On August 2, 2015, about 1600 Pacific daylight time, a Hudson Zodiac CH 601 XLB, N234S, experimental amateur-built airplane, was substantially damaged during a precautionary landing after experiencing a severe engine vibration near Hubbard, Oregon. The commercial pilot, sole occupant, was not injured. Visual meteorological conditions prevailed for the personal flight, which was being operated in accordance with 14 Code of Federal Regulations Part 91, and a flight plan was not filed. The local flight had departed the Valley View Airport (5S9), Estacada, Oregon, about 15 minutes prior to the accident, with Lenhardt Airpark (7S9), Hubbard, Oregon, as the planned destination. In a report submitted to the National Transportation Safety Board (NTSB) investigator-in-charge, the pilot reported that about 10 minutes prior to landing he noticed an [engine] vibration, and when the intensity increased he chose to land in a field. The pilot stated that prior to landing the engine shook violently, which resulted in him shutting the engine down. During the forced landing the airplane's left wing collided with a ground sprinkler system, which resulted in substantial damage to the airplane. On August 20, 2015, an NTSB air safety investigator, accompanied by two Federal Aviation Administration (FAA) inspectors, examined the airplane at the facilities of Premier Engines, located in Troutdale, Oregon. It was reported by an FAA inspector that the propeller flange was cracked through 5 of the 6 lightning holes in the flange, indicating a vibration in the propeller. The inspector stated that it was apparent that some bolts had lost torque or were not properly torqued, which allowed one blade to "bounce", and subsequently resulted in the reported vibration. The inspector added that the propeller was mounted on a heavily modified, formerly certified, propeller spacer. The spacer showed evidence of the builder [having attempted] to correct an out-of-track condition by "shaving " the spacer down on one side. The propeller bolt bushings installed in the spacer appeared to be "homemade" and of different lengths, with bolt holes drilled off center, and generally of poor quality. An examination of the airplane's engine revealed no anomalies that would have precluded normal operation.