According to the pilot, he lifted off the 1,800 foot soft sod runway after an 800 foot ground roll and as the airplane passed over the fence, he noticed the climb deteriorating. A scan of the instruments indicated the tachometer indicating 2,100 RPM versus the 2,500 RPM that would have been normal for takeoff. The airplane subsequently crossed a river and impacted the opposite bank. Two of the passengers stated that they thought the pilot had used most of the runway during the takeoff roll. Post-accident investigation revealed no evidence of pre-impact mechanical failure or malfunction. Tear down of the propeller indicated that the blades were at a pitch setting of about 20.5 degrees instead of the 12.5 degrees pitch which was normal for takeoff and climb. No explanation for the higher pitch setting was found during the investigation. A computer simulation of the accident scenario indicated that the blade angle would have been about 18.5 degrees for the conditions and power described by the pilot.