A propeller shaft is a device on which a propeller is attached to and transfers the power from the engine to the propeller. This terminology is typically used when discussing a ship or boat propeller as an airplane's version is typically mounted to a hub. The propeller shaft runs from the engine through a seal in an inboard engine application. It then runs through the hull and into a bearing just ahead of the propeller. The shaft must run true and straight, free of any bends or it will vibrate the vessel as well as pre-maturely wear out the bearings and seals.
The propeller shaft is typically made of hardened steel and incorporates a spline on the end of the shaft where the propeller mounts. This spline allows the propeller to be mounted onto the shaft without slipping or spinning free. Along with the spline, a threaded section of the shaft incorporates a nut and washer to tighten and secure the propeller to the shaft. This nut is usually a castle nut, which is a nut that resembles the tower of a castle and uses a cotter pin. The pin is placed through a hole drilled through the shaft, and then bent over to prevent the nut from loosening or backing off.
In an outboard engine application, the principle of the propeller shaft remains the same, but on a much shorter scale. The shaft on an outboard engine runs from a beveled gear in the engine's lower unit, through a sealed bearing and out of the casing. The same spline system is used to retain the propeller as with the castle nut.
In many outboard systems, a shear pin is used to prevent an object from coming in contact with the propeller and breaking the propeller shaft. A shear pin is a small metal bolt-like pin which is made of softer metal than the propeller shaft. When the propeller comes in contact with a hard object much like a rock or large underwater stone, the pin will shear off. This allows the shaft to continue rotating while the propeller stays still.
It is an easy task to raise the engine's lower unit out of the water and install a new shear pin with minor hand tools. Once the pin is replaced, the boat can once again be operated. If not for the shear pin, the shaft might be severely damaged or the propeller could be broken or damaged beyond use.