Tuesday, June 30, 2009

Controllable Pitch Propellers

Controllable Pitch Propellers are propelling device for ships or boats that provide more flexibility in the blade pitch variation which is important for ship maneuvering in restricted waters such as harbor or narrow bay areas. The use of controllable pitch propeller is increasing day by day. According to Llyods Register of Shipping, in 2000, the market share of this propulsion system reached 35 percent. The CP propellers are mostly installed in ships that need higher degree of maneuverability such as passenger ships, ferry, tug boat and trawler vessels. For military application, the installation of controllable pitch propellers in landing ship tanks greatly helps the ships to load and unload military personal and their logistic supplies easily in shallow waters.


Although controllable pitch propeller has higher degree of freedom for the operaton of ships or boats, its design, manufacturing, installation, and operation are more complex than the conventional fixed pitch propeller. In general, the controllable pitch or CP propeller has two operating mechanisms. They are pull-push rod and hub piston mechanisms which are depicted below:

cp-propellers.gif

The CP propeller can be controlled directly from the bridge or from the engine room using hydraulic or mechanical system. In the hydraulic system, the actuating force needed to change the pitch of the propeller is transmitted to the blades of propeller by piston in servo motor that pushes or pulls oil inside the shaft of the propeller. A servo motor assembly consists of control rod assembly, locknut, piston, and oil.
Controllable Pitch Propeller Schematic Operating Systems: (a) push-pull rod system and (b) hub piston system Source: page 22 of Marine Propellers and Propulsion second edition by John Carlton
Another advantage which controllable pitch propeller provides during the operation of a ship is that it can move the ship astern at full power. It means that the ship can move backward without the help of tugboat. Such capability cannot be found in conventional propelling system that uses fixed pitch propeller.

cp-propeller.jpg
Besides having better properties in maneuverability, CP-propellers, in general, has overall propulsion efficiency that is lower than fixed pitch propeller due to their higher boss diameter ratio (DB/D) which is around 0.24 to 0.32D. For fixed pitch propeller the boss diameter ratio (DB/D) is between 0.16 and 0.25. Marine screw propeller with higher boss diameter ratio tends to have lower efficiency caused by complex hydrodynamic problems such as cavitation and wake. by Charles Roring in Manokwari of West Papua Indonesia
Also read: Boat Propeller

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