The pilot reported that during a return flight, he elected to fly from the right pilot seat, to allow a cargo-handler to sit in the left pilot seat. The taxi to the runway was short and required little rudder input. However, during takeoff, as he added power, he "did not seem" to have full right rudder authority and he was unable to maintain the runway centerline. Subsequently, the airplane exited the left side of the runway, struck a small ridge, and came to rest nose down. The airplane sustained substantial damage to the fuselage and left-wing. The pilot further reported that, after the accident, he realized the airplane had stow-able rudder pedals on the right side, and that they had been placed in the "stowed" position. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.