The flight instructor in the tailwheel-equipped experimental amateur-built airplane, reported that he was providing aircraft transition training and tailwheel training to the pilot. The pilot taxied to the runway, and successfully completed three high speed takeoff rolls. He then taxied into position for a normal takeoff and the pilot requested that the flight instructor stay on the controls during the takeoff. The instructor reported that during takeoff with the pilot-receiving-instruction on the controls, as the airplane started to climb, the airplane drifted to the right, and settled back to the ground. The instructor applied left rudder and attempted to continue the takeoff, but the right main landing gear collided with a drainage culvert. The airplane spun to the right and came to rest upright. The airplane sustained substantial damage fuselage. Both pilots reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.