On July 19, 1999, about 0920 Alaska daylight time, a tundra tire equipped Piper PA-12 airplane, N3780M, sustained substantial damage during a forced landing about 28 nautical miles southwest of Dillingham, Alaska, at 58 degrees 54.42 minutes north latitude, 159 degrees 20.33 minutes west longitude. The solo commercial pilot was not injured. The airplane was being operated by the pilot under 14 CFR Part 91 as a business flight in support of his construction firm. The flight departed Togiak, Alaska, about 0900 for Dillingham. Visual meteorological conditions prevailed at the time of the accident, and no flight plan was filed. The pilot told the NTSB investigator-in-charge (IIC) during a telephone interview on July 20, that the airplane had been parked in the rain for two days prior to the accident, with the fuel tanks about 1/3 full. He said that he drained and checked the fuel tanks for contamination prior to the flight, with no anomalies noted. The pilot stated about 10 miles from Togiak, the engine momentarily ran rough, so he adjusted the throttle and applied carburetor heat, and the engine smoothed out. He said about 10 minutes later, cruising about 300 feet above the ground, the engine suddenly quit. The pilot landed the airplane on tundra, and the airplane nosed over, bending the left wing lift-struts. The pilot stated during the interview, and in his NTSB Pilot/Operator report, that he took off with about 14 gallons fuel, and suspects water was in the fuel.