44Controllablepitch propellers

Controllable-pitch propellers (CPP) are often used in practice. They feature the following advantages and disadvantages:

+ Fast stop manoeuvres are possible.

+ The main engine does not need to be reversible.

+ CPPs allow the main generator to be driven from the main engine which is efficient and cheap. Thus electricity can be generated with the efficiency of the main engine and using heavy fuel. Variable ship speeds can be obtained with constant propeller rpm as required by the generator.

— Fuel consumption is higher. The higher propeller rpm at lower speed is hydrodynamically suboptimal. CPPs require a thicker hub (0.3-0.32D). The pitch distribution is suboptimal. The usual almost constant pitch in the radial direction causes negative angles of attack at the outer radii at reduced pitch, thus slowing the ship down. Therefore CPPs usually have higher pitch at the outer radii and lower pitch at the inner radii. The higher pitch at the outer radii necessitates a larger propeller clearance.

— Higher costs for propeller.

The blades are mounted in either pivot or disc bearings. The pitch-control mechanism is usually controlled by oil pressure or, more rarely, pneumatically. CPPs may have three, four or five blades.

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