Sounds like the prop hub is spun. You can verify by taking a sharpie marker and drawing a line across the back of the prop from the outer edge across the inner hub. Take it out and give it gas. Stop the motor and look at the line. If the inner portion of the line is no longer lined up with the outer portion, the hub is spun.
If so, the prop can be re-hubbed by a prop shop for around 60 bucks (take it to your local marine dealer Merc, OMC, etc.) , you can buy a new one here at iboats stores, or you can buy a used one on ebay.
