Patent Publication Number: US-8109801-B2

Title: Boat propulsion system and control unit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a boat propulsion system and a control unit thereof. 
     2. Description of the Related Art 
     Conventionally, for example, as disclosed in JP-A-2007-283951, a control unit having a control lever for adjusting an accelerator opening is known. In the control unit disclosed in JP-A-2007-283951, the accelerator opening increases as an operating amount of the control lever increases. 
     For example, when a boat is leaving from or approaching to a dock or quay, or is trolling, it is preferable to finely adjust a boat propulsion speed by finely adjusting the rotational speed of a propeller. 
     However, in the control unit disclosed in JP-A-2007-283951, it is difficult to finely adjust the rotational speed of the propeller. 
     SUMMARY OF THE INVENTION 
     In order to overcome the problems described above, preferred embodiments of the present invention provide a boat propulsion system that can easily and accurately perform a fine adjustment of a rotational speed of a propeller. 
     A first boat propulsion system according to a preferred embodiment of the present invention includes a power source, a propeller, a control lever, an accelerator opening detection section, a sensitivity switching section, and a control device. The power source generates a turning force. The propeller is driven by the turning force of the power source. An accelerator opening is input to the control lever by an operation of an operator. The accelerator opening detection section detects an operating amount of the control lever. The accelerator opening detection section outputs the operating amount of the control lever. A degree of the accelerator opening relative to the operating amount of the control lever is switched by the sensitivity switching section operated by the operator. The sensitivity switching section outputs sensitivity that is the degree of accelerator opening relative to the input operating amount of the control lever as a sensitivity switching signal. The control device controls output of the power source based on the operating amount of the control lever and the sensitivity switching signal. 
     A second boat propulsion system according to another preferred embodiment of the present invention includes a power source, a propeller, a control lever, an accelerator opening detection section, a sensitivity switching section, and a control device. The power source generates a turning force. The propeller is driven by the turning force of the power source. An accelerator opening is input to the control lever by an operation of an operator. The accelerator opening detection section detects an operating amount of the control lever. The accelerator opening detection section outputs the accelerator opening corresponding to the operating amount of the control lever. Sensitivity that is the degree of the accelerator opening relative to the operating amount of the control lever output from the accelerator opening detection section is switched by the sensitivity switching section operated by the operator. The control device controls output of the power source based on the accelerator opening. 
     A control unit according to a preferred embodiment of the present invention is a control unit for a boat propulsion system. The boat propulsion system includes a power source, a propeller, and the control device. The power source generates a turning force. The propeller is driven by the turning force of the power source. The control device controls output of the power source based on the accelerator opening. The control unit according to a preferred embodiment of the present invention includes a control lever, an accelerator opening detection section, and a sensitivity switching section. An accelerator opening is input to the control lever by an operation of an operator. The accelerator opening detection section detects an operating amount of the control lever. The accelerator opening detection section outputs the accelerator opening corresponding to the operating amount of the control lever. Sensitivity that is the degree of the accelerator opening relative to the operating amount of the control lever output from the accelerator opening detection section is switched by the sensitivity switching section operated by the operator. 
     According to various preferred embodiments of the present invention, it is possible to realize a boat propulsion system that can easily and accurately perform a fine adjustment of a rotational speed of a propeller. 
     Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross sectional view of the stern portion of a boat in accordance with a first preferred embodiment of the present invention as viewed from a side. 
         FIG. 2  is a schematic configuration diagram showing the configuration of a thrust generating unit in accordance with the first preferred embodiment of the present invention. 
         FIG. 3  is a schematic cross sectional view of a shift mechanism in accordance with the first preferred embodiment of the present invention. 
         FIG. 4  is an oil circuit diagram in accordance with the first preferred embodiment of the present invention. 
         FIG. 5  is a control block diagram of the boat. 
         FIG. 6  is a table showing engaging states of first to third hydraulic clutches and the shift position of the shift mechanism. 
         FIG. 7  is a graph showing the relationship between a connecting force of the hydraulic clutch for shift change and a ratio of the rotational speed of an output shaft to the rotational speed of an input shaft. 
         FIG. 8  is a conceptual illustration showing a control lever. 
         FIG. 9  is a graph showing the relationship between an operating angle of the control lever and an accelerator opening. In the figure, M 1  represents the relationship between the operating angle of the control lever and the accelerator opening in a first mode. In the figure, M 2  represents the relationship between the operating angle of the control lever and the accelerator opening in a second mode. 
         FIG. 10  is a flowchart showing control of the rotational speed of the propeller in the first and the second modes. 
         FIG. 11  is a flowchart showing control of the rotational speed of the propeller in the second mode. 
         FIG. 12  is a map specifying the relationship between the accelerator opening and the rotational speed of the propeller. 
         FIG. 13  is a map specifying the relationship between the accelerator opening, a throttle opening, and the connecting force of the hydraulic clutch for shift change. A graph in a bold line specifies the throttle opening. A graph in a broken line specifies the connecting force of the hydraulic clutch for shift change. 
         FIG. 14  is a graph showing the relationship between the rotational speeds of the second and the third power transmission shafts and the accelerator opening in the case where the throttle opening and the connecting force of the hydraulic clutch for shift change are respectively controlled to the target value. 
         FIG. 15  is a graph showing the relationship between the accelerator opening and the rotational speeds of the second and the third power transmission shafts in the case where each of the hydraulic clutches for shift change is disengaged or engaged corresponding to the shift position. 
         FIG. 16  is a flowchart showing control of the rotational speed of the propeller in the first and the second modes in a second preferred embodiment of the present invention. 
         FIG. 17  is a flowchart showing control of the rotational speed of the propeller in the second mode in the second preferred embodiment of the present invention. 
         FIG. 18  is a control block diagram showing an example of adjustment control of the connecting force of the clutch performed in step S 33 . 
         FIG. 19  is a map for calculating adjusting amounts of the connecting forces of the clutches. 
         FIG. 20  is an example of a time chart showing the control performed in step S 33 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Example of preferred embodiments of the present invention will hereinafter be described by using an outboard motor  20  shown in  FIG. 1  as a boat propulsion system. The following preferred embodiments are mere examples of the preferred embodiments carrying out the present invention. The present invention is not limited to the following preferred embodiments. 
     A boat propulsion system according to a preferred embodiment of the present invention may be a so-called inboard motor or a so-called stern drive for example. The stern drive is also referred to as an inboard-outboard. The “stern drive” is a boat propulsion system in which at least a power source is installed on a hull. The “stern drive” also includes a system in which other components than a propulsion section are installed on a hull. 
     First Preferred Embodiment 
       FIG. 1  is a schematic partial cross-sectional view of a stern  11  of a boat  1  according to a first preferred embodiment, in a side view. As shown in  FIG. 1 , the boat  1  includes a hull  10  and an outboard motor  20 . The outboard motor  20  is attached to the stern  11  of the hull  10 . 
     General Structure of Outboard Motor  20   
     The outboard motor  20  includes an outboard motor body  21 , a tilt and trim mechanism  22 , and a bracket  23 . 
     The bracket  23  includes a mount bracket  24  and a swivel bracket  25 . The mount bracket  24  is fixed to the hull  10 . The swivel bracket  25  is swingable about a turning shaft  26  with respect to the mount bracket  24 . 
     The tilt and trim mechanism  22  performs a tilt operation and a trim operation of the outboard motor body  21 . Specifically, the tilt and trim mechanism  22  swings the swivel bracket  25  about the mount bracket  24 . 
     The outboard motor body  21  includes a casing  27 , a cowling  28 , and a thrust generating unit  29 . The thrust generating unit  29  is housed in the casing  27  and the cowling  28  except for a portion of a propulsion section  33  which will be described later. 
     As shown in  FIGS. 1 and 2 , the thrust generating unit  29  includes an engine  30 , a power transmission mechanism  32 , and the propulsion section  33 . 
     In this preferred embodiment, an example in which the outboard motor  20  has the engine  30  as a power source is described. However, the power source is not specifically limited as long as it can generate a turning force. For example, the power source may be an electric motor. 
     The engine  30  preferably is a fuel injection engine having a throttle body  87  shown in  FIG. 5 . In the engine  30 , the engine speed and the engine power are adjusted by adjusting the throttle opening. The engine  30  generates a turning force. As shown in  FIG. 1 , the engine  30  includes a crankshaft  31 . The engine  30  outputs the generated turning force via the crankshaft  31 . 
     The power transmission mechanism  32  is arranged between the engine  30  and the propulsion section  33 . The power transmission mechanism  32  transmits the turning force generated by the engine  30  to the propulsion section  33 . The transmission mechanism  32  includes a shift mechanism  34 , a speed reduction mechanism  37 , and a coupling mechanism  38 . 
     The shift mechanism  34  is connected to the crankshaft  31  of the engine  30 . As shown in  FIG. 2 , the shift mechanism  34  includes a gear ratio change mechanism  35  and a shift position change mechanism  36 . 
     The gear ratio change mechanism  35  changes the gear ratio between the engine  30  and the propulsion section  33  between a high speed gear ratio (HIGH) and a low speed gear ratio (LOW). Here, the “high speed gear ratio” is a gear ratio in which the ratio of the output side rotational speed to the input side rotational speed is relatively high. In contrast, the “low speed gear ratio” is a gear ratio in which the ratio of the output side rotational speed to the input side rotational speed is relatively low. 
     The shift position change mechanism  36  changes the shift position between a forward position a reverse position and a neutral position. 
     The speed reduction mechanism  37  is arranged between the shift mechanism  34  and the propulsion section  33 . The speed reduction mechanism  37  transmits the turning force from the shift mechanism  34  to the propulsion section  33  while reducing the rotational speed. Here, the structure of the speed reduction mechanism  37  is not particularly limited. For example, the speed reduction mechanism  37  may have a planetary gear mechanism. Further, the speed reduction mechanism  37  may have, for example, a speed reduction gear-set. 
     The coupling mechanism  38  is arranged between the speed reduction mechanism  37  and the propulsion section  33 . The coupling mechanism  38  includes a bevel gear-set (not shown). The coupling mechanism  38  transmits the turning force from the speed reduction mechanism  37  to the propulsion section  33  while changing the direction. 
     The propulsion section  33  includes a propeller shaft  40  and a propeller  41 . The propeller shaft  40  transmits the turning force from the coupling mechanism  38  to the propeller  41 . The propulsion section mechanism  33  converts the turning force generated by the engine  30  into thrust. 
     As shown in  FIG. 1 , the propeller  41  preferably includes two propellers, a first propeller  41   a  and a second propeller  41   b . The rotation direction of the first propeller  41   a  is opposite to that of the second propeller  41   b . When the turning force output from the power transmission mechanism  32  is in a forward direction, the first propeller  41   a  and the second propeller  41   b  rotate in the directions opposite with respect to each other, thereby generating thrust in a forward direction. Thus, the shift position is made forward. On the other hand, when the turning force output from the power transmission mechanism  32  is in a reverse direction, the first propeller  41   a  and the second propeller  41   b  respectively rotate in a direction opposite to that during forward movement. As a result, the thrust in the reverse direction is generated. Accordingly, the shift position is made reverse. 
     In this regard, the propeller  41  may include a single propeller or three or more propellers. 
     Detailed Structure of Shift Mechanism  34   
     Next, referring mainly to  FIG. 3 , the structure of the shift mechanism  34  in this preferred embodiment will be described in detail.  FIG. 3  shows a schematic structure of the shift mechanism  34 . Accordingly, the actual structure of the shift mechanism  34  is not precisely the same as that in  FIG. 3 . 
     The shift mechanism  34  includes a shift case  45 . The shift case  45  is generally cylindrical in appearance. The shift case  45  includes a first case  45   a , a second case  45   b , a third case  45   c , and a fourth case  45   d . The first case  45   a , the second case  45   b , the third case  45   c , and the fourth case  45   d  are integrally fixed preferably by bolts or other fastening or connecting elements. 
     Gear Ratio Change Mechanism  35   
     The gear ratio change mechanism  35  includes a first power transmission shaft  50  as an input shaft, a second power transmission shaft  51  as an output shaft, a planetary gear mechanism  52  as a shift gear-set, and a hydraulic clutch  53  for gear ratio change. 
     The planetary gear mechanism  52  transmits rotation of the first power transmission shaft  50  to the second power transmission shaft  51  at a low speed gear ratio (LOW) or a high speed gear ratio (HIGH). The gear ratio of the planetary gear mechanism  52  is changed by engaging or disengaging the hydraulic clutch  53  for gear ratio change. 
     The first power transmission shaft  50  and the second power transmission shaft  51  are arranged coaxially. The first power transmission shaft  50  is rotatably supported by the first case  45   a . The second power transmission shaft  51  is rotatably supported by the second case  45   b  and the third case  45   c . The first power transmission shaft  50  is connected to the crankshaft  31 . The first power transmission shaft  50  is also connected to the planetary gear mechanism  52 . 
     The planetary gear mechanism  52  includes a sun gear  54 , a ring gear  55 , a carrier  56 , and a plurality of planetary gears  57 . The ring gear  55  is formed generally cylindrical. On an inner periphery surface of the ring gear  55 , teeth are formed to mesh with the planetary gear  57 . The ring gear  55  is connected to the first power transmission shaft  50 . The ring gear  55  rotates together with the first power transmission shaft  50 . 
     The sun gear  54  is arranged within the ring gear  55 . The sun gear  54  rotates coaxially with the ring gear  55 . The sun gear  54  is attached to the second case  45   b  via a one-way clutch  58 . The one-way clutch  58  permits rotation in a forward direction while restrains rotation in a reverse direction. Therefore, the sun gear  54  can rotate forward while it cannot rotate reversely. 
     A plurality of the planetary gears  57  are arranged between the sun gear  54  and the ring gear  55 . Each planetary gear  57  meshes with both the sun gear  54  and the ring gear  55 . Each planetary gear  57  is rotatably supported by the carrier  56 . As a result, each of a plurality of the planetary gears  57  revolves around an axis of the first power transmission shaft  50  at the mutually same speed while rotating itself. 
     In this specification, “rotation” means that a member turns around an axis positioned within the member. In contrast, “revolution” means that a member turns around an axis positioned outside the member. 
     The carrier  56  is connected to the second power transmission shaft  51 . The carrier  56  rotates together with the second power transmission shaft  51 . 
     The hydraulic clutch  53  for gear ratio change is arranged between the carrier  56  and the sun gear  54 . In this preferred embodiment, the hydraulic clutch  53  for gear ratio change preferably is a wet type multi-plate clutch. However, in the present invention, the hydraulic clutch  53  for gear ratio change is not limited to a wet type multi-plate clutch. The hydraulic clutch  53  for gear ratio change may be a dry type multi-plate clutch or a so-called dog clutch, for example. 
     In this specification, the “multi-plate clutch” preferably is a clutch that includes a first member and a second member capable of rotating mutually with each other, one or plural first plates rotating together with the first member, and one or plural second plates rotating together with the second member, in which rotation between the first member and the second member is controlled by the pressurized contact between the first plates and the second plates. In this specification, “clutch” is not limited to an article that is arranged between an input shaft to which the turning force is input and an output shaft from which the turning force is output to connect or disconnect therebetween. 
     The hydraulic clutch  53  for gear ratio change includes a hydraulic piston  53   a  and a plate group  53   b  including clutch plates and friction plates. When the piston  53   a  is driven, the plate group  53   b  comes into pressurized contact. As a result, the hydraulic clutch  53  for gear ratio change is engaged. In contrast, when the piston  53   a  is not driven, the plate group  53   b  comes into non-pressurized contact. As a result, the hydraulic clutch  53  for gear ratio change is disengaged. 
     When the hydraulic clutch  53  for gear ratio change is engaged, the sun gear  54  and the carrier  56  become fixed each other. Accordingly, the sun gear  54  and the carrier  56  integrally rotate as the planetary gears  57  revolve. 
     Shift Position Change Mechanism  36   
     The shift position change mechanism  36  changes the shift position between a forward position, a reverse position and a neutral position. The shift position change mechanism  36  includes the second power transmission shaft  51  as an input shaft, a third power transmission shaft  59  as an output shaft, a planetary gear mechanism  60  as a rotational direction change mechanism, a first hydraulic clutch  61  for shift change, and a second hydraulic clutch  62  for shift change. 
     The first hydraulic clutch  61  for shift change and the second hydraulic clutch  62  for shift change connect or disconnect the second power transmission shaft  51  as an input shaft to or from the third power transmission shaft  59  as an output shaft. Specifically, connection between the second power transmission shaft  51  and the third power transmission shaft  59  changes by connecting or disconnecting the first hydraulic clutch  61  to or from the second hydraulic clutch  62 . In other words, the first hydraulic clutch  61  and the second hydraulic clutch  62  are devices for changing connection between the second power transmission shaft  51  and the third power transmission shaft  59 . Specifically, the rotational speed of the third power transmission shaft  59  with respect to the rotational speed of the second power transmission shaft  51  is adjusted by adjusting a connecting force between the first hydraulic clutch  61  and the second hydraulic clutch  62 . More specifically, the rotational direction of the third power transmission shaft  59  with respect to the rotational direction of the second power transmission shaft  51  and the ratio of the absolute value of the rotational speed of the third power transmission shaft  59  to the absolute value of the rotational speed of the second power transmission shaft  51  are adjusted by adjusting the connecting forces of the first hydraulic clutch  61  and the second hydraulic clutch  62 . 
     The planetary gear mechanism  52  changes the rotational direction of the third power transmission shaft  59  with respect to the rotational direction of the second power transmission shaft  51 . Specifically, the planetary gear mechanism  52  transmits the turning force of the second power transmission shaft  51  as a turning force in a forward direction or a reverse direction to the third power transmission shaft  59 . The rotational direction of the turning force transmitted by the planetary gear mechanism  52  is changed by engaging or disengaging the first hydraulic clutch  61  and the second hydraulic clutch  62 . 
     The third power transmission shaft  59  is rotatably supported by the third case  45   c  and the fourth case  45   d . The second power transmission shaft  51  and the third power transmission shaft  59  are arranged coaxially. In this preferred embodiment, the hydraulic clutches  61 ,  62  preferably are a wet type multiple-plate clutch. However, the hydraulic clutches  61 ,  62  may be a dog clutch, respectively, for example. 
     Here, the second power transmission shaft  51  is a common member to the gear ratio change mechanism  35  and the shift position change mechanism  36 . 
     The planetary gear mechanism  60  includes a sun gear  63 , a ring gear  64 , a plurality of planetary gears  65 , and a carrier  66 . 
     The carrier  66  is connected to the second power transmission shaft  51 . The carrier  66  rotates together with the second power transmission shaft  51 . Accordingly, as the second power transmission shaft  51  rotates, the carrier  66  rotates and a plurality of the planetary gears  65  mutually revolve at the same speed with each other. 
     A plurality of the planetary gears  65  mesh with the ring gear  64  and the sun gear  63 . The first hydraulic clutch  61  is arranged between the ring gear  64  and the third case  45   c . The first hydraulic clutch  61  includes a hydraulic piston  61   a  and a plate group  61   b  including clutch plates and friction plates. When the hydraulic piston  61   a  is driven, the plate group  61   b  comes into pressurized contact. This causes the first hydraulic clutch  61  to be engaged. As a result, the ring gear  64  is fixed to the third case  45   c  and disabled so as not to rotate. In contrast, when the piston  61   a  is not driven, the plate group  61   b  comes into non-pressurized contact. This causes the first hydraulic clutch  61  to be disengaged. As a result, the ring gear  64  is unfixed to the third case  45   c  and enabled to rotate. 
     The second hydraulic clutch  62  is arranged between the carrier  66  and the sun gear  63 . The second hydraulic clutch  62  includes a hydraulic piston  62   a  and a plate group  62   b  including clutch plates and friction plates. When the piston  62   a  is driven, the plate group  62   b  comes into pressurized contact. This causes the second hydraulic clutch  62  to be engaged. As a result, the carrier  66  and the sun gear  63  integrally rotate. In contrast, when the piston  62   a  is not driven, the plate group  62   b  comes into non-pressurized contact. This causes the second hydraulic clutch  62  to be disengaged. As a result, the ring gear  64  and the sun gear  63  are enabled to rotate separately. 
     Here, the reduction ratio of the planetary gear mechanism  60  is not limited to 1:1. The planetary gear mechanism  60  may have a reduction ratio other than 1:1. The reduction ratios may either be same or different between the case in which the planetary gear mechanism  60  transmits the turning force in the forward direction and the case in which the planetary gear mechanism  60  transmits the turning force in the reverse direction. 
     In this preferred embodiment, the description will be made of the case in which the planetary gear mechanism  60  has a reduction ratio other than 1:1 and the reduction ratios are different between the case in which the planetary gear mechanism  60  transmits the turning force in the forward direction and the case in which the planetary gear mechanism  60  transmits the turning force in the reverse direction. 
     Specifically, in this preferred embodiment, examples of approximate values of the ratio between the rotational speed of the first power transmission shaft  50  and the rotational speed of the third power transmission shaft  59  preferably is as follows. 
     High speed forward: 1:1, with a reduction ratio of 1 
     High speed reverse: 1:1.08, with a reduction ratio of 0.93 
     Low speed forward: 1:0.77, with a reduction ratio of 1.3 
     Low speed reverse: 1:0.83, with a reduction ratio of 1.21 
     As shown in  FIG. 2 , the shift mechanism  34  is controlled by the control device  91 . Specifically, the hydraulic clutch  53  for gear ratio change, the first hydraulic clutch  61 , and the second hydraulic clutch  62  are controlled by the control device  91 . 
     The control device  91  includes an actuator  70  and an electronic control unit (ECU)  86  as an electronic control unit. The actuator  70  engages and disengages the hydraulic clutch  53  for gear ratio change, the first hydraulic clutch  61 , and the second hydraulic clutch  62 . The ECU  86  controls the actuator  70 . 
     Specifically, as shown in  FIG. 4 , hydraulic cylinders  53   a ,  61   a ,  62   a  are driven by the actuator  70 . The actuator  70  includes an oil pump  71 , an oil passage  75 , an electromagnetic valve  72  for gear ratio change, an electromagnetic valve  73  for reverse shift connection, and an electromagnetic valve  74  for forward shift connection. 
     The oil pump  71  is connected to the hydraulic cylinders  53   a ,  61   a ,  62   a  with the oil passage  75 . The electromagnetic valve  72  for gear ratio change is arranged between the oil pump  71  and the hydraulic cylinder  53   a . Hydraulic pressure of the hydraulic cylinder  53   a  is adjusted by the electromagnetic valve  72  for gear ratio change. The electromagnetic valve  73  for reverse shift connection is arranged between the oil pump  71  and the hydraulic cylinder  61   a . Hydraulic pressure of the hydraulic cylinder  61   a  is adjusted by the electromagnetic valve  73  for reverse shift connection. The electromagnetic valve  74  for forward shift connection is arranged between the oil pump  71  and the hydraulic cylinder  62   a . Hydraulic pressure of the hydraulic cylinder  62   a  is adjusted by the electromagnetic valve  74  for forward shift connection. 
     Each of the electromagnetic valve  72  for gear ratio change, the electromagnetic valve  73  for reverse shift connection, and the electromagnetic valve  74  for forward shift connection is capable of gradually changing the cross-section area of the oil passage  75 . Accordingly, pressing forces of the cylinders  53   a ,  61   a ,  63   a  can be gradually changed by using the electromagnetic valve  72  for gear ratio change, the electromagnetic valve  73  for reverse shift connection, and the electromagnetic valve  74  for forward shift connection. This enables the hydraulic clutches  53 ,  61 ,  62  to gradually change their connecting forces. Therefore, as shown in  FIG. 7 , the ratio of the rotational speed of the third power transmission shaft  59  to that of the second power transmission shaft  51  can be adjusted. As a result, the ratio of the rotational speed of the third power transmission shaft  59  as an output shaft to the rotational speed of the second power transmission shaft  51  as an input shaft can be substantially adjusted in a continuous manner. 
     In this preferred embodiment, each of the electromagnetic valve  72  for gear ratio change, the electromagnetic valve  73  for reverse shift connection, and the electromagnetic valve  74  for forward shift connection is preferably configured by a PWM (Pulse Width Modulation) controlled solenoid. However, each of the electromagnetic valve  72  for gear ratio change, the electromagnetic valve  73  for reverse shift connection, and the electromagnetic valve  74  for forward shift connection may be configured by a valve other than a PWM controlled solenoid valve. For example, each of the electromagnetic valve  72  for gear ratio change, the electromagnetic valve  73  for reverse shift connection, and the electromagnetic valve  74  for forward shift connection may be configured by an on/off controlled solenoid valve. 
     Shift Operation of Shift Mechanism  34   
     Next, the description will be made of a shift operation of the shift mechanism  34  in details mainly with reference to  FIGS. 3 and 6 .  FIG. 6  is a table showing engaging states of the hydraulic clutches  53 ,  61 ,  62  and the shift position of the shift mechanism  34 . In the shift mechanism  34 , the shift position is changed by engaging or disengaging of the first to third hydraulic clutches  53 ,  61 ,  62 . 
     Shift Change Between Low Speed Gear Ratio and High Speed Gear Ratio 
     The shift change between the low speed gear ratio and the high speed gear ratio is made by the gear ratio change mechanism  35 . Specifically, the shift change between the low speed gear ratio and the high speed gear ratio is made by an operation of the hydraulic clutch  53  for gear ratio change. More specifically, when the hydraulic clutch  53  for gear ratio change is disengaged, the gear ratio of the gear ratio change mechanism  35  becomes “low speed gear ratio.” In contrast, when the hydraulic clutch  53  for gear ratio change is engaged, the gear ratio of the gear ratio change mechanism  35  becomes “high speed gear ratio.” 
     As shown in  FIG. 3 , the ring gear  55  is connected to the first power transmission shaft  50 . Accordingly, the ring gear  55  rotates in the forward direction as the first power transmission shaft  50  rotates. Here, when the hydraulic clutch  53  for gear ratio change is disengaged, the carrier  56  and the sun gear  54  mutually become rotatable. Accordingly, the planetary gears  57  revolve while rotating. As a result, the sun gear  54  attempts to rotate in the reverse direction. 
     However, as shown in  FIG. 6 , the one-way clutch  58  prevents rotation of the sun gear  54  in the reverse direction. Therefore, the sun gear  54  is fixed by the one-way clutch  58 . As a result, as the ring gear  55  rotates, the planetary gears  57  revolve between the sun gear  54  and the ring gear  55 , which causes the second power transmission shaft  51  to rotate together with the carrier  56 . In this case, since the planetary gears  57  rotate while revolving, the rotation of the first power transmission shaft  50  is decelerated and transmitted to the second power transmission shaft  51 . The gear ratio of the gear ratio change mechanism  35  is thus changed to the “low speed gear ratio.” 
     On the other hand, when the hydraulic clutch  53  for gear ratio change is engaged, the planetary gears  57  and the sun gear  54  rotate integrally with each other. Accordingly, rotation of the planetary gears  57  is prohibited. Thus, as the ring gear  55  rotates, the planetary gears  57 , the carrier  56 , and the sun gear  54  rotate in the forward direction at the same rotational speed as that of the ring gear  55 . Here, as shown in  FIG. 6 , the one-way clutch  58  permits rotation of the sun gear  54  in the forward direction. As a result, the first power transmission shaft  50  and the second power transmission shaft  51  rotate in the forward direction at a substantially same rotational speed. In other words, the turning force of the first power transmission shaft  50  is transmitted to the second power transmission shaft  51  at the same rotational speed and in the same rotational direction. The gear ratio of the gear ratio change mechanism  35  is thus changed to the “high speed gear ratio.” Changing between forward, reverse and neutral positions 
     A shift change is made between a forward or a reverse position and a neutral position in the shift position change mechanism  36 . Specifically, the first hydraulic clutch  61  and the second hydraulic clutch  62  are operated to change the shift position between a forward position, a reverse position and a neutral position. 
     When the first hydraulic clutch  61  is disengaged while the second hydraulic clutch  62  is engaged, the shift position of the shift position change mechanism  36  is made “forward.” When the first hydraulic clutch  61  is disengaged, the ring gear  64  is rotatable relative to the shift case  45 . When the second hydraulic clutch  62  is engaged, the carrier  66 , the sun gear  63 , and the third power transmission shaft  59  rotate integrally with each other. Therefore, when the first hydraulic clutch  61  is disengaged while the second hydraulic clutch  62  is engaged, the second power transmission shaft  51 , the carrier  66 , the sun gear  63 , and the third power transmission shaft  59  rotate integrally in the forward direction. The shift position of the shift position change mechanism  36  is thereby made “forward.” 
     When the first hydraulic clutch  61  is engaged while the second hydraulic clutch  62  is disengaged, the shift position of the shift position change mechanism  36  is made “reverse.” When the first hydraulic clutch  61  is engaged while the second hydraulic clutch  62  is disengaged, rotation of the ring gear  64  is restricted by the shift case  45 . On the other hand, the sun gear  63  is rotatable relative to the carrier  66 . Thus, as the second power transmission shaft  51  rotates in the forward direction, the planetary gears  65  revolve while rotating. As a result, the sun gear  63  and the third power transmission shaft  59  rotate in the reverse direction. The shift position of the shift position change mechanism  36  is thereby made “reverse.” 
     When both the first hydraulic clutch  61  and the second hydraulic clutch  62  are disengaged, the shift position of the shift position change mechanism  36  is made “neutral.” When the first hydraulic clutch  61  and the second hydraulic clutch  62  are both disengaged, the planetary gear mechanism  60  idles. Therefore, rotation of the second power transmission shaft  51  is not transmitted to the third power transmission shaft  59 . The shift position of the shift position change mechanism  36  is thereby made “neutral.” 
     Changing between the low speed gear ratio and the high speed gear ratio and the shift position change are performed as described above. Thus, as shown in  FIG. 6 , when the hydraulic clutch  53  for gear ratio change and the first hydraulic clutch  61  are disengaged while the second hydraulic clutch  62  is engaged, the shift position of the shift mechanism  34  is made “low speed forward.” 
     When the hydraulic clutch  53  for gear ratio change and the second hydraulic clutch  62  are engaged while the first hydraulic clutch  61  is disengaged, the shift position of the shift mechanism  34  is made “high speed forward.” 
     When the first hydraulic clutch  61  and the second hydraulic clutch  62  are both disengaged, the shift position of the shift mechanism  34  is made “neutral” regardless of the engaging state of the hydraulic clutch  53  for gear ratio change. 
     When the hydraulic clutch  53  for gear ratio change and the second hydraulic clutch  62  are disengaged while the first hydraulic clutch  61  is engaged, the shift position of the shift mechanism  34  is made “low speed reverse.” 
     Further, when the hydraulic clutch  53  for gear ratio change and the first hydraulic clutch  61  are engaged while the second hydraulic clutch  62  is disengaged, the shift position of the shift mechanism  34  is made “high speed reverse.” 
     Control Block of Boat  1   
     Now, description will be made of a control block of the boat  1  mainly with reference to  FIG. 5 . 
     First, description will be made of the control block of the outboard motor  20  with reference to  FIG. 5 . The outboard motor  20  is provided with the ECU  86 . The ECU  86  constitutes a portion of the control device  91  shown in  FIG. 2 . The ECU  86  controls each of mechanisms of the outboard motor  20 . 
     The ECU  86  includes a central processing unit (CPU)  86   a  as a computation section and a memory  86   b . The memory  86   b  stores various settings such as maps to be discussed later. The memory  86   b  is connected to the CPU  86   a . When the CPU  86   a  performs various calculations, it reads out necessary information stored in the memory  86   b . As needed, the CPU  86   a  outputs computation results to the memory  86   b  and causes the memory  86   b  to store the computation results. 
     The throttle body  87  of the engine  30  is connected to the ECU  86 . The throttle body  87  is controlled by the ECU  86 . The throttle opening of the engine  30  is thus controlled. Specifically, the throttle opening of the engine  30  is controlled based on an operating amount of a control lever  83  and a sensitivity switching signal. As a result, the output of the engine  30  is controlled. 
     An engine speed sensor  88  is also connected to the ECU  86 . The engine speed sensor  88  detects the rotational speed of the crankshaft  31  of the engine  30  shown in  FIG. 1 . The engine speed sensor  88  outputs the detected engine speed to the ECU  86 . 
     The propulsion section  33  is provided with a propeller speed sensor  90 . The propeller speed sensor  90  detects the rotational speed of the propeller  41 . The propeller speed sensor  90  outputs the detected rotational speed to the ECU  86 . The rotational speed of the propeller  41  is substantially the same as that of the propeller shaft  40 . Thus, the propeller speed sensor  90  may detect the rotational speed of the propeller shaft  40 . 
     The electromagnetic valve  72  for gear ratio change, the electromagnetic valve  74  for forward shift connection, and the electromagnetic valve  73  for reverse shift connection are connected to the ECU  86 . The ECU  86  controls opening/closing and the opening degrees of the electromagnetic valve  72  for gear ratio change, the electromagnetic valve  74  for forward shift connection, and the electromagnetic valve  73  for reverse shift connection. 
     As shown in  FIG. 5 , the boat  1  includes a local area network (LAN)  80 . The LAN  80  is extended over the hull  10 . In the boat  1 , signals are transmitted between devices via the LAN  80 . 
     The ECU  86  of the outboard motor  20 , a controller  82 , and a display device  81  are connected to the LAN  80 . The display device  81  displays information output from the ECU  86  and information output from the controller  82  to be discussed later. Specifically, the display device  81  displays a current speed, shift position, etc., of the boat  1 . 
     The controller  82  includes the control lever  83 , an accelerator opening sensor  84 , a shift position sensor  85 , and a mode selecting switch  92 . 
     A shift position and an accelerator opening are input to the control lever  83  by operations of a boat operator of the boat  1 . Specifically, when the boat operator operates the control lever  83 , the accelerator opening sensor  84  and the shift position sensor  85  detect the accelerator opening and the shift position, respectively, corresponding to the position of the control lever  83 . Each of the accelerator opening sensor  84  and the shift position sensor  85  is connected to the LAN  80 . The accelerator opening sensor  84  and the shift position sensor  85  transmit an accelerator opening signal and a shift position signal, respectively, to the LAN  80 . The ECU  86  receives, via the LAN  80 , the accelerator opening signal and the shift position signal output from the accelerator opening sensor  84  and the shift position sensor  85 . 
     Specifically, when a control portion  83   a  of the control lever  83  is positioned in the neutral area indicated by “N” in  FIG. 8 , the shift position sensor  85  outputs a shift position signal corresponding to the neutral position. When the control portion  83   a  of the control lever  83  is positioned in the forward area indicated by “F” in  FIG. 8 , the shift position sensor  85  outputs a shift position signal corresponding to the forward position. When the control portion  83   a  of the control lever  83  is positioned in the reverse area indicated by “R” in  FIG. 8 , the shift position sensor  85  outputs a shift position signal corresponding to the reverse position. 
     The accelerator opening sensor  84  detects an operating amount of the control portion  83   a . Specifically, the accelerator opening sensor  84  detects an operating angle θ that denotes how much the control portion  83   a  is operated from the middle position. The control portion  83   a  outputs the operating angle θ as an accelerator opening signal. 
     Either a first mode or a second mode is input to the mode selecting switch  92  shown in  FIG. 5  by an operation of the boat operator. Here, the “first mode” is a mode in which the degree of the accelerator opening is relatively large with respect to the operating angle θ of the control lever  83  as shown as M 1  in  FIG. 9 . In contrast, the “second mode” is a mode in which the degree of the accelerator opening is relatively small with respect to the operating angle θ of the control lever  83  as indicated by M 2  in  FIG. 9 . That is, in the first mode and the second mode, the degree of the accelerator opening with respect to the operating angle θ of the control lever  83  is different. 
     The mode selecting switch  92  outputs to the ECU  86  a signal corresponding to an input mode of either one of the first mode or the second mode. In this preferred embodiment, this “signal corresponding to an input mode” is the sensitivity switching signal. 
     When the boat operator operates the mode selecting switch  92  to select the first mode, the CPU  86   a  refers to the map M 1  shown in  FIG. 9  that is stored in the memory  86   b  to determine the accelerator opening based on the input accelerator opening signal. In contrast, when the boat operator operates the mode selecting switch  92  to select the second mode, the CPU  86   a  refers to the map M 2  shown in  FIG. 9  that is stored in the memory  86   b  to determine the accelerator opening based on the input accelerator opening signal. 
     Control of Boat  1   
     Now, description will be made of the control of the boat  1 . 
     Basic Control of Boat  1   
     When the control lever  83  is operated by the boat operator of the boat  1 , the accelerator opening sensor  84  and the shift position sensor  85  detect the accelerator opening and the shift position corresponding to the operating state of the control lever  83 . The detected accelerator opening and shift position are transmitted to the LAN  80 . The ECU  86  receives an accelerator opening signal and a shift position signal output via the LAN  80 . The ECU  86  controls the throttle body  87  and hydraulic clutches  61 ,  62  based on the accelerator opening obtained from the accelerator opening signal and the map shown in  FIG. 9 . The ECU  86  thus performs control of the rotational speed of the propeller. 
     The ECU  86  also controls the shift mechanism  34  according to the shift position signal. Specifically, in the case where a “low speed forward” shift position signal is received, the ECU  86  drives the electromagnetic valve  72  for gear ratio change to disengage the hydraulic clutch  53  for gear ratio change, and drives the electromagnetic valves  73 ,  74  for shift connection to disengage the first hydraulic clutch  61  and engage the second hydraulic clutch  62 . The shift position is thus changed to the “low speed forward” position. 
     Specific Control of Boat  1   
     (1) Control of Rotational Speed of Propeller in First Mode and Second Mode 
     When the outboard motor  20  is operated, the control shown in  FIG. 10  is repeated. As shown in  FIG. 10 , when the outboard motor  20  is operated, the mode is determined in step S 1 . If the mode is determined to be the first mode in step S 1 , the procedure proceeds to step S 2 . In step S 2 , the engine output is adjusted based on the accelerator opening without adjusting the connecting forces of the hydraulic clutches  61 ,  62  for shift change. The hydraulic clutches  61 ,  62  are adapted to be engaged or disengaged corresponding to the selected shift position. More specifically, the connecting forces of the hydraulic clutches  61 ,  62  preferably are substantially 0% or substantially 100%. 
     Accordingly, when either one of the hydraulic clutches  61 ,  62  is engaged, the rotational speed of the second power transmission shaft  51  as an input shaft is controlled to be substantially the same as dimensions of the rotational speed of the third power transmission shaft  59  as an output shaft. More specifically, the rotational speed of the second power transmission shaft  51  as an input shaft is controlled to be substantially the same as the rotational speed of the third power transmission shaft  59  as an output shaft. It should be noted that “substantially same rotational speed” means that the absolute value of the rotational speed is the same. In this regard, the rotational direction may be either same or reverse. 
     However, as described above, the reduction ratio of the planetary gear mechanism  60  may be other than 1:1. When the reduction ratio of the planetary gear mechanism  60  is not 1:1, the rotational speed of the second power transmission shaft  51  as an input shaft is not perfectly the same as the rotational speed of the third power transmission shaft  59  as an output shaft. In this preferred embodiment, “substantially same rotational speed” includes the case that has the difference of rotational speed of about 10%, for example. 
     On the other hand, if the mode is determined to be the second mode in step S 1 , the procedure proceeds to step S 3 . In step S 3 , the engine speed and the connecting forces of the hydraulic clutches  61 ,  62  are adjusted in response to the accelerator opening. Specific control of the rotational speed of the propeller in the second mode performed in step S 3  will be described hereinafter with reference mainly to  FIG. 11 . 
     As shown in  FIG. 11 , in the second mode, at first, a target rotational speed of the propeller, a target throttle opening, and target connecting forces of the hydraulic clutches  61 ,  62  are calculated in step S 31 . 
     Specifically, the CPU  86   a  reads out a map shown in  FIG. 12  stored in the memory  86   b . The map shown in  FIG. 12  specifies the relationship between the rotational speed of the propeller and the accelerator opening. The CPU  86   a  applies the accelerator opening calculated from the accelerator opening signal to the map shown in  FIG. 12  to calculate the target rotational speed of the propeller  41 . 
     The CPU  86   a  reads out a map shown in  FIG. 13  stored in the memory  86   b . The map shown in  FIG. 13  specifies the relationship between the accelerator opening, the throttle opening, and the target connecting forces of the hydraulic clutches  61 ,  62 . Specifically, a graph indicated with a solid line in  FIG. 13  specifies the throttle opening. A graph indicated with a broken line in  FIG. 13  specifies the connecting forces of the hydraulic clutches  61 ,  62 . The CPU  86   a  applies the calculated accelerator opening to the map shown in  FIG. 13  to calculate the target throttle opening and the target connecting forces of the hydraulic clutches  61 ,  62 . 
     Here, as shown in  FIG. 13 , when the accelerator opening is equal to or smaller than a predetermined accelerator opening A 1 , the target throttle opening becomes T 1  regardless of the accelerator opening. T 1  is set slightly larger than a throttle opening Ta at an idling state of the engine  30 . Therefore, when the accelerator opening is equal to or smaller than the predetermined accelerator opening A 1 , the engine speed is maintained generally constant. 
     In contrast, when the accelerator opening is larger than the predetermined accelerator opening A 1 , the target throttle opening increases as the accelerator opening increases. Thus, the engine speed is adjusted in response to the accelerator opening when the accelerator opening is larger than the predetermined accelerator opening A 1 . 
     Further, when the accelerator opening is equal to or smaller than the predetermined accelerator opening A 1 , the target connecting forces of the hydraulic clutches  61 ,  62  are set to increase as the accelerator opening increases. Also, when the accelerator opening is larger than the predetermined accelerator opening A 1  and smaller than A 2 , the target connecting forces of the hydraulic clutches  61 ,  62  are set to increase as the accelerator opening increases. However, the rate of the target connecting forces of the hydraulic clutches  61 ,  62  relative to the accelerator opening at a time when the accelerator opening is larger than the predetermined accelerator opening A 1  and smaller than A 2  is set smaller than the rate of the target connecting forces of the hydraulic clutches  61 ,  62  relative to the accelerator opening at a time when the accelerator opening is equal to or smaller than the predetermined accelerator opening A 1 . When the accelerator opening is equal to or larger than the predetermined accelerator opening A 2 , the connecting forces of the hydraulic clutches  61 ,  62  become constant regardless of the accelerator opening. 
     Accordingly, when both of the throttle opening and the connecting forces of the hydraulic clutches  61 ,  62  are controlled according to the target, the relationship between the rotational speeds of the second power transmission shaft  51  and the third power transmission shaft  59  is as shown in  FIG. 14 . 
     In  FIGS. 14 and 15 , a line denoted by a numeral “ 51 ” shows the rotational speed of the second power transmission shaft  51 . A line denoted by a numeral “ 59 ” shows the rotational speed of the third power transmission shaft  59 . 
     For convenience of description, graphs shown in  FIGS. 14 and 15  are a schematic graph assuming that loading conditions of the propeller  41  are constant. Since the loading conditions of the propeller  41  always vary, the actual relationship is not necessarily as shown in  FIGS. 14 and 15 . Additionally for convenience, the following description will also be made assuming that there is no load on the propeller  41 . 
     Specifically, as shown in  FIG. 14 , when the accelerator opening is equal to or smaller than the predetermined accelerator opening A 1 , the rotational speed of the second power transmission shaft  51  is a predetermined rotational speed r 2  and is generally constant. When the accelerator opening is larger than the predetermined accelerator opening A 1 , the rotational speed of the second power transmission shaft  51  increases as the accelerator opening increases. 
     On the other hand, when the accelerator opening is zero, the third power transmission shaft  59  does not substantially rotate. The rotational speed of the third power transmission shaft  59  increases as the accelerator opening increases from zero. When the accelerator opening is equal to the predetermined accelerator opening A 1 , the rotational speed of the second power transmission shaft  51  is approximately equal to the rotational speed of the third power transmission shaft  59 . When the accelerator opening is equal to the predetermined accelerator opening A 2 , the rotational speed of the second power transmission shaft  51  is substantially equal to the rotational speed of the third power transmission shaft  59 . 
     That is, when the accelerator opening is equal to the predetermined accelerator opening A 2 , the hydraulic clutches  61 ,  62  are substantially fully engaged. The hydraulic clutches  61 ,  62  are controlled in so-called half-clutch until the accelerator opening reaches the predetermined accelerator opening A 2 . The rotational speed of the third power transmission shaft  59  is thereby adjusted to be smaller than the rotational speed of the second power transmission shaft  51 . 
     In this preferred embodiment, step S 32  is performed following step S 31  as shown in  FIG. 11 . In step S 32 , the throttle opening is adjusted to the calculated target throttle opening by the CPU  86   a.    
     Next, in step S 33 , the connecting forces of the hydraulic clutches  61 ,  62  are adjusted by the CPU  86   a  in response to the actual rotational speed of the propeller detected by the propeller speed sensor  90 . Specific adjustment control of the connecting forces of the hydraulic clutches  61 ,  62  performed in step S 33  will be described hereinafter with reference mainly to  FIG. 18 . 
     As described above, in step S 31 , the CPU  86   a  calculates the target rotational speed of the propeller using the map in  FIG. 12  showing the relationship between the accelerator opening and the rotational speed of the propeller. Next, as shown in  FIG. 18 , the CPU  86   a  calculates a deviation of the actual rotational speed of the propeller from the target rotational speed of the propeller. An adjusting amount to the target connecting forces of the hydraulic clutches  61 ,  62  is calculated based on the above deviation multiplied by the control gain. Specifically, the CPU  86   a  applies a value (deviation X gain(G)) to a map shown in  FIG. 19  showing the relationship between the adjusting amount of the connecting forces of the hydraulic clutches  61 ,  62  and the value (deviation X gain(G)) to calculate the adjusting amount of the connecting forces of the hydraulic clutches  61 ,  62 . The CPU  86   a  obtains the connecting forces of the hydraulic clutches  61 ,  62  by adding the calculated adjusting amount of the connecting forces of the hydraulic clutches  61 ,  62  to the calculated target connecting forces of the hydraulic clutches  61 ,  62 . Thus, the CPU  86   a  adjusts the electromagnetic valves  73 ,  74  for shift connection based on the calculated connecting forces of the hydraulic clutches  61 ,  62  for shift change. 
     When the calculated connecting forces of the hydraulic clutches  61 ,  62  are in the range between 0 to 100%, the CPU  86   a  adjusts the electromagnetic valves  73 ,  74  so that the actual connecting forces of the hydraulic clutches  61 ,  62  are equal to the calculated connecting forces. When the calculated connecting forces of the hydraulic clutches  61 ,  62  are less than 0%, the CPU  86   a  adjusts the electromagnetic valves  73 ,  74  so that the connecting force of the opposite side clutch increases. Further, when the calculated connecting forces of the hydraulic clutches  61 ,  62  exceed 100%, the CPU  86   a  adjusts the electromagnetic valves  73 ,  74  so that either one of the connecting forces of the hydraulic clutches  61 ,  62  is equal to 100%. 
     In this case, the control gain is selected among the proportional gain, the integral gain, and the derivative gain in consideration of hydraulic responsiveness and mechanical inertia. Combination of two or more of the proportional gain, the integral gain, and the derivative gain may be used as the control gain. 
     Specific description will hereinafter be made referring to an example time chart shown in  FIG. 20 . 
     In the example shown in  FIG. 20 , the shift position of the shift position change mechanism  36  is made neutral at time t 1 . Next, the second mode is started at time t 2 . Accordingly, engaging states of the hydraulic clutches  61 ,  62  and the engine speed are controlled in response to the accelerator opening after time t 2  by step S 3 . 
     During a period between time t 2  and time t 3 , the target rotational speed of the propeller approaches zero. During a period between time t 2  and time t 3 , a deviation of the actual rotational speed of the propeller from the target rotational speed of the propeller is large. Accordingly, a control amount of the first hydraulic clutch  61  calculated by a computation shown in  FIG. 18  becomes less than 0%. Therefore, the connecting force of the second hydraulic clutch  62  is increased despite the fact that the target rotational speed of the propeller is in the forward side. As a result, the rotational speed of the propeller decreases so that the actual rotational speed of the propeller approaches the target rotational speed of the propeller. 
     During a period between time t 3  and time t 4 , a deviation of the actual rotational speed of the propeller from the target rotational speed of the propeller is small. Accordingly, a control amount of the first hydraulic clutch  61  calculated by a computation shown in  FIG. 18  is in the range between 0 to 100%. Therefore, the connecting force of the second hydraulic clutch  62  is increased according to the calculated control amount. 
     After time t 4 , the feedback control shown in  FIG. 18  becomes balanced. The connecting force of the first hydraulic clutch  61  is maintained slightly lower than the target connecting force after time t 4 . 
     As described above, in this preferred embodiment, a degree of the accelerator opening relative to the operating amount of the control lever  83  can be switched by switching the mode. Therefore, for example, advantages in adjusting the accelerator opening or the rotational speed of the propeller are further enhanced. Specifically, for example, if the mode is switched to a mode in which degree of the accelerator opening relative to the operating amount of the control lever  83  is relatively small, fine adjustment of the accelerator opening can be facilitated. This makes it easy to perform fine adjustment of the thrust, the propulsion speed, and the rotational speed of the propeller. For example, it becomes easy to finely adjust the thrust and the propulsion speed of the boat  1  during an operation of leaving from or approaching to a dock or quay, or while trolling. Also, if the mode is switched to a mode in which degree of the accelerator opening relative to the operating amount of the control lever  83  is relatively large, it is possible to adjust the thrust and the propulsion speed of the boat  1  promptly. 
     Especially, in this preferred embodiment, as described bellow, in the second mode in which the ratio of the rotational speed of the third power transmission shaft  59  to the rotational speed of the second power transmission shaft  51  can be finely adjusted, a degree of the accelerator opening relative to the operating amount of the control lever  83  is preferably small. Therefore, it is further easier to finely adjust the thrust and the propulsion speed of the boat  1 . 
     Also, in the first mode in which the hydraulic clutches  61 ,  62  are maintained to be either engaged or disengaged, a degree of the accelerator opening relative to the operating amount of the control lever  83  is preferably large. Therefore, it is easy to control the thrust and the propulsion speed of the boat  1  promptly. 
     In this preferred embodiment, engaging states of the hydraulic clutches  61 ,  62  are controlled in the second mode. The ratio of the rotational speed of the third power transmission shaft  59  to the rotational speed of the second power transmission shaft  51  can thereby be finely adjusted. This allows to control the rotational speed of the third power transmission shaft  59  more precisely. Accordingly, it is easy to finely adjust the thrust and the propulsion speed. Especially, it is easy to finely adjust the thrust and the propulsion speed sailing at a low speed range or at a very low speed range during an operation of leaving from or approaching to a dock or quay, or during trolling. 
     Here, “low speed range” is, for example, a speed range about 10 km/h to about 20 km/h. “Very low speed range” is, for example, a speed range about 0 to about 10 km/h. However, these ranges are merely non-limiting examples. Definitions of the low speed range and the very low speed range are different depending on the types of boat in which a boat propulsion system is mounted. 
     In this preferred embodiment, as shown in  FIG. 14 , the engaging states of the hydraulic clutches  61 ,  62  can be controlled in a manner that the rotational speed of the third power transmission shaft  59  substantially varies continuously from zero to the rotational speed of the second power transmission shaft  51 . Therefore, it is further easier to finely adjust the thrust and the propulsion speed. 
     For example, when the hydraulic clutches  61 ,  62  are controlled to be either disengaged or engaged corresponding to the shift position, and when the shift position is in a forward or a reverse position, the rotational speed of the second power transmission shaft  51  as an input shaft and the rotational speed of the third power transmission shaft  59  as an output shaft are controlled to be substantially the same as shown in  FIG. 15 . As shown in  FIG. 15 , this makes it difficult to adjust the rotational speed of the third power transmission shaft  59  to be lower than the rotational speed r 2  of the second power transmission shaft  51  at idling of the engine  30 . Therefore, it is difficult to adjust the rotational speed of the propeller to be lower than the predetermined rotational speed. As a result, it is difficult to generate little thrust. 
     In contrast, in this preferred embodiment, the hydraulic clutches  61 ,  62  are controlled by the ECU  86  to adjust the rotational speed of the third power transmission shaft  59  to be smaller than the rotational speed of the second power transmission shaft  51  in the second mode. Accordingly, as shown in  FIG. 14 , it is possible to adjust the rotational speed of the third power transmission shaft  59  to be lower than the rotational speed r 2  of the second power transmission shaft  51  at idling of the engine  30 . Therefore, it is possible to adjust the rotational speed of the propeller to be lower than the predetermined rotational speed. As a result, it is possible to generate further little thrust. This makes it easy to propel the boat  1  at low speed. 
     In this preferred embodiment, as described above, the engaging states of the hydraulic clutches  61 ,  62  can be controlled such that the rotational speed of the third power transmission shaft  59  substantially varies continuously from zero to the rotational speed of the second power transmission shaft  51 . This makes it possible to generate very little thrust. Accordingly, it is also possible to propel the boat  1  at very low speed. 
     However, a method for controlling the engaging states of the hydraulic clutches  61 ,  62  is not specifically limited. For example, as with this preferred embodiment, the engaging states of the hydraulic clutches  61 ,  62  may be controlled by adjusting the connecting forces of the hydraulic clutches  61 ,  62 . Also, the engaging states of the hydraulic clutches  61 ,  62  may be controlled by adjusting the connecting time of the hydraulic clutches  61 ,  62 . Specifically, the engaging states of the hydraulic clutches  61 ,  62  may be controlled by changing ratios between the time of connecting and the time of disconnecting of the hydraulic clutches  61 ,  62 . In other words, the engaging states of the hydraulic clutches  61 ,  62  may be controlled by adjusting the connecting time of the hydraulic clutches  61 ,  62  for each certain period. 
     When the connecting forces of the hydraulic clutches  61 ,  62  are adjusted, it is preferable to use a multi-plate type clutch for the hydraulic clutches  61 ,  62 , as described in the present preferred embodiment. When a hydraulic clutch is used for clutches  61 ,  62 , it is more preferable to use valves  72  to  74  that can gradually change hydraulic pressure. With the above configuration, it is easy to adjust the connecting forces of the hydraulic clutches  61 ,  62 . 
     On the other hand, when the connecting time of the hydraulic clutches  61 ,  62  is adjusted, either a dog clutch or a multi-plate type clutch may be used as the hydraulic clutches  61 ,  62 . 
     Second Preferred Embodiment 
     In the above first preferred embodiment, description was made of an example in which the mode selecting switch  92  as a sensitivity switching section outputs an operating amount of the control lever  83  and the control device  91  controls the throttle opening of the engine  30  based on the output operating amount of the control lever  83  and a mode output as a sensitivity switching signal. However, the present invention is not limited to this structure. 
     For example, sensitivity switching based on the mode may be made by the mode selecting switch  92 . Specifically, the mode selecting switch  92  may be configured to output the accelerator opening based on the operating amount of the control lever  83  and the selected mode. More specifically, the mode selecting switch  92  may output an accelerator opening calculated by applying the operating amount of the control lever  83  to a map shown in  FIG. 9 . 
     In this case, as with the above first preferred embodiment, advantages in adjusting the accelerator opening are further enhanced. Specifically, for example, fine adjustment of the accelerator opening and the thrust and the propulsion speed of the boat  1  as well as prompt adjustment of the accelerator opening and the thrust and the propulsion speed of the boat  1  can be achieved. 
     Other Modifications 
     In the above preferred embodiments, description was made of an example in which control of the hydraulic clutches  61 ,  62  for shift change as well as degree of the accelerator opening relative to the operating amount of the control lever are changed by switching the mode between the first and the second mode. However, the control of the hydraulic clutches  61 ,  62  and the degree of the accelerator opening relative to the operating amount of the control lever may be independently changed. Specifically, a switch to change the control of the hydraulic clutches  61 ,  62  may be provided separate from a switch to change the degree of the accelerator opening relative to the operating amount of the control lever. Further, only the switch to change the degree of the accelerator opening relative to the operating amount of the control lever may be provided. 
     In the above preferred embodiments, an example provided with the mode selecting switch  92  for switching between the first mode and the second mode was described. However, the mode selecting switch  92  is not essential for the present invention. 
     For example, the mode may be controlled by the ECU  86  to be the second mode automatically when the accelerator opening is equal to or smaller than a predetermined value and to be the first mode automatically when the accelerator opening is larger than the predetermined value. 
     In the above preferred embodiments, an example in which two modes having different degrees of the accelerator opening relative to the operating angle θ of the control lever  83  are selectable was described. However, the number of the mode is not limited to two. For example, three or more modes having different degrees of the accelerator opening relative to the operating angle θ of the control lever  83  may be selectable. 
     Specifically, for example, three modes that are a very low speed mode, a low speed mode, and a normal mode may be selectable. The very low speed mode is used in sailing at very low speed during leaving from or approaching to the dock or quay. In the very low speed mode, the degree of the accelerator opening relative to the operating angle θ of the control lever  83  is preferably smallest. The low speed mode is used in sailing at low speed during trolling. In the low speed mode, the degree of the accelerator opening relative to the operating angle θ of the control lever  83  is preferably relatively small. In the normal mode, the degree of the accelerator opening relative to the operating angle θ of the control lever  83  is preferably larger compared to the very low speed mode and the low speed mode. 
     In the above first preferred embodiment, a case where both the engaging states of the hydraulic clutches  61 ,  62  for shift change and the engine speed are preferably controlled in the second mode was described. However, only the engaging states of the hydraulic clutches  61 ,  62  may be controlled without controlling the engine speed in the second mode. In this preferred embodiment, a case where the engaging states of the hydraulic clutches  61 ,  62  are controlled without controlling the engine speed in the second mode will hereinafter be described. 
     In the following descriptions, components having substantially the same functions as those in the above first preferred embodiment are designated by the same reference numerals, and their detailed description is omitted. In this preferred embodiment,  FIGS. 1 to 9  will also be referred in common with the above first preferred embodiment. 
     In this preferred embodiment, as shown in  FIG. 16 , if the mode is determined to be the second mode in step S 1 , the procedure proceeds to step S 4 . In step S 4 , the engaging states of the hydraulic clutches  61 ,  62  for shift change are controlled in response to the accelerator opening. Thus, as shown in  FIG. 17 , step S 32  shown in  FIG. 11  is not performed, but step S 33  is performed following step S 31 . 
     In this case, it is also possible to finely adjust the thrust of the boat  1  and to generate very little thrust. 
     In the above preferred embodiments, an example in which the shift position change mechanism  36  preferably includes one planetary gear mechanism  60  and two clutches  61 ,  62  was described. In the present invention, however, the shift position change mechanism is not limited to this configuration. For example, the shift position change mechanism may include a forward/reverse change mechanism arranged in a coupling mechanism portion and a clutch for engaging or disengaging between the forward/reverse change mechanism and the engine  30 . 
     In the above preferred embodiments, the memory  86   b  in the ECU  86  mounted on the outboard motor  20  preferably stores a map for controlling the gear ratio change mechanism  35  and a map for controlling the shift position change mechanism  36 . In addition, the CPU  86   a  in the ECU  86  mounted on the outboard motor  20  preferably outputs control signals for controlling the electromagnetic valves  72 ,  73 ,  74 . 
     However, the present invention is not limited to this configuration. For example, the controller  82  mounted on the hull  10  may be provided with a memory as a storage section and a CPU as a computation section, in addition to or in place of the memory  86   b  and the CPU  86   a . In this case, the memory provided in the controller  82  may store a map for controlling the gear ratio change mechanism  35  and a map for controlling the shift position change mechanism  36 . In addition, the CPU provided in the controller  82  may output control signals for controlling the electromagnetic valves  72 ,  73 ,  74 . 
     In the above preferred embodiments, an example in which the ECU  86  preferably controls both the engine  30  and the electromagnetic valves  72 ,  73 ,  74  was described. However, the present invention is not limited hereto. For example, there may be separately provided an ECU for controlling the engine and an ECU for controlling the electromagnetic valves. 
     In the above preferred embodiments, the controller  82  is a so-called “electronic controller.” Here, the term “electronic controller” refers to a controller that converts an operating amount of the control lever  83  into an electric signal and outputs the electric signal to the LAN  80 . 
     In the present invention, however, the controller  82  may not necessarily be an electronic controller. For example, the controller  82  may be a so-called mechanical controller. Here, the term “mechanical controller” refers to a controller that includes a control lever and a wire connected to the control lever and that transmits the operating amount and direction of the control lever to the outboard motor as physical quantity of the operating amount and direction of the wire. 
     In the above preferred embodiments, an example in which the shift mechanism  34  has the gear ratio change mechanism  35  was described. However, the shift mechanism  34  may not have the gear ratio change mechanism  35 . For example, the shift mechanism  34  may only have the shift position change mechanism  36 . 
     In this specification, the connecting force of a clutch is a value representing an engaging state of the clutch. That is, “the connecting force of the hydraulic clutch  53  for gear ratio change is 100%,” for example, means that the hydraulic piston  53   a  is driven to bring the plate group  53   b  into completely pressurized contact and that the hydraulic clutch  53  for gear ratio change is completely engaged. On the other hand, “the connecting force of the hydraulic clutch  53  for gear ratio change is 0%,” for example, means that the hydraulic piston  53   a  is not driven to bring the plate group  53   b  into nonpressurized contact with each plate being separated and that the hydraulic clutch  53  for gear ratio change is completely disengaged. Further, “the connecting force of the hydraulic clutch  53  for gear ratio change is 80%,” for example, means that the hydraulic clutch  53  for gear ratio change is driven to bring the plate group  53   b  into pressurized contact to establish a so-called half-clutch state in which the drive torque transmitted from the first power transmission shaft  50  as an input shaft to the second power transmission shaft  51  as an output shaft, or the rotational speed of the second power transmission shaft  51 , is about 80% of the value when the gear hydraulic clutch  53  for ratio change is completely engaged. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.