Click image to view full size image
Controllable pitch propellers are used in ships that are required to operate at variable speeds even when operating at constant power requirements. Such ships are tugs,trawlers, ferries, ice breakers, and small scale warships that are equipped with gas turbines.The controllable pitch propellers can be used to run the ship in forward and astern direction both without the requirement to change the direction of rotation of the engine.
link:https://www.thrusterleader.net/controllable-pitch-propeller/
Controllable pitch propellers