Q: What is a shaft?
A: The propeller is attached to the shaft, the shaft runs from the engine through a seal, then the hull into a bearing then onto the propeller. The propeller shaft must be free of any bends and cracks or it will cause vibration and can erode the bearing and seals.  The shaft is made up of hardened steel, comes in various lengths and diameters. It is tapered at the end(s) with a threaded section where the propeller is attached along with a propnut and a cotter pin to hold it in place