, do not rely upon any of the above-described filtration processes alone. Boil the water for five minutes, or don't use it. If you are ever any doubt about the purity of the water it can be reliably disinfected by this boiling technique or adding bleach.

Keeping your onboard water supply safe and good tasting is not overly arduous. Watermakers and/or good catchment systems will guarantee the delivery of a flood of safe and pure water without relying on onshore supplies. Should you need to go for shore supplies some simple and inexpensive filtration systems, with a little added due diligence, will deliver clean water in and out of the tanks.

With thanks to:
Michael Harpur, Yacht Obsession.

How To Install a Watermaker

Spectra Watermaker handover

Camco 40043 TastePURE Water Filter with Flexible Hose Protector


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