30. State about controllable pitch propeller:

Controllable-Pitch Propeller Driving

a.Structure

It consists of two main parts:

-Blades can be turned around their axis.

-Mechanism for controlling pitch of the propeller. This mechanism is generally installed inside the propeller hub.

-Methods for controlling pitch of the propeller

-To vary pitch of the propeller, its blades must be turned around their vertical axis. This movement is carried out by hydraulic or mechanical means. Today, there are four common methods to control pitch of the propeller; the illustrations are showed as below.

b. Advantages

-It is not necessary to change the rotating direction and revolution of the main engine when reversing the direction and changing speed of the ship.

-This can be done by controlling  pitch of the propeller.

-The main engine is permitted to operate effectively even in heavy load condition and is protected from overload.

-Flexibility and longevity of the propulsion plant is high.

c. Disadvantages

-Mechanism of pitch control  is complicated.

-The quality of sealing equipment of the propeller hub is required.

-Therefore, it is very difficult to manufacture, assemble and repair.

-Cost of controllable pitch propeller is high.

-It is difficult to operate, especially determining the suitable working point between the main engine and the propeller.

-If the pitch ratio (H/D) and the propeller revolution (np) are selected unsuitably, the result power of the main engine and efficiency of the propulsion plant is reduced.

-Knowledge of the operator is required.

Application

-The using of controllable-pitch propeller driving mode becomes more suitable on naval ships, passenger ships, and workshop ships, fishing ships, rescue ships.

1- Gearing type

2- Turning type

3- Connecting rod type

4- Rack-and-pinion type