The prop is NOT threaded onto the shaft. The threads you see are for the prop nuts only. The prop slides onto the shaft, over the threaded end, and is held in place by a key that sits in a keyway cut into the shaft (with a reciprocal keyway cut into the prop hub.) To remove the prop, follow these steps:
1.- Remove cotter pin from shaft end
2.- Immobilize prop and remove prop nuts (a length of 2X2 run between the strut and rudder works well for this)
3.- Thread one nut back onto the end of the shaft (this will keep the prop from flying off the end of the shaft when it pops free)
4.- Attach a prop puller (or gear puller, if a prop puller is not available) to the prop and shaft
5.- Tighten the puller very tight (but do not overtighten)
6.- Bang on the puller with a large hammer at the after end to vibrate the prop loose
7.- If the prop does not come loose immediately, re-attach the puller in a new position and repeat step 6. Repeat as necessary
8.- Once the prop is off, be sure to retrieve the key. It may be in the keyway on the shaft or stuck inside the prop hub
9.- The prop WILL come off. The rule of thumb is Pressure, Persistence and Patience
Good luck.