Controllable-pitch propeller (CPP)

A controllable-pitch propeller carries blades that rotate about their spindle axis under hydraulic control through the hub and a bored shaft, so thrust and direction change without reversing the engine. It lets a constant-speed engine drive a shaft generator at fixed rpm while still varying ship speed, gives full astern thrust from ahead rotation, and suits tugs, ferries, and trawlers with wide load swings. The cost is hub complexity, lower peak open-water efficiency, and a larger boss.