Have you ever spent a day on the water, gotten home and just not felt all that great? Maybe you’re fatigued, have a headache and chalk it up to too much sun or beer. It could be, however, that these are also the symptoms of carbon monoxide (CO) poisoning, a boating safety problem more common than most people realize.

CO is an invisible, odorless and tasteless poison gas that is part of the engine exhaust. It’s potentially lethal, causing a handful of boating deaths every year and making countless others ill.

All gasoline powered boats make CO, the effect of which varies from boat to boat and operating conditions present. Since CO is heavier than air, it stays low to the water. Combined with the “station wagon” effect, or being trapped by a low-pressure area immediately behind a boat, the inboard stern area and transom platform are especially susceptible. CO levels at the swim step on some vessels have been measured as high as 26,000 parts per million (ppm). A level of 12,800 ppm results in death in less than 3 minutes.

A 2004 California law prohibits sitting on the swim platform while the motor is running, or hanging on to it while idling forward in gear, often referred to as teak surfing or platform dragging. Also prohibited is bodysurfing the wake behind a boat.

Wake surfing, a popular new activity on the Delta, is still legal in California. However, caution should be used when learning to perform this skill – for both the surfer and boat occupants. Extended periods near the transom, down low at water level exposes the surfer to higher than safe levels of CO as the boat is moving slowly with the engine working hard to plow the water necessary to create a surf wake. In addition, a portion of that same toxic cloud is also being sucked back into the boat, placing passengers at risk.

A study reported by Boat US in 2006 revealed that CO levels reach more than 1,000 ppm at the inside transom of many express cruisers. BoatTest.com performed a study with a 19-foot Correct Craft ski boat and found a CO peak of 434 ppm at the stern seat, operating at 7½ mph. This study also found a peak CO level of 58 ppm at 80 feet behind the boat and two feet off the water, right where the kids are sitting on the tube! By way of comparison, the EPA threshold of safety standard is 9 ppm for eight hours of exposure and 35 ppm for one hour.

Most people start to feel symptoms above 70 ppm, and 200 ppm is considered a point of evacuation. Since you can’t see it or smell it, knowing how bad the cloud is hard to determine without monitoring equipment. However, some simple operator guidelines can keep your exposure levels to a minimum.

First, limit idling in neutral and if necessary, keep the nose into the wind. Shut off the motor while people are climbing in and out or going for a quick swim. Next, ensure that cross ventilation exists so that CO doesn’t collect without any way to escape. If a side canvas is up, open a hatch to keep the air flowing through. Third, use wind to an advantage. Make that monster wake into the wind, or on a crosswind – but not downwind, where the CO cloud blows back into the boat. Finally, make some turns while pulling kids on the tube. The tube will cross an “S” trail of CO but not sit directly in its path as driving in a straight line does.

Being mindful of CO while boating can help prevent unnecessary illness and even tragedy. Keep that invisible cloud at bay to boat safe and have fun.

Jonathan Bloom is a USCG licensed Master and ASA certified sailing instructor. He can be reached via www.baydeltafun.com.