Patent Publication Number: US-2023140720-A1

Title: Marine propulsion system and marine vessel

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2021-180206 filed on Nov. 4, 2021. The entire contents of this application are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a marine propulsion system and a marine vessel. 
     2. Description of the Related Art 
     A marine propulsion system including a main propulsion device and an auxiliary propulsion device, both of which are attached to the stern of a hull, is known in general. Such a marine propulsion system is disclosed in Japanese Patent Laid-Open No. 2000-344193, for example. 
     Japanese Patent Laid-Open No. 2000-344193 discloses an automatic return navigation device including a main propulsion device attached to the stern of a hull, an auxiliary propulsion device attached to the stern of the hull, and a controller that performs a control to maintain the hull at a target point specified by a user. When a distance from the hull to the target point exceeds a predetermined distance with the main and auxiliary propulsion devices stopped, the controller drives only the auxiliary propulsion device to move (return) the hull to the target point. The controller stops the auxiliary propulsion device again when the hull returns to the target point. In the control to return the hull to the target point, only the auxiliary propulsion device is driven instead of driving both the main propulsion device and the auxiliary propulsion device. 
     Although not clearly described in Japanese Patent Laid-Open No. 2000-344193, conventionally, there has been known a fixed point holding (Stay Point™) control to maintain the orientation of a bow at a target orientation specified by a user and maintain the position of a hull at a target point specified by the user. When the control to return the hull to the target point described in Japanese Patent Laid-Open No. 2000-344193 is applied to such a fixed point holding control, a control is conceivably performed to drive only the auxiliary propulsion device instead of driving both the main propulsion device and the auxiliary propulsion device. However, in such a case, it is conceivably difficult for only the auxiliary propulsion device to move the hull while maintaining the orientation of the bow required for normal fixed point holding. In recent years, in the field of marine vessels, from the viewpoint of SDGs (Sustainable Development Goals), it is desired to reduce environmental burdens, such as reducing the amount of carbon dioxide emissions associated with driving of propulsion devices of marine vessels. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide marine propulsion systems and marine vessels that each achieve fixed point holding of hulls including main propulsion devices and auxiliary propulsion devices while reducing or minimizing environmental burdens associated with driving of propulsion devices. 
     A marine propulsion system according to a preferred embodiment of the present invention includes a main propulsion device to be attached to a stern of a hull and operable to rotate in a right-left direction to change a direction of a thrust, an auxiliary propulsion device to be attached to the stern, including an electric motor to drive an auxiliary thruster to generate a thrust, operable to rotate in the right-left direction to change a direction of the thrust, and having a maximum output smaller than a maximum output of the main propulsion device, and a controller configured or programmed to perform a fixed point holding control to maintain an orientation of a bow of the hull at a target orientation and maintain a position of the hull at a target point by causing the main propulsion device and the auxiliary propulsion device to cooperate with each other. 
     A marine propulsion system according to a preferred embodiment of the present invention includes the controller configured or programmed to perform the fixed point holding (Stay Point™) control to maintain the orientation of the bow of the hull at the target orientation and maintain the position of the hull at the target point by causing the main propulsion device and the auxiliary propulsion device to cooperate with each other. Accordingly, unlike a case in which only the auxiliary propulsion device is driven in the fixed point holding control, the main propulsion device and the auxiliary propulsion device are caused to cooperate with each other to flexibly rotate and move the hull so as to maintain the orientation of the bow of the hull at the target orientation and maintain the position of the hull at the target point. Furthermore, the auxiliary propulsion device includes the electric motor to drive the auxiliary thruster to generate a thrust. Accordingly, as compared with a case in which the main propulsion device is driven and the auxiliary propulsion device is an engine propulsion device, the amount of carbon dioxide emitted from the auxiliary propulsion device is reduced. Thus, the hull including the main propulsion device and the auxiliary propulsion device is held at a fixed point while environmental burdens associated with driving of the auxiliary propulsion device are reduced or minimized. 
     In a marine propulsion system according to a preferred embodiment of the present invention, the main propulsion device is preferably provided on a centerline of the hull in the right-left direction, and the auxiliary propulsion device is preferably provided to one side of the centerline of the hull in the right-left direction. Accordingly, in a marine vessel including the main propulsion device provided on the centerline of the hull in the right-left direction, and the auxiliary propulsion device provided to one side of the centerline of the hull in the right-left direction, the main propulsion device and the auxiliary propulsion device are caused to cooperate with each other to flexibly rotate and move the hull, and thus the directions and magnitudes of the thrusts of the main propulsion device and the auxiliary propulsion device are adjusted in the fixed point holding control. 
     In a marine propulsion system according to a preferred embodiment of the present invention, the auxiliary propulsion device preferably has a right-left rotatable angle range to change the direction of the thrust larger than a right-left rotatable angle range of the main propulsion device, and the controller is preferably configured or programmed to rotate the hull by driving the auxiliary propulsion device in the fixed point holding control. Accordingly, the hull is rotated (pivot-turned) by the electric motor-driven (electric) auxiliary propulsion device that has the right-left rotatable angle range to change the direction of the thrust larger than the right-left rotatable angle range of the main propulsion device such that a change in the position of the hull becomes smaller. 
     In a marine propulsion system according to a preferred embodiment of the present invention, the main propulsion device preferably includes an engine to drive a main thruster to generate a thrust and having a maximum value and a minimum value of a power range larger than a maximum value and a minimum value of a power range of the electric motor, and the controller is preferably configured or programmed to limit the power range of the engine by matching an upper limit value of the power range of the engine with the maximum value of the power range of the electric motor while the main propulsion device and the auxiliary propulsion device are caused to cooperate with each other to move the hull, and to limit the power range of the electric motor by matching a lower limit value of the power range of the electric motor with the minimum value of the power range of the engine while the main propulsion device and the auxiliary propulsion device are caused to cooperate with each other to move the hull. Accordingly, the power range of the engine and the power range of the electric motor are adjusted to be equivalent or substantially equivalent to each other, and thus when the main propulsion device and the auxiliary propulsion device are caused to cooperate with each other, the output of the engine and the output of the electric motor are prevented from being out of balance. 
     In a marine propulsion system according to a preferred embodiment of the present invention, the controller is preferably configured or programmed to rotate the hull by driving the auxiliary thruster to generate the thrust from the auxiliary propulsion device while a main thruster operable to generate a thrust from the main propulsion device is stopped when the hull is rotated to maintain the orientation of the bow at the target orientation in the fixed point holding control. Accordingly, the hull is rotated only by the electric auxiliary propulsion device, and thus the hull is quietly rotated. Furthermore, a thrust is generated only from the electric motor-driven (electric) auxiliary propulsion device during rotation of the hull, and thus environmental burdens during rotation of the hull are reduced. 
     In such a case, the controller is preferably configured or programmed to rotate the hull about a center of gravity of the hull on the spot while holding the position of the hull. Accordingly, the hull is rotated on the spot without changing the position of the hull, and thus the accuracy of maintaining the target point in the fixed point holding control is improved. 
     In a marine propulsion system according to a preferred embodiment of the present invention, the controller is preferably configured or programmed to, when the hull is moved to maintain the position of the hull at the target point in the fixed point holding control, move the hull laterally or diagonally while maintaining the orientation of the bow by simultaneously driving a main thruster to generate the thrust from the main propulsion device and the auxiliary thruster to generate the thrust from the auxiliary propulsion device, and move the hull in a forward-rearward direction by driving the main thruster while the auxiliary thruster is stopped. Accordingly, the main thruster and the auxiliary thruster are simultaneously driven (caused to cooperate with each other) such that the hull is moved laterally or diagonally while the orientation of the bow is maintained, and the hull is moved in the forward-rearward direction by driving only the main thruster. 
     In such a case, the controller is preferably configured or programmed to move the hull laterally or diagonally while maintaining the orientation of the bow by positioning an intersection of an output vector of the main thruster and an output vector of the auxiliary thruster on a straight line extending through a center of gravity of the hull and the target point and setting, in a direction from the center of gravity to the target point, a direction of a resultant force of the output vector of the main thruster and the output vector of the auxiliary thruster that indicates a moving direction of the hull. Accordingly, even when the main propulsion device and the auxiliary propulsion device are not provided in a well-balanced manner with respect to the centerline of the hull in the right-left direction, the hull is moved laterally or diagonally while the orientation of the bow is maintained. 
     In a marine propulsion system in which the main thruster and the auxiliary thruster are simultaneously driven to move the hull laterally or diagonally while the orientation of the bow is maintained, the controller is preferably configured or programmed to cause a direction of an output vector of the main thruster and a direction of an output vector of the auxiliary thruster to be opposite to each other in the forward-rearward direction when the hull is moved laterally or diagonally while the orientation of the bow is maintained. Accordingly, the direction of the output vector of the main thruster and the direction of the output vector of the auxiliary thruster are opposite to each other, and thus the hull is easily moved laterally or diagonally while the orientation of the bow is maintained. 
     In a marine propulsion system according to a preferred embodiment of the present invention, the controller is preferably configured or programmed to perform a control to rotate the hull to maintain the orientation of the bow at the target orientation and a control to move the hull to maintain the position of the hull at the target point at different timings in the fixed point holding control. Accordingly, rotating the hull to maintain the orientation of the bow at the target orientation and moving the hull to maintain the position of the hull at the target point are separated from each other such that a change in the position of the hull during rotation of the hull is reduced or prevented, and a change in the orientation of the bow during movement of the hull is reduced or prevented. 
     In a marine propulsion system according to a preferred embodiment of the present invention, the main propulsion device is preferably an engine outboard motor including an engine to drive a main propeller to generate a thrust and provided on a centerline of the hull in the right-left direction, and the auxiliary propulsion device is preferably an electric outboard motor including the electric motor to drive an auxiliary propeller corresponding to the auxiliary thruster and provided to one side of the centerline of the hull in the right-left direction. Accordingly, environmental burdens are reduced due to driving of the electric outboard motor, and the hull including the engine outboard motor and the electric outboard motor is held at a fixed point. 
     A marine propulsion system according to a preferred embodiment of the present invention includes a main propulsion device to be attached to a stern of a hull and operable to rotate in a right-left direction to change a direction of a thrust, an auxiliary propulsion device to be attached to the stern, operable to rotate in the right-left direction to change a direction of a thrust, and having a maximum output smaller than a maximum output of the main propulsion device, and a controller configured or programmed to perform a fixed point holding control to maintain an orientation of a bow of the hull at a target orientation and maintain a position of the hull at a target point by causing the main propulsion device and the auxiliary propulsion device to cooperate with each other. 
     A marine propulsion system according to a preferred embodiment of the present invention includes the controller configured or programmed to perform the fixed point holding (Stay Point™) control to maintain the orientation of the bow of the hull at the target orientation and maintain the position of the hull at the target point by causing the main propulsion device and the auxiliary propulsion device to cooperate with each other. Accordingly, unlike a case in which only the auxiliary propulsion device is driven in the fixed point holding control, the main propulsion device and the auxiliary propulsion device are caused to cooperate with each other to flexibly rotate and move the hull so as to maintain the orientation of the bow of the hull at the target orientation and maintain the position of the hull at the target point. Consequently, the hull including the main propulsion device and the auxiliary propulsion device is held at a fixed point. 
     A marine vessel according to a preferred embodiment of the present invention includes a hull, and a marine propulsion system provided on or in the hull. The marine propulsion system includes a main propulsion device attached to a stern of the hull and operable to rotate in a right-left direction to change a direction of a thrust, an auxiliary propulsion device attached to the stern, including an electric motor to drive an auxiliary thruster to generate a thrust, operable to rotate in the right-left direction to change a direction of the thrust, and having a maximum output smaller than a maximum output of the main propulsion device, and a controller configured or programmed to perform a fixed point holding control to maintain an orientation of a bow of the hull at a target orientation and maintain a position of the hull at a target point by causing the main propulsion device and the auxiliary propulsion device to cooperate with each other. 
     A marine vessel according to a preferred embodiment of the present invention includes the controller configured or programmed to perform the fixed point holding (Stay Point™) control to maintain the orientation of the bow of the hull at the target orientation and maintain the position of the hull at the target point by causing the main propulsion device and the auxiliary propulsion device to cooperate with each other. Accordingly, unlike a case in which only the auxiliary propulsion device is driven in the fixed point holding control, the main propulsion device and the auxiliary propulsion device are caused to cooperate with each other to flexibly rotate and move the hull so as to maintain the orientation of the bow of the hull at the target orientation and maintain the position of the hull at the target point. Furthermore, the auxiliary propulsion device includes the electric motor to drive the auxiliary thruster to generate a thrust. Accordingly, as compared with a case in which the main propulsion device is driven and the auxiliary propulsion device is an engine propulsion device, the amount of carbon dioxide emitted from the auxiliary propulsion device is reduced. Thus, the hull including the main propulsion device and the auxiliary propulsion device is held at a fixed point while environmental burdens associated with driving of the auxiliary propulsion device are reduced or minimized. 
     In a marine vessel according to a preferred embodiment of the present invention, the main propulsion device is preferably provided on a centerline of the hull in the right-left direction, and the auxiliary propulsion device is preferably provided to one side of the centerline of the hull in the right-left direction. Accordingly, in the marine vessel including the main propulsion device provided on the centerline of the hull in the right-left direction, and the auxiliary propulsion device provided to one side of the centerline of the hull in the right-left direction, the main propulsion device and the auxiliary propulsion device are caused to cooperate with each other to flexibly rotate and move the hull, and thus the directions and magnitudes of the thrusts of the main propulsion device and the auxiliary propulsion device are adjusted in the fixed point holding control. 
     In a marine vessel according to a preferred embodiment of the present invention, the auxiliary propulsion device preferably has a right-left rotatable angle range to change the direction of the thrust larger than a right-left rotatable angle range of the main propulsion device, and the controller is preferably configured or programmed to rotate the hull by driving the auxiliary propulsion device in the fixed point holding control. Accordingly, the hull is rotated (pivot-turned) by the electric motor-driven (electric) auxiliary propulsion device that has the right-left rotatable angle range to change the direction of the thrust larger than the right-left rotatable angle range of the main propulsion device such that a change in the position of the hull becomes smaller. 
     In a marine vessel according to a preferred embodiment of the present invention, the main propulsion device preferably includes an engine to drive a main thruster to generate a thrust and having a maximum value and a minimum value of a power range larger than a maximum value and a minimum value of a power range of the electric motor, and the controller is configured or programmed to limit the power range of the engine by matching an upper limit value of the power range of the engine with the maximum value of the power range of the electric motor while the main propulsion device and the auxiliary propulsion device are caused to cooperate with each other to move the hull, and limit the power range of the electric motor by matching a lower limit value of the power range of the electric motor with the minimum value of the power range of the engine while the main propulsion device and the auxiliary propulsion device are caused to cooperate with each other to move the hull. Accordingly, the power range of the engine and the power range of the electric motor are adjusted to be equivalent or substantially equivalent to each other, and thus when the main propulsion device and the auxiliary propulsion device are caused to cooperate with each other, the output of the engine and the output of the electric motor are prevented from being out of balance. 
     In a marine vessel according to a preferred embodiment of the present invention, the controller is preferably configured or programmed to rotate the hull by driving the auxiliary thruster to generate the thrust from the auxiliary propulsion device while a main thruster operable to generate a thrust from the main propulsion device is stopped when the hull is rotated to maintain the orientation of the bow at the target orientation in the fixed point holding control. Accordingly, the hull is rotated only by the electric auxiliary propulsion device, and thus the hull is quietly rotated. Furthermore, a thrust is generated only from the electric motor-driven (electric) auxiliary propulsion device during rotation of the hull, and thus environmental burdens during rotation of the hull are reduced. 
     In such a case, the controller is preferably configured or programmed to rotate the hull about a center of gravity of the hull on the spot while holding the position of the hull. Accordingly, the hull is rotated on the spot without changing the position of the hull, and thus the accuracy of maintaining the target point in the fixed point holding control is improved. 
     In a marine vessel according to a preferred embodiment of the present invention, the controller is preferably configured or programmed to, when the hull is moved to maintain the position of the hull at the target point in the fixed point holding control, move the hull laterally or diagonally while maintaining the orientation of the bow by simultaneously driving a main thruster to generate the thrust from the main propulsion device and the auxiliary thruster to generate the thrust from the auxiliary propulsion device, and move the hull in a forward-rearward direction by driving the main thruster while the auxiliary thruster is stopped. Accordingly, the main thruster and the auxiliary thruster are simultaneously driven (caused to cooperate with each other) such that the hull is moved laterally or diagonally while the orientation of the bow is maintained, and the hull is moved in the forward-rearward direction by driving only the main thruster. 
     In such a case, the controller is preferably configured or programmed to move the hull laterally or diagonally while maintaining the orientation of the bow by positioning an intersection of an output vector of the main thruster and an output vector of the auxiliary thruster on a straight line extending through a center of gravity of the hull and the target point and setting, in a direction from the center of gravity to the target point, a direction of a resultant force of the output vector of the main thruster and the output vector of the auxiliary thruster that indicates a moving direction of the hull. Accordingly, even when the main propulsion device and the auxiliary propulsion device are not provided in a well-balanced manner with respect to the centerline of the hull in the right-left direction, the hull is moved laterally or diagonally while the orientation of the bow is maintained. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic view showing a marine vessel including a marine propulsion system and a hull according to a preferred embodiment of the present invention. 
         FIG.  2    is a side view showing a main propulsion device of a marine propulsion system according to a preferred embodiment of the present invention. 
         FIG.  3    is a side view showing an auxiliary propulsion device of a marine propulsion system according to a preferred embodiment of the present invention. 
         FIG.  4    is a block diagram of a marine vessel including a marine propulsion system and a hull according to a preferred embodiment of the present invention. 
         FIG.  5    is a diagram illustrating the power range of an engine of a main propulsion device and the power range of an electric motor of an auxiliary propulsion device according to a preferred embodiment of the present invention. 
         FIG.  6    is a diagram showing a joystick of a marine propulsion system according to a preferred embodiment of the present invention. 
         FIG.  7    is a diagram showing a display example for a fixed point holding control of a display of a marine propulsion system according to a preferred embodiment of the present invention. 
         FIG.  8    is a diagram illustrating a control to rotate a hull by a controller of a marine propulsion system according to a preferred embodiment of the present invention. 
         FIG.  9    is a diagram illustrating a control to move a hull laterally and diagonally by a controller of a marine propulsion system according to a preferred embodiment of the present invention. 
         FIG.  10    is a diagram illustrating the relationship between the direction of the output vector of a main propeller and the direction of the output vector of an auxiliary propeller during lateral movement and diagonal movement of a hull according to a preferred embodiment of the present invention. 
         FIG.  11    is a flowchart of a control process for fixed point holding executed by a controller of a marine propulsion system according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention are hereinafter described with reference to the drawings. 
     The structure of a marine vessel  100  including a marine propulsion system  102  according to preferred embodiments of the present invention is now described with reference to  FIGS.  1  to  11   . 
     In the figures, arrow FWD represents the forward movement direction of the marine vessel  100  in a forward-rearward direction, and arrow BWD represents the rearward movement direction of the marine vessel  100  in the forward-rearward direction. Arrow R represents the starboard direction of the marine vessel  100  in a right-left direction (a direction perpendicular to the forward-rearward direction), and arrow L represents the portside direction of the marine vessel  100  in the right-left direction. 
     As shown in  FIG.  1   , the marine vessel  100  includes a hull  101  and the marine propulsion system  102  provided on or in the hull  101 . The hull  101  may be a hull of a fishing boat or a fishing vessel for a user to fish, or a relatively large hull such as a passenger vessel, for example. 
     The marine propulsion system  102  includes a main propulsion device  1 , an auxiliary propulsion device  2 , a joystick  3 , a display  4  that displays navigation-related information, etc., an orientation sensor  5   a,  a position sensor  5   b,  and a controller  6 . The joystick  3 , the display  4 , the orientation sensor  5   a,  the position sensor  5   b,  and the controller  6  are mounted on or in the hull  101 . 
     The marine propulsion system  102  (controller  6 ) performs a fixed point holding (Stay Point™) control to maintain the orientation T 1  (FWD) of a bow  101   a  of the hull  101  at a target orientation T 2  (see  FIG.  7   ) and maintain the position A 1  of the hull  101  at a target point A 2  (see  FIG.  7   ) by causing the main propulsion device  1  and the auxiliary propulsion device  2  to cooperate with each other. In the fixed point holding control, without the user maneuvering the marine vessel, the orientation of the hull  101  is automatically maintained at the target orientation T 2  specified by the user, and the position of the hull  101  is automatically maintained at the target point A 2  specified by the user. The fixed point holding control is described below in detail. 
     The “cooperate” described above refers to automatically driving the main propulsion device  1  and the auxiliary propulsion device  2  at the same time to adjust mutual rudder angles (orientations) and mutual outputs of the main propulsion device  1  and the auxiliary propulsion device  2  in the fixed point holding (Stay Point™) control. 
     Only one main propulsion device  1  shown in  FIGS.  2  and  4    is attached to the stern  101   b  (transom) of the hull  101 . The main propulsion device  1  is an engine outboard motor including an engine  12  to drive a main propeller  10  to generate a thrust. The main propulsion device  1  is provided on a centerline a of the hull  101  in the right-left direction. The main propulsion device  1  rotates in the right-left direction to change the direction of the thrust of the main propeller  10 . The main propeller  10  is an example of a “main thruster”. 
     The main propulsion device  1  includes a main propulsion device body  1   a  and a steering mechanism  1   b  provided on the main propulsion device body  1   a.  The main propulsion device body  1   a  is attached to the stern  101   b  of the hull  101  via the steering mechanism  1   b.    
     The main propulsion device body  1   a  includes the main propeller  10 , an engine control unit (ECU)  11 , the engine  12 , a cowling  13 , a shift actuator  14 , a drive shaft  15 , a gearing  16 , a propeller shaft  17 , and a steering control unit (SCU)  18 . 
     The ECU  11  is a control circuit, for example, and includes a central processing unit (CPU). The ECU  11  controls driving of the engine  12  based on a command from the controller  6 . 
     The engine  12  is a drive source for the main propeller  10 . The engine  12  is provided in an upper portion of the main propulsion device  1 , and is an internal combustion engine driven by explosive combustion of gasoline, light oil, or the like. The engine  12  is covered with the cowling  13 . As an example, the maximum output P 10  (see  FIG.  5   ) of the engine  12  is about 200 horsepower. 
     The shift actuator  14  switches the shift state of the main propulsion device  1  to any one of a forward movement state (shift F), a reverse movement state (shift R), and a neutral state (shift N) by switching the meshing of the gearing  16 . When the shift state of the main propulsion device  1  is in the forward movement state, a thrust is generated from the main propeller  10  toward the FWD side, and when the shift state is in the reverse movement state, a thrust is generated from the main propeller  10  toward the BWD side. When the shift state is in the neutral state, a thrust is not generated from the main propeller  10 . 
     When the shift state is switched, the meshing state of the gearing  16  of the main propulsion device  1  is changed, and thus a shift shock occurs in the gearing  16 . That is, when the shift state is switched, the gearing  16  of the main propulsion device  1  generates relatively loud noise and vibrations. 
     The drive shaft  15  is connected to a crankshaft (not shown) of the engine  12  so as to transmit a power from the engine  12 . The drive shaft  15  extends directly below the engine  12  with the main propeller  10  located in the water. 
     The gearing  16  transmits a rotational force from the drive shaft  15  to the propeller shaft  17 . The main propeller  10  is attached to a rear end of the propeller shaft  17 . The main propeller  10  generates a thrust in the axial direction of the propeller shaft  17  by rotating in the water. The main propeller  10  moves the hull  101  forward or rearward by switching the direction of a thrust between a forward direction and a rearward direction according to the rotational direction switched depending on the shift state. 
     The SCU  18  is a control circuit, for example, and includes a central processing unit (CPU). The SCU  18  controls driving of the steering mechanism  1   b  based on a command from the controller  6 . 
     The steering mechanism  1   b  rotates the main propulsion device body  1   a  in the right-left direction with a steering shaft  19  extending in an upward-downward direction as a central axis of rotation. That is, the steering mechanism  1   b  changes the orientation of the main propulsion device body  1   a  in the right-left direction. When the orientation of the main propulsion device body  1   a  in the right-left direction changes, the direction of the thrust of the main propeller  10  also changes according to the orientation of the main propulsion device body  1   a.    
     As an example, a right-left rotatable angle range θ 1  (see  FIG.  1   ) to change the direction of the thrust of the main propulsion device  1  is about 60 degrees (30 degrees on one side). As an example, the steering mechanism  1   b  includes a hydraulic cylinder (not shown) to apply a rotational force to the steering shaft  19 , an electric pump (not shown) to pressure-feed oil to drive the hydraulic cylinder, etc. 
     Only one auxiliary propulsion device  2  shown in  FIGS.  3  and  4    is attached to the stern  101   b  (transom) of the hull  101 . The auxiliary propulsion device  2  is an electric outboard motor including an electric motor  23  to drive an auxiliary propeller  20  to generate a thrust. The auxiliary propulsion device  2  is provided to one side of the centerline of the hull  101  in the right-left direction. Specifically, the auxiliary propulsion device  2  is located on the left side relative to the centerline a (see  FIG.  1   ) of the hull  101  in the right-left direction. The auxiliary propulsion device  2  rotates in the right-left direction to change the direction of the thrust of the auxiliary propeller  20 . The auxiliary propeller  20  is an example of an “auxiliary thruster”. 
     The auxiliary propulsion device  2  includes the auxiliary propeller  20 , a duct  21 , a motor control unit (MCU)  22 , the electric motor  23 , a cowling  24 , a steering control unit (SCU)  25 , and a steering mechanism  26 . 
     The duct  21  is provided in a lower portion of the auxiliary propulsion device  2  with the auxiliary propeller  20  located in the water. The duct  21  has a cylindrical shape and supports the auxiliary propeller  20  on the inner peripheral side such that the auxiliary propeller  20  is rotatable. In  FIG.  3   , the central position of rotation of the auxiliary propeller  20  is indicated by a central axis β. That is, the auxiliary propeller  20  generates a thrust in a direction along the central axis β. 
     The MCU  22  is a control circuit, for example, and includes a central processing unit (CPU). The MCU  22  controls driving of the electric motor  23  based on a command from the controller  6 . 
     The electric motor  23  is a drive source for the auxiliary propeller  20 . The electric motor  23  is driven by power from a battery (not shown) mounted in the hull  101 , for example. The maximum output P 20  of the electric motor  23  of the auxiliary propulsion device  2  is smaller than the maximum output P 10  of the engine  12  of the main propulsion device  1 . As an example, the maximum output P 20  (see  FIG.  5   ) of the electric motor  23  is about  20  horsepower. 
     The electric motor  23  includes a stator  23   a  integral and unitary with the duct  21  and a rotor  23   b  integral and unitary with the auxiliary propeller  20 . 
     The cowling  24  covers an upper portion of the auxiliary propulsion device  2  such that electrical wiring and the like are not exposed. The cowling  24  does not rotate in the right-left direction unlike the auxiliary propeller  20  when the direction of the thrust in the right-left direction is changed. That is, the auxiliary propulsion device  2  does not rotate the entire auxiliary propulsion device  2  (auxiliary propulsion device body) excluding the steering mechanism  26  in the right-left direction but rotates only a portion (such as the duct  21  and the auxiliary propeller  20 ) of the auxiliary propulsion device  2  on the lower side, unlike the main propulsion device  1  that rotates the entire main propulsion device body  1   a  excluding the steering mechanism  1   b  in the right-left direction. 
     Therefore, the auxiliary propulsion device  2  does not need to rotate a relatively large structure such as the engine  12  of the main propulsion device  1  in the right-left direction, and thus a right-left rotatable angle range θ 2  (see  FIG.  1   ) to change the direction of the thrust is relatively large. As an example, the right-left rotatable angle range θ 2  to change the direction of the thrust of the auxiliary propulsion device  2  is about 140 degrees (70 degrees on one side). 
     The auxiliary propeller  20  generates a thrust by rotating in the water. The drive source for the auxiliary propeller  20  is the electric motor  23 , and thus the auxiliary propeller  20  is able to freely switch between forward rotation, reverse rotation (the direction of the thrust in the forward-rearward direction), and stop without generating a shift shock unlike the main propulsion device  1 . 
     The SCU  25  is a control circuit, for example, and includes a central processing unit (CPU). The SCU  25  controls driving of the steering mechanism  26  based on a command from the controller  6 . 
     The steering mechanism  26  is built into the auxiliary propulsion device  2 . The steering mechanism  26  rotates the duct  21  in the right-left direction with a steering shaft  27  extending in the upward-downward direction as a central axis of rotation. When the orientation of the duct  21  in the right-left direction changes, the direction of the thrust of the auxiliary propeller  20  supported by the duct  21  also changes. 
     As an example, the steering mechanism  26  includes a reduction gear unit (not shown) to apply a rotational force to the steering shaft  27 , an electric motor (not shown) to drive the reduction gear unit, etc. 
     The power range P 1  of the engine  12  of the main propulsion device  1  and the power range P 2  of the electric motor  23  of the auxiliary propulsion device  2  are now described with reference to  FIG.  5   . 
     The maximum and minimum values of the power range P 1  of the engine  12  that drives the main propeller  10  are both larger than those of the electric motor  23  that drives the auxiliary propeller  20 . Specifically, the maximum value (maximum output P 10 ) of the power range P 1  of the engine  12  is larger than the maximum value (maximum output P 20 ) of the power range P 2  of the electric motor  23 . The minimum value (minimum output P 11 ) of the power range P 1  of the engine  12  is larger than the minimum value (minimum output P 21 ) of the power range P 2  of the electric motor  23 . 
     The upper limit value of the power range P 1  of the engine  12  is limited when the main propulsion device  1  and the auxiliary propulsion device  2  are caused to cooperate with each other in the fixed point holding (Stay Point™) control. The lower limit value of the power range P 2  of the electric motor  23  is limited when the main propulsion device  1  and the auxiliary propulsion device  2  are caused to cooperate with each other in the fixed point holding control. The details are described below. 
     The joystick  3  shown in  FIG.  6    is an operator to maneuver the marine vessel. The joystick  3  includes a main body  3   a  and a columnar stick  3   b  extending upward from the main body  3   a.  The stick  3   b  is a portion that is gripped by the user during maneuvering of the marine vessel. 
     The main body  3   a  includes a joystick button  30 , three buttons to start an automatic marine vessel maneuvering mode including a Stay Point™ button  31   a,  a Fish Point™ button  31   b,  and a drift button  31   c,  and a thrust adjustment operation button  32 . 
     The joystick button  30  receives operations to start and end a joystick mode. That is, the joystick button  30  switches between a normal state and a state (joystick mode) in which the joystick  3  is used to maneuver the marine vessel. In the normal state, the marine vessel is maneuvered using a remote control lever (not shown) to switch the shift state and adjust the engine speed, for example, and a steering wheel (not shown) to operate steering. 
     The Stay Point™ button  31   a  receives operations to start and end the Stay Point™ (fixed point holding) control. The Stay Point™ (fixed point holding) control refers to an automatic marine vessel maneuvering control to maintain the orientation T 1  of the bow  101   a  of the hull  101  at the target orientation T 2  and maintain the position Al of the hull  101  at the target point A 2 . 
     The Fish Point™ button  31   b  receives operations to start and end a Fish Point™ control. The Fish Point™ control refers to an automatic marine vessel maneuvering control to direct the stern  101   b  (or the bow  101   a ) of the hull  101  to the target point by rotating the hull  101  and maintain the hull  101 , the stern  101   b  (or the bow  101   a ) of which has been directed to the target point, at the target point by moving the hull  101  in the forward-rearward direction. The hull  101  does not move laterally in the Fish Point™ control. 
     The drift button  31   c  receives operations to start and end a drift control. The drift control refers to an automatic marine vessel maneuvering control to move the hull  101  by receiving external forces including wind and water flow while maintaining the orientation of the bow  101   a  of the hull  101  at the target orientation by rotating the hull  101 . 
     The thrust adjustment operation button  32  receives an operation to adjust the level of the thrust magnitude of the marine vessel  100  (the main propulsion device  1  and the auxiliary propulsion device  2 ). The thrust adjustment operation button  32  includes a plus button  32   a  to increase the level of the thrust magnitude and a minus button  32   b  to decrease the level of the thrust magnitude. 
     In the joystick mode, the marine vessel  100  moves in the tilting direction of the stick  3   b  while maintaining the orientation T 1  of the bow  101   a  based on a tilting operation of the stick  3   b  by the user. In such a case, the orientations of the bow  101   a  before and after the movement are parallel or substantially parallel to each other. Predetermined calibration is performed in advance on the marine vessel  100  (controller  6 ) by a boat builder or the like such that the tilting direction of the stick  3   b  matches the actual moving direction of the hull  101 . 
     In the joystick mode, the marine vessel  100  rotates in the twisting direction of the stick  3   b  based on a twisting operation of the stick  3   b  by the user. 
     In the joystick mode, the marine vessel  100  turns in the tilting and twisting directions of the stick  3   b  based on simultaneous tilting and twisting operations of the stick  3   b  by the user. The term “turn” indicates moving the hull  101  in the tilting direction of the stick  3   b  while gradually changing the orientation T 1  of the bow  101   a  in the twisting direction of the stick  3   b.    
     In the fixed point holding control, automatic marine vessel maneuvering is performed, and thus the stick  3   b  is not operated by the user. Furthermore, in the fixed point holding control, the marine vessel  100  only moves and rotates while maintaining the orientation T 1  of the bow  101   a,  and does not turn. 
     As shown in  FIG.  7   , the display  4  includes a touch panel  4   a.  As an example, when the Stay Point™ button  31   a  (see  FIG.  6   ) is operated to start the Stay Point™ control, the display  4  displays a simplified model D of the hull  101  and a surrounding map M around the hull  101  including an obstacle O around the hull  101 . 
     The display  4  receives the setting of the target orientation T 2  and the target point A 2  based on a user&#39;s touch operation on the touch panel  4   a.  The setting of the target orientation T 2  and the target point A 2  may be performed via another operator such as a panel operator (not shown). The display  4  displays the target orientation T 2  and the target point A 2  set on the surrounding map M. Furthermore, the display  4  displays the current orientation T 1  of the marine vessel  100  on the surrounding map M. 
     The orientation sensor  5   a  shown in  FIG.  1    measures the current orientation T 1  of the marine vessel  100 , which is the orientation (FWD) of the bow  101   a  of the marine vessel  100 . The orientation sensor  5   a  is used to determine whether or not the current orientation T 1  of the marine vessel  100  deviates from the target orientation T 2  in the fixed point holding control, for example. As an example, the orientation sensor  5   a  includes an electronic compass. 
     The position sensor  5   b  measures the current position A 1  of the hull  101 . The marine vessel  100  also acquires the current speed of the marine vessel  100  based on the time change of the current position A 1  of the hull  101  measured by the position sensor  5   b.  As an example, the position sensor  5   b  includes a global positioning system (GPS) device. 
     The controller  6  is a control circuit, for example, and includes a central processing unit (CPU). 
     The controller  6  performs the fixed point holding (Stay Point™) control to maintain the orientation T 1  of the bow  101   a  of the hull  101  at the target orientation T 2  and maintain the position A 1  of the hull  101  at the target point A 2  by causing the main propulsion device  1  and the auxiliary propulsion device  2  to cooperate with each other. 
     Specifically, in the fixed point holding control, the controller  6  corrects a deviation of the current orientation T 1  of the bow  101   a  of the hull  101  from the target orientation T 2  by rotating the hull  101  (brings the amount of deviation of the orientation closer to 0 by rotating the hull  101 ). The controller  6  calculates the amount of deviation of the orientation T 1  of the bow  101   a  based on the measurement value of the orientation sensor  5   a.    
     In the fixed point holding control, the controller  6  corrects a deviation of the current position A 1  of the hull  101  from the target point A 2  by moving the hull  101  (brings the amount of deviation of the position closer to 0 by moving the hull  101 ). The controller  6  calculates the amount of deviation of the position based on the measurement value of the position sensor  5   b.  The term “move” in the fixed point holding control indicates changing the position A 1  of the hull  101  while maintaining the orientation T 1  of the bow  101   a  of the hull  101 . 
     In the fixed point holding control, the controller  6  performs a control to rotate the hull  101  to maintain the orientation T 1  of the bow  101   a  at the target orientation T 2  and a control to move the hull  101  to maintain the position A 1  of the hull  101  at the target point A 2  at different timings. 
     The controller  6  limits the power range P 1  of the engine  12  by matching the upper limit value of the power range P 1  of the engine  12  with the maximum value (maximum output P 20 ) of the power range P 2  of the electric motor  23  while the main propulsion device  1  and the auxiliary propulsion device  2  are caused to cooperate with each other to move the hull  101  (see  FIG.  5   ). Furthermore, the controller  6  limits the power range P 2  of the electric motor  23  by matching the lower limit value of the power range P 2  of the electric motor  23  with the minimum value (minimum output P 11 ) of the power range P 1  of the engine  12  while the main propulsion device  1  and the auxiliary propulsion device  2  are caused to cooperate with each other to move the hull  101  (see  FIG.  5   ). 
     That is, the controller  6  sets a common power range for the engine  12  and the electric motor  23 , in which the upper limit values of the outputs are the same as each other and the lower limit values of the outputs are the same as each other while the main propulsion device  1  and the auxiliary propulsion device  2  are caused to cooperate with each other to move the hull  101  (see  FIG.  5   ). 
     As shown in  FIG.  8   , the controller  6  rotates the hull  101  by driving the auxiliary propeller  20  to generate a thrust from the auxiliary propulsion device  2  while the main propeller  10  that generates a thrust from the main propulsion device  1  is stopped when the hull  101  is rotated to maintain the orientation T 1  of the bow  101   a  at the target orientation T 2  in the fixed point holding control. 
     In such a case, the controller  6  rotates the hull  101  about the center of gravity of the hull  101  on the spot while holding the position A 1  of the hull  101  in the fixed point holding control. Such a so-called “pivot turning” is not able to be provided with the main propulsion device  1  (engine outboard motor) having a relatively small right-left rotatable angle range θ 1  (see  FIG.  1   ) to change the direction of the thrust. 
     As shown in  FIGS.  9  and  10   , the controller  6  moves the hull  101  in the forward-rearward direction by driving the main propeller  10  while the auxiliary propeller  20  is stopped when the hull  101  is moved to maintain the position A 1  of the hull  101  at the target point A 2  in the fixed point holding control. 
     The controller  6  moves the hull  101  laterally and diagonally while maintaining the orientation T 1  of the bow  101   a  by simultaneously driving the main propeller  10  that generates a thrust from the main propulsion device  1  and the auxiliary propeller  20  that generates a thrust from the auxiliary propulsion device  2  when the hull  101  is moved to maintain the position A 1  of the hull  101  at the target point A 2  in the fixed point holding control. 
     In such a case, the controller  6  moves the hull  101  laterally and diagonally while maintaining the orientation T 1  of the bow  101   a  by positioning an intersection I of the output vector V 1  of the main propeller  10  and the output vector V 2  of the auxiliary propeller  20  on a straight line SL extending through the center of gravity of the hull  101  and the target point A 2  and setting, in a direction from the center of gravity to the target point A 2 , the direction T 3  of the resultant force V 3  of the output vector V 1  of the main propeller  10  and the output vector V 2  of the auxiliary propeller  20  that indicates the moving direction of the hull  101 . 
     The controller  6  causes the direction of the output vector V 1  of the main propeller  10  and the direction of the output vector V 2  of the auxiliary propeller  20  to be opposite to each other in the forward-rearward direction when the hull  101  is moved laterally or diagonally while the orientation T 1  of the bow  101   a  is maintained. 
     A fixed point holding control process by the controller  6  of the marine propulsion system  102  is now described with reference to  FIG.  11   . In a flowchart shown in  FIG.  11   , it is assumed that the control process starts from a state in which the amount of deviation of the current orientation T 1  of the bow  101   a  from the target orientation T 2  exceeds an orientation threshold and the amount of deviation of the current position A 1  of the hull  101  from the target point A 2  exceeds a position threshold. In short, the control process to correct the orientation T 1  of the bow  101   a  relatively greatly deviated from the target orientation T 2  and the position A 1  of the hull  101  relatively greatly deviated from the target point A 2  is described below. 
     In step S 1 , the amount of deviation of the orientation T 1  of the bow  101   a  from the target orientation T 2  is calculated based on the measurement value of the orientation sensor  5   a.  Then, the process advances to step S 2 . 
     In step S 2 , the hull  101  is rotated by driving the auxiliary propeller  20  while the main propeller  10  is stopped. Then, the process advances to step S 3 . 
     In step S 3 , it is determined whether or not the amount of deviation of the orientation T 1  of the bow  101   a  is equal to or less than the orientation threshold. When the amount of deviation of the orientation T 1  of the bow  101   a  is equal to or less than the orientation threshold, the orientation T 1  of the bow  101   a  substantially matches the target orientation T 2 . In step S 3 , when the amount of deviation of the orientation T 1  of the bow  101   a  is equal to or less than the orientation threshold, the process advances to step S 4 , and when the amount of deviation of the orientation T 1  of the bow  101   a  is not equal to or less than the orientation threshold, the process returns to step S 1 . 
     In step S 4 , the amount of deviation of the position A 1  of the hull  101  from the target point A 2  is calculated based on the measurement value of the position sensor  5   b.  Then, the process advances to step S 5 . 
     In step S 5 , the hull  101  is moved toward the target point A 2  while the orientation T 1  of the bow  101   a  is maintained. At this time, when the hull  101  is moved in the forward-rearward direction, the hull  101  is moved by driving the main propeller  10  while the auxiliary propeller  20  is stopped. When the hull  101  is moved diagonally or laterally, the hull  101  is moved by simultaneously driving the auxiliary propeller  20  and the main propeller  10 . Then, the process advances to step S 6 . 
     In step S 6 , it is determined whether or not the amount of deviation of the position A 1  of the hull  101  is equal to or less than the position threshold. When the amount of deviation of the position A 1  of the hull  101  is equal to or less than the position threshold, the position A 1  of the hull  101  substantially matches the target point A 2 . In step S 6 , when the amount of deviation of the position A 1  of the hull  101  is equal to or less than the position threshold, the process proceeds to END, and when the amount of deviation of the position A 1  of the hull  101  is not equal to or less than the position threshold, the process returns to step S 4 . 
     According to the various preferred embodiments of the present invention described above, the following advantageous effects are achieved. 
     According to a preferred embodiment of the present invention, the marine propulsion system  102  includes the controller  6  configured or programmed to perform the fixed point holding (Stay Point™) control to maintain the orientation T 1  of the bow  101   a  of the hull  101  at the target orientation T 2  and maintain the position A 1  of the hull  101  at the target point A 2  by causing the main propulsion device  1  and the auxiliary propulsion device  2  to cooperate with each other. Accordingly, unlike a case in which only the auxiliary propulsion device is driven in the fixed point holding control, the main propulsion device  1  and the auxiliary propulsion device  2  are caused to cooperate with each other to flexibly rotate and move the hull  101  so as to maintain the orientation T 1  of the bow  101   a  of the hull  101  at the target orientation T 2  and maintain the position A 1  of the hull  101  at the target point A 2 . Furthermore, the auxiliary propulsion device  2  includes the electric motor  23  to drive the auxiliary propeller  20  to generate a thrust. Accordingly, as compared with a case in which the main propulsion device  1  is driven and the auxiliary propulsion device  2  is an engine propulsion device, the amount of carbon dioxide emitted from the auxiliary propulsion device  2  is reduced. Thus, the hull  101  including the main propulsion device  1  and the auxiliary propulsion device  2  is held at a fixed point while environmental burdens associated with driving of the auxiliary propulsion device  2  are reduced or minimized. 
     According to a preferred embodiment of the present invention, the main propulsion device  1  is provided on the centerline a of the hull  101  in the right-left direction, and the auxiliary propulsion device  2  is provided to one side of the centerline of the hull  101  in the right-left direction. Accordingly, in the marine vessel  100  including the main propulsion device  1  provided on the centerline a of the hull  101  in the right-left direction, and the auxiliary propulsion device  2  provided to one side of the centerline of the hull  101  in the right-left direction, the main propulsion device  1  and the auxiliary propulsion device  2  are caused to cooperate with each other to flexibly rotate and move the hull  101 , and thus the directions and magnitudes of the thrusts of the main propulsion device  1  and the auxiliary propulsion device  2  are adjusted in the fixed point holding control. 
     According to a preferred embodiment of the present invention, the auxiliary propulsion device  2  has the right-left rotatable angle range θ 2  to change the direction of the thrust larger than the right-left rotatable angle range θ 1  of the main propulsion device  1 , and the controller  6  is configured or programmed to rotate the hull  101  by driving the auxiliary propulsion device  2  in the fixed point holding control. Accordingly, the hull  101  is rotated (pivot-turned) by the electric motor-driven (electric) auxiliary propulsion device  2  that has the right-left rotatable angle range θ 2  to change the direction of the thrust larger than the right-left rotatable angle range θ 1  of the main propulsion device  1  such that a change in the position A 1  of the hull  101  becomes smaller. 
     According to a preferred embodiment of the present invention, the main propulsion device  1  includes the engine  12  to drive the main propeller  10  to generate a thrust, and the engine  12  has the maximum value and the minimum value of the power range P 1  larger than the maximum value and the minimum value of the power range of the electric motor  23 . The controller  6  is configured or programmed to limit the power range P 1  of the engine  12  by matching the upper limit value of the power range P 1  of the engine  12  with the maximum value of the power range P 2  of the electric motor  23  while the main propulsion device  1  and the auxiliary propulsion device  2  are caused to cooperate with each other to move the hull  101 , and to limit the power range P 2  of the electric motor  23  by matching the lower limit value of the power range P 2  of the electric motor  23  with the minimum value of the power range P 1  of the engine  12  while the main propulsion device  1  and the auxiliary propulsion device  2  are caused to cooperate with each other to move the hull  101 . Accordingly, the power range P 1  of the engine  12  and the power range P 2  of the electric motor  23  are adjusted to be equivalent or substantially equivalent to each other, and thus when the main propulsion device  1  and the auxiliary propulsion device  2  are caused to cooperate with each other, the output of the engine  12  and the output of the electric motor  23  are prevented from being out of balance. 
     According to a preferred embodiment of the present invention, the controller  6  is configured or programmed to rotate the hull  101  by driving the auxiliary propeller  20  to generate a thrust from the auxiliary propulsion device  2  while the main propeller  10  that generates a thrust from the main propulsion device  1  is stopped when the hull  101  is rotated to maintain the orientation T 1  of the bow  101   a  at the target orientation T 2  in the fixed point holding control. Accordingly, the hull  101  is rotated only by the electric auxiliary propulsion device  2 , and thus the hull  101  is quietly rotated. Furthermore, a thrust is generated only from the electric motor-driven (electric) auxiliary propulsion device  2  during rotation of the hull  101 , and thus environmental burdens during rotation of the hull  101  are reduced. 
     According to a preferred embodiment of the present invention, the controller  6  is configured or programmed to rotate the hull  101  about the center of gravity of the hull  101  on the spot while holding the position A 1  of the hull  101 . Accordingly, the hull  101  is rotated on the spot without changing the position A 1  of the hull  101 , and thus the accuracy of maintaining the target point A 2  in the fixed point holding control is improved. 
     According to a preferred embodiment of the present invention, the controller  6  is configured or programmed to, when the hull  101  is moved to maintain the position A 1  of the hull  101  at the target point A 2  in the fixed point holding control, move the hull  101  laterally or diagonally while maintaining the orientation T 1  of the bow  101   a  by simultaneously driving the main propeller  10  to generate a thrust from the main propulsion device  1  and the auxiliary propeller  20  to generate a thrust from the auxiliary propulsion device  2 , and move the hull  101  in the forward-rearward direction by driving the main propeller  10  while the auxiliary propeller  20  is stopped. Accordingly, the main propeller  10  and the auxiliary propeller  20  are simultaneously driven (caused to cooperate with each other) such that the hull  101  is moved laterally or diagonally while the orientation T 1  of the bow  101   a  is maintained, and the hull  101  is moved in the forward-rearward direction by driving only the main propeller  10 . 
     According to a preferred embodiment of the present invention, the controller  6  is configured or programmed to move the hull  101  laterally or diagonally while maintaining the orientation T 1  of the bow  101   a  by positioning the intersection I of the output vector V 1  of the main propeller  10  and the output vector V 2  of the auxiliary propeller  20  on the straight line SL extending through the center of gravity of the hull  101  and the target point A 2  and setting, in the direction from the center of gravity to the target point A 2 , the direction T 3  of the resultant force V 3  of the output vector V 1  of the main propeller  10  and the output vector V 2  of the auxiliary propeller  20  that indicates the moving direction of the hull  101 . Accordingly, even when the main propulsion device  1  and the auxiliary propulsion device  2  are not provided in a well-balanced manner with respect to the centerline a of the hull  101  in the right-left direction, the hull  101  is moved laterally or diagonally while the orientation T 1  of the bow  101   a  is maintained. 
     According to a preferred embodiment of the present invention, the controller  6  is configured or programmed to cause the direction of the output vector V 1  of the main propeller  10  and the direction of the output vector V 2  of the auxiliary propeller  20  to be opposite to each other in the forward-rearward direction when the hull  101  is moved laterally or diagonally while the orientation T 1  of the bow  101   a  is maintained. Accordingly, the direction of the output vector V 1  of the main propeller  10  and the direction of the output vector V 2  of the auxiliary propeller  20  are opposite to each other, and thus the hull  101  is easily moved laterally or diagonally while the orientation T 1  of the bow  101   a  is maintained. 
     According to a preferred embodiment of the present invention, the controller  6  is configured or programmed to perform a control to rotate the hull  101  to maintain the orientation T 1  of the bow  101   a  at the target orientation T 2  and a control to move the hull  101  to maintain the position Al of the hull  101  at the target point A 2  at the different timings in the fixed point holding control. Accordingly, rotating the hull  100  to maintain the orientation T 1  of the bow  101   a  at the target orientation T 2  and moving the hull  101  to maintain the position A 1  of the hull  101  at the target point A 2  are separated from each other such that a change in the position A 1  of the hull  101  during rotation of the hull  101  is reduced or prevented, and a change in the orientation T 1  of the bow  101   a  during movement of the hull  101  is reduced or prevented. 
     According to a preferred embodiment of the present invention, the main propulsion device  1  is an engine outboard motor including the engine  12  to drive the main propeller to generate a thrust and provided on the centerline α of the hull  101  in the right-left direction, and the auxiliary propulsion device  2  is an electric outboard motor including the electric motor  23  to drive the auxiliary propeller  20  and provided to one side of the centerline of the hull  101  in the right-left direction. Accordingly, environmental burdens are reduced due to driving of the electric outboard motor, and the hull  101  including the engine outboard motor and the electric outboard motor is held at a fixed point. 
     The preferred embodiments of the present invention described above are illustrative in all points and not restrictive. The extent of the present invention is not defined by the above description of the preferred embodiments but by the scope of the claims, and all modifications within the meaning and range equivalent or substantially equivalent to the scope of the claims are further included. 
     For example, while the process operations performed by the controller are described using a flowchart in a flow-driven manner in which processes are performed in order along a process flow for the convenience of illustration in preferred embodiments described above, the present invention is not restricted to this. In the present invention, the process operations performed by the controller may alternatively be performed in an event-driven manner in which the processes are performed on an event basis. In this case, the process operations performed by the controller may be performed in a complete event-driven manner or in a combination of an event-driven manner and a flow-driven manner. 
     While the marine propulsion system preferably includes only one main propulsion device in preferred embodiments described above, the present invention is not restricted to this. In the present invention, the marine propulsion system may alternatively include a plurality of main propulsion devices. 
     While the marine propulsion system preferably includes only one auxiliary propulsion device in preferred embodiments described above, the present invention is not restricted to this. In the present invention, the marine propulsion system may alternatively include a plurality of auxiliary propulsion devices. 
     While the main thruster of the main propulsion device is preferably the main propeller in preferred embodiments described above, the present invention is not restricted to this. In the present invention, the main thruster of the main propulsion device may alternatively be a jet that generates a thrust by jetting water. 
     While the auxiliary thruster of the auxiliary propulsion device is preferably the auxiliary propeller in preferred embodiments described above, the present invention is not restricted to this. In the present invention, the auxiliary thruster of the auxiliary propulsion device may alternatively be a jet that generates a thrust by jetting water. 
     While the main propulsion device is preferably provided on the centerline of the hull in the right-left direction in preferred embodiments described above, the present invention is not restricted to this. In the present invention, the main propulsion device may alternatively be shifted from the centerline of the hull in the right-left direction. 
     While the main propulsion device preferably includes the engine as a drive source for the main propeller in preferred embodiments described above, the present invention is not restricted to this. In the present invention, the main propulsion device may alternatively include an electric motor as a drive source for the main propeller. 
     While the main propulsion device and the auxiliary propulsion device are preferably outboard motors in preferred embodiments described above, the present invention is not restricted to this. In the present invention, the main propulsion device and the auxiliary propulsion device may alternatively be inboard-outboard motors, for example. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.