Complex sentence: The cause of the accident believed to be a malfunctioning overspeeding outboard propeller, resulting from the obstruction of metal particles in the regulator valves of the oil feed lines after a gear had been damaged when the supercharger of the corresponding engine was accelerated (shifted).
Split into simpler sentences: The cause of the accident is believed to be a malfunctioning overspeeding 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 (shifted).