A spun propeller means that the prop hub has spun inside the propeller. Once the hub spins inside of the propeller, the propeller will spin freely and not along with the hub. A hub can spin inside of a prop due to damage to the hub, damage to the inside diameter of the propeller or incorrect propeller installation. Fixing a spun propeller will require the assistance of a boat propeller repair shop to press the old hub out of the propeller and the new hub into the propeller.
Inspect the propeller first to determine if the hub has spun inside the propeller. Mark the front of the propeller and the propeller hub directly across from each other with a permanent marker. Run the propeller for about two minutes and then shut it off. Inspect the two marks to see if they are still lined up across from each other. If not, then the hub is spun inside the propeller.