On June 22, 2000, about 1530 Eastern Daylight Time, a homebuilt Avid Amphibian, N61HS, was substantially damaged during a forced landing after takeoff from the Shannon Airport (EZF), Fredericksburg, Virginia. The certificated commercial pilot sustained minor injuries. Visual meteorological conditions prevailed and no flight plan had been filed for the personal flight conducted under 14 CFR Part 91. In a telephone interview, the pilot said that about 1 1/2 to 2 weeks prior to the accident, he removed the airplane's engine, propeller, alternator, and magnetos to check wiring located beneath the airplane's instrument panel. After reassembling the components, he performed a successful engine ground test run-up, and a 30 minute test flight. During the test flight, the pilot noticed the engine RPM consistently went beyond the red line limit. After landing he confirmed that the propeller, which was an IVRO Magnum, ground adjustable propeller, was set to the same pitch prior to it's removal. On the day of the accident, the pilot adjusted the propeller pitch an additional 5 degrees to keep the RPM within limits. He then performed a normal ground run-up at EZF, and departed Runway 24 for a test flight. The pilot stated that the takeoff was normal and the engine sounded "smooth." After reaching an altitude of about 300 to 400 feet (agl), the airplane experienced a sharp reduction in power. The RPM dropped from 6,600 to 5,700; however, the engine sound remained constant. The pilot retarded the throttle and then advanced the throttle forward. The engine RPM increased momentarily then dropped to about 5,000 RPM. The pilot was unable to maintain altitude and performed a forced landing to a corn field. The pilot noted that the engine continued to run smoothly at the lower RPM, and at no time did the engine "sputter or cough." According to the pilot, the airplane had been operated about 270 hours. The airplane's engine, an Aero GT-1000, had been operated about 285 hours. Examination of the airplane by a Federal Aviation Administration inspector did not revealed any catastrophic airframe or engine malfunctions. Additionally, the airplane's engine ran smoothly during a post accident test run performed by the pilot; however, examination of the propeller reduction gear box revealed that the steel cage of the sprague clutch bearing was "cracked and partially disintegrated."