The phrase Virtual Propeller Guard broadly refers to methods used to detect the presence or likely presence of people or objects in the water that may be struck by or pulled into a boat propeller. Long ago we posted a more concise definition of Virtual Propeller Guards.

With increasing activity in the use of sensors to detect people at risk of being struck by a boat propeller we decided to try to bring most of the relevant patents on the topic together in one location.

In the instance of this post, we will only be referring to patented approaches using sensors to detect the presence of people in the water at the stern / behind the transom of a recreational boat. Those in the water may include swimmers, floaters, those on tubes, those on or at the swim ladder, water skiers, wake boarders, wake surfers, those standing in shallow water near the vessel, those riding a PWC, those in a Kayak or Stand Up Paddleboard (SUP), people being rescued, or any other conveyance or activity that may place people in the water behind the stern of a recreational power boat within a distance of about 10 feet of the boat propeller. Patents listed below may not cover the entire range of applications or distance described above.

The Volvo Penta image below is an example of these types of patents.

Volvo Penta U.S. Patent 8,195,381 sketch

Sensor Based Detection of People Near the Propeller

Brunswick / Mercury Marine

• U.S. Patent 6,273,771 Control System for a Marine Vessel issued to Brunswick 14 Aug. 2001 for a CANBUS integrated electronic control system for the entire boat that incorporates sensors that could be used to detect people and objects in the water in front of and behind the boat.
• U.S. Patent 6,354,892 issued to Brunswick on March 12, 2002. The patent describes a device (infrared sensor in a tube to limit sensor area) for detecting people near an operating propeller or bow thruster. It sounds alarms, deactivates the propeller or takes other appropriate action when people are in the danger zone.
• U.S. Patent 7,105,800 Detection System and Method for a Propeller Driven Marine Vessel With a False Triggering Prevention Capability, Assigned to Brunswick by Richard Staerzl, issued 12 Sept. 2006. This patent follows up on their # 6,354,892 patent for an infrared sensing device. The new patent teaches the method of sensing infrared with one sensor and visible light with the other and combining the results to prevent false triggers from reflected sunlight.
• U.S. Patent Application 2008/0174455 Method for Detecting a Source of Heat Near a Marine Vessel by Richard Staerzl of Mercury Marine published 24 July 2008. This is one of at least two recent follow up applications to their previous U.S. Patents 6,354,892 and 7,105,800. This application covers the concept of using two infrared sensor units (one on each side of the boat) with a total of four infrared sensing elements in an overlapping pattern to detect “marine mammals” in the water near the rear of the boat. It requires a positive signal from at least two of the four sensing elements to generate an alarm. False triggers can be reduced by not providing an alarm if only the two most inboard sensors detect the “heat emitting object”. They also point out the sensors could be placed under the deck of a pontoon boat and be protected from the sun, making sensing easier and more accurate.
• U.S. Patent Application 20080173818 Heat Source Sensor for a Marine Vessel by Richard Staerzl of Mercury Marine published 24 July 2008. This application is in tandem with the application above. it focuses primarily on the infrared sensors.
• U.S. Patent 7,476,862 Method for Detecting a Source of Heat Near a Marine Vessel. Issued 13 Jan 2009. Invented by Dick Staerzl and assigned to Brunswick. Another Mercury infrared patent trying to reduce false triggers on a virtual propeller guard.
• U.S. Patent 7,511,276 Heat Source Sensor for a Marine Vessel. Issued 31 March 2009. Invented by Richard Staerzl and assigned to Brunswick. Another Mercury infrared patent trying to reduce false positives on a virtual propeller guard.
• U.S. Patent 7,924,164 Method for Sensing the Presence of a Human Body Part Within a Region of a Machine Tool. Invented by Dick Staerzl and assigned to Brunswick. Issued 12 April 2011. This a manufacturing floor worker protection patent based on detecting parts of human(s) near manufacturing equipment at hazardous times, such as near the saw blade of a band saw. This patent is included here because it was assigned to Brunswick and invented by the same inventor of other patents listed here. Is basically a similar application on the shop floor vs at a boat propeller.
• U.S. Patent Application 2017/0323154 Person Detection in a Marine Environment. Assigned to Brunswick. Image detection system captures and scans images, then uses neural network(s) to detect people and objects in those images in order to detect objects (including people) in the water near a boat. our coverage.
• U.S. Patent 10,372,976 Person Detection in a Marine Environment. Assigned to Brunswick. Issued 6 August 2019. Use of a visual light camera with an Artificial Neural Network (ANN) trained to scan the images to detect people behind the boat. Once people are detected in harms way, a range of actions can be taken. This patent, and most other Virtual Propeller Guard patents by Brunswick utilizes the CANBus system described in U.S. Patent 6,273,771. our coverage.
• European Patent Application 3242248A1 Person Detection in a Marine Environment. Brunswick patent application related to U.S. patent above. Brunswick has filed the application for a large number of European countries and for Japan.

Volvo Penta

• U.S. Patent Application 2009/0276148 Safety System for Marine Vessels filed by Volvo Penta. Published 5 November 2009. Digital anchor providing safety to one or more persons in the water in the area behind the vessel.
• U.S. Patent Application 2012/0095628 Safety System for Marine Vessels assigned to Volvo Penta published 19 April 2012. A virtual propeller guard system in conjunction with a digital anchor (GPS anchor). See U.S. Patent 8,195,381.
• U.S. Patent 8,195,381 Safety System for Marine Vessels assigned to Volvo Penta and issued June 5, 2012. This patent describes the use of a digital anchor, a virtual anchor based on GPS signals and controlling the drive(s) in combination with infrared sensors to detect people in the water near the vessel and people in the water in the propeller danger zone. our coverage.
• U.S. Patent 8,271,155 Safety System for Marine Vessels assigned to Volvo Penta and issued September 18, 2012. Followup patent to their digital anchor patent, U.S. Patent 8,195,381.

Others

• U.S. Patent # 6,676,460, invented by Marvin Motsenbocker, issued 13 Jan 2004. A system using sensors to detect swimmers, animals and other objects in the water near a propeller. The system slows or stops propellers when they are detected.
• U.S. Patent 6969287 Electronic Shut Off Systems, invented by Marvin Mosenbocker, issued 29 November 2005. Methods to quickly stop a boat propeller.
• U.S. Patent 7,335,071 Electronic Shut Off Systems, invented by Marvin Motsenbocker, issued 26 February 2008. Virtual Propeller Guard for electric driven boat propellers.
• U.S. Patent 10,317,448 Human Sensing Using Electric Fields, and Associated Systems and Methods, issued to Swift Engineering on 11 June 2019. Resembles techniques used by Mercury/Brunswick in an operator presence detection patent, but used for around mobile devices including boats.

Very Early Efforts

• U.S. Patent 3,575,527 issued April 20, 1971. This system senses the presence of a hand near an electric fan blade and disconnects the fan. While this particular patent does not apply to a marine propeller, it is hard not to see the resemblance.
• U.S. Patent 3,805,723 issued to the U.S. Navy April 23, 1974. The patent teaches the use of “feelers” in front of a circular slow moving propeller on an underwater submersible used with divers. The feelers were tied to Microswitches to stop the propellers. Due to the slow rotation of the propellers, they can be rapidly stopped. This is the earliest reference we have seen to the use of Virtual Propeller Guards specifically for protecting people.

We Encourage Further Development in This Field of Boat Propeller Safety

We hope that by bringing these patents together, others will be encouraged to continue developments in the field of Virtual Propeller Guards using sensors to detect the presence of humans / people / swimmers near recreational boat propellers. We thank those already working in this field and encourage others to join in the effort as well.