Patent Description:
<CIT> discloses a watercraft that includes two outboard motors attached to the rear side of its hull and is capable of controlling the outboard motors by operating a joystick. The watercraft is capable of taking a forward behavior, a reverse behavior, a rightward translation behavior, a leftward translation behavior, a fixed-point right turning behavior, and a fixed-point left turning behavior according to the operation of the joystick. That is, the steering states and the shift positions of the two outboard motors are controlled so as to provide any of these watercraft behaviors. For the forward behavior and the reverse behavior, the two outboard motors are steered generally parallel at the same shift position (a forward shift position or a reverse shift position). For the rightward translation behavior and the leftward translation behavior, the two outboard motors are steered with their center lines extending in an inverted V-shape toward a hull moving center (i.e., the two outboard motors are steered in a toe-in orientation), and one of the two outboard motors is set in the forward shift position and the other outboard motor is set in the reverse shift position. For the fixed-point right turning behavior and the fixed-point left turning behavior, the two outboard motors are steered in parallel or substantially parallel, and one of the two outboard motors is set in the forward shift position and the other outboard motor is set in the reverse shift position.

It has been considered to control a watercraft behavior by controlling the steering states of plural propulsion devices in a manner different from that shown in <CIT>.

It is an object of the present invention to provide a watercraft propulsion system, a watercraft propulsion control method for controlling a watercraft that are each able to properly control watercraft behavior by controlling the steering states of a plurality of propulsion devices in a manner different from the conventional manner, and watercraft including the watercraft propulsion systems.

According to the present invention said object is solved by a watercraft propulsion system having the features of independent claim <NUM>. Moreover said object is also solved by a watercraft according to claim <NUM>. Preferred embodiments are laid down in the dependent claims.

Moreover, according to the present invention said object is solved by watercraft propulsion control method for controlling a watercraft having the features of independent claim <NUM>. Preferred embodiments are laid down in the dependent claims.

Further preferred embodiments provide watercraft propulsion systems that are each able to solve a problem which is likely to occur when the watercraft behavior is controlled by controlling the steering states of a plurality of propulsion devices in a manner different from the conventional manner, and watercraft including the watercraft propulsion systems.

In order to overcome the previously unrecognized and unsolved challenges described above, a preferred embodiment provides a watercraft propulsion system including a first propulsion device attachable to a hull in a steerable manner (in a laterally pivotable manner, a second propulsion device attachable to the hull adjacent to the first propulsion device in a steerable manner, a first steering device to steer the first propulsion device, a second steering device to steer the second propulsion device, and a controller configured or programmed to control the first propulsion device, the second propulsion device, the first steering device, and the second steering device. The controller is configured or programmed to determine whether or not a predetermined load torque increase condition is satisfied in which a steering load torque of the second propulsion device is likely to be increased by a water jet generated by the first propulsion device, and to perform a propulsive force restricting control to restrict the propulsive forces of the first propulsion device and the second propulsion device if the predetermined load torque increase condition is satisfied.

With this arrangement, if the water jet generated by one of the two adjacent propulsion devices (the first propulsion device) is likely to increase the steering load torque of the other propulsion device (the second propulsion device), the propulsive forces of the two propulsion devices (the first propulsion device and the second propulsion device) are restricted. Thus, the two propulsion devices are each steerable to a proper steering angle without any substantial influence of a water jet generated by either one of the propulsion devices, making it possible to apply the propulsive forces in proper directions to the hull. Thus, even if the steering states of the plurality of propulsion devices are controlled in a manner different from the conventional manner, the watercraft behavior is properly controlled. This makes it possible to steer the propulsion devices to the proper steering angles while preventing the steering load torque from being excessively increased.

In a preferred embodiment, the propulsive force restricting control includes a propulsive force reducing control to control the propulsive forces of the first propulsion device and the second propulsion device to be lower than their target propulsive forces until the second propulsion device is completely steered to its target steering angle.

With this arrangement, the propulsive forces of the two adjacent propulsion devices are controlled to be lower than their target propulsive forces until the other propulsion device (the second propulsion device) which is otherwise likely to be influenced by the waterjet generated by the one propulsion device (the first propulsion device) is completely steered to its target steering angle. This makes it possible to reliably steer the two propulsion devices to their target steering angles while preventing the steering load torque from being excessively increased.

In a preferred embodiment, the propulsive force restricting control includes a propulsive force generation prohibiting control to prohibit the first propulsion device and the second propulsion device from generating the propulsive forces until the second propulsion device is completely steered to its target steering angle.

With this arrangement, the two adjacent propulsion devices are prohibited from generating the propulsive forces until the other propulsion device (the second propulsion device) which is otherwise likely to be influenced by the water jet generated by the one propulsion device (the first propulsion device) is completely steered to its target steering angle. This makes it possible to reliably steer the two propulsion devices to their target steering angles while preventing the steering load torque from being excessively increased.

In a preferred embodiment, the predetermined load torque increase condition includes a steering angle condition such that a first target steering angle is set for the first propulsion device so as to direct the water jet generated by the first propulsion device toward the second propulsion device and a second target steering angle is set for the second propulsion device so as to steer the second propulsion device in a direction against the water jet.

With this arrangement, the propulsive force restricting control is performed when the steering load torque is otherwise likely to be excessively increased due to a steering angle relationship between the two adjacent propulsion devices (the first propulsion device and the second propulsion device). This makes it possible to reliably steer the two propulsion devices to their target steering angles.

In a preferred embodiment, the predetermined load torque increase condition includes a steering angle condition such that a first target steering angle and a second target steering angle are respectively set for the first propulsion device and the second propulsion device so as to steer the first propulsion device and the second propulsion device to move the rear ends of the first propulsion device and the second propulsion device toward each other.

With this arrangement, the propulsive force restricting control is performed when the steering load torque is otherwise likely to be excessively increased due to a steering angle relationship between the two adjacent propulsion devices (the first propulsion device and the second propulsion device). This makes it possible to reliably steer the two propulsion devices to the their target steering angles.

In a preferred embodiment, the controller has a plurality of control modes including a bow turning mode in which the first propulsion device generates a forward propulsive force and the second propulsion device generates a reverse propulsive force with the rear ends of the first propulsion device and the second propulsion device located closer to each other than the front ends of the first propulsion device and the second propulsion device. The predetermined load torque increase condition includes a condition such that the controller is in the bow turning mode.

With this arrangement, in the bow turning mode, the controller drives one of the two adjacent propulsion devices (the first propulsion device) forward and drives the other propulsion device (the second propulsion device) in reverse while steering the two adjacent propulsion devices in a V-shaped orientation (in a so-called toe-out orientation). Thus, a moment is applied to the hull to turn the bow of the hull (e.g., at a fixed point) by controlling the steering states of the two propulsion devices in a manner different from the conventional manner. In this case, the water jet generated by the propulsion device (the first propulsion device) driven forward is likely to apply a resistance to the steering of the propulsion device (the second propulsion device) driven in reverse. To cope with this, the propulsive force restricting control is performed in the bow turning mode, thus making it possible to steer the two propulsion devices to their target steering angles to properly turn the bow of the hull.

In a preferred embodiment, the second steering device includes an electric motor as its drive source. With this arrangement, the steering device (the second steering device) for the other propulsion device (the second propulsion device) which receives the water jet generated by the one propulsion device (the first propulsion device) includes the electric motor as its drive source. There is a possibility that the steering device (the second steering device) including the electric motor as its drive source cannot steer the corresponding propulsion device (the second propulsion device) to its target steering angle when the steering load torque is great. If the electric motor is stopped due to an excessively great steering load torque, for example, a fail-safe control is performed to prevent the flow of an excessively large drive current. Therefore, where the steering load torque is likely to be excessively increased, the propulsive force restricting control is performed, thus making it possible to properly control the steering angles of the propulsion devices while preventing the electric motor from being overloaded.

In a preferred embodiment, the predetermined load torque increase condition includes a steering angle condition such that the second propulsion device receives the water jet generated by the first propulsion device due to a steering angle relationship between the first propulsion device and the second propulsion device.

With this arrangement, the steering load torque is likely to be excessively increased when the other propulsion device (the second propulsion device) receives the water jet from the one propulsion device (the first propulsion device). In this case, the propulsive force restricting control is performed, thus making it possible to steer the propulsion devices to their target steering angles while preventing the steering load torque from being excessively increased.

In a preferred embodiment, the second propulsion device includes a rudder plate, and the predetermined load torque increase condition includes a steering angle condition such that the rudder plate receives the water jet generated by the first propulsion device due to a steering angle relationship between the first propulsion device and the second propulsion device.

With this arrangement, the steering load torque is likely to be excessively increased when the rudder plate of the other propulsion device (the second propulsion device) receives the water jet from the one propulsion device (the first propulsion device). In this case, the propulsive force restricting control is performed, thus making it possible to steer the propulsion devices to their target steering angles while preventing the steering load torque from being excessively increased.

In a preferred embodiment, at least one of the first propulsion device and the second propulsion device is an electric propulsion device that includes an electric motor as its drive source.

The electric propulsion device tends to be designed so as to have a wider steerable angle range (e.g., ±<NUM> degrees or wider) as compared with an engine propulsion device. Where the two propulsion devices, at least one of which is the electric propulsion device, are adjacent to each other, a water jet generated by the electric propulsion device is more likely to hinder the steering of the other propulsion device. In this case, the propulsive force restricting control is performed, thus making it possible to properly steer the two propulsion devices to their target steering angles.

Another preferred embodiment provides a watercraft propulsion system including at least two propulsion devices attachable to a hull in a steerable manner (in a laterally pivotable manner), at least two steering devices to respectively steer the at least two propulsion devices, and a controller configured or programmed to control the at least two propulsion devices and the at least two steering devices, and configured or programmed to include a plurality of control modes including a bow turning mode in which one of the at least two propulsion devices generates a forward propulsive force and another of the at least two propulsion devices generates a reverse propulsive force with the rear ends of the at least two propulsion devices located closer to each other than the front ends of the at least two propulsion devices.

With this arrangement, for example, one of two adjacent propulsion devices is driven forward and the other propulsion device is driven in reverse with the two adjacent propulsion devices steered in a V-shaped orientation (in a so-called toe-out orientation) in the bow turning mode. Thus, a moment is applied to the hull such that the bow of the hull can be turned (e.g., at a fixed point) by controlling the steering states of the two propulsion devices in a manner different from the conventional manner.

Another further preferred embodiment provides a watercraft propulsion system including at least two propulsion devices attachable to a hull in a steerable manner (in a laterally pivotable manner), at least two steering devices to respectively steer the at least two propulsion devices, and a controller configured or programmed to control the at least two propulsion devices and the at least two steering devices, and configured or programmed to include a plurality of control modes including a bow turning mode in which the at least two propulsion devices respectively generate propulsive forces tangentially of a circle about the turning center of the hull so as to respectively apply moments to the hull in the same turning direction about the turning center of the hull.

With this arrangement, the plurality of propulsion devices generate the propulsive forces tangentially of the circle about the turning center of the hull in the bow turning mode such that the moments are applied to the hull in the same turning direction. Thus, the bow of the hull can be turned (e.g., at a fixed point) by controlling the steering states of the plurality of propulsion devices in a manner different from the conventional manner.

Still another preferred embodiment provides a watercraft including a hull, and a watercraft propulsion system attached to the hull and having any of the above-described features.

<FIG> is a plan view showing an exemplary construction of a watercraft <NUM> mounted with a watercraft propulsion system <NUM> according to a preferred embodiment.

The watercraft <NUM> includes a hull <NUM>, and a plurality of electric outboard motors EM attached to the hull <NUM>. In the present preferred embodiment, two electric outboard motors EM are attached to the hull <NUM>. The electric outboard motors EM are examples of the propulsion devices, more specifically, examples of the electric propulsion device including the electric motor as its power source.

In the present preferred embodiment, the two electric outboard motors EM are attached to the stern <NUM> of the watercraft <NUM>. More specifically, the two electric outboard motors EM are disposed side by side transversely of the hull <NUM> in adjacent relation on the stern <NUM>. That is, no other propulsion device is disposed between the two electric outboard motors EM. For discrimination between the two electric outboard motors EM, one of the electric outboard motors EM disposed rightward relative to the other electric outboard motor EM is referred to as "starboard-side electric outboard motor EMs" and the other electric outboard motor EM disposed leftward relative to the one electric outboard motor EM is referred to as "port-side electric outboard motor EMp. " In this example, the starboard-side electric outboard motor EMs is disposed on the right side of a center line 2a extending anteroposteriorly of the hull <NUM>, and the port-side electric outboard motor EMp is disposed on the left side of the center line 2a. More specifically, the starboard-side electric outboard motor EMs and the port-side electric outboard motor EMp are disposed symmetrically with respect to the center line 2a.

A usable space <NUM> for passengers is provided inside the hull <NUM>. A helm seat <NUM> is provided in the usable space <NUM>. A steering wheel <NUM>, a remote control lever <NUM>, a joystick <NUM>, a gauge <NUM> (display panel) and the like are provided in association with the helm seat <NUM>. The steering wheel <NUM> is an operation element operable by an operator to change the course of the watercraft <NUM>. The remote control lever <NUM> is an operation element operable by the operator to change the magnitudes (outputs) and the directions (forward or reverse directions) of the propulsive forces of the electric outboard motors EM, and corresponds to an acceleration operation element. The joystick <NUM> is an operation element operable instead of the steering wheel <NUM> and the remote control lever <NUM> by the operator for watercraft maneuvering operation.

<FIG> is a side view showing the structure of the electric outboard motor EM by way of example, and <FIG> is a rear view of the electric outboard motor EM as seen from the rear side of the watercraft <NUM>.

The electric outboard motors EM each include a bracket <NUM> for attachment thereof to the hull <NUM>, and a propulsion device body <NUM>. The propulsion device body <NUM> is supported by the bracket <NUM>. The propulsion device body <NUM> includes a base <NUM> supported by the bracket <NUM>, an upper housing <NUM> extending downward from the base <NUM>, a tubular (duct-shaped) lower housing <NUM> disposed below the upper housing <NUM>, and a drive unit <NUM> disposed in the lower housing <NUM>. The propulsion device body <NUM> further includes a cover <NUM> that covers the base <NUM> from the lower side, and a cowl <NUM> that covers the base <NUM> from the upper side. A tilt unit <NUM> and a steering unit <NUM> are accommodated in a space defined by the cover <NUM> and the cowl <NUM>. Further, a buzzer <NUM> that generates sound when the tilt unit <NUM> is actuated may be accommodated in this space.

The drive unit <NUM> includes a propeller <NUM>, and an electric motor <NUM> that rotates the propeller <NUM>. The electric motor <NUM> includes a tubular rotor <NUM> to which the propeller <NUM> is fixed radially inward thereof, and a tubular stator <NUM> that surrounds the rotor <NUM> from the radially outside. The stator <NUM> is fixed to the lower housing <NUM>, and the rotor <NUM> is supported rotatably with respect to the lower housing <NUM>. The rotor <NUM> includes a plurality of permanent magnets <NUM> disposed circumferentially thereof. The stator <NUM> includes a plurality of coils <NUM> disposed circumferentially thereof. The rotor <NUM> is rotated by energizing the coils <NUM> such that the propeller <NUM> is correspondingly rotated to generate a propulsive force.

The tilt unit <NUM> includes a tilt cylinder <NUM> as a tilt actuator. The tilt cylinder <NUM> may be a hydraulic cylinder of electric pump type adapted to pump a hydraulic oil by an electric pump. One of opposite ends of the tilt cylinder <NUM> is connected to the lower support portion <NUM> of the bracket <NUM>, and the other end of the tilt cylinder <NUM> is connected to the base <NUM> via a cylinder connection bracket <NUM>. A tilt shaft <NUM> is supported by the upper support portion <NUM> of the bracket <NUM>, and the base <NUM> is connected to the bracket <NUM> via the tilt shaft <NUM> pivotally about the tilt shaft <NUM>. The tilt shaft <NUM> extends transversely of the hull <NUM>, so that the base <NUM> is pivotable upward and downward. Thus, the propulsion device body <NUM> is pivotable upward and downward about the tilt shaft <NUM>.

An expression "tilt-up" means that the propulsion device body <NUM> is pivoted upward about the tilt shaft <NUM>, and an expression "tilt-down" means that the propulsion device body <NUM> is pivoted downward about the tilt shaft <NUM>. The tilt cylinder <NUM> is driven to be extended and retracted such that the tilt-up and the tilt-down is achieved. The propeller <NUM> is moved up to an above-water position by the tilt-up such that the propulsion device body <NUM> is brought into a tilt-up state. Further, the propeller <NUM> is moved down to an underwater position by the tilt-down such that the propulsion device body <NUM> is brought into a tilt-down state. The tilt unit <NUM> is an exemplary lift device that moves up and down the propeller <NUM>.

A tilt angle sensor <NUM> is provided to detect a tilt angle (i.e., the angle of the propulsion device body <NUM> with respect to the bracket <NUM>) to detect the tilt-up state and the tilt-down state of the propulsion device body <NUM>. The tilt angle sensor <NUM> may be a position sensor that detects the position of the actuation rod of the tilt cylinder <NUM>.

The steering unit <NUM> includes a steering shaft <NUM> connected to the lower housing <NUM> and the upper housing <NUM>, and a steering actuator <NUM>. The steering actuator <NUM> generates a drive force to pivot the steering shaft <NUM> about its axis. Therefore, the lower housing <NUM> and the upper housing <NUM> are pivoted about the steering shaft <NUM> by driving the steering actuator <NUM> such that the direction of the propulsive force generated by the drive unit <NUM> is changeable leftward and rightward. The upper housing <NUM> has a plate shape that extends anteroposteriorly of the hull <NUM> in a neutral steering position, and functions as a rudder plate to be steered by the steering unit <NUM>. The steering unit <NUM> is an example of the steering device. In this example, the steering unit <NUM> is incorporated unitarily with the propulsion device body <NUM> in the electric outboard motor EM, but the steering device is not necessarily required to be incorporated in the electric outboard motor.

<FIG> is a block diagram showing an exemplary configuration of the watercraft propulsion system <NUM> provided in the watercraft <NUM>. The watercraft propulsion system <NUM> includes the two electric outboard motors EM (EMs, EMp).

The watercraft propulsion system <NUM> includes a main controller <NUM>. The main controller <NUM> is connected to an onboard network <NUM> (CAN: Control Area Network) provided in the hull <NUM>. Remote control ECUs <NUM> (<NUM>, 90p) respectively associated with the two electric outboard motors EM (EMs, EMp), a joystick unit <NUM>, a GPS (Global Positioning System) receiver <NUM>, an azimuth sensor <NUM>, and the like are connected to the onboard network <NUM>. The electric outboard motors EM each include a motor controller <NUM> and a steering controller <NUM>, which are connected to the associated remote control ECU <NUM> via an outboard motor control network <NUM>. The main controller <NUM> transmits and receives signals to/from various units connected to the onboard network <NUM> to control the electric outboard motors EM, and further controls other units. The main controller <NUM> has a plurality of control modes, and controls the units in predetermined manners according to the respective control modes.

A steering wheel unit <NUM> is connected to the outboard motor control network <NUM>. The steering wheel unit <NUM> outputs an operation angle signal indicating the operation angle of the steering wheel <NUM> to the outboard motor control network <NUM>. The operation angle signal is received by the remote control ECUs <NUM> and the steering controllers <NUM>. In response to the operation angle signal generated by the steering wheel unit <NUM> or steering angle commands respectively generated by the remote control ECUs <NUM>, the steering controllers <NUM> of the electric outboard motors EM respectively control the steering actuators <NUM> to control the steering angles of the electric outboard motors EM.

A remote control unit <NUM>, which generates an operation position signal indicating the operation position of the remote control lever <NUM>, is connected to the onboard network <NUM>. The remote control unit <NUM> includes a starboard-side remote control lever <NUM> and a port-side remote control lever 7p respectively associated with the starboard-side electric outboard motor EMs and the port-side electric outboard motor EMp. The remote control unit <NUM> outputs the operation position signal indicating the operation position of the remote control lever <NUM> to the onboard network <NUM>. The operation position signal is received by the remote control ECUs <NUM>. The remote control ECUs <NUM> each generate a propulsive force command. In response to the propulsive force command, the motor controller <NUM> controls the electric motor <NUM> to control the propulsive force of the electric outboard motor EM.

The joystick unit <NUM> generates an operation position signal indicating the operation position of the joystick <NUM>, and generates an operation signal indicating the operation of any of operation buttons <NUM> provided in the joystick unit <NUM>.

The remote control ECUs <NUM> are each able to output the propulsive force command to the corresponding motor controller <NUM> via the outboard motor control network <NUM>. The propulsive force command includes a shift command for forward drive, reverse drive or stop, and an output command for an output (specifically, a motor rotation speed). Further, the remote control ECUs <NUM> are each able to output the steering angle command to the corresponding steering controller <NUM> via the outboard motor control network <NUM>.

The remote control ECUs <NUM> each perform different control operations according to different control modes of the main controller <NUM>. In a control mode for watercraft maneuvering with the use of the steering wheel <NUM> and the remote control lever <NUM>, for example, the remote control ECUs <NUM> each generate the propulsive force command according to the operation position signal generated by the remote control unit <NUM>, and each apply the propulsive force command to the corresponding motor controller <NUM>. Further, the remote control ECUs <NUM> each command the corresponding steering controller <NUM> to conform to the operation angle signal generated by the steering wheel unit <NUM>. In a control mode for watercraft maneuvering without the use of the steering wheel <NUM> and the remote control lever <NUM>, on the other hand, the remote control ECUs <NUM> each conform to commands applied by the main controller <NUM>. That is, the main controller <NUM> generates the propulsive force command (the shift command and the output command) and the steering angle command, and the remote control ECUs <NUM> each output the propulsive force command (the shift command and the output command) and the steering angle command to the motor controller <NUM> and the steering controller <NUM>, respectively. In a control mode for watercraft maneuvering with the use of the joystick <NUM>, for example, the main controller <NUM> generates the propulsive force command (the shift command and the output command) and the steering angle command according to the signals generated by the joystick unit <NUM>. The magnitude and the direction (the forward direction or the reverse direction) of the propulsive force and the steering angle of each of the electric outboard motors EM are controlled according to the propulsive force command (the shift command and the output command) and the steering angle command thus generated.

The motor controller <NUM> and the steering controller <NUM> of each of the electric outboard motors EM are configured to actuate the electric outboard motor EM in response to the propulsive force command and the steering angle command applied from the corresponding remote control ECU <NUM>. As described above, the propulsive force command includes the shift command and the output command. The shift command is a rotation direction command for the stop, the forward rotation, or the reverse rotation of the propeller <NUM>. The output command is a command for the magnitude of the propulsive force to be generated, specifically, a command for the rotation speed. The steering angle command is a command for the steering angle. The motor controller <NUM> controls the electric motor <NUM> according to the shift command (rotation direction command) and the output command. Further, the steering controller <NUM> controls the steering actuator <NUM> according to the steering angle command.

The GPS receiver <NUM> detects the position of the watercraft <NUM> by receiving radio waves from an artificial satellite orbiting the earth, and outputs position data indicating the position of the watercraft <NUM> and speed data indicating the moving speed of the watercraft <NUM>. The main controller <NUM> acquires the position data and the speed data, which are used to control and display the position and/or the azimuth of the watercraft <NUM>.

The azimuth sensor <NUM> detects the azimuth of the watercraft <NUM>, and generates azimuth data, which is used by the main controller <NUM>.

The gauge <NUM> is connected to the main controller <NUM> via a control panel network <NUM>. The gauge <NUM> is a display device that displays various information for the watercraft maneuvering. The gauge <NUM> is connected to the remote control ECUs <NUM>, and to the motor controllers <NUM> and the steering controllers <NUM> of the electric outboard motors EM via the control panel network <NUM>. Thus, the gauge <NUM> can display information such as the operation states of the electric outboard motors EM, and the position and/or the azimuth of the watercraft <NUM>. The gauge <NUM> may include an input device <NUM> such as a touch panel and buttons. The input device <NUM> may be operated by the operator to set various settings and give various commands such that operation signals are outputted to the control panel network <NUM>.

A power switch unit <NUM> operable to turn on and off power supplies to the electric outboard motors EM is connected to the remote control ECUs <NUM>. The power switch unit <NUM> includes a plurality of power switches <NUM> (two power switches <NUM> in the present preferred embodiment) operable to separately turn on and off the starboard-side electric outboard motor EMs and the port-side electric outboard motor EMp.

With the power switches <NUM> turned on, the remote control ECUs <NUM> perform a power supply control to control the power supplies to the respective electric outboard motors EM. Specifically, power supply relays (not shown) respectively provided between batteries <NUM> (e.g., <NUM> V) and the electric outboard motors EM are turned on. The batteries <NUM> preferably include a plurality of batteries <NUM> (two batteries <NUM> in the present preferred embodiment) respectively provided in association with the electric outboard motors EM. When the power switches <NUM> are turned off, the remote control ECUs <NUM> respectively turn off the power supply relays to turn off the power supplies to the electric outboard motors EM. Electric outboard motor state information indicating whether or not the power supplies to the respective electric outboard motors EM are turned on is applied from the remote control ECUs <NUM> to the main controller <NUM> via the onboard network <NUM>.

An application switch panel <NUM> is further connected to the onboard network <NUM>. The application switch panel <NUM> includes a plurality of function switches <NUM> operable to apply predefined function commands. For example, the function switches <NUM> may include switches for automatic watercraft maneuvering commands. Specific examples of the function switches <NUM> may include switches for an automatic steering function of maintaining the azimuth of the watercraft <NUM>, for an automatic steering function of maintaining the course of the watercraft <NUM>, for an automatic steering function of causing the watercraft <NUM> to pass through a plurality of checkpoints sequentially, and for an automatic steering function of causing the watercraft <NUM> to sail along a predetermined pattern (zig-zag pattern, spiral pattern or the like). A function for the tilt-up or the tilt-down of the electric outboard motors EM may be assigned to one of the function switches <NUM>.

The main controller <NUM> is able to control the electric outboard motors EM in the plurality of control modes. The control mode of the main controller <NUM> can be classified into an ordinary mode, a joystick mode, or a holding mode in terms of operation system.

In the ordinary mode, a steering control operation is performed according to the operation angle signal generated by the steering wheel unit <NUM>, and a propulsive force control operation is performed according to the operation signal (operation position signal) of the remote control lever <NUM>. In the present preferred embodiment, the ordinary mode is a default control mode of the main controller <NUM>. In the steering control operation, specifically, the steering controllers <NUM> of the electric outboard motors EM respectively drive the steering actuators <NUM> according to the operation angle signal generated by the steering wheel unit <NUM> or the steering angle commands generated by the remote control ECUs <NUM>. Thus, the drive units <NUM> and the upper housings <NUM> of the electric outboard motors EM are steered leftward and rightward such that the propulsive force directions of the electric outboard motors EM are changed leftward and rightward with respect to the hull <NUM>. In the propulsive force control operation, specifically, the motor controllers <NUM> of the electric outboard motors EM respectively drive the electric motors <NUM> according to the propulsive force commands (the shift commands and the output commands) applied from the remote control ECUs <NUM> to the motor controllers <NUM>. Thus, the electric motors <NUM> are each controlled to a forward rotation state, a reverse rotation state, or a stop state, and the rotation speeds of the electric motors <NUM> are changed.

In the joystick mode, the steering control operation and the propulsive force control operation are performed according to the operation signal of the joystick <NUM> of the joystick unit <NUM>. The holding mode includes automatic watercraft maneuvering modes that are selectable by operating holding mode setting buttons <NUM>, <NUM>, <NUM> (see <FIG>) provided in the joystick unit <NUM> to perform the steering control operation and the propulsive force control operation so as to hold the position and/or the azimuth of the hull <NUM>.

<FIG> is a perspective view showing the structure of the joystick unit <NUM> by way of example. The joystick unit <NUM> includes the joystick <NUM>, which is inclinable forward, backward, leftward, and rightward (i.e., in all <NUM>-degree directions) and is pivotable (twistable) about its axis. In this example, the joystick unit <NUM> further includes a plurality of operation buttons <NUM>. The operation buttons <NUM> include a joystick button <NUM> and the holding mode setting buttons <NUM> to <NUM>.

The joystick button <NUM> is an operation element operable by the operator to select the control mode (watercraft maneuvering mode) utilizing the joystick <NUM>, i.e., the joystick mode.

The holding mode setting buttons <NUM>, <NUM>, <NUM> are operation buttons operable by the operator to select position/azimuth holding system control modes (examples of the holding mode). More specifically, the holding mode setting button <NUM> is operated to select a fixed point holding mode (Stay PointTM) in which the position and the bow azimuth (or the stern azimuth) of the watercraft <NUM> are maintained. The holding mode setting button <NUM> is operated to select a position holding mode (Fish PointTM) in which the position of the watercraft <NUM> is maintained but the bow azimuth (or the stern azimuth) of the watercraft <NUM> is not maintained. The holding mode setting button <NUM> is operated to select an azimuth holding mode (Drift PointTM) in which the bow azimuth (or the stern azimuth) of the watercraft <NUM> is maintained but the position of the watercraft <NUM> is not maintained.

In the joystick mode, the main controller <NUM> applies the steering angle command and the propulsive force command to the remote control ECUs <NUM>. The remote control ECUs <NUM> apply the steering angle command to the corresponding steering controllers <NUM>, and apply the propulsive force command to the corresponding motor controllers <NUM>. Thus, the steering control operation and the propulsive force control operation are performed on the electric outboard motors EM. In the steering control operation on the electric outboard motors EM, in this case, the steering controllers <NUM> of the electric outboard motors EM respectively drive the steering units <NUM> according to the steering angle command applied from the main controller <NUM> to the steering controllers <NUM> via the remote control ECUs <NUM>. Thus, the drive units <NUM> and the upper housings <NUM> of the electric outboard motors EM are pivoted leftward and rightward such that the propulsive force directions of the electric outboard motors EM are changed leftward and rightward with respect to the hull <NUM>. In the propulsive force control operation on the electric outboard motors EM, in this case, the motor controllers <NUM> of the electric outboard motors EM control the rotation directions and the rotation speeds of the electric motors <NUM> according to the propulsive force command (the shift command and the output command) applied from the main controller <NUM> to the motor controllers <NUM> of the electric outboard motors EM via the remote control ECUs <NUM>. Thus, the rotation directions of the propellers <NUM> are each set to a forward drive direction or a reverse drive direction, and the rotation speeds of the propellers <NUM> are changed.

<FIG>, <FIG>, <FIG>, and <FIG> are diagrams for describing two types of joystick modes, showing the operation states of the joystick <NUM> and the corresponding behaviors of the hull <NUM>. More specifically, <FIG> and <FIG> show exemplary operations to be performed in a first joystick mode in which the propulsive forces of the two electric outboard motors EM are both utilized. <FIG> and <FIG> show exemplary operations to be performed in a second joystick mode in which only one of the propulsive forces of the two electric outboard motors EM is utilized.

Based on the electric outboard motor state information, the main controller <NUM> detects whether the power supply mode is a dual mode in which the power supplies to the two electric outboard motors EM are both turned on or a single mode in which the power supply to only one of the two electric outboard motors EM is turned on. If the joystick mode is commanded by operating the joystick button <NUM> in the dual mode, the main controller <NUM> performs the control operation according to the first joystick mode. If the joystick mode is commanded by operating the joystick button <NUM> in the single mode, the main controller <NUM> performs the control operation according to the second joystick mode.

In the first joystick mode shown in <FIG> and <FIG>, the main controller <NUM> defines the inclination direction of the joystick <NUM> as an advancing direction command, and defines the inclination amount of the joystick <NUM> as a propulsive force magnitude command that indicates the magnitude of the propulsive force to be applied in the inclination direction. Further, the main controller <NUM> defines the pivoting direction of the joystick <NUM> about its axis (with respect to the neutral position of the joystick <NUM>) as a bow turning direction command, and defines the pivoting amount of the joystick <NUM> (with respect to the neutral position of the joystick <NUM>) as a bow turning speed command. For execution of these commands, the steering angle command and the propulsive force command are generated by the main controller <NUM> and applied to the steering controllers <NUM> and the motor controllers <NUM> of the electric outboard motors EM via the remote control ECUs <NUM>. Thus, the drive units <NUM> and the upper housings <NUM> of the respective electric outboard motors EM are steered to the steering angles according to the steering command, and the rotation directions and the rotation speeds of the electric motors <NUM> of the respective electric outboard motors EM are controlled so as to generate the propulsive forces according to the propulsive force command.

When the joystick <NUM> is inclined without being pivoted in the first joystick mode, the hull <NUM> is moved in a direction corresponding to the inclination direction of the joystick <NUM> without the bow turning, i.e., with its azimuth maintained. That is, the hull <NUM> is in a hull translation behavior. Examples of the hull translation behavior are shown in <FIG>. The steering states of the two electric outboard motors EM are typically such that the propulsive force action lines of the two electric outboard motors EM (extending along the respective propulsive force directions) cross each other in the hull <NUM>. That is, the two electric outboard motors EM are steered in an inverted V-shaped orientation as seen in plan (in a so-called toe-in orientation). With the electric outboard motors EM thus steered, one of the electric outboard motors EM is driven forward, and the other electric outboard motor EM is driven in reverse. Thus, the hull <NUM> translates in the direction of the resultant force of the propulsive forces generated by the two electric outboard motors EM. Where one of the electric outboard motors EM is driven forward and the other electric outboard motor EM is driven in reverse to generate propulsive forces of the same magnitude, for example, the hull <NUM> can translate laterally. The control mode of the main controller <NUM> in which the two electric outboard motors EM are controlled in the above-described manner to translate the hull <NUM> in the first joystick mode is referred to as "translation mode.

When the joystick <NUM> is inclined and pivoted in the first joystick mode, the hull <NUM> is in a hull behavior such that the bow is turned in a direction corresponding to the pivoting direction of the joystick <NUM> while the hull <NUM> is moved in a direction corresponding to the inclination direction of the joystick <NUM>. At this time, a moment is applied to the hull <NUM> by changing the steering angles and/or the outputs of the two electric outboard motors EM while keeping the inverted V-shaped orientation steering states of the two electric outboard motors EM. In this case, therefore, the control mode of the main controller <NUM> is the translation mode.

When the joystick <NUM> is pivoted (twisted) without being inclined in the first joystick mode, on the other hand, the bow of the hull <NUM> is turned in a direction corresponding to the pivoting direction of the joystick <NUM> without any substantial position change. That is, the hull <NUM> is in a fixed-point bow turning behavior. Examples of the fixed-point bow turning behavior are shown in <FIG>. In these examples, the steering states of the two electric outboard motors EM in the fixed-point bow turning behavior are such that the propulsive force action lines of the two electric outboard motors EM (extending along the respective propulsive force directions) cross each other behind the hull <NUM>. That is, the two electric outboard motors EM are steered in a V-shaped orientation as seen in plan (in a so-called toe-out orientation). With the electric outboard motors EM thus steered, one of the electric outboard motors EM is driven forward, and the other electric outboard motor EM is driven in reverse. Thus, the propulsive forces respectively generated by the two electric outboard motors EM each apply a moment to the hull <NUM> about the turning center of the hull <NUM> such that the hull <NUM> is brought into the fixed-point bow turning behavior. The control mode of the main controller <NUM> in which the two electric outboard motors EM are controlled in the above-described manner to turn the bow of the hull <NUM> in the first joystick mode is referred to as "bow turning mode.

In the second joystick mode shown in <FIG> and <FIG>, the propulsive force generated by only one of the two electric outboard motors EM is utilized and, therefore, the hull translation (see <FIG>) which utilizes the resultant force of the propulsive forces of the two electric outboard motors EM is impossible as shown in <FIG>. That is, the second joystick mode is a control mode that disables a certain hull behavior (specifically, the hull translation behavior) available in the first joystick mode. As shown in <FIG>, the propulsive force generated by only one of the electric outboard motors EM can apply the moment to the hull <NUM> about the turning center, so that the fixed-point bow turning behavior may be available.

In the second joystick mode, the main controller <NUM> defines the anteroposterior inclination of the joystick <NUM> as the propulsive force command (the shift command and the output command), and ignores the lateral inclination of the joystick <NUM>. That is, when the joystick <NUM> is inclined, only the anteroposterior directional component of the inclination direction of the joystick <NUM> serves as an effective input, and is defined as the propulsive force command. More specifically, if the anteroposterior directional component has a value indicating the forward inclination, the anteroposterior directional component is defined as a forward shift command. If the anteroposterior directional component has a value indicating the rearward inclination, the anteroposterior directional component is defined as a reverse shift command. Further, the magnitude of the anteroposterior directional component is defined as a command (output command) indicating the magnitude of the propulsive force. The propulsive force command (the shift command and the output command) thus defined is inputted from the main controller <NUM> to the motor controller <NUM> of the one electric outboard motor EM via the corresponding remote control ECU <NUM>. On the other hand, the main controller <NUM> defines the axial pivoting of the joystick <NUM> as the steering angle command in the second joystick mode. That is, the main controller <NUM> generates the steering angle command according to the axial pivoting direction and the axial pivoting amount of the joystick <NUM>, and inputs the steering angle command to the steering controller <NUM> of the one electric outboard motor EM via the corresponding remote control ECU <NUM>. When the joystick <NUM> is pivoted but not inclined, the main controller <NUM> may control the steering state of the one electric outboard motor EM in the bow turning mode (see <FIG>).

The motor controller <NUM> drives the corresponding electric motor <NUM> according to the propulsive force command, and the steering controller <NUM> drives the corresponding steering actuator <NUM> according to the steering angle command.

<FIG> is a block diagram showing the configuration of the steering actuator <NUM>. The steering actuator <NUM> includes a steering motor <NUM> (an electric motor for the steering). A current is supplied to the steering controller <NUM> to drive the steering motor <NUM>. A torque generated by the steering motor <NUM> is transmitted the steering shaft <NUM> (output shaft) via a deceleration mechanism <NUM> including a reduction gear <NUM> and a worm gear/wheel <NUM>. Thus, the steering motor <NUM> is driven to rotate the steering shaft <NUM> such that the electric outboard motor EM is steered. The rotation angle of the steering shaft <NUM> is detected as an actual steering angle by a steering angle sensor <NUM>, and the output signal of the steering angle sensor <NUM> is inputted to the steering controller <NUM>.

Electric power is supplied from the battery <NUM> to the steering controller <NUM> via a power supply circuit <NUM>. The steering angle command is applied from an upper-level controller to the steering controller <NUM>. The upper-level controller is the steering wheel unit <NUM> in the ordinary mode, and is the remote control ECU <NUM> in the joystick mode and the holding mode. The steering controller <NUM> controls a drive current to be supplied to the steering motor <NUM> through feed-back control so that the actual steering angle detected by the steering angle sensor <NUM> matches with the value of the steering angle command (steering angle command value). Further, the steering controller <NUM> applies information of the actual steering angle detected by the steering angle sensor <NUM> to the upper-level controller.

<FIG> is a schematic plan view for describing, in greater detail, the steering states (see <FIG>) in the bow turning mode effected in the dual mode. In the bow turning mode, the main controller <NUM> controls the steering states of the two electric outboard motors EM so that the rear ends of the electric outboard motors EM are located closer to each other than the front ends of the electric outboard motors EM. That is, as seen in plan, the two electric outboard motors EM are steered in the V-shaped orientation (i.e., the so-called toe-out orientation). At this time, the two electric outboard motors EM respectively generate the propulsive forces generally tangentially of a circle <NUM> about the turning center <NUM> of the hull <NUM>. In the bow turning mode, the main controller <NUM> generates the propulsive force command so as to drive one of the two electric outboard motors EM forward and drive the other electric outboard motor EM reverse. Thus, the two electric outboard motors EM respectively generate the propulsive forces generally tangentially of the circle <NUM> about the turning center <NUM> so as to apply moments to the hull <NUM> in the same turning direction <NUM> (clockwise in <FIG>) about the turning center <NUM>. In the example of <FIG>, the starboard-side electric outboard motor EMs is driven in reverse, and the port-side electric outboard motor EMp is driven forward such that the two electric outboard motors EM apply clockwise moments to the hull <NUM> about the turning center <NUM> of the hull <NUM>.

The target steering angles of the electric outboard motors EM in the bow turning mode are hereinafter each referred to as "bow turning mode steering angle. " The bow turning mode steering angle of the starboard-side electric outboard motor EMs and the bow turning mode steering angle of the port-side electric outboard motor EMp are respectively referred to as "bow turning mode starboard-side steering angle" and "bow turning mode port-side steering angle" for discrimination therebetween. The steering angles may be each defined with respect to the propulsive force directions parallel or substantially parallel to the anteroposterior direction of the hull <NUM> with the electric outboard motors EM each set in the neutral steering position. Where the steering angles of the electric outboard motors EM are each defined as zero degrees when the electric outboard motors EM are each set in the neutral steering position, the bow turning mode starboard-side steering angle and the bow turning mode port-side steering angle have different signs and substantially the same absolute value.

The electric outboard motors EM can be more easily designed so as to have a wider steerable angle range as compared with an engine outboard motor employing an engine as its drive source. Specifically, the electric outboard motors EM can be each designed to have a steerable angle range of ±<NUM> degrees or wider (e.g., ±<NUM> degrees), and can be designed even so as to have a steerable angle range of ±<NUM> degrees.

<FIG> is a perspective view showing a positional relationship between the two electric outboard motors EM in the bow turning mode. With the port-side electric outboard motor EMp driven forward, the propeller <NUM> discharges water from the front side to the rear side such that water jet <NUM> is generated rearward of the port-side electric outboard motor EMp. Where the electric outboard motors EM are steered in the V-shaped orientation (in the so-called toe-out orientation) in the bow turning mode, at least a portion of the water jet <NUM> hits the starboard-side electric outboard motor EMs to apply a counterclockwise moment to the starboard-side electric outboard motor EMs. Particularly, when the waterjet hits the upper housing <NUM> (rudder plate) of the starboard-side electric outboard motor EMs, the moment has a relatively great magnitude. Therefore, the steering load torque received by the steering actuator <NUM> of the starboard-side electric outboard motor EMs is increased by the influence of the water jet <NUM>.

<FIG> shows an exemplary operation to be performed at the start of the bow turning mode (according to a comparative example). In this example, the two electric outboard motors EM are each set in the neutral steering position with their propulsive force directions parallel or substantially parallel to the anteroposterior direction of the hull <NUM> at the initial stage immediately before the start of the bow turning mode. When the bow turning mode is started, the steering control operation is started to steer the two electric outboard motors EM in the V-shaped orientation (in the toe-out orientation) and, simultaneously, the propulsive force control operation is started to drive one of the two electric outboard motors EM forward and drive the other electric outboard motor EM reverse. In the example shown, the port-side electric outboard motor EMp is driven forward to generate a forward propulsive force and, therefore, water jet <NUM> is generated rearward of the port-side electric outboard motor EMp. Further, the starboard-side electric outboard motor EMs is driven in reverse to generate a reverse propulsive force and, therefore, water jet <NUM> is generated forward of the starboard-side electric outboard motor EMs. When the two electric outboard motors EM are steered to their bow turning mode steering angles in this state, the rear ends of the electric outboard motors EM are moved toward each other. Therefore, the water jet <NUM> generated rearward by the port-side electric outboard motor EMp hits the starboard-side electric outboard motor EMs, particularly hits the upper housing <NUM> (rudder plate) of the starboard-side electric outboard motor EMs. This increases the steering load torque of the starboard-side electric outboard motor EMs. Since the starboard-side electric outboard motor EMs should be steered in a direction against the water jet <NUM> to the bow turning mode starboard-side steering angle, the steering load torque is likely to be excessively increased.

<FIG> shows a change in the steering angle of the starboard-side electric outboard motor EMs in the exemplary operation shown in <FIG> shows a change in the drive current of the steering motor <NUM> of the starboard-side electric outboard motor EMs in the same operation. When the bow turning mode is started at time t0, the steering angle command value changes toward the bow turning mode steering angle, and reaches the bow turning mode steering angle at time t1. The actual steering angle changes following the steering angle command value through the feed-back control performed by the steering controller <NUM> (see <FIG>). As the steering operation proceeds, however, the steering load torque is increased by the influence of the water jet <NUM> generated by the port-side electric outboard motor EMp. At time t2 before the bow turning mode steering angle is reached, the output torque of the steering motor <NUM> is balanced against the steering load torque, so that the actual steering angle no longer changes. On the other hand, the steering controller <NUM> increases the drive current to be supplied to the steering motor <NUM> in order to eliminate a difference between the steering angle command value and the actual steering angle. Thus, at time t3, the drive current of the steering motor <NUM> exceeds the rated current of the steering motor <NUM>. The steering controller <NUM> detects this state and, at time t4, performs a fail-safe process, for example, to stop the current supply to the steering motor <NUM>.

<FIG> shows another exemplary operation to be performed at the start of the bow turning mode (according to a preferred embodiment). In this example, similarly, the two electric outboard motors EM are each set in the neutral steering position with their propulsive force directions parallel or substantially parallel to the anteroposterior direction of the hull <NUM> at the initial stage immediately before the start of the bow turning mode. When the bow turning mode is started, the main controller <NUM> performs the steering control operation to steer the two electric outboard motors EM in the V-shaped orientation (in the toe-out orientation), and performs a propulsive force restricting control operation to restrict the generation of the propulsive forces. Specifically, the propulsive force restricting control operation is a propulsive force reducing control operation in which the propulsive forces of the electric outboard motors EM are controlled to levels lower than the target propulsive forces until the actual steering angles of the electric outboard motors EM reach the bow turning mode steering angles. More specifically, the propulsive force restricting control operation may be a propulsive force generation prohibiting control operation in which the generation of the propulsive forces is prohibited by controlling the propulsive forces at zero until the actual steering angles of the electric outboard motors EM reach the bow turning mode steering angles.

When the actual steering angles of the electric outboard motors EM reach the bow turning mode steering angles, the main controller <NUM> starts the propulsive force control operation to drive one of the two electric outboard motors EM forward and drive the other electric outboard motor EM in reverse. In the example shown, the port-side electric outboard motor EMp is driven forward to generate a forward propulsive force to generate water jet <NUM> rearward of the port-side electric outboard motor EMp. Further, the starboard-side electric outboard motor EMs is driven in reverse to generate a reverse propulsive force to generate water jet <NUM> forward of the starboard-side electric outboard motor EMs.

<FIG> shows a change in the steering angle of the starboard-side electric outboard motor EMs in the exemplary operation shown in <FIG>, and <FIG> shows a change in the drive current of the steering motor <NUM> of the starboard-side electric outboard motor EMs in the same exemplary operation. When the bow turning mode is started at time t10, the steering angle command value changes toward the bow turning mode steering angle and, at time t11, reaches the bow turning mode steering angle. The actual steering angle changes following the steering angle command value through the feed-back control performed by the steering controller <NUM> (see <FIG>). Until the actual steering angle reaches the bow turning mode steering angle, the port-side electric outboard motor EMp generates no water jet or generates a weak water jet, so that the steering load torque does not substantially increase. Therefore, the actual steering angle reaches the bow turning mode steering angle at time t12. On the other hand, the steering controller <NUM> controls the supply of the drive current to the steering motor <NUM> so as to eliminate a difference between the steering angle command value and the actual steering angle. Unlike in the case of <FIG>, the actual steering angle reaches the bow turning mode steering angle at time t12. Therefore, the drive current does not continuously increase to more than the rated current, but the supply of the drive current to the steering motor <NUM> is stopped.

Thereafter, the port-side electric outboard motor EMp starts generating the propulsive force. Therefore, the water jet <NUM> of the port-side electric outboard motor EMp hits the starboard-side electric outboard motor EMs (particularly, hits the upper housing <NUM> (rudder plate)) such that the steering load torque occurs. However, the actual steering angle already reaches the bow turning mode steering angle. Therefore, the steering motor <NUM> is not driven, but the actual steering angle is maintained at the bow turning mode steering angle by the friction of the worm gear/wheel <NUM>.

<FIG> is a flowchart for describing an exemplary process to be performed by the main controller <NUM> at the start of the bow turning mode. When the joystick <NUM> is twisted in the neutral position, the main controller <NUM> starts the bow turning mode. The main controller <NUM> determines whether or not the bow turning mode is effected in the dual mode (see <FIG> and <FIG>) (Step S1). If the bow turning mode is effected in the dual mode (YES in Step S1), the main controller <NUM> performs the steering control operation to steer the two electric outboard motors EM in the V-shaped orientation (Step S2), and performs the propulsive force restricting control operation (the propulsive force reducing control operation or the propulsive force generation prohibiting control operation) to restrict (e.g., prohibit) the generation of the propulsive forces of the two electric outboard motors EM (Step S3). The main controller <NUM> acquires information of the actual steering angles of the two electric outboard motors EM from the steering controllers <NUM>, and continuously performs the propulsive force restricting control operation (the propulsive force reducing control operation or the propulsive force generation prohibiting control operation) until the actual steering angles reach the bow turning mode steering angles (NO in Step S4). If the actual steering angles of the two electric outboard motors EM reach the bow turning mode steering angles (YES in Step S4), the main controller <NUM> starts the propulsive force control operation to drive one of the two electric outboard motors EM forward and drive the other electric outboard motor EM in reverse according to the twisting direction and the twisting amount of the joystick <NUM> (Step S5).

If the bow turning mode is not effected in the dual mode (NO in Step S1), i.e., if the bow turning mode is effected in the single mode, the main controller <NUM> performs the steering control operation to control an energized one of the electric outboard motors EM to a steering angle corresponding to the twisting direction of the joystick <NUM> (Step S6). Simultaneously with the steering control operation, the main controller <NUM> may start the propulsive force control operation to cause the energized electric outboard motor EM to generate a target propulsive force corresponding to the twisting amount of the joystick <NUM> (Step S7). Alternatively, the main controller <NUM> may start the propulsive force control operation (Step S7) after the energized electric outboard motor EM is steered to a predetermined steering angle suitable for the fixed-point bow turning behavior.

The bow turning mode effected in the dual mode is an example of the predetermined load torque increase condition in which the water jet generated by one of the two electric outboard motors EM (first propulsion device) is likely to increase the steering load torque of the other electric outboard motor EM (second propulsion device). In the bow turning mode effected in the dual mode, the bow turning mode steering angle (first target steering angle) of one of the two electric outboard motors EM (first propulsion device) is set so that the water jet generated by the one electric outboard motor EM (first propulsion device) is directed toward the other outboard motor EM (second propulsion device). Then, the bow turning mode steering angle (second target steering angle) of the other electric outboard motor EM (second propulsion device) is set so that the other electric outboard motor EM (second propulsion device) is steered in a direction against the water jet. Therefore, the bow turning mode effected in the dual mode is an example of the steering angle condition such that the predetermined load torque increase condition is satisfied. This steering angle condition is a condition such that the bow turning mode steering angles (the first target steering angle and the second target steering angle) are set for the two electric outboard motors EM (the first propulsion device and the second propulsion device) so as to steer the two electric outboard motors EM to move the rear ends of the two electric outboard motors EM toward each other. Further, the steering angle condition is also a condition such that the other electric outboard motor EM (second propulsion device) (particularly, the upper housing <NUM> (rudder plate) of the other electric outboard motor EM) receives the water jet generated by the one electric outboard motor EM (first propulsion device) due to the steering angle relationship between the two electric outboard motors EM.

In a preferred embodiment described above, the two electric outboard motors EM are disposed side by side on the stern by way of example. Alternatively, as shown in <FIG>, three or more electric outboard motors EM may be attached to the hull <NUM>. <FIG> shows the steering states of three electric outboard motors EM attached to the hull <NUM> when the bow turning mode is effected in a triple mode in which the three electric outboard motors EM respectively generate propulsive forces. A starboard-side electric outboard motor EMs and a port-side electric outboard motor EMp are steered in a V-shaped orientation. The propulsive force direction of a middle electric outboard motor EMc extends generally transversely of the hull <NUM>. The bow turning mode steering angles and the operation states of the three electric outboard motors EM are controlled so that the three electric outboard motors EM respectively generate propulsive forces tangentially of a circle <NUM> about the turning center <NUM> of the hull <NUM> to apply moments to the hull <NUM> in the same turning direction <NUM> (clockwise in <FIG>) about the turning center <NUM> of the hull <NUM>. Thus, the three electric outboard motors EM can efficiently apply the moments to the hull <NUM> and, therefore, can smoothly turn the bow of the hull <NUM>. When the bow turning mode is started, the steering load torques of the respective electric outboard motors EM are likely to be increased by the influence of water jets generated by the adjacent electric outboard motors EM. To compensate for this, the propulsive force control operation is performed to generate predetermined target propulsive forces after the completion of the steering to the bow turning mode steering angles. Thus, the bow turning mode can be smoothly utilized for the watercraft maneuvering.

A preferred embodiment described above is directed to the exemplary case in which the load torque increase condition (steering angle condition) is satisfied when the control mode of the main controller <NUM> is brought into the bow turning mode. Where the load torque increase condition (steering angle condition) in which the water jet generated by one of the two adjacent propulsion devices is likely to excessively increase the steering load torque of the other propulsion device is satisfied in a control mode other than the bow turning mode, the two propulsion devices can be properly steered to the target steering angles by using preferred embodiments.

In a preferred embodiment described above, the electric outboard motors are used as the propulsion devices by way of example, but engine outboard motors each utilizing an engine as a drive source thereof may be used as the propulsion devices. Further, propulsion devices utilizing different types of prime movers may be used in combination (e.g., an engine outboard motor and an electric outboard motor may be used in combination).

In a preferred embodiment described above, the outboard motors are used as the propulsion devices by way of example, but inboard motors, inboard/outboard motors (stern drives), waterjet propulsion devices and other types of propulsion devices may be employed.

Claim 1:
A watercraft propulsion system (<NUM>) comprising:
a first propulsion device (EMs, EMp) configured to be attachable to a hull (<NUM>) of a watercraft (<NUM>) in a steerable manner;
a second propulsion device (EMp, EMs) configured to be attachable to the hull (<NUM>) in a steerable manner;
a first steering device (<NUM>) configured to steer the first propulsion device (EMs, EMp);
a second steering device (<NUM>) configured to steer the second propulsion device (EMp, EMs); and Z r charactered in that it further comprises a controller (<NUM>) configured or programmed to control the first propulsion device (EMs, EMp), the second propulsion device (EMp, EMs), the first steering device (<NUM>), and the second steering device (<NUM>), and configured or programmed to determine whether or not a predetermined load torque increase condition is satisfied in which a steering load torque of the second propulsion device (EMp, EMs) is likely to be increased by a water jet generated by the first propulsion device (EMs, EMp), and to perform a propulsive force restricting control (S3) to restrict a propulsive force of the first propulsion device (EMs, EMp) and a propulsive force of the second propulsion device (EMp, EMs) if it is determined that the predetermined load torque increase condition is satisfied.