28. State concept about indirect driving mode, advantage & disadvantage?

a. Feature

-There is intermediate equipment between the main engine and the propeller

-Revolution of the main engine is much higher than the optimum range of the propeller revolution (in some cases, may be the same).         

-Transmission ratio may be constant (one-stage reduction gear) or variable (multi-stage reduction gear).

-In fact, the multi-stage reduction gear is generally used (normally two stage for ahead and one stage for astern).

-Main engine is usually a high or medium-speed, four-stroke cycle, non-reversible diesel engine. In some special case such as LNG or air craft carrier, steam turbine is used.

b. Advantages

-Dimension and weight of the propulsion plant are small

-The same type of diesel engine can be used for many kinds of ships that have different propeller revolution rates.

-It is not necessary to directly reverse the main engine for reversing direction of the ship

-Two or more main engines can be used to drive only one propeller,

c. Disadvantages

-Transmission efficiency is lower than the direct driving.

-It is complex and difficult to operate, maintain and repair.

-Longevity of engine is shorter than direct driving engine.

Application

-The indirect driving mode is more suitable for the ship which requires high flexibility such as  naval ships, passenger ships and container ships.