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CPP

Controllable Pitch Propeller



Related Terms

PROPELLER PITCH

The ratio of the difference between the propeller pitch and actual distance travelled during one turn to the propeller pitch, expressed in percent.

PITCH

  1. The alternate rise and fall of the bow of a vessel proceeding through waves;
  2. The theoretical distance advanced by a propeller in one revolution;
  3. Tar and resin used for caulking between the planks of a wooden vessel.

CONTROLLABLE-PITCH PROPELLER

A ship propeller in which the pitch of the blades can be changed while the propeller is in motion; five types used for aircraft are twoposition, variable-pitch, constant-speed, feathering, and reversible-pitcH.

The hub servo type comprises (contain) of a pitch altering mechanism enclosed the propeller hub filled with oil from the header tank which situated above the water line to get static pressure so as to prevents ingress of seawater.

Design

The C.P.P contain four main parts:

  1. Propeller hub incorporating Servo, crank ring for turning blades and necessary seals.
  2. Oil distribution box that control oil flow to the hub servo and mounted at the forward end of the shaft.
  3. Hydraulic system (header tank, sump tank, pumps, filters and coolers)
  4. Control system (pneumatic or electric)

The propeller blades are bolted to their crank rings and a single piece hub body to facilitate blades changing. The servomotor consists of a piston rod with a piston which moves axially when pressure oil is fed to either side of the piston (double acting). The piston rod is equipped with 4 or 5 slotted-ears depends on the number of blades. The crank ring has two eccentric pins, 180 apart from each other, which fit into the holes of the slide shoes and which in turn fitted into the slots of the piston rod’s ears.

Working principle

When piston rod moves, the crank pin rings rotates with the circular movement transmitted via piston, piston rod, slots, sliding shoes and crank pins. Then the propeller blade, which is bolted to the crank pin, rings turn. The O.D box conveys pressurized oil into the valve rod and to lead return oil out from the hollow shafting. And also, via the control servo its control the movement of the valve rod that directs the high pressure to the desire hub piston. The movement of the servomotor is regulated both from the Engine Control room and bridge pneumatically or electrically. With the pneumatic system, the control servo motor is operated by the telemotor receiver. But with the electric system, the telemotor system is replace by the solenoid hydraulic valve. The ship fitted C.C.P with bridge control. The U.M.S notation should place a propeller pitch indicator; rpm indicator and engine overload indicators are placed at each station. Any required changed in speed or direction is obtained by the operation of the remote control signal lever at the control station, which than adjusted the propeller pitch, fuel setting and engine speed in rpm accordance with the present programmed. The control lever sends a specific signal via transmitter to the receivers on the OD box of the pitch control mechanism and engine governor. The transmitter combinator produces two signals,

  1. Rpm command signal, which through I/P converter (current to pneumatic) acts on the governor to move the fuel link rack to turn the engine at desire revolution.
  2. Pitch command signal which goes to the pitch control unit where it is compared with the actual measured value (feed back) of the pitch given by the pitch transmitter.

The computed deviation causes the pitch control unit to determine the output, which becomes the drive signal for solenoid control valve on the O.D box. This value moves the hub servomotor to bring about the necessary change in the propeller pitch. Whilst the load control unit receives two input signals from the rpm transmitter and fuel rack position transmitter. This unit computes the value of the engine load and compares it with the set value. Preset engine load value is equal to rpm multiply by fuel rack position. Than the load control unit generates an appropriate trimming signal to correct the drive signal for the solenoid control valve on the O.D box. Thus the load on the engine is not allowed to exceed the set value, irrespective of the position of the control lever and external conditions, weather, hull fouling, draft.

Emergency and fail safe arrangement

  1. Available and visual alarms are fitted in each station and operated for;
  2. Low oil pressure
  3. High oil temperature
  4. Low oil level in the tanks
  5. Failure of power supply
  6. There should be an alternative power supply for emergency control of the blade pitch.
  7. If the telemotor system fail, the propeller pitch can be manipulated directly from the O.D box by disconnecting the remote control system and changing to hand position.
  8. In the event of hydraulic system failure, the safety spring allows the propeller pitch to a full ahead position.
  9. Auxiliary hydraulic system must also be provided.
  10. In the case of failure of main servomotor, the system must have
  11. Emergency servo
  12. Mechanical link
  13. (a) Emergency servo It is fitted at the foreword end of the O.D box and oil is fed into this servomotor from a hand pump and moved the main servo piston rod so that the blade are in the full ahead position.
  14. (b) Mechanical link A stud is screwed through the distribution box end and thread into the piston rod, when this stud is tightened; the piston rod draws the main servo into full ahead position.

Advantages

  1. Increase the bollard pull about 30% of tugs and fishing vessels.
  2. Improved manoeuverabily
  3. (a) Better reversing and deceleration since pitch can be adjusted, to be maintained at maximum astern power.
  4. (b) Better crash stop facility.
  5. (c) No momentum being destroyed during maneuvering. Since there is no substantial mechanism is reversed in direction.
  6. No reversing gear required.
  7. Less stern tube wear.
  8. Few engine starts (lesser capacity of air compressor and bottle)
  9. U.M.S status and bridge control facility.

Disadvantages

  1. Heavier propeller
  2. Increase stress on tail shaft and stern tube.
  3. Complex mechanism and control system.
  4. Lower propulsive efficiency.
  5. Greater cavition damage on propeller blades.
  6. Increase risk of break down due to oil system failure, seal leakage, control system failure.
  7. Shorter survey period of one year.
  8. Higher cost.

FEATHERING PROPELLER

A variable pitch marine or airscrew propeller capable of increasing pitch beyond the normal high pitch value to the feathered position.

FOUL (OF PROPELLER)

A line, wire, net, etc., is wound round the propeller

SCREW

  1. propeller. 2. propeller-driven.

STERN FRAME

Large casting attached to th after end of the keel, incorporating the rudder gudgeons and propeller post in single-screw ships

PROPELLER

An arrangement which consists of hub and number of blades mounted on a rigid shaft protruding from the hull of a vessel, usually driven by an inboard motor.

TAIL SHAFT

The ships propeller shaft

INBOARD MOTOR

An engine mounted within the hull of a vessel, usually driving a fixed propeller by a shaft protruding through the stern. Generally used on larger vessels. See also stern drive and outboard motor.

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