On May 26, 2013, about 1130 eastern daylight time, an experimental, amateur-built Sonex, N634SX, operated by a private individual, sustained substantial damage when it impacted a canal, following a partial loss of engine power during cruise flight near Naples, Florida. The private pilot was seriously injured. The personal flight was conducted under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed for the flight that departed Naples Municipal Airport (APF), Naples, Florida, about 1125. No flight plan was filed for the planned flight to Immokalee Regional Airport (IMM), Immokalee, Florida. The pilot reported that the airplane was built in 2004 and he purchased it on March 3, 2011. Prior to the accident flight, the airplane had not flown for approximately 2 years while he refurbished it. It received an annual condition inspection on April 20, 2013. The pilot then operated the airplane for about 3 hours on the ground, conducting taxi tests and making minor adjustments to the airframe and engine. During the accident flight, he departed runway 5 at APF and flew east toward IMM. About 5 minutes after takeoff, at 2,000 feet mean sea level, then engine lost oil pressure, partial power, and began to vibrate. The pilot initially flew toward Marco Island Airport (MKY), Marco Island, Florida; however, the airplane was losing altitude and he did not think the airplane would make it to that airport. He then flew toward a golf course. During short final approach, he turned left to better align the airplane with a fairway, but the airplane stalled and impacted a canal prior to the golf course. The pilot added that the propeller continued to rotate until impact. The airplane was equipped with an Aerovee 2180, 80-horsepower, fuel-injected engine, which was a modified Volkswagen engine. At the time of its most recent annual condition inspection, the airplane and engine had accumulated about 34 total hours of operation. A Federal Aviation Administration (FAA) inspector reported that the accident site was about 1.25 miles from MKY. The wreckage was recovered from the canal about 3 days after the accident. Examination of the wreckage did not reveal any cracks in the engine case or oil residue in the engine compartment. With the exception of a disconnected oil breather tube, the FAA inspector did not observe any preimpact anomalies. The pilot added that the oil breather tube was "flimsy" and most likely separated during impact. He further stated that there was no evidence of an oil leak in the air or on the ground. He was also near the canal for 45 minutes after the accident and did not observe any fuel or oil sheen on the water. The pilot did not know why the engine lost oil pressure and power.