Controllable Pitch Propellor

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   Advantages
        o    Allow greater manoevrability
        o    Allow engines to operate at optimum revs
        o    Allow use of PTO alternators
        o    Removes need for reversing engines
        o    Reduced size of Air Start Compressors and receivers
        o    Improves propulsion efficiency at lower loads

     Disadvantages
        o    Greater initial cost
        o    Increased complexity and maintenance requirements
        o    Increase stern tube loading due to increase weight of assembly, the stern tube bearing diameter is larger to accept the larger diameter shaft required to allow room for OT tube
        o    Lower propulsive efficiency at maximum continuous rating
        o    Prop shaft must be removed outboard requiring rudder to be removed for all prop maintenance.
        o    Increased risk of pollution due to leak seals

The CPP consists of a flange mounted hub inside which a piston arrangement is moved fore and aft to rotate the blades by a crank arrangement.The piston is moved by hydraulic oil applied at high pressure (typically 140 bar) via an Oil transfer tube (OT tube) This tube has and inner and outer pipe through which Ahead and astern oil passes. The tube is ported at either end to allow oil flow and segregated by seals.
Oil is transfered to the tube via ports on the shaft circumference over which is mounted the OT box. This sits on the shaft on bearings and is prevented from rotation my a peg. The inner bore of the box is seperated into three sections. The ahead and astern and also an oil drain which is also attached to the hydraulic oil header to ensure that positive pressure exists in the hub and prevents oil or air ingress
The OT tube is rigidly attached to the piston, as the piston moves fore and aft so the entire length of the tube is moved in the same way. A feedback mechanism is attached to the tube, this also allows for checking of blade pitch position from within the engineroom.

Operation Modes
There are two main methods of operation of a veseel with a CPP.
Combinator-For varying demand signals both the engine revs and the pitch are adjusted to give optimum performance both in terms of manouevrability and response, and also economy and emissions.Constant speed- The engine operates at continuous revs ( normally design normal max working revs), demand signals vary CPP pitch only. This is particularly seen in engines operating PTO generator systems

Emergency running
In the event of CPP system hydraulic failure an arrangement is fitted to allow for mechanical locking of the CPP into a fixed ahead pitch position. This generally takes the form of a mechanical lock which secures the OT tube. Either hand or small auxiliary electric hydraulic pump is available for moving the pitch to the correct position