John Tylor and PBO’s experts share some valuable safety tips for avoiding carbon monoxide on a boat

Carbon monoxide on a boat: how to protect you and your crew

While most PBO readers are in the depths of winter, trying to stay warm with diesel heaters, on the other side of the globe we’re enjoying sunny evenings at anchor, with air-conditioning units blasting out a mini fog of diesel fumes.

Regardless of whether we sail in a warm or cool climate, we’re likely to be burning fossil fuels in enclosed spaces, so it’s worth reminding ourselves of the dangers of carbon monoxide.

What is carbon monoxide?

Carbon monoxide (CO) is a gas with no colour, odour or taste.

All hydrocarbon fuels exhaust a mix of nasty gases that includes the deadly CO.

To burn completely, fuels such as gas, LPG, coal, wood, paraffin, oil and petrol need an adequate supply of air.

When carbon is burnt, it should be fully oxidised, forming carbon dioxide, which comprises a carbon molecule with two oxygen molecules attached.

However, without adequate ventilation, the carbon only partially oxidises, binding to just one carbon molecule (CO) instead of two (CO2).

The bolts on the manifold had vibrated loose allowing exhaust gases to enter the living spaces

Causes of carbon monoxide on a boat: The bolts on the manifold had vibrated loose allowing exhaust gases to enter the living spaces

A build-up of this deadly gas can happen in a number of ways: through faulty or misused appliances, exhaust fumes from an engine or generator, escaped flue gases from solid fuel stoves or a short supply of air.

Tragically, each year many people die from this deadly poison.

While everyone is at risk, it is the very young, the old, those with anaemia, heart or respiratory disease that are especially vulnerable.

If you smoke, then you are even more at risk than the general population.

Our body’s red blood cells have the crucial job of transporting oxygen from our lungs around our body, in particular to our brain and heart.

A blue flame on a gas cooker

The healthy blue flame shows complete combustion

For some unfortunate reason, the haemoglobin in our red cells finds poisonous CO more than 200 times more attractive than the oxygen it is meant to carry.

So, with a blood stream now full of deadly gas, we quickly begin to experience its effects.

These are not always obvious, especially to the person affected as they immediately begin to feel drowsy and this can mask many other symptoms.

The poison also inhibits cell tissues using oxygen the blood may have been able to absorb.

The body takes a long time to purge CO, so symptoms will persist even after the affected person has been removed to safety because vital organs remain starved.

What can be done to reduce the risk of carbon monoxide on a boat?

Routinely, before any major trip or during the winter layover, have all fuel burning appliances checked by a qualified technician.

Appliances deteriorate with time and can become a health risk without warning to those on board.

Blocked flues are a common problem that will prevent exhaust gases from venting to the outside and so filling the cabin, and even a slight leak in an engine exhaust will push gases into living spaces.

A yellow flame on a gas cooker

The yellow flame from this gas jet shows incomplete combustion. It needs urgent attention

Small portable solid-fuel camping cookers are becoming more popular as they use cheap, readily available fuel and give a marvellous flavour to the food.

While they all come with a caution that they should not be used without adequate ventilation, in winter the temptation is to heat the cabin while the dinner is baking.

The problem is they not only consume oxygen but also exhaust gases, including CO, into the same space we occupy.

Without ventilation, people down below can increase their risk without realising it.

Gas stoves

Generally, our gas stoves (and some gas water heaters) vent into the internal space of the boat rather than outside, so make sure there is plenty of positive ventilation while these are in use.

Gas flames should burn with a blue flame; if you see yellow tinges in the flame, have the appliance checked out as this indicates incomplete combustion and a higher level of CO output.

Gas should be installed and checked by a gas safe engineer, who is marine endorsed.

A white CO alarm. A white CO alarm is essential for detecting carbon monoxide on a boat

A CO alarm is essential for altering you to carbon monoxide on a boat

Portable generators

Small portable petrol generators are becoming cheaper and more popular. The carbon monoxide output from these can be dangerously high.

PBO reported on a tragic case in Cumbria in 2013 where a mother and daughter were killed by fumes from the improvised exhaust of a boat generator, which became detached while they slept.

Exercise extreme care if using a generator as fumes can blow into the cabin and accumulate, even if you have it running outside.

And finally, never use a petrol-powered tool in the cabin without positive ventilation and a full-face respirator; never use one while you’re alone.

Heaters and lanterns

Other sources of CO include oil or kerosene-fuelled heaters and lanterns not vented to the outside.

Closing hatches to preserve all available heat or running heaters could exacerbate the problem.

A heater that takes its combustion air from outside and has a fully sealed combustion chamber exhausting outside would be a sensible investment.

A heater on a boat

The kerosene heater and oil lamp on the author’s boat. The heater exhausts outside: check the flue is clear

Cruiser Katy Bourke wrote in Cruising World (January 1982) of a fortunate escape while wintering in New York.

She was using two unvented kerosene heaters.

They tried to be careful about ventilation throughout the boat, but one particularly cold night “Bruce woke up in the middle of the night with a blinding headache (an early symptom of carbon monoxide poisoning). We had closed the boat up too tightly.”

Some sources have blamed candles, but the amount of gas produced by these is small, and in any case the boat owner runs a far greater risk of fire from the exposed flame.

What about a CO detector?

A CO detector is an essential piece of safety equipment for any boat, and marine surveyor Ben Sutcliffe-Davies recommends fitting one in every cabin.

But never let an alarm be a substitute for proper operation and maintenance of appliances, engines, etc.

There are several types of detectors available, but remember that the end-user (that is, you and me) has no reliable method of testing or calibrating the unit.

AA batteries in the back of a co alarm. A white CO alarm is essential for detecting carbon monoxide on a boat

CO alarm batteries should be replaced when the monitor starts to chirp and show a low battery light

It is possible to receive what you think is a false alarm (see panel, below), which can be difficult to verify.

CO is not smoke (although this is where we often find it) so we do not have a reliable method of checking the source of the gas.

Each alarm must be investigated with spaces thoroughly aired before declaring all safe.

CO detectors may have an active sensor that requires regular servicing or replacement, so if you have one make it part of your regular service programme.

Ensure your alarm is certified (BS EN 50291-2), test it routinely and never remove batteries except to replace them.

Why not just use a smoke detector?

A smoke detector will alert us to the presence of smoke which we can check out and locate the source.

There are two common types of technologies for smoke detectors: ionizing and photoelectric detectors.

Ionizing smoke detectors use a small chamber with a radioactive element to detect combustion products.

A white CO alarm is essential for detecting carbon monoxide on a boat

A CO alarm should be replaced seven years after installation

These are the most common because they work reasonably well and are cheap.

Photoelectric detectors use a photocell to detect smoke particles in the air sample.

These are generally recommended by Fire Authorities as they’re more sensitive.

Other detectors use a combination of both methods to improve reliability.

While carbon monoxide is often present in smoke, neither of these smoke detectors will sense it alone when it is at its most dangerous.

How to prevent poisoning

Don’t use a gas appliance if you suspect it’s not working properly.

Never close up the cabin, shut ventilators or any openings that supply fresh air while an unvented appliance is operating, even in the dead of winter.

Educate your crew about the sources and conditions that may result in CO poisoning and the treatment of any person affected.

Symptoms and treatment

CO poisoning can mimic the symptoms of some common illnesses, causing tiredness, shortness of breath, headaches and nausea.

It may be confused with sea sickness, flu, colds or other viral infections.

If unrecognised and the exposure continues, then it may progress to severe headaches, sleepiness, weakness, nausea, dizziness and vomiting.

A generator on the deck of a boat. Prevent carbon monoxide on a boat by checking where exhaust fumes go

Beware exhaust fumes from generators outside finding their way into the boat

If still untreated, then it can rapidly lead to confusion, loss of consciousness, brain damage and eventually death.

At any level of breathing this gas our health will be affected.

Carbon monoxide (CO) poisoning can happen within a matter of minutes and is responsible for more deaths than any other single poison.

If symptoms are linked to any particular appliance, then you should suspect CO poisoning.

Stop using the suspect appliance and move the victim and everyone else to a well-ventilated place and seek medical advice urgently.

Continues below…


Diesel engines are the power-source of choice in our boats for many reasons.

The fuel is less flammable than petrol; they are economical and generally more reliable due, in part, to the relative absence of complicated electrics.

But they still produce carbon monoxide, albeit in lower quantities than a comparable petrol engine.

They also exhaust a lethal cocktail of other irritants that do us long term damage.

These include very fine soot, (linked to lung disease and cancer), and nitrous oxide and nitrogen dioxide that can cause other respiratory disease.

Other by-products from our diesel engines are formaldehyde, benzene, sulphur dioxide, hydrogen sulphide, carbon dioxide, arsenic, and acetaldehyde – all very good reasons to avoid breathing any of it.

Beware cheap exhausts

PBO’s marine surveyor Ben Sutcliffe-Davies warns against the fire and CO risk of cheap exhausts bought online and fitted to marine diesel heaters.

“Personally I’d not put anything on my yacht that hasn’t got the correct CE or new UK mark,” says Ben.

“There are reports every year about carbon monoxide poisoning in the news and on the Marine Accident Investigation Branch website.”

After surveying three yachts in 2021 where fires had been caused by diesel heater failure, Ben bought an £8 diesel heater exhaust on the internet for a closer look.

Prevent carbon monoxide on a boat: Avoid A truck exhaust in chrome

Beware an £8 ‘truck’ exhaust like this

He discovered it was intended for a truck, not a yacht, and contained a small hole that on a vehicle would be used to expel any water vapour.

“In a yacht any vapour that drops out of this hole is going to include carbon monoxide or exhaust gases and they’re not able to escape being in your cabin or your locker,” he says on his YouTube video.

Boat owners with diesel heaters are urged to check the exhaust system to ensure a cheap road vehicle exhaust is not fitted – they’re fine for trucks, but not for boats.

Ben added: “If you have one, get rid of it, buy yourself the proper exhaust which is a stainless steel tube, clearly marked for marine use, and get it fitted professionally. I also recommend installing a carbon monoxide alarm in every cabin.”

Co detector gave me more than we bargained for

Stu Davies, PBO’s engine expert was puzzled when his CO detector starting bleeping with no apparent cause

We got back on the boat one evening and the carbon monoxide detector was pinging occasionally.

However, there was nothing on board being used that could produce CO.

I checked the batteries in the detector at 1.49V and they were all the same, so I was puzzled as to the cause of the alarm going off.

Then I noticed that the sticker on the bottom said replace the detector after seven years and that it would start chirping at or around that date to warn us to do so.

I assumed that was the cause of the noise so I took the batteries out and went to bed.

The next morning I did a recheck and confirmed the message to replace the detector and so removed it.

We went out in the morning and all seemed well, though I did note the BM1 voltmeter/ammeter was showing 13A charge which was unusual.

A boat battery

A failed battery led to gassing and the CO alarm being activated. Credit: Stu Davies

That afternoon I came down into the saloon and smelt something I recognised from three years ago when we had a battery charge runaway, and we’d changed the three service batteries (we have 3x110A batteries in parallel for the house bank).

It was ‘toasting’ the battery! The voltmeter was showing 14+V and 13A charge!

One of the batteries had failed and the chargers were thinking the battery was flat and so were whacking amps into it.

I quickly switched off the mains charger and solar controller plus the battery switches.

They’re in the port aft cabin, and the last thing needed now was a stray spark near the batteries.

I carefully opened up the battery compartment in the starboard aft cabin.

One battery was hot, very hot, and gassing. It was that gas (hydrogen and some hydrogen sulphide) that was setting off the CO detector.

It was interesting that the detector detected that gas even before we smelled it the next day.

I let the battery cool, then disconnected it and removed it.

After cooling I put the voltmeter on it and it registered 6.9V.

A note of caution here, batteries explode if not handled carefully.

If you are not confident about what you are doing, open all hatches and switch off the chargers and electric systems.

Let everything cool down and get an expert in to deal with it.

The moral of the story? If your CO detector goes off and its batteries are OK, then a check of the main boat batteries might be in order! CO is not the only gas that can set them off!

10 tips for staying safe from carbon monoxide on a boat

The Boat Safety Scheme (BSS), a public safety initiative owned by the Canal & River Trust and the Environment Agency, has excellent advice on everything from fire and electrical safety to solid fuel stoves, generators and petrol. See

  • Install fuel burning appliances in line with makers’ directions
  • Follow servicing guidelines; maintenance should be routine and competent – don’t allow bodged repairs, adjustments and adaptations
  • Always use appliances as per the instructions and never use cookers for space heating
  • Don’t block ventilation – appliance fuels like gas, coal, wood, oil, paraffin, etc. need sufficient air to burn safely
  • Don’t use charcoal barbecues in or near a cabin – only stone-cold charcoal is safe
  • Keep engine fumes out of the cabin space, never use a portable generator in or near a cabin
  • Learn about the danger signs and spot potential hazards before CO occurs
  • Deal with problems immediately, never use equipment you suspect has problems
  • Install at least one certified CO alarm and test it routinely

Enjoyed reading Carbon monoxide on a boat: how to protect you and your crew?

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