CONTROLLABLE PITCH PROPELLER :

In the control of vessels usinga Controllable Pitch Propeller, the engine speed, fuel rack setting and propeller pitch are preset in relationship to one - another for optimum performance. Any required change in ship speed or direction is obtained by the operation of a single lever at the control station which then adjusts the propeller pitch, fuel rack setting and engine speed in accordance with a predetermined programme. Thus when a change in ship's speed is required, the signal proportional to the change is fed to the governor and to the pitch setting mechanism.

The fuel racks are adjusted and assuming the engine load does not exceed the predetermined maximum for the speed chosen, the signal will adjust the pitch setting mechanism until the signal representing the actual pitch matches that of the desired pitch. Should there be an increase in load, the engine speed will tend to drop, causing the governor to move the fuel racks so that the engine speed is restored. In moving the fuei rack the governor also moves a transducer which produces a signal proportional to the overload. This signal is then fed into the pitch control system via a relay which gives a bias to the desired pitch signal, so that it no longer matches the actual pitch signal and the error then operates the pitch setting servomotor and mechanism, adjusting the propeller pitch. This adjustment continues until the actual pitch and the new modified desired value pitch signals match and the engine operates around the new loading condition.

In this arrangement, the input signal to the pitch change operates a variable stroke oil pump which pumps oil to the left hand side of the piston and receives oil from the right hand side, or vice-versa. As the piston moves it operates a feed - back mechanism which brings the pump stroke back to mid - position, holding the piston and the propeller pitch at the required position. With UMS notation a propeller pitch and shaft rpm indicator should be fitted at each control station and means should be provided to prevent the engine and shafting being subjected to excessive torque due to changes in pitch.

There should be an alternative power source (either a hand pump or diesei engine driven pump) provided for controlling pitch as well as a fail safe arrangement (with spring) to bring back the pitch to full - ahead mode during failure of hydraulic supply or controls. Audible and visual alarms should operate for : low hydraulic system pressure, low level in hydraulic supply tank, failure of power supply between bridge and hydraulic actuator and high oil temperature at the cooler outlet.

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