Carbon Monoxide Exposure Associated With Towed Watersports

Carbon monoxide is a lethal gas that is produced in engine exhaust and can accumulate, even in areas that appear to be well ventilated. This is not just an academic concern; there have been drowning deaths confirmed to be a result of carbon monoxide poisoning.

Many people have been on a boat and later in the day felt lethargic or had a headache. They typically blame it on the sun, food, beer, etc. The complaints may be due to Carbon monoxide poisoning.

A towed watersport includes all watersport activities that rely upon the boat to pull an individual or the boat wake to push them. This would include waterskiing, wakeboarding, wakesurfing, barefooting, tubing and towed inflatables.

Because Carbon Monoxide (CO) is colorless, tasteless, odorless and nonirritating, it can overcome the exposed person without warning. Carbon monoxide poisons primarily by tightly binding to hemoglobin in the blood forming carboxyhemoglobin, replacing oxygen, and reducing the oxygen carrying capacity of the blood. The following people are at increased risk: Children, people with pre-existing heart and lung conditions, people with asthma, and the unborn child. Recognizing early warning signs of CO poisoning is sometimes difficult because the symptoms of CO exposure, headache, dizziness and nausea, are common to many conditions. The severity of symptoms of CO exposure is influenced by:

Concentration of CO
How long the exposure lasts
The individual’s level of physical activity
Health related risk factors

Purpose of This Study
To better understand the potential for Carbon monoxide exposure to persons towed behind an inboard boat, to the occupants within the boat, and the effect that “Fresh Air Exhaust”™ has upon Carbon monoxide exposure

Test Plan
Record Carbon monoxide levels at various locations within and around an operating, direct drive, gasoline engine, inboard boat, operating at speeds and configurations that approximate towed watersport conditions.

Earnest, et al, and, Echt et al, conducted extensive measurements of CO concentration in boat engine exhaust (1, 2) and clearly demonstrate the potential for CO poisoning during boat operation. The U.S. Coast Guard/American Boating and Yacht Council (USCG/ABYC) “Carbon Monoxide Safe Distance Study” dated September 2003, and the Technical Services “Carbon Monoxide , Behind Boat Data” dated 9-8-2003, measured CO levels in locations aft of the test boat that approximated the exposure levels to individuals participating in towed watersports (3, 4). A summary of the USCG study and the Technical Services study has been written by this author (5).

– The typical carbureted V8 boat engine produces lethal levels of CO measured at the outlet/s of the exhaust pipes.
– Carbon monoxide concentrations are greatest near the transom of the tow boat.
– Carbon monoxide concentrations aft of the boat are greatest close to the water and in the centerline behind the boat.
– Carbon monoxide concentrations aft of the boat diminish as a function of the height above the water.
– Carbon monoxide concentrations do not diminish in linear proportion to the distance aft of the transom.
– Tuber’s and kneeboarder’s heads are closer to the water than the heads of someone standing upon a ski or board.
– Carbon monoxide concentrations around an operating inboard boat vary substantially, depending upon numerous factors such as, boat and swim platform design, engine load, boat speed, relative wind direction and speed, and air flow within the boat.

Test Protocols
– Test variables were speed, location relative to the stern of the boat, location within the boat, stock exhaust, and Fresh Air Exhaust installed.
– Testing terminated if the wind exceeded 5 mph.
– Carbon monoxide concentration, aft of the boat, at 5′ above the water, is lower than the concentration measured at 2′ above the water (3,4). Except for the wakesurfing tests, measurements aft of the boat in this study were made at 2′ above the water.
– It was observed that at 10 mph, with Fresh Air Exhaust installed, the visible exhaust bubbles end at about 100′ aft; therefore, one additional sampling was taken at that distance.
– Wakesurfing conditions were replicated by adding ballast sufficient to cause a 10 degree list to starboard. A Hobie Cat sailboat was surfed on the starboard side of the boat with the tow line slack for most of the duration of the test. The CO meter was located approximately 10′ aft, off center, and measurements were taken at 2′ and 5′ above the water.
– Measurements at the swim platform, transom, and back seat of the boat in this study were taken at 21″ above the respective surface as this would be the approximate face height of a large child or small adult sitting at those locations.
– Boat speeds were determined by GPS.

Test Equipment
Primary tow boat (source of engine exhaust).
1988 Correct Craft Ski Nautique – #CTC24196F888
351 cid carbureted gasoline engine

Secondary test boat
2001 Tige 20i – #T1XJ4398G001
351 CID throttle body fuel injected

Towed boat
16′ Hobie Cat sailboat with mast and sail removed. This boat allowed uninterrupted air flow to reach the CO monitor.

Carbon monoxide Monitor
Biosystems Toxi Ultra Single Sensor Gas Detector s/n G35933

Weather Conditions
Testing occurred over three days. The air temperature was between 64 and 76 degrees Fahrenheit, water temperature was between 62 and 64 degrees Fahrenheit, relative humidity was between 68% and 98%, wind direction was variable and wind speeds were between 0 and 4 mph during testing.

Wind Conditions and Peak Measurements
Carbon monoxide is a gas; therefore, it will rapidly diffuse into the surrounding air. It will diffuse equally in all directions. It does not move rearward, away from the boat. It stays where it is exhausted from the boat and diffuses into the air. As the boat moves forward, the towed rider moves forward into the plume. If there were no wind or air movement, the exhaust from a forward moving boat would form a cone that expanded in all directions with the CO becoming diluted by the air.

Even in a no wind condition, the movement of the boat forward creates air movement; the greater the speed of the boat, the greater the amount of air movement generated. This air movement mixes additional fresh air with the exhaust plume, thus causing a more rapid dilution of the CO.

The forward movement of the boat also creates eddy currents. Eddy currents can create pockets of boat exhaust that do not mix with the additional air. Eddy currents can also create pockets of exhaust gas that move forward, into the boat. Eddy currents change based upon many things, such as the configuration of the boat (for example open windows versus closed windows) and where people are sitting (creating additional eddy currents in their own air wake). Eddy currents are also affected by the speed of the boat. The CO concentration in the rear seat of the stock boat, at 5 mph, was only 1 ppm average and 3 ppm peak; at 10 mph though the measurements were 10 ppm average and 50 ppm peak.

The exhaust plume of the boat is also affected by wind. As wind speeds increase, there is additional mixing and diffusion of the exhaust gases. This is generally good, but unfortunately, the wind can also exacerbate the problem. If the boat was moving forward at 5 mph, into a 5 mph headwind, the eddy current for the rear seat passenger would now be 10 mph relative and could substantially increase their CO exposure. In another example, if a boat was moving forward at 10 mph but there was an 11 mph tailwind, the elevated levels of CO at the rear of the boat could be blown into the boat. Because of the wind’s potential effects, CO measurements were taken with wind speeds less than 5 mph.

The effect of the wind on the exhaust plume is also why peak CO measurements are important. Under worst case conditions, it is possible for wind speed and direction and the resulting eddy currents, to increase the average CO concentration towards the peak concentration.

Rear Seat Measurement Discrepancies
Our initial testing did not show significantly elevated levels of CO at the back seat of the boat as was measured in the USCG study (3). Upon further review, the USCG study, Photo 4, shows that the CO measurements were taken directly on top of deck above the transom, somewhat behind the back seat. Our measurements were made about 1 foot forward.

In this study, at 5 mph, there were substantial differences between the CO levels measured at the rear seat and at the transom, even though that distance is only about 2′ apart. It could be described that the exhaust gases aft of the back seat, of a forward moving boat, are part of the primary plume coming from the exhaust outlets, and any exhaust gases forward of this point are moved there by eddy or wind currents.

It is clear, based upon the studies to date, that there are likely dangerous levels of CO on or about the swim platform and the back of the boat. A child, sitting in the rear seat but facing rearwards, with their face level with the deck at the back of the boat, could inhale dangerous levels of CO. An adult sitting upright in the rear seat would likely create an eddy current in the air aft of them, thus drawing CO forward. If that adult were facing rearwards, they could inhale dangerous levels of CO.

Exhaust Gas Scrubbing
Industrial scrubbers that remove pollutants from exhaust gases are common. One method is to inject a fine mist/spray of water into the exhaust gases which “scrubs” the gases (wet scrubber), resulting in a cleaner air discharge. Fresh Air Exhaust acts as a wet scrubber by injecting the engine exhaust gases into water aerated by the boat’s propeller.

At the temperature of water in which we typically ski, CO is approximately 2% to 2.5% soluble. In order to remove all of the CO from the exhaust gases, a scrubber would need to mix 40 to 50 times the volume of water to the volume of the exhaust gases. It would appear that Fresh Air Exhaust is doing so when the propeller is turning approximately 2000 rpm (approximately 10 mph).

At 10 mph, it takes 6.81 seconds to travel 100′. At 10 mph the exhaust measured at 100′ aft is approximately 6 seconds old. With Fresh Air Exhaust installed the CO measurement at 100′ aft was 1 ppm average, 2 ppm peak. This indicates that the scrubbing action is nearly complete and that the CO measured was a part of the exhaust plume that escaped scrubbing.

Four series of CO measurements were taken aft of a 2001 Tige 20i (60′ and 80′ aft, 10 mph and 20 mph). All of the readings taken behind the Tige were 0 average and 0 peak. This indicates that the scrubbing action of Fresh Air Exhaust installed on the Tige is complete.

The current hypothesis is that the CO combines with free oxygen in the water to form CO2. Based upon the efficiency of industrial wet scrubbers, Fresh Air Exhaust likely reduces or eliminates Nitrous Oxides (NOx) and Particulate Matter (soot) from the engine exhaust as well. Fresh Air Exhaust may cause a substantial reduction in air pollution generated by inboard boats with no net increase in water pollution.

In order to wakesurf, a boat needs to be ballasted (weighted) heavier on the side that the surfer intends to ride. In this study, the tow boat was ballasted with water on the starboard side and the Hobie Cat was surfed on the starboard side. The Correct Craft boat used in this study produced a good surf wave when ballasted to a 10 degree list. When the boat is leaning like this, at the wakesurfing speed of 10 mph, the water passing the starboard transom flows towards the centerline. This water movement carries the exhaust gases emitted from stock exhaust outlets towards the port side of the boat. This exhaust diversion resulted in CO concentrations in the wakesurf zone of 1 ppm peak. The Technical Services study (4) showed similar findings with CO concentrations of 1 ppm average, 2 ppm peak.

The transom, swim platform, and exhaust outlet configuration for tournament style, inboard ski boats are similar. Many wakesurfers have commented that they infrequently smell exhaust fumes while wakesurfing, yet people in the boat complain of the exhaust smell. It is likely that most boats of this style, when properly ballasted for wakesurfing, will achieve wakesurf zones that have very little CO present. It is also likely that some boats, due to their transom, swim platform, and exhaust outlet configuration will not achieve these low concentrations.

The World Wakesurf Association advises all participants to be aware of CO dangers and to be alert to the smell of exhaust. By monitoring for the smell of exhaust fumes, and always wearing a life vest, there is little danger to a wakesurfer of CO induced drowning. As wakesurfers have noted, and these studies confirm, the greatest danger of CO exposure during wakesurfing is to occupants within the boat.

The USCG study (3) utilized a color coded display that helps to quickly visualize the reported data. A similar format is utilized in Appendix A, B, and C. Appendix B displays the CO concentrations relative to wakesurfing in an “overhead view”. Appendix C shows the approximate levels of exposure to various towed watersports participants. Appendix D places CO levels into perspective. Appendix E is a photograph of a person wakesurfing. Appendix F is a table listing the various CO measurements, taken at 10 second intervals, during the course of this study.

Corrections and Peer Review
This report is being distributed on the internet. Critique or corrections are requested. Any mistakes will be corrected and noted in Appendix G. Critique of a “peer review” nature will be added to Appendix G.

Conclusions – General
– A passenger riding in the boats tested could be exposed to unacceptable levels of CO, particularly between 5 mph (slow no-wake speeds) and 10 mph.
– Riding a towed inflatable (tube) should never be done, under any conditions, at speeds less than 10 mph. At speeds greater than 10 mph, tubers should not be towed in a straight line behind the center of the boat.
– Any rule that prohibits watersports activities conducted on a board or ski/s, such as skiing or surfing, within 20′ aft of the boat, is not supported by the studies.
– Platform dragging or swimming near an operating boat could be rapidly lethal due to CO poisoning.
– Sitting in the rear seat of an operating carbureted direct drive inboard boat potentially exposes a person to unacceptable levels of Carbon monoxide. Pregnant women, young children, and anyone with a pre-existing heart or lung condition should not sit at this location.

Conclusions – Fresh Air Exhaust
– At 5mph, Fresh Air Exhaust is very effective at moving the exhaust away from the back of the boat and substantially reducing the risk of CO exposure within the boat or on the swim platform. This reduction may be as high as 98% at the transom.
– While reducing the CO levels within and at the rear of the boat at 5mph, Fresh Air Exhaust caused the levels of CO to increase aft of the boat, compared to the stock exhaust. Fortunately, no towed watersport occurs at 5mph except tubing, and that should be avoided at 5mph with or without Fresh Air Exhaust installed.
– At 10 mph and 20 mph, Fresh Air Exhaust scrubs almost all CO from the boat’s exhaust, with minimal CO levels recorded at all points within the boat and aft of the boat.
– With Fresh Air Exhaust installed, the risk of Carbon monoxide poisoning at 5mph is greatly reduced, and at speeds of 10 mph or greater the risk is virtually eliminated.
Rider and Passenger Safety and Fresh Air Exhaust
– Wakesurfing, from the perspective of CO exposure, is safe in the boats tested and under the conditions tested. Carbon monoxide exposure of the occupants in the boat though is increased when the boat is heavy and moving at 10 mph. Fresh Air Exhaust nearly eliminates CO within and around a boat while wakesurfing. The safety of wakesurfers and boat occupants would be enhanced by the installation of Fresh Air Exhaust.
– Inflatables (tubes) and kneeboards should not be towed below 10 mph. At speeds greater than 10 mph, the safety of tubers and kneeboarders would be enhanced by the installation of Fresh Air Exhaust.
– The USCG study concluded that “Carbon monoxide levels at 5′ above the water in excess of 60′ behind the ski boat are low enough to not be of concern to recreational skiers”. While this is true for experienced riders who seldom stay directly behind the boat, young children with a 3′ or 4′ face height, which are more likely to be beginners that stay centered behind the boat, could be exposed to significant levels of CO. The safety of children participating in towed watersports would be enhanced by the installation of Fresh Air Exhaust.
– We tell our children not to drink alcohol, smoke, do drugs, or have sex; yet sometimes they do. We can also tell our teenagers, when they take off in the boat, not to platform drag, and hope they don’t do that either. The safety of our teenagers would be enhanced by the installation of Fresh Air Exhaust.
– Sitting in the rear seat of an inboard boat potentially exposes a person to unacceptable levels of CO. The safety of boat passengers would be enhanced by the installation of Fresh Air Exhaust.

Author’s Note
I have been around boats most of my life. I certainly have smelled exhaust gases while operating these boats. For the most part, I had very little concern for CO. If I had been aware of platform dragging when I was a teenager, I would have done it. When I began building and testing Fresh Air Exhaust, I was merely trying to move the exhaust further away from the wakesurfer. Even though I was aware of the potential dangers of CO and some of the drowning deaths associated with it, my attitude towards boat exhaust was still somewhat careless.

My attitude changed substantially once I actually began testing CO concentrations. Our first time testing CO levels, we let the boat idle near the pier to warm up. As I went to set up the Biosystems CO monitor at the back seat, the alarm on the monitor started blaring. The CO levels inside the boat, around the back seat, exceeded 1100 ppm. Since 1200 ppm is considered lethal, I suddenly had a new respect for CO levels present in boats.

A life vest can prevent Carbon monoxide induced drowning. When participating in towed watersports, ALWAYS wear a life vest. Always!

I sincerely thank all of the people that helped me with this study.

Larry Mann, DC


1. Earnest G.S., Echt, A., Dunn, K., Hammond, D., McCleery R., McCammon, J.B., Jones, A. [2003]. Carbon Monoxide Emissions and Exposures on Recreational Boats under various Operating Conditions Lake Mead, Nevada and Lake Powell, Arizona. USDHHS, PHS, CDC, NIOSH, Cincinnati, Ohio, EPHB No. 171-05ee2.

2. Echt, A., Earnest G.S., Dunn, K., Hammond, D., McCleery R., McCammon, J.B., Blade, L., Valladares, R. [2003]. Carbon Monoxide Emissions and Exposures on Recreational Boats under various Operating Conditions Lake Norman, N.C.. USDHHS, PHS, CDC, NIOSH, Cincinnati, Ohio, EPHB No. 171-31a.

3. USCG/ABYC [September 2003]. Carbon Monoxide Safe Distance Study presented at IBEX Miami, October 2003.

4. Mathew, R., [9/8/2003]. Carbon Monoxide, Behind Boat Data Technical Services, 401 W. Chicago St., Syracuse, IN 46567

5. Mann, L.W., [3/1/2004]. Carbon Monoxide Exposure While Operating an Inboard Boat and Related Water Sports Activities