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Carbon Monoxide Density Pattern Mapping from Recreational Boat Testing.


Recreational watercraft and activities associated with pleasure boating including waterskiing, wakeboarding, wakesurfing, and tubing have been popular among recreational boat users for many years. The vessels used for these types of activities are primarily gasoline driven, with trends continuing to move toward increasing power output and engine displacement. Regardless of recent trends, understanding of CO gas exposure levels in towed watersports activities is an important step toward minimizing the risk of injury or death from poisoning. Activities such as "teak surfing," or "platform dragging," are extremely dangerous and are prohibited by law. Users of all types of watercraft that expel CO gas should adhere to safe boating standards (available at and are advised to review and comply with proper warning labels, which are included for reference in the appendix.

In concurrence with the increased popularity of these activities, boat engine suppliers have refined designs for catalytic converter applications on the exhaust systems of these vessels, and they continue to explore methods for reducing overall emissions.

Although the understanding and treatment of CO poisoning has progressed significantly in the last 50 years, the demand for higher-output engines in watersports tow boats has also increased. Supercharged engines have become more popular during the past decade, and engines available from most major tow boat manufacturers now exceed 500 hp. This study is primarily concerned with CO near and behind moving boats. It is well known that buffeting and vacuum effects can bring CO into the rear seating area and cockpit of the boat. This article is not intended to analyze these effects in detail, although the test results include samples taken in these areas.


The nature of CO exposure behind recreational boats and watercraft has been known to a limited extent for many years. Most boat users are aware that CO can accumulate behind a moving vessel, but limited information has been published about exposure levels relative to towed watersports activities. In the early 2000s, a dangerous activity known as teak surfing, or platform dragging, became popular with some thrill-seeking watersports participants. Teak surfing is performed by holding directly onto the swim platform while the boat idles forward or accelerates to a higher speed under engine power. During teak surfing, the person's head is very near the exhaust outlets of the towing vessel. Exposure to CO during teak surfing can be extreme, as exhaust gasses are buffeted directly into the breathing zone of the person. The name "teak surfing" comes from the fact that swim platforms were traditionally made from teak wood during the years prior to 2004.

Following several fatal CO exposure events, formal investigations into the activity commenced. California became the first state to officially ban teak surfing in 2004 (CA Bill: 17 September 2004-AB-2222 Boating Safety). This bill also made it illegal to occupy the swim platform while motoring. Awareness of the CO-related injuries and fatalities that were occurring during that time period was growing, and many other states followed and officially banned teak surfing and similar activities. The U.S. Coast Guard (USCG) has issued strong warnings about the dangers of these types of activities and has encouraged all U.S. states to prohibit them.


Unlike teak surfing, more traditional towed watersports activities including waterskiing, wakeboarding, wakesurfing, and tubing have much lower CO exposure risk owing to the greater distance between the boat exhaust and the participant(s) during these activities. The sport of wakesurfing involves riding the wake generated by the tow vessel. The wakesurfer rides the boat wake at distances that are typically closer to the stern of the boat than waterskiing, wakeboarding, and tubing. Wakesurfing has gained popularity over the past 15 years and is becoming the most popular "towed" watersport in the United States. Figure 1 is a photograph of a person wakesurfing. This activity is performed by riding the wake of the tow vessel, usually after using a rope to "get up" and mount the surfboard from a still position floating in the water. Links to articles describing wakesurfing are available in the appendix.

This example depicts one of many positions (relative to the vessel) a person can take while wakesurfing. Some variables include the fore/aft position on the wake, the vertical position on the wake (high or low), leg flexion, torso flexion, the angle of the surfer, and so on. Additionally, wake size, shape, and position varies from one boat to another, and is modified by many factors including the position and number of passengers and ballast in the boat, the mechanical features and wake-shaping technologies the boat is equipped with, and the personal preferences of the captain and surfer. In modern "surf boats" many of these mechanical and ballast devices can be activated and modified while the boat is underway, and while the surfer is riding the wake.

This article does not address the multitude of possible positions and conditions which can be encountered during surfing. Following extensive observation and analysis of a wide range of wakesurfing, a series of CO measurements was collected behind the vessels described herein at an elevation of approximately three feet (3 feet) vertically from the water/wake level. This height was chosen as a good reference level for surfers of various age, size, style, position, and preference.

Prior Work

A 2003 publication by Earnest et al. and prepared by the USCG and Centers for Disease Control and Prevention titled "Carbon Monoxide Emissions and Exposures on Recreational Boats Under Various Operating Conditions" [1] presents the results of numerous CO tests. Concerns regarding CO poisoning were intensifying in the months and years prior to this study. Quoting the same: "The gathered data is of particular importance to ski boats and others that pull people behind boats in the water." Various vessels were used for the study ranging from a 36-foot cabin cruiser to an 18-foot aluminum fishing boat with an outboard motor. The results confirmed that CO is most likely to accumulate to dangerous levels near the stern of a given vessel under calm or light wind conditions. Proximity to the exhaust and air movement due to vessel movement were mentioned as directly relatable factors in the quantity of CO measured at a given location.

In April 2007, Garcia et al. from the National Institute for Occupational Safety and Health (NOISH) and USCG published the results of a series of tests which compared exhaust-treated and non-treated vessels, specifically comparing vessels fitted with catalytic exhaust control devices to those with no exhaust treatment [2]. In some modes of this testing, a small dinghy was towed at various speeds behind the test vessel with CO sensors mounted aboard. Samples were taken at 2 feet and 5 feet above the water surface, with highest readings taken closest to the surface. While this test method has been attempted by the authors, the challenges presented by the large moving wake (especially in surf mode) make testing using this methodology extremely difficult.

The present work is focused on boat types that are commonly used for towed water-sports and aims to describe the CO plume graphically by mapping its effective shape, whereas these previous studies were more oriented to reporting maximum exposure levels at a few discrete points.

The reader is advised to study these prior works, as they contain important findings.

Test Methodology

The measurement of a matrix of points was desired to allow mapping of CO levels at various locations near the stern of the test vessel. This was initially attempted by the use of a custom-built lightweight tubular carbon fiber framework. Twelve sampling positions at the stern of the test vessels were proscribed to allow simultaneous recording of the CO plume at multiple positions. This methodology was troublesome, as the framework was difficult to control and keep in the correct orientation with the water level. A single pole design was finally chosen for the tests. This methodology will be described later.

In addition to the matrix of 12 positions, samples were taken inside the test vessels near various seating areas. An example plan-view diagram showing the sensor positions is included in Figure 2.

Openings and walk-through gate doors were closed during testing. This allowed sampling of the "worst case scenario" for the given test vessel.

As all gasoline-powered vessels expel exhaust gasses at or near the stern of the vessel, the transom was used as a datum point for the sensor array. CO sensors were placed at 5-foot intervals in a grid pattern aft of the transom and also near the seating locations (also shown in Figure 2) inside the cockpit of the vessel.

Test Instruments

Lascar Electronics model EL-USB-CO carbon monoxide data loggers were used for all tests. Device specifications are listed in Table 1.

Test Protocol

Tests were performed as follows:

1. Tests performed in calm environment with negligible wind

2. All walk-through openings closed (window and gate)

3. No bimini tops or enclosures installed

4. CO monitors time synchronized and placed in locations

5. Vessel engine properly warmed up

6. Run test series

Test Vessels

Test vessels were selected to represent a range of commonly used boats for towed watersports. All vessels used large displacement 8-cylinder gasoline engines. A 24-foot 2001 "deck boat" with a 395 hp engine and Mercruiser Bravo III drive was used. This type of boat and engine/drive combination has been popular for many years among recreational boaters. The engine exhaust is routed through the drive system in this boat and exits through the hub of the propeller, expelling the exhaust into the prop wash below water level. This vessel does not employ any catalytic exhaust treatment.

The second test vessel was a 25-foot 2017 "surf boat" with integrated water ballast tanks and other wake-modifying components at the stern. This boat used a PCM XR7 supercharged 550 hp engine and was equipped with an exhaust pipe extension (Surf Pipe[R]) designed to expel the engine exhaust into the prop wash. This vessel was equipped with catalyzed exhaust treatment components.

A 1996 "wake boat" was the third test vessel. The wake boat expelled exhaust at the waterline directly below the swim platform and did not include any catalytic exhaust treatment. This vessel was equipped with a 395 hp Mercruiser engine.

Table 2 lists specifications for the three test boats. All boats had bow seating areas with closable walk-through windshields and doors to access the bow from the main cockpit. All were fueled with 91 octane premium gasoline. Windshields and bow access doors were closed for all tests.

This choice of test vessels represents a best-case scenario in terms of vessel design (none of these vessels has an enclosed cabin or airflow-limiting components), and worst-case scenarios for as-used configuration in terms of CO exposure risk. The 2001 deck boat exhausts into the prop wash, which may be preferable to venting exhaust at the waterline under the swim platform in some modes. The 2017 surf boat also vents into the prop wash, and also has (now mandated) exhaust treatment using catalytic converters. The 1996 wake boat vents exhaust directly at the waterline under the swim platform. The worst-case scenario for each boat was achieved by closing the windshield and walk-through openings. This can allow a portion of the exhaust gasses to collect in the cockpit, as will be expected when the boat is used is a closed condition. This CO exposure condition has been referred to in other publications as "backdrafting" or as the "station wagon effect."

Observations: Testing

Testing was performed by mounting the CO sensors to a composite pole (spaced at 5-foot intervals) and manually holding the pole in the desired position. Continuous tests were performed for two-minute time intervals, with the pole moved from portside to center, then to the starboard side, to record data from the various positions at the stern of the test vessel. This technique allowed the operator to hold the pole at a constant elevation during the testing and eliminated problems caused by waves and variations in the height of the sensors. The position of the pole was carefully controlled by the operator to align with the desired elevation and orientation for each test. See Figure 3.

All testing was performed with the pole approximately 3 feet above water level. Previous studies have recorded CO levels at various heights above water surface [3]. In addition to the samples recorded with the pole extended aft of the vessel, samples were recorded with the operator holding the pole crosswise to the stern of the vessel, the results of which are listed in the Results section. The height of the pole in these "crosswise" tests was also maintained at 3 feet from water level.


Test results are reported graphically in the figures that follow. Figure 4 is an example sensor target indictor and CO findings label. The location of the sensor is shown by the blue target. The circled values are the maximum and mean CO levels recorded at that target location during each two-minute sample period. Note that the circular labels for maximum and mean values are offset from the sensor locations.

Figures 5, 6, and 7 are results of tests performed with the sensor-equipped pole above the swim platform at a height of 3 feet above water level, transverse to the stern of the vessels. Since tests at this location resulted in the highest sensor readings across all boats, figures are included here to highlight the level of CO exposure that can be experienced on the swim platform while the engine is running. Full versions of these graphics are provided in the appendix. As shown, exposure to CO from exhaust gasses can be a concern if the vessel's engine is allowed to idle while people occupy areas in close proximity to the exhaust port such as the swim platform. Operators, passengers, and recreators are warned to avoid the stern of the vessel while the engine is running. Every boat captain should take care to ensure that the engine is off when people are on and around the swim platform.

For the deck boat, maximum values of nearly 900 parts per million (ppm) were recorded near the center of the swim platform, with average values between 400 and 500 ppm CO over the two-minute test period.

A comparable test on the surf boat gave a maximum reading of 253 ppm and an average of approximately 50 ppm. The lower values on the surf boat are probably due to the catalytic exhaust treatment on this vessel. Even with catalytic exhaust treatment, these areas on and around the vessel should not be occupied while the engine is running.

Peak values recorded from the crosswise testing on both the surf and wake boats are offset from the boat centerline and exhaust location. This is most likely due to slight variations in air movement during the tests, even though the tests were conducted in a calm environment.

Individual test results for each vessel and test configuration are included in the appendix. Table 3 lists maximum and mean values from each test, specifically listing the mean recorded at the same location as where the maximum value was recorded. For example, the maximum value for the Idle/Neutral/Crosswise test on the deck boat was 895 ppm. The mean at this same location during this test was 505 ppm. The reader can also refer to Figure 4 for the location of this reported value. (Note: No data is available for the Idle/Neutral test on the wake boat.)

From a design perspective, it is logical that expelling engine exhaust below the surface of the water and into the prop wash will help to transfer exhaust gasses farther aft of the stern of the vessel as compared to vessels which simply exhaust through the transom at water surface level. The tests done in Surf Mode show that this type of exhaust system design can result in lower CO exposures in the areas where a surfer would be located behind the vessel. The point where exhaust gasses rise to the surface will have significant influence on the results in these tests. Since the deck boat expels through the center of the propeller at a position deeper below the surface, it is reasonable that the results (in surf mode) will reflect lower CO readings at this distance and speed. Important factors in analysis of these systems are the forward speed of the vessel, the fluid motion relative to the vessel combined with exhaust gasses (prop wash), and the floatation rate of gasses coming to the surface from the prop wash. This article does not attempt to model the complex combinations and fluid dynamics of these factors.

Analysis and Findings

The NIOSH recommended CO exposure limit is 35 ppm (for workplace) CO exposure of up to 8 hours. NIOSH has also mandated a ceiling of 200 ppm. This means that workers are prohibited from being exposed to CO levels above 200 ppm for any period of time [4]. As shown in the results, all of the test vessels produced CO exposure conditions above 200 ppm in at least one test mode. These results should not be interpreted as an argument against towed watersports activities. By following simple safety rules (specifically as described on USCG warning and caution labels), any of the boats tested here can be used without risk of harm to passengers and participants.

NIOSH has published an "Immediately Dangerous to Life and Health" level of 1200 ppm for CO. Exposure to CO at levels nearing 900 ppm, such as was measured in an engine-idle mode above the swim platform on the deck boat, should be considered extremely hazardous. Passengers and participants must not be allowed to occupy the areas of the vessel near the transom while the engine is running.

The 2017 surf boat was equipped with the most modern CO mitigation technology of the boats tested here. This vessel has catalytic exhaust treatment and it expels exhaust below the water level into the prop wash. It produced the lowest overall CO exposure. Even with these improvements, some amount of exposure to CO can be expected while operating, riding in, surfing behind, or being towed behind any combustion engine-driven vessel. Advancements in technology, including catalytic exhaust gas treatment and prop-wash expulsion, are beneficial technologies that can help minimize CO exposure.

Even in a moving vessel, CO can accumulate in areas where users can be exposed. All of the vessels tested here are designed to be used in open configurations. The addition of bimini tops, side panel windows, and enclosures can increase risk if not ventilated properly.


This study presents findings from, and explores the challenges and factors associated with, measuring CO behind a moving vessel in these specific and limited real-world recreational environments. This is not a comprehensive analysis of CO exposure behind boats and does not address a variety of important factors relevant to the topic of CO exposure. Specifically

* The present study was undertaken with three V8 engine-equipped recreational boats. While these test vessels represent a sample group within the most popular segment in the towed-watersports category, useful future studies would include outboard engine-equipped vessels, personal watercraft (used as tow vessels as applied here), pontoon boats, and other types of vessels that are becoming popular among towed-watersports enthusiasts.

* This study is not a comprehensive analysis of the vast multitude of activities which can be engaged in around a recreational boat.

* No attempt was made to quantify the effect of catalytic converter-equipped vessels relative to identical vessels without catalytic exhaust treatment. A future study using two identical vessels with and without catalytic exhaust treatment would be informative. Similarly, vessels with few engine hours could be compared to those with many engine hours, various loading conditions could be applied to the same vessel, temperature effects could be analyzed, etc. This study is limited to the information contained herein.

* Environmental weather station data were analyzed relative to "time of test" sample data for the test locations. No reliable correlation was found and utilized in this study. This is fundamentally due to the ultrasensitive nature of air movement around the vessel in a real-world environment. Air currents follow a chaotic and variable pattern in every open environment. These air currents are further complicated by moving exhaust gasses and hot and cold surfaces on the vessel. Additional work in this area could include a controlled test environment where air movement could be carefully controlled.

* A single test elevation (3 feet above water surface) was used for all tests. The dynamic nature and shape of boat wakes, the motion of the water surface relative to the test vessel, the splashing and surface variabilities inherent to this type of boating, and the wide variety of rider positions that can be expected were all factors in the decision to measure at this elevation. Additional testing could be performed at other elevations. As advancements are made in CO sampling technology, splash-proof or water-resistant sensors may become available. This would allow closer monitoring of CO levels near the water surface, which may be applicable to some activities. This study is not an attempt to quantify CO exposure in every type of activity around a recreational vessel.


The findings presented here are intended to be used as a reference for research and discussion regarding CO poisoning and the water sports activities with which it has been associated. They are not intended to condemn or endorse any specific technologies or boat designs.

Exposure to CO gas can occur in any combustion engine-driven vessel and can reach hazardous levels when airflow patterns allow CO gas to accumulate in or around the vessel. Concentrations of CO gas will vary greatly for a given location depending on the speed of the vessel and airflow conditions. Wind speed (or the absence of wind) and wind direction are key factors in assessing CO exposure risk. When it comes to CO accumulation, concentration, and exposure relative to the types of vessels tested here, a little air movement goes a long way toward minimizing CO levels.

This test data shows that the highest levels of CO will be near the stern of the vessel and can accumulate around the swim platform and areas near the water in proximity to the exhaust outlet during idle and low speed operation. Avoiding these areas when the engine is running eliminates the risk of exposure to high levels of CO. When operators and participants follow good safety practices and obey safety laws, activities such as wake surfing, wake boarding, water skiing, and tubing do not place the participant is hazardous areas.

The highest CO levels can be expected to accumulate closest to the transom where the exhaust vents and near the water surface around the back of the boat. Under certain conditions, CO can accumulate inside the vessel where gasses are more protected from wind and airflow agitation.

While the vessel is underway, and under normal use circumstances such as those test conditions explored in this study, CO gasses are quickly dissipated by airflow around the vessel and other environmental factors.

Users of recreational vessels are wise to apply every available technology to reduce exposure to CO and should follow safe practices when engaging in powered watersports activities.

Contact Information

Mark H. Warner

Collision Safety Engineering, Orem, UT

Phone: 801 229 6200


(1.) Scott Earnest, G. et al., "Carbon Monoxide Emissions and Exposures on Recreational Boats under Various Operating Conditions," Report No.: EPHB 171-05ee2, U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Feb. 2003.

(2.) Garcia, A., David Marlow, G., Earnest, S., and Hall, R.M., "Evaluation of Carbon Monoxide Concentration with and without Catalytic Emission Controls from Gasoline Propulsion Engines," Report No.: EPHB 289-12a, U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health Division of Applied Research and Technology, Engineering and Physical Hazards Branch, Cincinnati, OH, Apr. 2007

(3.) Mann, L., "Carbon Monoxide Exposure Associated with Towed Watersports,", Mar. 2004.

(4.) Hall, R.M., Scott Earnest, G., Hammond, D.R., Dunn, K.H. et al., "A Summary of Research and Progress on Carbon Monoxide Exposure Control Solutions on Houseboats," Journal of Occupational and Environmental Hygiene 11(7):D92-D100, (2014), doi:1080/15459624.2014.895374.


A simple brief on the effects of CO exposure can be found here:

Tabulated values and a portion of the CO-related material from Occupational Safety and Health Administration (OSHA) are included in the following table. Linked references have been removed. The reader is encouraged to visit the online OSHA site for more details and accurate up-to-date information.

Carbon monoxide has over a 200-fold greater affinity for hemoglobin than has oxygen. Thus, it can make hemoglobin incapable of carrying oxygen to the tissues. The presence of CO-hemoglobin (COHb) interferes with the dissociation of the remaining oxyhemoglobin, further depriving the tissues of oxygen.

The signs and symptoms of CO poisoning include headache, nausea, weakness, dizziness, mental confusion, hallucinations, cyanosis, and depression of the S-T segment of an electrocardiogram. Although most injuries in survivors of CO poisoning occur to the central nervous system, it is likely that myocardial ischemia is the cause for many CO-induced deaths.

The uptake rate of CO by blood when air containing CO is breathed increases from 3 to 6 times between rest and heavy work. The uptake rate is also influenced by oxygen partial pressure and altitude.

Carbon monoxide can be removed through the lungs when CO-free air is breathed, with generally half of the CO being removed in 1 hour. Breathing of 100% oxygen removes CO quickly.

Acute poisoning from brief exposure to high concentrations rarely leads to permanent disability if recovery occurs. Chronic effects from repeated exposure to lower concentrations have been reported. These include visual and auditory disturbances and heart irregularities. Where poisoning has been long and severe, long-lasting mental and/or nerve damage has resulted.

The following table gives the levels of COHb in the blood which tend to form at equilibrium with various concentrations of CO in the air and the clinical effects observed:
Atmospheric CO   COHb in the blood   Symptoms
(ppm)            (%)

  70             10                  Shortness of breath upon vigorous
                                     exertion, possible tightness across
                                     the forehead
 120             20                  Shortness of breath with moderate
                                     exertion, occasional headache with
                                     throbbing in the temples
 220             30                  Decided headache,
                                     irritability, easy fatigability,
                                     disturbed judgment, possible
                                     dizziness, dimness of vision
 350-520         40-50               Headache, confusion, collapse,
                                     fainting upon exertion
 800-1220        60-70               Unconsciousness, intermittent
                                     convulsions, respiratory failure,
                                     death if exposure is prolonged
1950             80                  Rapidly fatal

Wakesurfing Links:

U.S. Coast Guard Warning Labels

USCG Helm Label

USCG Cabin Label

Test Results

The following pages graphically present full test results for each vessel, test condition, and sensor location.

Mark Warner, Collision Safety Engineering, L.C., USA


Received: 14 Jan 2018

Revised: 07 Aug 2018

Accepted: 14 Aug 2018

e-Available: 04 Oct 2018


Warner, M., "Carbon Monoxide Density Pattern Mapping from Recreational Boat Testing," SAE Int. J. Trans. Safety 6(2):107-131, 2018,


Carbon monoxide (CO) can cause brain damage or death.

Engine and generator exhaust contains odorless and colorless carbon monoxide gas.

Signs of carbon monoxide poisoning include nausea, headache, dizziness, drowsiness, and lack of consciousness.

Get fresh air if anyone shows signs of carbon monoxide poisoning.

See Owner's Manual for information regarding carbon monoxide poisoning.


Carbon monoxide (CO) can cause brain damage or death.

Carbon monoxide can be present in the cabin.

Signs of carbon monoxide poisoning include nausea, headache, dizziness, drowsiness, and lack of consciousness.

Get fresh air if anyone shows signs of carbon monoxide poisoning.

Get fresh air if carbon monoxide detector alarm sounds.

Carbon monoxide detector must be functioning at all times.

USCG Transom Label


Carbon monoxide (CO) can cause brain damage or death.

Engine and generator exhaust contains odorless and colorless carbon monoxide gas.

Carbon monoxide will be around the back of the boat when engines or generators are running.

Move to fresh air, if you feel nausea, headache, dizziness, or drowsiness.

TABLE 1 CO monitor specifications.

Model number             EL-USB-CO
Rated accuracy           Rated accuracy, [+ or -]6%.
Measurement range (CO)   3 ppm-1000 ppm
Logging rate             6 per minute (10 second interval)

TABLE 2 Test vessels/general specifications.

Test vessels     Drive type        Engine           Exhaust

2001 deck boat   Inboard/outboard  Merc 7.4L 395hp  Prop centered outlet
                 Mercruiser Bravo
1996 wake boat   Inboard           Merc 7.4L 395hp  Stern under the
                                                    swim platform
2017 surf boat   Inboard V drive   PCM XR7 550hp    Stern prop wash

TABLE 3 Maximum and mean values by boat type and test configuration.

CO gas test results      2001 deck   2017 surf   1996 wake boat
(CO, ppm)                boat        boat
                         Max  Mean   Max  Mean   Max       Mean

Idle/neutral/crosswise   895  505    253  76     695       457
Engine idle/neutral      306  124     20   7     No test   No test
5 MPH (wakeless)         233  131    152  27      40        14
Surf mode 10-12          129   43    100  46     636       151
Cruise mode - 30          18   12     13   4       8         4
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Author:Warner, Mark
Publication:SAE International Journal of Transportation Safety
Date:Jul 1, 2018
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