On July 12, 2009, about 1245 eastern daylight time, a Cessna 172N, N4773G, was substantially damaged during a precautionary landing in Rochester, Massachusetts. The certificated private pilot and three passengers were not injured. Visual meteorological conditions prevailed and no flight plan had been filed for the flight that departed Boire Airport (ASH), Nashua, New Hampshire, about 1140, destined for Martha's Vineyard Airport (MVY), Vineyard Haven, Massachusetts. The personal flight was conducted under Title 14 Code of Federal Regulations Part 91. The pilot reported that the airplane had descended to a cruise altitude of 1,500 feet when he began to feel "sporadic engine vibrations." The carburetor heat was still in the "on" position from the recent descent, and the pilot turned the carburetor heat to the "off" position, and enriched the fuel mixture. The vibration increased when engine power was added and was becoming worse. The pilot elected to reduce engine power and perform a precautionary landing to a field. The airplane subsequently touched down in a cranberry bog, and sustained substantial firewall and airframe damage. Examination of the wreckage by a Federal Aviation Administration (FAA) inspector did not reveal any mechanical malfunctions. The inspector noted that the No. 2 cylinder lower spark plug was fouled and that the compression on the No. 4 cylinder was lower than that of cylinders 1, 2 and 3; however, the reason for the severe engine vibration reported by the pilot could not be determined. According to maintenance records, the airplane had accumulated 5,311 total flight hours and had been operated for approximately 74 hours since its most recent inspection, a 100 hour inspection, which was performed on May 12, 2009. The engine had accumulated 3,668 total hours and 1,430 hours since major overhaul. It had accumulated 74 flight hours since inspection on May 12, 2009. The pilot reported 61 hours of total flight experience; all accumulated in the same make and model as the accident airplane. A weather observation taken at 1253, at the New Bedford Regional Airport (EWB), located about 8 miles southwest of the accident site, reported: clam winds; visibility 10 statute miles; ceiling 2,300 feet broken; temperature 24 degrees Celsius (C), dew point 18 degrees C; altimeter 29.87 inches of mercury.