The owner pilot had recently recovered his aircraft with new fabric, and is reported to have taxied it up and down the strip for approximately one hour before taking off. About 15 minutes after the aircraft became airborne, a witness reported that he heard a loud bang, followed by the engine noise stopping, and then observed the aircraft spiralling toward the ground from an estimated height of 600 feet. The aircraft impacted in an inverted attitude in a creek. The investigation revealed that the propeller had fractured in flight. It is considered likely, that due to the severe vibrations associated with the loss of part of the propeller blade, the pilot would have shut the engine down. The reason for the propeller failure could not be positively established. A modification to the engine installation had raised the aircraft thrust line and the pilot had carried out a number of additional modifications which moved the centre of gravity rearwards. It is considered probable that when the engine was shut down, a rapid pitch up occurred. Before the pilot could regain control, the aircraft stalled and fell into a spiral dive, from which there was insufficient height to recover.