Many Mercruisers (and I assume VPs as well) have reduction gear starters. These have a smaller, much faster spinning motor that drives a planetary gear set. The big ring in the gear set is made of plastic and can strip pretty easily. That would cause the same symptoms as described. Sadly, the gear is very easy to remove, and it has a part number, but no one sells it. You have to buy a new starter when it fails. I hate that....
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