Carbon monoxide (CO) is an odorless gas that is a combustion by-product of both gas and diesel engines. When inhaled by the human body it is dangerous because it interferes with the blood system and the brain. In small doses it may only result in temporary illness but in larger doses it can progress to brain damage with possible internal hemorrhaging and even death. The first symptom of CO poisoning is drowsiness and sometimes nausea, the later of which is most often associated with diesel produced CO.
CO by itself is odorless, but you can always be sure that it is present by the smell of engine exhaust. In fact this is the best way to detect CO but over a period of time people can become intolerant to the smell and cease noticing it. CO is heavier than air and will tend to collect in lower areas of the hull especially cabin spaces and sleeping quarters.
The most common method by which CO accumulates in cabin spaces is via leaking engine and generator exhaust systems. All exhaust systems need to be inspected frequently. Like on your car, they don’t last forever and require maintenance. All inboard engines both gas and diesel have water cooled exhaust systems. Any time the exhaust system shows evidence of a water leak, there is a serious potential for a CO leak therefore if it’s leaking water, it’s probably leaking CO.
Boats are also somewhat prone to what is known as the “station wagon effect”. When the vessel is moving under engine power a vacuum is created behind the boat, which can actually draw the fumes on board and into the cabin. This can occur at speeds as low as four or five knots. Even though this can’t always be prevented ensuring that all windows and hatches are open keeping the cabin well ventilated is the best insurance. Also sometimes a slight course change which can alter the wind direction can help as well.
The amount of CO produced by a diesel engine is less than half that of a gasoline engine but it is still dangerous. With diesel you are also being subjected to poisonous sulfur dioxide which is considerably less deadly, but it has a tendency to make you feel sicker. In rough water, it can increase the effects of, and often cause sea sickness. Long term exposure to diesel exhaust can do the same thing as short term exposure to gas exhaust. In either case, the condition has to be eliminated.
The installation of CO alarms in cabins is a good idea and they do work but like anything else they need to be maintained and kept in good order to be effective. Most surveyors that I know recommend their installation as part of a survey report and in reality it is just good common sense. They have become pretty much commonplace in our homes so why not out boats. The largest problem with alarms is that they are very sensitive to contaminants and when they do become contaminated they usually sound and then are disconnected and rendered inoperative.
The good news is that by simply being alert to the potential of the risk you can reduce the odds of this happening to you to nearly zero.