Yanmar preforms propeller guard mounting holes in D27 and D36 diesel outboard lower legs to prevent corrosion from drilled holes. From 2000 model year service manual.
Aspects of the Propeller Safety Debate chart
Houseboat Propeller Regulation Proposed and Withdrawn by USCG: Analysis by PGIC
History of Boat Propeller Safety
Mitigating Boat Propeller Injuries & Fatalities Chart