A friend of mine went to look at buying a 30 ft sailboat, but when he went to look at it, the owner mentioned in an offhand way that the prop had fallen off! Everything on the boat looked like it would be a great little sailer with a little work, but he decided not to buy it because of the prop.
When a prop falls off, it is usually from electrolysis. Significant damage like a prop falling off indicates that electrolysis had been attacking the boat for quite some time. It had eaten up the zincs, which are there to protect the other metals, and then destroyed enough of the prop ( and connecting bolts ) to have it fall off the boat!
When a prop falls off due to electrolysis, that is a serious problem. Most likely there are other problems waiting for the unsuspecting buyer. After the prop fell off, the electrical current probably moved on to attack the prop shaft and the through-hulls, weakening them and requiring replacement. This would add significant cost to the restoration had he decided to make the purchase.
Additionally, when a boat is neglected for a long time, you never know what other problems you will find. A prop falling off is a clear indication that you should walk away from the boat, no matter what the dream. Great deals on sailboats are always just around the corner.
How do you avoid electrolysis? Make sure you have zincs protecting your running gear, possibly add a galvanic isolator. Have your boat bottom inspected on a regular basis with a quality dive service. Letting things slide on a boat is always a bad idea. Costs mount significantly the longer a problem goes without being resolved. In the instance of this boat, a sailboat that could have been sold for a few thousand dollars had become more or less a give away boat.