This research was conducted to describe and quantify the nature and extent of impact injuries inflicted on a swimmer’s leg when struck by a particular cage-type propeller guard on a boat outboard motor. A specific objective was to determine a threshold velocity above which the injury would be considered to be sufficiently severe enough to result in loss of leg function. An outboard motor fitted with a cage-type prop guard was towed at various speeds on a platform attached to a centrifuge arm. The prop guard was impacted at decreasing velocities onto a series of embalmed human cadaver legs positioned stationary underwater and connected to the upper-body components of a Hybrid III test dummy. Measurements were made of:​ 1) the velocity of the impactor as it struck the cadaver legs, 2) the external response of the legs and attached Hybrid III components (via high-speed motion pictures and video), and 3) acceleration and force (for some of the tests). Post-impact analysis of the test legs included detailed radiographs, careful dissection, and evaluation of fractures to the tibia and fibula. Specific--tissue responses evaluated were bone fracture and fragmentation patterns. Six out of seven of the legs tested resulted in cornminuted fractures so severe that loss of leg function would be expected. The seventh impact, at the lowest velocity of 10.4 mph (16.7 km/h), resulted in a transverse fracture from which full recovery would be likely. As the next lowest velocity of impact was 13.6 mph (21.9 km/h), it was concluded that for the loading condition and population studied in this series of tests, the specific prop-guard cage would not be an effective device for preventing severe leg injury at boat velocities greater than or equal to about 13 mph (21 km/h).