Abstract:
A watercraft propulsion system has a control lever for giving instructions regarding operating mode and output power from a source of driving force. A controller sets a propeller driving mode according to instructions given from the control lever. In an embodiment, an electric motor and an engine are included in the propulsion system, and the controller simultaneously controls both the engine and motor based on inputs from the control lever and sensed conditions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is based on and claims priority under 35 U.S.C. § 119 to Japanese Patent Application Serial No. 2006-246081, filed on Sep. 11, 2006, the entire contents of which are expressly incorporated by reference herein. 
   BACKGROUND 
   1. Field of the Invention 
   The present invention relates to a watercraft propulsion system and its operating method. More specifically, it relates to a hybrid-type watercraft propulsion system having an engine and an electric motor as a source of driving force for a propeller. 
   2. Description of the Related Art 
   Japanese Publication No. 2004-257294 presents an example of a cruising method using so-called “acceleration assist,” in which the driving power of an engine is assisted by the driving power of an electric motor for the purpose of driving a power transmission device. In cruising with such acceleration assist, the output power of the electric motor is adjusted based on the operating speed of the control lever, throttle opening speed of the engine, the operating time of the control lever, and the engine rotation variation time. 
   This technique attempts to ensure consistency of the engine operation with the electric motor operation on the assumption that the engine is always running while under way. Therefore, such technique cannot prevent the exhaust gas and the noise emission during the trolling operation. 
   Japanese Publication No. 2006-036086 presents an example in which a throttle grip that can be operated in freely rotatable manner is provided on a bar handle, and a control switch is provided in the vicinity of the throttle grip. By operating the control switch, running and stopping of the engine and the electric motor, as well as the rotational direction of the electric motor, can be controlled. Also, the rotational speeds of the electric motor and engine can be adjusted according to the turning operation of the throttle grip. 
   However, two control means: the control switch and the throttle grip, must be used for controlling the running, stopping, and rotational speeds of the engine as well as the electric motor. Operation of such control means is troublesome. 
   SUMMARY OF THE INVENTION 
   Accordingly, there is a need in the art for a watercraft propulsion system and corresponding operating method by which an engine and an electric motor can be controlled easily, and at the same time the exhaust gas and the noise during the trolling operation can be suppressed. 
   In accordance with one embodiment, the present invention provides a watercraft propulsion system comprising an engine and an electric motor that are both configured to selectively drive a propeller. The system comprises a user instruction device and a controller. The user instruction device is configured so that a user can select a first or second forward operating mode and an output power within the selected operating mode. The controller is adapted to receive a signal from the instruction device indicative of the desired operating mode and output power and to control the engine and electric motor accordingly. The controller is configured to employ the electric motor to drive the propeller when the first forward operating mode is selected, and the controller is configured to employ the engine to drive the propeller when the second forward operating mode is selected. 
   In one such embodiment, the user instruction device comprises a control lever, and the position of the control lever simultaneously determines the selected operating mode and selected output power within the selected mode. In one embodiment, the control lever is rotatable about an axis, and the position of the control lever is controllable by one hand. 
   In a preferred embodiment, the first forward operation mode is a trolling mode, and the second forward operation mode is a regular cruising mode. In one such embodiment, the controller is configured so that a variation of the electric motor output power corresponding to a change in position of the control lever in the trolling mode is less than a variation of the engine output power corresponding to a change in position of the control lever in the regular cruising mode. 
   Another embodiment additionally comprises a battery for storing electric power to be supplied to the electric motor, a charge level detector configured to detect a charge level in the battery, and an electricity generator adapted to be driven by the engine to charge the battery. The charge level detector communicates with the controller, and the controller is configured to determine based on the detected charge level whether the engine should be run during the first forward operating mode in order to charge the battery. In one such embodiment, a connection between the engine and the propeller is disengaged in the first forward operating mode. 
   Another embodiment additionally comprises an engine speed detector adapted to communicate an engine speed to the controller, wherein the controller is configured to couple the engine to the propeller only when the engine exceeds a threshold engine speed. In another embodiment, the controller is configured to run the electric motor in addition to the engine during the second forward operation mode when the engine speed is below a second threshold value. 
   In yet another embodiment, the engine is operated at idle when the electric motor is driven by electric power from the battery in the first forward operating mode. 
   A yet further embodiment additionally comprises a battery for storing electric power to be supplied to the electric motor, a charge level detector configured to detect a charge level in the battery, and an electricity generator adapted to be driven by the engine to charge the battery, wherein the charge level detector communicates with the controller. The controller is configured so that if the detected charge level is below a threshold level when the electric motor is driving the propeller in the first forward operating mode, the controller restricts the output power of the electric motor. 
   A still further embodiment additionally comprises a battery for storing electric power to be supplied to the electric motor, a charge level detector configured to detect a charge level in the battery, and an electricity generator adapted to be driven by the engine to charge the battery, wherein the charge level detector communicates with the controller. The controller is configured so that if the detected charge level is below a threshold level when the electric motor is driving the propeller in the first forward operating mode, the controller simultaneously runs the electricity generator to charge the battery. 
   In accordance with another embodiment, A watercraft propulsion system comprises an engine and an electric motor that are both configured to selectively drive a propeller. The system comprises a user instruction device, a drive source switch, and a controller. The user instruction device is configured so that a user can select a first or second forward operating mode and an output power within the selected operating mode. The controller is adapted to receive a signal from the instruction device indicative of the desired operating mode and output power and to control the engine and electric motor accordingly. When the drive source switch is in a first position the controller is configured to employ the electric motor to drive the propeller when the first forward operating mode is selected and to employ the engine to drive the propeller when the second forward operating mode is selected. When the drive source switch is in a second position the controller is configured to employ the engine to drive the propeller when either the first or second forward operating mode is selected. 
   In accordance with yet another embodiment, the present invention provides a method for operating a watercraft propulsion system comprising an engine and an electric motor that are both configured to selectively drive a propeller and which are controlled by a controller. The method comprises receiving a user instruction selecting one of at least a first and second forward operating mode and selecting an output power in the selected operating mode. If the first forward operating mode is selected, the method comprises sensing a battery charge level and, if the sensed battery charge level is above a threshold level, driving the propeller with the electric motor. If the second forward operating mode is selected, the method directs driving the propeller with the engine. 
   In another embodiment, if the first forward operating mode is selected and the sensed battery charge level is below a second threshold, the electric motor is operated at a restricted output power level. 
   A further embodiment additionally comprises an electricity generator powered by the engine, wherein if the first forward operating mode is selected and the sensed battery charge level is below a second threshold, the engine is operated to drive the electricity generator simultaneously as the electric motor drives the propeller. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustration showing a watercraft propulsion system according to one embodiment. 
       FIG. 2  is an illustration showing an embodiment of a control lever. 
       FIG. 3  is a chart showing the corresponding relations between the control lever position and the output power from the source of driving force in accordance with an embodiment. 
       FIG. 4  is a flow chart showing the overall operation of the watercraft propulsion system. 
       FIG. 5  is a flow chart showing the operating mode determination process in step  17  of  FIG. 4 . 
       FIG. 6  is a flow chart showing an example of a forward operation process according to an embodiment. 
       FIG. 7  is a flow chart showing an example of determination process for the trolling operation according to an embodiment. 
       FIG. 8  is a flow chart showing another example of a forward operation process. 
       FIG. 9  is a flow chart showing still another example of a forward operation process. 
       FIG. 10  is a flow chart showing further still another a example of forward operation process. 
       FIG. 11  is a schematic illustration showing a watercraft propulsion system according to still another embodiment. 
       FIG. 12  is a flow chart showing an example of the forward operation process according to the embodiment of  FIG. 11 . 
       FIG. 13  is a flow chart showing another example of a forward operation process. 
       FIG. 14  is a flow chart showing still another example of a forward operation process. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Now, embodiments of this invention will be described below with reference to the drawings. 
   Referring to  FIG. 1 , a watercraft propulsion system  10  according to an embodiment is of a hybrid type having a propeller  12 , an engine  14  as a source of driving force for the propeller  12 , and an electric motor  16 . The illustrated embodiment is a motor-centered type watercraft propulsion system in which the electric motor  16  is disposed between the engine  14  and the propeller  12 . It should be noted that the watercraft propulsion system  10  and  10   a  (that will be described later) may be configured as an outboard motor or as a part of the watercraft. 
   In the illustrated watercraft propulsion system  10 , an electromagnetic clutch  22  is provided between a crankshaft  18  of the engine  14  and a rotor  20  of an electric motor  16 . The crankshaft  18  and the rotor  20  preferably are connected or separated by turning on or off the electromagnetic clutch  22 . A driveshaft  24  is joined to the rotor  20  of the electric motor  16 , and the driveshaft  24  is connected to the propeller  12  via a forward/reverse switching device  26 . The rotational direction of the propeller  12  is determined by the forward/reverse switching device  26 . The forward/reverse switching device  26  preferably is a common dog clutch as can be found on outboard motors, for instance, that is operated by an electric actuator. 
   An electric generator body  28  used for engine generation preferably is disposed on the top of the engine  14 , and the electric generator body  28  is provided on the upper end of the crankshaft  18 . Also, an exhaust pipe  30 , an ignition device  32  for ignition of the engine  14 , a throttle valve  34  for adjusting the amount of fuel delivered to the engine  14 , and an engine speed sensor  36  for detecting the engine rpm (revolutions per minute) preferably are provided on the engine  14 . A throttle motor  38  for driving the throttle valve  34 , and a throttle opening sensor  40  for detecting the throttle valve  34  preferably are provided on the throttle valve  34 . The exhaust pipe  30  preferably is provided so that the exhaust opening is located in the rearward of the propeller  12 . 
   The illustrated electric motor  16 , the electromagnetic clutch  22 , the forward/reverse switching device  26 , the electric generator body  28 , the ignition device  32 , the engine speed sensor  36 , the throttle motor  38 , and the throttle opening sensor  40  are connected to a controller  42 . Further, a main switch  44  for starting (ON) or stopping (OFF) the operation of the watercraft propulsion system  10 , a control lever  46  for giving instructions on the types of operating mode as well as on the amount of output power from the source of driving force, a drive/power generation switch  48  for selectively setting the driving function or power generation function, a trouble lamp  50  to make a trouble annunciation, a battery  52  composed of a 24V battery, for instance, and a battery voltage sensor  54  for detecting the voltage of the battery  52  preferably are connected to the controller  42 . 
   In the illustrated embodiment, the controller  42  receives signals indicating the opening of the throttle valve  34  by the throttle opening sensor  40 , signals indicating the rpm of the engine  14  by the engine speed sensor  36 , ON/OFF signals by the main switch  44 , lever position signals indicating the type of operating mode and the amount of output power from the source of driving force by the control lever  46 , setting signals indicating driving function or power generation function by the drive/power generation switch  48 , and signals indicating the battery voltage by the battery voltage sensor  54 . In addition, an electric power obtained by the engine generation at the electric generator body  28  charges up the battery  52  via the controller  42 . 
   The controller  42  preferably gives ignition instructions to the ignition device  32 , driving signals to the throttle motor  38 , ON/OFF signals to the electromagnetic clutch  22 , driving signals and the electric power from the battery  52  to the electric motor  16 , setting signals of forward or reverse to the forward/reverse switching device  26 , and lamp lighting signals to the trouble lamp  50 . 
   Additionally, the controller  42  preferably includes a memory  42   a . A program for implementing the operations shown in  FIGS. 4 through 10  preferably is stored in the memory  42   a . Further, operation data, a first prescribed value and a second prescribed value that are to be compared with the rpm of the engine  14 , a specified value to be compared with the battery voltage, a first threshold and a second threshold, table data showing the corresponding relation between the position of the control lever  46  and the output power from the source of driving force, and so on preferably are stored in the memory  42   a.    
   In the present embodiment, the control lever  46  represents an instruction means. The controller  42  represents a setting means and a first to a third determination means. The battery voltage sensor  54  represents the charge level detecting means. The engine speed sensor  36  represents the rpm detecting means. 
   In the present specification, the term “trolling mode” refers to the condition in which a watercraft is propelled forward in dead slow speed between 0 knot and several knots per hour. The “regular cruising mode” refers to the condition in which a watercraft is propelled in speed range greater than the trolling mode. 
   Next, referring to  FIGS. 2 and 3 , the relationship between the position of the control lever  46  and the types of operating mode or the output power from the source of driving force will be described in accordance with one embodiment. 
   As shown in  FIG. 2 , the control lever  46 , preferably rotatable in forward and backward, can give an instruction on the types of operating mode (regular cruising, trolling, stop, or reverse) by its lever position, and at the same time, it can give an instruction on the amount of output power from the source of driving force by its lever position, as shown in  FIG. 3 . 
   As best shown in  FIG. 3 , a prescribed range extending in the forward and backward direction around the neutral position of the control lever  46  is a stop mode, a prescribed range in the forward side of the stop mode range is a trolling mode, and a range farther in the forward side of the trolling mode range is a regular cruising mode. Also, a range in the backward section of the stop mode range is a reverse mode. In the illustrated embodiment, variation in the output power from the source of driving force relative to the displacement of the control lever  46  is smaller in the trolling mode compared with that in the regular cruising mode. 
   In this arrangement, instructions for the types of operating mode and the amount of output power from the source of driving force can be given easily and continuously by the rotating operation of the control lever  46 , resulting in the remarkable improvement of the controllability. In addition, it facilitates fine tuning of the output power in the trolling mode, and the speed control in the dead slow speed running. 
   Further, as shown in  FIG. 3 , the illustrated embodiment provides so called hysteresis, by which the mode switching position of the control lever  46  is different in the opening operation that moves the control lever farther from the neutral position in comparison with such position in the closing operation to move the control lever closer to the neutral position. In this way, some “play” is provided in the mode switching process, preventing frequent mode switching around the boundary of the abutting modes. 
   Overall operation of such an embodiment of a watercraft propulsion system will be described with reference to  FIG. 4 . 
   First, the system is initialized (step S 3 ) when the main switch  44  is pressed down to the ON state (step S 1 ). The system initialization includes setting of the electromagnetic clutch  22  at OFF state, for instance. 
   Next, the lever position signal of the control lever  46  is input into the controller  42  (step S 5 ), and the signal indicating the battery voltage detected by the battery voltage sensor  54  is input into the controller  42  (step S 7 ). Further, the setting signal from the drive/power generation switch  48  is input into the controller  42  (step S 9 ), followed by the input of the signal indicating the opening of the throttle valve  34  (throttle position) detected by the throttle opening sensor  40  (step S 11 ), and the input of the signal indicating the engine rpm detected by the engine speed sensor  36  (step S 13 ). The controller  42  in the present embodiment preferably detects any trouble in the watercraft propulsion system  10  based on these input information (step S 15 ), and the operating mode is determined if there is no trouble (step S 17 ). 
   The stopping process is implemented if the operating mode is the stop mode (step S 19 ), the forward operation process is implemented if it is a forward mode (step S 21 ), the reverse operation process is implemented if it is the reverse mode (step S 23 ), and the power generating process is implemented if it is the power generation mode (step S 25 ). Then, the process returns to the step S 5 . 
   On the contrary, if any trouble is detected in the watercraft propulsion system  10  in step S 15 , the trouble lamp  50  comes on according to the instruction by the controller  42  (step S 27 ), an irregular stop process preferably is implemented (step S 29 ), and the process is terminated. 
   Operating mode determination process shown as the step S 17  in  FIG. 4 , will next be explained in detail with reference to  FIG. 5 . 
   In this embodiment, the controller  42  first determines whether the setting signal from the drive/power generation switch  48  indicates power generation or driving (step S 51 ). If the signal indicates driving, the controller  42  determines whether the position of the control lever  46  has changed or not (step S 53 ). If the lever position has changed, determination is made whether the control lever  46  is at the neutral position or in the forward side of the neutral position (step S 55 ). If the control lever  46  is at the neutral position or in the forward side of the neutral position, then, determination is made whether the operating direction of the control lever  46  is in the forward-opening direction (step S 57 ). The operating direction of the control lever  46  can be determined based on the lever position in the previous control cycle and that in the present control cycle. 
   If the operating direction of the lever is determined to be in the forward-opening direction in step S 57 , then determination is made whether the control lever  46  is positioned in the stop range associated with the forward-opening operation (step S 59 ). If the lever is positioned in the stop range, the operating mode is determined to be the stop mode (step S 61 ). On the contrary, if the lever is not positioned in the stop range associated with the forward-opening operation in step S 59 , the operating mode is determined to be the forward mode (step S 63 ). When the operating mode is determined to be the forward mode, it is initially determined to be the trolling mode, and the forward/reverse switching device  26  is set in the forward mode. 
   If the operating direction of the control lever  46  is determined to be in the forward-closing direction in step S 57 , then, determination is made whether the lever is positioned in the stop range associated with the forward-closing operation (step S 65 ). If the lever is positioned in the stop range, the operating mode is determined to be the stop mode (step S 67 ). On the contrary, if the lever is not positioned in the stop range associated with the forward-closing operation in step S 65 , the operating mode is determined to be the forward mode (step S 69 ). 
   If the control lever  46  is in the reverse side relative to the neutral position in step S 55 , then, the process goes to step S 71 . In step S 71 , determination is made whether the operating direction of the control lever  46  is in the reverse-opening direction. If it is in the reverse-opening direction, determination is made whether the lever is positioned in the stop range associated with the reverse-opening operation (step S 73 ). If the lever is positioned in the stop range, the operating mode is determined to be the stop mode (step S 75 ). On the contrary, if the lever is not positioned in the stop range associated with the reverse-opening operation in step S 73 , the operating mode is determined to be the reverse mode (step S 77 ). 
   If the operating direction of the control lever  46  is determined to be in the reverse-closing direction in step S 71 , then, determination is made whether the lever is positioned in the stop range associated with the reverse-closing operation (step S 79 ). Then, if the lever is positioned in the stop range, the operating mode is determined to be the stop mode (step S 81 ). On the contrary, if the lever is not positioned in the stop range associated with the reverse-closing operation in step S 79 , the operating mode is determined to be the reverse mode (step S 83 ). 
   Further, if the drive/power generation switch  48  is set for power generation in step S 51 , the operating mode is determined to be the power generation mode (step S 85 ). 
   If there is no change in the position of the control lever  46  in step S 53 , determination is made whether the present mode is the power generation mode or not (step S 87 ). If the present mode is the power generation mode, the operating mode is determined to be the stop mode (step S 89 ). On the other hand, if the present mode is not the power generation mode in step S 87 , then, the present mode is maintained (step S 91 ). 
   Next, an operation example regarding the forward operation process shown in  FIG. 4  as the step S 21  will be described with reference to  FIG. 6 . 
   First, the controller  42  determines whether the operating mode is the trolling mode or not (step S 101 ). If it is the trolling mode, the controller  42  sets the propeller driving mode to the first mode, in which the propeller  12  is driven by the electric motor  16 , and the amount of output power from the electric motor  16  is adjusted according to the instruction given by the control lever  46 . In other words, the process goes to step S 103 , and the following process is implemented. 
   In step S 103 , the turning-off process for the electromagnetic clutch  22  is implemented. Then, the controller  42  determines whether the voltage of the battery  52  is below a specified value or not (step S 105 ). 
   If the battery voltage is below the specified value, the controller  42  determines the motor driving mode to be a third mode, in which the electric motor  16  is driven by electric power from the battery  52 , and in parallel, the electric power obtained by the engine generation is charged into the battery  52 . In other words, the process goes to step S 107 . Then, determination is made in step S 107  whether the engine has not been started yet. If the engine has not been started yet, the engine starting process is implemented (step S 109 ), and the process goes to step S 111 . 
   If the engine has already been started in step S 107 , the engine speed is controlled to obtain the prescribed amount of power generation, engine generation is implemented (step S 113 ), and then the process goes to step S 111 . 
   If the battery voltage exceeds the specified value in step S 105 , the controller  42  determines the motor driving mode to be in a fourth mode, in which the motor  16  is driven by electric power from the battery  52 . In other words, the process goes to step S 115 . Since the engine start is not required in the fourth mode, determination is made whether the engine  14  is stopping or not in step S 115 . If the engine  14  is stopping, the process goes to step S 111 , but if the engine  14  is running, the shutoff process for the ignition device  32  is implemented (step S 117 ), the throttle valve  34  is closed (S 119 ), and then the process goes to S 111 . 
   In step S 111 , the controller  42  calculates the electric motor driving current to be supplied to the electric motor  16 , with reference to the table data showing the corresponding relations of  FIG. 3 , so that the motor output power is obtained in response to the position of the control lever  46 . Then, a drastic change limiting process is implemented to prevent the electric motor driving current from changing sharply (step S 121 ), the positive rotation output process for the electric motor  16  is implemented (step S 123 ), and the process is terminated. 
   If the operating mode is not the trolling mode in step S 101 , it is determined to be the regular cruising mode. Consequently, the controller  42  sets the propeller driving mode to the second mode in which the propeller  12  is driven by the engine  14  and the amount of output power from the engine  14  is adjusted according to the instruction given by the control lever  46 . In other words, the process goes to step S 125 , and the following process is implemented. 
   Determination is made in step S 125  whether the engine has not been started yet. If the engine has not been started yet, the engine starting process is implemented (step S 127 ), and the process is terminated. 
   If the engine has already been started in step S 125 , determination is made whether the engine  14  is to be connected or not, namely, whether the electromagnetic clutch  22  is to be turned on or not (step S 129 ). Here, determination is made by the controller  42  based on the criteria that the engine speed is at or higher than the first prescribed value (1200 rpm, for example) or not. If the engine speed is at or higher than the first prescribed value, it is determined that the engine  14  can be connected, followed by the implementation of the electromagnetic clutch  22  turning-on process (step S 131 ), and the process goes to S 133 . Preferably the engine  14  and the propeller  12  are joined when the electromagnetic clutch  22  is turned on. 
   If the engine speed is below the first prescribed value and the condition does not allow the engine  14  to be connected yet in step S 129 , then, the process goes to step S 133  directly. 
   In step S 133 , determination is made whether the acceleration of electric motor  16  is required or not. Determination is made by the controller  42  based on the criteria that the speed of the engine  14  is at or higher than the second prescribed value (3000 rpm, for example) or not. If the speed of the engine  14  is below the second prescribed value, it is determined that the acceleration of the electric motor  16  is required, followed by implementing acceleration of the electric motor  16  (step S 135 ), and then the process goes to step S 137 . If acceleration of the electric motor  16  is determined not to be required in step S 133 , the electric motor  16  is stopped (step S 139 ), and the process goes to step S 137 . 
   In step S 137 , the controller  42  calculates the opening of the throttle valve  34  with reference to table data showing the corresponding relations of  FIG. 3 , so that the engine output power is obtained in response to the position of the control lever  46 . Then, a drastic change limiting process is implemented to prevent the opening of the throttle valve  34  from changing sharply (step S 141 ), the output power process for the engine  14  is implemented (step S 143 ), and the process is terminated. 
   According to the operation example described above, instructions for the types of operating mode as well as the amount of output power from the source of driving force can be given easily and continuously by the rotating operation of the single control lever  46 , resulting in the simple control of the engine  14  and the electric motor  16 . Especially, when the regular cruising mode is switched to the trolling mode by the control lever  46 , transition from the propeller being driven by the engine  14  into the propeller being driven by the electric motor  16  can be made smoothly. Further, shifting between the forward operation and the reverse operation can be made easily by the single control lever  46 . 
   Also, exhaust gas and the noise can be suppressed in trolling, because the propeller  12  is driven by the electric motor  20  when instruction is given to run in the trolling mode. 
   Further, in the first mode, the electric motor  16  is driven by the electric power supplied by the battery  52  when the battery voltage exceeds the specified value, while the motor  16  is driven by the electric power supplied by the battery  52 , and in parallel, the electric power obtained by the engine generation is charged into the battery  52  when the battery voltage is below the specified value. In this way, deterioration of the battery  52  due to over discharge can be prevented. 
   Additionally, in the first mode, when the electric power is supplied by engine generation using the engine  14 , with the propeller  12  being separated by turning-off the electromagnetic clutch  22 , engine speed can be controlled regardless of the rotational speed of the propeller  12 , and an adequate amount of charging power can be obtained. 
   Further, in the second mode, the engine  14  is connected to the propeller  12  by turning-on the electromagnetic clutch  22  when the speed of the engine  14  exceeds the first prescribed value, thus the engine  14  is connected to the propeller  12  smoothly. Additionally in the second mode, the electric motor  16  is also used to drive the propeller  12  until the speed of the engine  14  reaches the second prescribed value, thus acceleration of the watercraft is improved. 
   A preferred determination process to identify the trolling mode operation will be explained next with reference to  FIG. 7 . 
   First, the controller  42  determines whether the position of the control lever  46  has changed or not (step S 201 ), and if it has changed, determination is made whether the operating direction of the control lever  46  is in the opening direction (step S 203 ). 
   If the operating direction of the control lever  46  is in the opening direction, determination is made whether the control lever  46  is positioned in the trolling range associated with the opening operation (step S 205 ), then, if it is in the trolling range, the operating mode is determined to be the trolling mode, and the process is terminated. If the lever is not positioned in the trolling range in step S 205 , the operating mode is determined to be the regular cruising mode (step S 209 ), and the process is terminated. 
   If the operating direction of the control lever  46  is determined to be in the closing direction in step S 203 , determination is made whether the control lever  46  is positioned in the trolling range associated with the closing operation (step S 211 ). If it is in the trolling range, the operating mode is determined to be the trolling mode (step S 213 ), and the process is terminated. If the lever is not positioned in the trolling range in step S 211 , the operating mode is determined to be the regular cruising mode (step S 215 ), and the process is terminated. 
   Another operation example of an embodiment of the forward operation process will be explained with reference to  FIG. 8 . 
   First, determination is made whether the engine  14  has not been started yet (step S 301 ). If the engine has not been started yet, the turning-off process for the electromagnetic clutch  22  is implemented (step S 303 ), the engine starting process is implemented (step S 303 ), and the process goes to step S 307 . If the engine has already been started in step S 301 , the process goes to step S 307 . 
   In step S 307 , determination is made whether the operating mode is the trolling mode or not, and if it is the trolling mode, the controller  42  sets the propeller driving mode to the first mode. Then, the process goes to step S 309  to make determination whether the engine has not been started yet (step S 309 ). If the engine has not been started yet, the process goes to step S 311 , while if the engine has already been started, determination is made whether the voltage of the battery  52  is below the specified value or not (step S 313 ). If the battery voltage is below the specified value, the engine generation is implemented (step S 315 ), and the process goes to step S 311 . 
   If the battery voltage exceeds the specified value in step S 313 , the throttle valve  34  is controlled to the idling position by the controller  42  (step S 317 ), and the process goes to step S 311 . 
   In step S 311 , the controller  42  calculates the electric motor driving current with reference to the table data showing the corresponding relations of  FIG. 3 , so that the motor output power is obtained in response to the position of the operating lever  46 . Then, a drastic change limiting process is implemented to prevent the electric motor driving current from changing sharply (step S 319 ), the positive rotation output process for the electric motor  16  is implemented (step S 321 ), and the process is terminated. 
   If the operating mode is not the trolling mode in step S 307 , it is determined to be the regular cruising mode. Then, controller  42  sets the propeller driving mode to the second mode, the process goes to step S 323 , and the determination is made whether the engine  14  is to be connected or not, namely, whether the electromagnetic clutch  22  is to be turned on or not. If the engine speed is at or higher than the first prescribed value, it is determined that the engine  14  can be connected, and the electric motor  16  preferably is stopped (step S 325 ), followed by the implementation of the electromagnetic clutch  22  turning-on process (step S 327 ), then the process goes to S 329 . 
   If the engine speed is below the first prescribed value and the condition does not allow the engine  14  to be connected yet in step S 323 , then the process goes to step S 329  directly. 
   In step S 329 , the controller  42  calculates the opening of the throttle valve  34  with reference to table data showing the corresponding relations of  FIG. 3 , so that the engine output power is obtained in response to the position of the control lever  46 . A drastic change limiting process is implemented to prevent the opening of the throttle valve  34  from changing sharply (step S 331 ), the output power process for the engine  14  is implemented (step S 333 ), and the process is terminated. 
   In this operation example, the engine  14  is operated at idle while the electric motor  16  is driven by the electric power from the battery  52  in the first mode, allowing swift transition to the second mode without re-starting the engine  14  thereafter. 
   Another operation example of an embodiment of the forward operation process will be explained with reference to  FIG. 9 . 
   First, determination is made whether the operating mode is the trolling mode or not (step S 401 ), and if it is the trolling mode, the turning-off process for the electromagnetic clutch  22  is implemented (step S 403 ). Then the controller  42  compares the voltage of the battery  52  with the first threshold and the second threshold (step S 405 ). When the first threshold is below the battery voltage (first threshold &lt;battery voltage), the controller  42  sets the propeller driving mode to the first mode. 
   When the first threshold is below the battery voltage and the battery voltage is below the second threshold (first threshold &lt;battery voltage &lt;second threshold), the controller  42  determines the motor driving mode to be the third mode, and a determination is made whether the engine has not been started yet (step S 407 ). If the engine has not been started yet, the engine starting process is implemented (step S 409 ), and the process goes to step S 411 . 
   If the engine has already been started in step S 407 , the engine speed is controlled to obtain the prescribed amount of power generation, the engine generation is implemented (step S 413 ), and then, the process goes to step S 111 . 
   When the battery voltage is equal or larger than the second threshold (battery voltage≧second threshold) in step S 405 , then, the controller  42  determines the motor driving mode to be the fourth mode, and the determination is made whether the engine  14  is shut off (step S 415 ). If the engine  14  is stopped or stopping, the process preferably goes to step S 411 , but if the engine  14  is running, the shutoff process for the ignition device  32  is implemented (step S 417 ), the throttle valve  34  is closed (S 419 ), and the process goes to S 411 . 
   In step S 411 , the controller  42  calculates the electric motor driving current with reference to table data showing the corresponding relations of  FIG. 3 , so that the motor output power is obtained in response to the position of the operating lever  46 . The drastic change limiting process is implemented to prevent the electric motor driving current from changing sharply (step S 421 ), the positive rotation output process for the electric motor  16  is implemented (step S 423 ), and the process is terminated. 
   If the operating mode is not the trolling mode but the regular cruising mode in step S 401 , or if the battery voltage is equal to or less than the first threshold (battery voltage≦first threshold) in step S 405  even though the operating mode is the trolling mode, the controller  42  sets the propeller driving mode to the second mode, the process goes to S 425 , and the determination is made whether the engine has not been started yet. If the engine has not been started yet, the turning-off process for the electromagnetic clutch  22  is implemented (step S 427 ), the stopping process for the electric motor  16  is implemented (step S 429 ), the engine starting process is implemented (S 431 ), and the process is terminated. 
   If the engine has already been started in step S 425 , the electric motor stopping process is implemented (step S 433 ). Then, the controller  42  calculates the opening of the throttle valve  34  with reference to the table data showing the corresponding relations of  FIG. 3 , so that the engine output power is obtained in response to the position of the control lever  46  (step S 435 ), the drastic change limiting process is implemented to prevent the opening of the throttle valve  34  from changing sharply (step S 437 ), and the output power process for the engine  14  is implemented (step S 439 ). Then, determination is made whether the engine  14  is to be connected or not, namely, whether the electromagnetic clutch  22  is to be turned on or not (step S 442 ). If the engine speed is at or higher than the first prescribed value, it is determined that the engine  14  can be connected, followed by the implementation of the electromagnetic clutch  22  turning-on process (step S 443 ), and the process is terminated. On the other hand, if the engine speed is below the first prescribed value and the condition does not allow the engine  14  to be connected yet in step S 441 , the process is terminated. 
   According to this operation example, the propeller drive mode is set to the first mode or to the second mode taking account of not only the operating mode, but also comparison between the battery voltage and the first threshold. For instance, even if the operating mode is instructed to be the trolling mode, sometimes it is hard to drive the propeller  12  by the electric motor  16  in the first mode when the battery voltage is equal to or below the first threshold (battery voltage≦first threshold). In such cases, the second mode can be used to drive the propeller  12 . 
   Also in the first mode, the electric motor  16  is driven by the electric power from the battery  52  when the battery voltage is equal to or larger than the second threshold (battery voltage≦second threshold), or the electric motor  16  is driven by the battery while the battery is being charged by the engine generation when the first threshold is less than the battery voltage, and the battery voltage is less than the second threshold (first threshold&lt;battery voltage&lt;second threshold). Thus, deterioration of the battery due to over discharge can be prevented in this embodiment. 
   Still another operation example of an embodiment of the forward operation process will be explained with reference to  FIG. 10 . 
   In this embodiment, the engine watercraft propulsion system  10  is further provided with an engine switch  56  connected to the controller  42  as shown in  FIG. 1 . The engine switch  56  represents a switching means to change between using or not using the electric motor  16  for driving the propeller. 
   First, the controller  42  determines whether the operating mode is the trolling mode or not (step S 501 ), and if it is the trolling mode, determination is made whether the engine switch  56  is turned off or not (step S 503 ). If the engine switch  56  is turned off, the controller  42  sets the propeller driving mode to the first mode. The process goes to S 505 , and the turning-off process for the electromagnetic clutch  22  is implemented. Further, the determination is made whether the voltage of the battery  52  is below the specified value or not (step S 507 ). If the battery voltage is below the specified value, the controller  42  determines the motor driving mode to be the third mode, and the determination is made whether the engine has not been started yet (step S 509 ). If the engine has not been started yet, the engine starting process is implemented (step S 511 ), and the process goes to step S 513 . 
   If the engine has already been started in step S 509 , the engine speed is controlled to obtain the prescribed amount of power generation, the engine generation is implemented (step S 515 ), and then the process goes to step S 513 . 
   If the battery voltage exceeds the specified value in step S 405 , then the controller  42  determines the motor driving mode to be the fourth mode, and the determination is made whether the engine  14  is stopped (step S 517 ). If the engine  14  is stopping or has stopped, the process goes to step S 513 , but if the engine  14  is running, the shutoff process for the ignition device  32  is implemented (step S 519 ), the throttle valve  34  is closed (S 521 ), and then the process goes to S 513 . 
   In step S 513 , the controller  42  calculates the electric motor driving current with reference to the table data showing the corresponding relations of  FIG. 3 , so that the motor output power is obtained in response to the position of the operating lever  46 . A drastic change limiting process is implemented to prevent the electric motor driving current from changing sharply (step S 523 ), the positive rotation output process for the electric motor  16  is implemented (step S 525 ), and the process is terminated. 
   If the operating mode is not the trolling mode but the regular cruising mode, or if the engine switch  56  is turned on in step S 503  even though the operating mode is the trolling mode, the controller  42  sets the propeller driving mode to the second mode, the process goes to S 527 , and the determination is made whether the engine has not been started yet (step S 527 ). If the engine has not been started yet in step S 527 , the turning-off process for the electromagnetic clutch  22  is implemented (step S 529 ), the electric motor  16  is stopped (step S 531 ), the engine starting process is implemented (step S 533 ), and the process is terminated. 
   If the engine has already been started in step S 527 , determination is made whether the engine  14  is to be connected or not, namely, whether the electromagnetic clutch  22  is to be turned on or not (step S 535 ). If the engine speed is at or higher than the first prescribed value, it is determined that the engine  14  can be connected, followed by the implementation of the electromagnetic clutch  22  turning-on process (step S 537 ), and the process goes to S 539 . 
   If the engine speed is below the first prescribed value, the condition does not allow the engine  14  to be connected yet in step S 535 . The process then goes to step S 539  directly. 
   In step S 539 , the controller  42  calculates the opening of the throttle valve  34  with reference to the table data showing the corresponding relations of  FIG. 3 , so that the engine output power is obtained in response to the position of the control lever  46 . Then, the drastic change limiting process is implemented to prevent the opening of the throttle valve  34  from changing sharply (step S 541 ), the output power process for the engine  14  is implemented (step S 543 ), and the process is terminated. 
   According to this operation example, if the engine switch  56  is turned on in advance to avoid the use of electric motor  16 , the propeller  12  can be driven in the second mode, namely driven by the engine  14 , even if the operating mode is the trolling mode, allowing a flexible response to the operator&#39;s demand. 
   A watercraft propulsion system  10   a  according to yet another embodiment of the present invention will be described with reference to  FIG. 11 . 
   The illustrated watercraft propulsion system  10   a  is configured as the motor on top type in which the electric motor  16  is provided on the top of the engine  14  without using an electromagnetic clutch. In addition, the driveshaft  24  is joined to the lower end of the crankshaft  8  of the engine  14 , the rotor  20  of the electric motor  16  is joined to the upper end of the crankshaft  18 , and the electric generator body  28  is provided on the upper end of the rotor  20 . A program for implementing the operations shown in  FIGS. 12 through 14 , and other items are stored in the memory  42   a . The controller  42  preferably represents the setting means and the third to the fourth determination means. The rest of the configuration preferably is the same or similar to the watercraft propulsion system  10  described above, and the description for the duplicated part will be skipped. 
   An operation example regarding the forward operation process of the watercraft propulsion system  10   a  will be described with reference to  FIG. 12 . 
   First, determination is made whether the operating mode is the trolling mode or not (step S 601 ). If it is the trolling mode, the controller  42  sets the propeller driving mode to the first mode, the process goes to S 603 , and the controller  42  determines whether the voltage of the battery  52  is below specified value or not (step S 603 ). 
   If the battery voltage is below the specified value, the controller  42  determines that the motor driving mode be in a fifth mode, in which the output power of the electric motor  16  is restricted so that it is driven by the electric power from the battery  52 . In other words, the process goes to step S 605 . The restriction on the output power of the electric motor  16  is set in step S 605 , and the process goes to step S 607 . 
   If the battery voltage is not below the specified value, the controller  42  determines the motor driving mode to be a sixth mode, in which the electric motor  16  is driven by the electric power from the battery  52  without restricting the output power of the electric motor  16 . In other words, the process goes to step S 609 . The restriction on the output power of the electric motor  16  is removed in step S 609 , and the process goes to step S 607 . 
   In step S 607 , determination is made whether the engine  14  is shut off or not. If the engine  14  is running, the shutoff process for the ignition device  32  is implemented (step S 611 ), the throttle valve  34  is closed (S 613 ), and the process is terminated. 
   If the engine  14  is shut off in step S 607 , the controller  42  calculates the electric motor driving current with reference to the table data showing the corresponding relations of  FIG. 3 , so that the motor output power is obtained in response to the position of the operating lever  46  (step S 615 ). A drastic change limiting process for motor driving current is implemented (step S 617 ), the positive rotation output process for the electric motor  16  is implemented (step S 619 ), and the process is terminated. 
   If the operating mode is not the trolling mode in step S 601 , it is determined to be the regular cruising mode. The controller  42  then sets the propeller driving mode to the second mode, the process goes to step S 621 , and the determination is made whether the engine has not been started yet. If the engine has not been started yet, the engine starting process is implemented (step S 623 ) using the electric motor  16  provided on the top of the engine  14  as a starter motor, and then, the process is terminated. 
   If the engine has already been started in step S 621 , determination is made whether the acceleration of the electric motor  16  is required or not. If the acceleration of the electric motor  16  is required, the electric motor  16  is accelerated (step S 627 ) and the process goes to S 629 . If the acceleration of the electric motor  16  is not required in step S 625 , the electric motor  16  is stopped (step S 631 ), and the process goes to step S 629 . 
   In step S 629 , the controller  42  calculates the opening of the throttle valve  34  with reference to the table data showing the corresponding relations of  FIG. 3 , so that the engine output power is obtained in response to the position of the control lever  46 . A drastic change limiting process is implemented to prevent the opening of the throttle valve  34  from changing sharply (step S 633 ), the output power process for the engine  14  is implemented (step S 635 ), and the process is terminated. 
   According to this operation example, the propeller  12  can be driven without fail by adjusting the output power of the electric motor  16  in response to the battery voltage in the first mode, and the trolling operation can be implemented by the electric motor  16 . 
   Another operation example regarding an embodiment of the forward operation process will be explained with reference to  FIG. 13 . 
   First, determination is made whether the operating mode is the trolling mode or not (step S 701 ). If it is in the trolling mode, the process goes to S 703 , and the controller  42  determines whether the voltage of the battery  52  is below a specified value or not. If the battery voltage exceeds the specified value, the controller  42  sets the propeller driving mode to the first mode, and the determination is made whether the engine  14  is stopped or not (step S 705 ). If the engine  14  is running, the shutoff process for the ignition device  32  is implemented (step S 707 ), the throttle valve  34  is closed (S 709 ), and the process is terminated. 
   If the engine  14  is stopped in step S 705 , the controller  42  calculates the electric motor driving current with reference to the table data showing the corresponding relations of  FIG. 3 , so that the motor output power is obtained in response to the position of the operating lever  46  (step S 711 ). Then, a drastic change limiting process for motor driving current is implemented (step S 713 ), the positive rotation output process for the electric motor  16  is implemented (step S 715 ), and the process is terminated. In still another embodiment, after step S 709 , the process may progress to step S 711 . 
   If the operating mode is not the trolling mode but the regular cruising mode in step S 701 , or if the battery voltage is equal to or less than the specified value (battery voltage≦specified value) in step S 703  even though the operating mode is the trolling mode, the controller  42  sets the propeller driving mode to the second mode, the process goes to S 717 , and the determination is made whether the engine has not been started yet. If the engine has not been started yet, the engine starting process is implemented (step S 719 ), preferably using the electric motor  16  provided on the top of the engine  14  as a starter motor, and then the process is terminated. In another embodiment, after step S 719  the process moves to step S 721 , discussed below. 
   If the engine has already been started in step S 717 , the controller  42  calculates the opening of the throttle valve  34  with reference to the table data showing the corresponding relations of  FIG. 3 , so that the engine output power is obtained in response to the position of the control lever  46  (step S 721 ). A drastic change limiting process is implemented to prevent the opening of the throttle valve  34  from changing sharply (step S 723 ), the output power process for the engine  14  is implemented (step S 725 ), and the process is terminated. 
   According to this operation example, the propeller drive mode is set to the first mode or to the second mode taking account of not only the operation mode, but also comparison between the battery voltage and the specified value. For instance, even if the operating mode is instructed to be the trolling mode, sometimes it is hard to drive the propeller  12  by the electric motor  16  using the first mode when the battery voltage is equal to or below the specified value (battery voltage≦specified value). In such cases, the second mode can be used to drive the propeller  12 . 
   Still another operation example regarding a still further embodiment of the forward operation process will be explained with reference to  FIG. 14 . 
   To implement this operation, the watercraft propulsion system  10   a  is further provided with an engine switch  56  connected to the controller  42  as shown in phantom in  FIG. 11 . 
   First, determination is made whether the operating mode is the trolling mode or not (step S 801 ), and if it is the trolling mode, determination is made whether the engine switch  56  is turned off or not (step S 803 ). If the engine switch  56  is turned off, the controller  42  sets the propeller driving mode to the first mode, the process goes to S 805 , and determination is made whether the engine  14  is shut off or not. If the engine  14  is running, the shutoff process for the ignition device  32  is implemented (step S 807 ), the throttle valve  34  is closed (S 809 ), and the process is terminated. 
   If the engine  14  is stopped in step S 805 , the controller  42  calculates the electric motor driving current with reference to the table data showing the corresponding relations of  FIG. 3 , so that the motor output power is obtained in response to the position of the operating lever  46  (step S 811 ). Then, a drastic change limiting process is implemented to prevent the electric motor driving current from changing sharply (step S 813 ), the positive rotation output process for the electric motor  16  is implemented (step S 815 ), and the process is terminated. In another embodiment, after step S 809  the process moves to step S 811 . 
   If the operating mode is not the trolling mode but the regular cruising mode in step S 801 , or if the engine switch  56  is turned on in step S 803  even though the operating mode is the trolling mode, the controller  42  sets the propeller driving mode to the second mode, the process goes to S 817 , and the determination is made whether the engine has not bee started yet (step S 817 ). If the engine has not been started yet, the engine starting process is implemented (step S 819 ) preferably using the electric motor  16  provided on the top of the engine  14  as a starter motor, and the process is terminated. 
   If the engine has already been started in step S 817 , the controller  42  calculates the opening of the throttle valve  34  with reference to the table data showing the corresponding relations of  FIG. 3 , so that the engine output power is obtained in response to the position of the control lever  46  (step S 821 ). The drastic change limiting process preferably is implemented to prevent the opening of the throttle valve  34  from changing sharply (step S 823 ), the output power process for the engine  14  is implemented (step S 825 ), and the process is terminated. 
   According to this operation example, if the engine switch  56  is turned on in advance to avoid the use of electric motor  16 , the propeller  12  can be driven in the second mode, namely driven by the engine  14 , even if the operating mode is the trolling mode, allowing a flexible response to the operator&#39;s demand. 
   It is to be understood that the instruction means may have structure different than as in the embodiments discussed herein. For example, a joystick, knob, thumb-slide or the like and employ principles discussed herein. 
   Also, the battery voltage sensor  54  is used as the charge level detector in the embodiments described above, however, the charge level detector is not limited to the battery voltage sensor  54 , but some other means may be employed which detects the battery charge level based on the electric current and the elapsed time, for instance. 
   In a preferred embodiment, the principles discussed above are incorporated into an outboard motor. However, other watercraft propulsion systems can use these principles. For example, inboard motor systems and stern drives are also contemplated. 
   Although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. In addition, while a number of variations of the invention have been shown and described in detail, other modifications, which are within the scope of this invention, will be readily apparent to those of skill in the art based upon this disclosure. For example, it is contemplated that motors having somewhat different structure than in the illustrated embodiments could be used, and variations of the operational flow charts surely could be made while still employing inventive principles. It is also contemplated that various combinations or subcombinations of the specific features and aspects of the embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form varying modes of the disclosed invention. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow.