Roy. Welcome to the MB forum.
If the propshaft is threaded then as Bob says, one would expect the props to be threaded as well (to screw on).
I think that Bob meant to say the shafts would normally be stainless steel..? (which is what I would expect)
Does the hole go right through the prop-boss or is it blind?? Just possibly they were not threaded at the factory.
If you were wanting to r/c it, I would strongly suggest st/st shafts and "normal" screw-on and secure-with-a-locknut type model boat props. Do you know what diameter the shaft is? AND are the prop tubes simply a bit of brass tube or do they have bronze bearings in the ends?
Sorry to ask a lot of q`s but we need to know all the gory details.
Ashley