Carbon monoxide (CO) is a colorless, odorless, tasteless gas produced whenever a carbon-based fuel burns. On a boat, that means gasoline engines, generators, cooking ranges and heaters. The U.S. Coast Guard warns that CO can make you sick in seconds, and in high enough concentrations even a few breaths can be fatal. Every state boating course covers it.
Why it's easy to miss
Early symptoms of CO poisoning are headache, nausea, dizziness, weakness and irritated eyes, and they are easily mistaken for seasickness or alcohol intoxication. Someone who seems "just seasick" near the back of a boat may be breathing exhaust.
Related: What to Expect on Your State's Boating Safety Exam
Where CO collects
- Inside cabins and enclosed spaces, especially on older boats. State handbooks name exhaust leaks as the leading cause of CO deaths on boats.
- At the stern, from the "station wagon" or backdraft effect: when a boat is underway, especially at slow speed or with the bow high, exhaust can be pulled back into the cockpit and cabin.
- Around the swim platform and in the water behind the boat. This is why teak surfing (holding onto the swim platform while the boat moves) and dragging behind the boat are so dangerous. Handbooks warn against skiing or being towed within 20 feet of the boat.
- While idling or moving slowly, or with a tailwind, which can blow exhaust toward passengers.
- From generators and neighboring boats at a dock or raft-up, even when your own engine is off.
How to prevent it
- Install a marine-grade CO detector in living and sleeping spaces, and test it before each trip.
- Keep fresh air moving through the boat. Opening a forward hatch while underway helps counter the station wagon effect.
- Have exhaust systems inspected and fix leaks. Cold or poorly tuned engines produce more CO.
- Don't let anyone ride on the swim platform or hang off the stern while engines or generators are running.
- Seat children toward the front of the boat.
If you suspect CO poisoning
Move the person to fresh air, shut off the engine or generator, and get medical help right away. Symptoms that go away in fresh air still need a medical check.
On the exam
Expect questions on where CO builds up (stern, swim platform, enclosed cabins), what the early symptoms look like (seasickness), and the single best prevention step (a working CO detector plus fresh air). Try our boating safety exam practice questions.