The pilot reported that when the aircraft was about 300ft on final approach to runway 14, he noticed that the right engine rpm did not change as the the propeller controls were set to fine pitch. The aircraft then yawed to the right when it became necessary to introduce power to avoid undershooting the runway. Both propellers were selected to feather when the yawing moment became uncontrollable, at about 50ft AGL. The right wing struck the ground, the landing gear collapsed and the right wing was torn off during the ground slide as the aircraft slide off the runway onto the grass flight strip. The two occupants were not injured and evacuated the aircraft unassisted. Post flight inspection by a licenced aircraft maintenance engineer found that both propeller were on the coarse pitch stops and had not feathered. Inspection of the suspect right engine failed to find a fault which could have contributed to a powerloss. The pilot was under the impression that he had feather both propellers when he experienced directional control difficulties. Examination of both propeller hubs found that the propeller blades were on the coarse pitch stops. The propeller pitch control levers require a distinct movement to pass the coarse pitch stops into the feather position. It is apparent that the pilot did not position the levers where he intended. The pilot lost control of the aircraft on late final approach. The reason for the loss of control could not be determined from the evidence available.