Abstract:
A boat has a propulsion unit and a steering system. The steering system includes a steering device actuated by an electric actuator. A steering wheel operated by an operator is electrically connected to the actuator. Detectors collect data regarding one or more of operation status of the steering wheel, running status of the boat, status of the propulsion unit, status of the electric actuator, and the like. In certain situations, the electric actuator may not be capable of providing a quick steering response. A controller weighs detected data to identify such situations and controls a propulsive force of the propulsion unit so as to restrain the propulsive force if necessary to maintain good steering response.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based on and claims priority under 35 U.S.C. §119 to Japanese Patent Application Serial No. 2006-312157, filed on Nov. 17, 2006, the entire contents of which are expressly incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a boat having a steering system that uses an electric actuator. 
     2. Description of the Related Art 
     In conventional boat, the boat is steered in response to operation of the steering wheel. Japanese Patent Document No. JP-A-2005-254848 discloses an electric actuator that is actuated as an operator operates the steering wheel. An external force to the boat is detected during such steering, and a reaction torque is applied to the steering wheel based on the detected external force. Accordingly, the operator can feel the external force to the boat due to, for example, a water current, directly through the steering wheel, and thus can recognize the movement of the boat corresponding to such external force so as to react quickly. 
     When such a boat is under no external force, an operation feel of the steering wheel can be lighter. Unfortunately, when larger output is required for control surface deflection (high control surface deflection torque), and when the steering wheel is operated faster, output from the steering motor (electric actuator) becomes less responsive, resulting in a poor operation feel. 
     It should be noted that control surface deflection torque characteristics sufficient to cause control surface deflection may change depending on a number of conditions. For example,  FIG. 10  shows a change from control surface deflection force characteristic line A 1  to control surface deflection force characteristic line A 2 , depending on conditions such as the characteristics of the boat, a control surface angle, an operation speed, or the like. During some conditions, a control surface deflection force may exceed the limit of the motor ability, resulting in impaired responsiveness and a poorer operation feel. 
     Further, as shown in  FIG. 11 , some motor characteristics depend on the surroundings such as temperature. For example, when the temperature becomes high the motor characteristics may change from the state shown by motor characteristic line B 1  (solid line in the figure) to the state shown by motor characteristic line B 2  (broken line in the figure). Since the motor characteristics at high temperatures provide lower torque, a control surface deflection force required may not be obtained during light temperature conditions, resulting in impaired responsiveness and a poorer operation feel. 
     SUMMARY 
     Accordingly, there is a need in the art for a boat having a steering system that provides excellent responsiveness in varying conditions and which provides excellent operation feel during control surface deflection, depending on a running status of the boat. 
     In accordance with a preferred embodiment, the present invention provides a boat having a propulsion unit and a steering system comprising a steering wheel and a steering device. The steering device comprises an electric actuator. The steering wheel is operable by an operator and generates an actuation signal corresponding to steering wheel operation. The steering wheel is electrically connected to the electric actuator. The boat additionally comprises a controller adapted to control a propulsive force of the propulsion unit. The controller has at least one of an operation status portion adapted to obtain data concerning steering wheel operation, a running status portion adapted to obtain data concerning a running status of the boat, a propulsion unit status portion adapted to obtain data concerning a status of the propulsion unit, and an electric actuator status portion adapted to obtain data concerning a status of the electric actuator. The controller further comprises a propulsive force calculator adapted to calculate a propulsive force target based on data from at least one of the controller portions. The controller is configured to reduce or restrain the propulsive force of the propulsion unit to a level at or below the propulsive force target value. 
     In one embodiment, the operation status portion includes at least one of a control surface deflection force detector adapted to detect a control surface deflection force necessary to deflect a control surface of the boat, a load detector adapted to detect a load on the control surface, a steering operation detector adapted to detect a steering wheel operation angle, a steering wheel operation speed and a direction in which the steering wheel is operated, a control surface deflection detector adapted to detect a control surface deflection angle, a control surface deflection speed and a direction in which the control surface is deflected, corresponding to the steering wheel operation, and a deviation detector adapted to detect a deviation of a detected actual control surface deflection angle from a target control surface deflection angle corresponding to the steering wheel operation. 
     In another embodiment, the running status detector portion includes at least one of a weight detector adapted to detect at least one of a position of a waterline and a weight of the boat, a trim angle detector adapted to detect a trim angle of the boat, and a speed detector adapted to detect at least one of a speed and an acceleration of the boat. 
     In yet another embodiment, the propulsion unit status portion includes an operation storage adapted to store data concerning one or more of the installation number of the propulsion unit, an installation position of the propulsion unit relative to the boat, a rotational direction of a propeller of the propulsion unit, a propeller shape, a trim tab angle and a trim tab shape. 
     In a further embodiment, the electric actuator status portion includes a temperature detector adapted to detect a temperature of the electric actuator. 
     In a still further embodiment, the propulsion unit is an outboard motor. 
     In accordance with another preferred embodiment, the present invention provides a method of steering a boat comprising a propulsion unit, a steering wheel, and a steering system comprising an electric actuator adapted to deflect a control surface to effect steering. The method comprises obtaining an actuation signal corresponding to steering wheel operation, obtaining electronic data concerning at least one of a steering wheel operation, a running status of the boat, a status of the propulsion unit, and a status of the electric actuator. A propulsive force target is calculated based on the actuation signal corresponding to steering wheel operation and at least one of the electronic data. The propulsion unit is controlled so that a propulsive force of the propulsion unit is at or below the propulsive force target. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a boat in accordance with one embodiment. 
         FIG. 2  is an enlarged plan view of a steering device of the boat in accordance with the embodiment of  FIG. 1 . 
         FIG. 3  is a block diagram showing interactions of some systems and detectors in accordance with an embodiment. 
         FIG. 4  is a block diagram of aspects of an ECU in accordance with one embodiment. 
         FIG. 5  is a flowchart of a propulsive force control process in accordance with an embodiment. 
         FIG. 6  are graphs of a propulsive force control state depending on a control surface deflection status in accordance with an embodiment. 
         FIG. 7  are graphs showing effects of propulsive force control in accordance with an embodiment. 
         FIG. 8  are graphs of a propulsive force control state depending on a running status in accordance with one embodiment. 
         FIG. 9  are graphs showing certain characteristics during an acceleration in a running status in accordance with one embodiment. 
         FIG. 10  is a graph of deflection force characteristics, illustrating a relationship between control surface deflection torques and control surface deflection speeds. 
         FIG. 11  is a graph of motor characteristics, illustrating a relationship between torques generated by an electric motor and rotational speeds at different temperatures. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With initial reference to  FIGS. 1 to 8 , an embodiment of a boat has a hull  10  including a transom  11 . A “boat propulsion unit” is mounted to the transom  11  of the hull  10 . In the illustrated embodiment, the propulsion unit is an outboard motor  12  mounted to the transom  11  via clamp brackets  13 . The outboard motor  12  preferably is pivotable about a swivel shaft (steering pivot shaft)  14  that extends in a generally vertical direction. 
     A steering bracket  15  is fixed at the upper end of the swivel shaft  14 . The steering bracket  15  is coupled at its front end  15   a  to a steering device  16 . The steering device  16  is driven by operating a steering wheel  17  disposed in an operator&#39;s section of the hull  10 . 
     A remote control device  18  preferably is disposed in the operator&#39;s section in order to adjust a propulsive force of the outboard motor  12 . The outboard motor  12  is operated by operation of a lever  19  of the remote control device  18 . 
     In the embodiment, shown in  FIG. 2 , the steering device  16  includes a DD (direct drive) electric motor  20  that is attached to a threaded rod  21  extending in a width direction of the boat. The motor  20  is movable in the width direction of the boat along the threaded rod  21 . 
     The illustrated threaded rod  21  is supported at its ends by a pair of left and right supports  22 . The supports  22  are supported by a tilt shaft  23 . 
     The illustrated electric motor  20  has a coupling bracket  24  extending rearward. The coupling bracket  24  and steering bracket  15  are coupled with each other via a coupling pin  25 . 
     As a result, as the electric motor  20  is actuated to move in the width direction of the boat relative to the threaded rod  21 , the outboard motor  12  will pivot about the swivel shaft  14  via the coupling bracket  24  and the steering bracket  15 . As such, the meter actuates steering of boat by rotating the motor  12 . 
     With reference again to  FIG. 1 , the steering wheel  17  preferably is fixed to a steering wheel shaft  26 . At the proximal end of the steering shaft  26 , there is provided a steering wheel control unit  27 . The steering wheel control unit  27  includes a steering wheel operation angle sensor  28  for detecting an operation angle of the steering wheel  17 , and a reaction motor  29  for applying a desired reaction force to the steering wheel  17  during operation of the steering wheel  17  by the operator. 
     The steering wheel control unit  27  is connected to an electronic control unit (ECU)  33  via a signal cable  30 . The control unit  33  is connected to the electric motor  20  of the steering device  16 . The control unit  33  receives a signal from the steering wheel operation angle sensor  28 , controls the electric motor  20 , and controls the reaction motor  29 . 
     The remote control device  18  preferably is disposed in the vicinity of the steering wheel  17 , and is inclinable in a length direction of the boat. A position sensor  19   a  is provided in the remote control device  18  and detects the amount of operation of the operation lever  19 . The amount of operation is transmitted to the connected electronic control unit (ECU)  33  via a signal cable  31 . The propulsive force of the outboard motor  12  is controlled by a control device on the engine side, based on the signal. In this embodiment, the propulsive force can be controlled by adjusting a throttle opening degree, an ignition timing, an amount of injected fuel, and the like. 
     As shown in  FIG. 4 , the control unit  33  preferably includes operation status detection means  38  for detecting an operation status corresponding to an operator&#39;s steering wheel operation, running status detection means  39  for detecting a running status of the boat, outboard motor status recognition means  40  for recognizing a status of the outboard motor  12 , such as its installation number, and electric motor status detection means  41  for detecting a status of the electric motor  20 . 
     The control unit  33  also preferably includes propulsive force computation means  42  for computing a propulsive force target value to which the control unit  33  will reduce the propulsive force of the boat propulsion unit  12  when it determines that a load to the electric motor  20  during control surface deflection will increase beyond a threshold value if the propulsive force remains above the target value. This target value calculation preferably considers detection values from the operation status detection means  38 . This control unit  33  also preferably includes propulsive force restraint means  43  for restraining the propulsive force of the boat propulsion unit  12  in accordance with the propulsive force target value computed by the propulsive force computation means  42 . 
     As shown in  FIG. 3 , to the operation status detection means  38  there are connected: control surface deflection force detection means  53  for detecting a control surface deflection force required for control surface deflection corresponding to the operation of the steering wheel; load detection means  44  for detecting a load to the control surface; steering operation detection means  46  for detecting a steering wheel operation angle, a steering wheel operation speed and a direction in which the steering wheel is operated; and control surface deflection detection means  47  for detecting a deflection angle of the outboard motor  12 , a deflection speed of the outboard motor  12  and a direction in which the outboard motor  12  is deflected, corresponding to the operation of the steering wheel. The operation status detection means  38  also is connected with deviation detection means  45  for detecting a deviation of a detected actual control surface deflection angle from a target control surface deflection angle corresponding to the steering wheel operation. The steering wheel operation angle sensor  28  provided in the steering operation detection means  46  detects a steering wheel operation angle. 
     The numerous “means” introduced and discussed herein comprise detectors configured to detect the associated characteristics and generate an electronic signal that is communicated to the control unit  33  and/or to another detector. Such detectors may have any suitable structure, may employ one or more sensors working alone or in concert, may include stored data, may conduct calculations based upon sensor inputs and/or stored data, and the like. 
     To the running status detection means  39 , there preferably are connected weight detection means  48  for detecting at least one of the position of a waterline and the weight of the boat, trim angle detection means  49  for detecting a trim angle of the boat, and speed detection means  50  for detecting at least one of a speed and an acceleration of the boat, a propulsive force of the outboard motor  12 , and an engine rotational speed of the outboard motor  12 , as shown in  FIG. 3 . 
     Further, to the outboard motor status recognition means  40 , there preferably is connected operation storage means  51  for storing therein information on the installation number of the outboard motor  12 , the installation position of the outboard motor  12  relative to the boat, a rotational direction of a propeller of the outboard motor  12 , a propeller size, a propeller shape, a trim tab angle, a trim tab shape, and the like. Of course, the operation storage means  51  can be included in the ECU  33 . 
     Furthermore, the electric motor status detection means  41  preferably is connected with temperature detection means  52  for detecting a temperature of the electric motor  20 . The electric motor status detection means  41  preferably stores data on an output torque or the like relative to a rotational speed and a temperature of the electric motor  20 . 
     It is to be understood that the above-described list of means or detectors does not necessarily comprise an exhaustive list of all the detectors that can be used in embodiments of the inventions and neither does it represent a minimum list of detectors. Rather, it presents an example embodiment. It is contemplated that other embodiments may employ more or less detectors (means) and that such means may be somewhat different in configuration and in their electronic interconnections than as specifically described in this example embodiment. 
     With next reference to  FIG. 5 , operation of an embodiment will be described below. 
     As the operator first turns the steering wheel  17 , a signal will be transmitted from the steering wheel operation angle sensor  28  in the steering operation detection means  46  to the ECU  33 . Then, in step S 10  of  FIG. 5 , a target control surface deflection angle is detected, and in step S 11 , a target deviation is computed. 
     In step S 12 , the operation status detection means  38  detects an operation status. As used herein, the term “operation status” refers at least to: a control surface deflection torque required for deflecting the outboard motor  12 ; a load to the control surface; an operation angle and operation speed of the steering wheel; a direction in which the steering wheel is operated; a rotational angle, a rotational speed, a rotational direction, a control surface deflection speed, and a control surface deflection direction of the outboard motor  12  actuated in correspondence with a steering wheel operation; a deviation of a detected actual control surface deflection angle from a target control surface deflection angle corresponding to a steering wheel operation; and the like. 
     The control surface deflection torque is detected by the control surface deflection force detection means  53 . A load to the outboard motor  12  is detected by the load detection means  44 . The operation angle and the operation speed of the steering wheel and the direction in which the steering wheel is operated are detected by the steering operation detection means  46 . A rotational angle, a rotational speed, and a rotational direction of the outboard motor  12  actuated in correspondence with a steering wheel operation are detected by the control surface deflection detection means  47 . A deviation of a detected actual control surface deflection angle from a target control surface deflection angle corresponding to a steering wheel operation is detected by the deviation detection means  45 . Detection signals from those means are transmitted to the operation status detection means  38  to thereby detect the operation status. It is also possible to obtain the control surface deflection torque by computation. 
     In step S 13 , the running status detection means  39  detects a running status. As used herein, the term “running status” refers to at least one of the position of a waterline and the weight of the boat, at least one of a trim angle, a speed and an acceleration of the boat, a propulsive force of the outboard motor  12 , and such aspects concerning operational status of the hull and motor  12  relative the surrounding body of water. 
     The position of a waterline and the weight of the boat are detected by the weight detection means  48 . The trim angle of the boat is detected by the trim angle detection means  49 . The speed and the acceleration of the boat and the propulsive force of the outboard motor  12  are detected by the speed detection means  50 . The engine rotational speed is detected by a rotational speed sensor in the outboard motor  12 . Detection signals from those means are transmitted to the running status detection means  39  to thereby detect and/or calculate the running status. 
     In step S 14  the outboard motor status recognition means  40  recognizes a status of the outboard motor  12 . As used herein, the term “the status of the outboard motor  12 ”, refers to the installation number of the outboard motor  12 , the installation position of the outboard motor  12  relative to the boat and/or any other outboard motor that may also be mounted to the boat, a rotational direction of the propeller of the outboard motor  12 , a propeller size, a propeller shape, a trim tab angle, a trim tab shape, and the like. 
     Information on the installation number of the outboard motor  12 , the installation position of the outboard motor  12  relative to the boat, the rotational direction of the propeller of the outboard motor  12 , the propeller shape, the trim tab angle, the trim tab shape, and the like are stored in the operation storage means  51 . In this embodiment, such information is read and then transmitted to the outboard motor status recognition means  40  to thereby recognize the status of the outboard motor  12 . 
     Thereafter, in step S 15 , the electric motor status detection means  41  detects a status of the electric motor  20 . As used herein, the term “the status of the electric motor  20 ” refers to a temperature and a voltage of the electric motor  20 . Other motor characteristics, such as maintenance status and the like, can be detected and/or stored by this detector  41 . 
     The temperature of the electric motor  20  is detected by the temperature detection means  52 . A detection signal from the means  52  is transmitted to the electric motor status detection means  41  to thereby detect the status of the electric motor  20 . 
     Based on such detection values, in step S 16 , propulsive force computation means  42  in the ECU  33  computes a propulsive force for the outboard motor  12 . A deviation from the propulsive force of the running status detected by the running status detection means  39  is computed so as to set a propulsive force target value. In step S 17 , a signal to control/restrain the propulsive force is transmitted from the propulsive force restraint means  43  in the ECU  33  to the outboard motor  12 . Then, a throttle opening degree, an ignition timing, and/or an amount of injected fuel of the engine in the outboard motor  12 , and the like are adjusted to adjust and control the propulsion force to meet the target value. The process then returns to step S 10 . 
     As a result, during operation of the boat by the operator, since a propulsive force is restrained depending on a running status of the boat and the like in order to reduce a control surface deflection force to a level that can be accommodated within an advantageous performance range of the steering motor  20 , a sudden change of a control surface deflection operation and an excessive control surface deflection corresponding to a steering wheel operation can be prevented. Consequently, the electric motor  20  is actuated with excellent responsiveness in substantially all conditions, and thus the operator can obtain an excellent feel of operation. 
     In one preferred mode of operation, a propulsive force of the outboard motor  12  is controlled depending on a steering operation status. As a control surface deflection angle becomes larger, and as a control surface deflection speed becomes higher, larger control is performed to restrain a corresponding propulsive force. In a further embodiment, when a control surface deflection operation is performed in a direction affected by a reaction force of a propeller, a control is performed to restrain a corresponding propulsive force more than in a case in which a control surface deflection is operated in the opposite direction. 
     As a propulsive force becomes larger, a corresponding control surface deflection force incident to steering the boat becomes correspondingly larger as indicated in the relationship between propulsive forces and control surface deflection forces shown in (b) and (c) in  FIG. 6 . As shown, if a deflection angle is increased, the control surface deflection force associated with a corresponding propulsive force increases from amounts shown by a solid line to amounts shown by a broken line in (b). In a preferred embodiment that considers this relationship, an increase of a control surface deflection force is restrained from increasing by reducing the propulsive force when a control surface deflection angle is increased. 
     Similarly, when a control surface deflection speed is increased, a control surface deflection force relative to a corresponding propulsive force is increased from amounts shown by a solid line to amounts shown by a broken line in (c). In a preferred embodiment that considers this relationship, an increase of a control surface deflection force is restrained from increasing by reducing the propulsive force when a control surface deflection speed is increased. 
     A broken line in  FIG. 6  ( a ) shows the control surface deflection ability characteristics in one embodiment. This broken line represents the control surface deflection forces under which the control surface can be deflected in relation to control surface deflection speeds and control surface angles. As shown, when a control surface deflection angle and/or a control surface deflection speed are/is increased, the less control surface deflection force that can be handled by the steering device, and a steering operation may be out of the range of the control surface deflection ability characteristics. Such an out-of-range operation is represented by the solid line “a” in which a control surface is deflected without restraining the propulsive force. In a preferred embodiment, the propulsive force is restrained in order to restrain the control surface deflection force so that the relationship shown by the solid line “a” is changed to the relationship shown by solid line “b”. As the relationship represented by line “b” is within the range of the control surface deflection ability characteristics, control surface deflection responsiveness can be ensured. Accordingly, when the control surface deflection ability characteristics are as shown by the broken line, if a propulsive force is restrained, for example, from d 1  to d 2  as shown in  FIG. 7  ( a ), a corresponding control surface deflection force is also restrained from d 1  to d 2 . Reducing the control surface deflection force can restore the steering system to operating within the control surface deflection ability characteristics as shown in  FIG. 7  ( b ), in which control surface deflection force is reduced from e 1  to e 2 . 
     As shown in  FIG. 7  ( c ), when a propulsive force is not controlled, operation of the steering wheel  17  tends to yield a slow response in reaching a corresponding operation angle (control surface angle) during a period shown as a broken line in the drawing. When, on the other hand, a propulsive force is restrained in order to restrain a corresponding control surface deflection force as described above, the operation angle (control surface angle) can be promptly changed relative to time t as shown by a solid line in the drawing. Consequently, restraining propulsive force quickens steering response times. 
     In another preferred mode of operation, a propulsive force of the outboard motor  12  is controlled depending on a running status. A propulsive force is controlled to be small when the boat is running at a high speed, a load of the boat is heavy, a trim angle is in a trim-in position, the boat is accelerating or decelerating, and the like. If a propulsive force is increased, a control surface deflection force is increased in the relationship between propulsive forces and control surface deflection forces as shown by (b) and (c) of  FIG. 8 . In this relationship, if a deflection angle is increased, or if the weight of the boat, a trim angle, a running speed, and an acceleration are increased, a control surface deflection force relative to a propulsive force is increased as shown by a broken line in (b). However, even these amounts are increased, an increase of a control surface deflection force can be restrained by restraining a propulsive force. 
     Also, when a control surface deflection speed is increased, a control surface deflection force relative to a propulsive force is increased from amounts shown by a solid line to amounts shown by a broken line in (c). However, even if a deflection speed is increased, such increase of a control surface deflection force can be restrained by restraining a propulsive force. 
     A broken line in  FIG. 8  ( a ) shows the control surface deflection ability characteristics, illustrating the relationship between control surface deflection forces under which the control surface can be deflected by the steering device  16  and control surface deflection speeds and control surface angles. In a case in which the boat is running at a high speed, a load of the boat is heavy, a trim angle is in a trim-in position, the boat is accelerating or decelerating, and the like, when a control surface deflection angle and/or a control surface deflection speed are/is increased, an operation may be out of the range of the control surface deflection ability characteristics as shown by the solid line “a” while a control surface is deflected without restraining a propulsive force. In such a case, the propulsive force can be restrained in order to restrain the control surface deflection force so that the relationship shown by the solid line “a” is changed to the relationship shown by a solid line “b”. As the latter relationship is within the range of the control surface deflection ability characteristics, control surface deflection responsiveness can be ensured. 
     Further, during an abrupt acceleration such as from time t 1  to time t 2  as depicted in  FIG. 9  ( a ) or during an abrupt deceleration from time t 3  to time t 4  in the same drawing, rotational speeds of the engine are changed as shown by a solid line in  FIG. 9  ( a ). Accordingly, if the control surface is deflected during such an acceleration or deceleration, a control surface deflection force is abruptly increased as shown by a solid line in (b) during such an acceleration or deceleration, exceeding the range of control surface deflection ability characteristics. However, if a sudden change in a rotation speed of the engine is restrained as shown by the broken line in  FIG. 9  ( a ), the abrupt increase of control surface deflection forces can be mitigated as shown by a broken line in (b). As such, the control surface deflection forces can be prevented from exceeding the range of the control surface deflection ability. 
     In yet another preferred mode of operation, a propulsive force of the outboard motor  12  is controlled depending on a running status. A propulsive force is controlled to be small when the installation number of the outboard motor  12  is large. Depending on a rotational direction of a propeller provided to the outboard motor  12 , a reaction force from the propeller is generated in one direction. When a control surface is deflected in the direction counteracting such reaction force, a propulsive force of the outboard motor  12  is decreased more than the case in which the control surface is deflected in the opposite direction. 
     As to the installation position of the outboard motor  12 , in a boat embodiment having a plurality of outboard motors  12 , when the boat is driven with only part of the outboard motors  12  actually in operation, or when the individual outboard motors  12  are in different trim status (when the lower part of the individual outboard motor  12  has a different underwater depth), control surface deflection load characteristics will not be the same between control surface deflection to the left and control surface deflection to the right. Accordingly, a propulsive force preferably is adjusted depending on whether the outboard motor  12  generating the propulsive force is on the left side or the right side in the width direction of the boat, and/or depending on whether the outboard motor  12  has a smaller trim angle and thereby a deeper underwater depth is on the left side or the right side in the width direction of the boat (for example, the propulsive force is decreased when the control surface is returned from a deflected position to the side on which the outboard motor  12  of a deeper underwater depth is installed). 
     A propulsive force of the outboard motor  12  can also be controlled depending on a motor status. As the motor temperature rises, the motor characteristics described above tend to be exhibited as shown by the broken line in  FIG. 11 , and thus generally less torque will be outputted from the motor. Accordingly, a propulsive force from the outboard motor  12  preferably is decreased to thereby prevent exceeding the limit of the ability of the electric motor  20 . In addition, in a boat with a plurality of electric motors  20 , when it is driven with a small number of the electric motors  20  in operation, a propulsive force of the outboard motors  12  is decreased to thereby prevent exceeding the limit of the ability of the electric motor  20 . 
     In the above boats, the outboard motor  12  is deflected by the electric motor  20 . It is advantageous that an operation feel of the steering wheel  17  can be lighter; however, when larger torque is required for control surface deflection for example, when the operator operates the steering wheel  17  faster, output from the electric motor  20  may become less responsive, resulting in slow response of a control surface deflection operation. In this embodiment, however, in accordance with the motor characteristics of the electric motor  20 , a propulsive force of the outboard motor  12  is controlled in order to restrain a control surface deflection force to thereby prevent exceeding the limit of the motor characteristics of the electric motor. 
     By controlling propulsion force, the outboard motor  12  is deflected within the limit of the output of the electric motor  20  by operating the steering wheel  17 . Thus, a control surface deflection operation does not become slow in response. 
     It is a matter of course that while in the foregoing embodiment, the outboard motor  12  is used as the “boat propulsion unit,” the principles discussed herein are not limited to such structure, but can also use other structures such as a stern-drive. Further, embodiments disclosed herein includes the operation status detection means  38 , the running status detection means  39 , the outboard motor status recognition means  40  and the electric motor status detection means  41 . Other embodiments may appropriately have only one or more, of these structures. 
     Although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. In addition, while a number of variations of the invention have been shown and described in detail, other modifications, which are within the scope of this invention, will be readily apparent to those of skill in the art based upon this disclosure. It is also contemplated that various combinations or subcombinations of the specific features and aspects of the embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form varying modes of the disclosed invention. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow.