Patent Publication Number: US-8968040-B2

Title: Method of operating a marine vessel propulsion system, marine vessel propulsion system, and marine vessel including the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of operating a marine vessel propulsion system that includes an outboard motor. The present invention also relates to a marine vessel propulsion system with an outboard motor and a marine vessel including the same. 
     2. Description of the Related Art 
     An outboard motor is an example of a propulsion device for a marine vessel and includes a motor and a propeller driven by the motor. The outboard motor is attached to a stern of the marine vessel in a state enabling turning in right and left directions. The marine vessel is equipped with a steering apparatus to control a turning angle of the outboard motor. The steering apparatus turns the outboard motor in accordance with the operation of a steering handle by a marine vessel operator. In a case of a multiple installation arrangement in which a plurality of outboard motors are installed at the stern, the steering apparatus turns the plurality of outboard motors in synchronization. 
     U.S. 2010/0151750 A1 discloses an arrangement where a steering angle of a steering handle is detected by a steering angle sensor and a plurality of outboard motors are turned in accordance with the detection result. With the arrangement of U.S. 2010/0151750 A1, when there is a malfunction in the turning angle control of any of the outboard motors, the turning angle control of the corresponding outboard motor is stopped. A turning angle range of another normally functioning outboard motor is then restricted in accordance with the turning angle of the outboard motor with the turning angle control malfunction. The turning angle control of the normally functioning outboard motor is performed within the restricted turning angle range. Turning performance of the marine vessel can thus be secured while avoiding interference of the normally functioning outboard motor with the outboard motor with the turning angle control malfunction. 
     However, with the arrangement of U.S. 2010/0151750 A1, the control is complicated because the other normally functioning outboard motor must be restricted in its turning angle range in accordance with the turning angle of the outboard motor with the turning angle control malfunction. Also, depending on the turning angle of the outboard motor with the turning angle control malfunction, the turning angle range becomes extremely narrow in the other normally functioning outboard motor and a steerable angle range may become significantly restricted thereafter. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide a method of operating a marine vessel propulsion system, a marine vessel propulsion system, and a marine vessel including the same by which a turning performance of a marine vessel is secured by simple control when there is a malfunction in the turning angle control of at least one of a plurality of outboard motors. 
     In order to overcome the previously unrecognized and unsolved challenges described above, a first preferred embodiment of the present invention provides a method of operating a marine vessel propulsion system that includes a plurality of outboard motors, each including a motor and a propeller rotated by the motor, and a steering apparatus arranged to control turning angles of the plurality of the outboard motors and where the steering apparatus includes a steering member and a plurality of turning mechanisms arranged to turn each of the plurality of outboard motors individually in accordance with an operation of the steering member and each of the turning mechanisms includes a hydraulic cylinder including two cylinder chambers partitioned by a piston and a normally-closed bypass valve to put the two cylinder chambers of the hydraulic cylinder in communication with each other, the method of operating the marine vessel propulsion system including a step of judging whether or not there is a malfunction in a turning angle control of at least one of the outboard motors, a step where, when it is judged that there is the malfunction in the turning angle control of the at least one of the outboard motors, the bypass valve of the turning mechanism, corresponding to the at least one of the outboard motors judged to have the malfunction in the turning angle control, is put in an open state, and a power control step of keeping power transmission, between the motor and the propeller of the at least one of the outboard motors judged to have the malfunction in the turning angle control, in an interrupted state and meanwhile allowing power transmission between the motor and the propeller in the other outboard motor or motors while maintaining the open state of the bypass valve. “Motor” refers inclusively to an internal combustion engine, electric motor, or whatever type of engine that generates a vessel propulsion force. 
     With the method of the present preferred embodiment, when there is the malfunction in the turning angle control of the at least one of the outboard motors, the bypass valve of the turning mechanism, corresponding to the at least one of the outboard motors with the turning angle control malfunction (hereinafter referred to as the “malfunctioning outboard motor”), is put in the open state. The malfunctioning outboard motor is thus put in a state where, although the turning angle control thereof cannot be performed, it can be turned freely in the right and left directions. Also, the power transmission between the motor and the propeller of the malfunctioning outboard motor is kept in the interrupted state. Generation of a propulsive force by the malfunctioning outboard motor is thus stopped. 
     On the other hand, the power transmission between the motor and the propeller is allowed in the other outboard motor or motors (a normally functioning outboard motor without a turning angle control malfunction). A propulsive force can thus be generated by the normally functioning outboard motor. Also, the turning angle control of the normally functioning outboard motor is performed as is done normally and thus the turning performance of the marine vessel is secured by the turning angle control of the normally functioning outboard motor. Thus, even when there is a malfunction in the turning angle control of the at least one of the outboard motors, the marine vessel operator can steer the marine vessel by operating the steering member. 
     Also, with the method of the present preferred embodiment, unlike the prior art described in U.S. 2010/0151750 A1, a control to restrict the turning angle range of the normally functioning outboard motor in accordance with the turning angle of the malfunctioning outboard motor is not necessary. Control is thus easy in comparison to the arrangement and method described in U.S. 2010/0151750 A1. Also, with the present method, the turning angle range of the normally functioning outboard motor is not restricted in accordance with the turning angle of the malfunctioning outboard motor and thus, regardless of a turning angle at which the malfunction occurs, the normally functioning outboard motor can be turned in the same turning angle range as that before the malfunction so that adequate turning performance of the marine vessel is secured. 
     As mentioned above, the malfunctioning outboard motor is put in a state where the generation of a propulsive force is stopped but turning in the right and left directions is performed freely. Thus, when turning of the normally functioning outboard motor is performed while the marine vessel is moored, the malfunctioning outboard motor is turned by being pushed by the normally functioning outboard motor. Also, when the marine vessel is made to run by the propulsive force of the normally functioning outboard motor, the malfunctioning outboard motor is turned in the same direction as the other normally functioning outboard motor in accordance with a water stream that forms at a periphery of the malfunctioning outboard motor. A possibility of the normally functioning outboard motor contacting the malfunctioning outboard motor during the turning angle control of the normally functioning outboard motor is thus low, and even when contact of the malfunctioning outboard motor with the normally functioning outboard motor occurs, a load due to the contact is small. 
     The method of operating a marine vessel propulsion system may further include a step where, when it is judged that there is a malfunction in the turning angle control of at least one of the outboard motors, a marine vessel operator is urged to open the bypass valve of the turning mechanism corresponding to the at least one of the outboard motors judged to have the malfunction in the turning angle control. 
     With this method, when there is the malfunction in the turning angle control of the at least one of the outboard motors, the bypass valve of the turning mechanism corresponding to the malfunctioning outboard motor can be opened reliably by the marine vessel operator. 
     The method of operating a marine vessel propulsion system may further include a step of judging whether there is the malfunction in the turning angle control of all of the outboard motors or a there is the malfunction in the turning angle control of the at least one of the outboard motors, and a step where, when it is judged that there is the malfunction in the turning angle control of the at least one of the outboard motors, a marine vessel operator is urged to open the bypass valve of the turning mechanism corresponding to the at least one of the outboard motors judged to have a malfunction in the turning angle control. 
     With this method, when there is the malfunction in the turning angle control of the at least one of the outboard motors, the bypass valve of the turning mechanism corresponding to the malfunctioning outboard motor can be opened reliably by the marine vessel operator. 
     The method of operating a marine vessel propulsion system may further include a step where, when it is judged that there is the malfunction in the turning angle control of all of the outboard motors, the marine vessel operator is notified that there is the malfunction in the turning angle control of all of the outboard motors. 
     With this method, when there is the malfunction in the turning angle control of all of the outboard motors, the marine vessel operator is made aware of this. 
     The method of operating a marine vessel propulsion system may further include a step where, when it is judged that there is the malfunction in the turning angle control of all of the outboard motors, all of the outboard motors are moved to respective turning angle midpoints thereof and not less than one of the plurality of outboard motors is made to generate a propulsive force in the state where all of the outboard motors are fixed at the respective turning angle midpoints. 
     With this method, when there is the malfunction in the turning angle control of all of the outboard motors, the marine vessel can be made to turn by making use of an output difference among the plurality of outboard motors in the state where all of the outboard motors are fixed at the respective turning angle midpoints. Thus, even when there is the malfunction in the turning angle control of all of the outboard motors, the turning performance of the marine vessel is secured. 
     The method of operating a marine vessel propulsion system may further include a step where, when it is judged that there is the malfunction in the turning angle control of all of the outboard motors, the bypass valves of the turning mechanisms corresponding to all of the outboard motors are opened, all of the outboard motors are moved to the respective turning angle midpoints, and the bypass valves of the turning mechanisms corresponding to all of the outboard motors are thereafter closed. 
     With this method, even if all of the outboard motors are not positioned at the respective turning angle midpoints when there is the malfunction in the turning angle control of all of the outboard motors, all of the outboard motors can be moved to and fixed at the respective turning angle midpoints. 
     The method of operating a marine vessel propulsion system may further include a step where, when it is judged that there is the malfunction in the turning angle control of all of the outboard motors, a rotational speed of the motor is restricted to no more than a predetermined value in all of the outboard motors. 
     When there is the malfunction in the turning angle control of all of the outboard motors, the marine vessel can be turned by making use of the propulsive force of the outboard motors without performing turning angle control of the outboard motors. However, in this case, if the propulsive force of the outboard motors is too great, it may be difficult to obtain a turning behavior that is intended by the marine vessel operator. Thus, with the method of the present preferred embodiment, the rotational speed of the motor is restricted to no more than the predetermined value in all of the outboard motors when there is the malfunction in the turning angle control of all of the outboard motors. The propulsive force of the outboard motors are thus prevented from becoming too large and marine vessel maneuvering is made easy. 
     The power control step may include a step of restricting the rotational speed of the motor in the other outboard motor or motors to no more than a predetermined value. 
     If there is the malfunction in the turning angle control of the at least one of the outboard motors, the turning angle control of another normally functioning outboard motor is performed, wherein the normally functioning outboard motor may contact the malfunctioning outboard motor. Thus, with the method of the present preferred embodiment, when there is the malfunction in the turning angle control of the at least one of the outboard motors, the rotational speed of the motor is restricted to no more than the predetermined value in the other normally functioning outboard motor or motors. A load due to the contact of the normally functioning outboard motor with the malfunctioning outboard motor is thus significantly reduced or prevented. 
     The method of operating a marine vessel propulsion system may further include a step where, when it is judged that there is the malfunction in the turning angle control of the at least one of the outboard motors, the marine vessel is made to run at a vessel speed lower than a vessel speed corresponding to a maximum propulsive force that can be generated by all of the outboard motors. 
     With this method, when there is the malfunction in the turning angle control of the at least one of the outboard motors, the marine vessel runs at the vessel speed lower than the vessel speed corresponding to the maximum propulsive force that can be generated by all of the outboard motors and thus even when a normally functioning outboard motor contacts the malfunctioning outboard motor, the load due to the contact is significantly reduced. 
     A second preferred embodiment of the present invention provides a marine vessel propulsion system including a plurality of outboard motors, each including a motor and a propeller rotated by the motor, and a steering apparatus arranged to control turning angles of the plurality of the outboard motors and where the steering apparatus includes a steering member and a plurality of turning mechanisms to turn each of the plurality of outboard motors individually in accordance with an operation of the steering member, and each of the turning mechanisms includes a hydraulic cylinder including two cylinder chambers partitioned by a piston and a normally-closed bypass valve to put the two cylinder chambers of the hydraulic cylinder in communication with each other, the marine vessel propulsion system further including a malfunction judging unit arranged to judge whether or not there is the malfunction in the turning angle control of at least one of the outboard motors, a notifying unit that, when the malfunction judging unit judges that there is the malfunction in the turning angle control of the at least one of the outboard motors, urges the marine vessel operator to open the bypass valve of the turning mechanism corresponding to the at least one of the outboard motors judged to have the malfunction in the turning angle control, and a power control unit arranged to keep power transmission, between the motor and the propeller of the at least one of the outboard motors judged to have the malfunction in the turning angle control, in an interrupted state and meanwhile allowing power transmission between the motor and the propeller in the other outboard motor or motors. 
     With the present arrangement, when there is the malfunction in the turning angle control of the at least one of the outboard motors, the marine vessel operator is urged to open the bypass valve of the turning mechanism corresponding to the at least one of the outboard motors with the turning angle control malfunction (hereinafter referred to as the “malfunctioning outboard motor”). When the marine vessel operator opens the bypass valve of the turning mechanism corresponding to the malfunctioning outboard motor, the malfunctioning outboard motor is put in a state where, although the turning angle control thereof cannot be performed, it can be turned freely in the right and left directions. Also, the power transmission between the motor and the propeller of the malfunctioning outboard motor is kept in the interrupted state. Generation of a propulsive force by the malfunctioning outboard motor is thus stopped. 
     On the other hand, the power transmission between the motor and the propeller is allowed in the other outboard motor or motors (a normally functioning outboard motor without a turning angle control malfunction). A propulsive force can thus be generated by the normally functioning outboard motor. Also, the turning angle control of the normally functioning outboard motor is performed as is done normally and thus the turning performance of the marine vessel is secured by the turning angle control of the normally functioning outboard motor. Thus, even when there is a malfunction in the turning angle control of the at least one of the outboard motors, the marine vessel operator can steer the marine vessel by operating the steering member. 
     Also, with this arrangement, unlike the prior art described in U.S. 2010/0151750 A1, a control to restrict the turning angle range of the normally functioning outboard motor in accordance with the turning angle of the at least one of the outboard motors with the turning angle control malfunction is not necessary. Control is thus easy in comparison to the arrangement described in U.S. 2010/0151750 A1. 
     Also, the malfunctioning outboard motor is put in a state where the generation of a propulsive force is stopped but turning in the right and left directions can be performed freely. Thus, when the marine vessel is made to run by the propulsive force of the normally functioning outboard motor, the malfunctioning outboard motor is turned in the same direction as the other normally functioning outboard motor in accordance with a water stream at a periphery of the marine vessel. A possibility of the normally functioning outboard motor contacting the malfunctioning outboard motor during the turning angle control of the normally functioning outboard motor is thus low. Also, even when the normally functioning outboard motor contacts the malfunctioning outboard motor, a load due to the contact is small. 
     The marine vessel propulsion system may further include a restricting unit that, when the malfunction judging unit judges that there is the malfunction in the turning angle control of the at least one of the outboard motors, restricts a rotational speed of the motor to no more than a predetermined value in the at least one of the outboard motors judged to have a malfunction in the turning angle control. 
     As mentioned above, when there is the malfunction in the turning angle control of the at least one of the outboard motors, the power transmission between the motor and the propeller of the malfunctioning outboard motor is kept in the interrupted state and thus a rotational force of the motor of the malfunctioning outboard motor is not transmitted to the propeller. Thus, by restricting the rotational speed of the malfunctioning outboard motor to no more than the predetermined value, wasteful consumption of energy is significantly reduced or prevented. 
     Each of the bypass valves may be a manually opened/closed bypass valve. 
     Each of the bypass valves may be an automatically opened/closed type bypass valve. 
     The marine vessel propulsion system may further include a turning angle control stopping unit that, when the malfunction judging unit judges that there is the malfunction in the turning angle control of all of the outboard motors, stops the turning angle control of all of the outboard motors. With this arrangement, the turning angle control of all of the outboard motors can be stopped when there is a malfunction in the turning angle control of all of the outboard motors. 
     The malfunction judging unit may be arranged to judge that there is the malfunction in the turning angle control of all of the outboard motors when a malfunction due to an input system in common to all of the turning mechanisms is detected and to judge that there is the malfunction in the turning angle control of the at least one of the outboard motors when a malfunction due to output systems of the respective turning mechanisms is detected. 
     With this arrangement, when a malfunction due to the input system in common to all of the turning mechanisms is detected, it is judged that there is the malfunction in the turning angle control of all of the outboard motors. On the other hand, when the malfunction due to the output systems of the respective turning mechanisms is detected, it is judged that there is the malfunction in the turning angle control of the at least one of the outboard motors. 
     The malfunction due to the input system may include a malfunction of an operation amount detection sensor arranged to detect an operation amount of the steering member. Also, the malfunction due to the output systems includes a malfunction of turning angle sensors arranged to detect the turning angles of the outboard motors and a malfunction of the respective turning mechanisms. 
     A third preferred embodiment of the present invention provides a marine vessel including a hull and a marine vessel propulsion system attached to the hull. 
     With this arrangement, when there is the malfunction in the turning angle control of the at least one of the outboard motors, the marine vessel operator can open the bypass valve of the turning mechanism corresponding to the malfunctioning outboard motor. Also, the power transmission between the motor and the propeller of the malfunctioning outboard motor is kept in the interrupted state. On the other hand, the power transmission between the motor and the propeller in the other normally functioning outboard motor or motors is allowed. Also, the turning angle control of the normally functioning outboard motor is performed as is done normally and thus the turning performance of the marine vessel is secured by the turning angle control of the normally functioning outboard motor. Thus, even when there is the malfunction in the turning angle control of the at least one of the outboard motors, the marine vessel operator can steer the marine vessel by operating the steering member. 
     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 perspective view for describing an arrangement of a marine vessel according to a preferred embodiment of the present invention. 
         FIG. 2  is a schematic side view of an arrangement example of an outboard motor. 
         FIG. 3  is an arrangement diagram for describing an arrangement of a turning mechanism. 
         FIG. 4  is a block diagram for describing an electrical arrangement of a principal portion of the marine vessel. 
         FIGS. 5A to 5F  are schematic views for describing relationships of respective lever positions to adjust a propulsive force of the marine vessel and movements of a hull. 
         FIG. 6  is a flowchart of procedures of a basic target turning angle computing process performed by a main ECU and procedures of a motor control process performed by a turning ECU. 
         FIG. 7A  is a flowchart of a portion of procedures of a malfunction operation control process executed by the main ECU. 
         FIG. 7B  is a flowchart of a portion of the procedures of the malfunction operation control process executed by the main ECU. 
         FIG. 7C  is a flowchart of a portion of the procedures of the malfunction operation control process executed by the main ECU. 
         FIG. 8  is a schematic view of an example of an operation guidance screen displayed on a display in step S 24  of  FIG. 7A . 
         FIG. 9  is a schematic view of an example of an operation guidance screen displayed on the display in step S 29  of  FIG. 7A . 
         FIG. 10  is a schematic view of an example of an operation guidance screen displayed on the display in step S 35  of  FIG. 7C . 
         FIGS. 11A to 11C  are schematic views for specifically describing a process of steps S 21  to S 25  of  FIG. 7A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a perspective view for describing an arrangement of a marine vessel according to a preferred embodiment of the present invention. The marine vessel  1  includes a hull  2 , a plurality of outboard motors  3  as marine vessel propulsion devices, and a steering apparatus  4  that controls turning angles of the respective outboard motors  3 . Three outboard motors  3  are provided in the present preferred embodiment. The outboard motors  3  are aligned and attached along a stern of the hull  2  and are put in states enabling swinging (turning) in the right and left directions. When the three outboard motors are to be distinguished, the outboard motor disposed at a starboard side shall be referred to as the “starboard outboard motor  3 S,” the outboard motor disposed at a center shall be referred to as the “central outboard motor  3 C,” and the outboard motor disposed at a port side shall be referred to as the “port outboard motor  3 P.” Each of the outboard motors  3  includes an engine (internal combustion engine as an example of a motor) and a propeller (screw) and generates a propulsive force by rotation of the propeller by a driving force of the engine. 
     A marine vessel operator compartment  5  is provided at a front portion (stem portion) of the hull  2 . The marine vessel operator compartment  5  includes a steering handle  6  as a steering member, a remote controller  7 , an operation panel  8 , a display  9 , and a main ECU (electronic control unit)  10 . 
     A steering angle of the steering handle  6  is detected by a steering angle sensor  11  (see  FIG. 4 ). Also, three turning mechanisms  12  (see  FIG. 2  and  FIG. 3 ), respectively corresponding to the three outboard motors  3 , are provided at the stern. Each turning mechanism  12  includes an electric motor  102  (see  FIG. 3 ) as a turning actuator driven in accordance with the steering angle detected by the steering angle sensor  11 . The electric motors  102  of the three turning mechanisms  12  are controlled by a turning ECU  20  (see  FIG. 4 ). 
     The steering apparatus  4  includes the steering handle  6 , the steering angle sensor  11 , the main ECU  10 , the turning ECU  20 , the three turning mechanisms  12 , three turning angle sensors  112  (see  FIG. 3  and  FIG. 4 ) to be described below, etc. Due to the turning angle of each outboard motor  3  being controlled by the steering apparatus  4 , a direction of the propulsive force is changed and a heading direction of the marine vessel  1  is changed accordingly. 
     The remote controller  7  includes two levers, i.e. right and left levers  7 P and  7 S. Each of these levers  7 P and  7 S can be inclined forward and rearward. When the two levers  7 P and  7 S are to be distinguished, the lever disposed at a left side facing the stem shall be referred to as the “left lever  7 P” and the lever disposed at the right side facing the stem shall be referred to as the “right lever  7 S.” 
     Inclination positions of the levers  7 P and  7 S are respectively detected by potentiometers or other lever position sensors  13 P and  13 S (see  FIG. 4 ). The lever position sensor  13 P corresponds to the left lever  7 P and the lever position sensor  13 S corresponds to the right lever  7 S. 
     The display  9  displays states of the outboard motors  3 , an operation guidance screen, etc. The operation panel  8  includes three key switches  81 P,  81 C, and  81 S (“key switch  81 ,” when referred to collectively below) respectively corresponding to the three outboard motors  3 P,  3 C, and  3 S. 
     The key switches  81 P,  81 C, and  81 S are switches that are operated to turn on and off power supplies to the outboard motors  3 P,  3 C, and  3 S, respectively, and to start the engines of the outboard motors  3 P,  3 C, and  3 S, respectively. Specifically, by operating a key switch  81  from an off position to an on position, the power supply to the corresponding outboard motor  3  can be turned on. Further, by operating the key switch  81  from the on position to the start position, the engine of the corresponding outboard motor  3  can be started. Also, by operating the key switch  81  from the on position to the off position, the power supply to the corresponding outboard motor  3  can be put in the off state. 
       FIG. 2  is a schematic side view for describing an arrangement example in common to the three outboard motors  3 . 
     Each outboard motor  3  includes a propulsion unit  60  and an attachment mechanism  61  to attach the propulsion unit  60  to the hull  2 . The attachment mechanism  61  includes a clamp bracket  62  detachably fixed to a transom of the hull  2  and a swivel bracket  64  coupled to the clamp bracket  62  in a manner enabling pivoting around a tilt shaft  63  as a horizontal pivot axis. The propulsion unit  60  is attached to the swivel bracket  64  in a manner enabling pivoting around a steering shaft  65 . Thus, a turning angle (a direction angle defined by the direction of the propulsive force with respect to a centerline of the hull  2 ) can be changed by pivoting the propulsion unit  60  around the steering shaft  65 . Further, a trim angle of the propulsion unit  60  can be changed by pivoting the swivel bracket  64  around the tilt shaft  63 . The trim angle is an angle of attachment of the outboard motor  3  with respect to the hull  2 . 
     A housing of the propulsion unit  60  includes a top cowling  66 , an upper case  67 , and a lower case  68 . An engine  69  is installed as a drive source in the top cowling  66  with an axis of a crankshaft thereof extending vertically. A driveshaft  91  for power transmission is coupled to a lower end of the crankshaft of the engine  69  and vertically extends through the upper case  67  into the lower case  68 . 
     A propeller  90 , which is a propulsive force generating member, is rotatably attached to a rear side of a lower portion of the lower case  68 . A propeller shaft  92 , which is a rotation shaft of the propeller  90 , extends horizontally in the lower case  68 . The rotation of the driveshaft  91  is transmitted to the propeller shaft  92  via a shift mechanism  93 , which is a clutch mechanism. 
     The shift mechanism  93  includes a drive gear  93   a , defined by a beveled gear fixed to a lower end of the driveshaft  91 , a forward drive gear  93   b , defined by a beveled gear rotatably disposed on the propeller shaft  92 , a reverse drive gear  93   c , likewise defined by a beveled gear rotatably disposed on the propeller shaft  92 , and a dog clutch  93   d  disposed between the forward drive gear  93   b  and the reverse drive gear  93   c.    
     The forward drive gear  93   b  is meshed with the drive gear  93   a  from a front side, and the reverse drive gear  93   c  is meshed with the drive gear  93   a  from a rear side. The forward drive gear  93   b  and the reverse drive gear  93   c  are thus rotated in mutually opposite directions. 
     The dog clutch  93   d  is in spline engagement with the propeller shaft  92 . That is, the dog clutch  93   d  is axially slidable with respect to the propeller shaft  92 , but is not rotatable relative to the propeller shaft  92  and thus rotates together with the propeller shaft  92 . 
     The dog clutch  93   d  is slid along the propeller shaft  92  by axial pivoting of a shift rod  94  extending vertically parallel or substantially parallel to the driveshaft  91 . The shift position of the dog clutch  93   d  is thus controlled to be set at a forward drive position at which it is engaged with the forward drive gear  93   b , a reverse drive position at which it is engaged with the reverse drive gear  93   c , or a neutral position at which it is not engaged with either the forward drive gear  93   b  or the reverse drive gear  93   c.    
     When the dog clutch  93   d  is at the forward drive position, the rotation of the forward drive gear  93   b  is transmitted to the propeller shaft  92  via the dog clutch  93   d . The propeller  90  is thus rotated in one direction (forward drive direction) to generate a propulsive force in a direction of moving the hull  2  forward. On the other hand, when the dog clutch  93   d  is at the reverse drive position, the rotation of the reverse drive gear  93   c  is transmitted to the propeller shaft  92  via the dog clutch  93   d . The reverse drive gear  93   c  is rotated in a direction opposite to that of the forward drive gear  93   b , and the propeller  90  is thus rotated in an opposite direction (reverse drive direction) to generate a propulsive force in a direction of moving the hull  2  in reverse. When the dog clutch  93   d  is at the neutral position, the rotation of the driveshaft  91  is not transmitted to the propeller shaft  92 . That is, transmission path of a driving force between the engine  69  and the propeller  90  is cut off so that a propulsive force is not generated in either direction. 
     In relation to each engine  69 , a starter motor  45  is disposed to start the engine  69 . The starter motor  45  is controlled by the outboard motor ECU  30 . Also, a throttle actuator  48  is provided to actuate a throttle valve  52  of the engine  69  to change a throttle opening degree and thus change an intake air amount of the engine  69 . The throttle actuator  48  may include an electric motor. The operation of the throttle actuator  48  is controlled by the outboard motor ECU  30 . The engine  69  further includes an engine speed sensor  43  to detect the rotation of the crankshaft so as to detect the rotational speed of the engine  69 . 
     Also, in relation to the shift rod  94 , a shift actuator  49  to change the shift position of the dog clutch  93   d  is provided. The shift actuator  49  includes, for example, an electric motor, and operation thereof is controlled by the outboard motor ECU  30 . In relation to the shift actuator  49 , a shift position sensor  44  that detects the shift position of the shift mechanism  93  is provided. 
     The turning mechanism  12  is coupled to a steering arm  97  fixed to the propulsion unit  60 . By operating the turning mechanism  12 , the propulsion unit  60  is pivoted to the right and left around the steering shaft  65  and steering of the marine vessel  1  can thus be performed. 
       FIG. 3  is an arrangement diagram of an arrangement of the turning mechanism. 
     The turning mechanism  12  is preferably a hydraulic turning mechanism. The turning mechanism  12  includes a hydraulic pump  101 , an electric motor  102  to drive the hydraulic pump  101 , and a hydraulic cylinder  103 . 
     The hydraulic cylinder  103  is preferably a double-rod type double acting cylinder. The hydraulic cylinder  103  includes a cylinder tube  104 , a piston  105  provided inside the cylinder tube  104 , and a piston rod  106  connected to the piston  105 . The cylinder tube  104  and the piston rod  106  extend in a right/left direction. A space inside the cylinder tube  104  is partitioned by the piston  105  into a first cylinder chamber  107  at the left side and a second cylinder chamber  108  at the right side. The piston  105  is capable of moving relatively to the right and left inside the cylinder tube  104 . Actually, the right/left position of the piston  105  is fixed with respect to the hull  2  and the cylinder tube  104  moves to the right and left with respect to the piston  105 . 
     The first cylinder chamber  107  is connected to a first port of the hydraulic pump  101  via a first oil passage  109 . The second cylinder chamber  108  is connected to a second port of the hydraulic pump  101  via a second oil passage  110 . 
     One end portion and another end portion of the piston rod  106  respectively project axially outward from one end portion and another end portion of the cylinder tube  104 . The one end portion and the other end portion of the piston rod  106  are respectively coupled to two fixed arms  111 . The two fixed arms  111  are fixed to the swivel bracket  64 . The piston rod  106  is thus attached to the hull  2  via the swivel bracket  64  and the clamp bracket  62  (see  FIG. 2 ). The cylinder tube  104  is coupled to the steering arm  97  fixed to the outboard motor  3 . The cylinder tube  104  is guided by the piston rod  106  and is thus enabled to move in the right and left directions with respect to the hull  2 . The outboard motor  3  pivots to the right and left around the steering shaft  65  in accompaniment with the movement of the cylinder tube  104  in the right and left directions. 
     In the description that follows, a turning angle midpoint of an outboard motor  3  is a position of the outboard motor  3  at which a rotation axis Ap of the propeller  90  of the outboard motor  3  is parallel or substantially parallel to a straight line extending in a front/rear direction of the hull  2  in a plan view. Also, a position of the cylinder tube  104  with respect to the hull  2  when the outboard motor  3  is positioned at the turning angle midpoint shall be referred to as the turning angle midpoint position of the cylinder tube  104 . 
     The turning angle sensor  112  to detect the actual turning angle of the outboard motor  3  is provided in a vicinity of the hydraulic cylinder  103 . The turning angle sensor  112  detects an amount of movement of the cylinder tube  104  in both the right and left directions from the turning angle midpoint position of the cylinder tube  104 . The turning angle sensor  112 , for example, outputs the amount of movement of the cylinder tube  104  in the left direction from the turning angle midpoint position as a positive value and outputs the amount of movement in the right direction from the turning angle midpoint position as a negative value. The turning angle of the outboard motor  3  is detected based on the movement amount of the cylinder tube  104  from the turning angle midpoint position that is detected by the turning angle sensor  112 . 
     When the turning angle sensors  112  provided in the turning mechanisms  12  of the respective outboard motors  3 P,  3 C, and  3 S are to be distinguished, the turning angle sensor corresponding to the port outboard motor  3 P shall be referred to as the “turning angle sensor  112 P,” the turning angle sensor corresponding to the central outboard motor  3 C shall be referred to as the “turning angle sensor  112 C,” and the turning angle sensor corresponding to the starboard outboard motor  3 S shall be referred to as the “turning angle sensor  112 S.” 
     A first pilot check valve  113  is provided in a middle of the first oil passage  109 . A second pilot check valve  114  is provided in a middle of the second oil passage  110 . A pilot port of the first pilot check valve  113  is connected to a portion in the second oil passage  110  between the hydraulic pump  101  and the second pilot check valve  114 . A pilot port of the second pilot check valve  114  is connected to a portion in the first oil passage  109  between the hydraulic pump  101  and the first pilot check valve  113 . 
     The first pilot check valve  113  and the second pilot check valve  114  allow oil to flow through from the hydraulic pump  101  side to the hydraulic cylinder  103  side and blocks the flow of oil from the hydraulic cylinder  103  side to the hydraulic pump  101  side. However, each of the pilot check valves  113  and  114  is put in a state enabling reverse flow (flow through of oil from the hydraulic cylinder  103  side to the hydraulic pump  101  side) when a pilot pressure thereof become no less than a predetermined value. 
     The first oil passage  109  and the second oil passage  110  are connected, at portions closer to the hydraulic cylinder  103  than to the pilot check valves  113  and  114 , by a bypass oil passage  116  including a bypass valve  115 . In the present preferred embodiment, the bypass valve  115  is a manually opened/closed bypass valve that is opened and closed manually and is normally in a closed state. 
     The first port of the hydraulic pump  101  is further connected via a first check valve  117  to an oil tank  121  and connected via a first relief valve  118  to the oil tank  121 . Likewise, the second port of the hydraulic pump  101  is connected via a second check valve  119  to the oil tank  121  and connected via a relief valve  120  to the oil tank  121 . 
     The electric motor  102  is driven to rotate in a forward rotation direction or a reverse rotation direction to drive the hydraulic pump  101 . Specifically, an output shaft of the electric motor  102  is coupled to an input shaft of the hydraulic pump  101  and by rotation of the output shaft of the electric motor  102 , the input shaft of the hydraulic pump  101  is rotated to achieve driving of the hydraulic pump  101 . The electric motor  102  is, for example, a DC motor. When the electric motors  102  provided in the turning mechanisms  12  of the respective outboard motors  3 P,  3 C, and  3 S are to be distinguished, the electric motor corresponding to the port outboard motor  3 P shall be referred to as the “electric motor  102 P,” the electric motor corresponding to the central outboard motor  3 C shall be referred to as the “electric motor  102 C,” and the electric motor corresponding to the starboard outboard motor  3 S shall be referred to as the “electric motor  102 S.” 
     When the electric motor  102  is rotated in the forward rotation direction, the hydraulic pump  101  is rotated forwardly and, for example, oil inside the oil tank  121  is sucked into the hydraulic pump  101  via the second check valve  119  and discharged from the hydraulic pump  101  to the first oil passage  109 . The oil discharged to the first oil passage  109  is supplied via the first pilot check valve  113  and the first oil passage  109  to the first cylinder chamber  107  of the hydraulic cylinder  103 . The cylinder tube  104  is thus moved in the left direction with respect to the hull  2  so that a volume of the first cylinder chamber  107  increases. Due to this process, the pilot pressure input into the second pilot check valve  114  becomes no less than the predetermined pressure and thus the second pilot check valve  114  is put in the state enabling reverse flow. The oil inside the second cylinder chamber  108  is thus sucked via the second oil passage  110  and the second pilot check valve  114  into the hydraulic pump  101 . 
     When the electric motor  102  is rotated in the reverse rotation direction, the hydraulic pump  101  is rotated reversely and the oil inside the oil tank  121  is sucked into the hydraulic pump  101  via the first check valve  117  and discharged from the hydraulic pump  101  to the second oil passage  110 . The oil discharged to the second oil passage  110  is supplied via the second pilot check valve  114  and the second oil passage  110  to the second cylinder chamber  108  of the hydraulic cylinder  103 . The cylinder tube  104  is thus moved in the right direction with respect to the hull  2  so that a volume of the second cylinder chamber  108  increases. Due to this process, the pilot pressure input into the first pilot check valve  113  becomes no less than the predetermined pressure and thus the first pilot check valve  113  is put in the state enabling reverse flow. The oil inside the first cylinder chamber  107  is thus sucked via the first oil passage  109  and the first pilot check valve  113  into the hydraulic pump  101 . 
     When the rotation of the electric motor  102  is stopped and the hydraulic pump  101  is not driven, the flow through of oil inside the cylinder chambers  107  and  108  of the hydraulic cylinder  103  is disabled by the pilot check valves  113  and  114 . The movement of the cylinder tube  104  is thus disabled and the outboard motor  3  is put in a state of not being able to pivot around the steering shaft  65  (state of being fixed in turning angle). When in this state, the bypass valve  115  is opened, the hydraulic chambers  107  and  108  of the hydraulic cylinder  103  are put in communication with each other via the oil passages  109 ,  115 , and  110  and flow through of oil between the cylinder chambers  107  and  108  is enabled. The outboard motor  3  is thus put in a state of being able to pivot freely around the steering shaft  65  (freely turning state). When the bypass valve  115  is opened, even when the electric motor  102  is driven, the hydraulic cylinder  103  is not actuated. 
       FIG. 4  is a diagram for describing an electrical arrangement of a principal portion of the marine vessel  1 . 
     The operation panel  8 , the display  9 , the steering angle sensor  11 , and the lever position sensors  13 P and  13 S are connected to the main ECU  10 . The main ECU  10  includes a computer (microcomputer). The main ECU  10  is connected to a bus  15  that defines an inboard LAN (local area network). Also, a speed sensor  14  to detect a speed of the marine vessel  1  is connected to the bus  15 . 
     The outboard motors  3 S,  3 C, and  3 P include outboard motor ECUs  30 S,  30 C, and  30 P, respectively. The outboard motor ECU  30 P corresponds to the port outboard motor  3 P, the outboard motor ECU  30 C corresponds to the central outboard motor  3 C, and the outboard motor ECU  30 S corresponds to the starboard outboard motor  3 S. The outboard motor ECUs  30 S,  30 C, and  30 P are connected to the bus  15 . The outboard motor ECUs  30 S,  30 C, and  30 P are practically the same in internal arrangement and shall be referred to as the “outboard motor ECU  30 ” when referred to collectively below. 
     Each outboard motor ECU  30  includes a computer (microcomputer). A temperature sensor  41 , a hydraulic pressure sensor  42 , the engine speed sensor  43 , the shift position sensor  44 , a starter motor  45 , an ignition coil  46 , an injector  47 , the throttle actuator  48 , the shift actuator  49 , a fuel pump  50 , an oil pump  51 , etc., are connected to the outboard motor ECU  30 . 
     The starter motor  45  is a device to perform cranking of the engine. The injector  47  is a device that injects fuel into an air intake path of the engine. The throttle actuator  48  is a device that controls the throttle valve  52  to adjust the amount of air supplied to the air intake path of the engine. The ignition coil  46  is a device that increases a voltage applied to a spark plug (not shown). The spark plug is a device that discharges inside a combustion chamber of the engine to ignite a mixed gas inside the combustion chamber. The shift actuator  49  is a device that drives the shift mechanism  93  of the outboard motor. The fuel pump  50  is a device that pumps out fuel from a fuel tank (not shown) to supply the fuel to the injector  47 . The oil pump  51  is a device that circulates engine oil inside the engine. 
     The temperature sensor  41  detects a temperature of cooling water in the engine. The hydraulic pressure sensor  42  detects a pressure of the engine oil. The engine speed sensor  43  detects the rotational speed of the engine. The shift position sensor  44  detects the shift position of the shift mechanism  93  (shift position of the outboard motor). 
     The electric motors  102 P,  102 C, and  102 S and the turning angle sensors  112 P,  112 C, and  112 S of the turning mechanisms  12  respectively corresponding to the outboard motors  30 P,  30 C, and  30 S are connected to the turning ECU  20 . The turning ECU  20  is connected to the bus  15 . The turning ECU  20  includes drive circuits to drive the respective electric motors  102 P,  102 C, and  102 S and a computer (microcomputer) to control the drive circuits. 
     The computer of the main ECU  10  executes programs to achieve the functions of a plurality of function processing units. The function processing units include an electric power supply/starting control unit, a shift position etc., computing unit, a basic target turning angle computing unit, and a malfunction operation control unit. 
     Functions of the main ECU  10  as the electric power supply/starting control unit include performing, on the basis of an operation signal from a key switch  81  on the operation panel  8 , on/off control of the electric power supply of the corresponding outboard motor  3  and starting control of the engine of the corresponding outboard motor  3 . Functions of the main ECU  10  as the shift position etc., computing unit include performing a shift position etc., computing process of computing target shift positions and target engine speeds of the respective outboard motors  3  based on outputs of the lever position sensors  13 P and  13 S. Functions of the main ECU  10  as the basic target turning angle computing unit include performing a basic target turning angle computing process of computing basic target turning angles of the respective outboard motors  3  based on an output of the steering angle sensor  11 . Functions of the main ECU  10  as the malfunction operation control unit include performing a malfunction operation control process when there is a malfunction in the turning angle control of any of the outboard motors  3 . 
     These functions shall now be described in detail. 
     The functions of the main ECU  10  as the electric power supply/starting control unit areas follows. That is, when a key switch  81  is operated from the off position to the on position, the main ECU  10  turns on the electric power supply of the corresponding outboard motor ECU  30 . Also, when the key switch  81  is operated from the on position to the off position, the main ECU  10  turns off the electric power supply of the corresponding outboard motor  3 . Also, when the key switch  81  is operated from the on position to the start position, the main ECU  10  outputs an engine starting command to the corresponding outboard motor ECU  30  under a condition that the starting allowing conditions are met. The starting allowing conditions include the target shift position of the outboard motor  3 , computed by the main ECU  10 , is the neutral position and the actual shift position of the shift mechanism  93  of the corresponding outboard motor  3  is the neutral position. Information on the shift position of the shift mechanism  93  of each outboard motor  3  is sent from the corresponding outboard motor ECU  30  to the main ECU  10  via the bus  15 . 
     Upon receiving the engine starting command, the outboard motor ECU  30  performs an engine starting process. In the engine starting process, the outboard motor ECU  30  drives the starter motor  45 , the ignition coil  46 , and the injector  47  to perform fuel supply control and ignition control to start the engine. 
     Functions of the main ECU  10  as the shift position etc., computing unit shall now be described. Based on the output signals of the lever position sensors  13 S and  13 P, the main ECU  10  computes the target shift positions and the target engine speeds for the respective outboard motors  3  and transmits these to the corresponding outboard motor ECUs  30 . Each outboard motor ECU  30  controls the shift position and the engine speed of the corresponding outboard motor  3  based on the target shift position and the target engine speed that are transmitted from the main ECU  10 . Specifically, the outboard motor ECU  30  controls the shift actuator  49  so that the shift position of the outboard motor  3  becomes the target shift position and controls the throttle actuator  48  so that the engine speed becomes the target engine speed. Such control shall now be described in detail. 
     The shift position of each outboard motor  3  is controlled as follows. In the present preferred embodiment, the left lever  7 P is associated with the port outboard motor  3 P, the right lever  7 S is associated with the starboard outboard motor  3 S, and both levers  7 P and  7 S are associated with the central outboard motor  3 C. 
     When the left lever  7 P is inclined forward by no less than a predetermined amount from a predetermined neutral position, the shift position of the port outboard motor  3 P is set to the forward drive position and a propulsive force in the forward drive direction is generated from the corresponding outboard motor  3 P. The target engine speed is set at an idling engine speed up to the inclination position of the predetermined amount (forward drive shift-in position). When the left lever  7 P is inclined forward beyond the forward drive shift-in position, the target engine speed increases as the lever inclination amount increases. When the left lever  7 P is inclined rearward by no less than a predetermined amount from the neutral position, the shift position of the port outboard motor  3 P is set at the reverse drive position and a propulsive force in the reverse drive direction is generated from the port outboard motor  3 P. The target engine speed is set at the idling engine speed up to the inclination position of the predetermined amount (reverse drive shift-in position). When the left lever  7 P is inclined rearward beyond the reverse drive shift-in position, the target engine speed increases as the lever inclination amount increases. When the left lever  7 P is at the neutral position, the shift position of the port outboard motor  3 P is set at the neutral position and the outboard motor  3 P does not generate a propulsive force. 
     When the right lever  7 S is operated, the shift position and the engine speed of the starboard outboard motor  3 S are controlled in the same manner as in the above-described control of the shift position and the engine speed of the port outboard motor  3 P that is performed when the left lever  7 P is operated. 
     Further, the shift position of the central outboard motor  3 C is controlled as follows according to the operations of both levers  7 P and  7 S. That is, when the levers  7 P and  7 S are both inclined forward to no less than the forward drive shift-in positions from the neutral positions, the shift position of the central outboard motor  3 C is set at the forward drive position and a propulsive force in the forward drive direction is generated from the central outboard motor  3 C. When the levers  7 P and  7 S are both inclined rearward to no less than the reverse drive shift-in positions from the neutral positions, the shift position of the central outboard motor  3 C is controlled to be at the reverse drive position and a propulsive force in the reverse drive direction is generated from the central outboard motor  3 C. 
     The target engine speed is set to the idling engine speed if the inclination positions of both levers  7 P and  7 S are between the forward drive shift-in positions and the reverse drive shift-in positions. When the lever inclination positions are outside the ranges between both shift-in positions, the target engine speed is set according to the inclination amounts of both levers  7 P and  7 S. 
     If at least one of either of the levers  7 P and  7 S is at the neutral position, the shift position of the central outboard motor  3 C is set at the neutral position. The shift position of the central outboard motor  3 C is also set at the neutral position when one of the levers is inclined forward from the neutral position (for example, inclined forward relative to the forward drive shift-in position) and the other lever is inclined rearward from the neutral position (for example, inclined rearward relative to the reverse drive shift-in position). 
       FIGS. 5A-5F  are schematic views for describing relationships of the respective lever positions and movements of a hull. 
     When as shown in  FIG. 5A , the left lever  7 P is inclined forward (to an F side) relative to the neutral position and the right lever  7 S is at the neutral position, the shift position of the port outboard motor  3 P is set at the forward drive position and the shift positions of the other outboard motors  3 C and  3 S are set at the neutral positions. The hull  2  thus receives only the forward drive direction propulsive force of the port outboard motor  3 P and thus turns in the right direction. 
     When as shown in  FIG. 5B , the right lever  7 S is inclined forward (to the F side) relative to the neutral position and the left lever  7 P is at the neutral position, the shift position of the starboard outboard motor  3 S is set at the forward drive position and the shift positions of the other outboard motors  3 P and  3 C are set at the neutral positions. The hull  2  thus receives only the forward drive direction propulsive force of the starboard outboard motor  3 S and thus turns in the left direction. 
     When as shown in  FIG. 5C , both levers  7 P and  7 S are inclined forward (to the F side) relative to the neutral positions, the shift positions of all three outboard motors  3  are set at the forward drive positions. The hull  2  is thus driven forward by the forward drive direction propulsive forces of all three outboard motors  3 . 
     When as shown in  FIG. 5D , both levers  7 P and  7 S are inclined rearward (to an R side) relative to the neutral positions, the shift positions of all three outboard motors  3  are set at the reverse drive positions. The hull  2  is thus driven in reverse by the reverse drive direction propulsive forces of all three outboard motors  3 . 
       FIG. 5E  shows a state where the left lever  7 P is inclined rearward (to the R side) relative to the neutral position and the right lever  7 S is inclined forward (to the F side) relative to the neutral position. In this case, the shift position of the port outboard motor  3 P is set at the reverse drive position, the shift position of the starboard outboard motor  3 S is set at the forward drive position, and the shift position of the central outboard motor  3 C is set at the neutral position. The hull  2  is thus turned to the left by the reverse drive direction propulsive force of the port outboard motor  3 P and the forward drive direction propulsive force of the starboard outboard motor  3 S. 
       FIG. 5F  shows a state where the left lever  7 P is inclined forward (to the F side) relative to the neutral position and the right lever  7 S is inclined rearward (to the R side) relative to the neutral position. In this case, the shift position of the port outboard motor  3 P is set at the forward drive position, the shift position of the starboard outboard motor  3 S is set at the reverse drive position, and the shift position of the central outboard motor  3 C is set at the neutral position. The hull  2  is thus turned to the right by the forward drive direction propulsive force of the port outboard motor  3 P and the reverse drive direction propulsive force of the starboard outboard motor  3 S. 
     The functions of the main ECU  10  as the basic target turning angle computing unit and as the malfunction operation control unit shall be described below. 
     The computer of each outboard motor ECU  30  executes programs to achieve the functions of a plurality of function processing units. The plurality of function processing units include an engine starting process unit, a shift control unit, etc. A function of the outboard motor ECU  30  as the engine starting process unit is to perform the engine starting process. A function of the outboard motor ECU  30  as a shift control unit is to control the engine speed and the shift position based on the target engine speed and the target shift position computed by the main ECU  10 . 
     The computer of the turning ECU  20  executes programs to achieve the functions of a plurality of function processing units. The plurality of function processing units include a motor control unit, a malfunction monitoring unit, etc. A function of the turning ECU  20  as the motor control unit is to perform a motor control process to control the electric motors  102  of the turning mechanisms  12  of the respective outboard motors  3  based on the basic target turning angle computed by the main ECU  10 . A function of the turning ECU  20  as the malfunction monitoring unit is to monitor whether or not there is a malfunction in the turning angle control of the respective outboard motors  3 . 
     The function of the main ECU  10  as the basic target turning angle computing unit and the function of the turning ECU  20  as the motor control unit shall now be described with reference to  FIG. 6 . 
       FIG. 6  is a flowchart of procedures of the basic target turning angle computing process performed by the main ECU  10  and procedures of the motor control process performed by the turning ECU  20 . The reference target turning angle computing process and the motor control process shown in  FIG. 6  are performed repeatedly at every predetermined computation cycle. 
     The main ECU  10  acquires a steering angle θ based on the output of the steering angle sensor  11  (step S 1 ). The main ECU  10  then computes a basic target turning angle δo* in common to all of the outboard motors  3  based on the acquired steering angle θ and transmits it to the turning ECU  20  (step S 2 ). The main ECU  10 , for example, computes the basic target turning angle δo* corresponding to the acquired steering angle θ based on a map by which a relationship of the steering angle θ and the basic target turning angle δo* is stored in advance. 
     Upon receiving the basic target turning angle δo* transmitted from the main ECU  10  (step S 11 : YES), the turning ECU  20  computes target turning angles δ* of the respective outboard motors  3  based on the received basic target turning angle δo* (step S 12 ). For example, the turning ECU  20  computes the target turning angles δ* of the respective outboard motors  3  corresponding to the received basic target turning angle δo* based on a map by which a relationship of the basic target turning angle δo* and the target turning angles δ* of the respective outboard motors  3  is stored in advance. The turning ECU  20  may use the received basic target turning angle δo* as it is as the target turning angle δ* of each outboard motor  3 . 
     Thereafter, the turning ECU  20  uses the target turning angle δ* of each outboard motor  3  to perform feedback control of the electric motor  102  of the turning mechanism  12  of the corresponding outboard motor  3  (step S 13 ). Specifically, the turning ECU  20  drives the electric motor  102  of the turning mechanism  12  of each outboard motor  3  so that the actual turning angle δ of the corresponding outboard motor  3  detected by the turning angle sensor  112  approaches the target turning angle δ* of the corresponding outboard motor  3 . The turning angles of the respective outboard motors  3  are thus controlled in accordance with the steering angle of the steering handle  6 . 
     Details of the functions of the steering ECU  20  as the malfunction monitoring unit are as follows. When in a certain outboard motor  3  there is a malfunction of the turning mechanism  12  or the turning angle sensor  112 , the actual turning angle δ corresponding to the outboard motor  3  does not converge to the target turning angle δ* corresponding to the outboard motor  3 . The turning ECU  20  thus monitors, for each outboard motor  3 , whether or not a state where a difference between the actual turning angle δ and the target turning angle δ* is greater than a predetermined value has continued for no less than a predetermined time. When for a certain outboard motor  3  the state where the difference between the actual turning angle δ and the target turning angle δ* is greater than a predetermined value has continued for no less than the predetermined time, the turning ECU  20  judges that there is a malfunction in the turning angle control of the outboard motor  3  and provides notification of this condition to the main ECU  10 . 
     The functions of the main ECU  10  as the malfunction operation control unit shall now be described. 
       FIG. 7A ,  FIG. 7B , and  FIG. 7C  are flowcharts of procedures of the malfunction operation control process executed by the main ECU  10 . 
     The main ECU  10  monitors whether or not there is a malfunction in the turning angle control of each outboard motor  3  (step S 21 ). Malfunctions in the turning angle control of an outboard motor  3  include a malfunction of the steering angle sensor  11 , a malfunction of the turning mechanism  12 , a malfunction of the turning angle sensor  112 , etc. A malfunction of the steering angle sensor  11  is included among malfunctions due to an input system in common to all turning mechanisms  12 . A malfunction of the turning mechanism  12  or a malfunction of the turning angle sensor  112  is included among malfunctions due to output systems of the respective turning mechanisms  12 . 
     When there is a malfunction of the steering angle sensor  11 , the output signal of the steering angle sensor  11  is fixed at a predetermined value. The main ECU  10  can thus detect the malfunction of the steering angle sensor  11  (including disconnection of a signal line of the steering angle sensor  11 ) by monitoring the output signal of the steering angle sensor  11 . When the malfunction of the steering angle sensor  11  is detected, the turning angle control of none of the outboard motors  3  can be performed and thus the main ECU  10  judges that there is a malfunction in the turning angle control of all of the outboard motors  3 . 
     In a case where there is a malfunction in the turning angle control of any of the outboard motors  3  due to a malfunction, etc., of a turning mechanism  12  or a turning angle sensor  112 , a notification of this condition is provided from the turning ECU  20  to the main ECU  10  as mentioned above. The main ECU  10  can thus detect that there is a malfunction in the turning angle control of any of the outboard motors  3  and can recognize the outboard motor  3  with the malfunction in the turning angle control. 
     Upon detecting that there is a malfunction in the turning angle control of any of the outboard motors  3  among the three outboard motors  3  (step S 21 : YES), the main ECU  10  enters step S 22 . In step S 22 , the main ECU  10  judges whether there is a malfunction in the turning angle control of all outboard motors  3  or there is a malfunction in the turning angle control of one of the outboard motors  3  and stores the judgment result (step S 22 ). Specifically, the main ECU  10  sets an all-outboard-motor malfunction flag F (F=1) if it judges that there is a malfunction in the turning angle control of all outboard motors  3  and resets the all-outboard-motor malfunction flag F (F=0) if it judges that there is a malfunction in the turning angle control of one of the outboard motors  3 . 
     Thereafter, the main ECU  10  performs a process to forcibly decelerate a traveling speed of the marine vessel  1  (this process may hereinafter be referred to at times as the “forced deceleration process”) (step S 23 ). Specifically, the main ECU  10  fixes the target engine speeds for all outboard motors  3  at a predetermined speed regardless of the positions of the levers  7 P and  7 S and fixes the target shift positions for all outboard motors  3  at the neutral positions regardless of the positions of the levers  7 P and  7 S. The predetermined speed is set, for example, to an idling engine speed. The fixing of the target engine speed at the predetermined speed may also be performed by forcibly closing the throttle valves  52  fully. 
     Accordingly, at each outboard motor ECU  30 , a control of setting the engine speed of the corresponding outboard motor  3  at the predetermined speed and a control of setting the shift position of the corresponding outboard motor  3  at the neutral position are performed. The power transmission between the engine  69  and the propeller  90  is thus interrupted in all outboard motors  3  so that the generation of a propulsive force by all outboard motors  3  is stopped and the traveling speed of the marine vessel  1  is decelerated. 
     Also, the main ECU  10  displays, on the display  9 , an operation guidance screen to urge the marine vessel operator to operate the levers  7 P and  7 S to the neutral positions (step S 24 ). An example of the operation guidance screen displayed on the display in step S 24  is shown in  FIG. 8 . The operation guidance screen includes the emergency message: “There is a turning angle control malfunction.” and the operation guidance: “Operate the levers to the neutral positions.” The main ECU  10  waits for the levers  7 P and  7 S to be operated to the neutral positions (step S 25 ). Whether or not the levers  7 P and  7 S have been operated to the neutral positions is judged based on the output signals of the lever position sensors  13 P and  13 S. 
       FIGS. 11A-11C  are schematic views for specifically describing the process of steps S 21  to S 25 . 
     As shown in  FIG. 11A , a case where there is a malfunction in the turning angle control of the starboard outboard motor  3 S in a state where all three of the outboard motors  3  are generating propulsive forces in the forward drive direction and the hull  2  is being driven forward shall be presumed. In this case, the main ECU  10  detects that there is a malfunction in the turning angle control of the starboard outboard motor  3 S and resets the all-outboard-motor malfunction flag (F=0). Also, the main ECU  10  performs the “forced deceleration process.” The engine speeds of all outboard motors  3  are thus set at the predetermined speed regardless of the positions of the levers  7 P and  7 S and the shift positions of all outboard motors  3  are set at the neutral positions as shown in  FIG. 11B . Also, the main ECU  10  displays the operation guidance screen, such as shown in  FIG. 8 , on the display  9 . The marine vessel operator operates the levers  7 P and  7 S to the neutral positions as shown in  FIG. 11C  in accordance with the operation guidance screen. 
     When after the operation guidance screen (see  FIG. 8 ) has been displayed in step S 24 , the levers  7 P and  7 S are operated to the neutral positions by the marine vessel operator (step S 25  of  FIG. 7A : YES), the main ECU  10  ends the “forced deceleration process” that is currently being performed and restarts the normal shift position etc., computing process (step S 26 ). The target engine speeds and the target shift positions computed in accordance with the positions of the lever  7 P and  7 S are thus transmitted to the respective outboard motor ECUs  30 . 
     When the process of step S 26  ends, the main ECU  10  judges whether or not the all-outboard-motor malfunction flag F is set (F=1) (step S 27 ). If the all-outboard-motor malfunction flag F is set (F=1) (step S 27 : YES), that is, if there is a malfunction in the turning angle control of all outboard motors  3 , the main ECU  10  stops the turning angle control of all outboard motors  3  (step S 28 ). Specifically, the main ECU  10  transmits a turning angle control stopping command to stop the turning angle control of all outboard motors  3  to the turning ECU  20 . Upon receiving the turning angle control stopping command, the turning ECU  20  stops the motor control process for the turning mechanisms  12  of all outboard motors  3 . All outboard motors  3  are thus fixed at the turning angle position at that point and put in a state where turning is disabled. 
     Also, the main ECU  10  restricts the engine speeds of all outboard motors  3  to no more than a predetermined first restriction speed (step S 29 ). Specifically, the main ECU  10  restricts the target engine speeds transmitted to all outboard motor ECUs  30  to no more than the predetermined first restriction speed. More specifically, in a case where a target engine speed computed based on the positions of the lever  7 P and  7 S is higher than the first restriction speed, the main ECU  10  restricts the target engine speed to the first restriction speed. The propulsive forces of all outboard motors  3  are thus restricted. 
     Thereafter, the main ECU  10  displays, on the display  9 , an operation guidance screen to notify the marine vessel operator that there is a malfunction in the turning angle control of all outboard motors  3  and that steering should be performed by operating the levers  7 P and  7 S (step S 30 ). An example of the operation guidance screen displayed on the display  9  in step S 29  is shown in  FIG. 9 . The operation guidance screen includes the emergency message: “There is a turning angle control malfunction,” the character string: “All outboard motors” indicating that there is a malfunction in the turning angle control of all outboard motors, and the operation guidance: “Perform steering by operating the levers.” 
     By viewing the operation guidance screen, the marine vessel operator recognizes that there is a malfunction in the turning angle control of all outboard motors and that steering should be performed by operating the levers  7 P and  7 S. The marine vessel operator thus judges whether or not the turning angles of all outboard motors  3  are near the respective turning angle midpoints (step S 31 ). If the marine vessel operator judges that the turning angles of all outboard motors  3  are not near the turning angle midpoints (step S 31 : NO), he/she performs the following operation. That is, the marine vessel operator opens the bypass valves  115  of the turning mechanisms  12  corresponding to all outboard motors  3 , moves all outboard motors  3  to the respective turning angle midpoints manually and thereafter closes the bypass valves  115  of the turning mechanisms  12  corresponding to all outboard motors  3  (step S 32 ). The turning angles of all outboard motors  3  are thus fixed near the turning angle midpoints. Thereafter, the marine vessel operator operates the levers  7 P and  7 S to steer the marine vessel  1  (step S 33 ). 
     If the marine vessel operator judges that the turning angles of all outboard motors  3  are near the turning angle midpoints (step S 31 : YES), the marine vessel operator steers the marine vessel  1  by lever operations without performing the operation of step S 32  (step S 33 ). 
     In step S 33 , a turning operation of the marine vessel  1  by generation of a propulsive force at no less than one of the outboard motors  3  is performed in the state where the turning angles of all outboard motors  3  are fixed near the turning angle midpoints. That is, the turning operation of the marine vessel  1  is performed by output differences among the outboard motors  3 . For example, the turning operation of the marine vessel  1  is performed by the lever operations described in  FIG. 5A ,  FIG. 5B ,  FIG. 5E , and  FIG. 5F . A turning performance of the marine vessel  1  can thus be secured even when there is a malfunction in the turning angle control of all outboard motors  3 . 
     If in the case where there is a malfunction in the turning angle control of all outboard motors  3 , the propulsive forces of the outboard motors  3  become too great due to turning the marine vessel  1  by making use of the propulsive forces of the outboard motors  3  without performing turning angle control of the outboard motors  3 , it may be difficult to obtain a turning behavior intended by the marine vessel operator. Thus, in the present preferred embodiment, the engine speeds of all outboard motors  3  are restricted to no more than the predetermined first restriction speed in step S 29 . The propulsive forces of the outboard motors  3  can thus be prevented from becoming too large and the turning behavior intended by the marine vessel operator can be obtained readily. 
     In step S 30 , an operation guidance for making all outboard motors  3  move to the turning angle midpoints may be displayed in the operation guidance screen. In the case where a malfunction of all outboard motors  3  is detected, all outboard motors  3  may be forcibly controlled respectively to move to the turning angle midpoints automatically. 
     If in step S 27 , it is judged that the all-outboard-motor malfunction flag F is reset (F=0) (step S 27 : NO), that is, if there is a malfunction in the turning angle control of one of the outboard motors  3 , the main ECU  10  enters step S 34 . In step S 34 , the main ECU  10  stops the turning angle control of the outboard motor  3  with the turning angle control malfunction (hereinafter referred to as the “malfunctioning outboard motor”). Specifically, the main ECU  10  transmits a turning angle control stopping command to stop the turning angle control of the malfunctioning outboard motor  3  to the turning ECU  20 . Upon receiving the turning angle control stopping command, the turning ECU  20  stops the motor control process for the turning mechanism  12  of the malfunctioning outboard motor  3 . The malfunctioning outboard motor  3  is thus put in a state where the turning angle control is not performed. 
     Thereafter, the main ECU  10  displays, on the display  9 , an operation guidance screen to urge the marine vessel operator to open the bypass valve of the turning mechanism  12  corresponding to the outboard motor  3  with the turning angle control malfunction (step S 35 ). An example of the operation guidance screen displayed on the display  9  in step S 35  is shown in  FIG. 10 . The operation guidance screen includes the emergency message: “There is a turning angle control malfunction,” the character string: “Starboard outboard motor” indicating that there is a malfunction in the turning angle control of the starboard outboard motor, and the operation guidance: “Open the bypass valve of the malfunctioning outboard motor.” 
     Thereafter, the main ECU  10  performs a process to forcibly stop the generation of a propulsive force by the malfunctioning outboard motor  3  (hereinafter referred to at times as the “malfunctioning outboard motor propulsive force stopping process”) (step S 36 ). Specifically, the main ECU  10  fixes the target engine speed for the malfunctioning outboard motor  3  at a predetermined idle engine speed regardless of the positions of the levers  7 P and  7 S and fixes the target shift position for the malfunctioning outboard motor  3  at the neutral position regardless of the positions of the levers  7 P and  7 S. 
     Accordingly, at the outboard motor ECU  30  corresponding to the malfunctioning outboard motor  3 , a control of setting the engine speed of the malfunctioning outboard motor  3  at the idle engine speed and a control of setting the shift position of the malfunctioning outboard motor  3  at the neutral position are performed. The power transmission between the engine  69  and the propeller  90  is thus interrupted at the malfunctioning outboard motor  3  so that the generation of a propulsive force by the malfunctioning outboard motor  3  is stopped regardless of the positions of the levers  7 P and  7 S. The engine speed of the malfunctioning outboard motor  3  is set at the idle engine speed to prevent wasteful fuel consumption. Thereafter, the outboard motor ECU  30  corresponding to the malfunctioning outboard motor  3  keeps the shift position of the malfunctioning outboard motor  3  at the neutral position regardless of the positions of the levers  7 P and  7 S. The malfunctioning outboard motor  3  is thus kept in the state where the power transmission between the engine  69  and the propeller  90  is interrupted. 
     With the other normally functioning outboard motors  3 , such a propulsive force stopping process is not performed and thus control based on the normal shift position etc., computing process is performed. That is, the other normally functioning outboard motors  3  are kept in the state where the change of the shift position in accordance with the positions of the levers  7 P and  7 S is enabled and the power transmission between the engine  69  and the propeller  90  is allowed. The propulsive force can thus be secured by the other normally functioning outboard motors  3 . 
     Thereafter, the main ECU  10  restricts the engine speeds of the other normally functioning outboard motors  3  (the outboard motors without a malfunction in the turning angle control) to no more than a predetermined second restriction speed (step S 37 ). Specifically, the main ECU  10  restricts the target engine speeds transmitted to the other normally functioning outboard motors  3  to no more than the predetermined second restriction speed. More specifically, in a case where a target engine speed for a normally functioning outboard motor  3  computed based on the positions of the lever  7 P and  7 S is higher than the second restriction speed, the main ECU  10  restricts the target engine speed to the second restriction speed. 
     By viewing the operation guidance screen displayed in step S 35  (see  FIG. 10 ), the marine vessel operator recognizes that there is a malfunction in the turning angle control of one of the outboard motors  3  and that the bypass valve  115  of the turning mechanism  12  corresponding to the malfunctioning outboard motor  3  should be opened. The marine vessel operator thus opens the bypass valve  115  of the turning mechanism  12  corresponding to the malfunctioning outboard motor  3  (step S 38 ). The malfunctioning outboard motor  3  is thus put in a freely turning state of pivoting freely in the right and left directions even though the turning angle control thereof cannot be performed. The malfunctioning outboard motor  3  is thus put in a state of being pivotable under the influence of an external force due to the other adjacent outboard motors  3  or by a water stream. Here, the turning angle control of the malfunctioning outboard motor  3  is kept in the stopped state by step S 34 . However, even if the turning angle control of the malfunctioning outboard motor  3  is continued, the hydraulic cylinder  103  is not actuated by the driving of the electric motor  102  of the corresponding turning mechanism  12  because the bypass valve  115  of the turning mechanism  12  corresponding to the malfunctioning outboard motor  3  is open. 
     The marine vessel operator then steers the marine vessel  1  by operating the steering handle  6  while the bypass valve  115  is kept in the open state (step S 39 ). The turning mechanisms  12  corresponding to the normally functioning outboard motors  3  are kept in states of being capable of turning the corresponding outboard motors  3  (normal states) and thus the turning angle control of the normally functioning outboard motors  3  is performed as is done normally. The turning performance of the marine vessel  1  can thus be secured by the turning angle control of the normally functioning outboard motors  3 . 
     As mentioned above, in the case where there is a malfunction in the turning angle control of one of the outboard motors  3 , the malfunctioning outboard motor  3  is put in the state where the generation of a propulsive force is stopped and is put in the freely turning state. Thus, when the marine vessel  1  is made to travel by the propulsive force of the normally functioning outboard motors  3 , the malfunctioning outboard motor  3  is turned in the same direction as the other normally functioning outboard motors  3  so as to follow a water stream generated in a periphery thereof. A possibility of a normally functioning outboard motor  3  contacting the malfunctioning outboard motor  3  when the turning angle control of the normally functioning outboard motors  3  is performed is thus low. Also, even if a normally functioning outboard motor  3  contacts the malfunctioning outboard motor  3 , a load due to the contact is small. 
     Also, in this case, the propulsive force due to the malfunctioning outboard motor  3  is stopped and thus the marine vessel  1  travels at a lower vessel speed than a vessel speed due to a maximum propulsive force that can be generated from all outboard motors  3 . The load in the case where a normally functioning outboard motor  3  contacts the malfunctioning outboard motor  3  is thus significantly reduced. Especially, with the present preferred embodiment, the engine speeds of the normally functioning outboard motors  3  are restricted to no more than the second restriction speed in step S 37  and thus the load in the case where a normally functioning outboard motor  3  contacts the malfunctioning outboard motor  3  can be reduced even further. 
     Although preferred embodiments of the present invention have been described above, the present invention can be carried out in yet other modes as well. For example, with the preferred embodiments described above, the operation guidance screen that provides notification that the levers  7 P and  7 S should be operated to the neutral positions (see  FIG. 8 ) is displayed in step S 24  of  FIG. 7A . Apart from this, the operation guidance screen corresponding to the case where there is a malfunction in the turning angle control of all outboard motors  3  (see  FIG. 9 ) is displayed in step S 30  of  FIG. 7B  and the operation guidance screen corresponding to the case where there is a malfunction in the turning angle control of one of the outboard motors  3  (see  FIG. 10 ) is displayed in step S 35  of  FIG. 7C . However, the contents of the operation guidance screen of  FIG. 9  or the contents of the operation guidance screen of  FIG. 10  may be included, in accordance with the judgment result of step S 22  of  FIG. 7A , in the operation guidance screen of  FIG. 8  displayed in step S 24  of  FIG. 7A . 
     Although in the preferred embodiments described above, the engine speeds of all outboard motors  3  preferably are restricted to no more than the predetermined first restriction speed in step S 29  of  FIG. 7B , this process may be omitted. 
     In the preferred embodiments described above, the target engine speed for the malfunctioning outboard motor  3  is fixed at the predetermined idling engine speed and the target shift position for the malfunctioning outboard motor  3  is fixed at the neutral position in step S 36  of  FIG. 7B . However, in step S 36 , just the target shift position for the malfunctioning outboard motor  3  may be fixed at the neutral position without fixing the target engine speed for the malfunctioning outboard motor  3  at the predetermined idling engine speed. 
     Although in the preferred embodiments described above, the engine speeds of the other normally functioning outboard motors  3  are preferably restricted to no more than the predetermined second restriction speed in step S 37  of  FIG. 7B , this process may be omitted. 
     Although in the preferred embodiments described above, the bypass valve  115  preferably is a manually opened/closed bypass valve, it may instead be an automatically opened/closed bypass valve that is opened and closed by electric power. 
     Also, although in the preferred embodiments described above, the turning mechanisms  12  of the three outboard motors  3  are preferably controlled by a single turning ECU  20  in common thereto, the turning mechanisms  12  may instead be controlled by a plurality of turning ECUs provided in respective correspondence to the plurality of outboard motors  3 . 
     Also, although with the preferred embodiments described above, a case where the motor of each outboard motor preferably is an engine was described, the motor of each outboard motor may instead be an electric motor. 
     Also, although in the preferred embodiments described above, the turning mechanism  12  preferably is arranged to control the turning direction of the outboard motor by the rotation direction of the hydraulic pump  101 , an arrangement is also possible where a directional control valve, driven by an electric motor, is provided between the hydraulic pump  101  and the hydraulic cylinder  103 . With an arrangement provided with such a directional control valve, the hydraulic pump  101  is always rotatingly driven in a fixed direction and the turning direction of the outboard motor is controlled by control of the electric motor to drive the directional control valve. 
     Besides the above, various design changes may be applied within the scope of the matters described in the claims. 
     A non-limiting example of the correspondence between the components described in the claims and the components of the preferred embodiment described above is shown below: 
     motor: engine  69   
     steering member: steering handle  6   
     malfunction judging unit: main ECU  10 , turning ECU  20 , step S 21  of  FIG. 7A   
     notifying unit: display  9 , main ECU  10 , step S 35  of  FIG. 7C   
     power control unit: main ECU  10 , outboard motor ECU  30 , step S 36  of  FIG. 7C   
     restricting unit: main ECU  10 , outboard motor ECU  30 , step S 36  of  FIG. 7C   
     turning angle control stopping unit: main ECU  10 , turning ECU  20 , step S 28  of  FIG. 7B   
     The present application corresponds to Japanese Patent Application No. 2012-228656 filed on Oct. 16, 2012 in the Japan Patent Office, and the entire disclosure of which is incorporated herein by reference. 
     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.