Several years ago now, my prop shaft (1" dia. stainless) broke clean in two on my '88 Tristar also, although I didn't lose the prop.
I attributed it to a full speed manatee strike that happened several months previous (end of summer to next spring) so there was a long time between the strike and break, but not much motor time. The strut had an ever so slight bend in it (as I came to discover later), creating a stress to the shaft, but no damage to the prop blades at all, hence my thought at the time that no damage was done. I filed a claim with my boat insurance and they covered it. Replaced strut, shaft, cable, packing, and reconditioned prop.
When it gets replaced the strut, shaft log, and alignment should all be checked and brought into specs.
1988 Tristar 190 Sport, 351W w/GT-40's, 1:1 Velvetdrive