National Boating Safety Advisory Committee 1989 Subcommittee on Propeller Guards produced this sketch to show the increased cross sectional area created by propeller guards that could strike people.
Houseboat Propeller Regulation Proposed and Withdrawn by USCG: Analysis by PGIC
History of Boat Propeller Safety
Mitigating Boat Propeller Injuries & Fatalities Chart
Four Volume Review of the Boating Industry’s Boat Propeller Guard Product Liability Defense