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Avoiding carbon monoxide poisoning aboard a vessel

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What is the issue?
Carbon Monoxide (CO) is a by-product of carbon-based fuel products such as wood, paraffin, LPG, petrol and diesel. CO is produced when the fuel does not burn completely. It is a colourless, odourless, and tasteless gas that in high concentrations can be fatal, sometimes in as little as a few minutes.

CO kills by replacing the oxygen in the bloodstream that prevents essential supplies to your body tissues, heart, brain and other vital organs. Survivors of severe CO poisonings can be left with long-term brain damage such as poorer concentration or mood swings, etc. Even breathing-in low levels of CO over a long period will cause serious memory problems and concentration difficulty.

The risk of CO poisoning is extremely high in leisure boats. This is because of a tendency to run engines for long periods whilst charging batteries and/or when a vessel is becalmed where the engines may run for days on end. All that is required is an exhaust back-draft, due to a wind shift, or an exhaust leak and the integrity that keeps water out then serves to make the perfect container to collect gas and fumes. This makes CO the 'silent killer' of pleasure boaters.

Why address this?
This is a common killer in the UK. Each year boaters die or are made seriously ill from carbon monoxide poisoning.

How to address this?
Fit a suitable audible carbon monoxide alarm to the vessel. If you use a charging profile that has generators or engines running whilst people sleep, fit all sleeping quarters with their own individual alarms. Only if the boat has a single multi-use cabin, is one alarm OK. It is just not worth the risk.

When fitting the device (s) follow the alarm manufacturer’s installation instructions as far as the space and nature of the boat allow. If in any doubt call the manufacturer’s or supplier’s support line for advice.

I was unaware of this danger until a fellow cruiser lost two dear friends that nodded off whilst charging batteries at anchor. By chance, a drifting plastic bag was pulled into the water coolant intake blocking the pipe. This, in turn, caused the muffler to melt and the exhaust to vent out below decks leading to the fatalities. They just went to sleep naturally and never woke up again.


The effects of CO depend upon the volume of exposure. You can feel unwell even with very small doses over periods, for instance breathing the exhaust from nearby jet skis. But the greater the amount of CO there is in the air, as expressed below in parts-per-million, or the longer you are breathing in CO, the worse the symptoms may get:

  • • A slight headache will develop within two to three hours at an exposure of 200ppm

  • • A pronounced frontal headache will develop within one to two hours when exposed to 400 ppm.

  • • Light-headedness, queasiness and convulsions within an hour of exposure to 800 ppm. Anaesthetise within two hours.

  • • Headache, queasiness, light-headedness within twenty minutes when exposed to 1600 ppm. Fatal within ten minutes later.

  • • Headache, queasiness, light-headedness within ten minutes when exposed to 3200 ppm. Fatal within twenty minutes.

  • • Headache, queasiness, light-headedness within two minutes when exposed to 6400 ppm. Fatal within fifteen minutes.

Be vigilant as the lower exposure symptoms of CO poisoning can easily be mistaken for the flu, food poisoning, a hangover, seasickness or general tiredness. Ask yourself... ‘are people ill on my boat, but feel OK ashore?. But as we have seen, if you are asleep, you may not notice any symptoms as they develop and this is why the audible alarm is essential.


If you suspect you have carbon monoxide poisoning or the CO alarm activates, you need to act
fast. Get all people and pets out to fresh air as quickly as you can. Stay in the fresh air. If you can, on your way out, turn off appliances and engines, open doors, hatches, portals and awnings to allow fresh air to flow through the boat.

If anyone is exhibiting CO poisoning symptoms see a doctor and say you may have suffered carbon monoxide poisoning. Anyone with severe symptoms needs to get to a hospital as quickly as possible. Get a properly qualified person to find and fix the source of the problem before the vessel is used again.


In addition to having a CO2 alarm, and checking that it is functioning correctly, skippers and boat owners should be mindful of the safety of exhaust systems in daily use.

Routinely check the exhaust system of the boat or its generators. Inspect every part for leaks or problems including; manifolds, pipes, joints, hoses, clamps, silencers, and through-hull fittings. Ensure all exhaust clamps are in place and secure and that the rubber hoses are pliable and free of any kinks or have no heavy gear resting on them. When starting an engine, confirm that water flows from the exhaust outlet and listen for any tonal changes that would indicate there are problems.

Only install heating and cooking appliances that meet the latest standards and are suitable for use in boats. Have appliances properly installed and serviced routinely by competent fitters. Do not install or fix a portable generator inside any accommodation space.

Whether the boat is moving or moored, be mindful that under certain running and or wind conditions CO can be deflected or drawn in from engine exhausts. Avoid mooring too close to objects that obstruct exhaust port venting and be particularly vigilant when the air is still. Likewise, do not allow your exhaust to invade a nearby vessel and try to avoid running your engine whilst moored in crowded marinas.

With thanks to:
Michael Harpur, Yacht Obsession.
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