 The investigating Board believed the most likely cause of the accident to be a malfunctioning over-speeding outboard propeller caused by metal particles obstructing oil feed line regulator valves.The particles may have been formed by a gear that was damaged when the supercharger of the corresponding engine was accelerated (gear ratio shifted).The malfunctions of the propeller pitch might have provoked a flight disturbance and as a consequence the propeller may have sheared off.