Carbon Monoxide (CO) Safety on Boats

Printable checklist: Launch day

Carbon monoxide (CO) is a colorless, odorless and tasteless gas. The U.S. Coast Guard says it can harm and even kill you inside or outside your boat, whether you are underway, moored or anchored. You cannot see, smell or taste CO; in high enough concentrations, even a few breaths can be fatal. CO poisonings are preventable when you know the sources, where the gas can build up, the early symptoms and the Coast Guard's trip, monthly and annual checklists.

We do not invent ppm alarm thresholds, detector brand picks or medical dosing. Figures and rules below come from USCG Boating Safety pages and brochure, Discover Boating (NMMA) and the freely published ABYC TH-22 educational report hosted on uscgboating.org. Carriage of the rest of your federal safety gear (PFDs, extinguishers, visual distress signals and so on) is in required safety equipment by length; extinguisher counts, inspection and fire-emergency steps are deepened in boat fire extinguishers and fire safety—this guide expands that guide's carbon monoxide checklist note rather than replacing it. Confirm state overlays in State Rules.

What CO is and why boats produce it

From the Coast Guard CO dangers page and 2015 brochure: CO is produced when a carbon-based fuel—such as gasoline, propane, charcoal or oil—burns. Sources on a boat may include engines, gas generators, cooking ranges, and space and water heaters. Discover Boating adds that CO is inherent in engine exhaust and open-flame combustion, and that the gas can separate from those exhausts and travel through a boat in unexpected ways (for example, into a galley sink drain hose)—and it can also come from a neighboring boat.

ABYC TH-22 (educational report posted by USCG Boating): CO weighs about the same as air and distributes through a space; do not rely on smell or sight of other gases to detect it. The CO component of diesel exhaust is extremely low relative to gasoline-engine exhaust, but gasoline propulsion engines and generators remain the everyday recreational concern.

CO enters the bloodstream through the lungs and blocks the oxygen your body needs. Prolonged exposure to low concentrations or very quick exposure to high concentrations can kill you (USCG dangers page / brochure).

Where CO accumulates on boats

The Coast Guard accumulation page: CO can accumulate anywhere in or around your boat. How it can build up:

  • Inadequately ventilated canvas enclosures
  • Exhaust gas trapped in enclosed places
  • Blocked exhaust outlets
  • Another vessel's exhaust (the boat docked next to you can be just as deadly)
  • "Station wagon effect" or back drafting
  • At slow speeds, while idling, or stopped—and CO can remain at dangerous levels even after your engine or the other boat's engine is no longer running

ABYC TH-22 names backdrafting (station wagon effect) as air movement over or around a boat that creates a low-pressure / suction area around the stern and can raise CO levels on board. Relative wind, boat speed and trim angle all matter; under some conditions the low-pressure area can form elsewhere and let CO enter openings that are not at the stern. Adding or removing canvas, high bow / uneven trim, opening or closing ports and hatches, and slow speed with a following wind can all change onboard CO levels. Cockpit and deck drains can be an ingress path when cockpits or decks are enclosed with canvas or permanent structure.

Engines, generators, heaters and the swim platform

Keep people away from engine and generator exhaust outlets (USCG protect page). Coast Guard protection steps that matter every trip:

  • Maintain fresh air circulation throughout the boat; run exhaust blowers whenever the generator is operating.
  • 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.
  • Never enter areas under swim platforms where exhaust outlets are located unless the area has been properly ventilated.
  • Although CO can be present without the smell of exhaust, if you smell exhaust fumes, CO is also present—take immediate action to dissipate them.

ABYC TH-22: when the propulsion engine or generator is running, do not occupy aft lounging areas or the swim platform; do not swim under, around or near the exhaust outlet. Do not warm up or run propulsion engines for extended periods while stationary. Mooring or rafting next to other boats with engines or generators running can put your boat in a CO atmosphere even if your own engines are off. Portable gasoline generator sets: ABYC TH-22 says do not use this equipment on boats—their exhaust can raise CO in occupied spaces. Discover Boating separately warns never to connect a portable generator to a shore-power inlet (electrical hazard—see electrical and shore power safety).

Cabin fuel-burning appliances need adequate ventilation and proper maintenance (ABYC TH-22). Battery and shore-power care that supports alarms and blowers is in marine battery care and the electrical guide above. Exhaust and heater checks also belong in winterizing and spring commissioning.

Symptoms (often mistaken for seasickness)

USCG dangers page / brochure early symptoms: irritated eyes, headache, nausea, weakness and dizziness. They are often confused with seasickness or alcohol intoxication, so people may not get the help they need. The Coast Guard also notes that altitude, certain health problems and age increase CO's effects; people who smoke, consume alcohol, or have lung or heart problems are particularly susceptible—but anyone can be affected. Physical exertion accelerates how fast the blood absorbs CO.

Discover Boating: early signs (drowsiness and a slight headache) can feel like ordinary sleepiness; other symptoms include nausea, dizziness, loss of judgment, unconsciousness and convulsions. High concentration is deadly; so is lower concentration over several hours.

ABYC TH-22 lists a longer symptom sequence that can include watering/itchy eyes, flushed appearance, throbbing temples, inattentiveness, inability to think coherently, loss of coordination, ringing in the ears, tightness across the chest, headache, drowsiness, incoherence, slurred speech, nausea, dizziness, fatigue, vomiting, collapse and convulsions—order varies.

Treat symptoms of seasickness as possible CO poisoning until you are sure they are not (USCG protect page / brochure).

If you suspect CO — fresh air and call for help

USCG dangers page emergency steps (proceed with caution—the area may still be high in CO):

  1. Evaluate the situation and ventilate the area if possible.
  2. Evacuate and move affected people to fresh air.
  3. Observe the victim(s).
  4. Administer oxygen, if available.
  5. Contact medical help. If the victim is not breathing, perform rescue breathing or approved CPR until help arrives.
  6. Shut off potential CO sources if possible; correct ventilation / exhaust problems; investigate the source while you evacuate and ventilate.

USCG brochure: if CO is suspected, open all hatches, windows and ports; changing course and speed to put the boat heading into the wind can improve ventilation. Discover Boating: avoid exhaust buildup with a course change, or crack open an enclosure panel to increase airflow—fresh air is the most important first aid.

On the water, use VHF Channel 16 for distress and urgent help—see VHF marine radio basics. Do not wait if someone is collapsing or not breathing.

Where detectors belong — and what the Coast Guard asks you to do

USCG protect page / brochure: install and maintain CO alarms (brochure: marine-grade approved CO detectors) inside your boat. Do not ignore any alarm. Replace alarms as recommended by the alarm manufacturer. Every alarm requires immediate action.

Discover Boating: test CO detectors in the main cabin and any sleeping areas often; inspect engine and generator exhausts for dark sooty stains (exhaust gas) or white stains (cooling-water leaks).

ABYC TH-22: even with good design and maintenance, hazardous CO can still appear; supplement observation with a marine CO detection device in the accommodation space. Detection devices should meet ABYC A-24, Carbon Monoxide Detection Systems on Boats. TH-22 notes there are currently no known CO detectors for permanent installation in exterior areas, and that boats with enclosed accommodation compartments should not run engines or an auxiliary generator unless equipped with a functioning marine CO detector that complies with A-24. (We cite that standard by name only—we do not invent alarm set-points or brand lists.)

Federal equipment-by-length rules do not replace a working CO alarm habit when you have enclosed spaces, an engine or a generator—see the Coast Guard checklists below and the note in required safety equipment by length.

Coast Guard trip, monthly and annual checklists

Print and use the Coast Guard's lists (uscgboating.org carbon monoxide checklist). Do not operate your boat without doing the following.

Each trip

  • Know where CO exhaust outlets are on your vessel.
  • Educate all passengers about CO poisoning symptoms and where CO may accumulate.
  • When docked or rafted with another boat, be aware of that boat's exhaust.
  • Confirm that water flows from the exhaust outlet when engines and the generator start.
  • Listen for any change in exhaust sound (possible exhaust component failure).
  • Test each CO alarm by pressing the test button.

Once a month

  • Make sure all exhaust clamps are in place and secure.
  • Look for exhaust leaking from system components—rust and/or black streaking, water leaks, or corroded or cracked fittings.
  • Inspect rubber exhaust hoses for burned, cracked or deteriorated sections; hoses should be pliable and free of kinks.

Once a year — have a qualified marine technician

  • Replace exhaust hoses if cracking, charring or deterioration is found.
  • Ensure engines and generators are properly tuned and well maintained.
  • Inspect each water pump impeller and housing; replace if worn; confirm cooling systems work.
  • Inspect metallic exhaust components (cylinder head / gasket, exhaust manifold, water injection elbow, threaded adapter nipple) for cracking, rusting, leaking or loosening.
  • Clean, inspect and confirm proper operation of the generator cooling-water anti-siphon valve (if equipped).

Trip and monthly CO bullets also appear in the maintenance calendar by season and the first-season checklist. Put CO alarm tests on the same pre-launch pass as the printable Launch Day checklist. Book a free Vessel Safety Check with the Coast Guard Auxiliary. Budget detectors and exhaust work in the True Cost tool if you are still outfitting.

Sources

Related tools