Patent Publication Number: US-7909670-B2

Title: Marine propulsion system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a marine propulsion system. More specifically, the present invention relates to a marine propulsion system including an engine. 
     2. Description of the Related Art 
     Conventionally, marine propulsion units (marine propulsion systems) including an engine are known in the art (for example, see JP-A-Hei 9-263294). JP-A-Hei 9-263294 discloses a marine propulsion unit including an engine and a power transmission mechanism transmitting a driving force of the engine to a propeller in a certain fixed reduction ratio. The marine propulsion unit is constructed such that the driving force of the engine is directly transmitted to the propeller via the power transmission mechanism and the rotational speed of the propeller increases proportionally with an increase in the engine speed. 
     However, the marine propulsion unit disclosed in JP-A-Hei 9-263294 has a problem in which it is difficult to improve acceleration performance in a low speed position when the speed reduction ratio of the power transmission mechanism is set to achieve a larger maximum speed. Conversely, there is a problem that it is difficult to achieve a larger maximum speed when the reduction ratio of the power transmission mechanism is set to improve the acceleration performance in the low speed position. In other words, the marine propulsion unit disclosed in JP-A-Hei 9-263294 has a problem in which it is difficult for the user to achieve both an acceleration performance and a maximum speed approaching the levels that he/she desires. 
     SUMMARY OF THE INVENTION 
     In order to overcome the problems described above, preferred embodiments of the present invention provide a marine propulsion system in which both an acceleration performance and a maximum speed can approach levels that a user desires. 
     To achieve this, a marine propulsion system in accordance with a preferred embodiment of the present invention includes an engine; a propeller arranged to be rotated by the engine; a transmission mechanism arranged to operate in at least a low speed reduction ratio and a high speed reduction ratio, and arranged to transmit a driving force of the engine to the propeller with a speed thereof shifted to one of the low speed reduction ratio and the high speed reduction ratio during a forward travel and a reverse travel; and an operation portion operable by a user and arranged to shift the speed reduction ratio of the transmission mechanism to the low speed reduction ratio in at least the reverse travel. 
     As described above, the marine propulsion system in accordance with the above preferred embodiment includes a transmission mechanism arranged to transmit the driving force generated by the engine to the propeller with the speed shifted to one of the low speed reduction ratio and the high speed reduction ratio. The transmission mechanism is arranged such that the driving force generated by the engine can be transmitted to the propeller with the speed shifted to the low speed reduction ratio. Accordingly, an acceleration performance in the low speed position can be improved. Further, the transmission mechanism is constructed such that the driving force generated by the engine can be transmitted to the propeller with the speed shifted to the high speed reduction ratio. This allows a larger maximum speed to be obtained. As a result, both the acceleration performance and the maximum speed can approach levels that the user desires. The user operates the operation portion and thereby can arbitrarily shift the speed reduction ratio of the transmission mechanism to the low speed reduction ratio in the reverse travel. 
     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 perspective view showing a boat in which a marine propulsion system in accordance with a preferred embodiment of the present invention is installed. 
         FIG. 2  is a block diagram showing an arrangement of the marine propulsion system in accordance with a preferred embodiment of the present invention. 
         FIG. 3  is a side view illustrating an arrangement of a control lever section of the boat shown in  FIG. 1 . 
         FIG. 4  is a front view showing a lever of the control lever section shown in  FIG. 3 . 
         FIG. 5  is a side view showing a mode switching lever of the boat shown in  FIG. 1 . 
         FIG. 6  is a cross-sectional view illustrating an arrangement of a marine propulsion system main body of the marine propulsion system shown in  FIG. 1 . 
         FIG. 7  is a cross-sectional view illustrating an arrangement of a transmission mechanism of the marine propulsion system main body of the marine propulsion system shown in  FIG. 1 . 
         FIG. 8  is a cross-sectional view taken along line  100 - 100  of  FIG. 7 . 
         FIG. 9  is a cross-sectional view taken along line  200 - 200  of  FIG. 7 . 
         FIG. 10  is a diagram illustrating a shift inhibition control map for a manual mode of the marine propulsion system shown in  FIG. 1 . 
         FIG. 11  is a diagram illustrating a shift redirection control map for the manual mode of the marine propulsion system shown in  FIG. 1 . 
         FIG. 12  is a diagram illustrating a shift control map for an automatic mode of the marine propulsion system shown in  FIG. 1 . 
         FIG. 13  is a flowchart for demonstrating mode switch steps of the marine propulsion system shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described hereinafter with reference to the drawings. 
       FIG. 1  is a perspective view showing a boat in which a marine propulsion system in accordance with a preferred embodiment of the present invention is installed.  FIG. 2  is a block diagram showing a construction of the marine propulsion system in accordance with a preferred embodiment of the present invention.  FIGS. 3 through 9  are drawings specifically illustrating the construction of the marine propulsion system in accordance with the preferred embodiment shown in  FIG. 1 . In the figures, arrow FWD indicates the forward travel direction of the boat, and arrow BWD indicates the reverse travel direction of the boat. First, the construction of a boat  1  and the marine propulsion system installed in the boat  1  in accordance with a preferred embodiment of the present invention will be described with reference to  FIGS. 1 through 9 . 
     As shown in  FIG. 1 , the boat  1  in accordance with the present preferred embodiment has a hull  2  floating on the water surface, two outboard motors  3  mounted on rear portions of the hull  2  to propel the hull  2 , a steering section  4  arranged to steer the hull  2 , a control lever section  5  disposed in a vicinity of the steering section  4  and including a lever  5   a  arranged to turn in the fore-and-aft direction, and a display section  6  disposed in a vicinity of the control lever section  5 . As shown in  FIG. 2 , the outboard motors  3 , the control lever section  5 , and the display section  6  are connected together by each of common LAN cables  7  and  8 . In the present preferred embodiment, the boat  1  has a mode switching lever  9  arranged to switch between a manual mode that a user can select a speed reduction ratio and an automatic mode in which the reduction ratio is automatically selected based on operation on the lever  5   a . The mode switching lever  9  is arranged to turn to an MT position corresponding to the manual mode and an AT position corresponding to the automatic mode. The user turns the mode switching lever  9  to the MT position or the AT position and thereby can select the desired mode. The boat propulsion system is preferably provided with the outboard motors  3 , the steering section  4 , the control lever section  5 , the display section  6 , the common LAN cables  7  and  8 , and the mode switching lever  9 . 
     As shown in  FIG. 1 , the two outboard motors  3  are preferably symmetrically disposed with respect to the center in the width direction (directions of arrows X 1  and X 2 ) of the hull  2 . The outboard motor  3  is covered by a casing  300 . The casing  300  is preferably made of resin and has a function to protect the inside of the outboard motor  3  from water and so forth. The outboard motor  3  includes an engine  31 , two propellers  32   a  and  32   b  (see  FIG. 6 ) arranged to convert the driving force of the engine  31  into a propulsion force of the boat  1 , a transmission mechanism  33  arranged to transmit the driving force generated by the engine  31  to the propellers  32   a  and  32   b  with a speed thereof shifted to a low speed reduction ratio (approx. 1.33:1.00) and to a high speed reduction ratio (approx. 1.0:1.0), and an ECU (electronic control unit)  34  arranged to electrically control the engine  31  and the transmission mechanism  33 . The ECU  34  is an example of a “control portion” of a preferred embodiment of the present invention. An engine speed sensor  35  arranged to detect the engine speed of the engine  31  and an electronic throttle device  36  arranged to control the throttle opening of a throttle valve (not shown) of the engine  31  based on an accelerator opening signal described below are connected to the ECU  34 . The engine speed sensor  35  is disposed in a vicinity of a crankshaft  301  (see  FIG. 6 ) of the engine  31 . The engine speed sensor  35  performs the functions of detecting the rotational speed of the crankshaft  301  and transmitting the detected rotational speed of the crankshaft  301  to the ECU  34 . The rotational speed of the crankshaft  301  is an example of “engine speed” of a preferred embodiment of the present invention. The electronic throttle device  36  controls the throttle opening of the throttle valve (not shown) of the engine  31  based on the accelerator opening signal from the ECU  34  and also has a function to transmit the throttle opening to the ECU  34  and the control portion  52  described below. 
     In the present preferred embodiment, the ECU  34  has a function to generate an electromagnetic hydraulic pressure control valve driving signal based on a speed changing gear shift signal and a shift position signal sent by the control portion  52  of the control lever section  5  described below. An electromagnetic hydraulic pressure control valve  37  is connected to the ECU  34 . The ECU  34  generates a control signal to send the electromagnetic hydraulic pressure control valve driving signal to the electromagnetic hydraulic pressure control valve  37 . The electromagnetic hydraulic pressure control valve  37  is driven based on the electromagnetic hydraulic pressure control valve driving signal, and thereby the transmission mechanism  33  is controlled. Construction and operation of the transmission mechanism  33  will be described below in detail. 
     In the present preferred embodiment, as shown in  FIG. 4 , the lever  5   a  of the control lever section  5  has a gear shift switch  5   b  with which the user selects the speed reduction ratio in the manual mode. The gear shift switch  5   b  can preferably be set to a shift-down position (depressed position) S 1  corresponding to the low speed reduction ratio and a shift-up position (protruding position) S 2  corresponding to the high speed reduction ratio by operation of the user. The gear shift switch  5   b  is an example of an “operation portion” of a preferred embodiment of the present invention. The control lever section  5  preferably includes a memory portion  51  in which a shift inhibition control map, a shift redirection control map, and a shift control map described below are stored; and the control portion  52  arranged to generate signals (speed changing gear shift signal, shift position signal, accelerator opening signal, mode signal) to be sent to the ECU  34 . The control lever section  5  preferably further includes a shift position sensor  53  arranged to detect the shift position of the lever  5   a  and an accelerator position sensor  54  arranged to detect the opening of the lever (accelerator) opened or closed by operation on the lever  5   a . The shift position sensor  53  is provided to detect which shift position the lever  5   a  is positioned among a neutral position, a position in front of the neutral position, and a position in the rear of the neutral position. The memory portion  51  and the control portion  52  are connected together. The control portion  52  is capable of reading out the shift control map and so forth stored in the memory portion  51 . The control portion  52  is connected to both the shift position sensor  53  and the accelerator position sensor  54 . Thereby, the control portion  52  can obtain a detection signal (shift position signal) detected by the shift position sensor  53  and the accelerator opening signal detected by the accelerator position sensor  54 . The control portion  52  is connected to the gear shift switch  5   b  and the mode switching lever  9 . The control portion  52  is connected to the gear shift switch  5   b , and thereby can obtain a gear selection signal corresponding to a gear selected by the user in the manual mode. The control portion  52  is connected to the mode switching lever  9 , and thereby can obtain a mode selection signal corresponding to the mode (manual mode or automatic mode) selected by the user. 
     The control portion  52  is preferably connected to both of the common LAN cables  7  and  8 . Each of the common LAN cables  7  and  8  is connected to the ECU  34 . The common LAN cables have functions to transmit a signal generated by the control portion  52  to the ECU  34  and to transmit a signal generated by the ECU  34  to the control portion  52 . In other words, each of the common LAN cables  7  and  8  is capable of communication between the control portion  52  and the ECU  34 . The common LAN cable  8  is provided electrically independently of the common LAN cable  7 . 
     Specifically, the control portion  52  transmits the shift position signal of the lever  5   a  detected by the shift position sensor  53  to the display section  6  and the ECU  34  via the common LAN cable  7 . The control portion  52  transmits the shift position signal not via the common LAN cable  8  but only via the common LAN cable  7 . The control portion  52  transmits the mode selection signal obtained from the mode switching lever  9  to the display section  6  as the mode signal. The control portion  52  transmits the accelerator opening signal detected by the accelerator position sensor  54  to the ECU  34  not via the common LAN cable  7  but only via the common LAN cable  8 . The control portion  52  is capable of receiving an engine speed signal sent from the ECU  34  via the common LAN cable  8 . 
     In the present preferred embodiment, the control portion  52  has a function to electrically control a shift between the reduction ratios of the transmission mechanism  33  based on operation of the control lever section  5  by a user in the automatic mode. Specifically, the control portion  52  has a function to generate the speed changing gear shift signal arranged to control the transmission mechanism  33  so that it shifts to the low speed or the high speed reduction ratio based on the shift control map provided by the accelerator opening and engine speed stored in the memory portion  51 . The control portion  52  has a function to generate the speed changing gear shift signal arranged to control the transmission mechanism  33  so that it shifts to the low speed or the high speed reduction ratio based on the position of the gear shift switch  5   b  in the manual mode. The control portion  52  sends the generated speed changing gear shift signal to the display section  6  and the ECU  34  via the common LAN cables  7  and  8 . 
     The transmission mechanism  33  is controlled so that the hull  2  can travel forward when the lever  5   a  of the control lever section  5  is turned forward (direction of arrow FWD) (see  FIG. 3 ). The transmission mechanism  33  is controlled so that it retains a neutral state in which the hull  2  is propelled neither forward nor rearward when the lever  5   a  is not turned in the fore-and-aft direction, as is the lever  5   a  shown in solid lines in  FIG. 3 . The transmission mechanism  33  is controlled so that the hull  2  can travel rearward when the lever  5   a  of the control lever section  5  is turned rearward (direction opposite to arrow FWD) (see  FIG. 3 ). 
     The transmission mechanism  33  makes a shift-in operation (release from the neutral state) with the throttle valve (not shown) of the engine  31  fully closed (idling state) when the lever  5   a  of the control lever  5  is turned to position FWD 1  in  FIG. 3 . The throttle valve (not shown) of the engine  31  fully opens when the lever  5   a  of the control lever section  5  is turned to position FWD 2  in  FIG. 3 . 
     Similarly to the case that the lever  5   a  of the control lever section  5  is turned in the direction of arrow FWD, when the lever  5   a  is turned to position BWD 1  in  FIG. 3  in the direction opposite to the direction of arrow FWD, the transmission mechanism  33  makes a shift-in operation (release from the neutral state) with the throttle valve (not shown) of the engine  31  fully closed (idling state). The throttle valve (not shown) of the engine  31  fully opens when the lever  5   a  of the control lever  5  is turned to position BWD 2  in  FIG. 3 . 
     The display section  6  preferably includes a speed display  61  indicating the traveling speed of the boat  1 , a shift position display  62  indicating the shift position of the lever  5   a  of the control lever section  5 , a gear display  63  indicating the gear in the engaged state in the transmission mechanism  33 , and a mode display  64  indicating the mode manual mode (MT) or automatic mode (AT)) selected by the user. The traveling speed of the boat  1  displayed on the speed display  61  is calculated by the ECU  34  based on the engine speed sensor  35  and the intake state of the engine  31 . Calculated data about the traveling speed of the boat  1  is transmitted to the display section  6  via the common LAN cables  7  and  8 . The shift position displayed on the shift position display  62  is displayed based on the shift position signal sent from the control portion  52  of the control lever section  5 . The gear in the engaged state in the transmission mechanism  33  displayed on the gear display  63  is displayed based on the speed changing gear shift signal sent from the control portion  52 . The mode displayed on the mode display  64  is displayed based on the mode signal sent from the control portion  52 . In other words, the display section  6  has a function to inform the user (operator of the boat) about the traveling state of the boat  1 . 
     Next, construction of the engine  31  and the transmission mechanism  33  will be described. As shown in  FIG. 6 , the crankshaft  301  of the engine  31  rotates around axial line L 1 . The engine  31  generates a driving force through the rotation of the crankshaft  301 . An upper portion of an upper transmission shaft  311  of the transmission mechanism  33  is connected to the crankshaft  301 . The upper transmission shaft  311  is disposed along axial line L 1  and rotates around axial line L 1  together with the rotation of the crankshaft  301 . 
     The transmission mechanism  33  includes the upper transmission shaft  311  described above to which the driving force of the engine  31  is input, an upper transmission section  310  capable of shifting so that the boat  1  can perform either high speed travel or low speed travel, and a lower transmission section  330  capable of shifting so that the boat  1  can perform either forward travel or reverse travel. In other words, the transmission mechanism  33  is arranged to be capable of transmitting the driving force generated by the engine  31  to the propellers  32   a  and  32   b  with the speed shifted to the low speed reduction ratio (approx. 1.33:1) and the high speed reduction ratio (approx. 1:1) in the forward travel and also capable of transmitting driving force to the propellers  32   a  and  32   b  with the speed shifted to the low speed reduction ratio and the high speed reduction ratio in the reverse travel. 
     As shown in  FIG. 7 , the upper transmission section  310  preferably includes the upper transmission shaft  311  described above, a planetary gear section  312  arranged to reduce the rotational speed of the driving force of the upper transmission shaft  311 , a clutch  313  and a one-way clutch  314  arranged to control a rotation of the planetary gear section  312 , an intermediate shaft  315  to which driving force of the upper transmission shaft  311  is transmitted via the planetary gear section  312 , and an upper case section  316  defining a contour of the upper transmission section  310  with a plurality of members. The intermediate shaft  315  rotates at a rotational speed that is substantially not reduced when compared to the rotational speed of the upper transmission shaft  311  when the clutch  313  is in the engaged state. On the other hand, when the clutch  313  is in the disengaged state, the planetary gear section  312  rotates, and thus the intermediate shaft  315  rotates at a reduced rotational speed compared to the rotational speed of the upper transmission shaft  311 . 
     Specifically, a ring gear  317  is provided on a lower portion of the upper transmission shaft  311 . A flange member  318  is fitted to an upper portion of the intermediate shaft  315  by spline-fitting. The flange member  318  is disposed inside the ring gear  317  (on a side opposing axial line L 1 ). As shown in  FIGS. 7 and 8 , four shaft members  319  are fixed to a flange  318   a  of the flange member  318 . Four planetary gears  320  are rotatably mounted on the respective four shaft members  319 . Each of the planetary gears  320  is meshed with the ring gear  317 . Each of the four planetary gears  320  is meshed with a sun gear  321  arranged to rotate around axial line L 1 . As shown in  FIG. 7 , the sun gear  321  is supported by the one-way clutch  314 . The one-way clutch  314  is mounted on the upper case section  316  and can rotate only in direction A. Thereby, the sun gear  321  is arranged to rotate in only one direction (direction A). 
     The clutch  313  is preferably a wet type multi-plate clutch. The clutch  313  includes an outer case section  313   a  supported rotatably only in direction A by the one-way clutch  314 , a plurality of clutch plates  313   b  disposed in an inner periphery of the outer case section  313   a  at certain intervals from each other, an inner case section  313   c  at least partially disposed inside the outer case  313   a , and a plurality of clutch plates  313   d  mounted on the inner case section  313   c  and disposed in spaces between the plurality of clutch plates  313   b . The clutch  313  enters the engaged state in which the outer case section  313   a  and the inner case section  313   c  rotate unitarily when the clutch plates  313   b  of the outer case section  313   a  and the clutch plates  313   d  of the inner case section  313   c  contact with each other. Meanwhile, the clutch  313  enters the disengaged state in which the outer case section  313   a  and the inner case section  313   c  do not unitarily rotate when the clutch plates  313   b  of the outer case section  313   a  and the clutch plates  313   d  of the inner case section  313   c  are separated from each other. 
     Specifically, a piston  313   e  slidable on an inner peripheral surface of the outer case section  313   a  is disposed in the outer case section  313   a . The piston  313   e  moves the plurality of the clutch plates  313   b  of the outer case section  313   a  in a direction in which the piston  313   e  slides on the inner peripheral surface of the outer case section  313   a . A compression coil spring  313   f  is disposed in the outer case section  313   a . The compression coil spring  313   f  is disposed to urge the piston  313   e  in a direction in which the clutch plates  313   b  of the outer case section  313   a  are separated from the clutch plates  313   d  of the inner case section  313   c . The piston  313   e  slides on the inner peripheral surface of the outer case section  313   a  against a reaction of the compression coil spring  313   f  when the electromagnetic hydraulic pressure control valve  37  described above increases pressure of the oil flowing through an oil passage  316   a  of the upper case section  316 . Accordingly, the pressure of the oil flowing through the oil passage  316   a  of the upper case section  316  is increased or reduced, thereby allowing contact and separation between the clutch plates  313   b  of the outer case section  313   a  and the clutch plates  313   d  of the inner case section  313   c . Therefore, the clutch  313  can be engaged or disengaged. 
     Lower ends of the four shaft members  319  are mounted on an upper portion of the inner case section  313   c . In other words, the inner case section  313   c  is connected to the flange member  318  on which each of upper portions of the four shaft members  319  are mounted via the four shaft members  319 . Thereby, the inner case section  313   c , the flange member  318 , and the shaft members  319  can simultaneously rotate around axial line L 1 . 
     The planetary gear section  312  and the clutch  313  are constructed as described above. Therefore, when the clutch  313  is disengaged, the ring gear  317  rotates in direction A together with the upper transmission shaft  311  rotating in direction A. In this case, the sun gear  321  does not rotate in direction B opposite to direction A. Therefore, as shown in  FIG. 8 , each of the planetary gears  320  rotates around the shaft member  319  in direction A 1  and at the same time revolves around axial line L 1  in direction A 2  together with the shaft member  319 . Thereby, the flange member  318  (see  FIG. 7 ) rotates around axial line L 1  in direction A while the shaft members  319  revolve in direction A 2 . As a result, the intermediate shaft  315  fitted to the flange member  318  by spline-fitting, for example, can be rotated around axial line L 1  in direction A at the reduced rotational speed when compared to the rotational speed of the upper transmission shaft  311 . 
     The planetary gear section  312  and the clutch  313  are constructed as described above. Accordingly, when the clutch  313  is engaged, the ring gear  317  rotates in direction A together with the upper transmission shaft  311  rotating in direction A. In this case, the sun gear  321  does not rotate in direction B opposite to direction A. Therefore, each of the planetary gears  320  rotates around the shaft member  319  in direction A 1  and at the same time revolves around axial line L 1  in direction A 2  together with the shaft member  319 . At this point, since the clutch  313  is engaged, the outer case section  313   a  (see  FIG. 7 ) of the clutch  313  rotates in direction A together with the one-way clutch  314  (see  FIG. 7 ). Thereby, the sun gear  321  rotates around axial line L 1  in direction A. Therefore, the planetary gears  320  substantially do not rotate around the shaft members  319 , but the shaft members  319  revolve around axial line L 1  in direction A. Accordingly, the flange member  318  rotates at a speed generally equivalent to the rotational speed of the upper transmission shaft  311  since the speed is not substantially reduced by the planetary gears  320 . As a result, the intermediate shaft  315  can be rotated around axial line L 1  in direction A at the speed generally equivalent to the rotational speed of the upper transmission shaft  311 . 
     As shown in  FIG. 7 , the lower transmission section  330  is provided below the upper transmission section  310 . The lower transmission section  330  includes an intermediate transmission shaft  331  connected to the intermediate shaft  315 , a planetary gear section  332  arranged to reduce the rotational speed of the driving force of the intermediate transmission shaft  331 , forward-reverse switching clutches  333  and  334  arranged to control rotation of the planetary gear section  332 , a lower transmission shaft  335  to which the driving force of the intermediate transmission shaft  331  is transmitted via the planetary gear section  332 , and a lower case section  336  defining a contour of the lower transmission section  330 . Further, the lower transmission section  330  is arranged such that the lower transmission shaft  335  rotates in a direction (direction B) opposite to the rotational direction (direction A) of the intermediate shaft  315  (the upper transmission shaft  311 ) when the forward-reverse switching clutch  333  is engaged and the forward-reverse switching clutch  334  is disengaged. In this case, the lower transmission section  330  does not rotate propeller  32   b  but rotates only the propeller  32   a  so that the boat  1  can travel rearward. On the other hand, the lower transmission section  330  is constructed such that the lower transmission shaft  335  rotates in the same direction as the rotational direction (direction A) of the intermediate shaft  315  (the upper transmission shaft  311 ) when the forward-reverse switching clutch  333  is disengaged and the forward-reverse switching clutch  334  is engaged. In this case, the lower transmission section  330  rotates the propeller  32   a  in a direction opposite to the case of the reverse travel of the boat  1  and rotates the propeller  32   b  in a direction opposite to the rotational direction of the propeller  32   a  so that the boat  1  can travel forward. The lower transmission section  330  is constructed so that the forward-reverse switching clutches  333  and  334  are both not engaged at the same time. The lower transmission section  330  is constructed so that rotation of the intermediate shaft  315  (the upper transmission shaft  311 ) is not transmitted to the lower transmission shaft  335  (the lower transmission section  330  becomes the neutral state) when both the forward-reverse switching clutches  333  and  334  are in the disengaged state. 
     Specifically, the intermediate transmission shaft  331  rotates together with the intermediate shaft  315 . A flange  337  is provided on a lower portion of the intermediate transmission shaft  331 . As shown in  FIGS. 7 and 9 , three inner shaft members  338  and three outer shaft members  339  are fixed to the flange  337 . Three inner planetary gears  340  are rotatably mounted on the respective three inner shaft members  338 . Each of the inner planetary gears  340  is meshed with the sun gear  343  described below. Three outer planetary gears  341  are rotatably mounted on the respective three outer shaft members  339 . Each of the three outer planetary gears  341  are meshed with the inner planetary gear  340  and with a ring gear  342  described later. 
     The forward-reverse switching clutch  333  is provided in an upper portion of the lower case section  336 . The forward-reverse switching clutch  333  is preferably a wet type multi-plate clutch. A portion thereof is provided with a recess  336   a  of the lower case section  336 . The forward-reverse switching clutch  333  includes a plurality of clutch plates  333   a  disposed in an inner periphery of the recess  336   a  at certain intervals from each other, an inner case section  333   b  at least partially disposed inside the recess  336   a , and a plurality of clutch plates  333   c  mounted on the inner case section  333   b  and disposed in spaces between the plurality of clutch plates  333   a . The forward-reverse switching clutch  333  is constructed such that the lower case section  336  restrains rotation of the inner case section  333   b  when the clutch plates  333   a  of the recess  336   a  and the clutch plates  333   c  of the inner case section  333   b  contact with each other. Meanwhile, the forward-reverse switching clutch  333  is constructed such that the inner case section  333   b  freely rotates with respect to the lower case section  336  when the clutch plates  333   a  of the recess  336   a  and the clutch plates  333   c  of the inner case section  333   b  are separated from each other. 
     Specifically, a piston  333   d  slidable on an inner peripheral surface of the recess  336   a  is disposed in the recess  336   a  of the lower case section  336 . The piston  333   d  moves the clutch plates  333   a  of the recess  336   a  in a direction in which the piston  333   d  slides on the inner peripheral surface of the recess  336   a . A compression coil spring  333   e  is disposed in the recess  336   a  of the lower case section  336 . The compression coil spring  333   e  is disposed to urge the piston  333   d  in a direction in which the clutch plates  333   a  of the recess  336   a  are separated from the clutch plates  333   c  of the inner case section  333   b . The piston  333   d  slides on the inner peripheral surface of the recess  336   a  against reaction of the compression coil spring  333   e  when the electromagnetic hydraulic pressure control valve  37  described above increases the pressure of oil flowing through an oil passage  336   b  of the lower case section  336 . Accordingly, the pressure of the oil flowing through the oil passage  336   b  of the lower case section  336  is increased or reduced, thereby allowing engagement and disengagement of the forward-reverse switching clutch  333 . 
     A ring-shaped ring gear  342  is mounted in the inner case section  333   b  of the forward-reverse switching clutch  333 . As shown in  FIGS. 7 and 9 , the ring gear  342  is meshed with the three outer planetary gears  341 . 
     As shown in  FIG. 7 , the forward-reverse switching clutch  334  is provided in a lower portion of the lower case section  336  and is preferably a wet type multi-plate clutch. The forward-reverse switching clutch  334  mainly includes an outer case section  334   a , a plurality of clutch plates  334   b  disposed in an inner periphery of the outer case section  334   a  at certain intervals from each other, an inner case section  334   c  at least partially disposed inside the outer case  334   a , and a plurality of clutch plates  334   d  mounted on the inner case section  334   c  and disposed in spaces between the plurality of clutch plates  334   b . The forward-reverse switching clutch  334  is constructed such that the inner case section  334   c  and the outer case section  334   a  unitarily rotate around axial line L 1  when the clutch plates  334   b  of the outer case section  334   a  and the clutch plates  334   d  of the inner case section  334   c  contact with each other. On the other hand, the forward-reverse switching clutch  334  is constructed such that the inner case section  334   c  freely rotates with respect to the outer case section  334   a  when the clutch plates  334   b  of the outer case section  334   a  and the clutch plates  334   d  of the inner case section  334   c  are separated from each other. 
     Specifically, a piston  334   e  slidable on an inner peripheral surface of the outer case section  334   a  is disposed in the outer case section  334   a . The piston  334   e  moves the plurality of the clutch plates  334   b  of the outer case section  334   a  in a direction in which the piston  334   e  slides when it slides on the inner peripheral surface of the outer case section  334   a . A compression coil spring  334   f  is disposed in the outer case section  334   a . The compression coil spring  334   f  is disposed to urge the piston  334   e  in a direction in which the clutch plates  334   b  of the outer case section  334   a  are separated from the clutch plates  334   d  of the inner case section  334   c . The piston  334   e  slides on the inner peripheral surface of the outer case section  334   a  against reaction of the compression coil spring  334   f  when the electromagnetic hydraulic pressure control valve  37  described above increases the pressure of the oil flowing through an oil passage  336   c  of the lower case section  336 . Accordingly, the pressure of the oil flowing through the oil passage  336   c  of the lower case section  336  is increased or reduced, thereby allowing engagement and disengagement of the forward-reverse switching clutch  334 . 
     The three inner shaft members  338  and the three outer shaft members  339  are fixed to the inner case section  334   c  of the forward-reverse switching clutch  334 . In other words, the inner case section  334   c  is connected to the flange  337  by the three inner shaft members  338  and the three outer shaft members  339  and rotates around axial line L 1  together with the flange  337 . The outer case section  334   a  of the forward-reverse switching clutch  334  is mounted on the lower transmission shaft  335  and rotates around axial line L 1  together with the lower transmission shaft  335 . 
     The sun gear  343  is unitarily formed with an upper portion of the lower transmission shaft  335 . As shown in  FIG. 9 , the sun gear  343  is meshed with the inner planetary gears  340  as described above. The inner planetary gears  340  are meshed with the outer planetary gears  341  meshed with the ring gear  342 . The sun gear  343  rotates around axial line L 1  in direction B via the inner planetary gears  340  and the outer planetary gears  341  when the flange  337  rotates in direction A together with the intermediate transmission shaft  331  rotating around axial line L 1  in direction A when the ring gear  342  does not rotate due to engagement of the forward-reverse switching clutch  333 . 
     The planetary gear section  332 , the forward-reverse switching clutches  333  and  334  are constructed as described above. Thereby, when the forward-reverse switching clutch  333  is engaged, the ring gear  342  mounted on the inner case section  333   b  is fixed to the lower case section  336 . At this point, the forward-reverse switching clutch  334  is disengaged as described above. Therefore, the outer case section  334   a  and the inner case section  334   c  of the forward-reverse switching clutch  334  can separately rotate. In this case, when the flange  337  rotates around axial line L 1  in direction A together with the intermediate transmission shaft  331  rotating around axial line L 1  in direction A, each of the three inner shaft members  338  and the three outer shaft members  339  revolves around axial line L 1  in direction A. The outer planetary gears  341  mounted on the outer shaft members  339  rotate around the outer shaft members  339  in direction B 1 . The inner planetary gears  340  rotate around the inner shaft members  338  in direction A 3  together with rotation of the outer planetary gears  341 . Accordingly, the sun gear  343  rotates around axial line L 1  in direction B. As a result, as shown in  FIG. 7 , the lower transmission shaft  335  rotates around axial line L 1  in direction B together with the outer case section  334   a  although the inner case section  334   c  rotates around axial line L 1  in direction A. Accordingly, the lower transmission shaft  335  can be rotated in the direction (direction B) opposite to the rotational direction (direction A) of the intermediate shaft  315  (the upper transmission shaft  311 ) when the forward-reverse switching clutch  333  is in the engaged state and the forward-reverse switching clutch  334  is in the disengaged state. The planetary gear section  332  and the forward-reverse switching clutches  333  and  334  are constructed as described above. Thereby, when the forward-reverse switching clutch  333  is disengaged, the ring gear  342  mounted on the inner case section  333   b  can freely rotate with respect to the lower case section  336 . In this case, the forward-reverse switching clutch  334  can become either the engaged state or the disengaged state. 
     Next, descriptions will be made about a case when the forward-reverse switching clutch  334  is engaged. When the flange  337  rotates in direction A together with the intermediate transmission shaft  331  rotating around axial line L 1  in direction A, each of the three inner shaft members  338  and the three outer shaft members  339  revolves around axial line L 1  in direction A as shown in  FIG. 9 . In this case, the ring gear  342  meshed with the outer planetary gears  341  freely rotate. Therefore, the inner planetary gears  340  and the outer planetary gears  341  are idle. In other words, the driving force of the intermediate transmission shaft  331  is not transmitted to the sun gear  343 . Meanwhile, since the forward-reverse switching clutch  334  is engaged, as shown in  FIG. 7 , the outer case section  334   a  rotates around axial line L 1  in direction A together with rotation around axial line L 1  in direction A of the inner case section  334   c  which can rotate around axial line L 1  in direction A together with the three inner shaft members  338  and the three outer shaft members  339 . Accordingly, the lower transmission shaft  335  rotates around axial line L 1  in direction A together with the outer case section  334   a . As a result, the lower transmission shaft  335  can be rotated in the same direction as the rotational direction (direction A) of the intermediate shaft  315  (the upper transmission shaft  311 ) when the forward-reverse switching clutch  333  is in the disengaged state and the forward-reverse switching clutch  334  is in the engaged state. 
     As shown in  FIG. 6 , a speed reducing device  344  is provided below the transmission mechanism  33 . The lower transmission shaft  335  of the transmission mechanism  33  is input to the speed reducing device  344 . The speed reducing device  344  has a function to reduce the rotational speed of a driving force input by the lower transmission shaft  335 . A drive shaft  345  is provided below the speed reducing device  344 . The drive shaft  345  rotates in the same direction as the lower transmission shaft  335 . A bevel gear  345   a  is provided in a lower portion of the drive shaft  345 . 
     A bevel gear  346   a  of an inner output shaft  346  and a bevel gear  347   a  of an outer output shaft  347  are meshed with the bevel gear  345   a  of the drive shaft  345 . The inner output shaft  346  is arranged to extend rearward (direction of arrow BWD). The propeller  32   b  described above is mounted on a portion of the inner output shaft  346  in the direction of arrow BWD. The outer output shaft  347  is arranged to extend in the direction of arrow BWD similarly to the inner output shaft  346 . The propeller  32   a  described above is mounted on a portion of the outer output shaft  347  in the direction of arrow BWD. The outer output shaft  347  is hollow. The inner output shaft  346  is inserted in a cavity of the outer output shaft  347 . The inner output shaft  346  and the outer output shaft  347  can rotate independently of each other. 
     The bevel gear  346   a  is meshed with a side of the bevel gear  345   a  in the direction of arrow FWD. The bevel gear  347   a  is meshed with a side of the bevel gear  345   a  in the direction of arrow BWD. Thereby, when the bevel gear  346   a  rotates, the inner output shaft  346  and the outer output shaft  347  rotate in directions opposite to each other. 
     Specifically, the bevel gear  346   a  rotates in direction A 4  when the drive shaft  345  rotates in direction A. The propeller  32   b  rotates in direction A 4  via the inner output shaft  346  together with rotation of the bevel gear  346   a  in direction A 4 . Further, when the drive shaft  345  rotates in direction A, the bevel gear  347   a  rotates in direction B 2 . The propeller  32   a  rotates in direction B 2  via the outer output shaft  347  together with rotation of the bevel gear  347   a  in direction B 2 . The propeller  32   a  rotates in direction B 2  and the propeller  32   b  rotates in direction A 4  (direction opposite to direction B 2 ). Thereby, the boat  1  travels in the direction of arrow FWD (forward travel direction). 
     Further, when the drive shaft  345  rotates in direction B, the bevel gear  346   a  rotates in direction B 2 . The propeller  32   b  rotates in direction B 2  via the inner output shaft  346  together with rotation of the bevel gear  346   a  in direction B 2 . The bevel gear  347   a  rotates in direction A 4  when the drive shaft  345  rotates in direction B. In this case, the outer output shaft  347  does not rotate in direction A 4 . The propeller  32   a  rotates in neither direction A 4  nor direction B 2 . In other words, only the propeller  32   b  rotates in direction A 4 . The propeller  32   b  rotates in direction B 2 , and thereby the boat  1  travels in the direction of arrow BWD (reverse travel direction). 
       FIG. 10  is a diagram illustrating the shift inhibition control map for the manual mode of the marine propulsion system in accordance with a preferred embodiment of the present invention.  FIG. 11  is a diagram illustrating the shift redirection control map for the manual mode of the marine propulsion system in accordance with a preferred embodiment of the present invention. Next, the shift control in the manual mode will be described in detail with reference to  FIGS. 4 ,  10 , and  11 . 
     In the manual mode, the user operates the gear shift switch  5   b , and thereby can arbitrarily shift gears. However, a shift in response to an operation of the user may cause an adverse effect on the engine depending on a state of the engine at the time of the shift. Therefore, in the present preferred embodiment, a shift is not executed even though the user operates the gear shift switch  5   b  in particular engine states. Specifically, the control portion  52  does not execute a shift-down operation when the engine speed is larger than a certain threshold value, and the control portion  52  does not execute a shift-up operation when the engine speed is smaller than a certain threshold value. In the present preferred embodiment, the shift inhibition control map is used as described above. The shift inhibition control map is an example of a “first shift control map” of a preferred embodiment of the present invention. 
     As shown in  FIG. 10 , the shift inhibition control map in accordance with the present preferred embodiment is defined by the relationship between the engine speed of the engine  31  and the throttle opening. The vertical axis represents the engine speed and the horizontal axis represents the throttle opening on the shift inhibition control map. The shift inhibition control map includes a shift-down inhibition range R 1  in which the shift from the high speed to the low speed is inhibited, a shift-up inhibition range R 2  in which the shift from the low speed to the high speed is inhibited, and a shift execution range R 3  provided between the shift-down inhibition range R 1  and the shift-up inhibition range R 2  in which a shift is executed as instructed by the user (operation on the gear shift switch  5   b ). The shift-down inhibition range R 1  is provided in a zone for large engine speeds. The shift-up inhibition range is provided in a zone for small engine speed. The shift-down inhibition range R 1 , the shift-up inhibition range R 2 , and the shift execution range R 3  are defined by a shift-down inhibition referential curve D and a shift-up inhibition referential curve U. The shift-down inhibition referential curve D is a curve which passes through a limit engine speed N L  in a range that the throttle opening is equal to or less than a certain value, and also a curve on which the engine speed becomes smaller as the throttle opening becomes larger in a range that the throttle opening is equal to or larger than a certain value. The limit engine speed N L  is a speed that the engine speed of the engine  31  may exceed an allowable engine speed of the engine  31 . The shift-up inhibition referential curve U is a curve that the engine speed gradually becomes smaller as the throttle opening becomes larger in a range that the throttle opening is relatively small. Specifically, the shift-up inhibition referential curve U is set such that the engine speed gradually becomes smaller from a value larger than an idling engine speed N I  to a value smaller than the idling engine speed N I  as the throttle opening becomes larger, and reaches a minimum value N M . The shift-up inhibition referential curve U is a curve in which the engine speed gradually becomes larger as the throttle opening becomes larger in a range that the throttle opening is relatively large. Specifically, the shift-up inhibition referential curve U is set such that the engine speed gradually increases and becomes larger than the idling engine speed N I  as the throttle opening becomes larger after the engine speed reaches the minimum value N M . The threshold values for determinations on permissions for shift-up and shift-down operation are determined by the control portion  52  based on the shift-down inhibition referential line D, the shift-up inhibition referential line U, and the throttle opening. A shift control map for shift-down operation in accordance with the present preferred embodiment is used in common for both the forward travel and the reverse travel. 
     In the present preferred embodiment, the control portion  52  and the ECU  34  do not execute a shift when the engine speed and the throttle opening of the boat  1  is in the shift-down inhibition range R 1  or the shift-up inhibition range R 2  on the shift inhibition control map. In other words, when the user depresses the gear shift switch  5   b  and the gear shift switch  5   b  is positioned in the shift-down position S 1  (see  FIG. 4 ), the shift-down operation is not executed if the engine speed and the throttle opening is in the shift-down inhibition range R 1  at a point that the user makes an instruction to shift. When the user makes the gear shift switch  5   b  protrude to a position higher than the shift-down position S 1  and the gear shift switch  5   b  is positioned in the shift-up position S 2  (see  FIG. 4 ), the shift-up operation is not executed if the engine speed and the throttle opening is in the shift-up inhibition range R 2  at a point that the user makes an instruction to shift. When a shift is not executed in such a manner, a shift is executed after the engine speed and the throttle opening enter the shift execution range R 3 . 
     In the present preferred embodiment, the shift-down operation is made when the shift-up operation is made in the manual mode and the engine speed largely decreases after the shift-up operation compared to the engine speed before the shift-up operation. In the present preferred embodiment, the shift redirection control map is used to perform the controls described above. The shift redirection control map is an example of a “second shift control map” of a preferred embodiment of the present invention. 
     As shown in  FIG. 11 , the shift redirection control map in accordance with the present preferred embodiment is provided by the relationship between the engine speed of the engine  31  and the decrease proportion of the engine speed after the shift-up operation to the engine speed before the shift-up operation. The vertical axis represents the decrease proportion of the engine speed and the horizontal axis represents the engine speed before the shift-up operation on the shift redirection control map. The shift redirection control map includes a shift-up cancellation range R 4  in which the shift-down operation is again made after the shift-up operation is made once and shift-up acceptance range R 5  in which no shift-down operation is made after the shift-up operation. The boundary curve T between the shift-up cancellation range R 4  and the shift-up acceptance range R 5  is a curve that the decrease proportion of the engine speed becomes larger as the engine speed becomes larger. A threshold (a decrease proportion of an engine speed) for a determination about whether the shift-down operation is again made or not is determined by the control portion  52  based on the boundary curve T and the engine speed. The shift redirection control map in accordance with the present preferred embodiment is commonly used for both the forward travel and the rearward travel. 
       FIG. 12  is a map illustrating the shift control map for the automatic mode of the marine propulsion system in accordance with a preferred embodiment of the present invention. Next, descriptions will be made in detail about the shift control map for the automatic mode and shift control with use of the map with reference to  FIG. 12 . 
     The gear shift switch  5   b  is ineffective in the automatic mode. A shift is automatically made in response to operation (accelerator operation) on the lever  5   a  by the user. The control portion  52  determines a timing to execute a shift based on the shift control map shown in  FIG. 12 . 
     As shown in  FIG. 12 , the shift control map for the automatic mode is provided by the relationship between the engine speed of the engine  31  and the accelerator opening. The vertical axis represents the engine speed and the horizontal axis represents the accelerator opening on the shift control map. The shift control map includes a shift-down range R 6  providing the low speed reduction ratio, a shift-up range R 7  providing the high speed reduction ratio, and a dead zone range R 8  provided at a boundary between the shift-down range R 6  and the shift-up range R 7 . The shift control map in accordance with the present preferred embodiment is used in common for the forward travel and the rearward travel. 
     In the automatic mode, when a locus P on the shift control map given by the engine speed and the accelerator opening of the boat  1  enters the shift-down range R 6  from the shift-up range R 7  via the dead zone range R 8  (locus P 1  indicated in  FIG. 12 ), the control portion  52  and the ECU  34  controls the transmission mechanism  33  to make shift-down operation (shift from the high speed reduction ratio to the low speed reduction ratio). When a locus P given by the engine speed and the accelerator opening enters the shift-up range R 7  from the shift-down range R 6  via the dead zone range R 8  (locus P 2  indicated in  FIG. 12 ), the control portion  52  and the ECU  34  controls the transmission mechanism  33  to make a shift-up operation (shift from the low speed reduction ratio to the high speed reduction ratio). The dead zone range R 8  is provided to prevent frequent shifts between the reduction ratios. A shift is not made when a locus just enters the dead zone range R 8  from the shift-up range R 7  or just enters the dead zone range R 8  from the shift-down range R 6 . 
     In the present preferred embodiment, the user operates the mode switching lever  9 , and thereby can switch between the manual mode and the automatic mode. Since the boat  1  may suddenly accelerate or decelerate when a switch between the modes is made when the engine load is large, a switch between the modes is not executed when the engine load is large.  FIG. 13  is a flowchart demonstrating mode switch steps of the marine propulsion system in accordance with a preferred embodiment of the present invention. Next, the mode switch steps of the marine propulsion system will be described with reference to  FIG. 13 . A series of steps demonstrated in the flowchart are made approximately every 100 milliseconds, for example. Either the manual mode or the automatic mode is always selected. 
     The user operates the mode switching lever  9  to switch the modes. In a case of selecting the manual mode, the mode switching lever  9  is positioned in the MT position corresponding to the manual mode. In a case of selecting the automatic mode, the mode switching lever  9  is positioned in the AT position corresponding to the automatic mode. Thereby, the mode selection signal corresponding to the manual mode or the automatic mode is sent from the mode switching lever  9  to the control portion  52 . At this point, the control portion  52  determines whether the mode switching lever  9  is positioned in the MT position or not in step S 1  in  FIG. 13 . Specifically, the control portion  52  determines the position of the mode switching lever  9  based on the mode selection signal received from the mode switching lever  9 . 
     When the mode switching lever  9  is positioned in the MT position, the control portion  52  determines whether the manual mode is selected or not in step S 2 . If the manual mode is selected, the mode switch steps end without executing a switch between the modes. 
     If the manual mode is not selected (the automatic mode is selected), it is a case when the automatic mode is selected in the control portion  52  although the mode switching lever  9  is positioned in the MT position. Therefore, a switch from the automatic mode to the manual mode is executed. Now, in step S 3 , the control portion  52  determines whether the position of the lever  5   a  is the neutral position or not. If the lever  5   a  is in the neutral position, the accelerator opening (the opening of the lever  5   a ) is zero, and the engine speed is an idling speed. Therefore, the engine load is not large, and thus the manual mode is selected in step S 6 . Accordingly, the mode switch steps end. 
     If the lever  5   a  is not in the neutral position, the lever  5   a  is turned forward or rearward, and the accelerator is opened (the engine load is larger than that of the idling time). The control portion  52  determines whether a switch between the modes is made or not based on the magnitude of the engine load. In other words, in step S 4 , the control portion  52  determines whether the accelerator opening (the opening of the lever  5   a ) is equal to or less than a certain threshold value X or not. If the accelerator opening (the opening of the lever  5   a ) is equal to or less than the threshold value X, the control portion  52  determines whether the engine speed is equal to or less than a certain threshold value Y or not in step S 5 . If the accelerator opening is equal to or less than the threshold value X and the engine speed is equal to or less than the threshold value Y, the control portion  52  determines that the engine load is not large and selects the manual mode in step S 6 . The mode switch steps end. 
     If the accelerator opening is larger than the threshold value X or the engine speed is larger than the threshold value Y, the control portion  52  determines that the engine load is large. The mode switch steps end without executing a switch between the modes. 
     If the mode switching lever  9  is in the AT position corresponding to the automatic mode in step S 1 , the control portion  52  determines whether the automatic mode is selected or not in step S 7 . If the automatic mode is selected, the mode switch steps end without executing a switch between the modes. If the manual mode is selected, it is a state that the automatic mode is not selected in the control portion  52  although the mode switching lever  9  is positioned in the AT position corresponding to the automatic mode. Therefore, the control portion  52  determines whether a switch to the automatic mode is made or not based on the magnitude of the engine load. 
     In other words, the control portion  52  determines whether the position of the lever  5   a  is the neutral position or not in step S 8 . If the lever  5   a  is in the neutral position, the accelerator opening (the opening of the lever  5   a ) is zero, and the engine speed is the idling speed. Therefore, the engine load is not large, and thus the automatic mode is selected in step S 11 . Accordingly, the mode switch steps end. 
     If the lever  5   a  is not in the neutral position, the control portion  52  determines whether a switch between the modes is made or not based on the magnitude of the engine load. In other words, the control portion  52  determines whether the accelerator opening (the opening of the lever  5   a ) is equal to or less than the threshold value X or not in step S 9 . If the accelerator opening (the opening of the lever  5   a ) is equal to or less than the threshold value X, the control portion  52  determines whether the engine speed is equal to or less than the threshold value Y or not in step S 10 . If the accelerator opening is equal to or less than the threshold value X and the engine speed is equal to or less than the threshold value Y, the control portion  52  determines that the engine load is not large and selects the automatic mode in step S 11 . Accordingly, the mode switch steps end. 
     If the accelerator opening is larger than the threshold value X or the engine speed is larger than the threshold value Y, the control portion  52  determines that the engine load is large. The mode switch steps end without executing a switch to the automatic mode. A switch between the modes is executed as described above in the present preferred embodiment. 
     In various preferred embodiments described above, the driving force generated by the engine  31  can be transmitted to the propellers  32   a  and  32   b  when the transmission mechanism is shifted down. Accordingly, acceleration performance in the low speed position can be improved. Further, the driving force generated by the engine  31  can be transmitted to the propellers  32   a  and  32   b  in a state that the transmission mechanism is shifted up. This allows a larger maximum speed to be obtained. As a result, both the acceleration performance and the maximum speed can approach levels that the user desires. The user operates the gear shift switch  5   b  in the manual mode, and thereby can arbitrarily make the shift-down or shift-up operation in the forward and reverse travels. 
     In the various preferred embodiments described above, the threshold value of the engine speed arranged to inhibit the shift-down operation is determined based on the shift inhibition control map and the throttle opening. If the engine speed is larger than the threshold value, a control is made so that the shift-down operation is not executed. Thereby, the execution of the shift-down operation can be prevented when the user operates the gear shift switch  5   b  to the shift-down position in a state that the engine speed is large and thereby provides an instruction for the shift-down operation. Accordingly, the engine speed can be prevented from further increasing due to the execution of the shift-down operation in the state that the engine speed is large. This allows prevention of over-revving of the engine in the shift-down operation. 
     In the various preferred embodiments described above, the threshold value for the shift-down inhibition is set to decrease as the throttle opening of the engine becomes larger. Thereby, the execution of the shift-down operation can be prevented when the throttle opening is large and the engine output is large. Accordingly, the execution of the shift-down operation can be prevented when a large shift shock occurs due to the large engine output. 
     In the various preferred embodiments described above, the threshold value of the engine speed arranged to inhibit the shift-up operation is determined based on the shift inhibition control map and the throttle opening. If the engine speed is smaller than the threshold value, control is made so that the shift-up operation is not executed. Thereby, the execution of the shift-up operation can be prevented when the user operates the gear shift switch  5   b  to the shift-up position in a state that the engine speed is small and thereby provides an instruction for the shift-up operation. Accordingly, the engine speed can be prevented from further decreasing due to the shift-up operation in the state that the engine speed is small. Therefore, an engine stall can be prevented during the shift-up operation. 
     In the various preferred embodiments described above, the threshold value of the engine speed for the shift-up inhibition is set to become larger as the throttle opening of the engine  31  becomes larger in the range in which the throttle opening is relatively large. Thereby, the execution of the shift-up operation can be prevented when the throttle opening is large and the engine speed is small. The low speed reduction ratio is preferable to the high speed reduction ratio since torque is required when the throttle opening is large and the engine speed is small (such as a case when the propellers  32   a  or  32   b  is entangled with waterweeds, for example). The shift-up operation is inhibited in the state that torque is required, thereby allowing prevention of a torque decrease as a result of shift-up in the state that torque is required. 
     In the various preferred embodiments described above, the shift-down operation is again made when the decrease proportion of the engine speed after the shift-up operation to the engine speed before the shift-up operation is larger than a certain threshold value in the shift-up operation. Accordingly, the engine speed and the torque can be increased by shifting down when the engine speed largely decreases in the shift-up operation. This allows a prevention of an engine stall during the shift-up operation. 
     In the various preferred embodiments described above, the threshold value (decrease proportion of the engine speed) for the shift redirection is set to become larger as the engine speed becomes larger. Accordingly, the shift redirection (shift-up operation) is not made when the engine speed is large although the engine speed decreases substantially. An engine stall is not likely to occur when the engine speed is large, even if the decrease proportion of the engine speed is large. Accordingly, an engine stall can be prevented while the shift-up operation is made to reflect an intention of the user as accurately as possible. 
     In the various preferred embodiments described above, the mode switching lever  9  arranged to switch between the manual mode and the automatic mode is provided. Thereby, the user can freely make a switch between the manual mode and the automatic mode. 
     In the various preferred embodiments described above, a switch between the modes is executed when the accelerator opening (the opening of the lever  5   a ) is equal to or less than the threshold value X and the engine speed is equal to or less than the threshold value Y when the user operates the mode switching lever  9 . This allows a prevention of sudden acceleration or deceleration due to states of the gear shift switch  5   b  and the lever  5   a  of the control lever section  5  when the user mistakenly operates the mode switch lever  9  in a state that the engine load is large. 
     It should be understood that the preferred embodiments disclosed in the foregoing are merely exemplary, and do not limit the present invention. It is intended that the scope of the present invention be defined not by the preferred embodiments discussed in the foregoing descriptions but solely by the appended claims. Further, the present invention includes all modifications within meanings equivalent to the claims and the scope thereof. 
     For example, in the above preferred embodiments, descriptions are made about a marine propulsion system including the two outboard motors in which the engine and the propellers are disposed outside of the hull as an exemplary case. However, the present invention is not limited to this case, and can be applied to other marine propulsion systems including a stern drive in which an engine is fixed to a hull and an inboard motor in which an engine and a propeller are fixed to a hull, for example. The present invention can also be applied to a marine propulsion system including a single outboard motor. 
     In the above preferred embodiments, descriptions are made about a case when the horizontal axis of the shift redirection control map represents the engine speed. However, the present invention is not limited to this case, and the horizontal axis may instead represent the throttle opening, for example. 
     In the above preferred embodiments, descriptions are made about a case when the horizontal axis of the shift control map (see  FIG. 12 ) for the automatic mode represents the accelerator opening. However, the present invention is not limited to this case, and the horizontal axis may represent the throttle opening, in other words, the opening of the throttle valve provided in an intake passage of the engine, for example. 
     In the above preferred embodiments, descriptions are made about a case when a switch can be made between the manual mode and the automatic mode. However, the present invention is not limited to this case, and the mode may be fixed to the manual mode, for example. 
     In the above preferred embodiments, descriptions are made about a case when the gear shift switch  5   b  is provided as the operation portion that the user operates for a shift in the manual mode. However, the present invention is not limited to this case, and the operation portion may have any desirable form. 
     In the above preferred embodiments, descriptions are made about a case when the shift-up inhibition referential line U is set in a manner such that the engine speed becomes smaller as the throttle opening becomes larger in the range of the shift inhibition control map for small throttle opening. However, the present invention is not limited to this case, and the referential line U can be set so that the engine speed is a constant value (minimum value N M ) in the range for small throttle opening, for example. The minimum value N M  of the shift-up inhibition referential line U may be set to a value larger than the idling speed N I . 
     In the above preferred embodiments, descriptions are made about a marine propulsion system including an outboard motor having the two propellers as an exemplary case. However, the present invention is not limited to this case, and can be applied to other marine propulsion systems including an outboard motor having a single, three, or more propellers, for example. 
     In the above preferred embodiments, descriptions are made about a case when the maps (the shift inhibition control map and the shift redirection control map for the manual mode and the shift control map for the automatic mode) for the reverse travel of the boat have configurations similar to the maps for the forward travel of the boat. However, the present invention is not limited to this case. Two maps, in which one is dedicated to forward travel and the other is dedicated to reverse travel, could also be provided, for example. 
     In the above preferred embodiments, descriptions are made about a case when the control portion and the ECU are connected together by the common LAN cables and thereby communication can be made. However, the present invention is not limited to this case. Communication between the control portion and the ECU may be achieved by wireless communication, for example. 
     In the above preferred embodiments, the shift position signal is transmitted from the control portion to the ECU via only the common LAN cable  7 . The accelerator opening signal is transmitted from the control portion to the ECU via only the common LAN cable  8 . However, the present invention is not limited to this case. Both the shift position signal and the accelerator opening signal may be transmitted from the control portion to the ECU by the same common LAN cable, for example. Furthermore, the shift position signal may be transmitted from the control portion to the ECU via only the common LAN cable  8 , or the accelerator opening signal may be transmitted from the control portion to the ECU via only the common LAN cable  7 . 
     In the above preferred embodiments, the rotational speed of the crankshaft is used as an example of the engine speed. However, the present invention is not limited to this case. For example, the rotational speeds of members (shafts) other than the crankshaft that rotate together with rotation of the crankshaft in the engine such as propeller and output shaft may be used as the engine speed. 
     In the above preferred embodiments, descriptions are made about a case when the horizontal axis of the shift control map for the automatic mode represents the accelerator opening. However, the present invention is not limited to this case, but the horizontal axis may represent the throttle opening (the opening of the throttle valve provided in the intake passage of the engine), for example. 
     In the preferred embodiments described above, descriptions are made about a case when the two outboard motors are provided. However, the present invention is not limited to this case, but one, three, or more outboard motors may be provided, for example. In a case of having a plurality of outboard motors, timings for a shift or shift inhibition may be synchronized among all the outboard motors. In this case, one of the outboard motors is used as a main outboard motor, and thereby shift control may be made for the other outboard motors simultaneously with a shift control of the transmission mechanism of the main outboard motor. Specifically, the shift control may be made in the following manner. The control portion  52  outputs the “speed changing gear shift signal” or a “shift inhibition signal” to the ECU of the main outboard motor based on the shift control map and the shift inhibition control map stored in the memory portion  51  of the control lever section  5 . The ECU of the main outboard motor outputs a “driving signal” or “non-driving state retaining signal” to its own electromagnetic hydraulic pressure control valve  37  based on the “speed changing gear shift signal” or “shift inhibition signal”. Thereby, the upper transmission section  310  is shifted to the low speed position or inhibited from shifting. The ECU of the main outboard motor outputs the “driving signal” or “non-driving state retaining signal” to the ECUs installed in the other outboard motors via the common LAN. The ECUs of the other outboard motors output the “driving signal” or “non-driving state retaining signal” to their own electromagnetic hydraulic pressure control valves  37  based on the signals sent from the ECU of the main outboard motor. Thereby, the upper transmission section  310  of the main outboard motor and the upper transmission sections  310  of the outboard motors other than the main outboard motor are shifted to the low speed position or inhibited from shifting in a synchronized manner. 
     Each of the plurality of the outboard motors may output the shift control signal not only to its own transmission mechanism but also to the transmission mechanisms of the other outboard motors. In addition, each of the transmission mechanisms may make a shift based on the shift control signal sent the earliest among the shift control signals from the plurality of ECUs. Specifically, the shift control may be made in the following manner. The control portion  52  outputs the “speed changing gear shift signal” or a “shift inhibition signal” to the ECU of every outboard motor based on the shift control map and the shift inhibition control map stored in the memory portion  51  of the control lever section  5 . The ECU of each of the outboard motors outputs the “driving signal” or “non-driving state retaining signal” to its own electromagnetic hydraulic pressure control valve  37  based on the “speed changing gear shift signal” or “shift inhibition signal” and at the same time outputs the “driving signal” or “non-driving state retaining signal” to the electromagnetic hydraulic pressure control valves  37  of the other outboard motors via the common LAN. A switch between a driving state and a non-driving state is made in the electromagnetic hydraulic pressure control valve  37  of each of the outboard motors based on the “driving signal” or “non-driving state retaining signal” most recently sent. Thereby, the upper transmission section  310  of each of the plurality of the outboard motors is shifted to the low speed position or inhibited from shifting in a synchronized manner. Further, in this case, an ECU other than the ECU  34  controlling the engine may be provided in the outboard motor. The maps may be stored in the ECU. The control signals may be output from the ECU. 
     As described above, when timings for shifts and shift inhibition are synchronized among all the outboard motors, the control portion  52  of the control lever section  5  outputs the “speed changing gear shift signal” or “shift inhibition signal” if any of the following conditions is satisfied. The control portion  52  outputs the “speed changing gear shift signal” or “shift inhibition signal” if an operating state of at least any one of the plurality of outboard motors satisfies a condition for a shift or shift inhibition or if the operating state of a particular outboard motor among the plurality of the outboard motors satisfies the condition for a shift or shift inhibition. 
     In the above preferred embodiments, descriptions are made about a case when the shift control map and the shift inhibition control map are stored in the memory portion  51  included in the control lever section  5  and the control signals for making the transmission mechanism  33  shift the reduction ratios is output from the control portion  52  included in the control lever section  5 . However, the present invention is not limited to this case. The shift control map and the shift inhibition control map may be stored in the ECU  34  provided in the outboard motor, for example. In this case, the control signals may be output from the ECU  34  in which the shift control map and the shift inhibition control map are stored. 
     In the above preferred embodiments, descriptions are made about a case when the shift between forward, neutral, and reverse is made by the lower transmission section  330  electrically controlled by the ECU  34 . However, the present invention is not limited to this case. For example, the shift between forward, neutral, and reverse may be made by a forward-reverse switching mechanism defined with a pair of bevel gear and dog clutch as disclosed in JP-A-Hei 9-263294. 
     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.