Proper boat ventilation and a decent carbon monoxide alarm are essential for any yacht with accommodation, says Ben Sutcliffe-Davies

How often do you think about boat ventilation?

One of the most amazing things about owning a yacht is how little red tape there is, or even laws to follow, compared to life ashore.

The rules that do exist are generally there for good reason.

If you own a boat on inland waterways it is now mandatory to have at least one carbon monoxide alarm.

On the coastal yachts I survey, it baffles me why so many don’t have any kind of carbon monoxide alarm.

Diesel heaters must have the marine specific exhaust. The one made for trucks has a drain that can leak carbon monoxide

Diesel heaters must have the marine specific exhaust. The one made for trucks has a drain that can leak carbon monoxide

Over the last few years there have been an increasing number of fatalities reported by MAIB, for instance, from the build-up of carbon monoxide in craft.

These incidents most often occur when the crew are asleep and faulty systems are being run, such as poorly-fitted diesel heaters with the wrong type of exhaust system, generators left running and improperly ventilated, or engines with exhaust failure.

There have even been incidents where a vessel moored nearby has discharged enough fumes in a marina environment to fill someone else’s accommodation space.

It should take little convincing that if you don’t already have at least one alarm, you need to buy one and fit it properly.

Ideally buy two; they are readily available online costing little more than £10 and the standard kind for homes is also suitable on a boat.

The inland waterways Boat Safety Scheme ( provides lots of information about the appropriate locations of where a carbon monoxide alarm should be positioned.

Dome vents are fixed open, allowing good airflow

Dome vents are fixed open, allowing good airflow

As a minimum, any locations where people sleep should have an alarm as well as main areas.

It’s also worth testing alarms and ensuring they are loud enough and close enough to wake you up, especially if you are a heavy sleeper.

In the middle of winter when its sub zero outside, it’s tempting to try to eliminate chilly draughts.

I did just this as a teenager living on a house boat when I was apprenticing as a boat builder.

I still remember my dad coming aboard on a very cold day with about a foot of snow outside, losing his rag with me and ranting on and on about how stupid we had been and how dangerous it was to cover the cabin vents with the gas stove and gas heater on. He had a point.

How we survived when I look back I don’t know.

Dorade vents are essential to maintain air-flow down below

Dorade vents are essential to maintain air-flow down below

When sailing in summer, there are generally enough hatches open, with less heating and cooking required, to ensure that the boat is well ventilated.

As autumn sets in and temperatures drop, particularly overnight, it is a good time to take a quick look at the ventilation on your boat.

All boats should have fixed open ventilation, positioned at a low level where possible.

The most common low-level venting is normally through the companionway louvre vents, and overboard via an open transom or cockpit drains.

High-level venting is provided by dorade or dome vents on the coachroof, and should be located far enough forwards to create a flow of air through the boat.

Even when closed, closable dome vents should still allow enough ventilation to meet the minimum calculated requirements.

The venting requirements are worked out from a formula that include the number of people on board, and takes in the kW size of each appliance fitted within the accommodation – there are
a set of very important calculations made to ensure that the right amount of venting is provided.

The formula for this calculation can be found on several websites including the Boat Safety Scheme (visit

The formula for calculating the fixed boat ventilation requirements is as follows:

V= [2200 x U] + [650 x P] +[550 x H] + [440 x F]

where: V= ventilation requirement (in mm2)

U = input rating for all unflued appliances (inc. cokers) (in kW)

P = number of persons for which the accommodation space is designed

H = normal output rating of all open-flued solid fuel appliances (in kW)

F = input rating for all open-flued appliances (in kW)

Regular maintenance is key to good boat ventilation

Many vents include gauzes and filters to keep out flies or dirt, but can become clogged over time.

These need checking on a regular basis so they don’t start to restrict airflow. The venting calculated must not be able to be closed.

I can’t stress how important it is not to cover or block these vents at any time, no matter how tempting!

More and more owners live onboard for more extended periods of time.

Gas stoves are a prime source of carbon monoxide which is why boat ventilation is so important

Gas stoves are a prime source of carbon monoxide which is why boat ventilation is so important

With increased power demands, many also run a portable generator.

If this is on deck near a vent, or worse, in the cockpit, it may well be that the exhaust isn’t discharged over the side, but into the cockpit and down into the accommodation.

It’s such a common practice that many are surprised when they realise how much of a carbon monoxide poisoning risk this poses.

One other issue can be caused from the fact that most engines in yachts are fitted with no proper method of sealing off the engine compartment or lack good, extracted air venting overboard.

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If you have a leaking exhaust system in these scenarios it could be a significant issue.

If you intend to sail into the winter months, please ensure you buy at least one alarm and resist covering your vents.

Check your exhaust systems for possible leaks, and get your heating systems properly serviced, making sure to check that it is fitted with a marine-specific exhaust unit.

Any air extractors you have fitted should be effective and fully functioning.

Even being shouted at by my late dad for covering vents is better than carbon monoxide poisoning.

Carbon monoxide poisoning

Carbon monoxide has no smell or taste. Once inhaled it enters your bloodstream and prevents your blood being able to carry oxygen as normal.

Low-level symptoms are not always obvious, but include a headache, dizziness, nausea, tiredness and confusion, stomach pain and difficulty breathing.

More serious poisoning can cause brain damage and death.

The NHS recommends seeking medical advice from your GP if you think you have been exposed to low levels of carbon monoxide, and going straight to A&E if you think you have been exposed to high levels.

You will then need to have your boat checked for sources of carbon monoxide and any problems rectified before you stay on board again.