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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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