Propellers can be distinguished into two types, namely a Fixed Pitch Propeller (FPP) and a Controllable Pitch Propeller (CPP).
The Fixed Pitch Propeller is casted as one block and its shape is optimized for one operating point. A gearbox is needed to generate astern thrust. The first design of a Controllable Pitch Propeller dates back to 1903 and was initiated to improve maneuverability. In such propellers, the blades can be rotated around axes normal to the driving shafts by means of a hydraulic cylinder inside the hub and an eccentric pin-slot mechanism which transforms the linear motion of the yoke into a rotational movement of the blades.
The rotation of the blades around the spindle axis is called pitch. If the pitch range is large enough, the propeller can produce astern thrust and a reversing gearbox is not needed anymore. This type of propeller is most common on ships, where it is required to sail efficiently at two different load conditions, i.e. towing or running free, and on ships that sail to ports with limited or no tug assistance. Therefore, a CPP can mostly be found on harbour or ocean-going tugs, dredgers, cruise ships, ferries, frigates and cargo vessels.
A correctly sized CPP can be efficient for a wide range of ship speeds, since the pitch can be adjusted to absorb all the power that the engine is capable of generating. The working of this diagram is explained with an example.
A propeller is optimized for efficiency at a certain pitch α2 and shaft speed n3. This operating point, A, requires a certain power of the diesel engine and results in a ship speed u4.
When the ship speed must be decreased to u3, a FPP can only reduce the shaft speed to n2. A CPP can also adjust the pitch to α3, and reaches operating point, C, which is more efficient due to less fuel consumption. Other advantages of a CPP, such as sailing efficiently at varying load and weather conditions, can be substantiated in a similar way. The downside of a CPP is the large hub which decreases the efficiency and causes cavitations. Another drawback is the mechanical complexity which limits transmission power.
Overall, technical characteristics including : high reliability and supportability; fem analysis to key parts, low working oil pressure, good sealing and friendly to environment, shaft line-alignment designing and vibration calculation, hydrodynamic design with high propulsion efficiency and it is also simple an d easy to operate and maintain.The are of use vary from propulsion devices in inland vessels to offshore mixed ships, working boats, fishing boats, pelagic vessels and war ships.
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Link to this article：Series controllable pitch is also known as CPP
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