Exam Question

### (a) Why Keys and Keyways Have Been Eliminated
The elimination of keys and keyways in modern large propeller installations is primarily driven by the need to enhance the structural integrity and reliability of the propulsion shafting. The key reasons are:
* **Stress Concentration:** A keyway, particularly one with sharp corners like the traditional sled runner type, acts as a significant stress concentrator. Under the high and fluctuating torque loads from the main engine, these stress concentrations become initiation points for fatigue cracks. Over time, these cracks can propagate, leading to catastrophic shaft failure.
* **Fretting Corrosion:** The key is a separate component fitted between the shaft and the propeller hub. Despite a tight fit, minute relative movements (microslip) can occur under cyclic loading. This movement, in a corrosive seawater environment, leads to fretting corrosion, which degrades the mating surfaces, loosens the fit, and creates additional sites for fatigue crack initiation.