I have a 1973 25hp Evinrude outboard handed down to me by my Dad. After over 30 years service, I have spun 2 props in the last couple years. What causes props to do this? And, can they be repaired? If so, how? My use of the term "Spun Prop" is the prop seems to work fine, until you get out of idle and then the motor revs without going anyplace.