The pilot/owner reported that during straight and level flight, he felt a “shudder” and heard a “thump” and the airplane’s engine over sped. He executed a forced landing to a field, but the airplane’s right main landing gear separated from the airplane. The airplane came to rest on the remaining landing gear and the right wing. Postaccident examination revealed that the airplane’s pusher propeller had separated from the engine. During the separation, the propeller struck the left wing, tearing the fabric covering and substantially damaging internal structure. The forced landing resulted in substantial damage to the fuselage lower longerons. Photographic evidence supplied by the pilot showed that the threaded portion of the 6 bolts that retained the propeller were still in the engine propeller hub. The propeller and the unthreaded portion of the propeller bolts were not recovered. The pilot reported that other than the propeller separation, there were no preexisting mechanical problems with the airplane. He noted that the accident could have been prevented by performing a better inspection under the propeller hub, noting that the propeller bolts are not visible during daily inspections. The airplane was not registered, and the pilot did not hold a Federal Aviation Administration issued pilot certificate. He reported that he thought the operation of the airplane was covered under Title 14 Code of Federal Regulations (CFR) Part 103 – Ultralight Vehicles. 14 CFR 103 details the operation of ultralight vehicles that do not require a pilot certificate, a medical certificate, or registration of the vehicle. The vehicle requirements include an empty weight of less than 254 pounds, a fuel capacity not exceeding 5 gallons, a maximum level flight speed of 55 kts., and a stall speed of no more than 24 kts. Although the airplane was not weighed following the accident, research indicated that the accident airplane may have achieved all the requirements, except the fuel limitation. The accident airplane was equipped with a 10-gallon fuel tank. As such, the airplane should have been registered, and the pilot should have been certificated.