Circle of Death Boat Propeller Accident Invention

Circle of Death Boat Propeller Accidents Can be Detected and Prevented

System for Detecting and Stopping Circling Boats Invented by the Propeller Guard Information Center (PGIC)

We maintain an extensive list of technologies with potential applications to preventing or mitigating propeller injuries on our <2011 Propeller Safety Technologies page. This invention would have been posted there, but was moved here due to its length. We may post additional updates here in the future, but we will no change the original posting to maintain its integrity as the original disclosure.

Annually, many people operating recreational boats are ejected or fall overboard, the outboard motor or stern drive swings to one side, the unmanned boat goes into a tight circle known as the Circle of Death, and they are struck by the propeller. Many are struck multiple times as the boat continues to circle. Many have been killed.

The invention below addresses this problem. We are making a public disclosure of it to encourage its widespread application.

If you have any comments or questions about the System of Automatically Detecting and Stopping Boats in the Circle of Death, please contact us. Also, if your company has an active interest in trying to manufacture a system to stop circle of death boats please contact us.

The original disclosure is between the two lines below and will not be altered in the future (except for keeping the links and code current, and correcting spelling errors).

  • Posted 17 June 2010. PGIC Invention –

    System For Automatically Detecting and Stopping Boats in the Circle of Death

  • We claim the the propeller safety invention described below and now make a public disclosure of it and place all parts of it not previously patented by others into the public domain for use by all who wish to manufacture or use them. The world is certainly welcome to patent improvements to this invention but the basic concepts not previously patented by others are open to all.

    Reference: Our previous Circle of Death posting on our <2011 Propeller Safety Technologies page dated 24 July August 2007.

    Problem- Many small outboard boats (especially those without power steering) have a substantial torque on the steering wheel or tiller handle called steering torque. Steering torque results from the rotating propeller. If the wheel or tiller is released or the operator is ejected (such as by a boat wake, or by striking a submerged object, or by releasing the tiller handle while underway) the motor will slam to one side or the other (typically to the left) and the boat will begin circling tightly to the other side (typically to the right). This abrupt change in direction often ejects the operator or others on board, and is called the Circle of Death due to the high probability those in the water will be struck by the propeller of the unmanned boat and be killed. The same events can happen on stern drive boats, especially those without power steering.

    The You Tube video at the right shows how tight a small boat can circle for hours on end, and the difficulties in trying to stop one in the Circle of Death.

    The Circle of Death can result from a chain of events, such as the boat strikes a large wake or submerged object and the operator is ejected, then boat begins to circle abruptly, and other passengers are ejected.

    Circle of Death video

    The Circle of Death can also be originated by mechanical failures to the steering system that allow the drive to immediately swing to one side.

    Operators can also be ejected from the stern while they are inspecting an idling engine or propeller and the boat jumps in gear, or they may be rope starting the engine and fall in when the boat jumps into gear. Both these scenarios can result in a Circle of Death.

    If no one else is onboard, the boat will typically circle till it runs out of gas or rescue efforts are able to stop it (often by trying to re-board the boat while it is still circling or by fouling the propeller with a rope).

    Sometimes there is somebody left onboard, but they are unable to stop the Circle of Death (like young children or pets). Those in the water have to repeatedly try to dodge the circling boat and its propeller as the boat bears down on them again and again. Also, if others are not able to rescue them, they may be in the water for long periods of time, unable to re-board the circling boat, and drown. Some may attempt to re-board the circling boat from the water and be struck by the propeller.

    People ejected into the water wearing life jackets / PFD’s may not be able to dive beneath the approaching circling boat due the added floatation of the PFD, especially not repeatedly. Several people can be ejected at once, all exposed to the circling boat and its propeller. Parents or loved ones may take the boat / propeller strike in an effort to shield their children or loved one.

    Traditional Prevention Solutions –

    The prescribed solution is an emergency ignition cut-off switch (lanyard) attached to the operators clothing and clipped to the corresponding kill switch key at the helm. However, many operators do not wear them. Additionally, many older boats are not equipped with emergency ignition cut-off switches. 

    In recent years, virtual lanyards have been advanced as an alternative (a tag to be worn by the operator that can detect if they are too far away from the helm, or have fallen into the water). Others have proposed infrared detectors to detect the operators presence near the helm, or switches in the floor or seat to determine operator presence.

    Invention Description –

    This invention adapts the “Virtual Lanyard” concept used by MariTech and Autotether that kills the engine by an inline relay switch, pulling out the lanyard key, by blocking the flow of fuel to the engine, or by some other means. Instead of detecting wet pendants / tags, it uses sensors to detect the boat is tightly circling. Once the “Circle of Death” is detected, the engine is killed via one of the methods listed earlier, or shifted to neutral and possibly throttled back.

    Possible Options –

    Possible options and enhancements include:

    • Use of more than one method of detecting the “Circle of Death” to reduce false positives.
    • An override button to allow continuous tight circling under power when desired by the operator.
    • An alarm or warning may be sounded or lighted for a few seconds allowing an operator still on the boat to push an override switch that would allow the boat to continue to circle under their override command.
    • A test procedure to verify the system in operating condition.
    • A reset button to allow normal operation after the boat had been stopped by the system.
    • The override and reset buttons might be the same button.

    Defining the Circle of Death

    The system would contain a definition of the Circle of Death to check one or more variables against to determine when a Circle of Death was underway. That definition might include the use of a timer.

    The “Circle of Death” could be defined as for a given vessel by one or more of the definitions below.

    • The boat has been going in a tight circle to the left/right at a speed greater than “x” miles per hour for more than “y” seconds”.
    • The boat has completed “x” similar tight circles in the last “y” seconds”.
    • The boat has been leaning hard to port/starboard for “x” seconds and the bow is pitched up.
    • The boat is almost spinning on itself.
    • Some other definition

    Detecting the Circle of Death

    Some Methods of Detecting the Circle of Death

    • Orientation of the Boat for a Period of Time
      • Pitch, Yaw, Roll sensors
      • Mercury Switches
      • Inclinometers
      • Tilt sensors
    • Pitch, Yaw, Roll of the boat for the last few minutes (detecting circling)

    • Nature Signals of Orientation of Boat for a Period of Time
    • Wind speeds (are an indicator, but not always an exact relationship)
    • Changes in wind direction (as you circle)
    • Changes in orientation of the sun
    • Changes in orientation of shadows within the boat
    • Changes in orientation of a landmark with respect to the boat Centrifugal Force
    • Centrifugal force sensors (such as ball in a tube with a spring)
    • MEMs centrifugal force sensors (very small ones)
    • Lap Counters
      • Compass lap counters (counts number of times you spin the compass, there are numerous patents for these devices)
      • Magnetic Heading Sensors (counts number of times you spin the compass, there are numerous patents for these devices)
      • Doppler Radio Direction Finders (identify direction toward a radio source and the boat is circling with respect to it)
      • Hall effect lap counters (use to check for a compass going around thru several positions as it spins – U.S. Patent 5,844,960
      • GPS lap counters
    • Other Variables of Interest:
      • Historic Boat trajectory for the last few minutes
        • GPS historic output for the last “x” seconds
        • Oscillating wind direction and wind speeds
        • Boat is rolled hard to one side
        • Heading sensors
        • Detect a compass needle relative to rim of the compass (if needle is spinning in the compass you are circling)
        • Test water coming into water pump for presence of marine exhaust chemicals and gasses (you are circling in your own exhaust)
        • Position of drive relative to going straight (drive swung to one side)
        • Position of steering wheel, tiller handle, steering cables, steering cylinder rod extension, and other steering links (indicating the drive is swung to one side)
        • Position of sun viewed from the boat
        • Shade line inside the boat (shade line is moving from side to side if boat is rotating in non overhead sun)
        • Centrifugal force sensors (throw ball to outside against a spring when the boat is circling)
        • Pendulum output to detect lean
        • Pedometer can detect lean of the boat
        • Other methods
      • Drive is down and prop is turning
        • Trim
        • Speedometer
        • Other means
      • Boat Speed history for the last few minutes
        • Engine noise level or vibration (tied to boat speed)
        • Throttle position
        • Boat speed pickups (electronic speedometer feed, pitot tube, GPS speeds)
        • Other methods
      • Boat being in forward gear or not
        • Throttle/gearshift position
        • Electronic throttles and shifter output
        • Current boat trajectory
        • Accelerometer reading forward accelerations over the last few minutes
        • Other methods
      • Has Anybody Been Ejected?
        • Activation of lanyard or virtual lanyard systems
        • Detecting operator presence at the operators station by infrared sensors, by sensors in the seat, or by sensors in the floor. These are sometimes called Passive Occupant Sensing systems.
        • Capacitance at the steering wheel (a small boat is underway at above “x” miles per hour, but nobody has touched the steering wheel for “y” seconds).
        • Screams
        • Other ejection detection variables

    Boats currently using systems similar to Brunswick / Mercury Marine’s Smart Craft gauges already have easy access to many of the these variables.

    A traditional lanyard AND/OR a virtual lanyard could also be part of the system. When you are wearing the lanyard you have the normal instant stop when the operator goes overboard or leaves the helm. If you are not wearing one of those devices, it defaults to the “Circle of Death” protection mode.

    Some Other Possible Components

    Stopping circling boats does NOT require the wireless features needed by the pendants / tags associated with virtual lanyards and would thus be much cheaper. It could even just be sold as a separate inline “add on” to the fuel line. Small totally self contained unit with one or more sensors to detect circling, a timer, a valve, and a connection with the one of the boat’s electrical power systems. The system could optionally be battery powered, windup powered, cocked, solar powered, powered by centrifugal force, or powered by some other means.

    While it does not require wireless features, it could still incorporate them in certain versions of the design.

    Existing tilt switch systems integrating time delays (boat leans over “x” degrees for “y” seconds) could also be employed. The final result could just be a small block in an external fuel line with an arrow indicating direction of fuel flow and another one indicating which side goes to the inside of the spinning boat. Or the system might be designed detect circling in either direction to avoid the RH LH propeller and RH LH drive rotation issues. One version might store up enough rotational energy from the circling boat to power any sensors and close the fuel valve creating a totally free standing, self acting unit.

    Another possible configuration is to use detect the “Circle of Death” using various combinations of sensors previously mentioned, then generate an output to interrupt the ignition circuit of the outboard motor similar to U.S. Patent 6,276,974 (a PropGuard / MariTech patent) in which a relay is used to interrupt the ignition circuit.

    The same systems (shut off the fuel or interrupt the ignition) could include similar detection methods with similar outputs when the boat is going in reverse in a tight circle.

    Possible Implementations

    Several combinations of sensors and methods could be used to detect and stop a boat in the “Circle of Death”. Just a few examples are given below:

    • An system watching the output of an electronic compass detects the boat is rapidly circling. Once the boat has completed one tight loop, an alarm sounds. If no one presses the override button within “z” seconds, the engine is killed by an ignition relay switch similar to that used by MariTech with the gate switches. A reset button is provided to reset the system.
    • An algorithm processing GPS output detects the vessel is circling in a tight circle at speed. Once the boat has completed one tight loop, an alarm sounds. If no one presses the override button within “z” seconds, the engine is killed by an ignition relay switch similar to that used by MariTech with the gate switches. A reset button is provided to reset the system.
    • A compass lap counter is used to count laps in conjunction with a timer. If the boat completes once circle in less than “x” seconds, an alarm sounds. If no one presses the override button within “z” seconds, the engine is killed by an ignition relay switch similar to that used by MariTech with the gate switches. A reset button is provided to reset the system.
    • On a stern drive, the steering lever comes thru the gimbal housing to the interior of the boat to be attached to the steering cable or a power steering system. The position of the steering level reflects the current steering angle of the drive. A proximity switch (or a contact switch) could determine when the drive is steered all the way to one side (or two switches could decide if it is steered all the way to either side). The boat speedometer input/output can determine if the drive is running in forward at a speed greater than “x” mph (fast enough to be of concern to those in the water). Combined with a timer, the system can determine how long the boat has been circling (steering all the way to one side with forward speed greater than “x” mph. A lookup matrix could be calculated in advance of boat speeds and approximately how long it takes the boat to complete one circle steered all the way to one side at any given speed. Once the boat has been calculated to have completed a full circle (steered all the way to one side at speed “x” for time “y” that would have completed more than one circle according to the lookup matrix, an alarm sounds. If no one presses the override button within “z” seconds, the engine is killed by an ignition relay switch similar to that used by MariTech with the gate switches. A reset button is provided to reset the system.
    • A centrifugal force sensor (spring loaded ball in a tube) is used to detect the boat is in a tight turn. The switch is armed when the boat it turned on and turned off when the engine is killed. If the centrifugal force is greater than “x” the boat is in a hard turn at a speed high enough to be of concern to those in the water. A timer is used to monitor the length of time the centrifugal force is greater than “x”. Once that time exceeds “y” (time to complete greater than one full circle”) an alarm sounds. If no one presses the override button within “z” seconds, the engine is killed by an ignition relay switch similar to that used by MariTech with the gate switches. A reset button is provided to reset the system.
    • A radio system has a seek control similar to the one in your car that allows it to find the next station with a signal strength above a given level. That radio and seeker system are combined with a Radio Doppler Direction Finder to find the direction to a station with a strong signal. The direction to the station is monitored. If the direction to the station is moving with respect to the boat and the difference between the direction to the station and the orientation of the boat is following a pattern consistent with the boat circling, once the boat has completed in excess of one full circle, and that circle was completed in less than “x” seconds, an alarm sounds. If no one presses the override button within “z” seconds, the engine is killed by an ignition relay switch similar to that used by MariTech with the gate switches. A reset button is provided to reset the system.For all the examples above, even if the boat is going in tight circles at very slow speeds, the system will similarly sound an alarm and then shut off the boat after it completes three full circles if the override is not pressed.


    • We are aware several of these sensors in and of themselves may not individually positively confirm the boat is going forward and circling, put certain combinations of them can reliably do so.
    • The output from some of the sensors may need to be ran through a smoothing algorithm to reduce some noise in the signal due to randomness surrounding a boat in the water (it is bouncing in the water an may not even be going the same way around the circle at every single reading – for example, it may hit a way and retrograde for a reading or two, then resume its path).
    • This invention is NOT meant to replace the use of traditional lanyard engine emergency kill switches. Traditional switches will work much faster as they kill the engine the moment the operator goes overboard, not several seconds later after the boat may have already hit the operator.
    • There have been some instances in which a very young child or pet may still being in the boat so it is not totally unmanned. Their potential presence may effect the exact combinations of sensors selected.
    • GPS tends to underestimate speeds in tight circles (it captures individual points (positions) and calculates speeds between those points, not around the perimeter).
    • GPS circling algorithms are used in the glider industry as they circle in thermals. They are also used with GPS’s on soaring birds.
    • Compass Lap Counters, predominately in use for counting laps in swimming pools and for joggers on oval running tracks are currently covered in several patents.
    • Directions for numerous Doppler Radio Direction Finder build it yourself kits are online. These kits can determine a boat circling by seeing the direction to the transmitter go around the circle with respect to the orientation of the boat.
    • Radio, compass, magnetic, and solar direction finders can be forced to not only determine a reversing in direction, but also to ascertain the boat went all the way around in a circle. For example, U.S. Patent 5,844,960 forces the system to go through 36 compass positions as it circles to count a lap.
    • Optimally, all the components could be located in one “brick” behind the dash near the ignition and emergency engine shut-off switch allowing the system to be easily integrated into the vessel.
    • The Circle of Death stopping system could also be made available as a retro-fit kit for existing boats.
    • Fuzzy logic could be used with several inputs to help verify the boat is circling and reduce false positives.
    • Many boats have certain characteristics or other systems onboard lending them toward one type of system vs. another (using one method to detect circling vs. another.)
    • Boat velocity may be a bit difficult to detect due to the boat’s constantly changing heading. Prop thrust might be detected instead (stress on the transom, load on the gear system, sound of the engine, etc).


    • Comparison of Simplifying Line Algorithms for Recreational Boating Trajectory Dedensification. Yan Wu and Ronald Pelot. Book Chapter in: Lecture Notes in Geoinformation and Cartography. Geomatic Solutions for Disaster Management. Jonathan Li, Sisi Zlatanova, and Andrea Fabbri. 2007
    • Characterizing Recreational Boating Patterns Based on GPS Trajectory Points. Yan Wu. Phd thesis. Dalhousie University. 2007.
    • Classification of Recreational Boat Types Based on Trajectory Patterns. R. Pelot and Yan Wu. Pattern Recognition Letters. Vol.28. (2007). Pgs. 1987-1994.
    • U.S. Patent 6,276,974 assigned to PropGuard (now MariTech Industries)
    • Comparing Bird and Human Soaring Strategies. Zsuzsa Akos, Mate Nagy, and Tamas Vicsek. Proceedings of the National Academy of Sciences of the United States of America (PNAS) Vol.105. No. 11. (March 18, 2008.) Pgs. 4139-4143.
    • Doppler Direction Finder: Radio Direction Finder Kit. Ramsey Electronics Model No. DDF1. product manual is also available from that page.Retrieved 17 June 2010.
    • U.S. Patent 4,530,105 Lap Counter. Jonathan Rabinowitz. Counts the times you break the north south line.
    • U.S. Patent 4,869,575 Headware-mounted Periscope Display Device. James Kubik assigned to Iota Instrument Company. Swimming glasses include a magnetic lap counter that generates a pulse at every direction change and counts the pulses to count the laps.
    • U.S. Patent 5,125,010 Lap Counting System. Lewis Lee and Lester Wallace. Counts laps based on distance between a radio transmitter and receiver. Counts a lap when distance between receiver and transmitter becomes less than the communication range.
    • U.S. Patent 5,661,398. Two Channel Eight Bearings Magnetic Resolver Circuit Using Differential Amplifiers for Lap Counting and Timing. Ka Yiu Sham and Phillip Lim-Kong Wong. Assigned to Acumen.
    • U.S. Patent 5,844,960 Synchronized Voltage Controlled Oscillator Lap Counting Circuit. Ka Yiu Sham and Phillip Lim-Kong Wong. Assigned to Acumen. – a Hall-effect lap counter detects user going through 36 different compass positions to complete a lap.
    • U.S. Patent 7,641,590 Swimming Lap Counter. Raymond Chan. Assigned to IDT Technology Ltd.
    • U.S. Patent Application 20060098772 Distance Meter. Akseli Reho. Compass/magnetic lap counter for use on oval running tracks.
    • U.S. Patent Application 20020151810 Wrist-based Fitness Monitoring Devices. Philip Lim-Kong Wong. Wristwatch with a compass/magnetic lap counter in it.

    The line above concludes the original disclosure of the System For Automatically Detecting and Stopping Boats in the Circle of Death

    Later Follow Up Thoughts

    18 June 2010 – How this invention compares to traditional lanyards and to MariTech virtual lifeline.

    Just as this invention is not meant to replace traditional lanyard kill switches (they activate immediately as the person is ejected and the key is pulled from its slot), it is similarly NOT superior to MariTech’s virtual lifeline / CAST system which activates when the person hits the water. Our invention will not activate until the circle of death is underway (at least some portion of a circle has been completed, and possible after after one or more circles have been completed).

    The advantage this system has over traditional lanyard switches is its a passive system. You do not have to connect up. Many boats now have emergency engine shut-off switches, but few operators actually wear them.

    This system is superior in those instances (in the instances in which the operator is not wearing a lanyard.)

    The advantages of this system over MariTech’s virtual lanyard are cost and no need to wear tags. This invention does not require a wireless system to operate (it costs less) and some boats already have everything they need (GPS to detect tight circling at speed, Smart Craft gauges, etc.). Those boats just need a system overlay to monitor the variables and make the decisions, plus some means of killing the engine.

    If I was operating a boat and fell out I would much rather be wearing a traditional lanyard kill switch or a MariTech virtual lifeline than this invention. The problem is people are not wearing the alternatives and this is a totally passive system that will cost less than the MariTech system. People never see it. They just operate the boat and if everybody falls out, the system eventually stops the boat if it is circling tightly.

    18 June 2010 – Brunswick Smart Boat

    Back in 2001, Brunswick was issued U.S. Patent 6,273,771 for a CANBUS integrated electronic control system for an entire boat (many of the sensors were talking to a control system that allowed more intelligent decisions to be made. Basically, all the sensors and devices can talk to one another. Back when that patent was issued, we noted its possible application to propeller safety (some sensors detecting the presence or probably presence of people could talk to other devices on the vessel (blow the horn, throttle back, stop the drive, etc.). Brunswick CANBUS Smart Boat lends itself to the Circle of Death prevention system as well.

    18 June 2010 – The Existing Lanyard Kill Switch

    As boats get more complex, there are more reasons to slow down or kill an engine. It seems like some sort of standard engine key relay that could accept plugged in inputs (just like plugging in a USB cable) would allow multiple systems access to the switch, reducing costs. The current existing lanyard kill switches could just be one of those inputs (the key plugs in just like everything else. The system just knows that if that key is pulled out, the engine is stopped.

    30 May 2012 – Fuel Inlet Designed to Prevent Circling

    The fuel line to the tank (or elsewhere along its length) could be designed to prevent fuel from entering or prevent fuel from passing through it if the boat was in strong, continuous rotation. A ball or weight flying to the outside after a few rotations of a short radius could either prevent fuel from entering or block it from passing.

    Or the fuel line creates a tortuous path the fuel cannot traverse when the boat is in a strong, constant rotation.