Carbon Monoxide on Boats: Where It Comes From, CO Alarms and Warning Signs

The Coast Guard says CO can build up anywhere in or around a boat, including:

• Under canvas enclosures, camper tops and bimini curtains with poor airflow
• In enclosed spaces where exhaust is trapped
• Near blocked exhaust outlets
• From another boat's exhaust next to you at a dock, raft-up or anchorage
• Through the "station wagon effect" (back drafting): when the boat moves slowly, idles or sits stopped, exhaust can be pulled back into the cockpit and cabin. BoatUS reports this with cockpit-to-cabin doors open, and under a sailboat's dodger when motoring downwind.

CO can stay at dangerous levels even after the engine is turned off.

The swim platform and the space just behind the transom can have extreme CO levels.

• Never sit on, ride on or hang off the swim platform or back deck while engines or the generator are running.
• Never "teak surf" (being towed or body-surfing on the boat's wake right behind the transom). The Coast Guard says it is never safe.
• Do not swim or let kids play near exhaust outlets, and never go under a swim platform where exhaust exits unless the area has been aired out.

BoatUS recommends staying at least 20 feet from boats running an engine or generator, especially if the wind is blowing their exhaust toward you. At a raft-up, agree that nobody runs a generator, or put that boat on the downwind end.

BoatUS recommends a working CO alarm tested to UL 2034 and approved for marine use; household alarms are tested to a different standard and may not hold up in a marine environment. ABYC (as cited by BoatUS) recommends a CO alarm in every enclosed accommodation space, especially any cabin below the waterline.

Hard-wired marine alarms that run off the boat's 12-volt system avoid dead batteries; some can also shut down the generator when they sense CO, or show alerts on your chartplotter. BoatUS points out that an alert on the helm display helps when the alarm itself cannot be heard over the engines. If yours is hard-wired, remember it only works while it has power: know whether it stays on when you turn the battery switch off for the night.

Follow the alarm maker's instructions. General guidance from BoatUS and ABYC:

• In or near sleeping areas, where you will hear it at night
• Away from hatches, ports, vents and fans, where fresh air would dilute the sample and delay the alarm
• Not in dead-air corners where air does not circulate
• Protected from rain, spray and direct sunlight
• Not right next to a stove or heater; your alarm's manual gives a minimum distance

The Coast Guard's per-trip checklist includes pressing the test button on every CO alarm. If an alarm does not sound, replace its battery (if it has one) or the alarm before anyone sleeps aboard. A "chirp" from somewhere you cannot find often means a weak battery in a CO alarm, BoatUS notes.

CO sensors wear out. Fireboy-Xintex says its marine CO detectors enter an end-of-life warning at about 4 years and 11 months (a beep every 30 seconds and a red light), and at five years the alarm can no longer be silenced and the unit must be replaced. Other brands have their own lifespan printed on the label. Write the install date on each alarm and set a reminder in your boat profile.

From the Coast Guard's checklists:

• Each trip: know where every exhaust outlet is, confirm cooling water comes out of the exhaust when engines and the generator start, and listen for any change in exhaust sound. BoatUS adds: make sure vents, heater and stove flues and exhaust outlets are not blocked by debris, seaweed or algae, and remember a dock line, fender or swim platform gear can also block an outlet.
• Monthly: make sure exhaust clamps are tight, and look for rust or black streaks, water leaks, or corroded or cracked fittings. Inspect rubber exhaust hoses for burned, cracked or stiff sections.
• Yearly, by a qualified technician: replace damaged exhaust hoses, tune engines and generators, inspect water pump impellers and housings, check metal exhaust parts (manifolds, risers/elbows and the nipple between them) for cracks, rust or leaks, and service the generator's anti-siphon valve if it has one.

If you ever smell exhaust inside the boat, CO is present: act on it.

The Coast Guard says to run the exhaust blowers whenever the generator is operating. BoatUS advises not running the generator while you sleep, even for air conditioning, unless you have a CO alarm system designed to shut the generator off when it detects CO. Never run a portable gas generator on board or on the swim platform.

Treat any of these as possible CO poisoning, especially if several people feel ill at once or symptoms ease in fresh air:

• Headache, dizziness, weakness, sleepiness
• Nausea or vomiting
• Irritated eyes
• Confusion, or acting "drunk" without drinking

The Coast Guard's advice: treat seasickness symptoms as possible CO poisoning, get the person into fresh air right away and seek medical attention unless you are sure it is not CO.

Based on the Coast Guard's emergency guidance:

1. Move everyone to fresh air on deck, upwind of exhausts.
2. Shut off engines, generator and any flame appliances if you can do it safely.
3. Open hatches, ports and windows to ventilate.
4. Call 911, or the Coast Guard on VHF 16 if you are on the water. Give oxygen if you have it.
5. If someone is not breathing, start rescue breathing or CPR if trained, and continue until help arrives.
6. Do not run the boat again until the source is found and fixed by a technician.

Before you leave the dock, tell guests about CO: where exhausts are, no riding on the swim platform, and that feeling "seasick" should be reported. The Coast Guard recommends a free Vessel Safety Check from the Coast Guard Auxiliary or a partner organization; examiners look at safety equipment and can point out risks.