BMatthews is right on the flex shaft and bearings. A flex won't work since it will unwind when you try to start the engine. On a solid shaft, you need to keep bearings no further apart than 5 or 6 inches. Any further apart and the shaft will whip on you and destroy the bearings and the shaft(been there done that). I would recommend going with somewhere in the 8-10mm range on the shaft, as you will be putting a lot of torque on a 17.5" shaft when spinning an 11" prop with a .61.