Patent Publication Number: US-2023133897-A1

Title: Marine propulsion system and marine vessel

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Pat. Application No. 2021-180208 filed on Nov. 4, 2021. The entire contents of this application are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a marine propulsion system and a marine vessel. 
     Description of the Related Art 
     A marine propulsion system including a main propulsion device and an auxiliary propulsion device is known in general. Such a marine propulsion system is disclosed in Japanese Pat. Laid-Open No. 2000-168692, for example. 
     Japanese Pat. Laid-Open No. 2000-168692 discloses a marine vessel position control device including a main propulsion device, a pair of auxiliary propulsion devices provided respectively on the right side and the left side of the main propulsion device, and a controller to perform a control to maintain the position of a hull at a predetermined position by driving only the auxiliary propulsion devices to cause a bow to face the wind while the orientation of the bow is directed to the windward side. The pair of auxiliary propulsion devices are attached to the hull while being inclined with respect to a centerline of the hull in a right-left direction in a plan view such that a rotational moment is applied to the hull when a thrust is generated, and turn the hull when the thrust is generated. The turning by the rotational moment indicates gradually changing the orientation of the bow while moving forward. Therefore, the position of the hull is not maintained when the hull is turned. 
     Although not clearly described in Japanese Pat. Laid-Open No. 2000-168692, conventionally, there has been known a drift control to move a hull under external forces including wind and water flow while maintaining the orientation of a bow of the hull at a target orientation by rotating the hull. When the control to maintain the position of the hull at the predetermined position by causing the bow to face the wind while the orientation of the bow is directed to the windward side described in Japanese Pat. Laid-Open No. 2000-168692 is applied to such a drift control, a control is conceivably performed to drive only the auxiliary propulsion devices instead of driving both the main propulsion device and the auxiliary propulsion devices. However, in such a case, it is necessary to make a turn in which the position of the hull is not maintained in order to move the hull to the predetermined position with only the auxiliary propulsion devices. Therefore, it takes a relatively long time to direct the bow to the target orientation, and thus the orientation maintenance performance of the bow is conceivably low. 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 improve orientation maintenance performance of bows in drift controls while reducing 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 including a main thruster to generate a thrust, and operable to rotate in a right-left direction to change a direction of a thrust, an auxiliary propulsion device 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 drift control to move a hull under external forces including wind and water flow while maintaining an orientation of a bow of the hull at a target orientation by rotating the hull. The controller is configured or programmed to maintain the orientation of the bow at the target orientation by rotating the hull by driving the auxiliary thruster while stopping the main thruster in the drift control. The term “rotating the hull” indicates changing the orientation of the bow while maintaining the position of the hull, unlike turning of the hull accompanied by forward or rearward movement of the hull. 
     A marine propulsion system according to a preferred embodiment of the present invention includes the controller configured or programmed to perform a control to maintain the orientation of the bow at the target orientation by rotating the hull by driving the auxiliary thruster operable to rotate in the right-left direction to change the direction of the thrust while stopping the main thruster operable to generate a thrust from the main propulsion device in the drift control to move the hull under the external forces including wind and water flow while maintaining the orientation of the bow of the hull at the target orientation by rotating the hull. Accordingly, while the position of the hull is maintained, the hull is rotated by the auxiliary propulsion device by rotating the auxiliary propulsion device in the right-left direction. In other words, the rotation of the hull changes the orientation of the hull in a short period of time, unlike turning of the hull accompanied by forward movement of the hull. Consequently, the orientation maintenance performance of the bow in the drift control is improved. Furthermore, the auxiliary propulsion device includes the electric motor to drive the auxiliary thruster to generate a thrust. Accordingly, the amount of carbon dioxide emitted from the auxiliary propulsion device is reduced as compared with a case in which the auxiliary propulsion device is an engine propulsion device. Thus, the orientation maintenance performance of the bow in the drift control is improved while environmental burdens associated with driving of the propulsion devices are reduced as much as possible. 
     In a marine propulsion system according to a preferred embodiment of the present invention, the main propulsion device is preferably to be attached to a stern of the hull and is preferably provided on a centerline of the hull in the right-left direction, and the auxiliary propulsion device is preferably to be attached to the stern and is preferably provided to one side of the centerline of the hull in the right-left direction. Accordingly, the auxiliary propulsion device is spaced farther apart from the center of gravity of the hull as compared with the main propulsion device, and thus a relatively large rotational moment is generated by the auxiliary propulsion device at the time of rotating the hull. Therefore, the hull is more quickly rotated. 
     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 while stopping the main thruster without rotating the main propulsion device in the right-left direction in the drift control. Accordingly, the main propulsion device is not rotated in the right-left direction when the hull is rotated, and thus the hull is prevented from swinging due to rotation of the main propulsion device in the right-left direction. Furthermore, noise generated from the main propulsion device is reduced, and thus scaring away fish during fishing, for example, is reduced or prevented. 
     In such a case, the main propulsion device is preferably operable to maintain a rudder angle of the main thruster at a rudder angle along a centerline of the hull in the right-left direction while stopping the main thruster when the hull is rotated by driving the auxiliary thruster in the drift control. Accordingly, when the hull is rotated, the main propulsion device is kept on standby at the rudder angle along the centerline of the hull in the right-left direction, which corresponds to the rudder angle of the main thruster, and thus a thrust is immediately generated in the forward-rearward direction from the main thruster without changing the rudder angle of the main propulsion device after the rotation is completed in the drift track mode in which the hull is moved toward the target point using the external forces and forward movement. 
     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. 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. 
     A marine propulsion system according to a preferred embodiment of the present invention preferably further includes a mode switching operator to receive an operation to switch between a normal drift mode in which the drift control is performed using only the external forces as a power source to move the hull directed to the target orientation while the main thruster of the main propulsion device is stopped, and a drift track mode in which the drift control is performed to move the hull toward a target point using the thrust of at least one of the main thruster or the auxiliary thruster in addition to the external forces as the power source. Accordingly, the normal drift mode and the drift track mode are easily switched by the mode switching operator. 
     In such a case, the controller is preferably configured or programmed to start the drift track mode in either a first driving state in which the external forces and the main thruster are used as the power source to move the hull toward the target point, or a second driving state in which the external forces and the auxiliary thruster are used as the power source to move the hull toward the target point when the normal drift mode is switched to the drift track mode by the mode switching operator. Accordingly, in the first driving state, the hull is moved toward the target point by the main propulsion device, which has the maximum output larger than the maximum output of the auxiliary propulsion device, and thus the hull is moved faster as compared with a case in which the hull is moved toward the target point by the auxiliary propulsion device. In the second driving state, the hull is rotated and moved toward the target point by the auxiliary thruster driven by the electric motor, and thus quietness in the drift control is improved while environmental burdens are reduced. 
     A marine propulsion system that starts the drift track mode in the first driving state or the second driving state preferably further includes a thrust adjustment operator to receive an operation to adjust thrust magnitudes of the main propulsion device and the auxiliary propulsion device, and the controller is preferably configured or programmed to start the drift track mode in the first driving state when the normal drift mode is switched to the drift track mode by the mode switching operator, and switch the first driving state to the second driving state based on the thrust adjustment operator receiving an operation to change the thrust magnitudes to predetermined levels or less in the drift track mode. Accordingly, the first driving state is easily switched to the second driving state in response to an operation on the thrust adjustment operator to lower the thrust levels, and the hull is moved toward the target point. 
     In a marine propulsion system including the mode switching operator, the controller is preferably configured or programmed to automatically switch from the drift track mode to the normal drift mode when the hull reaches the target point and another target point is not specified. Accordingly, even when the hull reaches the target point and another target point is not specified, the drift track mode is automatically switched to the normal drift mode, and thus the drift control is continued. 
     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 corresponding to the main thruster 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 drift control is performed on the hull including the engine outboard motor and the electric outboard motor. 
     A marine propulsion system according to a preferred embodiment of the present invention includes a main propulsion device operable to rotate in a right-left direction to change a direction of a thrust, an auxiliary propulsion device 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 drift control to move a hull under external forces including wind and water flow while maintaining an orientation of a bow of the hull at a target orientation by rotating the hull. The controller is configured or programmed to maintain the orientation of the bow at the target orientation by rotating the hull by driving an auxiliary thruster to generate the thrust from the auxiliary propulsion device while stopping a main thruster operable to generate the thrust from the main propulsion device in the drift control. 
     A marine propulsion system according to a preferred embodiment of the present invention includes the controller configured or programmed to perform a control to maintain the orientation of the bow at the target orientation by rotating the hull by driving the auxiliary thruster operable to rotate in the right-left direction to change the direction of the thrust while stopping the main thruster operable to generate a thrust from the main propulsion device in the drift control to move the hull under the external forces including wind and water flow while maintaining the orientation of the bow of the hull at the target orientation by rotating the hull. Accordingly, while the position of the hull is maintained, the hull is rotated by the auxiliary propulsion device by rotating the auxiliary propulsion device in the right-left direction. In other words, the rotation of the hull changes the orientation of the hull in a short period of time by the auxiliary propulsion device while the position of the hull is maintained, unlike turning of the hull accompanied by forward movement of the hull. Consequently, the orientation maintenance performance of the bow in the drift control is improved. 
     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 operable to rotate in a right-left direction to change a direction of a thrust, an auxiliary propulsion device 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 drift control to move the hull under external forces including wind and water flow while maintaining an orientation of a bow of the hull at a target orientation by rotating the hull. The controller is configured or programmed to maintain the orientation of the bow at the target orientation by rotating the hull by driving the auxiliary thruster while stopping a main thruster operable to generate the thrust from the main propulsion device in the drift control. 
     A marine vessel according to a preferred embodiment of the present invention includes the controller configured or programmed to perform a control to maintain the orientation of the bow at the target orientation by rotating the hull by driving the auxiliary thruster operable to rotate in the right-left direction to change the direction of the thrust while stopping the main thruster operable to generate a thrust from the main propulsion device in the drift control to move the hull under the external forces including wind and water flow while maintaining the orientation of the bow of the hull at the target orientation by rotating the hull. Accordingly, while the position of the hull is maintained, the hull is rotated by the auxiliary propulsion device by rotating the auxiliary propulsion device in the right-left direction. In other words, the rotation of the hull changes the orientation of the hull in a short period of time, unlike turning of the hull accompanied by forward movement of the hull. Consequently, the orientation maintenance performance of the bow in the drift control is improved. Furthermore, the auxiliary propulsion device includes the electric motor to drive the auxiliary thruster to generate a thrust. Accordingly, the amount of carbon dioxide emitted from the auxiliary propulsion device is reduced as compared with a case in which the auxiliary propulsion device is an engine propulsion device. Thus, the orientation maintenance performance of the bow in the drift control is improved while environmental burdens associated with driving of the propulsion devices are reduced as much as possible. 
     In a marine vessel according to a preferred embodiment of the present invention, the main propulsion device is preferably attached to a stern of the hull and is preferably provided on a centerline of the hull in the right-left direction, and the auxiliary propulsion device is preferably attached to the stern and is preferably provided to one side of the centerline of the hull in the right-left direction. Accordingly, the auxiliary propulsion device is spaced farther apart from the center of gravity of the hull as compared with the main propulsion device, and thus a relatively large rotational moment is generated by the auxiliary propulsion device at the time of rotating the hull. Therefore, the hull is more quickly rotated. 
     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 while stopping the main thruster without rotating the main propulsion device in the right-left direction in the drift control. Accordingly, the main propulsion device is not rotated in the right-left direction when the hull is rotated, and thus the hull is prevented from swinging due to rotation of the main propulsion device in the right-left direction. Furthermore, noise generated from the main propulsion device is reduced, and thus scaring away fish during fishing, for example, is reduced or prevented. 
     In such a case, the main propulsion device is preferably operable to maintain a rudder angle of the main thruster at a rudder angle along a centerline of the hull in the right-left direction while stopping the main thruster when the hull is rotated by driving the auxiliary thruster in the drift control. Accordingly, when the hull is rotated, the main propulsion device is kept on standby at the rudder angle along the centerline of the hull in the right-left direction, which corresponds to the rudder angle of the main thruster, and thus a thrust is immediately generated in the forward-rearward direction from the main thruster without changing the rudder angle of the main propulsion device after the rotation is completed in the drift track mode in which the hull is moved toward the target point using the external forces and forward movement. 
     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. 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 marine propulsion system preferably further includes a mode switching operator to receive an operation to switch between a normal drift mode in which the drift control is performed using only the external forces as a power source to move the hull directed to the target orientation while the main thruster of the main propulsion device is stopped and a drift track mode in which the drift control is performed to move the hull toward a target point using the thrust of at least one of the main thruster or the auxiliary thruster in addition to the external forces as the power source. Accordingly, the normal drift mode and the drift track mode are easily switched by the mode switching operator. 
     In such a case, the controller is preferably configured or programmed to start the drift track mode in either a first driving state in which the external forces and the main thruster are used as the power source to move the hull toward the target point, or a second driving state in which the external forces and the auxiliary thruster are used as the power source to move the hull toward the target point when the normal drift mode is switched to the drift track mode by the mode switching operator. Accordingly, in the first driving state, the hull is moved toward the target point by the main propulsion device, which has the maximum output larger than the maximum output of the auxiliary propulsion device, and thus the hull is moved faster as compared with a case in which the hull is moved toward the target point by the auxiliary propulsion device. In the second driving state, the hull is rotated and moved toward the target point by the auxiliary thruster driven by the electric motor, and thus quietness in the drift control is improved while environmental burdens are reduced. 
     In a marine vessel that starts the drift track mode in the first driving state or the second driving state, the marine propulsion system preferably further includes a thrust adjustment operator to receive an operation to adjust thrust magnitudes of the main propulsion device and the auxiliary propulsion device, and the controller is preferably configured or programmed to start the drift track mode in the first driving state when the normal drift mode is switched to the drift track mode by the mode switching operator, and switch the first driving state to the second driving state based on the thrust adjustment operator receiving an operation to change the thrust magnitudes to predetermined levels or less in the drift track mode. Accordingly, the first driving state is easily switched to the second driving state in response to an operation on the thrust adjustment operator to lower the thrust levels, and the hull is moved toward the target point. 
     In a marine vessel including the mode switching operator, the controller is preferably configured or programmed to automatically switch from the drift track mode to the normal drift mode when the hull reaches the target point and another target point is not specified. Accordingly, even when the hull reaches the target point and another target point is not specified, the drift track mode is automatically switched to the normal drift mode, and thus the drift control is continued. 
     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 a power range of an engine of a main propulsion device and a 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 an example for a drift 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 normal drift mode of a drift control by a controller of a marine propulsion system according to a preferred embodiment of the present invention. 
         FIG.  9    is a diagram illustrating a drift track mode of a drift control 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  9   . 
     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 operation panel  40 , an orientation sensor  5   a , a position sensor  5   b , and a controller  6 . The joystick  3 , the display  4 , the operation panel  40 , 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 drift control to move the hull  101  under external forces F including wind and water flow while maintaining the orientation T 1  of a bow  101   a  of the hull  101  at a target orientation T 2  by rotating the hull  101  (see  FIG.  7   ). 
     In the drift control (normal drift mode), the controller  6  rotates the hull  101  to maintain the orientation T 1  of the bow  101   a  at the target orientation T 2  by driving an auxiliary propeller  20  while stopping a main propeller  10  that generates a thrust from the main propulsion device  1 . In the drift control, the hull  101  is automatically rotated without the user maneuvering the marine vessel. The drift control is described below in detail. The main propeller  10  is an example of a “main thruster”. The auxiliary propeller  20  is an example of an “auxiliary thruster”. 
     Only one main propulsion device  1  shown in  FIGS.  2  and  4    is attached to a stern  101   b  (transom) of the hull  101 . The main propulsion device  1  is an engine outboard motor including an engine  12  to drive the main propeller  10  to generate a thrust. The main propulsion device  1  is provided on a centerline α 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 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 the 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 θ1 (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 the 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 α (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 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 on 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 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 (registered trademark) button  31   a , a Fish Point® button  31   b , and a drift button  31   c , and a thrust adjustment operation button  32 . The thrust adjustment operation button  32  is an example of a “thrust adjustment operator”. 
     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 a 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 and maintain the position of the hull  101  at a target point A 1 . 
     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 A 1  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 A 1 , at the target point A 1  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 the drift control. The drift control refers to an automatic marine vessel maneuvering control to move the hull  101  under the external forces F (see  FIG.  7   ) including wind and water flow while maintaining the orientation T 1  of the bow  101   a  of the hull  101  at the target orientation T 2  by rotating the hull  101 , as described above. 
     The drift control has two modes including a normal drift mode and a drift track mode. In the normal drift mode, the drift control is performed using only the external forces F as a power source to move the hull  101  directed to the target orientation T 2  while the main propeller  10  of the main propulsion device  1  is stopped. In the drift track mode, the drift control is performed to move the hull  101  toward the target point A 1  using the thrust of at least one of the main propeller  10  or the auxiliary propeller  20  in addition to the external forces F as the power source. In the drift track mode, the controller  6  generates a thrust in the forward-rearward direction mainly from at least one of the main propeller  10  or the auxiliary propeller  20 . 
     The normal drift mode and the drift track mode are switched by a mode switching operation button  40   a  on the operation panel  40  (see  FIG.  1   ). The mode switching operation button  40   a  receives an operation to switch between the normal drift mode and the drift track mode, and is one of various buttons provided on the operation panel  40 . The mode switching operation button  40   a  is an example of a “mode switching operator”. 
     The controller  6  starts a control from the normal drift mode instead of the drift track mode when the drift control is started by the drift button  31   c . 
     The controller  6  starts the drift track mode in a first driving state when the drift control is started by the drift button  31   c  and the normal drift mode is switched to the drift track mode by the mode switching operation button  40   a . The first driving state refers to the driving state of the marine propulsion system  102  in which the external forces F and the main propeller  10  are used as power sources to move the hull  101  toward the target point A 1 . 
     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. 
     As an example, there are five levels including levels 1 to 5 to set the level of the thrust magnitude. At level 1, the thrust magnitude is the smallest, and the thrust magnitude gradually increases in the order of levels 2, 3, 4, and 5. When the drift control (normal drift mode) is started, the level of the thrust magnitude is automatically set to level 2, which is the second smallest from the bottom, at the time of start. 
     When the set level is level 2, 3, 4, or 5 in the drift track mode, the auxiliary propulsion device  2  rotates the hull  101 , and the main propulsion devices  1  moves the hull  101  in the forward-rearward direction. When the set level is level 1 in the drift track mode, the auxiliary propulsion device  2  rotates and moves the hull  101  in the forward-rearward direction. 
     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 T 1  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 drift control, automatic marine vessel maneuvering is performed, and thus the stick  3   b  is not operated by the user. 
     As shown in  FIG.  7   , the display  4  includes a touch panel  4   a . As an example, when the drift button  31   c  (see  FIG.  6   ) is operated to start the drift 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  based on a user’s touch operation on the touch panel  4   a  in the drift control. Furthermore, the display  4  receives the setting of the target point A 1  in the drift track mode of the drift control. The setting of the target orientation T 2  and the target point A 1  may be performed via another operator such as the operation panel  40  (see  FIG.  1   ). The display  4  displays the target orientation T 2  and the target point A 1  set on the surrounding map M. The display  4  also 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  is deviated from the target orientation T 2  in the drift 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 0  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 0  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 drift control to move the hull  101  under the external forces F including wind and water flow while maintaining the orientation T 1  of the bow  101   a  of the hull  101  at the target orientation T 2  by rotating the hull  101 . In the drift control, the controller  6  rotates the hull  101  to maintain the orientation T 1  of the bow  101   a  at the target orientation T 2  by driving the auxiliary propeller  20  while stopping the main propeller  10  that generates a thrust from the main propulsion device  1 . 
     In the drift control, the controller  6  rotates the hull  101  by driving the auxiliary propeller  20  while stopping the main propeller  10  without rotating the main propulsion device  1  in the right-left direction. 
     In such a case, the main propulsion device  1  maintains the rudder angle of the main propeller  10  at a rudder angle along the centerline α of the hull  101  in the right-left direction while stopping the main propeller  10  when the hull  101  is rotated by driving the auxiliary propeller  20  in the drift control. 
     As described above, the controller  6  starts the drift track mode in the first driving state when the normal drift mode is switched to the drift track mode by the mode switching operation button  40   a . In the drift track mode, the controller  6   switches the first driving state to a second driving state based on the thrust adjustment operation button  32  receiving an operation to change the level of the thrust magnitude to a predetermined level or less. The second driving state refers to the driving state of the marine propulsion system  102  in which the external forces F and the auxiliary propeller  20  are used as power sources to move the hull  101  toward the target point A 1 . 
     Specifically, in the drift track mode control, the controller  6  switches the first driving state in which the hull  101  is moved in the forward-rearward direction by driving the main propeller  10  while the auxiliary propeller  20  is stopped to the second driving state in which the hull  101  is moved in the forward-rearward direction by driving the auxiliary propeller  20  while the main propeller  10  is stopped based on the thrust adjustment operation button  32  receiving the operation to change the level of the thrust magnitude to the predetermined level or less (the operation to change level 2 to level 1). The controller  6  causes the auxiliary propeller  20  to generate a thrust to rotate the hull  101  in both the first driving state and the second driving state. 
     The normal drift mode of the drift control is now described with reference to  FIG.  8   . The controller  6  moves the hull  101  only by the external forces F including wind and water flow in the normal drift mode. When the orientation T 1  of the bow  101   a  is deviated from the target orientation T 2  at this time, the controller  6  corrects the misorientation by rotating the hull  101  by driving the auxiliary propeller  20  while stopping the main propeller  10 . 
     The drift track mode of the drift control is now described with reference to  FIG.  9   . The controller  6  moves the hull  101  toward the target point A 1  by the external forces F including wind and water flow that act on the hull  101  and the main propulsion device  1  in the drift track mode. The controller  6  rotates the hull  101  only with the auxiliary propulsion device  2  as in the normal drift mode. In the drift track mode, the hull  101  is moved toward the target point A 1  specified by the user. One or more target points A 1  may be specified. The drift track mode is used when the external forces F including wind and water flow that act on the hull  101  are not directed straight toward the target point A 1 , for example. 
     In the drift track mode, the controller  6  adjusts the thrust of the main propeller  10  to direct the resultant force of the external forces F including wind and water flow and the forward or rearward thrust of the main propeller  10  to the target point A 1 . 
     In the marine propulsion system  102 , the controller  6  is configured or programmed to automatically switch from the drift track mode to the normal drift mode when the hull  101  reaches the target point A 1  and another target point A 1  is not specified. That is, the marine propulsion system  102  switches to the normal drift mode and continues the drift control when the hull  101  reaches the final target point A 1  in the drift track mode. 
     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 a control to maintain the orientation T 1  of the bow  101   a  at the target orientation T 2  by rotating the hull  101  by driving the auxiliary propeller  20  operable to rotate in the right-left direction to change the direction of the thrust while stopping the main propeller  10  operable to generate a thrust from the main propulsion device  1  in the drift control to move the hull  101  under the external forces F including wind and water flow while maintaining the orientation T 1  of the bow  101   a  of the hull  101  at the target orientation T 2  by rotating the hull  101 . Accordingly, while the position of the hull  101  is maintained, the hull  101  is rotated by the auxiliary propulsion device  2  by rotating the auxiliary propulsion device  2  in the right-left direction. In other words, the rotation of the hull  101  changes the orientation of the hull  101  in a short period of time, unlike turning of the hull  101  accompanied by forward movement of the hull  101 . Consequently, the orientation maintenance performance of the bow  101   a  in the drift control is improved. Furthermore, the auxiliary propulsion device  2  includes the electric motor  23  to drive the auxiliary propeller  20  to generate a thrust. Accordingly, the amount of carbon dioxide emitted from the auxiliary propulsion device  2  is reduced as compared with a case in which the auxiliary propulsion device  2  is an engine propulsion device. Thus, the orientation maintenance performance of the bow  101   a  in the drift control is improved while environmental burdens associated with driving of the propulsion devices are reduced as much as possible. 
     According to a preferred embodiment of the present invention, the main propulsion device  1  is attached to the stern  101   b  of the hull  101  and is provided on the centerline α of the hull  101  in the right-left direction, and the auxiliary propulsion device  2  is attached to the stern  101   b  and is provided to one side of the centerline of the hull  101  in the right-left direction. Accordingly, the auxiliary propulsion device  2  is spaced farther apart from the center of gravity of the hull  101  as compared with the main propulsion device  1 , and thus a relatively large rotational moment is generated by the auxiliary propulsion device  2  at the time of rotating the hull  101 . Therefore, the hull  101  is more quickly rotated. 
     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  while stopping the main propeller  10  without rotating the main propulsion device  1  in the right-left direction in the drift control. Accordingly, the main propulsion device  1  is not rotated in the right-left direction when the hull  101  is rotated, and thus the hull  101  is prevented from swinging due to rotation of the main propulsion device  1  in the right-left direction. Furthermore, noise generated from the main propulsion device  1  is reduced, and thus scaring away fish during fishing, for example, is reduced or prevented. 
     According to a preferred embodiment of the present invention, the main propulsion device  1  is operable to maintain the rudder angle of the main propeller  10  at the rudder angle along the centerline α of the hull  101  in the right-left direction while stopping the main propeller  10  when the hull  101  is rotated by driving the auxiliary propeller  20  in the drift control. Accordingly, when the hull  101  is rotated, the main propulsion device  1  is kept on standby at the rudder angle along the centerline α of the hull  101  in the right-left direction, which corresponds to the rudder angle of the main propeller  10 , and thus a thrust is immediately generated in the forward-rearward direction from the main propeller  10  without changing the rudder angle of the main propulsion device  1  after the rotation is completed in the drift track mode in which the hull  101  is moved toward the target point A 1  using the external forces F and forward movement. 
     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 of the main propulsion device  1 . 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 θ2 to change the direction of the thrust larger than the right-left rotatable angle range of the main propulsion device  1  such that a change in the position of the hull  101  becomes smaller. 
     According to a preferred embodiment of the present invention, the marine propulsion system  102  further includes the mode switching operation button  40   a  to receive an operation to switch between the normal drift mode in which the drift control is performed using only the external forces F as a power source to move the hull  101  directed to the target orientation T 2  while the main propeller  10  of the main propulsion device  1  is stopped and the drift track mode in which the drift control is performed to move the hull  101  toward the target point A 1  using the thrust of at least one of the main propeller  10  or the auxiliary propeller  20  in addition to the external forces F as the power source. Accordingly, the normal drift mode and the drift track mode are easily switched by the mode switching operation button  40   a . 
     According to a preferred embodiment of the present invention, the controller  6  is configured or programmed to start the drift track mode in either the first driving state in which the external forces F and the main propeller  10  are used as the power sources to move the hull  101  toward the target point A 1  or the second driving state in which the external forces F and the auxiliary propeller  20  are used as the power sources to move the hull  101  toward the target point A 1  when the normal drift mode is switched to the drift track mode by the mode switching operation button  40   a . Accordingly, in the first driving state, the hull  101  is moved toward the target point A 1  by the main propulsion device  1 , which has the maximum output P 10  larger than the maximum output of the auxiliary propulsion device  2 , and thus the hull  101  is moved faster as compared with a case in which the hull  101  is moved toward the target point A 1  by the auxiliary propulsion device  2 . In the second driving state, the hull  101  is rotated and moved toward the target point A 1  by the auxiliary propeller  20  driven by the electric motor  23 , and thus quietness in the drift control is improved while environmental burdens are reduced. 
     According to a preferred embodiment of the present invention, the marine propulsion system  102  further includes the thrust adjustment operation button  32  to receive an operation to adjust the levels of the thrust magnitudes of the main propulsion device  1  and the auxiliary propulsion device  2 , and the controller  6  is configured or programmed to start the drift track mode in the first driving state when the normal drift mode is switched to the drift track mode by the mode switching operation button  40   a , and to switch the first driving state to the second driving state based on the thrust adjustment operation button  32  receiving the operation to change the levels of the thrust magnitudes to the predetermined levels or less in the drift track mode. Accordingly, the first driving state is easily switched to the second driving state in response to an operation on the thrust adjustment operation button  32  to lower the thrust levels, and the hull  101  is moved toward the target point A 1 . 
     According to a preferred embodiment of the present invention, the controller  6  is configured or programmed to automatically switch from the drift track mode to the normal drift mode when the hull  101  reaches the target point A 1  and another target point A 1  is not specified. Accordingly, even when the hull  101  reaches the target point A 1  and another target point A 1  is not specified, the drift track mode is automatically switched to the normal drift mode, and thus the drift control is continued. 
     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  10  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 drift control is performed on the hull  101  including the engine outboard motor and the electric outboard motor. 
     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 to the scope of the claims are further included. 
     For example, 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 a preferred embodiment of 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 a preferred embodiment of 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 a preferred embodiment of 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 a preferred embodiment of 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 a preferred embodiment of 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 a preferred embodiment of 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 a preferred embodiment of 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.