A very extensive annotated list of U.S. and International recreational boat propeller guard patents and other propeller safety device patents. The list includes several propeller guard patents from the early 1900’s.

Recent Propeller Guard Patents and Propeller Safety Patents

Links are provided to several of the patents below. For those without links, most can be viewed from Google Patents, just enter the patent number in the search box, or from the U.S. Patent and Trademark Office site. Just enter the patent number in the Query search box to see full text of the patents. A tiff viewer will be required to see the images on the USPTO site.

Most mechanical propeller guards are in Patent Class 440 (Marine Propulsion) and subclass 71 (Propeller Guard) or 72 (Propeller Guard – Cage or Screen). Some helpful search terms are “propeller guard”, propeller injury, manatee propeller, “propeller protector”, “propeller deflector”. You may also find International Patent Classes B63H5/16 and B63H5/165 useful.


  • U.S. Patent Application 2016/0340009 Propeller Guarding Assembly. Snap on propeller guard assembly. Invented by Stevens. Published 24 November 2016.
  • U.S. Patent 9,499,245 Boat Propeller Nut. Invented by Guy Taylor. 3PO propeller nut. Issued 22 November 2016.
  • U.S. Patent 9,475,550 Propeller Protection Shield for a Boat and Method of Use Thereof. Invented by Cory FormyDuval. Sort of a weighted tarp drug behind the boat between people and the propeller. Separates people from outboard and stern drive propellers. Issued 25 October 2016.
  • U.S. Patent 9,359,053 Propeller Cover. Drawstring bag cover for propeller. Invented by Hughes. Issued 7 June 2016.
  • U.S. Patent 9,266,592. Protector for Propeller. Invented by Peelman and Todd. Bag for temporary protecting propellers. Issued 23 February 2016.
  • U.S. Patent 8,939,807 Propeller Protection Apparatus. Invented by Bartley Jones. Deflection structure under the boat including a rolling tire(s) for the boat to slide on in shallow water. Issued 27 January 2015.
  • U.S. Patent 8,803,711. System and Methods for Displaying Operational Characteristics of Marine Vessels. Invented by Steven J. Gonring. Assigned to Brunswick Corporation. Issued 12 August 2014. Mercury Marine’s Moving Propeller Alert patent.
  • U.S. Patent Application US 2014/0065903 Guard and Method for Protecting Wildlife. Published 6 March 2014. Invented by Frederick Stearn. A propeller guard targeting manatee applications. It tries to keep them off the propeller, leading edge of the drive, and skeg while creating minimal resistance.
  • U.S. Patent 8,656,967 Cover for a Boat Motor. Issued 25 February 2014. Clothlike cover for lower unit of an outboard motor or stern drive to prevent contact with the propeller when anchored, beached, trailered, etc.. Also protects lower unit from the weather.
  • a series of applications:
    • WO Patent Application WO2014020548 A1. Propeller Including Discrete Blade Edge Cover Member. Invented by Russell Ian Hawkins. Published 6 February 2014.
    • WO Patent Application WO2014020551 A1. Propeller Incorporating a Secondary Propulsion System. Invented by Russell Ian Hawkins. Published 6 February 2014.
    • WO Patent Application WO2014020550 A1. Propeller Including a Blade Back Flow Guide. Invented by Russell Ian Hawkins. Published 6 February 2014.

    These pertain to the Mag Blade safety propeller being developed in South Africa. Our understanding is that current plans are to market it for its increases in performance, not for human safety.

  • U.S. Patent 8,590,470 Protective Cover Kit for a Marine Propeller. Issued 26 November 2013 to Randal Ploch. Prop bag for installing when in the water to prevent marine growth.
  • U.S. Patent 8,579,669 Gear Housing for an Aquatic Vessel Breakaway Safety System for an Aquatic Vessel and Aquatic Vessel. Issued November 12, 2013 to Volvo Penta. A version of their IPS pod drive that can rotate back and up when it strikes an object by crushing some of its construction, then breaking off from the vessel. The IPS is a tractor drive (propeller is in front of the case). Brunswick tried to address similar issues with their Mercury Marine through hull drive, the Zeus, in Brunswick’s U.S. Patent 8,062,082 issued November 22, 2011. We list these patents here because they illustrate the possibility of reducing blunt trauma issues by collapsing the drive. They also provide some other opportunities we will enlarge on in the future.
  • U.S. Patent Application 2013/0189884 Propeller Protection Apparatus invented by Jones and Hendirkson. Published July 25, 2013. Designed for small boats it is intended to protect the propeller when beaching. The device includes deflecting members and wheels to protect the propeller.
  • U.S. Patent 8,491,268 Safety Propeller invented by Colin Chamberlin of Australia. Issued July 23, 2013. The Australian Environmental Safety Propeller.
  • U.S. Patent Application US 2012/0309243 Propeller Protection Device invented by Elizabeth Johnson published December 6, 2012. Three or four bars placed longitudinally along the bottom of the vessel sloping down to the stern. The intent is to gradually slide oncoming people and marine life down below the oncoming propeller. It is a deflector somewhat similar to the LyfGard. The lady is in Norcross Georgia, home of LyfGard, so they may be related. The patent application only has three claims of which one is an independent claim. It appears to have been written by someone using a template.
  • 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.
  • U.S. Patent 8,257,121 for a “Boat Propeller Shield” assigned to Guy Taylor and issued on September 4, 2012. Guy Taylor’s 3PO Propeller Shield automatically swings up from his hexagonal shaped Navigator guard when underway to reduce drag. The shield automatically falls down to protect people behind the boat when the boat is at rest or backing up. We think this approach (flip up rear shields) holds great promise for reducing drag of conventional propeller guards. Prior to issuance of this patent, it had been several years since a conventional propeller guard U.S. patent had been issued that we are aware of.
  • 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. The patent is discussed in depth on our Volvo Penta Patents Virtual Propeller Guard System post.
  • U.S. Patent Application 20120095628 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 8,070,541 Underwater Protective Growth System to Inhibit Marine Growth. Invented by Michael Novack, IV. Issued 6 December 2011. A bag is temporarily attachable to a propeller mounted on a boat to prevent marine growth on the propeller. While this bag is not designed to be a propeller guard, it is in this family of patents so we listed it.
  • U.S. Patent Application US 2011/0009018 Boat Propeller Shield by Guy Taylor filed 9 June 2010 and published 13 January 2011. The patent covers the rear flap for his Navigator 3PO guard. While we are thrilled to see Mr. Taylor’s efforts with “flapping” rear screens we see some serious difficulties in getting this patent through the system in light of our previous art in this field. See “The Flapper” we posted 13 October 2006 on our <2011 Propeller Safety Technologies page. Please note we never update our own invention posts and they can be date verified on archive.org.
  • U.S. Patent 7,854,637 Multiple Venturi Nozzle System for Watercraft assigned to Marine Propulsion Technologies. Adjusting the conical inclination of their nozzle rings to direct the water passing over it into the area of the propeller increasing thrust pressure.
  • U.S. Patent Application US 2010/0291816 Breakaway Safety System assigned to Volvo Penta published 18 November 2010. Electronic detection system explosively ejects the propeller when it is approaching hazards under certain conditions. The invention appears to focus on protecting the boat, drive, and propeller from groundings and striking submerged objects, but could possibly be of use in protecting humans, or at least be of interest to those exploring the use of similar systems for protecting humans.
  • U.S. Patent Application US 2010/0167605 Multiple Venturi Nozzle System for Watercraft invented by Wilderich Schultz and Terry Smith (Marine Propulsion Technologies guard) published 1 July 2010. Patent application covers conical inclination of one or more of their nozzle rings to create the Venturi Effect and increase thrust pressure.
  • U.S. Patent 7,722,418 Energy Dissipation Valves for Hydraulic Cylinders. Issued 25 May 2010. Assigned to Teleflex. Trim Cylinder relief valve system for log strikes that appears like it might also be able to reduce blunt trauma impact to people or manatees struck by guards. They use very stiff springs with low preloads to allow the balls to modulate and control internal pressures during low energy impacts. (They try to accomplish what Mercury Marine tried with U.S. Patent 3,999,502 and 4,050,359, and they do it with fewer parts in a neater package for less cost.)
  • U.S. Patent 7,690,320 Enclosure for a Marine Drive Unit by Christopher Tool. Issued 6 April 2010. Frame like structure at rear of boat trailer that swings up and encircles the propeller when the boat is on the trailer. It protects the drive unit, reduces the potential for theft, and protects people from the propeller when the boat is on the trailer.
  • U.S. Patent Application 20100111703 Safety Propeller by Colin Chamberlain of Australia. Published 6 May 2010. The Australian Environmental Safety Propeller that won their 2009 invention of the year contest. Looks like a normal prop with blunted leading edges.
  • U.S. Patent 7,631,610 Variable Area Trim Tab and Added Propeller Guard Including Tunnel Propellers. Issued 15 December 2009. A sliding trim tab provides a variable lift area, especially for tunnel boats while also serving as a propeller guard (inlet screen to the prop the patent specifically mentions as providing safety for marine life and swimmers).
  • China Patent CN201350968 No Title. Published 25 November 2009. Looks like a conventional cage type guard in the sketch available at esp@cnet.
  • U.S. Patent 7,553,205 Electromechanical Spring Powered Actuator. Issued 30 June 2009. Invented by Anthony Viggiano and Martin LoSchiavo. Autotether with spring powered actuator.
  • 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 7,497,181 Sailboat Safety System for a Person Falling Overboard. Issued 3 March 2009. Invented by Anthony Viggiano. Autotether’s sailboat version.
  • U.S. Patent 7,448,925 Safety Shut-Off Device for Vehicle Having a Rotary On-Off Switch. Issued 11 November 2008. Invented by Anthony Viggiano. Autotether’s rotating switch version.
  • 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 trigers on a virtual propeller guard.
  • U.S. Patent 7,445,532 Safe Efficient Outboard Motor Assembly. Issued 4 November 2008 to Innerspace Corporation. Another Innerspace pump jet patent.
  • U.S. Patent Application US 2008/0265095 Energy Asorbing Impact Band and Method. Published 30 Oct. 2008 and assigned to Boeing. Described a “bird band” that can be placed on the leading edges of aircraft surfaces to absorb energy from impacted birds. It also helps deflect them once struck. This same concept might be applied to cage and ring type propeller guards to reduce blunt trauma.
  • U.S. Design Patent D579,400 Propeller Guard. Issued 28 Oct 2008 to Enviroprop. Another Enviroprop patent.
  • U.S. Patent Application 2008/0220670 Propeller Shield Apparatus for Trolling Motor. Published 11 Sept 2008. Sort of a bent “whale tail” above the prop. The inventor says it will improve manueverability and stability. It does not appear to be targeted at protecting people.
  • U.S. Patent Application 20080174455 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. PGIC comment – I found “marine mammals” and “heat emitting object” an interesting selection of terms. It will be interesting to see what might actually be granted from their application as much of it appears to be common knowledge. We do wish them well with their continuing efforts in this area and offer our assistance if they want any help. I will try to contact them and offer our help again shortly.
  • 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 7,371,140 Protective Marine Vessel and Drive issued 13 May 2008 to Brunswick. Another followup patent on their 7,234,983 patent for a twin thru hull drive partially recessed up into the boat.
  • U.S. Patent 7,371,139 Nozzle Driven Propulsion for Marine Craft issued 13 May 2008. Invented by Jose Abella. Variation of Kort Nozzle for higher speed vessels.
  • U.S. Patent Application 20080096447 Stern Drive Motor or Outboard Motor That Can Rotate 360 Degrees and Still Go Vertical or Horizontal or Both at the Same Time at Any Given Time published 24 April 2008. Invented by Douglas De Masi. This patent application written by the inventor or others unskilled in the art of writing patent applications includes the use of sensors to detect people, marine life, and obstacles near the drive. His “protective automatic safety system” could also be retrofit to existing drives. The sensors will not be “fooled by the boats motor, warmth of the body heat, sunlight, darkness of the clouds weather, water movement, and any thing else”. Just before the drive or propeller strike something, the system stops the propeller and raises the drive from the water. While several of his ideas are nice, they are little more of a “wish list.” Mr De Masi provides no details on how several “grandiose” concepts mentioned in his patent application could actually be accomplished. We do not mean to dimminish his efforts, and wish him well. Others reading his application may well be inspired to follow up on the “wish list”, figure out how to accomplish some of them and bring them to reality.
  • U.S. Patent 7,357,687 Marine Propulsion Steering System issued 15 Apr 2008 to Navatek. Steerable nozzle surrounding a propeller. The patent says the design helps prevent striking underwater objects such as groundings.

  • Colin Chamberlain’s Safety Propeller that won the ABC TV Australia New Inventors Show in November 2009

  • World Patent WO2008040049 Safety Propeller (Australian Patent AU2006233263(A1) published 10 April 2008. Some areas of the leading edge of the blade are thickened to provide less sharp of an edge.

  • U.S. Patent 7,355,518 Cordless Lanyard System Using E-Field issued 8 April 2008 to Brunwswick, invented by Richard Staerzl. Detects the presence or absence of the operator within the defined operating zone near the helm by detecting changes in an electric field. The patent includes a nice review of other patents in this area, as well as some of the relevant literature.
  • U.S. Patent Application 2008/0073008 Propeller Guard issued 27 March 2008. Strap on guard for AVIATION propellers when not in use.
  • Japan Patent JP2008062844 Propeller Guard, and Ship and Outboard Engine Equipped With Same. Published 21 March 2008. Assigned to Yamaha Motor. Uses several rods to prevent “twining” of foreign debris and still achieve a low drag resistance. The bars extend rearward, radially from the stern tube (torpedo) of an outboard or stern drive.
  • U.S. Patent 7,335,071 Electronic Shut Off Systems issued 26 Feb 2008. Invented by Marvin Motsenbocker and assigned to Maruta Electric Boatworks. Rapid shut off device for use as an electronic propeller guard. This patent is related to other patents by the same inventor.
  • A Series of four U.S. Design Patents D562,211, D562,212, D562,213, D562,214 Universal Propeller Guard design patents issued 19 Feb 2008 and assigned to Enviroprop.
  • Two U.S. Design Patents D562,209 and D562,210 Propeller Guard Bumper Rail design patents issued 19 Feb 2008 and assigned to Enviroprop. Appears to be a spring loaded bumper that provides either some sort of impact cushion to the guard (improves guard survival if it strikes something) or some sort of impact cushion to the person, marine life or object being struck or both. Since it is a design patent, they are unable to elaborate on its use or application, it only covers what it looks like.
  • U.S. Patent Application 2008/0020658 Outboard Motor With Reverse Shift published 24 Jan 2008. Small shallow running outboard that includes a deflecting plate propeller guard to protect the propeller.
  • U.S. Patent 7,291,047 Protective Casing for Water Craft Propellers issued 6 Nov 2007 invented by Roy Jones. Case/shield that is manually installed around the propeller when the prop is not in use to prevent damage to the propeller and/or injuries to people. Is for use in the water, in transit (like on a trailer), and when the boat is stored.
  • U.S. Patent Application 2007/0249242 Safe Efficient Outboard Motor Assembly published 25 Oct 2007 assigned to Innerspace Corporation. An outboard pumpjet. See Patent U.S. Patent 7,264,519 for a patented device from the same group.
  • U.S. Design Patent D553,551 Propeller Screen Guard assigned to Enviroprop issued 23 Oct 2007. A another volcano shaped screen guard from Enviroprop. It appears to have a hole where the lava would come out.
  • U.S. Patent Application 2007/0224892 Protective Marine Vessel and Drive published 27 Sept 2007. This Brunswick application appears to be a “follow up” on their 7,234,983 patent for a twin thru hull drive partially recessed up into the boat. The background and summary section of the application fail to mention any reasons for why they want to do this (attempt to limit damage to props and people?) One the claims is for a breakaway skeg.
  • U.S. Patent 7,270,584 Inboard/Outboard Motor Protector With Underwater Hydrofoil issued 18 Sept 2007 to Mark Judd Mitchell. Device to protect motors from impact (debris deflector) as well as to provide a hydrofoil to improve efficiency of the motor, ride and speed.
  • U.S. Patent 7,267,589 System and Apparatus for Improving Safety and Thrust from Hydro-Drive Device issued 11 Sep 2007 to Enviroprop. Uses vanes that can change their shape due to thrust and can also change the exit diameter of the shroud due to thrust. Varying vane shape and exit diameter is done to improve performance.
  • U.S. Patent 7,264,519 Safe Efficient Outboard Motor Assembly issued 4 Sept 2007 assigned to Innerspace Corporation of Covina California. An impeller jet outboard.
  • U.S. Patent 7,247,063 issued 24 July 2007 by Marc Lemchen. Is a propeller hazard, Carbon Monoxide / Carbon Dioxide warning device that is activated by the turning of the propeller. It mentions Mercury’s Verado drive being so quiet, some are not sure it is turning, exposing them to risk of injury.
  • U.S. Patent 7,244,153 Propeller Cover. Issued 17 July 2007. Cover for trolling motor propeller when not in use.
  • U.S. Patent 7,237,503 Protective Platform for Boat issued 3 July 2007 to Stagecoach Adventures. Large “flip up” flap at rear of boat can “flip down” to cover drive for safer entry and exit from the water, then be flipped up and out of the way when boat is under way. The platform has a foldup swim ladder mounted to its underside that can be deployed when the flap is down. Another version has steps mounted to its underside or steps cut into it. It purpose is to reduce propeller injuries.
  • U.S. Patent 7,234,983 Protective Marine Vessel and Drive issued 26 June 2007 to Brunswick. Twin tunnel drive with raised drives in “protective positions relative to the keel”. Another Zeus drive patent, it focuses on protecting the drive.
  • U.S. Patent 7,247,063 Warning Device for Marine Engines For Small Craft. Marc S. Lemchen. Issued 24 July 2007. Sensors detect rotation of the propeller or CO/CO2 levels and generate an alarm when certain settings are reached. The summary indicates one of the intended uses is Mercury’s Verado drive which is relatively quiet. Some may not be aware it is actually running.
  • U.S. Patent 7,241,193 Variable Marine Jet Propulsion issued 10 July 2007 by Jeff Jordan (Intellijet Marine). Variable pitch pump jet increases efficiency and allows reverse pitch, eliminating the need for a “bucket”. This patent is listed here because the Intellijet is on several lists of propeller injury avoidance devices.
  • U.S. Patent Application 2007/0135000 Outboard Jet Drive Marine Propulsion System published 14 June 2007 and assigned to Sword Marine Technology. Another OPS / JetPac drive patent. Listed here because the USCG lists them as a Propeller Injury Protection Device.
  • U.S. Patent 7,228,810 Powerboat With Disappearing Tunnel issued 12 June 2007. Invented by Ralph Brown. Tunnel has flaps and drive can be raised or lowered to run as a tunnel or not. When in tunnel mode it provides additional protection to the propeller and marine life, when lowered it runs with less drag.
  • U.S. Patent 7,229,331 Shroud for Hydro Thrust Device issued 12 June 2007. Invented by George Norman and assigned to Envrioprop. Duct to improve thrust and safety of trolling motors.
  • U.S. Patent Application 2007/0128045 Boat Propeller Safety Cover published 7 June 2007, invented by Johnna McNerney. Soft, flexible pouch to act as a cover for the propeller on the drive when the boat is docked or anchored to help prevent injuries.
  • U.S. Patent 7,201,619 Safety Shut-Off for a Powered Vehicle issued 10 April 2007 to Autotether for a wireless lanyard that physically pulls out the “key”. Invented by Anthony Viggiano.
  • U.S. Patent Application 2007/0049137 Warning Device for Marine Engines for Small Craft issued 1 March 2007 for a warning device that indicates the propeller is turning. The inventor cites very quiet Verado drives and houseboats as specific possible applications. We find this similar to recent discussions surrounding hybrid cars that hit blind people because they cant hear them coming. Some groups are promoting adding some sound to the hybrids.
  • U.S. Patent D537,028 “Ultimate Propeller Guard” issued 20 Feb 2007, invented by George Norman (he is on some of the Enviroprop patents). Long flat sided duct with some small holes in it.
  • U.S. Patent 7,172,477 Safety Propeller issued 6 Feb 2007. Propeller with a “collar” around the outside diameter of the blades. Somewhat like RingProp, but the “ring” has slits in it to allow water through, while still protecting humans and animals. Claim #5 says the front and rear guard blades have a discrete angle and pitch such that an engaging object is immediately propelled away, therby preventing contact. Its a bit hard to understand how this happens while still moving the boat forward.
  • U.S. Design Patent D536,295 Propeller Screen Guard issued 6 Feb 2007. Invented by George Norman (he is on some of the Enviroprop patents). Guard is shaped somewhat like a volcano with the core left out of it (big hole down the middle).
  • U.S. Design Patent D536,296 Universal Propeller Guard issued 6 Feb 2007. Invented by George Norman (he is listed on some of the Enviroprop patents). Variant of D536,295.
  • U.S. Patent Application 2006 0284007 Propeller Guard For Aircraft published 21 Dec 2006. Represents a recent attempt at the somewhat similar problem in light sport aircraft.
  • U.S. Patent Application 2006 0258232 Safe Efficient Outboard Motor Assembly published 16 November 2006 invented by Calvin Gongwer. An outboard drive with an impeller to reduce propeller injuries without incurring some of the problems that can be created by conventional propeller guards in certain situations.

  • 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.PGIC comments:

    1. The voting process is similar to that used to combine infrared and motion detectors in burglar alarms (heat plus motion means somebody is really out there), except in this case, heat plus strong visible reflected light means nobody is out there.The patent does a nice job of citing some previous work in this area, plus some from related fields. It was good to see us (rbbi.com / PGIC) cited FOUR times in the citations. We noticed the patent was actually filed back on 23 October 2003 (it has been in the system for almost three years), hinting it probably did not go through the patent system very smoothly. We checked the PAIR system and noticed they started out with 38 claims and did have some problems getting them issued. Its still interesting how they can write a patent like this and never say the word “guard” (except for the times it was in the titles of our documents cited by them).
    2. We posted this statement about false signals on our site back in 2002 “Many types of sensors might be used to detect people in the water. Some of those will also generate false readings in some situations. For example, if you try to detect body heat, you might also detect a warm boat dock near your boat before you start your engine. If you try to detect motion, a swimmer might be motionless. But tying both sensors together (with possibly even more technologies) and making some decisions based on current parameters (maybe including boat speed, engine RPM, etc) detection could be improved and false signals reduced. Cognitive Sensors and Sensor Data Fusion are techniques being employed in other industries that might lead to solutions in ours.” Hmm, sound a bit similar?
    3. Glad to see Brunswick was working on it, but this method was old news 4 years ago (actually long before then). Hope we see some action in the marketplace and not just on paper soon.


  • U.S. Patent 7,083,482 Marine Engine Stopping Device invented by Keith Jackson and Harry Bouge, another MariTech Virtual Lifeline / CAST patent.
  • U.S. Patent Application 2006/0166571 Shroud for Hydro Thrust Device published 27 July 2006. Two semi-circular brackets couple around a trolling motor housing. The graphic looks like a stern drive.
  • U.S. Patent 7,081,028 Portable Control Device Uses as a Security and Safety Component of a Marine Propulsion System, assigned to Brunswick, issued 25 July 2006. Small portable device allows starting the engine without a key and detects if you move too far from the operators station (thrown overboard) and stops the engine. Somewhat similar to MariTech’s virtual lifeline.
  • U.S. Patent Application 2006/0137594 Protective Platform for a Boat published 29 June 2006 is a flip out and down platform that stows up on the boat above the drive. it deploys out over and beyond the drive and may include a fold down ladder or incorporate steps as well. Its purpose is to allow swimmers to enter and exit without getting quite as close to the propeller.
  • U.S. Patent Application Publication US 2006/0105643 published 18 May 2006 covers Maritech’s Virtual Lifeline / CAST system and also mentions their swim ladder switch. Inventors are listed as Harry Bouge and Keith Jackson.
  • U.S. Patent 7,011,559 Obstacle Guard for a Boat Motor describes a clamp on guard primarily intended for protecting the propeller. Its top member mounts above the anti-cavitation plate and the lower member “cradles” the skeg.
  • U.S. Patent 6,986,689 System and Apparatus for Improving Safety and Thrust From Hydro-Drive Device assigned to EnviroProp. A vaned shroud that goes around conventional outboard and stern drive propellers providing increased protection.
  • U.S. Patent 6,976,891 Underwater Sound and Bubbles for Powered Watercraft issued 20 Dec 2005 describes injecting air into the water from electric powered boats and sailboats to create noise (like power boats create noise) so swimmers and marine life can detect the presence of and direction of oncoming “previously quiet” boats.
  • U.S. Patent 6,971,933 Rear Light for Propeller to Facilitate Safe Trailer Transport of the Motor Boat issued 6 Dec 2005. Basically, a tail light is installed on the prop to improve its visibility when trailering. This device might reduce “parking lot” prop injuries of people walking by and cutting themselves on a prop in the dark.
  • U.S. Patent 6,969,287 Electronic Shutoff Systems issued 29 Nov 2005 to Marvin Motsebocker is a follow on to his U.S. Patent # 6,676,460 for an Electronic Propeller Guard. This patent focuses on the use of magnets for coupling or braking the propeller shaft. They also act as a variable transmission to provide electronic control of torque and gear reduction.
  • U.S. Patent Application US 2005/0245146 System and Apparatus for Improving Safety and Thrust from a Hydro-Drive Device published 3 Nov 2005 is a shroud type device for lower units. Although not specifically assigned to Envrioprop, it comes from them.
  • U.S. Patent Application US2005/0214126 Propeller Cover published 29 Sept 2005 is reuseable cover for trolling motor propellers. It covers the prop when the trolling motor is not in the water reducing damage to the prop, scratches on things near it, and injuries to those near the motor.
  • U.S. Patent 6,916,217 Boat Propeller Shield System issued 12 July 2005 to Omni Ventures Inc. is a zippered shield system to protect the propeller and people from it when it is NOT in the water.
  • U.S. Patent 6,896,565 Protective Cage for an Outboard Motor and Holding Device Thereof issued 24 May 2005 to a German inventor. An outboard cage.
  • U.S. Patent 6,821,167 Outboard Motor issued 23 Nov 2004 to Ishigaki (Japanese water jet company). Outboard motor with an impeller, sort of a cross between and outboard and a water jet. Same unit appears in the patent applications as US 2003/0104733.
  • U.S. Patent 6,817,911 Hydro-Max Motorboat Anti-Slippage Shroud issued 16 Nov 2004. Patent focuses on reducing prop slip, but also frequently points out safety advantages of the shroud.
  • U.S. Patent 6,783,411 Propeller Protector Slipper. A “slip on” protector for use when the prop is not being used. It can be used in or out of the water and in transit. It is not for the same purpose as conventional propeller guards. A very similar device by the same inventor is the basis of an earlier patent, U.S. Patent 6,609,938.
  • U.S. Patent 6,736,686 Motorboat Propeller Safety Enclosure issued 18 May 2004. Device is for protecting boat propeller when boat is in transit (like on a trailer) and increases its visibility.

  • Maruta Electric Boatworks Patents Electronic Propeller Guard

  • U.S. Patent # 6,676,460 issued 13 Jan 2004, is a system using sensors to detect swimmers, animals and other objects in the water near a propeller. The system slows or stops the propeller when they are detected. It references our work in the area of Virtual Propeller Guards.

  • Sweden Patent SE522039 Propeller Guard for Boat with Flag Rudder and Onboard Motor, Comprises Cage Manuoeuvered Using Two Arms. Issued 7 Jan 2004. Cage does not interfere with the rudder behind it.
  • 6,609,938 Propeller Protector Slipper issued 26 Aug 2003. Big boot to go over propeller when achored, drifting, docked, in storage, or on a trailer to prevent propeller injuries and protect the prop.
  • U.S. Patent Application 2003-114052 Thrust Enhancing Propeller Guard Assembly. Invented by William Schultz and Gregory Morrell. Published 19 June 2003. One of the Marine Propulsion Technologies (MPT) patents.
  • U.S. Patent 6,544,082 Manatee Protection Installation on a Boat issued 8 Apr 2003. System can be deployed or retracted beneath boat / propeller.
  • U.S. Patent 6,503,110 Lower Unit Guard for an Outboard Motor issued 7 Jan 2003. A deflector in front of and below the lower unit /skeg.

  • Infrared “Virtual Lanyard” Kill Switch

  • U.S. Patent # 6,450,845 Passive Occupant Sensing System for a Watercraft. By Dick Snyder, assigned to Brunswick on 17 Sept. 2002. An infrared system detects if the operator is in the boat driving seat or not (somewhat like a lanyard kill switch) and takes the appropriate actions if the boat is underway and the driver is gone (likely overboard).

  • LyfGard Propeller Deflector Patent

  • U.S. Patent # 6,416,371 issued to Donald C. Johnson of Georgia on July 9, 2002. The patent describes the use of rigid vanes along the bottom of the boat to deflect debris and people from direct impact with the propeller. It also discusses the use of deflectors in combination with sensors to detect people in the path of the boat and deactivate the propeller by shutting off the flow of fuel, disengaging the propeller or stopping the engine to prevent striking them. It results from an application filed August 9, 2000. The device / company is now online at LyfGard
  • We are a bit puzzled the patent examiner did not cite our prior art on this one. We developed and published the basic sensor idea several years before this patent application was filed. I realize it is only an element of the patent in combination with the deflectors, but still seems unusual it was not cited. I left a message for the primary examiner pertaining to this patent on 11 July 2002. I also mentioned the recent Brunswick patent (6,354,892), suggested he enter the words “virtual propeller guard” in the Google search engine to find this page, and asked why our work in this area, including the 1999 online presentation was not cited as prior art. He contacted me by phone 12 July, 2002. He said although he was listed as the primary examiner on this patent, most of the work was done by the junior examiner and he provided oversight. They are unable to comment on specific patents, but did offer some general reasons why our work might not be cited. He said examiners are supposed to make use of NPL (Non Patent Literature) as part of the process. Most of that literature today is in electronic format, including journals. Our material may not have been cited because the examiner felt its content was duplicated by the patents cited, duplicated in other art cited, or not relevant to this specific patent. I asked him if he would please forward the info concerning our site to the junior examiner or others working in this area to make sure they were aware of its existence in the future. He said he does forward items he thinks may be useful. I thanked him for his help.

    It is also odd the deflection method is described in the patent, but no details are provided for the sensor system.



    Brunswick patents safety device for vessel

  • 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. We placed the basic concept of using infrared / infrared and other sensors as alternatives to conventional propeller guards in the public domain in our 1999 Virtual Propeller Guard Presentation and our even earlier in our 1997 Stealth Prop Guard concept to encourage development of Virtual Propeller Guards. We are very happy to see a major player creating intellectual property in this field.
  • Several drive manufacturers have avoided prop guard research, fearing the introduction of a prop guard, would indicate their existing open prop products were unsafe. Brunswick’s patent indicates use of their concept on a bow thruster in the abstract drawing, probably a carry over of those fears. The claims are carefully worded to avoid use of the word propeller (it looks for heat signals from near the surface of the water in the proximity of the propulsion device). In fact the words “propeller” and “guard” do NOT appear anywhere in the patent.

    At this stage, Virtual Propeller Guards are most effective when the vessel is at rest. When underway, things happen too fast for them to be effective, unless they are coupled with forward looking sensors in the bow. Even at low speeds, the drive leg itself can cause deadly injuries to those in the water. A large portion of propeller injuries occur when the boat is a rest or maneuvering. The patent makes no mention of these considerations, probably because they chose to leave out the “P” word.

    All that aside, we are very happy to see Brunswick activity in this area and hope others will soon follow their lead if they have not already done so. We look forward to seeing Virtual Propeller Guards on the market, in use and saving lives.

    We also noticed the patent was issued in less than a year (filed March 14, 2001, issued March 12, 2002). The U.S. Patent & Trademark Office (USPTO) says it is very rare for applications to go through that fast. It would have to be a “perfect application” which they almost never receive of move through on a “Petition to Make Special”. “Petition to Make Special” can be requested in some circumstances. Brunswick may have been able to have met one of those requirements. If they went forward, even partially, based on a prior art search, seems odd they did not cite our presentation posted in January 1999.

    The patent might be useful as a bargaining chip in court cases to show they are working on the problem.


More Recent Propeller Guard Patents and Propeller Safety Patents

Note- Many cage type patents are in U.S. Patent Class 416 subclass 247R

  • U.S. Patent Application 2004/0126242 Boat Propeller Guard Device published 1 July 2004 uses flexible guard members attached directly to the sharp edges of propellers.
  • GB2375747 Published 27 Nov 2002. This patent from Great Britain describes an outboard prop guard that can be made from a single piece of metal or a casting. It attaches to the skeg and cavitation plate.
  • U.S. Patent 6,478,640 Propeller Safety Guard for Outboard and Stern Drive Motors issued 12 Nov 2002 is a screen/cage type guard.
  • U.S. Patent 6,475,045 Thrust Enhancing Propeller Guard Assembly issued 5 Nov 2002 utilizes a series of rings. Invented by William Schultz and Gregory Morrell, assigned to Gregory Morrell (PropGuard).

  • U.S. Patent 6,416,371 Propeller Deflector issued 9 Jul 2002. Inclined vanes on bottom of boat hull deflect objects away from the propeller. System also includes a sensor to detect objects in water near path of boat and is designed to prevent loss of human and animal life in the propeller. Another early virtual propeller guard type patent.
  • U.S. Patent Application 2002 0052159 Life-Saving System invented by Eguchi published 2 May 2002. Person wears tag around their neck. If they go overboard, the boat returns to near that spot, alerts selected people, lowers a life belt and rope ladder.

  • U.S. Patent 6,317,050 Water Entry Alarm System issued 13 Nov 2001 assigned to Pool Alarm Corporation. Harness worn by person includes a water sensor that is automatically switched off when the harness is removed.
  • U.S. Patent 6,302,050 issued to an independent inventor on October 16, 2001 is an example of a device that folds down over a drive for use when a boat is at rest.

  • U.S. Patent 6,276,974 issued to Propguard Inc. (now MariTech) on 21 August 2001 for a swim ladder interlock switch to help reduce swimmer propeller injuries.

  • 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.
  • New Zealand Patent 337434 Encircling Propeller Guard With Straight and Beveled Portions. Published 25 May 2001.
  • 6,190,220 Protective Cover and Mehtod for Protecting a Boat Propeller Shaft issued 20 Feb 2001. Cone shaped cover to put over propeller shaft when prop is removed to protect the shaft and allow more frequent removal of the prop. Cites having the prop off makes the boat safer to those around it when the boat is out of the water, as well as the prop can be stored safety and prevent damage to it.
  • U.S. Patent 6,179,673 Outboard Motor Protection Apparatus issued 30 Jun 2001. A vertical leg attached to the transom in front of an outboard motor acts as a deflector and hits objects before the drive.
  • U.S. Patent 6,168,483 Deflecting Propeller Guard issued 2 Jan 2001 uses a deflector in front of and below the propeller.
  • U.S. Patent 6,162,104 Hydofoil Propeller Guard issued 19 Dec 2000. Duct type guard with large foil that goes over anticavitation plate.
  • U.S. Patent 6,159,062 High Performance Propeller Guard issued 12 Dec 2002, invented by Guy Taylor, Jr. Uses a ring with several holes, slots and ports in it. See U.S. Patent 5,098,321 for an earlier version of this unit and New Zealand Patent NZ242046.
  • U.S. Patent 6,157,303 Water Safety Portable Transmitter and Receiver issued 5 Dec 2000 to Terrapin Communications of Canada. Transmitter to be worn by children near a swimming poolto indicate immersion in the water. (This “tag” technology was later teamed with MariTech).
  • U.S. Patent Application 2001/0036776 Propeller Safety Guard for Outboard and Stern Drive Motors. Published 1 Nov. 2001 to Gerard P. Raps of Surrey Canada ( Adventure Marine ).
  • 6,152,064 Protective Cover for a Propeller issued 28 Nov 2000. Buoyant protective sleeve for use when anchored, at rest or during transport to protect swimmers and others from injury. It must be secured in place for use, then removed before getting underway.
  • U.S. Patents 6,102,661 and 5,405,243 – RingProp and its predecessors received these Patents for their ringed propellers that claim to reduce propeller injuries. Their propeller entered the U.S. market in 2004.
  • Ireland Patent IE20000233 Publication date: 2001-10-03 Propeller Guard or Propeller Ring. – “Crab” design which is “Crab Propeller Ring”. Based on the principles of crab legs and mandibles. Similar to GB2375747.
  • Australian Patent AU 200053620 A1 published 2001.03.01 describes a cage type guard.
  • U.S. Patent 5,975,969 assigned to Cal-Neva Marine Company 2 Nov. 1999. This unit has been on the market the last several years as the “Hydro-Foil Propeller Guard”. It is a ring type unit that attaches to the anti-cavitation plate and also expands the “wings” of the plate.
  • U.S. Patent 5,964,626 Tractor Pump Jet issued 12 Oct 1999 to OMC. Followup patent to 5,846,103. Forward facing pump jet eliminates contact with prop.
  • U.S. Design Patent D424,503 Propeller Guard Having Maneuvering Vanes. Issued 9 May 2000.
  • U.S. Patent 5,928,042 Propeller Guard looks like the later 6,159,062 patent.
  • U.S. Patent 5,904,603 Lockable Propeller Guard issued 18 May 1999. Lockable cover for drive propeller when drive is not in use to protect swimmers, manatees and other mammals.
  • 5,890,937 Propeller Shroud issued 6 Apr. 1999. Vertical members along side the propeller slop from front to back to protect the prop and swimmers.
  • 5,846,103 Tractor Pump Jet issued 8 Dec 1998 to OMC. Forward facing pump jet protects swimmers, divers, water skiers from contacting a prop.
  • U.S. Patent 5,759,075 Propeller Guard PropGuard, Inc.’s cage type patent.
  • 5,664,975 Reflective Propeller Safety Cover issued 28 Sept 1994. Reflective bag cover for props on drives when boat is out of the water. Primary purpose is to prevent rear end collisions, but does provide a level of protection to people.
  • 5,660,136 Drive Unit Safety Boot issued 22 Aug 1997. Collapsible drive cover to prevent injury to swimmers and others in the area. Must be installed when boat is at rest, and removed before starting the drive.
  • U.S. Patent 5,501,622 Marine Mammal Guard issued 26 mar 1996. Special protective housing for outboard motor lower unit to prevent animals and other objects from coming into contact with the propeller.
  • 5,494,465 Propeller Protector issued 27 Feb 1996. Shield that mounts on outboards when not in use to protect people from the propeller, especially targets small boats that are frequently beached.
  • U.S. Patent 5,470,262 Propeller Enclosure assigned to Bustillo Investment Corp of Puerto Rico on 28 Nov. 1995. A fence around the lower unit combines with a ring around the propeller.
  • The recent Johnson patent, 6,416,371, cites two early day versions of Virtual Propeller Guards we had not previously identified:
    • U.S. Patent 2,124,497 issued July 19,1938. This patent teaches the use of sensors in front of the propeller to warn of underwater obstacles and floating debris. Their output is used to disconnect the power plant from the propeller. Its primary purpose is to protect the propeller from damage.
    • 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 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 also discovered an early sensor guard product for use on electric fans

    • 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. Some of the “thinking” and wording may be useful to Virtual Propeller Guard inventors.
    • We also recently discovered a very old “Virtual Propeller Guard” type patent.

    • U.S. Patent 2,124,497 Safety Device for Marine Power Plants drops a mechanical sensor (sort of a very crude depth finder) from the bow of the boat to detect shallow rocks and debris that might impact the drive and decelerates the drive before impact to reduce propeller damage.
  • New Zealand Patent NZ242046 Propeller Guard Ring For Boat Outboard Motor; Ports With Inclined Axes With Respect to Propeller Axis. Invented by Guy Taylor, Jr. of the U.S. (and later of Three-P-O Navigator Ring)
  • 5,389,021 Motoboat Propeller Safety Shroud issued 14 Feb 1995. Shroud surrounds the prop. Includes some “relative dimensional values” that may be of interest to some.
  • D344,456 Boat Propeller Cover issued 22 Feb 1994. Case to carry lock around propellers.
  • 5,344,346 Driveshaft Housing Attachment issued 6 Sept 1994. Quickly installed and removed, it provides some protection for the prop and swimmers.
  • 5,246,345 Boat Propeller Cover issued 21 Sept 1993. Lockable cover to prevent theft and reduce injuries.
  • U.S. Patent 5,238,432 Marine Drive Unit Impact Avoidance System issued 24 Aug 1993. Sort of a mechanical Virtual Propeller Guard. A brace in front of the drive strikes the obstruction causing the drive to swing up and avoid impact. Its purpose is to protect the drive.
  • Japan Patent # JP5310188 (Japan) Safety Device for Small Ship. A “net type” prop guard swings out of the way at high speeds, but comes back over the prop at low speeds, specifically to protect divers. This design tries to have the best of both worlds (protection when needed the most and no performance degradation when underway at speed.).


Several older propeller guard patents are listed below for historical reference.

  • U.S. Patent 5,237,133 Safety Ignition Switch issued 17 Aug 1993 – a lanyard type boat ignition switch (kills engine if operator falls overboard).
  • U.S. Patent 5,224,889 Propeller Guard issued 6 Jul 1993. A foot / skid mounts to lower unit below the propeller to protect it from underwater obstacles.
  • U.S. Patent 5,213,525 Jacketed Marine Propeller issued 25 May 1993. Venturi type jacket around propeller helps “reduce the possibility of injury to swimmers, divers, and water skiers.”
  • U.S. Patent 5,207,605 Outboard Propeller Guard issued 4 May 1993. Plastic “wing like” guard attaches to sides of the motor housing or skeg.
  • U.S. Patent 5,205,766 Propeller Guard invented by Arsenault, a Canadian, issued 27 Apr 1993. A swept back, cage type unit.
  • U.S. Patent 5,190,148 Marine Key Switch issued 2 Mar 1993 – a lanyard type boat ignition switch (kills engine if operator falls overboard).
  • Australian Patent AU4082389 Propeller Guard published 1 March 1990 invented by Mason Wayne Jeffrey.
  • U.S. Patent 5,178,565 Lower Unit Guard issued 12 Jan 1993. Attaches to lower unit to prevent direct impact with submerged hazards. It descends from the cavitation plate to below the skeg.
  • U.S. Patent 5,176,550 Engine Propeller Blade and Lower Unit Protector issued 5 Jan 1993. Cage type primarily for protecting prop and drive. Now known as Prop Buddy built by Robert Hooper.
  • U.S. Patent 5,145,428 Shrouded Propeller System for a Sailboat issued 8 Sept 1992. Kort nozzle applied to a standard outboard to make it applicable for sailboat use (nonplaning type hulls). The nozzle creates a propeller guard.
  • U.S. Patent 5,105,755 Pontoon Gate With Safety Switch. Issued to Mastercrafters Corporation (a Genmar Company) on 21 April 1992. A pontoon boat gate with a safety interlock switch to prevent the engine from operating if the gate is open. It was a forerunner to the MariTech swim ladder interlock system and also included an override in case the system failed. It was an early virtual propeller guard (anticipated people in the water near the prop).
  • U.S. Patent 5,098,321 High Performance Boat Prop Guard with High Strength Attachment Bracket issued 24 Mar 1992. Invented by Guy Taylor, Jr. See U.S. Patent 6,159,062.
  • U.S. Design Patent 322,593 Boat Propeller Shield issued 24 Dec. 1991. Duct type guard.
  • U.S. Design Patent D322,256 Propeller Guard for Boat Motor issued 10 Dec 1991. A cage type guard shown on an outboard.
  • U.S. Design Patent D322,074 Outboard Propeller Guard issued 3 Dec 1991. Solid framework that bolts on the anti-cavitation plate and the skeg and wraps around the propeller.
  • 5,066,254 Boat Propeller Guard. 19 Nov. 1991. Cage type with no rear screen.
  • 5,044,884 Safety Propeller issued 5 Sept 1989 to Trustees of the University of Pennsylvania. “Ringed” propeller somewhat similar to RingProp, but with a crushable, hollow member upstream of the blades to provide energy absorption in event of a striking a person or animal. They specifically mention absorbing energy to protect a human skull at impacts to 30-35 mph. This is the first patent we recall covering a prop guard designed to cushion human impact and to reduce blunt trauma at moderate speeds..
  • 5,009,620 Propeller Guard. 1991. Louis Feranda, Sr.
  • Soviet Union Patent SU1643326 Propeller Guard issued 23 Apr 1991.
  • 5,007,869 Prop-Saver (Propeller Guard Device) issued 16 Apr. 1991. A “fin” attaches to the lower unit in below the propeller to protect the propeller from underwater obstacles.
  • 4,957,459 Propeller Shroud with Load Bearing Structure. 1990. Cage type design by Dick Snyder at Brunswick.
  • 4,826,461 Propeller Protector. 1990. Brunswick.
  • 4,718,871 Aquatic Growth Cutter issued 12 Jan 1988. Vertical blades in front of the gearcase cut kelp and other seaweed that could foul the prop. Can include a mesh screen. Is almost an anti-prop guard as far as personal safety goes. It could cut you before you hit the prop.
  • 4,680,017 Motorboat Propeller Guard for Improved Performance issued 14 Jul 1987. Ring type guard with screen over the end.
  • 4,637,801 Thrust Enhancing Propeller Duct Assembly for Water Craft issued 20 Jan 1987. Ring type device “serves as a safety factor for swimmers.” Invented by William Schultz. Early version of Prop Guard (from Prop Guard Marine) and Thrustor (from Marine Propulsion Technologies)
  • 4,565,533 Boat Propeller Guard. 1986. Manford H. Springer.(Canada)
  • Japanese Patent JP61105295 Vessel Equipped With Propeller Guard Apparatus assigned to Yanmar Diesel Engine Co. Retractable guard is used to prevent “clinging of nets and ropes in a fixed-shore-net water area”, then retracted to allow higher speeds in open water. This is one of very few propeller guard patents assigned to a boating company.
  • 4,305,143 Automatic Man Overboard Sensor and Rescue System. 8 Dec 1981. Boater wears a sensor that activates when they fall in. It is “heard” by sensors on the boat and an alarm is sounded.
  • 4,304,558 Marine Propulsion Device Including Propeller Shroud. OMC.
  • 4,411,631 Propeller Guard. Eero Makinen, Espoo and Stefan Gordin. (Finland)
  • 4,235,183 Marine Propulsion Device Including Propeller Protection Means issued 25 Nov 1980. Invented by Ralph S. Evinrude and assigned to OMC. Flat fins extend horizontally from both sides of the gearcase in front of the propeller.
  • U.S. Design Patent D257,353 Outboard Motor Protector issued 14 Oct 1980. A very open structure slides over the lower gear case and provides some protection for it.
  • 4,106,425 Marine Propulsion Unit with Protected Screw. 1978. Ludwig Gruber, Freilasing. (Germany).
  • 4,096,819 Marine Propulsion Device Including Propeller Protection Means issued to Ralph S. Evinrude 27 June 1978, assigned to OMC. Lower unit has fins protruding horizontally from propeller area resembling the anticavitation plate above the propeller.
  • 4,078,516 Propeller Guard issued 14 Mar 1978. Cage type by Donald G. Balius – one of the first prop guards in the USPTO fully searchable group of U.S. Patents.
  • 4,070,984 Outboard Motor and Weed Guard Therefor issued 31 Jan 1978. Cage type design.
  • 4,057,028 Outboard Motor Propeller Guard issued 8 Nov 1977. Guard consists of a cable running below the propeller, then upwards behind the prop. If the cable strikes something, it shifts the drive to neutral.
  • 4,013,033 Weed Guard for Trolling Motors issued 22 Mar 1977. Resembles cage type guards.
  • 3,965,845 Boat Propeller Protective Structure issued 29 Jun 1976. Structural pieces for attachment to inboard propeller shaft support structure to provide additional protection for the propeller from debris.
  • 3,889,624 Retractable Propeller Guard for Outboard Motor or Stern Drive Unit. 1975. Donald G. Balius. This is an early version of the Flapper guard concept we proposed much later.
  • Australian Patent AU5825773 Propeller Guard published 23 Jan 1975.
  • 3,805,723 Safety Cut-Off for Propellers – stops Hazelton propellers on impact in submersible / diver applications.
  • 3,802,377 Lower Unit Weed Guard issued 9 Apr. 1974. A swept back cage type design targeting trolling motor applications.
  • 3,774,720 Power-Operted Retractable Ladder for Pleasure Boats. Invented by Cecil Carton Hovey of Florida. Issued 27 Nov. 1973. This power operated extendable and retractable boat ladder included a interlock to prevent the ignition safety switch from being closed when the ladder was extended. It was a forerunner to the MariTech swim ladder interlock system, and one of the earlier virtual propeller guards (anticipated people in the water near the prop).
  • 3,658,028 Tubular Thrust Propeller issued 25 Apr 1972. Large hollow tube with a nose cone surrounds the prop.
  • 3,433,195 Hydrofoil Device for Outboard Motors issued 18 Mar 1969. “Bolt on” foil attaches to gearcase creating horizontal “wings” in front of the propeller. Also serves “as a propeller guard.”
  • 3,150,631 Demountable Stern Drive Assembly issued 29 Sept 1964. Tractor type stern drive (forward facing prop) has a “cushioned” propeller guard in front of it.
  • 3,149,605 Outboard Propulsion Unit Steering Assist Apparatus issued 22 Sep 1964. Large duct around prop reduces steering effort and prevents debris impact.
  • 3,099,240 Boat Construction issued 30 Jul 1963. Multiple outboard drives are used with ducts designed to raise the boat further from the water when underway at speed. Those ducts also serve as propeller guards.
  • 3,075,491 Propeller Guard for Outboard Motors issued 29 Jan 1983 to Frank Hogan. A series of tines in a ring around the propeller that appears to be slightly open at the inlet and rear in an effort to reduce impact on flow to and from the propeller.
  • 3,057.320 Boat Transom Propulsion Unit issued 9 Oct 1962. Inboard engine pivots through the transom (like has been suggested with todays outboards mounted as an inboard) with a guard around the prop.
  • 3,035,538 Propeller Guards issued 12 May 1962. Non metalic cage that sweeps to a point in front of the gearcase. One of the early prop guard patents to mention protecting swimmers and skin divers from coming in contact with the propeller.
  • 3,027,864 Propeller Guard issued 3 Apr 1962 to Virgil L. Polson. Bent tines (somewhat like a pitchfork) sweep back from the front of the drive to protect the propeller and avoid “injury to surfboard and water-ski enthusiasts.” Although I share the same last name with Mr. Polson, I am not sure who he is. It looks like he lives in Napa California and was born in Illinois in 1907. We will research this further in the future. I suspect he is a descendant of or kin to Virgil Lansden Polson b. 8 Oct 1860 Marion Co. IA and lived in Fredonia, Wilson County KS.
  • 3,025,825 Weed, etc. Guard for Outboard Motors issued 20 Mar 1962. Looks like a spring type garden rake suspended in front of the lower unit to absorb some of the shock of impact and cause the unit to “ride up” over obstacles, while serving as a weed guard.
  • 2,985,133 Propeller Guard issued 23 May 1961. For inboard type drives, its structure extends below the shaft, reaches back, then rises to a circular guard around the prop.
  • 2,983,246 Propeller Guard for Outboard Motor Boat issued to Audre Manley 9 May 1961. A light weight non-metalic prop guard cage. One of the early prop guard patents to mention protecting swimmers. The unit actually looks quite modern.
  • 2,965,065 Hydraulic Jet Propulsion Units for Boats issued 20 Dec 1960. Unit looks like an outboard, but has a water jet on it instead of a propeller. Its impeller is enclosed. This patent was included as an example of an early water jet – outboard hybrid.
  • 2,963,000 Propeller Guard and Rudder Device issued 6 Dec. 1960. One of the early prop guard patents to mention protecting people. Includes the phrases – “prevents the human body from coming into contact … preventing cutting and mutilation of persons.
  • U.S. Design Patent 187,407 Transon Drive Unit for a Boat. This patent from England covers a unique drive mounting to a circular plate receiver on the transom with a large duct around the propeller. The duct appears to also be a propeller guard.
  • 2,916,010 Fishing Line Propeller Guard issued 8 Dec. 1959. An open cage with no rear cover.
  • 2,894,477 Propeller Guard and Deflector for Outboard Motors issued 14 July 1959. Futuristic, stylish looking wings extend out along the side of the prop and from the bottom of the gearcase. Looks very modernistic.
  • 2,775,949 Torpedo Propeller Guard Lock issued 1 Jan 1957. Temporary removeable cap that pins on over props on torpedoes to prevent damage to them and to prevent injury to those handling them.
  • 2,717,570 Skid Attachment for an Outboard Motor issued 13 Sept 1955. A foot/skid mounts to lower unit below the prop to protect it from underwater obstacles.

  • 2,706,960 Outboard Motor Guard issued 26 April 1955 to T.F. Crew. This is one of the earliest cage type guards to specifically focus on protecting people. The first sentence of the patent includes: “to prevent the loss or injury of the hands, arms, legs or other parts of the body of those who may accidentally come in contact therewith during the operation of the motor.” Interestingly, the patent cites several live bait boxes (meshed boxes for carrying small fish with swing up and open lids / minnow buckets) as prior art.

  • 2,706,461 Combined Outboard Motor Propeller Guard and Runner issued 19 Apr. 1955. A “ski” is mounted below the drive to protect the prop.
  • 2,559,179 Propeller Guard for Outboard Motors issued 3 July 1951. Several vertical bands in front of the drive protect the propeller.
  • 2,551,371 Thrust Stabilizer for Outboard Motors issued 1 May 1951. Large duct with a nozzle surrounds the propeller to reduce waves produced by the prop.
  • 2,470,874 Propeller Guard for Outboard Motors issued 24 May 1949. A weed guard type unit that extends down in front of the drive with two tongs that reach out on each side of the propeller.
  • 2,468,890 Propeller Guard and Deflector for Outboard Motors issued 3 May 1949. Deflector type guard.
  • 2,444,217 Weedless Propeller Guard
  • 2,364,426 Boat With Means of Enclosing the Propeller When Not in Use issued 5 Dec 1944. Auxillary sailboat drive is covered when not in use or when boat is stored to protect the prop and reduce drag.
  • 2,355,842 Protector for Outboard Motors
  • 2,319,640 Propeller Guard
  • 2,244,217 Weedless Propeller Guard issued 3 Jun 1941. A cage type screen.
  • 2,213,610 Boat Propulsion Apparatus issued 25 Apr 1938. Removable duct type guard on an outboard.
  • 2,140,099 Outboard Motor Propeller Guard issued 13 Dec. 1938. Tines extend around the sides of the prop.
  • 2,136,628 Propeller and Rudder Guard for Outboard Motors issued 15 Nov. 1938. A cage type screen.
  • 2,135,162 Propeller Guard awarded to J.R. Benson 1 Nov. 1938. Some references apparently incorrectly cite this cage guard patent as the first prop guard patent. This is probably due to it being the earliest patent listed in the “Review of the State of the Art of Swimmer Protection From Outboard Motor Propellers” by Robert Taggart (one of the earliest reviews of this field published on 16 Feb 1979).
  • 1,869,977 Propeller Guard issued 2 Aug 1932 to B.C. Modin. Attaches to the shaft support of an inboard drive right in front of the propeller. Is shaped somewhat like a star with the point being very thin all the way out from the center of the star.
  • 1,734,119 Spring Cushioned Drive for Propellers issued 5 Nov 1929 assigned to Ole Evinrude. A spring laterally mounted in front of the prop takes up the shock of impacts (prop winds up the spring) to prevent prop damage (sort of an early day elastomeric hub).
  • 1,713,446 Propeller Guard issued 14 may 1929. Designed for use with divers to prevent entangling diving hoses and lines in the prop. Does not specifically mention “people” safety. Is an early effort at two positions. It is moved to the “protective position” when divers are working and away from the prop when underway.
  • 1,620,129 Propeller Guard issued 8 Mar 1927. Rear portion can be raised or lowered into position when needed. Says it is to prevent diving lifelines and hoses from fouling in the prop. It does no mention protecting divers from the prop. Jay O’Sullivan told the OMC-Decker Jury this was the first outboard motor propeller guard. We are not disputing his comments, we are only pointing out the inventor was talking about preventing hoses and lifelines from fouling the prop, not protecting people. U.S. Patent 2,706,960 (listed above) is one of the earliest cage guard patents to actually describe protecting people.
  • 1,196,176 Spiral Propeller for Ships issued 29 Aug 1916. Long screw like propeller operating in a tube to protect it from damage.
  • 1,092,960 Boat-Propeller issued 14 Apr 1914. Ducted prop for work in shallow water that may strike bottom or run foul of mooring ropes.
  • 1,065,089 Speed Accelerator and Propeller Guard issued 17 June 1913. Duct type guard.
  • 1,028,333 Propeller issued 4 Jun 1912 to E.B. Desenberg and E.S. Pilsworth. A shaft drive through the transom has a guard extending out below the propeller “for use in water filled with water plants, logs, or other debris. A u-joint external to the boat has it looking somewhat like an Arneson drive.
  • 1,009,635 Propeller Guard issued 21 Nov 1911. Spokes stick out from the rear of the vessel to the outside diameter of the prop.
  • 983,587 Propeller Guard issued 7 Feb 1911. Attempts to deflect grass and vegitation away from the propeller to prevent clogging.
  • 951,608 Propeller Guard issued 8 Mar 1910. Ski like bars stick out below the prop.
  • 662,522 Propeller for Boats awarded to S.H. Butterworth 27 Nov. 1900. A canister full of holes surrounds the propeller especially adapted to canal boats to prevent the washing of banks as the boat passes (shore erosion). It also mentions protecting against “drift wood and the like.”
  • 19,656 Canal Boat Propeller issued 16 March 1858. Recessed propeller and guard for use in protecting the propeller in canals.
  • 5,224 Screw Propeller issued 7 Aug 1847. Long screw with about 3 full turns is enclosed in a tube to protect it from impacts.

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