The aircraft was in cruise flight at about 5,600 feet mean sea level when the pilot heard a loud noise followed by severe vibrations. The pilot pulled the fuel mixture control and entered a power-off forced landing. On short final, the pilot noticed wires in his glide path. He maneuvered the aircraft under the wires. He landed on uneven terrain and the aircraft nosed over. Investigation revealed that about 17 inches of one propeller blade outboard section failed due to fatigue. The propeller was cut down from a 74' diameter to a diameter of 68' and put on the aircraft with a Lycoming 0-320 E engine. This is an unapproved combination and the propeller failed after about 20 hours of flight time in that combination. An independent laboratory examined the propeller and concluded that the fatigue failure was as a result of a forging defect. The NTSB laboratory reviewed that report and determined that the independent laboratory's conclusion was not substantiated.