Seen this a few times before. Screw threads are normally RH threads so having a prop that screws onto the shaft clockwise when looking at the shaft end means that to ensure the prop stays on, the shaft should rotate anti - clock so you need a prop that generates thrust when rotating anti - clock.
Before anyone jumps in with "It's a PT boat, so multi propped", retrieval wouldn't have been a problem, one of them would have been going the "Right" way.
For the clock rotation fraternity, a locknut on the shaft in front of the prop would not go amiss.
Regards Ian.