First, is there a lock nut on the main nut? Can you get it loose. Wrenches on both the prop nut and the lock nut.

If there is room for two more nuts on the threads, screw one almost to the nut holding the prop, screw the second up tight to the first one. Lock them together, really tight. Make sure you have at least four turns on the last nut, or you may strip the threads. Once you have the two new nuts jamed together, the hold the nut holding the prop and with a second wrench on the last nut, try to screw the shaft in, IE turn the nut to the right. You should have less than 1/2 turn between the prop nut and the new combination. As soon as the prop nut loosens, unlock the new nuts and remove them. You should be home free then.

A second method that might work, if you have a wood prop, is to invert the prop into a pan of water, just enough to cover the prop and leave it over night. The wood should swell enough to lock the prop to the thrust washer.

In any case, keep the vice grips and pliers away from the engine. You stand a chance of cracking the crank case at the front bearing, damaging the front bearing, damaging the thrust washer, and maybe the crank. If the two previous don't work, sacrifice the prop, Cut it out, being very careful not to cut into the crank or washers. Cut and chip. Be careful not to bend the shaft.

One last thing comes to mind, and this would be the last thing to try, well try it before trying to cut the prop out. Very carefully put some thin Ca glue at the prop to thrust washer seam. Don't get it behind the washer, or it will screw up bearings and seal the thrust washer to the crank shaft. Give the CA a couple hours to set, then try to take the nut off again. If you are lucky, all you will have to do is to seperate the prop from the thrust washer. Stick the prop and washer in the freezer and leave them for a couple hours. There should be enough difference between the thermal expansion between the prop and the washer to break them loose.

Don