On May 15, 2004, at 1622 central daylight time, a Cooke Rotorway Exec amateur-built helicopter, N319RC, was substantially damaged following a partial loss of engine power while hovering in preparation for takeoff from a boat dock located on Lake Conroe, near Willis, Texas. The helicopter was registered to and operated by the pilot. The commercial pilot/builder, sole occupant of the helicopter, was not injured. Visual meteorological conditions prevailed and no flight plan was filed for the personal flight conducted under 14 Code of Federal Regulations Part 91. The local flight was originating at the time of the accident. The 3,200-hour pilot reported in the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2) that after running the engine up to operating temperatures, he completed his pre-takeoff checklist. He lifted off from the boat dock to a 3-foot hover for about 20 to 30 seconds, and then started to move the helicopter over the water. Subsequently, he heard a ratcheting sound followed by a loss of engine power. He attempted to land back on the boat dock, but the helicopter descended quickly and the right landing skid and the main rotor blades contacted the dock. The helicopter came to rest upright in a boat slip in about 4-feet of water, adjacent to the dock. Two Federal Aviation Administration (FAA) inspectors performed an on-scene examination of the helicopter. According to one of the inspectors, the helicopter had been recovered from the water and the main rotor blades were already removed. The aft portion of the tail boom, the main rotor blades, and the main rotor system were damaged. Examination of the engine revealed that the #1 cylinder exhaust valve keeper retaining clip was disengaged. The pilot reported to one of the FAA inspectors that he had recently sent the #1 cylinder to Rotorway for repair. The cylinder was reinstalled the helicopter approximately 10 flight hours prior to the accident.