On December 29,1998, about 1645 hours Pacific standard time, an Ercoupe 415-C, N99371, incurred an in-flight failure of the crankshaft and subsequent separation of the propeller near San Ysabel, California. The airplane sustained substantial damage when the nose gear collapsed during the off-field forced landing. The private pilot, the sole occupant, was operating the borrowed airplane under the provisions of 14 CFR Part 91, and was not injured. The local personal flight originated from Ramona, California, about 1630. Visual meteorological conditions prevailed and no flight plan was filed. The pilot stated he was in cruise flight at 4,500 feet and approaching rising terrain. As he added power to climb, the engine started to vibrate. He looked down to check the mixture and carburetor heat and felt bad vibrations and heard a noise. He saw a flash out of the corner of his left eye, followed by a bang on the left wing. He noticed the top leading edge of the left wing was dented, heard a high whining sound and saw that the propeller had separated. He set up for best glide speed, looked for a place to land, and broadcast an emergency call on the common traffic advisory frequency. He estimated his speed at touchdown was just above stall speed. The left main landing gear touched down first, followed by the nose gear, then the right main gear. The terrain was uneven and the nose gear eventually collapsed. He estimated the airplane stopped within 100 feet. He turned the emergency locator transmitter (ELT) off and tried to radio for help. It did not work, so he secured the cockpit and walked to a farmhouse. Examination of the remaining crankshaft piece by the Safety Board's materials laboratory determined the crankshaft separated forward of the oil-slinger flange. The fracture spiraled around the crankshaft and suffered a large amount of mechanical damage. The laboratory's factual report noted a fatigue region intersected this damaged area. The laboratory determined from the crack arrest markings that the cracking initiated from a location approximately midway between the interior and exterior surfaces of the crankshaft wall. The fatigue region was flat and contained areas of multiple crack arrest marks. This fatigue region propagated through approximately 270 degrees of the circumference of the part. The remainder of the fracture surface, located between the end of the fatigue region and the origin, was rough and oriented on an angle. The external surface of the crankshaft revealed scattered corrosion pits on the fractured end and a region of fretting. The fretting was adjacent to a keyway used to secure the propeller to the hub. The report estimated the position of the fatigue initiation area approximately corresponded to the depth and location of the edge of the keyway slot.