Patent Publication Number: US-7214164-B2

Title: Shift operation control system

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a shift operation control system for outboard motors including a forward/reverse-switching mechanism that switches between a forward position, a neutral position, and a reverse position in response to the operation of a shift operation lever. 
   2. Description of the Related Art 
   Conventionally, the outboard motors are equipped with a forward/reverse-switching mechanism disposed between a drive shaft connected (or gear-connected) to the crankshaft of an engine and a propeller shaft as the rotating shaft of a propeller, for switching between the forward position, the neutral position, and the reverse position. 
   The forward/reverse-switching mechanism is comprised of a drive gear fixed to the lower end of the drive shaft, a forward gear and a reverse gear rotatably disposed on the propeller shaft, which are meshed with the drive gear, and a dog clutch disposed between the forward gear and the reverse gear, for being shifted to one of the neutral position, the forward position, and the reverse position, whereby the shift position is switched between the neutral position, the forward position, and the reverse position. 
   In this type of forward/reverse-switching mechanism, when a shift operation is carried out from a state in which one of the forward gear and the reverse gear is meshed with the dog clutch, the dog clutch is sometimes difficult to pull off and remains meshed with the gear. To overcome this problem, there has been proposed a system for facilitating the shift operation, in which load applied to the shift operation lever (a force for operating the shift operation lever or a pulling force) during the shift operation is detected, and when the detected load is not lower than a predetermined load (load setting value), the output from the associated engine (engine output) is reduced by turning ignition off (misfiring control) or the like (see e.g. Japanese Laid-Open Patent Publications (Kokai) No. S63-137098, S63-195094, H01-182196, and H02-216391). 
   Further, when the shift operation is carried out on the onboard motor to switch from the neutral position to the forward position or the reverse position, the dog clutch instantaneously connects between a rotating part of the engine and a gear part of the propeller, which generates a substantial shock (impact) when the gear part of the propeller is in stoppage. Further, immediately after the shift operation, a hull in which the outboard motor is installed receives shocks (vibrations) generated due to the propeller thrust. To eliminate these inconveniences, there has been proposed a method in which when the engine is idling and the shift operation lever is detected to have left the neutral position, the engine speed is controlled to be reduced, and when a shift operation is detected, the control for reducing the engine speed is cancelled (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2001-152897). 
   However, in the case of a conventional control method used in the system described above, in which the load acting on the shift operation lever during operation thereof is detected, and when the detected load is not lower than the predetermined load, the engine output is reduced, it is difficult to accurately detect the load acting on the shift operation lever due to the influence of the magnitude and stability (torque variation) of drive torque of the engine of the outboard motor, and practically difficult to set the load setting value. 
   Further, according to this conventional control method, the engine output (drive torque) is not reduced until after the load on the shift operation lever (i.e. the magnitude of a force for operating the shift operation lever) has reached the predetermined value during the shift operation, and therefore the force required for the shift operation becomes larger than the predetermined value before the drive torque is actually reduced, which makes it impossible to easily operate the shift operation lever. 
   On the other hand, to reduce the shock (shift shock) occurring during the shift operation, it is essential to quickly detect the shift operation. If a switch for detecting the shift operation is configured to detect the shift lever leaving from the neutral position in earlier timing, a shift operation not according to the operator&#39;s intention to carry out the shift operation lever is detected, and therefore an erroneous detection tends to occur. Conversely, if the switch is configured to detect the shift lever leaving from the neutral position in later timing, the detection of the shift operation becomes too late to reduce the shift shock sufficiently. 
   Further, in the conventional method of lowering the engine speed upon detection of the shift operation, a state of operation or position of the shift operation lever after the shift operation is carried out cannot be detected, and therefore, if the shift operation lever is stopped at a position which is neither the neutral position nor the forward or reverse position, the engine speed reduction control continues to be carried out. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a shift operation control system for outboard motors, which is capable of reducing load acting on a shift operation lever during a shift operation and a shock occurring during the shift operation, to thereby facilitate the shift operation. 
   To attain the above object, the present invention provides a shift operation control system for an outboard motor including an engine, and a forward/reverse-switching mechanism having a forward gear, a reverse gear, a dog clutch, and a shift operation lever, wherein an output from the engine is selectively transmitted to the forward gear or the reverse gear, for driving the forward gear or the reverse gear, and the dog clutch is actuated by a shift operation using the shift operation lever to switch a shift position between a forward position in which the dog clutch is meshed with the forward gear, a reverse position in which the dog clutch is meshed with the reverse gear, and a neutral position in which the dog clutch is not meshed with either the forward gear or the reverse gear, comprising a shift operation-detecting device that continuously detects the shift operation by the shift operation lever, an engine output changing control device that changes the output from the engine, and a control device that is operable when an early stage of the shift operation is detected by the shift operation-detecting device, to carry out control for changing the output from the engine using the engine output changing control device to change the output from the engine and then cancel the control for changing the output from the engine. 
   With the arrangement of the shift operation control system according to the present invention, the early stage of the shift operation, i.e. a state just before the operating force applied by the operator to the shift operation lever is increased is detected, and the engine output is reduced when it is predicted that the operating force applied to the shift operation lever will be increased, whereby the load acting on the shift operation lever or a shock occurring during the shift operation is reduced, which facilitates the shift operation. 
   Preferably, the control device carries out the control for changing the output from the engine using the engine output changing control device when an early stage of the shift operation from the forward position to the neutral position or from the reverse position to the neutral position is detected by the shift operation-detecting device, and a speed of the engine upon the detection of the early stage is not lower than a predetermined engine speed value, and cancels the control for changing the output from the engine when switching of the shift position to the neutral position is detected by the shift operation-detecting device. 
   With this arrangement, the early stage of the shift operation from the forward position to the neutral position or from the reverse position to the neutral position is detected, and when the engine speed detected upon the detection of the early stage is not lower than a predetermined engine speed value, i.e. when it is predicted that the operating force applied to the shift operation lever will increase, the engine output is reduced, whereby the shift operation can be carried out with a force smaller than that required in the prior art, which makes it easy to perform the shift operation. 
   More preferably, the control device carries out the control for changing the output from the engine when the early stage of the shift operation is detected by the shift operation-detecting device, and a throttle opening of the engine is not larger than a predetermined opening value. 
   With this arrangement, if, upon detection of the early stage of the shift operation by the shift operation detecting device, the throttle opening is not smaller than a predetermined value, control is performed to change the engine output. As a result, it is possible to prevent an erroneous detection of the shift operation when an external force other than that for the shift operation acts on the shift operation lever e.g. when a rapid acceleration is carried out, a jump occurs during running of the boat, or on other occasions. 
   More preferably, the control device carries out the control for changing the output from the engine when the early stage of the shift operation is detected by the shift operation-detecting device, and intake negative pressure in the engine is not higher than a predetermined negative pressure value. 
   With this arrangement, if, upon detection of the early stage of the shift operation by the shift operation detecting device, the intake negative pressure is not higher than a predetermined value, control is performed to change the engine output, which makes it possible to avoid an increase in the force for operating the shift operation lever (shifting force) occurring when the follow-up of the engine speed is delayed with respect to a sudden opening or closing of a throttle at throttle snap. 
   More preferably, the control device determines that the shift operation is in the early stage when it is detected by the shift operation-detecting device that a shift position of the shift operation lever is in a vicinity of the forward position or the reverse position, and an amount of change in the detected shift position is not smaller than a predetermined change amount value. 
   With this arrangement, when the shift operation detecting device detects that the shift position is in the vicinity of the forward position or the reverse position, and at the same time, the amount of change in the detected shift position is not less than a predetermined value, the control device determines that the shift operation is in the early stage, so that it is possible to accurately and promptly determine whether or not the shift operation is being carried out, by preventing erroneous detection or delayed detection of the shift operation due to assemblage errors of the shift operation detecting device and variations in the output thereof. 
   More preferably, the engine output changing control device changes the output from the engine by carrying out ignition control based on a number of consecutive misfires or an ignition timing retardation value, the number of consecutive misfires or the ignition timing retardation value being set in advance on an engine speed region-by-engine speed region basis. 
   With this arrangement, the engine output changing control device changes the engine output by carrying out firing control based on the number of consecutive misfires or the ignition timing retardation value set on an engine speed region-by-engine speed region basis. As a result, the engine output can be reduced while preventing engine stalling under various conditions. 
   More preferably, the engine output changing control device sets an allowable slope of reduction in the speed of the engine per each predetermined number of times of ignition, and changes the number of consecutive misfires or the ignition timing retardation value when a slope of reduction in the speed of the engine exceeds the allowable slope set in advance. 
   With this arrangement, the engine output changing control device sets the allowable slope of reduction in the engine speed with respect to each predetermined number of times of ignition, and when the set allowable slope of reduction exceeds a predetermined value, the number of consecutive misfires or the ignition timing retardation value is changed. This makes it possible to ensure the advantageous effects of preventing engine stalling. 
   More preferably, the control device sets a lower limit of an allowable slope of reduction in the speed of the engine per each predetermined number of times of ignition, and cancels the control for changing the output from the engine when the speed of the engine exceeds the lower limit of the allowable slope of reduction in the speed of the engine. 
   With this arrangement, the engine output changing control device sets the lower limit of the allowable slope of reduction in the engine speed per each predetermined number of times of ignition, and when the slope of reduction in the engine speed exceeds the set lower limit value, the control for changing the engine output is canceled. This makes it possible to prevent engine stalling during changing of the engine output. 
   Preferably, the control device carries out the control for changing the output from the engine using the engine output changing control device when an early stage of the shift operation from the neutral position to the forward position or from the neutral position to the reverse position is detected by the shift operation-detecting device, and cancels the control for changing the output from the engine after a predetermined time period elapses after completion of the control for changing the output from the engine. 
   With this arrangement, when the shift operation detecting device detects the early stage of the shift operation from the neutral position to the forward position or from the neutral position to the reverse position is detected, control is performed to change the engine output, and after the lapse of the predetermined time period, the control for changing the engine output is canceled. This makes it possible to reduce a shock at the time of shift operation and hence carry out the shift operation with ease. 
   More preferably, the control device determines that the shift operation is in the early stage when it is detected by the shift operation-detecting device that a shift position of the shift operation lever is in a vicinity of the neutral position, and an amount of change in the detected shift position exceeds a predetermined change amount value set for the neutral position. 
   With this arrangement, when the shift position detected by the shift operation detecting device is in the vicinity of the neutral position, and at the same time the amount of change in the detected shift position is not smaller than the predetermined value and exceeds the predetermined value set for the neutral position, the control device determines that the shift operation is in its early stage. As a result, it is possible to promptly and accurately determine whether a shift operation is being carried out the shift operation by preventing erroneous detection or delayed detection of the shift operation due to assemblage errors in the shift operation detecting device or variations in the output from the shift operation detecting device. 
   Preferably, the shift operation-detecting device includes a learning device that detects a change in an output value from the shift operation-detecting device, the output value being produced when the shift position is the neutral position, the learning device being operable when the output value is changed, to learn the changed output value. 
   With this arrangement, when the shift operation detecting device detects a change in the engine output value at the neutral position, the engine output value after the change is learned by the learning device. As a result, even if the engine output value at the neutral position is changed due to aging or the like, it is possible to promptly and accurately detect the shift operation. 
   More preferably, the shift operation-detecting device includes a determining device that determines a shift operation speed from an amount of change in the detected shift position, and the control device carries out the control for changing the output from the engine when it is determined by the determining device that the shift operation speed is not higher than a predetermined value. 
   With this arrangement, the determining device of the shift operation detecting device determines the shift operation speed from the amount of change in the detected shift position, and the control device changes the engine output when the shift operation speed is not lower than the predetermined value (predetermined output change amount). This makes it possible to prevent a stubble or unstable engine rotation by limiting the engine output change control at rapid acceleration. 
   More preferably, the control device includes a determining device that determines a speed of a boat in which the outboard motor is installed, based on the speed of the engine detected when the shift operation from the forward position to the neutral position is detected, and a duration over which the neutral position has been maintained, and the control device carries out the control for changing the output from the engine when it is determined by the determining device that the speed of the boat is not higher than a predetermined value. 
   With this arrangement, the control device determines the running speed of the boat from the engine speed during the shift operation from the forward position to the neutral position and the duration over which the neutral position has been maintained, and if the running speed of the boat is not higher than the predetermined value, the engine output is changed. This makes it possible to reduce a shock occurring during the shift operation and facilitate the shift operation. 
   More preferably, the engine output changing control device includes an intake air amount control device that controls an amount of intake air supplied to the engine, and the engine output changing control device changes the output from the engine by misfiring control or ignition timing control, while causing the intake air amount control device to increase the amount of intake air. 
   With this arrangement, the intake air amount control device of the engine output changing control device controls the amount of intake air supplied to the engine, and the engine output is changed through misfiring or ignition timing retardation, and through increase of the intake air amount by the intake air amount control device. As a result, it is possible to promptly reduce the engine output while preventing engine stalling even under various conditions. 
   More preferably, the control device immediately cancels the control for changing the output from the engine when predetermined conditions for canceling the control for changing the output from the engine are satisfied before the predetermined time period elapses. 
   With this arrangement, when the predetermined conditions for canceling the engine output change control are satisfied before the lapse of the predetermined time period, the control device immediately cancels the control for changing the engine output. As a result, it is possible to promptly reduce the engine output while preventing engine stalling even under various conditions. 
   Preferably, the forward/reverse-switching mechanism has a first-actuated driving part inside the outboard motor, and the shift operation-detecting device is disposed at the first-actuated driving part. 
   With this arrangement, the shift operation detecting device is disposed at the first-actuated driving part of the forward/reverse switching mechanism inside the outboard motor. As a result, it is possible to accurately detect the shift operation by the operator without being influenced by a play or hysteresis of the forward/reverse-switching mechanism. 
   Preferably, the outboard motor has a remote control box for remotely operating the forward/reverse-switching mechanism inside the outboard motor, and the shift operation lever has a drive part disposed in the remote control box, and the shift operation-detecting device is disposed in the drive part of the shift operation lever. 
   With this arrangement, the shift operation detecting device is disposed close to the driving part of the shift operation lever disposed in the remote control box for remotely controlling the forward/reverse-switching mechanism of the outboard motor. This makes it possible to detect the shift operation by the operator in earlier timing with higher accuracy without being influenced by a play or hysteresis of lots of linkages disposed between the remote control box and the outboard motor. 
   The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view, partly in longitudinal cross-section, showing the construction of an outboard motor to which is applied a shift operation control system according to a first embodiment of the present invention; 
       FIG. 2  is a cross-sectional view of component parts inside an engine cover of the outboard motor in  FIG. 1 ; 
       FIG. 3  is a fragmentary enlarged view taken on line A—A in  FIG. 2 ; 
       FIG. 4  is an enlarged view showing a shift position detector and its vicinity in  FIG. 2 ; 
       FIG. 5  is a diagram showing the relationship between an output from the shift position detector and the shift position; 
       FIGS. 6A to 6C  are diagrams showing output waveforms from the shift position detector, in which 
       FIG. 6A  shows the case where load acting on a shift operation lever is low; 
       FIG. 6B  shows the case where the load acting on the shift operation lever is medium; and 
       FIG. 6C  shows the case where the load acting on the shift position lever is high; 
       FIG. 7  is a schematic block diagram showing the arrangement of a control system of the outboard motor in  FIG. 1 ; 
       FIG. 8  is a flowchart showing an engine output reduction control process for the outboard motor in  FIG. 1 ; 
       FIG. 9  is a diagram useful in explaining a method of detecting a shift operation based on the output from the shift position detector; 
       FIG. 10  is a diagram useful in explaining another method of detecting a shift operation based on the output from the shift position detector; 
     FIGS.  11 A and  11 B 4  are timing diagrams useful in explaining a method of controlling ignition during the engine output reduction control process, in which: 
       FIG. 11A  shows changes in the number of consecutive misfires made according to the reduction of the engine speed; and 
       FIG. 11B  shows changes in the number of consecutive misfires made according to a slope of reduction in the engine speed; 
       FIGS. 12A and 12B  are diagrams useful in explaining timing of the engine output reduction control (execution timing thereof), in which: 
       FIG. 12A  is a diagram showing an example of timing in which the engine output reduction control is carried out when a shift operation is detected; and 
       FIG. 12B  is a diagram showing an example of the output from the shift position detector when an erroneous operation is carried out; 
       FIG. 13  is a schematic diagram showing another example of the disposition of the shift position detector; 
       FIGS. 14A to 14C  are schematic diagrams showing another example of the disposition of the shift position detector, in which: 
       FIG. 14A  is a top view of the shift position detector and its vicinity; 
       FIG. 14B  is a view showing a part of  FIG. 14A  in transverse cross-section; and 
       FIG. 14C  is a view showing another part of  FIG. 14A  in transverse cross-section; 
       FIGS. 15A and 15B  are fragmentary enlarged views of the shift position detector in  FIGS. 14A to 14C , in which: 
       FIG. 15A  is a fragmentary cross-sectional view taken on line B—B of  FIG. 14A ; and 
       FIG. 15B  is an enlarged view of a detection lever and its vicinity in  FIG. 14B ; 
       FIG. 16  is a flowchart showing an engine output reduction control process carried out by a shift operation control system according to a second embodiment of the present invention; 
       FIG. 17A  is a diagram useful in explaining a method of detecting a shift operation based on the output from the shift position detector; 
       FIG. 17B  is a diagram showing changes in the engine speed when the output from the shift position detector is changed; 
       FIG. 18  is a diagram useful in explaining the method of detecting a shift operation, employed by the shift operation control system according to the second embodiment; 
       FIG. 19  is a diagram useful in explaining another method of detecting a shift operation based on the output from the shift position detector; 
       FIG. 20  is a diagram useful in explaining timing of engine output reduction control carried out at rapid acceleration; 
       FIG. 21  is a diagram useful in explaining timing of the engine output reduction control (execution timing thereof) when an shift operation is detected; 
       FIG. 22A  is a diagram useful in explaining timing in which the engine output reduction control is carried out when the shift operation speed is high; 
       FIG. 22B  is a diagram useful in explaining timing in which the engine output reduction control is carried out when the shift operation speed is low; and 
       FIG. 23  is a diagram showing the procedure of a process in which the shift lever is switched from a position F to a position N when the boat is traveling at some speed. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention will now be described in detail below with reference to the drawings showing preferred embodiments thereof. 
     FIG. 1  is a fragmentary longitudinal cross-sectional view showing the construction of an outboard motor to which is applied a shift operation control system according to a first embodiment of the present invention. 
   As shown in  FIG. 1 , the outboard motor  1  includes an engine cover  2  which can be divided into an upper half and a lower half, a drive shaft housing  3 , and a gear housing  4 . Disposed under the engine cover  2  is an engine  5 , which is a water-cooled four-cycle six-cylinder V-type engine having a crankshaft, not shown, substantially perpendicularly (vertically) extending therein. 
   A drive shaft  6  connected (or gear-connected) to the crankshaft of the engine  5  extends from the lower end of the engine  5  through the drive shaft housing  3  into the gear housing  4  within which is disposed a forward/reverse switching mechanism  9 . 
   The forward/reverse switching mechanism  9  is comprised of a drive gear  9   a  implemented by a bevel gear fixed to the lower end of the drive shaft  6 , a forward gear  9   b  and a reverse gear  9   c  implemented by respective bevel gears rotatably disposed on a propeller shaft  8 , which are meshed with the drive gear  9   a , and a dog clutch  9   d  disposed between the forward gear  9   b  and the reverse gear  9   c,  for being shifted to one of the neutral position, the forward position, and the reverse position, whereby the shift position is switched between the neutral position, the forward position, and the reverse position. 
   A shift rod  10  extending parallel with the drive shaft  6  through the drive shaft housing  3  is pivotally movable to move the dog clutch  9   d  into mesh with either the forward gear  9   b  or the reverse gear  9   c,  or place the same in an intermediate position therebetween in which it is not meshed with either of them, whereby the transfer of torque from the drive shaft  6  to the propeller shaft  8  is permitted or inhibited. 
   The outboard motor  1  is horizontally pivotally supported by a swivel bracket  11 , with an upper part of the swivel bracket  11  being supported on a clamp bracket  13  for vertically pivotal movement via a tilt shaft  12 . When the clamp bracket  13  is removably fixed to a stern frame of a hull, not shown, the outboard motor  1  is mounted on the hull such that it can be pivoted horizontally (in a steering direction) and vertically (in a tilting direction). 
   Further, in front of the engine  5 , there is disposed an electrical equipment box  14  containing electrical control components including an ECM (Engine Control Module)  15 . 
     FIG. 2  is a transverse cross-sectional view of component parts inside the engine cover  2  of the outboard motor  1  in  FIG. 1 . 
   In  FIG. 2 , a remote control box  20  is disposed within the hull, and includes a shift operation lever  21  for carrying out a shift operation (switching operation) to switch between three positions forward position, neutral position, and reverse position. The shift operation lever  21  is connected to a shift lever  23  inside the outboard motor  1  via a remote control cable  22  comprised of inner and outer cables. The shift lever  23  is connected to a shift rod  10  via a plurality of shift operation links  25   a ,  25   b , and  25   c . A shift position detector  24  is disposed at an upper end of a pivot  23   a  of the shift lever  23 . 
     FIG. 3  is a fragmentary cross-sectional view showing the arrangement of a sequence of components from the shift lever  23  to the shift rod  10 . 
   In  FIG. 3 , when the operator operates the shift operation lever  21 , the shift lever  23  connected thereto via the remote control cable  22  is pivotally moved about its pivot  23   a , and the pivotal movement of the shift lever  23  in turn drives the shift operation links  25   a ,  25   b , and  25   c  connected to the shift lever  23  via the pivot  23   a  to thereby pivotally move the shift rod  10 . This causes movement of the dog clutch  9   d  of the forward/reverse-switching mechanism  9  to perform switching (shifting) between the three positions: forward position, neutral position, and reverse position. 
   A damper  25   d  implemented by a resilient member formed e.g. of rubber is interposed between connecting ends of the shift operation links  25   a  and  25   b . The damper  25  may be interposed between the shift operation links  25   b  and  25   c.    
   For example, when a shift operation from the forward (F) position to the neutral (N) position, or a shift operation in an opposite direction is performed, the gears are sometimes not instantly disengaged so that the movement of the shift rod  10  is stopped for a moment. In this case, due to the stoppage, the output value from the shift position detector  24  instantly stops changing to exhibit a flat waveform, which can cause a delay in the start of engine output change control. Further, the momentary stoppage of the shift rod makes the operator feel a sense of disorder or feel the shift operation lever  21  heavy to operate. To permit smooth pivotal movement of the shift lever  23  without being stopped, the above-mentioned damper  25   d  is provided. 
     FIG. 4  is an enlarged view showing the shift position detector  24  and its vicinity in  FIG. 2 . 
   As shown in  FIG. 4 , the shift position detector  24 , which is connected to the ECM  15  via a signal line, not shown, is disposed on the pivot  23   a  of the shift lever  23 . The shift position detector  24  is implemented by a rotary variable resistor which generates an output (voltage) corresponding to the rotational position of the shift lever  23 , as shown in  FIG. 5 , based on which the shift position is continuously detected, thereby detecting whether or not a shift operation is performed, shift operation timing, and a shift position. 
   The shift position detector  24  is disposed at a first-actuated rotary part (drive part) of a shift mechanism including the shift lever  23 , the shift operation links  25   a ,  25   b , and  25   c , and the forward/reverse-switching mechanism  9  inside the outboard motor  1 , and therefore it is suited or correctly detecting the shift operation of the operator in early timing. In addition, the pivot of the rotary variable resistor of the shift position detector  24  is directly connected to the upper end of the pivot  23   a  of the shift lever  23 , which enables designing of the shift mechanism compact in size using a small number of component parts and facilitates the assemblage, and further, the pivot of the rotary variable resistor is directed downward, which improves the waterproof reliability of the shift mechanism. Further, by coupling the pivot  23   a  to the pivot of the rotary variable resistor via a resin member or the like, it is possible to reduce vibrations that the shift position detector  24  receives from the pivot  23   a  of the shift lever  23 . 
     FIGS. 6A to 6C  are diagrams showing output waveforms from the shift position detector  24 , in which  FIG. 6A  shows the case where load acting on the shift operation lever  21  is low,  FIG. 6B  shows the case where the load is medium, and  FIG. 6C  shows the case where the load is high. 
   As shown in  FIGS. 6A to 6C , when the shift operation lever  21  is operated for a shift from the F position to the N position, the output from the shift position detector  24  is changed from a voltage C to a voltage B, and when the shift operation lever  21  is operated for a shift from the R position to the N position, the output from the shift position detector  24  is changed from a voltage A to the voltage B. 
   When the shift operation is performed from the F position to the N position at a low engine speed, as shown in  FIG. 6A , there occurs a slight delay Ta between timing (time point) T 1  in which the shift position detector  24  detects a shift operation and timing (time point) T 2  in which the dog clutch  9   d  is disengaged from the forward gear  9   b  to set the neutral position. However, load (operating force) applied to the shift operation lever  21  is small. 
   When the shift operation is performed from the F position to the N position at a medium engine speed, as shown in  FIG. 6B , there occurs a delay Tb between timing (time point) T 3  in which the shift position detector  24  detects a shift operation and timing (time point) T 4  in which the dog clutch  9   d  is disengaged from the forward gear  9   b  to set the neutral position. The delay Tb and the delay Ta are in the relationship of Tb&gt;Ta. In other words, the timing in which the dog clutch  9   d  is disengaged from the forward gear  9   b  to set the neutral position after detection of the shift operation is more delayed than in the case of the  FIG. 6A , and an increased force is applied to the shift operation lever  21  for operation thereof. 
   When the shift operation is performed from the F position to the N position at a high engine speed, as shown in  FIG. 6C , there occurs a delay Tc between timing (time point) T 5  in which the shift position detector  24  detects a shift operation and timing (time point) T 6  in which the dog clutch  9   d  is disengaged from the forward gear  9   b  to set the neutral position. The delay Tc and the delay Tb are in the relationship of Tc&gt;Tb. In other words, the timing in which the dog clutch  9   d  is disengaged from the forward gear  9   b  to set the neutral position after detection of the shift operation is much more delayed. 
   As described above, in spite of the shift position detector  24  detecting a substantial displacement (movement or rotation) of the shift operation lever  21 , the state continues in which the dog clutch  9   d  is not disengaged from the forward gear  9   b,  and the operator increases the operating force applied to the shift operation lever  21 , so that the operating force is accumulated in the play of component parts in a force transmission path from the remote control box  20  to the dog clutch  9   d,  such as the shift cable  22  (as expansion and deformation thereof). The instant the dog clutch  9   d  is disengaged from the forward gear  9   b,  the force accumulated moves the shift operation lever  21  beyond the neutral position, and thereafter, the operator, who felt the detent force and noticed the fact that the shift operation lever has been moved beyond the neutral position, operates the shift operation lever  21  back to the neutral position. In this state, the operator not only has to exert a very large force to move the shift operation lever  21 , but also perform an additional operation to bring the shift operation lever  21  from a position beyond the neutral position back to the neutral position. 
   The present invention eliminates the above-described inconvenience as follows: The drive torque (engine output) is not reduced after detecting that the operating force applied to the shift operation lever during a shift position reaches a predetermined value, but an engine output reduction (change) control process is carried out in which an early stage of the shift operation before the operator increases the operating force applied to the shift operation lever  21  is detected, and from the engine speed detected upon detection of the early stage of the shift operation, the engine output is reduced based on a prediction of the magnitude of the operating force to be applied to the shift operation lever  21 . This makes it possible to perform a shift operation with a much smaller force than in the prior art. 
     FIG. 7  is a schematic block diagram of a control system  30  of the outboard motor  1 . 
   As shown in  FIG. 7 , the control system  30  is comprised of a central processing unit (CPU)  31  including a RAM and a ROM, an EERPOM (Electrically Erasable and Programmable ROM)  32 , a communication interface (I/F)  33  connected to an external communication device  38 , an input circuit  34 , an output circuit  35 , an ignition device  36  connected to an external ignition coil, and a power supply circuit  37 . 
   Connected to the input circuit  34  are an external camshaft signal detector  39 , a crank angle signal detector (engine speed detector)  40 , a throttle opening detector. 41 , an intake pressure detector  42 , an atmospheric pressure detector  43 , an intake air temperature detector  44 , an engine temperature detector (coolant temperature detector)  45 , an engine tilt angle detector  46 , the shift position detector  24 , and a stop switch  47 . 
   Connected to the output circuit  35  are an external fuel injector  48 , an air amount regulator/actuator  49  comprised of a stepper motor and a solenoid valve, a display device  50  comprised of an LED, a beeper, a tachometer, and a trim meter, and a fuel pump  51 . 
   The CPU  31  of the control system  30  calculates the intake air amount based on detection signals from associated ones of the above-mentioned detectors, calculates an optimum fuel injection amount based on a value of the intake air amount obtained by subjecting the detected intake air amount to various corrections, and delivers a drive signal indicative of a duty ratio corresponding to the calculated fuel injection amount to the injector  48  via the output circuit  35 . This makes it possible to cause the injector  48  to inject the optimum amount of fuel suited to the calculated intake air amount by duty ratio control. 
     FIG. 8  is a flowchart showing an engine output reduction control process for the outboard motor  1 . This process is executed by the CPU  31  of the control system  30  based on a predetermined program stored in the ROM of the CPU  31 . 
   As shown in  FIG. 8 , first, the CPU  31  determines based on the output from the shift position detector  24  whether or not the shift operation lever  21  is in the F position or in the R position (i.e. whether the shift operation lever  21  is in a position other than the N position) (step S 100 ). 
   Next, when the shift operation lever  21  is in the F position or in the R position, the CPU  31  determines whether or riot the operator has performed a shift operation (step S 101 ). To reduce the operating force applied to the shift operation lever  21  when the shift operation lever  21  is shifted from the F position or the R position to the N position, it is of key importance to detect the shift operation intended by the operator in earlier timing and more accurately. To this end, not only the shift position of the shift operation lever  21  being shifted but also the amount of change in the output from the shift posit-on detector  24  occurring when the shift operation is performed is detected. 
   Due to assemblage errors of the shift position detector  24  and variations in tolerances of the associated component parts, it is difficult to promptly detect the shift position only from a determination of the position (absolute value) of the shift operation lever  21  being shifted. Further, in view of aging of the shift operation detector  24  and the associated component parts (change in the output due to wear of the component parts, variable resistor, and so forth), a correct determination cannot be carried out only from the absolute value of the shift position. 
   For example, as shown in  FIG. 10 , assuming that the output from the shift position detector  24  when the shift operation lever  21  is in the F position varies between an output C 1  and an output C 2  due to assemblage errors or the shift position detector  24  and variations in tolerances of the associated component parts, if the detection of a shift operation is carried out with reference to a predetermined absolute value c for shift operation determination, a delay of Δt in shift operation detection timing is caused by the output C 2  compared with the case of the output C 1 . In this case, in spite of a substantial change in the output from the shift position detector  24  (the operating force is accumulated in component parts in the force transmission path from the remote control box  20  to the dot clutch, such as the cables and links), the shift operation cannot be detected, and therefore the engine output change control function does not start to work, resulting in the operator being required to exert a very large operating force. Further, if the shift operation is detected with reference to another absolute value c′, the output C 1  is erroneously detected as indicating the N position. 
   To overcome the above problem, a comparison is made between the present value of the output from the shift position detector  24  and a value of the same detected a predetermined time period earlier. More specifically, the output from the shift position detector  24  is measured at short sampling time intervals (e.g. 10 ms), and an amount of change (Va−Vb) between an output voltage Vb in the present timing (time point) Tb and an output voltage Va in timing (time point) Ta earlier than the present timing Tb by a predetermined time period (e.g. 200 ms) is calculated, and when the amount of change in the output is not smaller than a predetermined value A (e.g. 100 mV), it is determined that a shift operation has been performed. This prevents a momentary change in the output caused by noise or some external force (impact) or the like applied on the shift position detector  24  from being erroneously detected as a shift operation having been performed. 
   Further, the control system  30  may be provided with a learning function, i.e. the capability of learning the output value from the shift position detector  24  when the shift operation lever  21  is in the F position or in the R position, and when the output from the shift position detector  24  is changed from the learned value by not less than a predetermined value Vc, it may be determined that a shift operation has been performed. As the method of learning the output value, when the same output value is detected a predetermined number of times when the shift operation lever  21  is in the F position, the output value is learned as the learned value for the F position. For example, as shown in  FIG. 10 , the output voltage C 2  from the shift position detector  24  is set to the learned value, and when the output voltage C 2  from the shift position detector  24  is changed by not less than the predetermined value Vc, it is determined that a shift operation has been performed. Even when the value of the output voltage from the shift position detector  24  when in the F position is changed to the output voltage C 1  due to aging or the like, the output voltage C 1  is set to the learned value by the learning function, it is possible to detect a shift operation with accuracy. 
   Further, the output from the shift position detector  24  when the shift operation lever  21  is in the F position or the R position is constantly measured at very short sampling time intervals, and when the measured value is changed by an amount not less than the predetermined value Vc, it may be determined that a shift operation has been performed. For example, as shown in  FIG. 10 , the output voltage C 2  from the shift position detector  24  may be measured as a normal output at predetermined time intervals and stored, and when the output voltage C 2  from the shift position detector  24  is changed by not less than the predetermined value Vc, it may be determined that a shift operation has been performed. In this case, even when the normal output from the shift position detector  24  is changed to the output voltage C 1  due to aging or the like, it is possible to accurately detect the shift operation with accuracy. 
   As described above, by constantly determining the present shift position of the shift operation lever  21  based on the output from the shift position detector  24 , and constantly detecting the amount of change in the output from the shift position detector  24 , it is possible to detect a shift operation as soon and accurately as possible. 
   Next, it is determined whether or not the throttle opening is not larger than a predetermined opening (small opening) (step S 102 ). 
   In the case of outboard motors, normally, a part of the outboard motor mounted on a fixed mounting part (clamp bracket or the like) or the stern of a hull is displaced with respect to the fixed mounting part due to propeller thrust of the outboard motor at rapid acceleration. Further, a fixed part that fixes in position a throttle cable for opening and closing the throttle and a fixed part that fixes in position a shift cable for performing the shift operation are disposed close to each other, and when the throttle is rapidly fully opened, these fixed parts are sometimes displaced by slight amounts. In such a case or when an external force is applied to the fixed parts due to a jump of the boat during running, the shift position detector  24  detects the displacement to erroneously detect that a shift operation has been performed, though no actual shift operation has been performed. 
   To overcome the problem of such an erroneous detection, in the present embodiment, the engine output reduction control process is carried out only when the throttle opening is not larger than the predetermined opening (small opening). 
   In a general outboard motor, a throttle and shift operation mechanism is provided in which a remote control box is used to perform the throttle operation and the shift operation by a single common shift operation lever. Specifically, by moving the shift operation lever forward from the neutral position, the shift position is shifted to the forward position, and then, as the shift operation lever is tilted forward from the forward position, the throttle can be operated from a fully closed position to a wide open position. 
   Conversely, by moving the shift operation lever rearward from the neutral position, the shift position is shifted to the reverse position, and then, as the shift operation lever is tilted rearward from the reverse position, the throttle can be operated from the fully closed position to a medium position. Due to this structure, in the case of outboard motors, when a shift operation is performed, the throttle is substantially fully closed irrespective of whether the engine speed is high or low. Therefore, by carrying out the engine output reduction control process only when the throttle opening is not larger than the predetermined opening, it is possible to eliminate the inconvenience of erroneously detecting a displacement occurring at rapid acceleration as a shift operation. 
   Next, the engine speed is detected, and it is determined whether or not it is necessary to carry out the engine output reduction control process (step S 103  in  FIG. 8 ). This is because when the engine speed is not higher than a predetermined value the drive torque of the engine is also low, and therefore the engine output reduction control process reed not be carried out. 
   Then, it is determined whether or not intake negative pressure is not higher than a predetermined negative value (step S 104 ). That is, if the throttle is rapidly closed for rapid deceleration when the engine is in a steady operating condition in a medium-to-high engine speed region, the engine speed largely drops even without execution of the engine output reduction control, and hence it is easy to perform the shift operation. Further, execution of the engine output reduction control under such a condition of the engine may cause engine stalling, and therefore there is no need to carry out the engine output reduction control process. 
   On the other hand, at a throttle snap in which the throttle is rapidly opened and then rapidly closed (rapid acceleration→rapid deceleration) or the like, the engine speed cannot immediately follow up the rapid opening and closing of the throttle, so that without execution of the engine output reduction control process, it is necessary to apply a large operating force for performing the shift operation immediately after such a throttle snap. 
   To eliminate this inconvenience, in the present embodiment, the intake pressure is detected when a shift operation is detected to thereby distinguish between a rapid deceleration from a steady operating condition in a medium-to-high engine speed region and a rapid deceleration caused by a throttle snap. 
   When the throttle is rapidly closed from the medium-to-high engine speed region, even after the throttle valve is closed to reduce the intake air amount, the engine speed progressively lowers while maintaining the engine speed to some level, so that the intake negative pressure becomes very large. Therefore, by detecting the intake pressure upon detection of a shift operation, it is possible to distinguish between a rapid deceleration from a steady operating condition in a medium-to-high engine speed region and a rapid deceleration caused by a throttle snap. 
   As another embodiment, it is also possible to determine whether or not the engine output reduction control should be executed, by determining the engine operating condition before the throttle is rapidly closed, e.g. whether or not the operation of the engine in a medium-to-high engine speed region has continued for not less than a predetermined time period. 
   Next, in a step S 105  in  FIG. 8 , the engine output reduction control process, i.e. the engine output reduction control through misfiring control is carried out. 
   After detection of the shift operation, it is necessary to decrease the engine speed as soon as possible insofar as engine stalling is prevented from occurring. The demand for rapidly reducing the engine speed is contradictory to the requirement of prevention of engine stalling, and in addition, in the case of the outboard motor  1 , the hull in which the outboard motor  1  is installed and the propeller which is mounted on the outboard motor  1  can vary in construction, and therefore it is necessary to control the engine speed to an optimum value dependent on the hull (size, weight, and loadage of the boat) in which the outboard motor  1  is installed. 
   Therefore, in reducing the engine output, it is effective to carry out the misfiring control (interruption of an ignition signal) starting with a cylinder to be ignited immediately after detection of the shift operation, instead of specifying in advance cylinders to be misfired, of the engine  5 . The method of reducing the engine output is not limited to the misfiring control, but it is also possible to control the fuel injection amount or the intake air amount (an intake air bypass amount) to thereby reduce the drive torque of the engine. However, in the control of the fuel injection amount and the intake air amount, the changed fuel injection amount or the changed intake air amount does not take effect to reduce the drive force only few combustion cycles until after the start of the control, when the changed amount of fuel or intake air enters the combustion chamber. In contrast, the ignition control (misfiring control or ignition timing retardation control) is very effective since the drive force (torque) starts to be reduced immediately after the start of the control. 
   According to the present engine output reduction control, when the shift operation is detected, the engine speed is detected by the crank angle signal detector  40 , and the misfiring control (ignition signal interruption) is carried out depending on the engine speed detected at the time. 
   
     
       
         
             
             
           
             
                 
               TABLE 1 
             
           
          
             
                 
                 
             
             
                 
               Engine Speed Region (rpm) 
             
          
         
         
             
             
             
             
             
             
             
          
             
                 
               N1 
               N2 
               N3 
               N4 
               N5 
               N6 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
             
             
          
             
               Number of Consecutive Misfires 
               A 
               B 
               C 
               D 
               E 
               F 
             
             
                 
             
          
         
       
     
   
   For example, assuming that the engine speed detected upon detection of the shift operation is 2800 rpm which belongs to an N 5  region of the engine speed, the misfiring control is immediately carried out by setting the number of consecutive misfires to a number E. For example, when E= 6  holds, as shown in  FIG. 11A , the misfiring control is started with the ignition timing of the next cylinder to be ignited (( 1 ) in  FIG. 11A ), and misfire is caused six consecutive times (# 5 →# 4 →# 3 →# 2 →# 1 →# 6 ), and then the next cylinder (# 5 ) is ignited. Thereafter, six consecutive misfires are carried out in the same manner. This misfiring control reduces the engine speed, and when the engine speed is in an N 4  engine speed region in Table 1, the number of consecutive misfires is changed to a number D (e.g. D=3), to execution of three consecutive misfires. The settings in Table 1 are configured depending on the types of the engine  5  and the propeller  7  which are mounted on the outboard motor  1 , and the hull in which the outboard motor  1  is installed. 
   On the other hand, when the engine speed is rapidly reduced during the consecutive misfiring control, engine stalling can occur. Therefore, an allowable slope of reduction in the engine speed (an amount of drop in the engine speed per ignition) as shown in Table 2 is set on an engine speed region-by-engine speed region basis, and when the detected slope of reduction in the engine speed exceeds the set allowable value (predetermined value), the number of consecutive misfires is automatically changed. 
   
     
       
         
             
             
           
             
                 
               TABLE 2 
             
           
          
             
                 
                 
             
             
                 
               Engine Speed Region (rpm) 
             
          
         
         
             
             
             
             
             
             
             
          
             
                 
               N1 
               N2 
               N3 
               N4 
               N5 
               N6 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
             
             
          
             
               Predetermined Values of 
               a 
               b 
               c 
               d 
               e 
               f 
             
             
               Deceleration for Changing 
             
             
               Number of Misfires 
             
             
                 
             
          
         
       
     
   
   The CPU  31  calculates a degree of deceleration (slope of reduction in the engine speed) during the misfiring control, and when the calculated value exceeds a predetermined value (“a” to “f”) in Table 2, the number of consecutive misfires is automatically changed from X to (X−1) For example, as shown in  FIG. 11B , when the degree of deceleration exceeds a predetermined value “e” when six consecutive misfires are being executed with the engine speed being in the N 5  region, more specifically, when the engine speed drops by not less than 300 rpm during one ignition (misfire), the setting of the consecutive misfiring is changed from the six consecutive misfires to ( 6 - 1 ) i.e. five consecutive misfires. If the degree of deceleration still exceeds the predetermined value “e”, the setting is further changed to ( 5 - 1 ) consecutive misfires. It is also possible to change the setting to (X−n (n: 2, 3, . . . )). The settings in Table 2 are configured depending on the types of the engine  5  installed in the outboard motor  1  and the like, similarly to Table 1. 
   Further, a lower limit value of the slope of reduction in the engine speed is set on an engine speed region-by-engine speed region basis, as shown in Table 3, and when the detected slope exceeds the lower limit value, the engine speed reduction control is canceled. 
   
     
       
         
             
             
           
             
                 
               TABLE 3 
             
           
          
             
                 
                 
             
             
                 
               Engine Speed Region (rpm) 
             
          
         
         
             
             
             
             
             
             
             
          
             
                 
               N1 
               N2 
               N3 
               N4 
               N5 
               N6 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
             
             
          
             
               Lower Limit Values of 
               g 
               h 
               i 
               j 
               k 
               l 
             
             
               Deceleration for Cancelling 
             
             
               Misfiring Control 
             
             
                 
             
          
         
       
     
   
   The CPU  31  calculates the degree of deceleration during the misfiring control, and when the calculated value exceeds the lower limit value (“g” to “l”) in Table 3, the engine output reduction control is canceled. For example, when the degree of deceleration exceeds a predetermined value “k” when the six consecutive misfires are being executed with the engine speed being in the N 5  region, more specifically, when the engine speed drops by not less than 500 rpm during one iginition (misfire), it is judged that engine stalling will occur, so that the engine output reduction control is immediately canceled (step S 107 , referred to hereinafter). The settings in Table 3 are set depending on the types of the engine  5  installed in the outboard motor  1  and the like, similarly to Tables 1 and 2. 
   Although in the embodiment described above, during the engine output reduction control, the consecutive misfiring control is carried out based on the settings in Tables 1 to 3, the method of reducing the engine output is not limited to the misfiring control, but the engine output may be reduced by changing the ignition timing to a predetermined retardation value which is set on an engine speed region-by-engine speed region basis. 
   By the way, when the shift operation is detected, if the engine speed is low (e.g. the engine speed is in the N 1  region), the engine output is small, and the required shift operating force is also small, so that it is not necessary to carry out the misfiring control consecutively a plurality of times. In this low engine speed region, to improve the shift feeling instead of reducing the shift operating force, timing is predicted in which the dog clutch  9   d  is disengaged from the forward gear  9   b  or the reverse gear  9   c  to set the neutral position, and based on the prediction, misfiring control or ignition timing retardation control is carried out. 
   In this respect, in the present embodiment, the timing in which the engine output reduction control is executed is varied depending on whether the engine speed is high or low (trawling). More specifically, when the engine speed is high, the shift operation is detected as soon and accurately possible, and the torque (engine output) is reduced as soon as possible so as to reduce the engine speed. In contrast, when the engine speed is low, since the danger of engine stalling increases if the misfiring control is consecutively executed a plurality of times, so that the timing in which one-time misfiring or ignition timing retardation is executed is made coincident with the timing in which the dog clutch  9   d  is disengaged from the forward gear  9   b  or the reverse gear  9   c  to set the neutral position (misfiring or ignition timing retardation is carried out immediately before the shift position is set to the neutral position). 
   Therefore, according to the present embodiment, when the engine speed is low, the timing in which the dog clutch  9   d  is disengaged to set the neutral position is predicted (calculated) from a ratio of change in the output (slope of change in the output) from the shift position detector  24 . 
     FIGS. 12A and 12B  are diagrams useful in explaining timing of the engine output reduction control (execution timing thereof), in which  FIG. 12A  is a diagram showing an example of timing in which the engine output reduction control is carried out when a shift operation is detected, and  FIG. 12B  is a diagram showing an example of the output from the shift position detector when an erroneous operation is carried out. 
   In  FIG. 12A , reference symbol A designates timing in which the shift position detector  24  detects the shift operation, and reference symbol D designates timing in which the dog clutch  9   d  is actually disengaged from the forward gear  9   b  to set the neutral position. 
   Reference symbol “a” designates a change in the engine speed occurring when the engine output reduction control is executed immediately after the shift operation is detected by the shirt position detector  24 . In this case, only after the engine output is reduced to reduce the engine speed and then the engine speed is recovered by termination of the engine output reduction control, the dog clutch  9   d  is disengaged from the forward gear  9   b  to set the neutral position. Therefore, the shift operating force for the shifting operation cannot be reduced. 
   In contrast, reference symbol “c” designates a change in the engine speed occurring when the timing D is calculated (predicted) in which the dog clutch  9   d  is to be disengaged from the forward gear  9   b  to set the neutral position, based on the ratio of change in the output from the shift position detector  24  from the timing A to the timing D, and the engine output reduction control starts to be carried out in timing C. In this case, in timing in which the dog clutch  9   d  is disengaged from the forward gear  9   b  to set the neutral position, the engine output reduction control process is carried out to reduce the engine speed, so that the shift operating force for the shift operation can be reduced. 
   Further, when the engine speed is low, a sufficient time margin is allowed after the timing A in which the shift operation is first detected and before the timing C in which the engine output reduction control is carried out, so that whether the shift operation is being continued is determined even after the shift operation is detected through calculation from the ratio of change in the output from the shift position detector  24 , to thereby increase the accuracy of detection of the shift operation, and from the ratio of change, timing in which the dog clutch  9   d  is to be disengaged from the forward gear  9   b  to set the neutral position is predicted (calculated). This control makes it possible to prevent erroneous control from being carried out when a slight displacement of the shift operation lever  21  not intended by the operator who grips the shift operation lever  21  (throttle and shift lever) is detected (e.g. as shown in  FIG. 12B ). 
   On the other hand, when the engine speed is in a low speed region, the shift operation lever  21  of the remote control box  20  is in the vicinity of a fully closed throttle position and hence in a region permitting a shift operation, and therefore the output from the shift operation detector  24  changes due to a slight displacement of the shift operation lever  21 . Therefore, although the motion of the operator who grips the shift operation lever  21  (a motion of the operator when he does not intend to carry out the shift operation) can be erroneously detected as the shift operation, only a displacement of the shift operation lever  21  continuously occurring over a predetermined time period is determined as the shift operation through the above control, which makes it possible to detect a displacement of the shift operation lever  21  intended by the operator for performing the shift operation in a manner discriminated from the displacement of the same not intended by operator. 
   Among shift operations at low engine speed, a shift operation for shifting from the F position to the N position, and further to the R position is carried out by the operator with a positive intention, so that the shift operating force for performing the shift operating force is strong and hence the ratio of change in the output from the shift position detector  24  is large. 
   When the engine speed is in a medium speed region, the shift operation lever  21  of the remote control box  20  is in a position corresponding to the throttle being open, and the remote control box  20  is configured such that in the above state of the throttle, the shift operation lever  21  is immovable. Therefore, the motion of the operator holding the shift operation lever  21  is not transmitted to the shift position detector  24 . 
   Next, when the shift position detector  24  detects that the shift position has been switched to the N position (YES to the step S 106  in  FIG. 8 ), the engine output reduction control is canceled. Whether or not the shift position is the N position can be determined by providing a predetermined threshold value E, as shown in  FIG. 12A . However, the shift position can vary due to assemblage errors and the tolerance of component parts, and further, the position where the shift position is switched to the N position can vary even with the same engine, depending on the shift operating force applied to the shift operation lever for performing the shift operation, and therefore the most accurate detection can be carried out when the shift position is at a point (timing D in  FIG. 12A ) where the amount of change in the output from the shift position detector  24  largely changes. This point corresponds to the moment where the dog clutch  9   d  is disengaged from the forward gear  9   b  to set the neutral position, and the shift operation lever  21  is suddenly pivotally moved. 
   Further, when the slope of reduction in the engine speed exceeds the lower limit value (YES to the step S 107 ), the engine output reduction control is immediately canceled. Further, even when the engine speed drops below the predetermined value (YES to the step S 108 ), the engine output reduction control is canceled. 
   When the slope of change in the output from the shift position detector  24  is inverted, it is judged that the shift operation has been canceled midway during the shift operation (YES to the step S 109 ), and the engine output reduction control is canceled. 
   Further, in anticipation for the case where the conditions for canceling the control are not satisfied due to failure of the shift operation detector  24 , the crank angle signal detector  40 , etc., a predetermined upper limit control time period is provided. Normally, the shift operation is completed in a short time period (Δt seconds), and therefore, if the control continues longer than such a short time period, it is judged that there exists some abnormality, and the engine output reduction control is forcibly canceled before engine stalling occurs (YES to the step S 110 ). For example, the above predetermined upper limit control time period is set to about three times longer (3Δt seconds) than the time period over which the shift operation is normally carried out. 
   Although in the above-described embodiment, the shift position detector  24  is disposed at the first-actuated rotary part of the shift mechanism, it may be disposed at any location insofar as the location is between the remote control box  20  and the dog clutch  9   d  in the outboard motor  1 . However, between the remote control box  20  and the dog clutch  9   d,  the remote control cable  22  and a lot of linkages are disposed, so that due to the presence of a play and hysteresis of each of these component parts, it is desirable to dispose the shift operation detector  24  at a location closer to the operator, i.e. the remote control box  20 . 
     FIG. 13  is a schematic diagram showing another example of the disposition of the shift position detector  24 . 
   As shown in  FIG. 13 , the shift position detector  24  is disposed such that the pivot of the rotary variable resistor  24  is directly coupled to the pivot  21   a  of the shift operation lever  21 . This makes it possible to detect the shift operation earlier and more accurately. Further, as described hereinabove, the shift operation lever  21  of the remote control box  20  is also adapted to carry out operations for opening and closing the throttle, providing the merit that not only the shift operation but also the throttle opening can be detected at the same time. 
     FIGS. 14A to 14C  and  FIGS. 15A and 15B  are schematic diagrams showing still another example of the disposition of the shift position detector  24 . 
   In  FIGS. 14A to 14C  and  FIGS. 15A and 15B , reference numeral  23   a  designates a shift lever pivot,  61  a forward/reverse switching mechanism-forming bracket,  62  a detection lever-pivoting protrusion fixed to the shift lever  23 ,  63  a detection lever,  64  a detent plate as a wave-shaped plate for generation of a click at each shift position,  65  a ball and spring-contained component part for setting the click in the shifting operation, and  66  a detection lever fixing plate. 
   The detection lever  63  is fixed to the variable resistor pivot within the shift position detector  24 , and is urged against the shift lever-pivoting protrusion  62  in an anticlockwise direction by a spring mechanism in the shift position detector  24 , as indicated by the arrow X in  FIG. 15B . With this arrangement, the detection lever-pivoting protrusion  62  which pivotally moves together with the shift lever  23  pivotally moves the detection lever  63  about the variable resistor pivot, whereby the shift operation can be detected. 
   A contact surface  63   a  of the detection lever  63  in contact with the detection lever-pivoting protrusion  62  is formed as a curved surface, which is advantageous in that the detection lever  63  can be designed in the amount of rotation (angle) with some freedom with respect to the amount of rotation (angle) of the shift lever  23 . This also makes it possible to increase the detecting resolution only in regions of angles particularly important for the shift detection (e.g. only regions where the shift position is changed from F to N, from R to N, etc.). For example, for a 1-degree change of the shift lever position, the detection lever  63  may be pivotally moved through an increased angle only in the particular regions, whereby a nonlinear characteristic of the output from the shift position detector  24  can be obtained as shown in  FIG. 5 . 
   Further, this arrangement has the merit that the lever ratio of the detection lever  63  can be designed as desired, and the angle of movement of the detection lever  63  (angle B° in  FIG. 15B ) can be made larger with respect to the angle of movement of the shift lever  23  (angle A° in  FIG. 15B ), thus enabling the use of the maximum pivotal angle range of the variable resistor. 
   Further, in the present arrangement, the pivot of the shift lever  23  is not directly connected to the pivot of the variable resistor, built the spring mechanism is provided to absorb vibrations, to thereby prevent wear of the variable resistor pivot and wear of the internal resistance material and the brush caused by vibrations and provide excellent vibration resistance for the shift operation detector  24 . At the same time, the shift operation detector  24  has a freedom of disposition thereof, and a compact construction can be realized which makes use of unused (available) space in the engine. 
   As described above, according to the present embodiment, the shift operation by the shift operation lever  21  is continuously detected by the shift position detector  24 , and when an early stage of the shift operation from the forward position to the neutral position or from the reverse position to the neutral position is detected, the engine output reduction control is carried out if the engine speed at the time of detection is not less than a predetermined value, and when the shift position detector  24  detects that the shift position has been switched to the neutral position, the engine output reduction control is canceled. Therefore, when the shift operation is at an early stage, i.e. the shift operation lever  21  is in a state before the operator increases the shift operating force applied to the shift operation lever  21 , and at the same time the engine speed is not less than the predetermined value, i.e. when it is predicted that the shift operating force applied to the shift operation lever  21  is to increase, the engine output is reduced, whereby an operating force smaller than that required in the prior art is sufficient to perform the shift operation, thus facilitating the shift operation. 
   Further, when the shift position detector  24  detects the early stage of the shift operation, and at the same time the throttle opening is not smaller than a predetermined value, control is performed to change the engine output. As a result, an erroneous detection of the shift operation can be prevented, which may be made when an external force other than the shift operating force for performing the shirt operation acts on the shift operation lever  21  e.g. at a rapid acceleration or upon a jump of the boat during running. 
   Further, when the shift operation detector  24  detects the early stage of the shift operation, and at the same time the intake negative pressure is not higher than a predetermined value, control is performed to change the engine output, which makes it possible to avoid an increase in the shift operating force occurring when the engine speed cannot immediately follow up sudden opening or closing of the throttle at a throttle snap. 
   Further, when the shift position detector  24  detects that the shift position is in the vicinity of the forward position or the reverse position, and at the same time, the amount of change in the detected shift position is not less than a predetermined value, the CPU  31  determines that the shift operation is in its early stage. As a result, it is possible to accurately and promptly determine whether or not the shift operation is being carried out, by preventing erroneous detection or delayed detection of the shift operation due to assemblage errors of the shift operation detector  24  and variations in the output thereof. 
   Further, the CPU  31  changes the engine output by carrying out firing control based on the number of consecutive misfires or the ignition timing retardation value set on an engine speed region-by-engine speed region basis. As a result, the engine output can be reduced while preventing engine stalling under various conditions. 
   Further, the CPU  31  sets the allowable slope of reduction in the engine speed with respect to each predetermined number of times of ignition, and when the set allowable slope of reduction exceeds a predetermined value, the number of consecutive misfires or the ignition timing retardation value is changed. This makes it possible to ensure the advantageous effects of promptly reducing the engine output while preventing engine stalling. 
   Also, the CPU  31  sets the lower limit value of the allowable slope of reduction in the engine speed per each predetermined number of times of ignition, and when the slope of reduction in the engine speed exceeds the set lower limit value, the control for changing the engine output is canceled. This makes it possible to prevent engine stalling during changing of the engine output. 
   Further, the shift position detector  24  is disposed at the first-actuated driving part of the forward/reverse switching mechanism  9  inside the outboard motor  1 . As a result, it is possible to accurately detect the shift operation by the operator without being influenced by a play or hysteresis of the forward/reverse-switching mechanism  9  inside the outboard motor  1 . 
   Further, the shift position detector  24  is disposed close to the driving part of the shift operation lever  21  disposed in the remote control box  20  for remotely controlling the forward/reverse-switching mechanism of the outboard motor  1 . This makes it possible to detect the shift operation by the operator in earlier timing with higher accuracy without being influenced by a play or hysteresis of lots of linkages disposed between the remote control box  20  and the outboard motor  1 . 
   Although in the first embodiment described above, the step S 101  in  FIG. 8  is executed before the steps S 102  to S 104  are executed, it may be executed after execution of the step S 104 . Further, the order of execution of the steps S 102  to S 104  is not limited to the description given with reference to  FIG. 8 . 
   Next, a description will be given of a second embodiment of the present invention. In the first embodiment described above, the shift operation by the shift operation lever  21  is continuously detected by the shift position detector  24 , and when an early stage of the shift operation from the forward (F) position to the neutral (N) position or from the reverse (R) position to the neutral (N) position is detected, the engine output reduction control is carried out. The second embodiment is distinguished from the first embodiment in that when an early stage of a shift operation from the neutral (N) position to the forward (F) position or from the neutral (N) position to the reverse (R) position is detected, the engine output reduction control is carried out. The other parts of the configuration of the present embodiment are the same as those of the first embodiment, and therefore detailed description thereof is omitted. In the following description, the elements and parts which correspond to those of the first embodiment are designated by identical reference numerals. 
     FIG. 16  is a flowchart showing an engine output reduction control process carried out by a shift operation control system according to the second embodiment. This process is executed by the CPU  31  based on a predetermined program stored in the ROM of the CPU  31 . 
   As shown in  FIG. 16 , first, the CPU  31  determines, based on the output from the shift position detector  24 , whether or not the shift operation lever  21  is in the neutral (N) position (step S 201 ). If the result of the determination shows that the shift operation lever  21  is in the N position, it is determined whether or not the shift operation has been carried out by the operator (steps S 202  to S 205 ). 
   As stated above as to the first embodiment, due to variations caused by assemblage errors of the shift position detector  24  and tolerances of the associated component parts, and aging, it is difficult to promptly detect the shift position only from a determination of the position (absolute value) resulting from the shift operation. 
   For example, as shown in  FIG. 17A , assuming that the output from the shift position detector  24  when the shift operation lever  21  is in the N position varies from an output B 1  to an output B 2 , if the detection of the shift operation is carried out with reference to a predetermined absolute value Bt for shift operation determination, a delay of Δtin shift operation detection timing is caused by the output B 1  compared with the case of the output B 2 . 
   On the other hand, if the output from the shift position detector  24  assumes the output B 2 , there arises the inconvenience of erroneously detecting a subtle movement of the shift operation lever  21  causes by a load unconsciously applied to the shift operation lever  21  or the like, as the shift operation being carried out. For example, when the operator grips the shift operation lever  21  by hand, as shown in  FIG. 17B , the load unconsciously applied to the shift operation lever  21  or the like changes the output from the shift operation detector  24 . If the engine output reduction control, described hereinafter, is carried out upon detection of the change, a change in the engine speed not intended by the operator is produced as indicated by reference numeral  60 . Further, as indicated by reference numeral  61  in  FIG. 17B , even when the shift position is brought back to the N position after detection of the shift operation, a change occurs in the engine speed. 
   To avoid these inconveniences, as shown in  FIG. 18 , the output from the shift position detector  24  (e.g. the value B 1  or B 2 ) produced when the shift position is the N position is stored as a learned neutral (N) value, and insensitive zones each having a margin b are set for the learned N value, and when the output from the shift position detector  24  goes beyond the insensitive zone, and at the same time the amount of change in the output from the shift position detector  24  exceeds a predetermined value A, it is determined that a shift operation has been carried out from the N position to the F position (or from the N position to the R position). 
   The learned N value is stored as the output value of the shift position detector  24  when the output from the shift position detector  24  is within the range of “b” to “c” in  FIG. 5 , and the amount of change in the output voltage from the shift position detector  24  has continued to be within a predetermined range over a predetermined time period. For example, assuming that the initial value of the learned N value is designated by b 1 , and the present output value from the shift position detector  24  is designated by b 2  (b 2 &lt;b 1 ), and if the output voltage has been stable in the vicinity of the value b 2  with the amount of change remaining within the above predetermined range, a value obtained by subtracting a predetermined very small value b 3  from the value b 1  is set to a new learned N value (learned N value=b 1 −b 3 ). 
   On the other hand, assuming that the output from the shift position detector  24  is designated by b 4  (b 4 &gt;b 1 ), and has continued to be stable in the vicinity of b 4  for a predetermined time period with the amount of change in the output voltage remaining within the predetermined range, a value obtained by adding the value b 3  to the value b 1  is set to a new learned N value (learned N value=b 1 +b 3 ). By repeatedly carrying out this process, the learned N value converges, and even if the output value when the shift position is the N position has changed due to aging or the like, the new learned N value is set by the learning function, so that it is possible to promptly and accurately detect whether or not the shift operation has been carried out. 
   Further, as indicated by reference numeral  61  in FIG.  17 B, even when the shift operation lever  21  is brought back to the N position after detection of the shift operation, a change occurs in the engine speed. Therefore, when the output from the shift position detector  24  goes beyond the insensitive zone having the margin b, and at the same time the amount of change in the output from the shift position detector  24  exceeds the predetermined value, it is determined that a shift operation has been carried out. 
   The comparison between output values from the shift position detector  24  is made not between the present value and the immediately preceding value of the output from the shift position detector  24 , but between the present value and a value detected a predetermined time period earlier. More specifically, as shown in  FIG. 19 , the output from the shift position detector  24  is measured at short sampling time intervals (e.g. 10 ms), and an amount of change (Va−Vb) between an output voltage Vb in the present timing (time point) Tb and an output voltage Va in timing (time point) Ta earlier than the present time point Ta by a predetermined time period (e.g. 200 ms) is calculated, and when the amount of change in the output is not smaller than the predetermined value A (e.g. 100 mV) in the increasing direction (in the decreasing direction when switched from the N position to the R position), it is determined that a shift operation has been carried out. 
   This makes it possible to provide control such that even when the output from the shift position detector  24  has changed beyond the insensitive zone set for the learned N value with the margin b, the engine output reduction control is not readily carried out. To prevent an erroneous detection due to noise or an external force (impact) applied to the shift position detector  24 , it may be consecutively determined a plurality of times whether or not the amount of change in the output is not smaller than the predetermined value A. 
   Referring again to  FIG. 16 , in a step S 202 , a shift operation speed is detected from the amount (slope) of change in the output, and it is determined whether or not the shift operation speed is larger than the predetermined value A (amount of change in the output&gt;A). If the result of the determination shows that the predetermined value A is exceeded (YES to the step S 202 ), it is determined that rapid acceleration is being carried out, and then the present process returns to the step S 201 . This is because if the engine output reduction control is carried out during rapid acceleration in which the shift operation from the N position to the F position is rapidly carried out, as shown in  FIG. 20 , the engine speed once drops to cause a stumble or unstable engine rotation  62 , resulting in degraded accelerability. To avoid this inconvenience, within the insensitive zone as well, when the amount of change in the output from the shift position detector  24  exceeds the predetermined value A, it is judged that rapid acceleration is being performed, so that the engine output reduction control is not carried out. 
   In the case of outboard motors, the same lever is used both for the shift operation and the throttle operation, and accordingly, when rapid acceleration is carried out from the N position, as shown in  FIG. 20 , the output from the shift position detector  24  suddenly changes before the throttle is suddenly opened. Therefore, from the amount (slope) of change in the output from the shift position detector  24 , it is possible to distinguish the rapid acceleration from a shift operation under a steady condition. 
   When the amount of change in the output from the shift position detector  24  does not exceed the predetermined value A (NO to the step S 202 ), it is determined whether or not the output value from the shift position detector  24  is larger than (learned N value−b) and smaller than (learned N value+b) (step S 203 ). This determines, as described above, whether or not the output value from the shift position detector  24  is within the insensitive zone having the margin b set for the learned N value. 
   If the result of the determination in the step S 203  shows that the output value from the shift position detector  24  is not within the range of (learned N value−b) to (learned N value+b), it is determined whether or not the amount (slope) of change in the output is not larger than the predetermined value A and not smaller than a predetermined value B (step S 204 ). The predetermined value B (B&lt;A) is an arbitrary value set for detecting the slope of change in the output from the shift position detector  24  to predict shift IN timing. Further, as shown in  FIG. 19 , the amount of change in the output is determined by a comparison of the present value with the output value produced the predetermined time period earlier. The predetermined time period is rather short, and when it is shortest, it is equal to the sampling time period. Therefore, the amount of change in the output can be regarded as a value having the same meaning as the slope of change in the output from the shift position detector  24 , so that the slope may be used in place of the amount of change. However, when using the slope, the predetermined values A and B should be also replaced by respective values for use with the slope. If the result of the determination in the step S 204  shows that the amount (slope) of change in the output is within the range of the predetermined value A to the predetermined value B), it is determined that a shift operation, has been carried out, and the process proceeds to a step S 206 . 
   On the other hand, the result of the determination in the step S 204  shows that the amount (slope) of change in the output is not within the range of the predetermined value A to the predetermined value B) (NO to the step S 204 ), it is further determined whether or not the amount (slope) of change in the output is smaller than the predetermined value B and not smaller than a predetermined value C (step S 205 ). This determines whether or not the shift operation speed is low. The predetermined value C is an arbitrary value set in association with the values A and B (A&gt;B&gt;C). 
   If the result of the determination in the step S 205  shows that the amount (slope) of change in the output is not within the range of the predetermined value B to the predetermined value C) (NO to the step S 205 ), it is judged that the shift operation speed is low, and then the process returns to the step S 201  so as to detect the shift operation a plurality of times. This makes it possible to improve the accuracy in the detection of the shift operation. 
   On the other hand, if the result of the determination in the step S 205  shows that the amount (slope) of change in the output is within the range of the predetermined value B to the predetermined value C) (YES to the step S 205 ), it is judged that a shift operation has been carried out, and the process proceeds to the step S 206 . Also when the amount of change in the output is reduced to 0 or the slope of the change in the output is an opposite slope, the engine output reduction control is not carried out, either. 
   Next, in the step S 206 , the engine output reduction control is carried out. 
   After the shift operation is detected, it takes a very short time to switch the shift position from the N position to the F position, so that it is necessary to reduce the engine speed as promptly as possible insofar as engine stalling is prevented. To attain this goal, it is effective to instantly carry out the ignition timing retardation control (or misfiring control) for the engine  5  starting with the cylinder which is to be ignited immediately after the shift operation is detected without specifying cylinders to be subjected to the ignition timing retardation control (or misfiring control). 
   In the present engine output reduction control, even after the shift position is switched from the N position to the F position, the engine output reduction control is continued for a while to reduce the engine speed, and thereafter the engine speed is progressively recovered to the normal rotational speed, more specifically, the retarded ignition timing is progressively restored to the normal ignition timing, whereby a shock caused by the shift operation is further reduced. 
   The method of reducing the drive torque of the engine is not limited to the ignition timing retardation control or the misfiring control described above, but the control of fuel injection amount or the intake air amount (control of the bypass intake air) may be carried out to reduce the drive torque. However, with the control of the fuel injection amount or the intake air amount, the changed fuel injection amount or the changed intake air amount enters the combustion chamber to reduce the drive force only few combustion cycles after the start of the control. In contrast, the ignition control (misfiring control or ignition timing retardation control) is very effective since the drive force starts to be reduced immediately after the start of the control. 
     FIG. 21  is a timing diagram useful in explaining the engine output reduction control process. 
   In  FIG. 21 , reference numeral t 1  designates a time point the shift operation is started, t 2  a time point the output value goes beyond the insensitive zone, and t 3  a time point the amount of change in the output exceeds the predetermined value in the increasing direction. 
   In the present engine output reduction control, as shown in  FIG. 21 , when the shift operation is detected at the time point t 3 , the normal ignition timing is immediately retarded by a predetermined amount (ignition timing retardation amount “a”) to carry out the ignition timing retardation control. 
   On the other hand, if the engine speed is suddenly reduced during execution of the ignition timing retardation control, there is a possibility of engine stalling. To prevent engine stalling, an allowable slope of reduction in the engine speed (amount of drop in the engine speed per each ignition) is set (a predetermined reference drop value per each ignition is set), and the amount of the drop in the engine speed is constantly detected through calculation. When the amount of drop in the engine speed is larger than the predetermined drop value, to prevent engine stalling, the ignition timing retardation amount is changed from the amount “a” to the amount “a′” to change the controlled variable in the engine output reduction control. 
   Further, to prevent engine stalling, the intake air amount may be controlled as indicated by reference numeral  63  in  FIG. 21 . When the intake air amount control is carried out, the ignition timing retardation (or misfiring) is executed, and at the same time the intake air amount is increased (reference numeral  63  in  FIG. 21 ) in anticipation for a sudden drop in the engine speed during the shift operation (e.g. when load from the propeller is high). 
   When the load from the propeller is high and the amount of drop in the engine speed is larger than the predetermined drop value, there is a fear of engine stalling, and therefore the engine output reduction control is canceled and the ignition timing control is carried out to restore the retarded ignition timing to the normal ignition timing or advance the ignition timing, and at the same time, the intake air amount increased in advance is controlled, for prevention of engine stalling. 
   The control of the intake air amount is not employed during the engine output reduction control in which the instant engine output reduction (with a large effect) is necessary, since the changed intake air amount enters the combustion chamber to reduce the drive force only few combustion cycles after the start of the control. 
   In the present engine output reduction control, the shift operation speed may be detected from the amount (slope) of change in output from the shift position detector  24 , and the controlled variable in the engine output reduction control may be changed according to the shift operation speed. As shown in  FIG. 22B , when the shift operation speed is low, it is preferable to carry out the detection of the shift operation a plurality of times. On the other hand, as shown in  FIG. 22A , when the shift operation speed is high, it is determined that rapid acceleration is being carried out. 
   Further, in the present engine output reduction control, the controlled variable of the engine output reduction control may be changed depending on the warmed-up condition of the engine. When the engine is cold, the engine has increased friction. Therefore, it is determined whether or not the engine is warmed up, from the engine temperature or the coolant temperature, and if the engine is warmed up, the controlled variable of the engine output reduction control is changed. 
   Moreover, during the engine output reduction control, tachometer output is controlled. More specifically, though for a short time period, during the engine output reduction control, the engine speed becomes much lower than the normal idling speed so that the pointer of the tachometer largely drops to give a sense of disorder or uneasy feeling to the operator. To eliminate this inconvenience, during the engine output reduction control, the tachometer output is controlled so as to prevent the pointer from indicating an engine speed value lower than a predetermined value. 
   After execution of the engine output reduction control, when the amount of drop in the engine speed is larger than the predetermined drop value, the controlled variable in the engine output reduction control is reduced. Further, when the amount of drop in the engine speed is larger than a predetermined threshold value, the engine output reduction control is canceled, and engine output increase control is carried out as required. 
   Referring again to  FIG. 16 , when the amount of drop in the engine speed (slope of reduction in the engine speed) exceeds the lower limit value (YES to a step S 207 ), there is a fear of engine stalling, and therefore the engine output reduction control is immediately canceled. Further, when the engine speed becomes lower than a predetermined value (YES to a step S 208 ), the engine output reduction control is cancelled. 
   Moreover, when the slope of change in the output from the shift position detector  24  has been inverted, it is judged that the shift operation had been canceled midway (YES to a step S 209 ), and therefore the engine output reduction control is canceled. The engine output reduction control may be canceled when the shift position has been changed from the N position to the F position (or the R position). 
   Next, it is determined whether or not a predetermined time period (as designated by symbol “c” in  FIG. 21 ) has elapsed from the start of the engine output reduction control (step S 210 ), and when the predetermined time period has elapsed, the ignition timing is progressively restored from the retarded ignition timing to the normal ignition timing (step S 211 ). The predetermined time period “c” is set depending on the hull (shape, weight, and loadage thereof) in which the outboard motor is installed and the load from the propeller. Also when the direction of the shift operation has been inverted, or when the warm-up condition has been changed, the ignition timing may be restored to the normal ignition timing. 
   Further, although in the detection of the shift operation, control is performed such that a subtle motion of the shift operation lever  21  (due to unconscious application of load on the shift operation lever  21  or variation in the output from the shift position detector  24 ) is prevented from being erroneously detected as the shift operation, the detection of the amount of change in the output from the shift operation detector  24  may be continued even after the shift operation is detected, and when a change in the output from the shift operation detector  24  in the opposite direction is detected as shown in  FIG. 17  or no change in the output is detected (e g. the shift operation lever  21  is stopped and fixed midway during the shift operation), the engine output reduction control may be immediately canceled. 
   Although in the engine output reduction control described above, the case of shifting the shift position from the N position to the F position is given, the engine output reduction control may be carried out in respective different manners according to the shift operation from the N position to the F position and that from the N position to the R position, by setting controlled variables of the engine output reduction control, including the ignition timing retardation amount and the intake air amount, the slope of reduction in the engine speed, the lower limit value of the slope of reduction in the engine speed, and so forth, for each of the different operating directions of the shift operation. 
   Next, a description will be given of a process carried out when the shift operation lever  21  is switched from the F position to the N position while the boat is running at some speed. 
     FIG. 23  is a diagram showing the process carried out when the shift lever  21  is switched from the F position to the N position when the boat is traveling at some speed. 
   As shown in  FIG. 23 , first, it is determined whether or not the shift operation from the F position to the N position has been carried out (step S 300 ), and then, the direction (forward or reverse) and speed of running of the boat are determined based on the engine speed detected when the shift operation from the F position to the N position was detected and the duration over which the shift operation lever has been held in the N position after being shifted to the N position (steps S 301  to S 304 ). 
   For example, there is a case where even when a sequence of operations, i.e. full throttle operation→rapid deceleration→shift operation from the F position to the N position→maintaining the idling engine speed are carried out, or even when the idling condition is further continued for 10 seconds from the above state, the boat still runs at some speed. Therefore, if it is configured such that the engine output reduction control is carried out only when the idling state has been continued, when the shift operation is repeatedly carried out during stoppage or very low running speed of the engine (F→N→R→N→F→N→R), the engine output reduction control does not work, and a shock occurs during the shift operation. 
   To overcome the problem, the engine speed at the time of detection of the shift operation is detected (step S 301 ), and from the detected engine speed, a time period Xn over which the engine output reduction control is inhibited when the shift operation from the N position to the R position is carried out, and a time period Yn before transition to engine output increase control is determined when the shift operation from the N position to the R position is carried out are set based on Table 4 (step S 302 ). In Table 4, there are set time periods X 1  to Xn and Y 1  to Yn. 
   
     
       
         
             
             
           
             
                 
               TABLE 4 
             
           
          
             
                 
                 
             
             
                 
               Engine Speed Region (rpm) 
             
          
         
         
             
             
             
             
             
             
          
             
                 
               N1–N2 
               N2–N3 
               N3–N4 
               . . . 
               Nn- 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
             
          
             
               Set Time Period 
               X1 
               X2 
               X3 
               . . . 
               Xn 
             
             
               Set Time Period 
               Y1 
               Y2 
               Y3 
               . . . 
               Yn 
             
             
                 
             
          
         
       
     
   
   Next, after the shift operation from the N position to the R position is detected (step S 303 ), a calculation is made of elapsed time t from the detection of the shift operation from the F position to the N position and before the detection of the shift operation from the N position to the R position. 
   When a comparison of the calculated elapsed time t with the time period Xn (n: 1, 2, . . . , n) set in the step S 302  (step S 305 ) shows t&lt;Xn (YES to the step S 305 ), it is judged that the running speed of the boat in the forward direction is high, and therefore the engine output increase control is carried out when the shift operation from the N position to the R position is being carried out (step S 310 ). 
   On the other hand, if Xn≦t≦Yn holds (YES to the step S 306 ), it is judged that the boat is running at some speed in the forward direction, and when the shift operation from the N position to the R position is being carried out, the engine output reduction control is inhibited (step S 309 ). Further, when t&gt;Yn holds (YES to the step S 307 ), the engine output reduction control is allowed (step S 308 ). This makes it possible to reduce a shift shock even when the shift operation from the N position to the R position is carried out in a state where the boat is still running at some speed after execution of the shift operation from the F position to the N position. 
   For example, when the hull is running at full throttle and an engine speed of 6000 rpm, if the shift operation from the F position to the N position is carried out, it can be easily determined that the running speed will be high immediately after execution of the shift operation. Therefore, when the engine speed Na is set to a range of 5000 to 6000 rpm, the time period Xa to 10 seconds, and the time period Ya to 20 seconds, if the shift operation from the N position to the R position is carried out after the lapse of 5 seconds after execution of the shift operation from the N position to the R position, the engine output increase control is carried out. 
   On the other hand, if the shift operation from the N position to the R position is carried out after the lapse of 15 seconds, the engine output reduction control is inhibited. Further, if the shift operation from the N position to the R position is carried out after the lapse of 25 seconds, the engine output reduction control is allowed and executed. 
   As described above, according to the present embodiment, the shift position detector  24  continuously detects the shift operation by the shift operation lever  21 , and when the shift operation detector  24  detects an early stage of the shift operation from the neutral position to the forward position or from the neutral position to the reverse position, control is performed to change the engine output, and after the lapse of a predetermined time period, the control for changing the engine output is canceled. Thus, the early stage of the shift operation, i.e. a state just before the operator increases the shift operating force applied to the shift operation lever  21  is detected, and the engine output is reduced, whereby a shock occurring during the shift operation can be reduced to facilitate the shift operation. 
   Further, when the shift position detected by the shift operation detector  24  is in the vicinity of the neutral position, and at the same time the detected amount of change the output from the shift operation detector  24  is not smaller than the predetermined value A, and exceeds the predetermined value “b” set for the neutral position, it is determined that the shift operation is in its early stage. As a result, it is possible to promptly and accurately determine whether a shift operation is being carried out by preventing erroneous detection or delayed detection of the shift operation due to assemblage errors in the shift operation detecting device  24  or variations in the output from the shift operation detecting device  24 . 
   Further, when the shift position detector  24  detects a change in the output value indicative of the neutral position, the output value indicative of the neutral position after the change is learned. Therefore, even if the output value indicative of the neutral position is changed due to aging or the like, it is possible to promptly and accurately detect the shift operation since the learned neutral (N) value is set by the learning function. 
   Further, from the amount of change in the shift position detected by the shift position detector  24 , the shift operation speed is determined, and when the shift operation speed is not larger than the predetermined value (predetermined output change amount) A, the engine output is changed. This makes it possible to prevent a stumble or unstable engine rotation by limiting the engine output change control at rapid acceleration. 
   Moreover, the running speed of the boat is determined from the engine speed during the shift operation from the forward position to the neutral position and the duration over which the neutral position has been maintained, and if the running speed of the boat is not higher than the predetermined value, the engine output is changed. This makes it possible to reduce a shock occurring during the shift operation and facilitate the shift operation. 
   Also, the engine output is changed through misfiring or ignition timing retardation, and through increase of the intake air amount. Therefore, it is possible to promptly reduce the engine output while preventing engine stalling even under various conditions. 
   Further, when the predetermined conditions for canceling the engine output change control are satisfied before the lapse of the predetermined time period, the control for changing the engine output is immediately canceled. As a result, it is possible to promptly reduce the engine output while preventing engine stalling even under various conditions.