Don’t Ignore CO Monitors

One boater's story of a false alarm that turned out to be the real thing.

CO monitor

Dockside Confessional: Monitor Meltdown

A few years back, I was working at a large marine dealership in Florida and had the chance to take a brandnew express cruiser down the Intracoastal Waterway to a boat show. Everything was going great when I pulled into a marina for the night, then just after I fell asleep, the CO monitor in the salon started sounding. It wouldn’t stop, but the engines and the generator were off, so I wasn’t all that worried about poisoning from exhaust fumes.

There wasn’t a cutoff switch for the alarm on the breaker panel, and every time I hit the unit’s reset button, it would just start up again in a minute or so. To make things worse, the alarm in my stateroom started chirping as well. By then, I remembered someone once told me the chemicals in a new fi berglass boat could set off a monitor alarm, so I fi gured that was the problem. I disabled both alarms and went back to bed.

The next morning when I opened the hatch to check the engine oil, there was a strong odor coming from the batteries, and they were hot to the touch. It turns out the automatic battery charger malfunctioned and wouldn’t turn off. It was overcharging the batteries all night, and that’s probably what the monitors detected.

The Confessor Replies

First, it’s never a good idea to ignore or disable CO monitors. Carbon monoxide poisoning is the leading cause of accidental poisoning deaths in America. This toxic gas is hard to detect because it’s odorless, colorless and tasteless, and its effects sneak up on you, impairing motor and memory functions before you realize there is a problem.

Engines, fuel oil heaters and generators are the leading causes of CO buildup in boats. So in a way, you were somewhat justified in downplaying the alarm warnings if none of these were in operation. That said, any time an alarm sounds you should watch for telltale symptoms of CO poisoning, such as headache, dizziness, disorientation and nausea. If things don’t feel right, get out of the cabin and into fresh air, then open the boat and ventilate thoroughly.

Yes, it is true there are certain substances besides CO that can cause these alarms to sound. In your case, it is likely the overcharged batteries were venting hydrogen gas. It’s also worth noting that, because these monitors are usually tied directly into the boat’s 12-volt system and are always on, they will begin to alarm when battery voltage drops signifi cantly. This can actually provide a sort of early warning system for battery drain.

In your case, ignoring the alarms didn’t put you at risk, but had you investigated more thoroughly when they sounded, you might have found the malfunctioning charger hours earlier and prevented potential damage to the batteries. Overcharging can be very detrimental to marine batteries, and both wet-cell and sealed gel-cell varieties are susceptible.

The U.S. Coast Guard is asking all boat owners and operators to help reduce fatalities, injuries, property damage, and associated healthcare costs related to recreational boating accidents by taking personal responsibility for their own safety and the safety of their passengers. Essential steps include: wearing a life jacket at all times and requiring passengers to do the same; never boating under the influence (BUI); successfully completing a boating safety course; and getting a Vessel Safety Check (VSC) annually from local U.S. Coast Guard Auxiliary, United States Power Squadrons(r), or your state boating agency’s Vessel Examiners. The U.S. Coast Guard reminds all boaters to “Boat Responsibly!” For more tips on boating safety, visit