Boating Safety

Insidious Carbon Monoxide

carbon monoxide

Don't be a victim of Carbon Monoxide (CO) onboard your vessel. Know the potential sources of CO on your boat.

It's difficult to comprehend that when we head out on the open water, with a fresh breeze, clear air and nary a boat nearby we could be a victim of something we usually associate with confined spaces and poor ventilation–Carbon Monoxide (CO). Onboard our vessels, we have a few potential sources of CO such as gasoline-powered engines and generators and propane cooking and heating appliances. Exhausts should dissipate harmful gases away from the boat, however experts in occupational health and safety, search and rescue and ambulatory care are identifying emerging issues for boaters.

back drafting, co

Back drafting can occur when a boat is operated at high bow angle. (Image courtesy of U.S. Coast Guard)

You might be asking, "Why is CO seen as an emerging issue if boats and engines have been around together for a long time?" Well, we have known that CO has been present with engines on boats, but we never realized how potent. In fact, it can hang around even in idyllic settings and with new hull designs and boating activities; people are now in locations where they weren't before and where CO has always been. Unfortunately, we have probably misdiagnosed CO poisoning in past boating incidents as well.

What is Carbon Monoxide?

CO is a by-product of combustion that tags along with better-known hydrocarbon exhaust gases such as Carbon Dioxide, various sulphides/sulphates and a whole slew of other "pollutants" from the operation of engines and some cooking and heating appliances. But CO has unique attributes–it is colorless, odorless, is non-irritating and has a chemical structure that makes it more attractive to haemoglobin than oxygen. Interesting combination.

Since CO is odorless, you would have no idea that it may be present if it wasn't for the smell of other exhaust gases. Since it is invisible, there is no way to visually identify that CO may be present if the temperature or dew point is such that typically smoky exhaust gases are not evident. Since it is non-irritating, your eyes will not water nor will you experience any burning sensations in your throat or nose if it is present and you are immersed in it. And it is potent, surprisingly potent.

co, nearby boats

Exhaust from nearby vessels can send CO into your boat's cabin or cockpit. (Image courtesy of U.S. Coast Guard)

Unlike automobile engines, marine engines have very limited pollution controls–if the pollution controls found on automobile engines were placed on marine engines, "mileage" would drop drastically, something boaters would balk at with current fuel prices. Some new outboards have made incredible strides in reductions of various gases. All manufacturers' 4-stroke outboards have certainly reduced emissions and BRP's 2-stroke e-Tech engines have shown emissions with nominal CO levels, even better than the 4-strokes!

co poisoning

However, it is the older 2-strokes and the ol' inboard engines and generators that produce quite a bit of CO. The US National Institute for Occupational Safety and Health (NIOSH) has carried out extensive studies testing various aspects of CO and boats (www.cdc.gov/niosh/topics/coboating). The one finding that was breath-taking was the comparison of CO output from a single marine engine relative to a current automobile engine. Take a guess: what is the CO output of one inboard marine engine equivalent to in terms of car engines? It's 188 car engines!

The Anecdotes

CO, genset, boating

Kids (and adults) love to slip off slides into the water when anchoring for the afternoon hiding under swim platforms and between hulls–right where CO is accumulating from the generator.

CO should dissipate away from a boat, especially a moving one, so how do CO and people come together?

1. Single hull vessels are the predominant hull type on the water, but a growing popular design is the deck boat–either based on pontoons or twin hulls, they may have a simple canopy covering or a fully enclosed cabin (house) structure. Houseboats have all the amenities such as fridges, stoves, Stereo/TV and air conditioning. Anchoring for the afternoon can be a great way to spend a summer day with cool beer, the A/C on inside and lot's of fun in the water outside. But it's that generator that is quietly running and exhausting near the waterline either in between the hulls or out of sight under the swim platform. Kids (and adults) love to slip off slides into the water, hide under swim platforms and venture between hulls–right where CO is accumulating.

2. Towing activities have grown from simple water skiing to wake boarding, knee boarding and tubing. It's always more entertaining to have a large quarter wave for folks to rocket over and the best way to create a large one is to either stay off plane at low revs or fill a water bladder in the stern of the boat to keep from getting on plane with more revs. That high bow, low stern trimming creates a huge "hole" in the water just aft of the boat and forward of the quarter wave that thrill seekers have found is a great place to hang off the swim platform (usually made of teak on boats designed for towing) and go "teak-surfing". Unfortunately, that hole is also a great place for CO to accumulate and teak surfers can be there one moment and gone the next.

3. Another enjoyable activity is finding your friends at an anchorage or wharf and rafting up together–perhaps for the afternoon or the night. There may be batteries to charge or gennies to run and therefore exhausts to emit gases. If one vessel's exhaust is located such that gases get trapped between vessels or the wharf face, as the warm gases rise up to escape they quickly cool, descend and possibly waft through open ports or hatches and into sleeping quarters.

4. The time-honored story of the empty circling boat of the (supposedly) drunken angler who was trolling and passed out and went over the side is now being challenged. If the angler was trolling with the wind on a damp day those fumes had nowhere to go but to waft back over the boat.

5. Even larger "cabin cruisers" or express yachts may have an issue. If the design of the vessel is such, air movement around either side or top of the cabin structure while underway can create a vacuum or station-wagon-effect near the after deck. If aft doors, hatches or ports are open, these fumes can strangely be sucked into the cabin.

All of these scenarios indicate that for the most part, CO is down low on the water and therefore should not pose a threat to people up on deck and in cabins. But these same scenarios show that fumes can find their way into upper areas and people are certainly able to find their way down to the fumes.


Another factor that makes CO dangerous for boaters is its symptoms–they mimic seasickness. If it's not enough that CO is odorless, non-irritating, invisible and potent, it just so happens that a person who is being poisoned by it looks like they are simply getting green around the gills.

signs of CO poisoning

Knowing the signs of carbon monoxide (CO) poisoning is an important part of CO safety.

When we breathe, the oxygen entering our lungs meets up with haemoglobin that it binds to and then takes a tour through our body. Incredibly, CO is actually more attractive to haemoglobin than oxygen so a breath of air that contains CO sees the CO take the place of the oxygen resulting in varying levels of carboxyhaemoglobin, the stable binding of the CO and haemoglobin. This is why only a few breaths can have grave effects and why breathing a high concentration can make a person lose consciousness incredibly quickly–you may be breathing but the oxygen isn't getting a chance to get into your blood.

Initial signs of CO poisoning are fatigue, nausea, vomiting and a slight headache. More moderate exposure will lead to dizziness, shortness of breath, a fast heart rate, mental confusion and a throbbing headache. Eventually a person will faint, become unconscious, have convulsions and cardio-respiratory failure and without removal from the contaminated area, will die. Of course, if that person is swimming and becomes dizzy or disorientated in the water it's not difficult to see how they could simply slip under the surface.

These symptoms are more pronounced in children or pets due to their smaller body mass. Disorientation can occur so quickly for any person that calling for help or self-rescue just doesn't occur.

Dealing with It

With any activity there are risks, be it driving (that moose), skiing (that tree), camping (that bear) or boating–to enjoy the activity you mitigate the risks. First you learn about dangers so that you are aware of what can happen; then you take steps to steer clear of them so that an activity can remain enjoyable; and ultimately you have an action plan to deal with the danger if and when you come face to face with it.

The dangers of CO have been described above. Steering clear of them includes being aware of where the sources are and not immersing yourself or permitting others into a potential CO environment and not letting the CO enter the area where you are.

The most important part of "dealing with it" is taking action if you have identified CO is present, perhaps with a sounding of an alarm or if you have come across someone suffering from the symptoms. The catch phrase is –Evacuate, Ventilate, Investigate and Take Corrective Action–don't ignore it as it could happen again. But be very careful, if someone has been overcome by CO in an enclosed space, going in to get him or her may make you the next casualty. Have an emergency plan, share it with your crew and conduct safety drills.

CO Detection

More and more vessel manufacturers are installing CO detectors and in fact this is a requirement for certain classes of boats based on the American Boat and Yacht Council (ABYC) Standard A24. Most boats in North America are built for sale in the U.S., thus they are built to meet the ABYC Standard.

The certain classes of vessels that now have them are ones with gas engines or generators and that have enclosed accommodations with sleeping areas. Although CO detectors are not required on larger vessels that have diesel engines; many manufactures install them with the reasoning that CO may come aboard uninvited from another vessel. If you are looking at purchasing a new boat, ask about detectors but if you have or are looking at purchasing an older boat, they most likely do not have them as those vessels predated the identification of the CO issue.

co detectors

There are after-market detectors specifically designed for a marine application (available at local chandleries) that can be installed as easily as a smoke detector. The manufacturer's instructions are usually extensive, detailing how they work, how and where they should be mounted and how to respond to an alarm. Most are battery operated but it is possible to purchase units that can be wired into 12-volt systems. There have been issues with dampness affecting the efficiency of detectors so not only is a marine environment problematic but also mounting them near heads/showers is cautioned.

Go Boating, Have Fun.

CO is indeed one slippery character, but many aspects of boating have a risky flipside–it just so happens that we make ourselves aware of them, prepare for instances where they might sneak up on us and have a great time on the water. Remember, you can't see it, smell it or taste it but if you are running an engine or cooking or heating with propane be aware that you are creating it. Be one step ahead, breathe easy and enjoy your time on the water.