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Controllable Pitch Propellers (CPP)

Some propellers are "controllable pitch", which means that the blades can be turned to increase or decrease pitch. Unlike most boats, these props always turn in the same direction. To go in reverse, the angle of the blades is turned backwards. The Navy was concerned with the failure of hydraulic system and propeller blades going astern. Boswell developed a concept of balanced spindle torque. This peculiar blade shape was to give a balanced spindle torque. Also, the blade stayed in position if the hydraulic system failed. Improved hub bolt stress, vibration and cavitation. The blades are skewed forward and aft. The peculiar looking blades work successfully. Every manufacturer in the world is using their technology, which was not patented by the Carderock Division. Then the Navy decided they could use controllable pitch propellers (CPP).

For ships which normally operate at widely varying speeds and propeller loadings (towboats, rescue vessels, trawlers, and ferryboats), the application of controllable-pitch (rotatable-blade) propellers permits the use of full engine power at rated rpm under all operational conditions, ensuring maximum thrust production, utmost flexibility, and maneuverability. Since these propellers are also reversible, they permit the use of nonreversible machinery (gas turbines). The hydraulic or electric servomotor for adjusting the pitch of the blades requires a hollow tailshaft for its operation. The propeller pitch can be directly controlled from the ship's bridge. In each case the operational advantages of the controllable-pitch screw must be weighed against the disadvantages of more complex construction and higher manufacturing cost.

The first Navy use resulted in a problem -- the Barbey failure. The Barbey was a DE-1052, equipped with an experimental propeller. Once at sea, all of the blades fell off the propeller during maneuvers. The Barbey captain was rather unhappy and so was the US Navy. It had been decided that the DD-963 would have a controlled pitch propeller and be gas turbine driven. The Navy had planned for this whole class of ship to be equipped with this propeller and it had failed. Stress analysis confirmed that the DD-963 (designed by Kamawa and Bird Johnson) would have the same problem.

A consortium of hydro, structures, materials and acoustics experts was established at DTMB and fixed the problem. The problem was that the horsepower in the turbine had been increased. In turn, to increase the propeller's strength, the crank ring was heat treated, which caused susceptibility to cracks. There were a lot of unhappy admirals over this propeller incident, because restrictions had to be placed on the DD-963 Class. Because of this incident, Rickover decided the Carderock Division should be the US Navy's design agent for propellers. Results of the research effort affected all CPPs worldwide.

Prior to the mid 1960s, simple Beam Theory was used to determine the strength of blades. Development of highly-skewed propellers required better stress analysis, particularly in backing. At about the same time, the Carderock Division Structures Directorate was developing a finite element computer program for stress analysis of propeller blades. From this point forward, Structures began to play a regular role in the development of propellers. Shen-Eppler Blade sections were designed for viscous flow and to operate in a three-dimensional wake (1979). This led to a joint program with Dutch and the successful propeller DDG-51 ATD.




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Page last modified: 07-07-2011 12:57:43 ZULU