On March 31, 1998, at 1100 mountain standard time, a Aeronca 7DC, N4484E, received substantial damage when it struck rocks during takeoff roll from a field 60 miles south of Gillette, Wyoming. The private pilot and sole occupant was not injured. The flight was a local area flight operating under Title 14 CFR Part 91 and no flight plan was filed. Visual meteorological conditions prevailed. According to information provided by the pilot via his written report (NTSB Form 6120.1/2) and a telephone interview, after about two hours of flight the aircraft was in cruise at about 100 feet above ground level (agl) when the engine lost all power. The pilot performed a forced landing in a field and drained water from the fuel. After he got the engine restarted, he attempted a takeoff from the field and his main landing gear entered a snow covered area which caused the aircraft to swerve into rocks. In his telephone interview, the pilot said he normally did not refuel the aircraft until he was preparing for the next flight, which often was at an interval of several days or longer. Thus, his aircraft was sitting with partially filled fuel tanks in an environment where the differential temperature over a 24 hour period varies greatly. According to information provided by several aircraft manufacturers while this investigator was investigating similar accidents where water was found in the fuel, partially filled fuel tanks create a condition in which the fuel tank performs like a "still" and the tank will manufacture water by condensation. The greater the temperature variation, the more water is manufactured.