Carbon monoxide does not give you a sea story to tell afterwards, because the people it catches do not always wake up. It is the one cabin hazard that genuinely frightens me, more than gas explosions, more than fire, because it works silently on a sleeping crew and the early symptoms feel like nothing worth waking up for. A canal boat in autumn, hatches shut against the cold, a heater running, a generator on the bank: that is the textbook setup for a tragedy, and it happens most years to people who were not careless, just unlucky and unprotected.
What the gas actually is
Carbon monoxide is colourless and odourless. You cannot see it, smell it or taste it. It is produced whenever a carbon fuel burns without enough oxygen: petrol engines, generators, gas heaters, solid-fuel stoves, even a hob left running too long in a sealed space. On the water there is a wide menu of sources, and the canal-boat cabin in cold weather is where they all come together with the worst possible ventilation.
The reason it kills is biochemical. CO binds to the haemoglobin in your blood far more readily than oxygen does, so even a modest concentration in the air slowly starves your body of oxygen. The effect is cumulative. Low levels that you breathe for hours build up in your bloodstream gradually, over a whole night or even across several days, until you reach a critical point without ever having felt acutely ill.
The numbers that matter
CO concentration is measured in parts per million, and the relationship between concentration, time and harm is what every boater should carry in their head.
- Around 200 ppm sustained for a couple of hours produces a slight headache. It feels like a dull ordinary headache, which is exactly the problem.
- At 200 ppm and above, prolonged exposure causes real physical symptoms and is fatal over a period of hours.
- At around 800 ppm, CO can be fatal within minutes.
- Studies of the air outside houseboats have recorded readings well above 1,200 ppm, the level regarded as immediately dangerous to life.
The cruel part is the symptom list: headache, nausea, dizziness, drowsiness, confusion, slowed reactions. Every one of those reads as tiredness, seasickness, a hangover or a bug. A crew quietly being poisoned will tend to lie down and rest, which is the worst thing they can do.
Why the canal cabin is the danger zone
A coastal yacht spends much of its time with the cockpit open and air moving. A canal boat in cold or wet weather does the opposite: it buttons up tight, runs a heater, and traps the crew in a small sealed volume for the night. If you run a diesel or gas heater to keep warm through a French winter on the canals, ventilation and a working alarm are the difference between waking up and not. My piece on choosing a heater for canal winter cruising in France goes into the heating options; this article is about not dying of the by-product.
Generators are a specific menace. Never run a portable generator in or near a cabin. In one investigated death the CO came from a drain plug in the generator's exhaust muffler that had worked loose and dropped into the bilge, so the exhaust was venting inside the boat without anyone realising. The crew did nothing wrong that they could see. That is the nature of this gas.
Fit the alarm. There is no excuse not to.
A certified carbon monoxide alarm is cheap, it runs for years on its own battery, and it is the single thing most likely to save your life aboard. In the UK, since the first of April 2019, a CO alarm has been required on all boats within the scope of the Boat Safety Scheme that have permanent accommodation. Whether or not your boat is in scope, fit one anyway.
Buy one certified to the marine-relevant standard, BS EN 50291-2, which is the version designed for boats and other vehicles rather than houses. The standard defines how the alarm responds: at 50 ppm it must sound between 60 and 90 minutes; at 100 ppm it must sound within 40 minutes and not before 10; at 300 ppm it sounds within 3 minutes. Those response curves are tuned so the alarm ignores brief harmless puffs but wakes you well before a dangerous dose accumulates.
Fitting and living with it:
- Mount it where you sleep, at roughly head height, away from cooking fumes that would cause nuisance alarms.
- Test it on the button at the start of every trip and routinely afterwards.
- Never remove the battery to silence it. If it goes off, it has a reason.
- Replace it at the end-of-life date marked on the unit; sensors degrade over years.
Ventilation is not optional
An alarm tells you there is a problem. Ventilation stops the problem happening. Permanent fixed ventilation, low and high vents that you cannot accidentally block, is what keeps the air in a sealed cabin breathable while an appliance burns. The instinct on a cold night is to close every gap, and that instinct is the one to fight. A flueless gas heater in particular consumes the cabin's oxygen and dumps its combustion products straight into the living space; it needs generous air or it must not run while you sleep.
This connects directly to the wider gas discipline aboard. A gas heater or hob that burns incompletely produces more CO, so the same care that keeps your gas safety aboard for the French cruise in order, sound regulators, no leaks, well-maintained appliances, also keeps your CO production low. A dirty, ageing, badly tuned appliance is both a leak risk and a CO risk. It is also worth knowing what you are burning: if you have switched to local cylinders on a long stay, the notes on Calor versus French gas bottles cover getting the right regulator and a clean supply, which matters for complete combustion as much as for refilling.
Solid-fuel stoves and the autumn temptation
The little solid-fuel stove is the heart of canal-boat life through a French autumn, and it is also a CO source that behaves differently from gas. A stove with a blocked or poorly drawing flue, or one that is damped right down overnight to keep in, can smoulder incompletely and push CO back into the cabin instead of up the chimney. The risk rises exactly when comfort tempts you to shut everything tight and bank the fire down for the night. Keep the flue swept and drawing, never run a stove with a restricted air supply in a sealed cabin, and treat a smoky, sluggish fire as a warning rather than a minor nuisance. A stove that will not draw properly is telling you something about your ventilation as much as your fuel.
The same goes for the moment you raft up or moor stern-to against another boat. Your neighbour's engine exhaust, generator or stove can vent CO that drifts into your cabin through an open hatch, and you have no control over their maintenance. On a still night in a crowded canal basin, the gas does not respect the gap between hulls. Your alarm is your last line of defence against a problem that started on someone else's boat.
A short survival checklist for the canals
- CO alarm certified to BS EN 50291-2, fitted near the sleeping area, tested and in date.
- Permanent low and high ventilation that cannot be blocked, kept clear even when it is cold.
- Heater and cooker serviced; flues clear; no flueless heater running while anyone sleeps.
- No generator run in or near the cabin, ever, and the exhaust system checked for leaks.
- Crew briefed on the symptoms: unexplained headache or drowsiness in the cabin means get into fresh air first, ask questions second.
Carbon monoxide is the rare hazard where the equipment that protects you costs less than a couple of marina nights and asks almost nothing of you. Fit the alarm, keep the vents open, look after the appliances, and brief the crew to treat a cabin headache as a warning rather than a nuisance. The boaters this gas takes are almost never reckless. They are simply the ones who did not have a working alarm above the bunk on the wrong night.
