This topic covers the situation where the propeller is not spinning at the same speed as propeller shaft, therefore reducing forward thrust. This is normally due to the rubber bushing in the centre of the propeller hub having failed and starting to slip. It is the purpose of this rubber bushing to absorb shocks of the propeller hitting an object underwater and stopping those shocks damaging the gearbox.
Older outboard motors have sheer pins instead of rubber bushings. These shear pins fracture when the propeller hits an object allowing the propeller to spin on the shaft. In this case the propeller must be removed and the shear pin replaced.
Bushings have the advantages of being able to absorb a certain amount of knocks and keep working, shear pins have the advantage of being easily repaired with a small-to-carry spare part.
If you need to replace your propeller, visit the Solas Prop Finder.