The pilot stated that he recently installed a newly rebuilt engine in his unregistered experimental amateur-built airplane and completed a 1-hour break-in procedure the day prior to the accident. He was departing from a soft farmer’s field and he believed the engine got wet or the propeller was damaged by mud or rocks during the takeoff, reducing thrust. The airplane gained only 40 to 50 feet of altitude before it impacted trees. The airplane incurred substantial damage to the empennage. The reason for loss of engine power was not determined. The pilot stated he should have taken the airplane back to his home base and not attempted the takeoff from the soft field.