Fireboat prop
Welcome to the site.
db appears to be helping you so you are in good hands. Original Fireboat kits had a built in slot as db says but if yours doesn't then a vertical hole at the correct distance from the transom (stern) can be elongated (by file) to take the prop shaft at the correct angle. I use epoxy glue to fill the slot but car body filler works just as well. Good luck
Dave
Dave