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The Low Down: Fresh Water On Board

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This article is written with the dual aim of educating and preparing yacht Engineers, their Captains, Operators, Owners and Management Companies to best plan and comply with the regulations and good practice for the production, treatment, storage and testing of fresh water held on board a yacht.

The Background

Do you drink the fresh water that you make on board? If not, why not? Do you spend lots of money on bottled water adding to the plastic production and recycling problem? Mounting evidence suggests that fresh water made from sea water using a correctly maintained Reverse Osmosis Plant (RO plant) or even plain tap water may be better for you than some of the plethora of bottled waters available.

This will depend on the condition of the plant membranes and the storage tanks on your vessel but also where the water was made and what treatment systems are on board. Tap water in most countries is usually perfectly suitable and in some cases better than the bottled variety. 

Tap water pic 1


The MLC Minimum Requirements Regulations, in Part 8, make provisions covering food and catering for seafarers working on merchant ships, commercially operated yachts and other sea-going vessels. In accordance with the provisions of these regulations, pleasure vessels, fishing vessels, ships of traditional build, warships, naval auxiliaries and vessels which are not ordinarily engaged in commercial activities are not subject to these regulations, but any yacht of any size that is commercially operated must comply.

Must or Should or Best Practice?

Whether you are commercially operated or not should not be the deciding factor on whether you routinely sample and test the fresh water made on board. Even if you only drink bottled water, illness can be transmitted through washing, brushing teeth, showering and food preparation with contaminated water.

The 2014 MSN 1845(M) Maritime Labour Convention, 2006: Food and Catering: Provision of Food and Fresh Water, states – “The ship-owner and Master of a ship must ensure that food and drinking water of appropriate quality, nutritional value and quantity is provided free of charge to meet the needs of those on board.” 

Interestingly Annex 2 uses the word should in its guidance rather than must, so what should we do? It would be foolhardy to ignore the wording in MSN 1845 (Regulation 34) which also states that the drinking water must be suitable in respect of quantity, nutritional value, quality and “ does not contain anything which is likely to cause sickness or injury to health.”  How do you know unless you test it?

Waiting until someone falls ill is not a good quality control measure. The RO plant manufacturers and test kit providers (and I incidentally) agree drinking water should be sampled and tested on common sense and best practice grounds.

MSN 1845 however makes no mention of the mandatory recording of tests. Surely one would be on thin ice trying to explain the difference to a less than cooperative Port State Inspector, or to answer some fairly searching questions if someone falls ill on board as a result of ingesting contaminated water.

safe drinking water

What Should I Drink?

I regularly canvass students on my Engineering courses as to whether they drink the water made on board. Generally they fall into two distinct categories of either “no way, have you seen the sea water off the coast here?” or “of course we do, it’s better for you and cheaper”. The bottled water industry is worth over £1.7 billion in the UK alone and even bigger I suspect here in France. 

There is a view that the expensive bottled water may contain harmful bacteria that wouldn’t be allowed in tap water and there are controversial concerns that have linked the plastic used with cancer-causing chemicals, but official data are hard to find. Sadly some bottled water is merely tap water that has been distilled to remove most of the minerals.

Clever marketing has made it an essential accessory for some folk, lest they are seen without a trendy brand in hand on a hot day, swigging voraciously as though it tastes better than good quality tap water.

bottle water


As an ex-smoker of advancing years, my taste buds are probably past their best and some of the bottled waters seem tasteless and a waste of money. At sea 'potable' water made from flash distillation plants or RO plants passed through hydrated lime is a good thirst quencher. 

The fewer the minerals, including sodium chloride (salt), the less taste and the more acidic it becomes. There was a view that the purer the water the better – not so! In fact pure distilled water is bad for you due to acidity (pH) levels. I find switching from bottled to tap and back again on a regular basis provides a good balance ashore and even switching brands quite refreshing. However at sea it was always the ship's made water for me.


Sea water that is to be treated on ships should be taken from areas relatively free from pollution, including air pollution. Twenty miles from land is generally considered to be a safe distance but it may be in excess of the twenty miles in some cases. Judgment should be used based on a risk assessment which should include consideration of the possible effect that ship operations might have on the quality of the water intake.

The seawater inlets (sea chests) should be located forward and, if possible, on the opposite side of the ship from the overboard waste water, sewage and ballast tanks discharge outlets. Sea water should pass through suitable strainers and filters ranging from 80 microns down to 20 microns before entering the water making equipment. (One micron is one thousandth of one millimetre by the way).

salt water

Maintenance and storage

All elements of the freshwater production, treatment and delivery system including filters, pumps, hot water heaters, tanks etc. should be inspected, cleaned and flushed out and items replaced where appropriate, according to the manufacturer’s instructions and the planned maintenance system. The maintenance recommended in MSN 1845 is self-explanatory regarding annual super chlorination and the cleaning of fresh water systems including filling hoses and fittings. Annex 2 of the MSN also gives good advice on some of the dangers associated with tank coatings.


Water made on board is usually tested automatically for salt content by a device called a salinometer. This measures the salt content by electrical conductivity and is measured in micro-siemens. It will alarm if the salt level is too high and the product water is then dumped overboard. Since the quality of the water is controlled by the membrane back pressure it is possible to strip out too many minerals and salts such that the product is tasteless and acidic. There is normally no alarm for this.

This is another good reason for testing. Many vessels will pass the made water (known technically as permeate) through carbon filters to remove odour and limestone, or hydrated lime filters to add minerals to improve taste and restore a more neutral acidity. Another good reason for testing.

We are not done yet. Made water may still have bacteria, viruses or pathogens that were not boiled to death in a distillation plant (steam) or trapped in the pores of the membrane, so it is treated yet again. This time either Ultra Violet (UV) light or Silver Ion sterilisers are used. If UV is used then the made water has to be chlorinated to 0.2ppm to kill any remaining bacteria as this is a temporary sterilisation process that interferes with the bacteria’s DNA. Silver ION is considered permanent so chlorination is not necessary.

fresh water

On the downside, these treatments may be doing too much or too little – another good reason to test. With all these processes in place you can see why this water may be better for you than from other sources.


Taking a regular sample and testing on board or sending it off for analysis is mandatory for commercially operated yachts, and for others its a good idea. Regular tests can be undertaken on board by commercially available kits and most are easy to use.

The basic test kits are usually looking for the free chlorine or chloramine levels where 0.2ppm is recommended for water taken from shore mains or barge. More comprehensive test kits also check the levels of Silver Ion, E-coli, pH, colour, Copper, Coliform, Iron, Legionella and other bacteria without specialist training and provide log books for the recording of results.

What should I do?

The commonly held best practice in the industry is to conduct some form of risk assessment undertaken by the Chief Engineer in consultation with the Master. A comprehensive and well run maintenance system will include fresh water treatment, tank cleaning and water testing.

If you are “in class” your Surveyor can advise. Likewise your DPA can do the same if you are in an “ISM” or “mini ISM”  Regular testing and the results of such will stand you in good stead with everyone on board, not to mention Port State Inspectors or if your vessel is quarantined due to ill health.

Head of Engineering at Bluewateryachting and Member of the International Association of Maritime Institutes.

Thirsty for more info?
MSN 1845 (M) - Maritime Labour Convention, 2006: Food and Catering - Provision of Food and Fresh Water

WHO: Water Sanitation & Health

*Image credits: Flickr/ blog spot/Flickr

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