HP Motor Requirements
It may be a shear-pin in your prop that has, well, sheared! It is the connection of choice years ago between the prop and the drive. If your prop hit anything, or became entangled, the shear-pin was "sacrificed" in order to prevent more costly damage to the motor. Problem was, those damn shear-pins always seemed to let lose for no appearant reason. I recall that our club had a whaler with a 25hp "John-rude" OB that went through a pin a week!
Easy fix though. Pull the prop, drive out the remains of the old pin and install a new one.
Your engine is probably turning the prop through friction just enough to make it seem like its working, but won''t put any torque to the water.