So Mike, here's my two cents worth as a boat
owner for more than 40 years and also as a marine
professional in the engine business for more than 30 years....
As a sailor, if props disappeared as a result of running aground, I and a lot of others would have replaced numerous props over the years. And if you snagged someones anchor
chain, the evidence would be all over the boat bottom in the form of scratches and gouges on the shaft, strut, rudder
But, as someone who worked in the engine business, I can tell you that props fall off shafts all the time, especially sail drives and stern drives....And the vast majority
of these times it is the result of improper installation
. Installers forget cotter pins, forget to fold over lock tabs, reuse locking bolts when they shouldn't, apply improper torque with dinky little screwdrivers, etc. The list is endless. One of the classic scenarios is the boat that pulls into the slip and shifts into reverse to slow the boat, but it just keeps on going. The prop pulled itself off the shaft in reverse because the nut or cone is gone and the prop is now lying on the bottom right below the boat.
Occasionally the rubber hub in an I/O or saildrive
prop can fail, and the prop disappears, but these usually leave the remains of the hub still connected to the shaft as evidence.
Inboard prop loss is even more suspect.....a properly installed inboard propeller almost never falls off a shaft. You can go into boatyards
all over the place and see props with blades folded back from grounding or hitting logs
and the prop remained on the shaft. Boatyards
don't invest in heavy- duty hydraulic prop pullers just because they like to spend money!
So if a prop parts
company with a shaft, most of the time the prop wasn't installed correctly.