Carbon monoxide (CO) on a boat comes from anything that burns fuel: the propulsion engines, the generator, a propane or gasoline cabin heater or stove, and exhaust from a neighboring boat at the dock or rafted alongside. It has no color and no smell. The protection is a marine CO alarm, listed to UL 2034 for recreational boats, in every enclosed space where people sleep or spend time. Press the test button on each one before every trip and replace each alarm before the date its maker gives. If an alarm sounds, or anyone feels flu-like or seasick after time near exhaust, get everyone into fresh air and call for help.
AIRLOCK makes odor eliminators. It does not sell CO detectors, and nothing it makes detects, reduces, or removes carbon monoxide. A musty cabin and a CO problem are two separate things.
It's natural to picture CO staying back at the exhaust outlets, low on the transom and pointed away from everyone aboard. Coast Guard guidance warns about it on the swim platform, on the back deck, in the cockpit, and inside the cabin. Each of those spots gets its CO a different way, and one of them works while the boat is running forward, away from its own exhaust, and still pulls the fumes back aboard. The paths come right after the alarm steps, because those steps should never wait.
If a CO alarm sounds (or someone feels sick near exhaust)
Use these steps when an alarm goes off, and use them when there's no alarm but someone aboard has symptoms after time near exhaust.
- Get everyone into fresh air right away, on deck, the dock, or shore, and away from the exhaust outlets.
- Call for help: 911, the Coast Guard on VHF Channel 16, or local responders, whichever fits where you are.
- Stay out of closed cabins until the boat is ventilated and responders or a qualified technician say it's safe.
- Get medical help for anyone with symptoms.
- Find and fix the source before the next trip: an exhaust leak, a generator or engine problem, canvas that traps fumes, or a neighboring boat's exhaust path.
Never silence a working alarm and keep using the boat.
Where carbon monoxide comes from on a boat
CO is in the fumes produced any time fuel burns. The CDC's list of sources includes vehicles, small engines, stoves, lanterns, grills, and furnaces. Aboard, that means the propulsion engines, the generator, a propane or gasoline cabin heater or stove, and the grill. It can also come from somebody else's boat. ABYC's summary of its 2025 A-24 revision points out that the source of CO can be on another boat, which matters at a crowded dock or in a raft-up.
The engines get most of the attention. The Coast Guard has also issued a specific advisory about gasoline generators, and it points at the hours when the boat isn't underway at all.
How CO reaches the places people sit and sleep
Exhaust travels once it leaves the boat. These are the paths described in US Coast Guard recreational-boating guidance and related investigations, starting at the stern and working forward.
The swim platform and the cavity under it
A gasoline generator with transom exhaust keeps running while people swim off the stern or sit on the swim platform. The Coast Guard advises turning those generators off whenever the swim platform is in use, and keeping swimmers out of the cavity between the swim platform and the hull, where exhaust can collect. This is the generator advisory. It was written for a boat sitting moored or at anchor with the generator on, the same hours people spend on the swim platform.
The back deck
USCG Boating says never sit, teak surf, or hang on the back deck or swim platform while the engines are running. Teak surfing is never a safe activity.
Underway: the station-wagon effect
This is the path that carries exhaust forward. When the boat is underway or idling, exhaust can be drawn back into the cockpit or cabin, much like a station wagon pulls its own exhaust in through an open tailgate. Canvas enclosures and limited forward ventilation make it worse, and some canvas setups trap more than others. The Coast Guard's answer is to keep fresh air moving through the boat at all times.
At the dock or rafted up
When the boat is docked or rafted alongside another, the other boat's exhaust can get into your cabin or cockpit. The Coast Guard's trip checklist includes keeping an eye on that exhaust.
Leaks in your own exhaust system
Cracked hoses, loose clamps, rust or black streaking, and a change in the exhaust sound are warning signs. Each time the engines and generator start, confirm that water flows from the exhaust outlet.
Fuel-burning gear in or near the cabin
Never use generators, grills, camp stoves, or other gasoline, propane, or charcoal-burning devices inside an enclosed cabin, or outside near an open hatch or port that feeds the cabin. Open hatches and ports come up again when it's time to mount the detector, for a different reason.
Follow any of these paths to its end and it finishes in the same few places: the cockpit and the cabin, where people sit, eat, and sleep. Those are the spaces ABYC wants a detector watching.
CO symptoms: seasick, or CO?
Seasickness and CO poisoning can look alike on the water. The CDC lists headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion, often described as "flu-like." USCG Boating's rule is to treat seasickness-like symptoms as possible CO: get the person into fresh air and seek medical attention unless you're sure it isn't CO.
People who are asleep or under the influence can die from CO before they have any symptoms. A guest asleep in the forward berth won't feel a thing, so the alarm in that berth has to do the noticing.
CO can be present with no exhaust smell at all. If you do smell exhaust fumes, the Coast Guard says CO is present too, so air the boat out. The other smell aboard, the musty one waiting when the cabin's been shut all week, tells you nothing about CO either way, and that mix-up gets settled further down.
Does my boat need a marine CO detector?
ABYC standard A-24 (Installation of Carbon Monoxide Detectors and Alarms) calls for carbon monoxide detectors on boats with enclosed accommodation spaces. ABYC's public summary of the 2025 revision describes the spaces to monitor as "habitable spaces": spaces surrounded by permanent structure with provision for sleeping, cooking, eating, navigation or steering, or washing or toilet use. On a boat, that reads as the berths, the galley, the salon or dinette, the head, and an enclosed helm. Storage lockers, open cockpits, and engine rooms typically do not require detectors.
The detector itself should be tested by an independent laboratory to UL 2034, including the sections for recreational boats. Use a unit marked for marine or recreational-boat use. A household plug-in alarm was never tested for boat conditions.
Where to mount a marine CO detector
Mount and locate each detector per its listing and ABYC A-24. Under the published A-24 text, detectors monitor the contiguous cabin space and each sleeping space closed off by solid bulkheads or structure and permanent doors. A sleeping space separated only by a curtain does not need an extra detector.
ABYC says to avoid mounting a detector where:
- rain, spray, water, or sunlight can damage it
- hatches, ports, or forced ventilation dilute the air the sensor samples
- corners have poor natural air circulation
That second item is the open hatch again. Fresh air through a hatch is good for the people aboard and bad for a detector mounted right beside it, because the sensor ends up sampling diluted air instead of cabin air.
Follow the detector maker's height and wiring instructions. If the unit is wired through a breaker, ABYC calls for a block or other multistep means so nobody turns that breaker off by accident.
Testing, power, and replacement
Pressing the test button on each CO alarm every time you go out is on the Coast Guard's trip checklist. The CPSC notes that the button checks the alarm's circuitry, not the sensor itself. It also can't show whether the alarm will have power when it matters, or whether it has outlived its service life.
- Power. Hard-wired marine detectors need a live supply. Marine CO detector manuals say the alarm must be connected to the correct power source and, for best results, stay powered at all times. On a wired unit, a dead house battery or a tripped breaker can cut that supply. Some 12V models carry a backup battery and some run on a sealed internal battery alone, so check your detector's manual for how yours is powered and what its low-voltage and fault signals mean.
- Age. Replace each alarm before the date on its label or in its manual. Service life varies by model, so don't run one past the maker's limit, and follow the maker's schedule for deeper checks.
- Older boats. Replace any alarm that's been disconnected, or whose housing doesn't show it's marine-rated, with current marine CO detection. USCG Boating has long urged owners of older boats to check this.
The same Coast Guard checklist covers the exhaust system behind every path above:
- Every trip: know where the exhaust outlets are, confirm water flows from the exhaust outlet when the engines and generator start, and listen for any change in exhaust sound.
- Monthly: check that the exhaust clamps are in place and secure, look for leaks such as rust or black streaking, and inspect the rubber exhaust hoses.
- Yearly: have a qualified marine technician check the hoses, the water pump, the metallic exhaust parts, and the generator anti-siphon valve if the boat has one.
A musty cabin smell is not carbon monoxide
A musty, mildew, or head smell in a closed cabin is an odor and moisture problem. Carbon monoxide has no smell, so a cabin that smells fine proves nothing about CO, and a cabin that smells musty proves nothing either. Exhaust is the one smell that points to CO. Never treat a CO alarm as something an air purifier can handle. If you can't tell whether someone is seasick or breathing CO, get them into fresh air and get medical help. For the odor side of a closed cabin, see why boats smell and why boats smell after storage.
What AIRLOCK is not
AIRLOCK makes odor eliminators. An odor eliminator is not a CO detector and not a safety device. No air purifier or odor product, AIRLOCK included, protects anyone from carbon monoxide. AIRLOCK products do not detect, filter, reduce, or remove CO, and AIRLOCK does not sell CO detectors. On a boat, the protection is a working, in-date marine CO alarm, a maintained exhaust system, and the habits on this page. Only the alarm can warn you about CO nobody aboard can smell. The same detector and exhaust habits for RVs are on our RV carbon monoxide and propane safety page.
Boat carbon monoxide FAQ
Does my boat need a carbon monoxide detector?
If the boat has an enclosed cabin where people sleep, cook, eat, steer, or use the head, ABYC A-24 calls for CO detection in those spaces, with detectors tested to UL 2034 for recreational boats. Open cockpits, storage lockers, and engine rooms typically don't need one. When in doubt, follow the standard and your builder's guidance.
Where should a boat CO detector be installed?
In the contiguous cabin space and in each sleeping space separated by solid bulkheads and permanent doors, following ABYC A-24 and the detector's manual. Keep it out of rain, spray, and sunlight, away from hatches, ports, and forced ventilation that dilute the air it samples, and out of corners with poor air circulation.
How often should I test a marine CO alarm?
Every time you go out. USCG Boating's trip checklist says to press the test button on each CO alarm before each trip. The CPSC notes the button checks the circuitry, not the sensor, so also follow the maker's schedule for deeper checks and replace the alarm before its end-of-life date.
What causes carbon monoxide on a boat?
Anything that burns fuel: propulsion engines, a generator, cabin heaters or stoves, grills, and exhaust from another boat. CO tends to build up near the swim platform and transom, under canvas enclosures, and in the cockpit or cabin when exhaust is drawn back aboard underway, which is called the station-wagon effect.
Is teak surfing dangerous because of CO?
Yes. USCG Boating says never sit, teak surf, or hang on the back deck or swim platform while the engines are running, and that teak surfing is never a safe activity. Engine and generator exhaust leaves at the back of the boat and can collect around the swim platform.
Can an air purifier protect against carbon monoxide on a boat?
No. No air purifier or odor product, AIRLOCK included, detects, filters, or removes carbon monoxide. An odor eliminator is not a safety device. Protection on a boat comes from working marine CO alarms, a maintained exhaust system, and fresh air habits. If an alarm sounds, get everyone into fresh air.