Admittedly, propeller shafts aren't too exciting to look at. The propeller
goes on one end, and the engine (or the gearbox) on the other end. The
object midway along the shaft is the engine coupling.
This is the nut
that holds the propeller on the end of the propeller shaft. The propeller
shaft is tapered to match the taper inside the propeller. Identical slots
are routed on the propeller shaft, and inside the propeller to make a 'key
way'. A block of steel called a 'key' fits in these slots so when the shaft
turns the propeller turns. This is torturous service to say the least.
The fits must be exact or the pieces will not stay together. See Causes
of Propeller Hub Cracking by LL Walker, Jr. This nut has been tightened,
and a strap is welded to the propeller shaft to keep it from becoming loose.