Volvo Penta Patents Virtual Propeller Guard System

On June 5, 2012, Volvo Penta was issued U.S. Patent 8,195,381 for a Safety System for Marine Vessels. The patent focuses on detecting people in the water in conjunction with digital anchors. UPDATE note – on September 18, 2012 Volvo Penta was issued another patent, U.S. Patent 8,271,155 for technologies surrounding this same concept.

GPS anchors, also known as digital anchors, are computer systems that keep a boat or vessel in place on the surface of the water. They first gained popularity in drilling rigs and have since spread to recreational boats, and more specifically to larger, twin engine recreational boats. The boat operator can punch a button (or touch a touch screen) and the vessel will stay in place based on GPS signals.

In recreational craft and smaller working vessels, sometimes operate with only one person on board. That person might launch the digital anchor then leave the boat and get in the water to tend to the vessel, to tend to nets, to perform a dive, or for other reasons.

Volvo Penta Patent 8,195,381

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

If the digital anchor fails, the boat might leave the area stranding the boat operator. The digital anchor might decide to make a correction just as the boat operator is approaching the stern and they could be struck by the propeller.

To minimize these occurrences, Volvo Penta proposes detecting people in the water near the boat. If someone is near the boat the system knows the boat should not leave the area. If someone is in the water within the propeller danger zone, the motors can be stopped.

Volvo Penta uses infrared sensors to detect people in the water. The system uses a moving average difference system to help eliminate false signals. It also uses a variable threshold detection level based on solar radiation, wind speed at the boat, water temperature near the boat, and wave amplitude.

The moving averages themselves are also adjusted according to the solar radiation (sunlight), wind speed, wave amplitude, and temperature of the water in that “cell” / pixel / quadrant.

Infrared sources can be worn by the boat operator (as a headband or on a life jacket) to improve detection. These sources can be pulsed in a pattern known by the detection system to further reduce errors.

Volvo also specifies a range of infrared wavelengths to be sensed to reduce interference. A radiation wavelength of 10um to 800nm is recommended. More preferably in a wavelength range of 5um to 1um.

The patent description includes the mention of a proprietary CANbus system (of the nature of Mercury’s SmartCraft system).

One of their patent claims covers angularly stabilizing at least one of the sensors to compensate for the bouncing of the boat. This is a good idea we have not previously seen in efforts to detect people in the water from boats. While Volvo says the sensors could be stabilized with a gyro (gyroscope), we can envision them using some of the optical stabilization techniques now used in cameras to improve lining up the sequential images.

We noticed Volvo also filed a claim for the software associated with operating the safety system.

Congratulations to Volvo Penta for their efforts to further the field of virtual propeller guards (detecting people in the water and taking the appropriate actions). We look forward to further developments in this field from Volvo and hope to see similar developments from others.

The Volvo Penta Patent’s Path to Issue

The patent was filed on November 30, 2006. Taking close to 6 years to issue indicates it probably had some challenges going through the system. We checked the USPTO PAIR system to follow its path to issue. It was a continuation of a foreign filing and did not begin to make progress in the system until late April 2009. It looks like most of the lost time was just going through the system and not spent in fighting out the claims.

All the claims that were granted concerning detecting people in the water were in conjunction with a digital anchor (they do not apply to boats not using digital anchors).

The Volvo patent does cite the Staerzl patent by Brunswick / Mercury Marine 7,105,800. After the Volvo patent was filed, Volvo submitted it as an information disclosure.

PAIR does show the patent has one child (another patent is in process based off this patent). The continuity data identifies the child as 13/309,896 filed on December 2, 2011 amd having the same invention name as this patent (Safety System for Marine Vessels). PAIR says this patent (the one issued in June 2012) was a continuation of 12/447,659. The child still in the system has been sent a notice of allowance as of early June 2012, so it will probably be issued in the future.

It looks like the prosecution of the patent was relatively normal. Several of their claims were rejected as being unpatentable over Knight (U.S. 5,386,368) in view of Staerzl (U.S. 7,105,800) (digital anchors were known as were infrared detection systems). The USPTO claims it would have been obvious to combine the two inventions (digital anchor with infrared detection). Several dependent claims were rejected because they were based on an independent claim that was rejected. The whole process is somewhat normal. Volvo Penta rewrote the claims around the objections, and the patent was issued.

Volvo did have to significantly restrict their first independent claim. The original first claim was for a digital anchor with a sensor system to detect people in the water. The issued first claim specifies the sensor system is an infrared system and includes several specifications of that system. While it is common for the first independent claim to be restricted some by the patent examiner, this was a pretty tight restriction or narrowing of their original claim.

With Regards to Some of Our Work in this Area

Our site (Propeller Guard Information Center) is often cited as a reference in patents of this nature. We suspect it was not cited this time because it is already cited several times in the Staerzl patent.

Interestingly, the patent description mentions the possibility of using audio sensors to detect the operator in the water shouting. We first put this idea (detecting shouts to reduce false alarms) forward back on February 2, 2009 in our “System for Reducing False Alarms in Virtual Propeller Guard Systems Using Infrared Sensors” which can be seen on our Prop Technologies <2011 page. Volvo does NOT include that element (detecting shouts) in their patent claims. We previously promoted the use of neural networks to improve the detection of people in the water using similar sensors. Volvo's patent says:

“Computation of a moving average is a most reliable approach to detecting a presence of the one or more persons in the water in comparison to neural network-type detection or template comparison detection which are also within the scope of the present invention.”

Volvo says calculating moving averages of quadrant values (or moving averages of differences of quadrant values) is a better approach than using neural networks. While we agree, moving averages is a technique that holds promise, we see moving averages as being within the confines of those that would be explored by neural networks. Interestingly, while Volvo says moving averages is better,

Volvo then says, while its moving average detection method is better than neural networks, their method includes neural networks too.

We posted the use of neural networks in detecting people in the water to the public domain log ago. We also point them to a paper we cited earlier, “Improving Classification Performance of Sonar Targets by Applying General Regression Neural Network with PCA” by Erkmen and Yildirim. Expert Systems with Applications. 2007.

The use of moving averages has long been practiced in classification of sonar targets. Studies have applied the use of genetic algorithms (which we have also suggested as means of processing these signals) and the use of neural networks to processing sonar signals. These are direct corollaries to processing the signals from infrared detectors used in virtual propeller guards.

We find it interesting that Volvo paid to have this patent issued in mid 2012, when Brunswick has publicly said in the past that they have given up on their similar efforts. Brunswick says it was just too hard to detect people in the water against the sun, waves, heat, and other challenges. Volvo issuing this patent will hopefully encourage Brunswick and others to give this approach another look.

If anybody out there would like to seriously pursue a project like this, please contact us as we have countless resources in this area.

Comments About How the Patent Describes the Need for the Invention

The patent states the invention is needed because people sometimes leave vessels with digital anchors by going into the water for one reason or another (to tend nets, to dive, etc.). While this is true, countless operators enter the water from normal non-digital anchor boats (including by being ejected) and could use the same level of protection. That point is completely overlooked by the patent.

We recognize systems of this nature come with an associated economic cost that is perhaps better born by larger more complex vessels, but the need remains the same. The industry seems to recognize the need for them in larger, more expensive vessels, but ignores the need on smaller ones that are much more likely to have people near them in the water.

We are not saying systems of this nature (infrared detection systems) should be immediately installed on all small vessels. We are saying that appropriate steps to improve the safety of operators on smaller vessels from propellers should be taken.


Volvo Penta was issued a related patent, U.S. Patent 8,271,155 Safety System for Marine Vessels on September 18, 2012.


  1. The idea of this patent is good , but there is a much easier way. Autotether manufactures a MOB system that is very inexpensive typically installs in seconds and without tools, usable on all boats and motor types. The operator wears a small sensor of FOB if he were to fall into the water or go voluntarily in the water at anchor or while underway, the sensor immediately senses this and disables the engine.
    It is failsafe design and currently being used by thousands of boaters around the world. What is interesting when surveyed 96% of the people who have purchased this system use it when ever they are on their vessel, verses less than 5% boater use the emergency kill switch lanyard.

  2. While Autotether, Virtual Lifeline, and other FOB based virtual lanyard systems offer some great ease of use advantages over traditional lanyard systems, we think the ability to detect people in the water holds great promise in protecting those not wearing FOBs (those not from your boat, swimmers, etc.).

    Meanwhile, we encourage everybody to evaluate their situation and consider Autotether, Virtual Lifeline, and similar systems when appropriate.

    If you would like to get a rough idea the risk of someone being hit by your boat propeller AND what types of devices, training, and behaviors might best help you alleviate those risks visit our Developing a Consumer Guide for the Selection of Propeller Guards and Other Propeller Safety Devices page. Our guide is far from perfected, but it can give you some idea of your risk, areas of exposure, and how to reduce those risks. Again, that tool is NOT professional advice. We posted it to generate some comments and stimulate others to contribute to its improvement.

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