Controllable pitch propellers (fig. 12-22) are used in some naval ships. Controllable pitch propellers give a ship excellent maneu-verability and allow the propellers to develop maximum thrust at any given shaft rpm. Ships with controllable pitch propellers require no reversing gear since the direction of the propeller thrust can be changed without changing the direction of shaft rotation. Con-trollable pitch systems are widely used on diesel-driven ships,. while gas turbine powered ships use controllable pitch as the only means available for providing reverse thrust. Controllable pitch propeller systems may be controlled from the bridge or from the engineroom through piping inside a hollow propulsion shaft to the propeller hub. Hydraulic or mechanical controls are used to apply the actuating force required to change the position, or angle of the pitch, of the propeller blades.

A hydraulic system is the most widely used means of providing the force required to change the pitch of a controllable pitch propeller. In this type of system, a valve-positioning mechanism actuates an oil control valve. The oil control valve permits hydraulic oil, under pressure, to be introduced to either side of a piston (which is connected to the propeller blade) and at the same time allows for the controlled discharge of hydraulic oil from the other side of the piston. This action repositions the piston and thus changes the pitch of the propeller blades.

Some controllable pitch propellers have mechanical means for providing the blade actuating force necessary to change the pitch of the blades. In these designs, a worm screw and crosshead nut are used instead of the hydraulic devices for transmitting the actuating force. The torque required for ro-tating the worm screw is supplied either by an electric motor or by the main propulsion plant through pneumatic brakes.

In most installations, propeller pitch and engine power are controlled through a single lever. Movement of the lever causes both engine speed and pitch to change to suit the powering condition ordered. In emergencies, and in ships without single lever control, the propeller pitch may be changed independently of the engine power setting. Under this condition, overspeeding of the engine can result if the pitch is set too low, or overtorquing of the engine can result if the pitch is set too high.

Instructions regarding the limitations of engine speeds at different propeller pitches have been issued to all ships equipped with controllable pitch propellers. In addition, the special operating and maintenance instructions for these propellers should be consulted before any overhaul or repairs are undertaken.