Have Boat Questions? Ask a Boat Repair Expert.
This sounds like a classic prop hub spun out. Inside the prop, between the output shaft and the prop is a rubber bushing. If you remove the prop nut and look down the end of the shaft you will see this black donut looking thing. They fail all the time. When trying to get up to speed the prop is under full load so if it is going to slip that is when it will happen. Then running at top speed the prop is not under that much load so the failing hub will hold.
Thanks for asking.