NSW Maritime (Australia) propeller guard position stamped dated May 8, 2009. Says many organizations are now using them on outboard powered boats.
Houseboat Propeller Regulation Proposed and Withdrawn by USCG: Analysis by PGIC
History of Boat Propeller Safety
Mitigating Boat Propeller Injuries & Fatalities Chart
Approaches to Prevent Outboard Motors From Flipping Into Boats After Striking Floating or Submerged Objects
Why Outboards Used in Bass Tournaments Disproportionately Break Off & Flip into Boats Compared to Other Outboard Motors
Four Volume Review of the Boating Industry’s Boat Propeller Guard Product Liability Defense