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The inside of the prop uses a hard, press fit rubber hub. It's design is that if you ever hit the bottom, the hub would slip and sacrifice itself so you don't damage the rest of the drive. They spin if you hit something, and they also can spin if they are old props, as rubber does dry rot and shrink over time. Once they have spun, they will grab somewhat at lower rpms, and then let go when you really put the power down.
You can send the prop out to have it re-hubbed, but there is usually some down time with that. Most folks will buy a new prop so they can use the boat right away, and then send out their old prop, and keep it for a backup when they get it back in.
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If it is slipping out of gear, its going to make an absolute racket. It would sound like a clothes dryer full of nickles. You couldn't miss the noise if you wanted to, is it making a racket?
Here is the easiest way to tell if the hub is bad.
Pull the prop, get some white out or some paint, and make a mark on the inside of the props hub. Across the rubber hub, and on to the metal part of the hub. This is called a witness mark. Go run the boat until the prop slips, then pull the prop back off. If the marks line up, then hub is good. If the marks are off, the hub is bad.