Information in the paragraph below comes from Prop Guards and Alternative Propulsion. Charlotte Harbor Magazine. January 2003.
The first patent for a ring-cage prop guard design was issued in 1938 to J. R. Benson. Its main purpose was to protect the propeller and lower gear case from impacts with rocks and other debris. Additional ring-type guards came on the scene in 1943, and by 1964, eighteen designs had been patented and placed on the market. They included mesh cages, forked, and semi-ducted styles.
Note – the history above also appears a few other places online. The 1938 Benson patent they mention, but do not supply the patent number for, is probably U.S. Patent 2,135,162 awarded to J.R. Benson 1 Nov. 1938. We have noticed a few patents pre-dating the Benson patent, including U.S. Patent 1,620,129 Propeller Guard issued to G. Peterson 8 Mar 1927 and U.S. Patent 662,522 Propeller for Boats awarded to S.H. Butterworth 27 Nov. 1900. The second being specifically targeted at canal boats to reduce shore erosion, but also mentions protecting the boat propeller from driftwood.
About 1960 – 1962 several propeller guard patents also started mentioning how they could protect humans, swimmers and skin divers from coming in contact with the propeller. See our Propeller Guard Patents page for additional information.
Today, things that “look like” propeller guards / prop guards seem more focused in their application. They tend to target one of several functions such as providing protection to the propeller, improving performance, keeping weeds out of the propeller, protecting manatees, keeping fishing lines out of the propeller, or protecting people.
We have since complete a History of Recreational Boat Propeller Safety Issues and the Propeller Safety Movement that covers very early propeller guards to the most recently proposed propeller safety devices, and much more.