Patent Publication Number: US-2018029682-A1

Title: Ship propulsion system, ship, and ship propulsion method

Description:
TECHNICAL FIELD 
     The present invention relates to a ship propulsion system, a ship, and a ship propulsion method. 
     This application claims priority based on Japanese Patent Application No. 2015-031546 filed in Japan on Feb. 20, 2015, the content of which is hereby incorporated by reference. 
     BACKGROUND ART 
     In the related art, PTL 1 discloses a ship propulsion system that selects one of electric power generated by a shaft generator driven by a main engine (engine) for driving a main propeller in the ship that is provided with a contra rotating propeller system (CRP) and electric power generated by a main generator for supplying the electric power for use in a ship and supplies the selected electric power to a motor for driving a stern-side propeller. 
     When this ship propulsion system is used, the electric power that is generated by the shaft generator which is driven by the high-fuel economy main engine is supplied to the motor for the stern-side propeller during normal navigation. When the speed of the ship is required to be raised due to a delay in ship operation schedule or the like, this ship propulsion system does not use the shaft generator, the main engine is used only for the driving of the main propeller, and the stern-side propeller is driven by the electric power generated by the main generator. Accordingly, power serving propulsion can be increased and the speed of the ship can be raised. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] PCT Japanese Translation Patent Publication No. 2014-505621 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the case of this ship propulsion system, however, an increase in the speed of the ship that is assumed during the design of the ship entails an increase in the size of the main engine and an increase in initial cost. The larger the main engine, the wider the space it occupies in an engine room. This, in turn, results in a narrow maintenance space. Once the engine room is expanded for a maintenance space to be ensured, the engine room is flooded to a larger extent in the event of damage. In this case, the width of the ship should be increased for restoration performance to be ensured, and this exacerbates its propulsion performance and fuel economy. 
     The present invention provides a ship propulsion system, a ship, and a ship propulsion method for addressing the problems described above. 
     Solution to Problem 
     According to a first aspect of the present invention, a ship propulsion system includes a main generator supplying electric power into a ship, an electric power distribution unit distributing the electric power from the main generator, a first electric motor allowing a first rotary shaft to be driven to rotate by the electric power input via the electric power distribution unit, a main propeller rotating with the first rotary shaft, a second electric motor allowing a second rotary shaft to be driven to rotate by the electric power input via the electric power distribution unit, and a stern-side propeller placed on a stern side of the main propeller and rotating with the second rotary shaft. 
     According to a second aspect of the present invention, the ship propulsion system further includes a main engine allowing the first rotary shaft to be driven to rotate, in which the first electric motor compensates for a shortfall in the rotation of the main propeller attributable to the main engine by allowing the first rotary shaft to be driven to rotate. 
     According to a third aspect of the present invention, the first electric motor has a generator generating electric power from the rotation of the first rotary shaft attributable to output of the main engine and supplying the electric power to the electric power distribution unit, and the electric power distribution unit supplies the second electric motor with the electric power supplied by the first electric motor. 
     According to a fourth aspect of the present invention, the ship propulsion system further includes a control unit switching operations of the first electric motor, and the control unit switches between allowing the first rotary shaft to be driven to rotate by the first electric motor in response to thrust required for the ship and supplying electric power to the electric power distribution unit by generating the electric power from the rotation of the main engine. 
     In a fifth aspect of the present invention, a plurality of the main engines with regard to the single main propeller and a decelerator connected to the plurality of main engines are provided. 
     According to a sixth aspect of the present invention, the main propeller is a controllable pitch propeller and the first electric motor is driven to rotate at a constant speed of rotation. 
     According to a seventh aspect of the present invention, the main propeller is a fixed pitch propeller. 
     In an eighth aspect of the present invention, the single main propeller or a plurality of the main propellers and the single stern-side propeller placed to face each of the main propellers are provided. 
     In a ninth aspect of the present invention, two units of the stern-side propeller axisymmetrically placed, about a rotational axis of the main propeller as an axis of symmetry, with respect to the single main propeller are provided. 
     According to a tenth aspect of the present invention, a ship includes the ship propulsion system according to any one of the first to ninth aspects described above. 
     According to an eleventh aspect of the present invention, a ship propulsion method includes allowing electric power to be supplied from a main generator supplying the electric power into a ship, allowing the electric power from the main generator to be distributed by an electric power distribution unit, allowing a first rotary shaft to be driven to rotate by a first electric motor by the electric power input via the electric power distribution unit, allowing a main propeller to be rotated with the first rotary shaft, allowing a second rotary shaft to be driven to rotate by a second electric motor by the electric power input via the electric power distribution unit, and allowing a stern-side propeller placed on a stern side of the main propeller to be rotated with the second rotary shaft. 
     Advantageous Effects of Invention 
     According to the ship propulsion system, ship, and ship propulsion method described above, a high-performance ship propulsion system can be obtained that is capable of preventing an increase in the size of a main engine. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a first diagram illustrating an example of a ship propulsion system according to a first embodiment of the present invention. 
         FIG. 2  is a second diagram illustrating an example of the ship propulsion system according to the first embodiment of the present invention. 
         FIG. 3  is a third diagram illustrating an example of the ship propulsion system according to the first embodiment of the present invention. 
         FIG. 4  is a fourth diagram illustrating an example of the ship propulsion system according to the first embodiment of the present invention. 
         FIG. 5  is a fifth diagram illustrating an example of the ship propulsion system according to the first embodiment of the present invention. 
         FIG. 6  is a sixth diagram illustrating an example of the ship propulsion system according to the first embodiment of the present invention. 
         FIG. 7  is a seventh diagram illustrating an example of the ship propulsion system according to the first embodiment of the present invention. 
         FIG. 8  is a diagram illustrating an example of a ship propulsion system according to a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Hereinafter, a ship propulsion system according to a first embodiment of the present invention will be described with reference to  FIGS. 1 to 7 . 
       FIG. 1  is a first diagram illustrating an example of the ship propulsion system according to the first embodiment of the present invention. 
     A ship propulsion system  1  is a ship propulsion system that is provided in a cargo ship, a passenger boat such as a ferry, or the like. 
     As illustrated in  FIG. 1 , the ship propulsion system  1  is provided with a main generator  10 A, a main generator  10 B, a main generator  10 C, a switchboard  11 , a transformer  12 , an inverter  13 , a pod propeller  14 , a motor  15 , a stern-side propeller  16 , a main propulsion engine  20 , a shaft generator motor  21 , a thyristor inverter  22 , a synchronous governor  23 , and a control device  30 . The main generators  10 A to  10 C are collectively referred to as a main generator  10 . 
     The main generators  10 A,  10 B, and  10 C are generators installed in the ship and supply electric power to facilities in the ship. The main generators  10 A to  10 C are connected to the switchboard  11 . 
     The switchboard  11  is connected to the main generator  10 A, the main generator  10 B, the main generator  10 C, the transformer  12 , the thyristor inverter  22 , and equipment (not illustrated) in the ship. The switchboard  11  performs opening and closing of an electric circuit and switching of electrical systems between the main generators  10 A to  10 C and the transformer  12 , the thyristor inverter  22 , and the equipment (not illustrated) in the ship. For example, the switchboard  11  distributes the electric power generated by the main generators  10 A to  10 C to the electrical systems supplying the electric power to the transformer  12  and the equipment in the ship. Under a predetermined condition, the switchboard  11  supplies the electric power to the thyristor inverter  22  as well as the transformer  12  and the equipment in the ship. Under a predetermined condition, the switchboard  11  supplies electric power generated by the shaft generator motor  21  (described later) to the transformer  12  and the equipment in the ship. 
     The transformer  12  steps down the voltage of the electric power output from the switchboard  11  to a predetermined voltage so that it can be supplied to the inverter  13 . 
     The inverter  13  controls the voltage and frequency output by the transformer  12  at a desired voltage and a desired frequency so that the rotation speed of the stern-side propeller  16  reaches a desired rotation speed. 
     The pod propeller  14  is a cocoon-shaped propulsion device placed behind a main propeller  24 . The pod propeller  14  is configured to be provided with the motor  15  and the stern-side propeller  16 . 
     The motor  15  is an electric motor that is built into the pod propeller  14 . The motor  15  is driven to rotate by the electric power that is controlled by the inverter  13 . 
     The stern-side propeller  16  is coaxially connected to a rotary shaft of the motor  15  via a shaft  17  and rotates as a result of the driving of the motor  15 . The stern-side propeller  16  is disposed on a stern side of the main propeller  24 . The stern-side propeller  16  adds to thrust from the main propeller  24  or is used as a side thruster when the ship comes alongside a pier or moves away from it. 
     The main propulsion engine  20  is, for example, a diesel engine. The main propulsion engine  20  is connected to the main propeller  24  via a shaft  27 . The main propulsion engine  20  rotates the main propeller  24  by allowing the shaft  27  to be driven to rotate. A gas turbine, a steam turbine, a nuclear turbine, an electric motor, or the like can be applied to the main propulsion engine  20  as a device for allowing the shaft  27  to be driven to rotate. 
     The shaft generator motor  21  is a device that is provided with a generator which generates electric power from the rotation of the shaft  27  resulting from output of the main propulsion engine  20  and a motor which is driven to rotate by the electric power generated by the main generator  10 . The shaft generator motor  21  is disposed on the shaft  27 . The shaft generator motor  21  may be placed on a stern side of the main propulsion engine  20  or may be placed on its bow side as well. A mode in which the shaft generator motor  21  is operated as a generator is called an electric power generation mode, and a mode in which the shaft generator motor  21  is operated as a motor is called an electric mode. The control device  30  switches the operating modes of the shaft generator motor  21  between the electric power generation mode and the electric mode in response to, for example, the thrust that is required for the ship. 
     A first gear, for example, is coaxially disposed on a rotary shaft of the shaft generator motor  21  with a second gear coaxially disposed on the shaft  27 . The first gear is fitted with the second gear and the shaft generator motor  21  and the shaft  27  are connected to each other via these gears. When the shaft generator motor  21  is operated in the electric mode, the shaft generator motor  21  is driven to rotate by the electric power from the main generator  10 , this rotation is transmitted to the second gear via the first gear, and the shaft  27  is rotated. In the case of the electric mode, not only rotational power from the main propulsion engine  20  but also rotational power from the shaft generator motor  21  is input to the shaft  27  and the rotational power from the main propulsion engine  20  and the rotational power from the shaft generator motor  21  rotate the main propeller  24  via the shaft  27 . When the shaft generator motor  21  is operated in the electric mode, the power from the shaft generator motor  21  assists in the rotation of the main propeller  24  as described above. 
     When the shaft generator motor  21  is operated in the electric power generation mode, the shaft  27  is driven to rotate by the main propulsion engine  20 , this rotation is transmitted to the first gear via the second gear, and the rotary shaft of the shaft generator motor  21  is rotated. When the shaft generator motor  21  is operated in the electric power generation mode, the shaft generator motor  21  generates electric power from the rotation of the shaft  27  as described above. 
     The thyristor inverter  22  is a constant frequency device for suppressing fluctuations in the frequency of the electric power that is generated by the shaft generator motor  21  or the electric power that is output from the switchboard  11  and stabilizing it. 
     The synchronous governor  23  is a device that controls the voltage and frequency of the electric power that is generated by the shaft generator motor  21  or the voltage and frequency of the electric power that is output from the switchboard  11 . The thyristor inverter  22  and the synchronous governor  23  control the electric power generated by the shaft generator motor  21  and output it to the switchboard  11 . The thyristor inverter  22  and the synchronous governor  23  control the electric power output from the switchboard  11  and output it to the shaft generator motor  21 . 
     The main propeller  24  is connected to the main propulsion engine  20  via the shaft  27 , is rotated by the shaft  27  being driven to rotate, and generates most of the thrust that is required for the ship to travel. When the shaft generator motor  21  is operated in the electric mode, the main propeller  24  is rotated by power assisted by not only the output of the main propulsion engine  20  but also the output of the shaft generator motor  21 . According to  FIG. 1 , the main propeller  24  and the stern-side propeller  16  are provided to coaxially face each other and form a set of contra rotating propeller system (CRP). 
     The ship propulsion system  1  is provided with the main generator (main generators  10 A,  10 B, and  10 C) supplying the electric power into the ship, the electric power distribution unit (switchboard  11 ) distributing the electric power of the main generator, the first electric motor (shaft generator motor  21 ) allowing the first rotary shaft (shaft  27 ) to be driven to rotate by the electric power input via the electric power distribution unit, the main propeller (main propeller  24 ) rotating with the first rotary shaft, the second electric motor (motor  15 ) allowing the second rotary shaft (shaft  17 ) to be driven to rotate by the electric power input via the electric power distribution unit, the stern-side propeller (stern-side propeller  16 ) placed on the stern side of the main propeller and rotating with the second rotary shaft, and the main engine (main propulsion engine  20 ) allowing the first rotary shaft to be driven to rotate. The first electric motor compensates for a shortfall in the rotation of the main propeller by the main engine by allowing the first rotary shaft to be driven to rotate. The control unit (control device  30 ) switches between allowing the first rotary shaft to be driven to rotate by the first electric motor in response to the thrust required for the ship and supplying electric power to the electric power distribution unit by generating the electric power from the rotation of the main engine. 
     Hereinafter, an example of the operation of the ship propulsion system  1  during ship propulsion will be described. 
     Operation Example 1 
     The main generators  10 A to  10 C output the generated electric power to the switchboard  11 , and the switchboard outputs some of the input electric power to the equipment in the ship as intra-ship electric power. The switchboard  11  outputs some of the input electric power to the transformer  12 . This electric power is supplied to the motor  15  through the transformer  12  and the inverter  13  and is used as electric power for driving the stern-side propeller  16 . The main propulsion engine  20 , in the meantime, rotates the main propeller  24  via the shaft  27 . In operation example 1, the ship is propelled by the main propeller  24  being rotated by the output of the main propulsion engine  20  and the stern-side propeller  16  being rotated by the electric power supplied from the main generators  10 A to  10 C as described above. 
     Operation Example 2 
     Another example of the operation of the ship propulsion system  1  during ship propulsion will be described below. As in the case of operation example 1, the electric power from the main generators  10 A to  10 C is used for the rotation of the stern-side propeller  16  and the equipment in the ship. The control device  30  operates the shaft generator motor  21  in the electric power generation mode on condition that, for example, the speed of the ship is equal to or less than a predetermined speed. As a result, the power from the main propulsion engine  20  is used for rotating not only the main propeller  24  but also the shaft generator motor  21 . The shaft generator motor  21  is rotated with the shaft  27  by the output of the main propulsion engine  20  and generates electric power. The shaft generator motor  21  outputs the generated electric power to the switchboard  11  via the thyristor inverter  22 . The control device  30  controls the switchboard  11  and switches the electrical systems for the electric power generated by the shaft generator motor  21  to be supplied to the transformer  12 . The switchboard  11  outputs the electric power input from the shaft generator motor  21  to the transformer  12 . The electric power input to the transformer  12  is output to the motor  15  via the inverter  13  and is used for the rotation of the stern-side propeller  16 . In operation example 2, the electric power generated by the shaft generator motor  21  is used for the rotation of the stern-side propeller  16  as described above. As a result, the electric power generated by the main generators  10 A to  10 C can be preferentially allotted to the other equipment in the ship. Alternatively, energy can be saved by the electric power generated by the main generators  10 A to  10 C being reduced. The electric power generated by the shaft generator motor  21  may be used as electric power for the equipment in the ship. 
     This operation example 2 can be used in a scene where, for example, the ship does not require a large amount of thrust (a high speed is not required for the ship) and the output of the main propulsion engine  20  is enough for the rotation of the main propeller  24 . In the case of a ship propulsion system that does not have the shaft generator motor  21 , the main generators  10 A to  10 C need to be large or a larger number of the main generators  10  need to be provided in case the intra-ship electric power increases. In general, a high level of fuel economy is achieved when the main propulsion engine  20  has a high level of operation efficiency and the output of the main propulsion engine  20  is preferentially used. When it comes to the main generators  10 A to  10 C, in the meantime, power management is performed in many cases for control to reach the optimal number of units in operation in response to load situations. According to operation example 2 of the present embodiment, electric power generation by the shaft generator motor  21  can be performed by the use of the output of the main propulsion engine  20  and this electric power can be supplied, and thus an increase in the capacity of the main generator  10  can be prevented and initial costs can be reduced. In addition, energy can be saved by the number of the main generators  10  in operation being reduced. 
     Operation Example 3 
     Yet another example of the operation of the ship propulsion system  1  during ship propulsion will be described below. As is the case with operation examples 1 and 2, the electric power from the main generators  10 A to  10 C is used for the rotation of the stern-side propeller  16  and the equipment in the ship. The output of the main propulsion engine  20  is used for the rotation of the main propeller  24 . In addition, in this operation example 3, the control device  30  controls the switchboard  11  and performs electric power system switching for the electric power generated by the main generators  10 A to  10 C to be output to the thyristor inverter  22  on condition that, for example, the speed of the ship exceeds a predetermined speed. The electric power generated by the main generators  10 A to  10 C is supplied to the shaft generator motor  21  via the thyristor inverter  22  and the synchronous governor  23 . The control device  30  operates the shaft generator motor  21  in the electric mode. By the shaft generator motor  21  being operated as the motor, the shaft generator motor  21  is driven to rotate by some of the electric power generated by the main generators  10 A to  10 C being used as its power source. The rotational power of the shaft generator motor  21  assists in the rotation of the main propeller  24  attributable to the output of the main propulsion engine  20 . In other words, in operation example 3, some of the electric power generated by the main generators  10 A to  10 C is used for assisting in the propulsion of the main propeller  24  by the shaft generator being used as the motor. 
     This operation example 3 can be used in a scene where, for example, the ship requires a large amount of thrust and a high speed is required for the ship. In the case of a ship propulsion system that does not have the shaft generator motor  21 , the main propulsion engine  20  that is provided for it needs to be large for the speed of the ship to be ensured. According to operation example 3 of the present embodiment, however, a shortfall in the rotational power of the main propulsion engine  20  can be compensated for by the rotational power of the shaft generator motor  21  attributable to the electric power generated by the main generators  10 A to  10 C and used for assisting in the propulsion of the main propeller  24 . Accordingly, a large main propulsion engine  20  assuming an increase in the speed of the ship does not have to be provided, and thus initial costs can be reduced. By adding to an output for regular use (such as an output equivalent to 80% of a rated output), the main propulsion engine  20  is capable of yielding a high propulsion output with the output for regular use as it is. Accordingly, the load on the main propulsion engine  20  can be reduced and its exhaustion can be prevented, which leads to maintenance cost reduction. In a case where the main propulsion engine  20  has a trouble, the ship can be propelled by the main propeller  24  being rotated as a result of the operation of the shaft generator motor  21  in the electric mode. 
     The ship propulsion system  1  according to the present embodiment can have the following configurations as illustrated in  FIGS. 2 to 7 . 
     (Single-Engine Single-Shaft CRP (FPP)) 
       FIG. 2  is a second diagram illustrating an example of the ship propulsion system according to the first embodiment of the present invention. 
     Illustrated in  FIG. 2  is a ship propulsion system  2  that is configured to have one main propulsion engine and one main propeller. The ship propulsion system  2  illustrated in  FIG. 2  is a contra rotating propeller-type propulsion system, in which its stern-side propeller and main propeller are disposed to face each other at coaxial positions in proximity to each other. 
     As illustrated in  FIG. 2 , the ship propulsion system  2  is provided with the main generators  10 A,  10 B, and  10 C, the switchboard  11 , the transformer  12 , the inverter  13 , the motor  15 , an FPP  161 , a low-speed diesel engine  201 , the shaft generator motor  21 , the thyristor inverter  22 , the synchronous governor  23 , and the main propeller  24 . 
     In the ship propulsion system  2  illustrated in  FIG. 2 , the motor  15  is disposed not in the pod propeller but in the ship. The motor  15  transmits a rotation operation to the shaft  17  by means of a mechanical transmission mechanism (not illustrated) and rotates the FPP  161 . The ship propulsion system  2  illustrated in  FIG. 2  is provided with the fixed pitch propeller (FPP)  161  as an example of the stern-side propeller  16  and the low-speed diesel engine  201 , which has a relatively low speed of rotation at a time of rate operation, as an example of the main propulsion engine  20 . The rest of the configuration and operation (operation example 1 to operation example 3) of the ship propulsion system  2  illustrated in  FIG. 2  is similar to the case of  FIG. 1 . 
     (Single-Engine Single-Shaft CRP (CPP)) 
       FIG. 3  is a third diagram illustrating an example of the ship propulsion system according to the first embodiment of the present invention. 
     A ship propulsion system  3  illustrated in  FIG. 3  is a contra rotating propeller-type propulsion system that is configured to have one main propulsion engine and one main propeller as is the case with  FIG. 2 . 
     As illustrated in  FIG. 3 , the ship propulsion system  3  is provided with the main generators  10 A,  10 B, and  10 C, the switchboard  11 , the motor  15 , a CPP  162 , the low-speed diesel engine  201 , the shaft generator motor  21 , the thyristor inverter  22 , the synchronous governor  23 , and the main propeller  24 . 
     In the ship propulsion system  3  illustrated in  FIG. 3 , the motor  15  is disposed in the ship and the rotation operation is transmitted to the shaft  17  by the mechanical transmission mechanism (not illustrated) as is the case with the ship propulsion system  2  illustrated in  FIG. 2 . The ship propulsion system  3  illustrated in  FIG. 3  is provided with the controllable pitch propeller (CPP)  162  as an example of the stern-side propeller  16  and the low-speed diesel engine  201  as an example of the main propulsion engine  20 . The CPP  162  is capable of obtaining thrust required for a change in propeller pitch (blade angle) with a rotation speed remaining constant whereas the FPP  161  obtains thrust required for rotation speed control. Accordingly, in the case of the configuration illustrated in  FIG. 3 , the transformer  12  and the inverter  13  required for rotation speed control are not provided unlike in  FIG. 2 , in which the FPP is provided. The rest of the configuration and operation of the ship propulsion system  3  illustrated in  FIG. 3  is similar to the case of  FIG. 2 . 
     (Double-Engine Single-Shaft CRP (FPP)) 
       FIG. 4  is a fourth diagram illustrating an example of the ship propulsion system according to the first embodiment of the present invention. 
     Illustrated in  FIG. 4  is a ship propulsion system  4  that is configured to have two main propulsion engines and one main propeller. In the case of the configuration illustrated in  FIG. 4 , two stern-side propellers are axisymmetrically placed, about the rotational axis of the main propeller as an axis of symmetry, with respect to the single main propeller. 
     As illustrated in  FIG. 4 , the ship propulsion system  4  is provided with the main generators  10 A,  10 B, and  10 C, the switchboard  11 , a transformer  12 A, a transformer  12 B, an inverter  13 A, an inverter  13 B, a motor  15 A, a motor  15 B, an FPP  161 A, an FPP  161 B, a medium-speed diesel engine  202 A, a medium-speed diesel engine  202 B, the shaft generator motor  21 , the thyristor inverter  22 , the synchronous governor  23 , the main propeller  24 , and a decelerator  25 . 
     The switchboard  11  is connected to the main generators  10 A to  10 C, the transformer  12 A, the transformer  12 B, and the thyristor inverter  22 . The transformer  12 A is connected to the inverter  13 A, and the inverter  13 A is connected to the motor  15 A. The motor  15 A is connected to a shaft  17 A via a mechanical transmission mechanism. Likewise, the transformer  12 B is connected to the inverter  13 B and the inverter  13 B is connected to the motor  15 B. The motor  15 B is connected to a shaft  17 B via a mechanical transmission mechanism. 
     The decelerator  25  is connected to the medium-speed diesel engine  202 A and the medium-speed diesel engine  202 B. The decelerator  25  is connected to the main propeller  24  via the shaft  27 . The medium speed of the medium-speed diesel engine  202  shows that its speed of rotation at a time of rated operation is moderate, and the decelerator rotates the main propeller  24  with the speeds of rotation of the medium-speed diesel engine  202 A and the medium-speed diesel engine  202 B reduced. The shaft generator motor  21  is disposed on the shaft  27 . The shaft generator motor  21  is connected to the thyristor inverter  22 . The thyristor inverter  22  is connected to the synchronous governor  23 . 
     The ship propulsion system  4  illustrated in  FIG. 4  is provided with the two FPPs  161 A and  161 B as examples of the stern-side propeller  16  and the two medium-speed diesel engines  202 A and  202 B as examples of the main propulsion engine  20 . 
     Hereinafter, the transformer  12 A and the transformer  12 B will be collectively referred to as the transformer  12 . Likewise, the inverter  13 A and the inverter  13 B will be collectively referred to as the inverter  13 . The motor  15 A and the motor  15 B will be collectively referred to as the motor  15 . The FPP  161 A and the FPP  161 B will be collectively referred to as the FPP  161 . The medium-speed diesel engine  202 A and the medium-speed diesel engine  202 B will be collectively referred to as the medium-speed diesel engine  202 . 
     Description of the above-described operation examples 1 to 3 in the ship propulsion system  4  illustrated in  FIG. 4  will be conducted below. In operation example 1, the electric power generated by the main generators  10 A to  10 C is supplied to the transformer  12 A and the transformer  12 B via the switchboard  11 . In addition, the transformer  12 A outputs the input electric power to the inverter  13 A after converting the voltage of the input electric power. The inverter  13 A outputs the input electric power to the motor  15 A after controlling the frequency of the input electric power. The motor  15 A rotates the FPP  161 A via the shaft  17 A. Likewise, the transformer  12 B outputs the input electric power to the inverter  13 B after converting the voltage of the input electric power. The inverter  13 B outputs the input electric power to the motor  15 B after controlling the frequency of the input electric power. The motor  15 B rotates the FPP  161 B via the shaft  17 B. The decelerator  25  reduces the speed of rotation by inputting the output of the medium-speed diesel engine  202 A and the output of the medium-speed diesel engine  202 B and rotates the main propeller  24  via the shaft  27 . 
     In the case of operation example 2, the control device  30  operates the shaft generator motor  21  in the electric power generation mode. The shaft generator motor  21  outputs the generated electric power to the thyristor inverter  22 , and the thyristor inverter  22  outputs it to the switchboard  11 . The synchronous governor  23  controls the voltage and frequency of the electric power that the thyristor inverter  22  outputs to the switchboard  11 . The control device  30  switches the electrical systems of the switchboard  11  as described with reference to  FIG. 1 . The switchboard  11  outputs the electric power input from the thyristor inverter  22  to the transformer  12 A and the transformer  12 B. The electric power output to the transformer  12 A and the electric power output to the transformer  12 B are used as power sources for the FPP  161 A and the FPP  161 B, respectively. Alternatively, the switchboard  11  may use the electric power input from the thyristor inverter  22  as electric power for the facilities in the ship. As a result, the electric power generation capacity of the main generators  10 A to  10 C and the number of those in operation can be optimized. 
     In the case of operation example 3, the control device  30  operates the shaft generator motor  21  in the electric mode. The control device  30  switches the electrical systems of the switchboard  11  as described with reference to  FIG. 1 . The switchboard  11  outputs some of the electric power generated by the main generators  10 A to  10 C to the shaft generator motor  21  via the thyristor inverter  22 , and the shaft generator motor  21  assists in the rotation of the main propeller  24  attributable to the main propulsion engine  20  by being driven to rotate. 
     In the case of the ship propulsion system  4  illustrated in  FIG. 4 , the FPP  161 A and the FPP  161 B are, for example, stern-side propellers disposed at azimuth propellers. The azimuth propeller is a propeller that has a function as a rudder as well as a ship propulsion function by the propeller itself rotating. The azimuth propeller can be used as a thruster as well. For example, not only can the azimuth propeller be used as a stern thruster when the ship comes alongside a pier or moves away from it, it also allows the ship to come alongside the pier or move away from it in a smooth manner in a narrow structure by facilitating hydraulic power adjustment in front-back and left-right directions because it is capable of performing a 360-degree turn. The ship propulsion system  4  illustrated in  FIG. 4  is provided with two azimuth propellers along with the two medium-speed diesel engines  202 , and thus it is capable of ensuring redundancy. 
     (Double-Engine Single-Shaft CRP (CPP)) 
       FIG. 5  is a fifth diagram illustrating an example of the ship propulsion system according to the first embodiment of the present invention. 
     Illustrated in  FIG. 5  is a ship propulsion system  5  that is configured to have two main propulsion engines and one main propeller as in  FIG. 4 . 
     As illustrated in  FIG. 5 , the ship propulsion system  5  is provided with the main generators  10 A,  10 B, and  10 C, the switchboard  11 , the motor  15 A, the motor  15 B, a CPP  162 A, a CPP  162 B, the medium-speed diesel engine  202 A, the medium-speed diesel engine  202 B, the shaft generator motor  21 , the thyristor inverter  22 , the synchronous governor  23 , the main propeller  24 , and the decelerator  25 . The ship propulsion system  5  illustrated in  FIG. 5  is provided with the two CPPs  162 A and  162 B as examples of the stern-side propeller  16  and the two medium-speed diesel engines  202 A and  202 B as examples of the main propulsion engine  20 . The stern-side propeller in the ship propulsion system  5  illustrated in  FIG. 5  is the CPP, and thus it is not provided with the transformer  12  and the inverter  13  unlike in  FIG. 4 . 
     The rest of the configuration and operation of the ship propulsion system  5  illustrated in  FIG. 5  is similar to the case of  FIG. 4 . 
     (Double-Engine Double-Shaft CRP (FPP)) 
       FIG. 6  is a sixth diagram illustrating an example of the ship propulsion system according to the first embodiment of the present invention. 
     Illustrated in  FIG. 6  is a ship propulsion system  6  that is configured to have two main propulsion engines and two main propellers. 
     As illustrated in  FIG. 6 , the ship propulsion system  6  is provided with the main generators  10 A,  10 B, and  10 C, the switchboard  11 , the transformer  12 A, the transformer  12 B, the inverter  13 A, the inverter  13 B, the motor  15 A, the motor  15 B, the FPP  161 A, the FPP  161 B, the medium-speed diesel engine  202 A, the medium-speed diesel engine  202 B, a shaft generator motor  21 A, a shaft generator motor  21 B, a thyristor inverter  22 A, a thyristor inverter  22 B, a synchronous governor  23 A, a synchronous governor  23 B, a main propeller  24 A, and a main propeller  24 B. The ship propulsion system  6  illustrated in  FIG. 6  is provided with the two FPPs  161 A and  161 B as examples of the stern-side propeller  16  and the two medium-speed diesel engines  202 A and  202 B as examples of the main propulsion engine  20 . 
     The switchboard  11  is connected to the main generators  10 A to  10 C, the transformers  12 A and  12 B, and the thyristor inverters  22 A and  22 B. The transformer  12 , the inverter  13 , the motor  15 , and the FPP  161  have a connection relationship similar to that illustrated in  FIG. 4 . The medium-speed diesel engine  202 A is connected to the main propeller  24 A via a shaft  27 A. The shaft generator motor  21 A is disposed on the shaft  27 A. The shaft generator motor  21 A is connected to the thyristor inverter  22 A. The thyristor inverter  22 A is connected to the synchronous governor  23 A. Likewise, the medium-speed diesel engine  202 B is connected to the main propeller  24 B via a shaft  27 B. The shaft generator motor  21 B is disposed on the shaft  27 B. The shaft generator motor  21 B is connected to the thyristor inverter  22 B. The thyristor inverter  22 B is connected to the synchronous governor  23 B. 
     Hereinafter, the medium-speed diesel engine  202 A and the medium-speed diesel engine  202 B will be collectively referred to as the medium-speed diesel engine  202 . The shaft generator motor  21 A and the shaft generator motor  21 B will be collectively referred to as the shaft generator motor  21 . The thyristor inverter  22 A and the thyristor inverter  22 B will be collectively referred to as the thyristor inverter  22 . The synchronous governor  23 A and the synchronous governor  23 B will be collectively referred to as the synchronous governor  23 . The main propeller  24 A and the main propeller  24 B will be collectively referred to as the main propeller  24 . 
     In a case where the operation according to the above-described operation example 1 is performed in the ship propulsion system  6  illustrated in  FIG. 6 , the electric power that is generated by the main generators  10 A to  10 C is supplied to the transformer  12 A and the transformer  12 B via the switchboard  11 . In addition, the electric power with a voltage converted by the transformer  12 A and the electric power with a voltage converted by the transformer  12 B are supplied to the motor  15 A and the motor  15 B after frequency control by the inverter  13 A and the inverter  13 B, respectively. The motor  15 A rotates the FPP  161 A via the shaft  17 A, and the motor  15 B rotates the FPP  161 B via the shaft  17 B. The medium-speed diesel engine  202 A rotates the main propeller  24 A via the shaft  27 A, and the medium-speed diesel engine  202 B rotates the main propeller  24 B via the shaft  27 B. 
     In the case of operation example 2, the control device  30  operates the shaft generator motor  21 A in the electric power generation mode. The shaft generator motor  21 A generates electric power by being driven to rotate with the shaft  27 A by the output of the medium-speed diesel engine  202 A. The shaft generator motor  21 A outputs the generated electric power to the thyristor inverter  22 A. The thyristor inverter  22 A outputs the input electric power to the switchboard  11 . The synchronous governor  23 A controls the voltage and frequency of the electric power that the thyristor inverter  22 A outputs to the switchboard  11 . Likewise, the shaft generator motor  21 B is operated in the electric power generation mode and outputs the generated electric power to the thyristor inverter  22 B. The thyristor inverter  22 B outputs the input electric power to the switchboard  11 . The control device  30  switches the electrical systems of the switchboard  11  as described with reference to  FIG. 1 . The switchboard  11  outputs the electric power input from the thyristor inverters  22 A and  22 B and some of the electric power input from the main generators  10 A to  10 C to the transformer  12 A and the transformer  12 B and uses them as power sources for the FPP  161 A and the FPP  161 B or the like. 
     In the case of operation example 3, the control device  30  switches the electrical systems of the switchboard  11  as described with reference to  FIG. 1 . The switchboard  11  outputs some of the electric power generated by the main generators  10 A to  10 C to the thyristor inverters  22 A and  22 B. The control device  30  operates the shaft generator motors  21 A and  21 B in the electric mode. The shaft generator motor  21 A assists in the rotation of the main propeller  24 A by being driven to rotate with the electric power input from the thyristor inverter  22 A. Likewise, the shaft generator motor  21 B assists in the rotation of the main propeller  24 B by being driven to rotate with the electric power input from the thyristor inverter  22 B. 
     The FPP  161 A may be placed coaxially with the main propeller  24 A and in proximity thereto, such that one set of contra rotating propeller system is realized, as illustrated in  FIG. 6 . Likewise, the FPP  161 B and the main propeller  24 B may be coaxially placed to constitute one set of contra rotating propeller system. In the case of  FIG. 6 , two sets of propulsion systems are provided based on the double-engine double-shaft configuration, and thus redundancy can be ensured. In addition, application of the ship propulsion system  6  according to the present embodiment allows the electric power generated by the main generator  10  to be used as a power source for the main propeller  24  or allows the electric power generated by the shaft generator motor  21  by the use of the power from the medium-speed diesel engine  202  to be used as a power source for the FPP  161 , and thus power source redundancy can be improved. 
     (Double-Engine Double-Shaft CRP (CPP)) 
       FIG. 7  is a seventh diagram illustrating an example of the ship propulsion system according to the first embodiment of the present invention. 
     Illustrated in  FIG. 7  is a ship propulsion system  7  that is configured to have two main propulsion engines and two main propellers as in  FIG. 6 . 
     As illustrated in  FIG. 7 , the ship propulsion system  7  is provided with the main generators  10 A,  10 B, and  10 C, the switchboard  11 , the motor  15 A, the motor  15 B, the CPP  162 A, the CPP  162 B, the medium-speed diesel engine  202 A, the medium-speed diesel engine  202 B, the shaft generator motor  21 A, the shaft generator motor  21 B, the thyristor inverter  22 A, the thyristor inverter  22 B, the synchronous governor  23 A, the synchronous governor  23 B, the main propeller  24 A, and the main propeller  24 B. The ship propulsion system  7  illustrated in  FIG. 7  is provided with the two CPPs  162 A and  162 B as examples of the stern-side propeller  16  and the two medium-speed diesel engines  202 A and  202 B as examples of the main propulsion engine  20 . In the case of  FIG. 7 , the transformer  12  and the inverter  13  are absent and the switchboard  11  is connected to the motor  15 . The rest of the configuration and operation of the ship propulsion system  7  illustrated in  FIG. 7  is similar to the case of  FIG. 6 . 
     According to the present embodiment, the output of the main propulsion engine  20  can be used as a power source for the stern-side propeller  16  and the main generator  10  can be operated in an energy-saving manner in a case where the main propulsion engine  20  has a surplus output while the main propeller  24  is driven by the use of the high-fuel economy and large-capacity main propulsion engine  20 . When the power source of the main propeller  24  is insufficient, the electric power generated by the main generators  10 A to  10 C can be added to the power source of the main propeller  24 . Even in a scene where the use of the main propulsion engine  20  is limited by a stormy weather, for example, a propulsion loss attributable to the stormy weather is reduced since the main propeller  24  can be rotated by the electric power from the main generators  10 A to  10 C. 
     Since the plurality of main generators  10  and main propulsion engines  20  are provided as the power sources for the main propeller  24  and the stern-side propeller  16 , redundancy can be given and operation can still be performed even if any one of them fails. 
     Low-output navigation with the main propulsion engine  20  is likely to lead to problems in the form of soot accumulation, deterioration in terms of maintainability, and so on. However, since the plurality of main generators  10  is provided, the main generator  10  can be operated in compliance with an optimal output for navigation and at a control point of high fuel efficiency. 
     The configuration in which the motor  15  is disposed in the ship is illustrated as an example in  FIGS. 2 to 7 . However, the motor  15  may be disposed in the pod propeller as in  FIG. 1  instead. The number of the main generators  10  may not be three. 
     Second Embodiment 
     Hereinafter, the control device  30  according to a second embodiment of the present invention will be described with reference to  FIG. 8 . The same reference numerals will be used to refer to similar components of the first embodiment, and detailed description thereof will be omitted. 
       FIG. 8  is a diagram illustrating an example of a ship propulsion system according to the second embodiment of the present invention. 
     As illustrated in  FIG. 8 , a ship propulsion system  8  is provided with the main generators  10 A,  10 B, and  10 C, the switchboard  11 , the transformer  12 , the inverter  13 , the pod propeller  14 , the motor  15 , the stern-side propeller  16 , a main propeller  241 , and a propulsion electric motor  26 . 
     The propulsion electric motor  26  is driven to rotate by the electric power generated by the main generator  10  via the switchboard  11  and rotates the main propeller  241 . The propulsion electric motor  26  is, for example, a motor. 
     The main generators  10 A to  10 C are connected to the switchboard  11 . The switchboard  11  is connected to the main generator  10 A, the main generator  10 B, the main generator  10 C, the transformer  12 , the propulsion electric motor  26 , and the equipment (not illustrated) in the ship. The transformer  12  is connected to the inverter  13 . The inverter  13  is connected to the motor  15  disposed in the pod propeller  14 . The motor  15  is connected to the stern-side propeller  16  via the shaft  17 . The propulsion electric motor  26  is connected to the main propeller  241  via the shaft  27 . In the present embodiment, the motor  15  and the propulsion electric motor  26  are driven to rotate by electric power supply from the main generator  10 . The main propeller  241  is a CPP. 
     The ship propulsion system  8  is provided with the main generator (main generators  10 A to  10 C) supplying the electric power into the ship, the electric power distribution unit (switchboard  11 ) distributing the electric power of the main generator, the first electric motor (propulsion electric motor  26 ) allowing the first rotary shaft (shaft  27 ) to be driven to rotate by the electric power input via the electric power distribution unit, the main propeller (main propeller  241 ) rotating with the first rotary shaft, the second electric motor (motor  15 ) allowing the second rotary shaft (shaft  17 ) to be driven to rotate by the electric power input via the electric power distribution unit, and the stern-side propeller (stern-side propeller  16 ) placed on the stern side of the main propeller and rotating with the second rotary shaft. 
     An operation of the ship propulsion system  8  according to the second embodiment will be described. In the second embodiment, the main generator  10  is the only power source. The control device  30  calculates the electric power that allows the main propeller  241  to rotate, the electric power that allows the stern-side propeller  16  to rotate, and the electric power that is supplied to the facilities in the ship, and controls the output and the number of the main generators  10  in operation. The switchboard  11  supplies each system with the electric power generated by the main generator  10 . The inverter  13  controls the frequency of the electric power input from the transformer  12  and rotates the motor at a desired speed of rotation. The propulsion electric motor  26  is driven to rotate at a constant speed by the electric power input via the switchboard  11 . The propulsion electric motor  26  rotates the main propeller  241  via the shaft  27 . The main propeller  241  is the CPP, and the control device  30  performs control for desired thrust to be obtained through a change in the propeller pitch of the main propeller  241 . 
     According to the present embodiment, a system with less vibration and noise than ship propulsion systems provided with existing main propulsion engines, which include the first embodiment, can be obtained. With propulsion systems based on low-speed diesel engines, for example, it is difficult to meet noise standards stipulated by law or the like. With the present embodiment, however, noise reduction can be achieved. 
     Configurations such as the main propulsion engine  20  and the shaft generator motor  21  can be omitted in the present embodiment, and thus the efficiency of equipment placement in an engine room can be improved along with work efficiency in the engine room. 
     Since the plurality of main generators  10  are provided as power sources, redundancy can be given and operation can still be performed even if any one of the main generators fails. 
     In the second embodiment, the motor  15  may be provided not in the pod propeller  14  but in the hull of the ship as in the first embodiment. The stern-side propeller may be a CPP and the transformer  12  and the inverter  13  may be omitted as well. In addition, two units of the stern-side propeller  16  (FPP or CPP) may be provided with respect to the main propeller  241  as illustrated in  FIG. 4 . Also possible is a double-shaft configuration that is provided with two sets of CRPs, each of which has one set of main propeller  241  and stern-side propeller  16 , as illustrated in  FIG. 6 . The main propeller may be an FPP, an inverter may be disposed between the switchboard  11  and the propulsion electric motor  26 , and the rotation speed of the propulsion electric motor  26  may be controlled by the inverter in the second embodiment as well. 
     Components of the embodiments described above can be appropriately replaced with other known components without departing from the scope of the present invention. The technical scope of the present invention is not limited to the embodiments described above, and various modifications can be added thereto without departing from the scope of the present invention. 
     INDUSTRIAL APPLICABILITY 
     According to the ship propulsion system, ship, and ship propulsion method described above, a high-performance ship propulsion system can be obtained that is capable of preventing an increase in the size of a main engine. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10  Main generator 
               11  Switchboard 
               12  Transformer 
               13  Inverter 
               14  Pod propeller 
               15  Motor 
               16  Stern-side propeller 
               17  Shaft 
               20  Main propulsion engine 
               21  Shaft generator motor 
               22  Thyristor inverter 
               23  Synchronous governor 
               24  Main propeller 
               25  Decelerator 
               26  Propulsion electric motor 
               27  Shaft 
               30  Control device 
               161  FPP 
               162  CPP 
               201  Low-speed diesel engine 
               202  Medium-speed diesel engine