Ultra-simply, here’s how the PropSmith works. One plate threads onto the shaft. The other plate follows, but freely. Long bolts pass through both plates and thread into the tapped holes in the propeller hub. Short bolts separate the two plates as they are tightened, thus pulling the propeller.
Many small yards can’t justify $10,000 or so for a full set of PropSmiths that cover each size shaft, but a single PropSmith for one boat is around $800. That’s a worthwhile investment when traveling off the beaten path, particularly for boats with metric shafts or twin-engine vessels with left-handed threads on the starboard shaft, a foible common to boats built in Taiwan.