27. State concept about direct driving mode, advantage & disadvantage?

a. Feature

-There is not any intermediate equipment between the main engine and the propeller.

-Revolution of the propeller is the same  main engine revolution.

-It is commonly applied on the large and medium-sized ships.

-In a direct driving propulsion plant, the main engine is a kind of low-speed, large power, direct reversing diesel engine.

The most common is low-speed; two- strokes cycle diesel engine with crosshead.

b. Advantages

-Driving efficiency is high (97 - 98%)

-Main engine has low specific fuel consumption (SFC= 120 g/hp.h)

-The propeller efficiency is high

-Its structure is simple and easy to maintain.

-It is safe in working, reduces noise and vibration and has high longevity.

c. Disadvantages

-The main engine has large dimensions and weight so dimensions and weight of the propulsion plant are increased, therefore transportation ability of the ship is reduced.

-The direct driving mode is not suitable for ships with small displacement, or height of the engine room is limited.

- To reverse rotating direction of the propeller, the main engine must be reversed so it is not flexible.

Application

-The direct driving mode is suitable for tanker ships,  general cargo ships or bulk carriers.