41.  State advantages and disadvantages of indirect driving mode?

Advantages

- Dimension and weight of the propulsion plant are small. It reduces dimensions of the engine room and increases the cargo transportation ability of the ship. At the same power, the weight of the high or medium-speed diesel engine is 40% lighter than a low-speed diesel engine.

- 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. This can be done by the multi-stage reduction gear therefore, the flexibility of the propulsion plant and the longevity of the main engine will be increased.

- Two or more main engines can be used to drive only one propeller, thus increases the flexibility of the propulsion plant and the main engines can be effectively operated in every operating condition (even when light load).

Disadvantages

- Transmission efficiency is lower than the direct driving.

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

- Longevity is shorter.

The indirect driving mode is more suitable for naval ships, passenger ships and container ships.

42.  State features of indirect driving and electric driving mode?

- There is intermediate equipment between the main engine and the propeller to transmit power. In the indirect driving mode, intermediate equipment may be clutch (friction clutch, hydraulic clutch) or clutch and speed reducer (hydraulic speed reducer, reduction gear).

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

Ratio between revolution of the main engine and the propeller is called transmission ratio:

i=n/np        (p be")                        

n: Revolution of the main engine.

np: Revolution of the propeller.

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 for ahead, one for astern).

- In the indirect driving mode, the main engine is usually a high or medium-speed, four-stroke cycle, non-reversible diesel engine.

Electric driving mode:

- There is no mechanical connection between the main engine and the propeller.

- There are two times of energy transformation

44.  State advantages and disadvantages of electric driving mode?

Advantages

- The same as the indirect driving mode, the main engine is usually a high or medium-speed therefore, the dimension and weight of the propulsion plant is small and thus reduces the dimensions of the engine room.

- It is not necessary to reverse the main engine when reversing the direction of the ship. This can be carried out by reversing the direction of the electric motor. Therefore the longevity of the main engine and the flexibility of the propulsion plant will be increased.

- Revolution of the propeller can be changed while the main engine is still working at a constant speed. This can be done by changing revolution of the motor. Therefore the main engine can always be run at the most economic region.

- It is not necessary to have a long shafting and many intermediate bearings, so the main engine can be located in the best suitable position in the engine room.

- When the ship is stably sailing on the sea, the main generator can satisfy a part of the electric energy requirements of the ship.

Disadvantages

- Transmission efficiency is much lower than the two kinds listed above.

- Overload ability of the electric motor is less.

- It is complex, not economic and expensive.

- Longevity is shorter.

Electric driving mode is more suitable for naval ships, fishing ships...

45.  Describe the specification of structure and methods for controlling pitch of propeller?

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.

b. 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

46.  State advantages and disadvantages of driving mode using controllable pitch propeller.

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 the pitch of the propeller.

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

- Flexibility and longevity of the propulsion plant are high.

Disadvantage

- Mechanisms are complex. The quality sealing of the propeller hub is required. Therefore, it is very difficult to manufacture, assemble and repair. It is expensive.

- 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 is reduced.

- Knowledge of the operator is required.

Controllable-pitch propeller driving becomes more suitable for use on naval ships, passenger ships, and workshop ships, fishing ships, rescue ships.

47.  Describe the structure and operational principle of hydraulic driving equipment?

In a hydraulic driving, the main equipment consists of a centrifugal pump and a hydraulic turbine (Figure 3.3).

A centrifugal pump (1) is installed on shaft of the main engine; a hydraulic turbine (2) with its axis is connected to the propeller shaft. When the main engine is working, liquid from tank (6) will fill up the centrifugal pump and supplied to the hydraulic line (3) with high kinetic energy. The liquid is then ejected with high speed into the hydraulic turbine. Therefore, the hydraulic turbine is turned and drives the propeller shaft. Liquid going out of the turbine is returned to the tank and makes a circulating circuit.

Thus mechanical energy of the main engine is transmitted to the propeller by the hydraulic mechanism and working liquid.

48.  State features and applications of hydraulic driving equipment?

- Clutch and unclutch crankshaft of the main engine and the propeller shaft.

- Extend working region of the propulsion plant. The main engine will not be overloaded in rough sea conditions.

- It permits a centering error between the crankshaft and the propeller shaft.

- Reversible hydraulic driving equipment can be used.

- Hydraulic-driving equipment is applied in the propulsion plant as a hydraulic clutch or a hydraulic speed reducer.

49.  Describe the structure and operational principle of the hydraulic clutch?

 Features

- When the hydraulic clutch is working stably, torque on the receiving shaft is not changed.

- Centrifugal pump and hydraulic turbine have the same dimension.

- High transmission efficiency.

1- Centrifugal pump

2- Hydraulic turbine

3- Cover

4- Actuating shaft

5- Receiving shaft

A hydraulic clutch consists of centrifugal pump (1) and hydraulic turbine (2) placed inside a cover (3). The pump shaft is connected to the crankshaft of the main engine. The turbine shaft is connected to the propeller shaft. When the hydraulic clutch is out of liquid, the pump and the turbine have no mechanical connection. The pump is freely turned, and the hydraulic clutch is in idle condition.

When filling up the hydraulic clutch with liquid, the liquid with high kinetic energy is supplied by the pump into the hydraulic turbine thus turbine revolves and drives the propeller shaft. So the torque generated by the main engine is transmitted to the propeller shaft and drives the propeller.

To stop the propeller shaft, the liquid inside the hydraulic clutch is drained out, the pump is freely turned again, and the turbine is stopped.