Yes exactly. Yea it would not hold at all if you just tightened against the round shaft. It needs a good flat and don't forget the set screw tip needs to be ground flat also. Remove the tapered tip that just makes it so it doesn't hold as well and will easily work forward if you accidentally overtighten the prop nut. Remember just snug not tight against the prop and when you set the drive dog make sure to leave enough for the prop and nut to go on probably like 1/4" or thereabouts because the next prop you buy could have a even longer hub and you wouldn't be able to use it unless you bought a whole new shaft and started over. Don't forget at least a 3/8" gap between the drive/bushing and the drive dog. Does that drive you bought have integral bushings pressed in it or does it use a slip in bushing ?