Abstract:
A method for controlling the movement of a marine vessel rotates one of a pair of marine propulsion devices and controls the thrust magnitudes of two marine propulsion devices. A joystick is provided to allow the operator of the marine vessel to select port-starboard, forward-reverse, and rotational direction commands that are interpreted by a controller which then changes the angular position of at least one of a pair of marine propulsion devices relative to its steering axis.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally related to a maneuvering system for a marine vessel and, more particularly, to a system by which a marine vessel can be maneuvered through the use of a joystick and two or more independently steerable marine propulsion devices. 
     2. Description of the Related Art 
     Many different systems are known to those skilled in the art of marine vessel design for steering a marine vessel. In addition, numerous techniques have been developed by which an operator of a marine vessel can use a joystick to command various movements of the marine vessel. Many of these systems utilize two or more independently steerable marine propulsion devices that are manipulated in order to result in a desired movement of the marine vessel. These systems are particularly useful in docking a pleasure craft. Some of these maneuvering systems incorporate a bow thruster in combination with other propulsion devices, such as sterndrive units or outboard motors. However, some marine propulsion systems have been developed which provide adequate maneuvering of the marine vessel through the use of two independently steerable marine propulsion devices without the need for a bow thruster. 
     U.S. Pat. No. 3,521,589, which issued to Kemp on Jul. 21, 1970, describes an underwater vessel that comprises a shell having a pair of propelling and maneuvering devices mounted on opposite sides thereof. Each of the propelling devices includes a transversely extending hollow gooseneck, having an arm-receiving opening in one side thereof. An L-shaped arm assembly includes a hollow base portion extending coaxially through the gooseneck and a hollow secondary portion extending transversely of the base portion, geared thereto, and projecting through the arm-receiving opening. 
     U.S. Pat. No. 4,220,111, which issued to Krautkremer et al. on Sep. 2, 1980, describes a drive and control device for a watercraft having at least one pair of steerable propellers. The pair of steerable propellers is spaced longitudinally along the center of the watercraft from a center of lateral resistance on the watercraft. The steerable propellers are also spaced equidistant from the longitudinal centerline and on opposite sides thereof. 
     U.S. Pat. No. 4,691,659, which issued to Ito et al. on Sep. 8, 1987, describes an apparatus for steering joystick of ship. The rotation angles around the X and Y axes due to the operation of a joystick lever having degrees of freedom of three axes are respectively detected by two rotation angle detectors. A push button switch is provided at the edge of the joystick lever. When the push button switch is not operated, the operating direction and operation amount of the joystick lever are calculated on the basis of the outputs of two rotation angle detectors. When the push button switch was operated, the operating direction and operation amount of the joystick lever are calculated from the output of either one of two rotation angle detectors. 
     U.S. Pat. No. 4,947,782, which issued to Takahashi on Aug. 14, 1990, describes a remotely operated vehicle. The vehicle is provided with not less than three thrusters arranged in the longitudinal direction of a vehicle body. The center of gravity of the vehicle body includes a pendulum and the center of buoyancy of the vehicle body including the pendulum are set in agreement with each other and the pendulum is provided so that it can be turned around a Y-axis extending in the lateral direction of the vehicle body and passing the center of gravity thereof. 
     U.S. Pat. No. 5,090,929, which issued to Rieben on Feb. 25, 1992, describes a paired motor system for a small boat propulsion and steerage. The motors provide a steerable propelling system for small boats. Each motor drives a propeller carried in an elongate channel communicating with each lateral side of a boat beneath the water line to one boat end to move water through the channels for boat propulsion. The electric motors are of variable speed, reversible and separately controlled by a joystick-type control device to provide differential control of motor speed to allow steerage. 
     U.S. Pat. No. 5,924,379, which issued to Masini et al. on Jul. 20, 1999, discloses an actuating mechanism with an improved mounting structure. The mechanism is provided with supported members that extend away from the centerline of a cylinder bore, piston and actuator rod of an actuation mechanism that uses pressure to move the piston within a cylinder bore. Two support members are attached to a cylinder housing and provided with mounting holes. The two support members are spaced apart from the cylinder housing to allow external support structure to be placed between the cylinder housing and the two support members. Appropriate fasteners, such as bolts, attach each of the two support members to the external support structure in such a way that the cylinder housing can pivot about an axis extending through both bolts. Most importantly, a line extending through the supporting bolts intersect the cylinder bore at a place between its opposing ends. This reduces the required space necessary to allow the cylinder to pivot properly. 
     U.S. Pat. No. 6,142,841, which issued to Alexander et al. on Nov. 7, 2000, discloses a waterjet docking control system for a marine vessel. The maneuvering control system utilizes pressurized liquid at three or more positions on a marine vessel in order to selectively create thrust that moves the marine vessel into desired locations and according to chosen movements. A source of pressurized liquid, such as a pump or a jet pump propulsion system, is connected to a plurality of distribution conduits which, in turn, are connected to a plurality of outlet conduits. In any of the embodiments of the invention, a joystick control can be used to select or deselect each of the outlet conduits and, in certain embodiments, to select the direction of operation of an associated reversible motor. 
     U.S. Pat. No. 6,234,853, which issued to Lanyi et al. on May 22, 2001, discloses a simplified docking method and apparatus for a multiple engine marine vessel. A docking system is provided which utilizes the marine propulsion unit of a marine vessel, under the control of an engine control unit that receives command signals from a joystick or pushbutton device, to respond to a maneuver command from the marine operator. The docking system does not require additional propulsion devices other than those normally used to operate the marine vessel under normal conditions. The docking or maneuvering system of the invention uses two marine propulsion units to respond to an operator&#39;s command signal and allows the operator to select forward or reverse commands in combination with clockwise or counterclockwise rotational commands either in combination with each other or alone. 
     U.S. Pat. No. 6,276,977, which issued to Treinen et al. on Aug. 21, 2001, discloses an integrated hydraulic steering actuator. The actuator is provided for an outboard motor system in which the cylinder and piston of the actuator are disposed within a cylindrical cavity inside a cylindrical portion of a swivel bracket. The piston within the cylinder of the actuator is attached to at least one rod that extends through clearance holes of a clamp bracket and is connected to a steering arm of an outboard motor. 
     U.S. Pat. No. 6,406,340, which issued to Fetchko et al. on Jun. 18, 2002, describes a twin outboard motor hydraulic steering system. The steering assembly applies a force to the tiller arms of twin marine, outboard propulsion units and rotates the propulsion units about a steering axis between a center position and hard over positions to each side of the center position. A tie bar is pivotally connected to the steering apparatus and pivotally connected to the tiller arm of a second propulsion unit. 
     U.S. Pat. No. 6,447,349, which issued to Fadeley et al. on Sep. 10, 2002, describes a stick control system for watercraft boats. The boat has a reversing bucket for controlling forward/reverse thrust and a rotatable nozzle for controlling sideward forces. A bucket position sensor is connected to the reversing bucket, and the bucket is controlled using the output of the position sensor to enable the bucket to automatically moved to a neutral thrust position. 
     U.S. Pat. No. 6,684,803, which issued to Dickson on Feb. 3, 2004, describes a watercraft steering apparatus with a joystick. The apparatus includes a movable two directional joystick including a steering arm, a depressible throttle trigger affixed to an upper end portion of the joystick, and a pulley system including a steering cable attached to a lower end of the steering arm, the steering cable extending around pulleys affixed to the starboard or port side of the watercraft in matching pairs. 
     U.S. Pat. No. 6,755,703, which issued to Erickson on Jun. 29, 2004, discloses a hydraulically assisted gear shift mechanism for a marine propulsion device. The mechanism is for use in conjunction with a gear shift device and provides a hydraulic cylinder and piston combination connected by a linkage to a gear shift mechanism. Hydraulic pressure can be provided by a pump used in association with either a power trim system or a power steering system. Hydraulic valves are used to pressure selected regions of the hydraulic cylinder in order to actuate a piston which is connected, by an actuator, to the gear shift mechanism. 
     U.S. Pat. No. 6,896,563, which issued to Dickson on May 24, 2005, describes a joystick steering apparatus for a watercraft. The apparatus includes a joystick comprising at least three movable interconnected swinging arms with a first and third one of the swinging arms being generally vertically oriented and a second one of the swinging arms being generally horizontally oriented. It also includes a mechanical housing supporting the joystick and at least one mechanism movably connecting the joystick apparatus to an outdrive of the watercraft. 
     U.S. Pat. No. 6,994,046, which issued to Kaji et al. on Feb. 7, 2006, describes a marine vessel running controlling apparatus. The apparatus controls running of a marine vessel and includes a pair of propulsion systems which respectively generate propulsive forces on a rear port side and a rear starboard side of the hull and a pair of steering mechanisms which respectively change steering angles defined by directions of the propulsive forces with respect to the hull. The apparatus includes a target combined propulsive force acquiring section, a target movement angle acquiring section, a steering controlling section which controls the steering angles of the respective steering mechanisms such that a turning angular speed of the hull is substantially equal to a predetermined target angular speed, a target propulsive force calculating section which calculates target propulsive forces to be generated from the respective propulsion systems based on the target combined propulsive force, the target movement angle and the steering angles of the respective steering mechanisms, and a propulsive force controlling section which controls the respective propulsion systems so as to attain the target propulsive forces. 
     U.S. Pat. RE39,032, which issued to Gonring et al. on Mar. 21, 2006, discloses a multi-purpose control mechanism for a marine vessel. The mechanism allows the operator of a marine vessel to use the mechanism as both a standard throttle and a gear selection device and, alternatively, as a multi-axes joystick command device. The control mechanism comprises a base portion and a lever that is movable relative to the base portion along with a distal member that is attached to the lever for rotation about a central axis of the lever. A primary control signal is provided by the multi-purpose control mechanism when the marine vessel is operated in a first mode in which the control signal provides information relating to engine speed and gear selection. The mechanism can also operate in a second or docking mode and provide first, second and third secondary control signals relating to desired maneuvers of the marine vessel. 
     U.S. patent application Ser. No. 11/248,482 (M09992), which was filed on Oct. 12, 2005 by Bradley et al., discloses a method for maneuvering a marine vessel in response to a manually operable control device. The marine vessel is maneuvered by independently rotating first and second marine propulsion devices about their respective steering axes in response to commands received from a manually operable control device, such as a joystick. The marine propulsion devices are aligned with their thrust vectors intersecting at a point on a centerline of the marine vessel and, when no rotational movement is commanded, at the center of gravity of the marine vessel. Internal combustion engines are provided to drive the marine propulsion devices. The steering axes of the two marine propulsion devices are generally vertical and parallel to each other. The two steering axes extend through a bottom surface of the hull of the marine vessel. 
     U.S. patent application Ser. No. 11/248,483 (M09993), which was filed on Oct. 12, 2005 by Bradley et al., discloses a method for positioning a marine vessel. A vessel positioning system maneuvers a marine vessel in such a way that the vessel maintains its global position and heading in accordance with a desired position and heading selected by the operator of the marine vessel. When used in conjunction with a joystick, the operator of the marine vessel can place the system in a station keeping enable mode and the system then maintains the desired position obtained upon the initial change of the joystick from an active mode to an inactive mode. In this way, the operator can selectively maneuver the marine vessel manually and, when the joystick is released, the vessel will maintain the position in which it was at the instant the operator stopped maneuvering it with the joystick. 
     U.S. patent application Ser. No. 11/365,175 (M09981), which was filed by Griffiths et al. on Mar. 1, 2006, discloses a selectively lockable marine propulsion device. A steering system for a marine vessel is provided with a connecting link attached to first and second marine propulsion devices. The connecting link is selectively disposable in first and second states of operation which either require synchronous rotation of the first and second marine propulsion devices or, alternatively, independent rotation of the two marine propulsion devices. This allows both marine propulsion devices to be operated by a single actuator or, alternatively, independent maneuvering of the two marine propulsion devices during certain types of docking procedures. 
     The patents described above are hereby expressly incorporated by reference in the description of the present invention. 
     SUMMARY OF THE INVENTION 
     A method for controlling the movement of a marine vessel, in accordance with a preferred embodiment of the present invention, comprises the steps of attaching first and second marine propulsion devices to the marine vessel for rotation about first and second steering axes, connecting first and second steering actuators to the first and second marine propulsion devices, respectively, providing first and second manually manipulatable control devices which are configured to receive position-related commands from an operator of the marine vessel, providing a controller connected in signal communication with the first and second manually manipulatable control devices, receiving a set of vessel maneuver commands from the second manually manipulatable control device, calculating first and second thrust magnitudes for the first and second marine propulsion devices, respectively, and an angular position of the second marine propulsion device which will achieve the appropriate maneuver of the marine vessel, causing the first and second marine propulsion devices to generate the first and second thrust magnitudes, and causing the second marine propulsion device to move to the angular position. 
     In a preferred embodiment of the present invention, it further comprises the step of connecting a third steering actuator to the first and second marine propulsion devices. The third steering actuator is selectively configurable in a first state wherein the first and second marine propulsion devices are rigidly attached together for synchronous rotation about their respective steering axes and a second state wherein the first and second marine propulsion devices are movable relative to each other. In a preferred embodiment of the present invention, it further comprises the step of causing the first marine propulsion device to move to a fixed position during maneuvering steps. The fixed position disposes a propeller shaft of the first marine propulsion device in parallel association with a central axis of the marine vessel extending from a transom to a bow of the marine vessel. This central axis is generally aligned with a keel of the marine vessel. 
     In a particularly preferred embodiment of the present invention, it further comprises the steps of using first and second steering actuators to respond to the vessel movement commands. The first and second marine propulsion devices are sterndrive units in a preferred embodiment of the present invention and the sterndrive units are attached to a transom of the marine vessel. The second manually manipulatable control device is a joystick in a preferred embodiment of the present invention and the first manually manipulatable control device is a steering wheel. The controller comprises a microprocessor in a preferred embodiment of the present invention and the vessel maneuver commands comprise a left-right command, a forward-reverse command, and a rotate command. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more fully and completely understood from a reading of the description of the preferred embodiment in conjunction with the drawings, in which: 
         FIGS. 1-4  are schematic illustrations showing various movements of marine propulsion devices relative to a marine vessel; 
         FIGS. 5A-5E  show various maneuvering movements of a marine vessel that can be achieved through the use of a preferred embodiment of the present invention; 
         FIGS. 6A-6C  illustrate various characteristics of a joystick device; 
         FIGS. 7-9  show various movements of marine propulsion devices in response to activation of steering actuators used in a preferred embodiment of the present invention; and 
         FIG. 10  is a table showing hypothetical joystick commands and the thrusts and angular positions of marine propulsion devices which achieve those joystick commands. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Throughout the description of the preferred embodiment of the present invention, like components will be identified by like reference numerals. 
       FIG. 1  is a simplified schematic representation of a marine vessel  10  with a first marine propulsion device  11  and a second marine propulsion device  12  attached to a transom  14  of the marine vessel. Arrows  17  and  18  represent exemplary thrusts provided by the first and second marine propulsion devices. Although both arrows in  FIG. 1  illustrate forward thrusts which exert forces on the marine vessel  10  to cause it to move in a forward direction, it should be understood that either one or both of the marine propulsion devices,  11  and  12 , can be operated in reverse gear to provide the opposite effect. 
     Point  20  in  FIG. 1  represents an effective center of gravity (i.e. center of turn) for the marine vessel  10 . It should be understood that the location of point  20  is not, in all cases, the actual center of gravity of the marine vessel  10 . Because of many factors, the effective center of gravity  20  can be at a different location than the actual center of gravity that would be calculated by analyzing the weight distribution of the various components of the marine vessel. In addition, it should be understood that maneuvering a boat  10  in a body of water results in reactive forces exerted against the hull of the boat by the wind and the water. In other words, as various maneuvering thrusts are exerted by the first and second marine propulsion devices,  11  and  12 , the hull of the boat pushes against the water in front of it. That water exerts a reaction force against the hull. As a result, the effective center of gravity identified as point  20  in  FIG. 1  can change in response to different sets of forces and reactions exerted on the hull of the marine vessel  10 . This concept is recognized by those skilled in the art and is referred to as the instantaneous center of turn in U.S. Pat. No. 6,234,853 and as the instantaneous center in U.S. Pat. No. 6,994,046. 
     With continued reference to  FIG. 1 , the distance between the instantaneous center of turn, or instantaneous center of gravity  20 , and the transom  14  is identified as dimension A and the dimension between the steering axes,  21  and  22 , of the first and second marine propulsion devices is identified as dimension B. Both of the marine propulsion devices are rotatable about their individual steering axes,  21  and  22 . Dashed line  26  represents a possible range of travel of the thrust  18  of the second marine propulsion device  12  about its steering axis  22 . This angle, between dashed line  26  and dashed line  28  is identified as angle θ. In a preferred embodiment of the present invention, both marine propulsion devices are rotatable about their individual steering axes,  21  and  22 , in both clockwise and counterclockwise directions. 
     One possible configuration of the marine vessel  10  and its marine propulsion devices,  11  and  12 , is shown in  FIG. 2 . Both thrust vectors,  17  and  18 , are directed through the center of gravity  20 , or instantaneous center of turn. Under the condition shown in  FIG. 2 , no effective moment about the center of gravity  20  would exist. As a result, all movement of the marine vessel  10  would occur without rotation of the boat about the center of gravity  20 . The various possible maneuvering actions will be described in greater detail below. 
       FIG. 3  illustrates the marine vessel  10  with the first marine propulsion device  11  aligned with a line  30  that is generally parallel with a centerline  32  of the marine vessel. The centerline  32  is generally aligned with a keel of the marine vessel that extends from its transom  14  to the bow  36 . The second marine propulsion device  12  is turned, at angle θ, so that its thrust vector  18  is aligned with dashed line  26 . The thrust vector  18  is resolved in  FIG. 3  to a Y axis vector  40  and an X axis vector  42 . For purposes of this description, the X axis represents a line  44  that is generally perpendicular to the centerline  32  and exerts a force on the marine vessel  10  in a left or right direction. The Y axis is generally parallel to the centerline  32  and exerts a force on the marine vessel  10  in a forward or reverse direction. 
       FIG. 4  illustrates the marine vessel  10  with the marine propulsion devices,  11  and  12 , rotated about their respective steering axes,  21  and  22 , to direct their thrusts,  17  and  18 , in a parallel direction to each other. This type of configuration would result when the marine vessel  10  was operated to steer the boat in a convention manner which typically would incorporate a steering wheel. When operating in this way, the thrusts,  17  and  18 , of the first and second marine propulsion devices are typically parallel and the marine propulsion devices are typically rotated in synchrony about their respective steering axes,  21  and  22 . 
     With continued reference to  FIGS. 1-4 , it can be seen that two individually steerable marine propulsion devices allow the operator of a marine vessel a wide variety of maneuvering actions that can be used to cause the boat to move through many different maneuvering and docking motions. 
       FIGS. 5A-5E  are simplified illustrations showing the many different movements that can be accomplished through the use of two individually steerable marine propulsion devices. In  FIG. 5A , the boat  10  is shown as being capable of moving to the left  46  and to the right  48  without any forward or reverse motion and without any rotation about its instantaneous center of turn.  FIG. 5B , on the other hand, shows the marine vessel  10  moving only forward  50  or backward  52  to the positions identified by dashed line representations of the boat.  FIG. 5C  shows a combination of forward and starboard motions of the marine vessel  10 . The forward movement is represented by dashed arrow  56  and the starboard movement is represented by dashed arrow  58 . The resultant vector  60  causes the marine vessel  10  to move toward the position represented by dashed lines in  FIG. 5C .  FIG. 5D  illustrates a rotation  62  of the marine vessel  10  without any movement in a forward, reverse, left, or right direction. The rotation  62  is about the instantaneous center of turn  20 . The maneuver represented in  FIG. 5D  can be particularly advantageous during docking maneuvers.  FIG. 5E  illustrates a combination of rotation  62  and a translation  60  which is both forward and in the starboard direction. This combination of translation and rotation results in the marine vessel  10  being located in the position represented by dashed lines in  FIG. 5E . 
     With continued reference to  FIGS. 5A-5E , it can be seen that many different types of maneuvering motions are possible with two marine propulsion devices that are independently steerable and which can exert both forward and reverse thrusts of selectable magnitude. The concepts described above in conjunction with  FIGS. 1-4  and  5 A- 5 E, are also described in U.S. Pat. Nos. 3,521,586 and 4,220,111. In addition, the maneuvering capabilities of such a marine vessel are also described in U.S. Pat. Nos. 6,234,853 and 6,994,046 and in the patent applications filed by Bradley et al. and described above. 
       FIG. 6A  is a simplified schematic representation of a joystick device  70 . It typically comprises a shaft  72  and a handle  74 . The shaft  72  is movable, as represented by dashed line arrow  76  in numerous directions relative to the base  78 .  FIG. 6B  illustrates the shaft  72  and handle  74  in three different positions which vary by the magnitude of its angular movement. Arrows  80  and  82  show the different magnitude of movement. In a typical joystick application, the degree of movement away from a generally vertical position, as shown in  FIG. 6A , represents the analogous magnitude of an actual movement command selected by an operator.  FIG. 6C  is a top view of the joystick device  70  in which the handle  74  is in a central, or neutral, position. The handle  74  can be manually manipulated in a forward F, rearward R, port P or starboard S direction. In addition, it can be rotated about the centerline  86  of the rod  72  as represented by arrow M. 
     With continued reference  6 A- 6 C, a marine vessel operator can manipulate the joystick to provide maneuver commands that will cause the marine vessel to move in a forward-reverse, port-starboard, or rotating movement or any combination of these movements. 
     It is well known that a marine vessel can be caused to achieve many different types of motions in response to joystick commands provided by the operator of the marine vessel. However, known maneuvering systems cause both of the marine propulsion devices to be steered in cooperation with each other. Typically, this steering is performed in synchrony. In certain situations, it can be beneficial if the maneuvering, such as during docking procedures, can be accomplished without requiring motion of both marine propulsion devices. It is recognized that in certain complex systems, it is possible to logically disconnect a steering wheel (e.g. by interrupting signal communication with the drives) from its associated actuators which are configured to cause rotation of the marine propulsion devices about their respective steering axes. This logical disconnection is preferred during joystick docking maneuvers so that the steering wheel does not continually rotate in response to rotation of the marine propulsion devices about their steering axes. However, in smaller marine vessels with less complicated control systems, it would also be preferred if the steering wheel could be stationary during these joystick docking maneuvers. 
     An additional consideration, in relation to marine vessels with two or more marine propulsion devices, is the potential loss of hydraulic pressure of one of the two steering systems. In many marine applications, a rigid bar or link is connected between the steering arms of the marine propulsion devices. Naturally, if the marine propulsion devices are to be independently steered, the link must be removable. However, if one of the two hydraulic steering systems experiences a failure, a rigid link can be extremely helpful in allowing the operator of the marine vessel to steer one of the two marine propulsion devices and have the rigid link cause the other marine propulsion device to move in synchrony with the working hydraulic system. 
       FIG. 7  is a simplified representation of a preferred embodiment of the present invention. The dashed line  10  represents the aft portion of the marine vessel and reference numeral  14  indicates the approximate location of the transom of the marine vessel. First and second manually manipulatable control devices,  101  and  102 , are shown connected in signal communication with a controller  106 . The first manually manipulatable control device  101  is a steering wheel  110 . The steering wheel is shown attached to a console  112 . The second manually manipulatable control device  102  is illustrated as the joystick device  70  described above in conjunction with  FIGS. 6A-6C . 
     In a preferred embodiment of the present invention, the steering actuators are hydraulic actuators. However, it should be clearly understood that this is not a requirement in all embodiments. Alternative types of actuators (e.g. electric motors, pneumatic actuators) can be used within the scope of the present invention. 
     With continued reference to  FIG. 7 , a first steering actuator  121  and a second steering actuator  122  are shown connected to steering arms,  131  and  132 , of the first and second marine propulsion devices,  11  and  12 . The first and second steering actuators,  121  and  122 , are configured to cause the steering arms,  131  and  132 , to rotate about the steering axes,  21  and  22 . Actuation of the first and second steering actuators is caused by the valve  140  which is controlled by the controller  106 . A hydraulic pump  144  provides pressurized hydraulic fluid which is conducted through the valve  140  in a selective manner in order to actuate the steering actuators. As an example, if the operator of the marine vessel  10  turns the steering wheel  110  in a clockwise rotation to steer the boat toward the right, pressurized hydraulic fluid would be conducted, by the valve, into hydraulic conduits  148  and  150 . If, on the other hand, the steering wheel  110  was rotated in a counterclockwise rotation by the operator to cause the boat  10  to turn toward the left, pressurized fluid would be conducted into conduits  154  and  156 . Naturally, the valve  140  would conduct the return hydraulic fluid through the other conduits associated with the first and second steering actuators,  121  and  122 . 
     With continued reference to  FIG. 7 , a third steering actuator  123  is connected to both the first and second steering arms,  131  and  132 , and serves to alternately lock the steering arms together or allow them to rotate independently about their respective steering axes,  21  and  22 . When valve  160  is closed by the controller  106 , hydraulic fluid is not permitted to flow between the chambers identified by reference numerals  164  and  165 . Since the hydraulic fluid cannot flow around the piston  167  and through the valve  160 , the position of the piston  167  is locked relative to the position of its cylinder  169 . Alternatively, by allowing fluid to flow through the valve  160 , the controller can unlock the third steering actuator  123  and permit the two marine propulsion devices to rotate about their respective steering axes independent from each other. The result of the changing of the status of valve  160  and the third steering actuator  123  is to change the movement of the first and second marine propulsion devices,  11  and  12 , from the synchronous motion described above in conjunction with  FIG. 4  and the independent motion described above in conjunction with  FIGS. 2 and 3 . Throughout the description of the preferred embodiment of the present invention, it should be understood that the physical position of the first and second steering actuators,  121  and  122 , relative to the steering arms,  131  and  132 , is not limiting to the scope of the present invention. In other words, the first steering actuator  121  could be located to the right of the first steering arm  131  rather than to the left as shown in  FIG. 7 . Similarly, the steering actuators can be incorporated integrally with transom brackets of the marine propulsion devices. The illustration in FIG.  7  is schematic and intended to show an exemplary functional relationship between the various components. 
     With continued reference to  FIG. 7 , it can be seen that the first, second, and third steering actuators can be operated in many different ways. For example, during conventional steering of the marine vessel  10  with the steering wheel  110 , valve  160  can be closed so that the third steering actuator  123  acts as a rigid bar or link between the first and second steering arms,  131  and  132 . In this state, the first and second steering actuators,  121  and  122 , can be operated in synchrony with both of the steering actuators exerting force on their associated steering arms. The third steering actuator  123  would maintain the two steering arms in parallel association with each other so that hydraulic pressure conducted into either of the hydraulic conduits  154  and  156  or into hydraulic conduits  148  and  150  will cause synchronous rotation of the first and second marine propulsion devices,  11  and  12 . Alternatively, with the third steering actuator acting as a rigid rod, the valve  140  can allow free flow of hydraulic fluid through conduits  150  and  156  to deactivate the effect of the second steering actuator  122  on the steering effort. As a result, the first steering actuator  121 , acting in cooperation with the third steering actuator and closed valve  160 , can steer both marine propulsion devices with the piston of the second steering actuator  122  merely moving in response to movement of the second steering arm  132  to which it is connected. Alternatively, the first steering actuator  121  can be deactivated in this way, with the valve  140  allowing a free flow of hydraulic fluid through conduits  148  and  154 , and the second steering actuator  122  can cause both marine propulsion devices to rotate about their steering axes. In both of these hypothetical situations, valve  160  is closed by the controller  106  to lock the position of piston  167  relative to cylinder  169 . As a result, the third steering actuator  123  acts as a rigid bar between the steering arms  131  and  132 . 
     With continued reference to  FIG. 7 , it should also be understood that the controller  106  can open valve  160  to allow a free flow of hydraulic fluid between chambers  164  and  165 . As a result, piston  167  can move freely within chamber  165  and the two steering arms,  131  and  132 , can move independently from each other. This allows the first and second steering actuators,  121  and  122 , to control their associated steering arms independently of each other. This type of motion allows the results illustrated in  FIGS. 2 and 3 . When the operator of the marine vessel is using the joystick  70 , the controller  106  would typically open valve  160  to allow independent rotation of the marine propulsion devices,  11  and  12 . Alternatively, when the operator of the marine vessel is using the steering wheel  110 , the controller  106  would typically close the valve  160  so that the third steering actuator  123  locks the steering arms together and causes the two marine propulsion devices to rotate in synchrony with each other under the control of one or both of the first and second steering actuators,  121  and  122 . 
       FIG. 8  illustrates the system described above in conjunction with  FIG. 7 , but with the second marine propulsion device  12  rotated about its steering axis  22  independent of the position of the first marine propulsion device  11 . The valve  140 , controlled by the controller  106 , causes pressurized hydraulic fluid to flow through conduit  150  with returning fluid flowing through conduit  156 . Similarly, valve  160  is opened so that hydraulic fluid can flow from chamber  164  to chamber  165 . In a preferred embodiment of the present invention, the controller  106  would also cause the valve  140  to prevent fluid flow through conduits  148  and  154 . This locks the position of the piston of the first steering actuator  121  in place and prevents rotation of the first marine propulsion device  11  about its steering axis  21 . As a result, the operator of the marine vessel is able to use the joystick  70  to maneuver the marine vessel. Also, in this embodiment of the present invention, the first marine propulsion device  11  is locked in a forward position and all maneuvering motions are achieved through the rotation of only the second marine propulsion device  12 . 
       FIG. 9  shows the configuration similar to  FIGS. 7 and 8 , but with the second marine propulsion device  12  rotated in an opposite direction than illustrated in  FIG. 8 . The movement shown in  FIG. 9  would be accomplished by the controller  106  controlling the valve  140  to cause a flow of pressurized hydraulic fluid to flow through conduit  156  and return from the second steering actuator  122  through conduit  150 . The valve  140  would be controlled to prevent flow through conduits  148  and  154  in order to lock the first steering arm  131  in place as described above. 
     When the operator of the marine vessel is controlling the boat with the joystick during maneuvering procedures, the thrusts,  17  and  18 , provided by the two marine propulsion devices are controlled, along with the gear selection of both devices, in the manner described in U.S. Pat. No. 4,220,111 or 4,947,782. Control of the thrust vectors,  17  and  18 , can also be performed in the manner described in U.S. Pat. No. 6,234,853 or 6,994,046. Since the techniques used to control the direction and magnitude of the thrusts provided by marine propulsion devices are well known to those skilled in the art and described above in several United States patents, those techniques will not be described in detail herein. Typically, systems of that type use a digital throttle and shift (DTS) system in which a microprocessor transmits control signals to a controller within each marine propulsion device. 
     It should be understood that, in a preferred embodiment of the present invention, all maneuvering motions are achieved through the movement of only the second marine propulsion device  12  with the first marine propulsion device  11  locked in a forward position as illustrated in  FIGS. 8 and 9 . Although the present invention allows various alternative operations, the capability of the preferred embodiment shown in  FIGS. 8 and 9  is illustrated in the table of  FIG. 10 . 
     With continued reference to  FIG. 9 , an alternative embodiment of the present invention could eliminate the first steering actuator  121  (i.e. the one illustrated on the port side), connect the valve  140  to the third steering actuator  123  (i.e. the one in the center), and use the second steering actuator  122  (i.e. the actuator at the starboard position) as a “locked” actuator during docking maneuvers. The steering would then be achieved through the use of the third steering actuator  123 , in response to a joystick, while the locked actuator  122  would prevent the steering wheel from moving during the docking procedure. 
     With reference to  FIG. 10 , twenty-seven examples of joystick positions are shown in combination with the resulting thrust magnitude for the port and starboard marine propulsion devices,  11  and  12 , and the magnitude of angle θ illustrated in  FIGS. 8 and 9 . It should be understood that the magnitudes in the table of  FIG. 10  are exemplary and selected solely to show that numerous combinations of magnitudes, both positive and negative, can be achieved and satisfied through the sole movement of the second marine propulsion device  12 . The exemplary numbers in the table of  FIG. 10  do not represent specific units (e.g. pounds or foot-pounds), but are used to show the ability of the system to satisfy the various commands received from the joystick. As described above in conjunction with  FIGS. 6A-6C , the handle  74  of the joystick can be moved forward, backward, to the left, to the right, and rotated to convey the desire of the marine vessel operator to the controller  106 . Naturally, a maximum movement in any direction would represent some associated maximum analogous level of thrust in that direction or some maximum moment of rotation. In the table of  FIG. 10 , Example 1 represents a movement of the joystick handle  74  toward the right to request a thrust of 10 units (e.g. pounds), a movement forward to request a forward thrust of 10 units and a rotation of the handle  74  to represent a request of 10 units (e.g. foot-pounds) moment. As shown in the table, the controller  106  would determine that a port thrust  17  of 41.667 units in a forward direction, a starboard thrust  18  of −33.208 units and an angle θ of −17.526 degrees for the second marine propulsion device  12  would result in the joystick request. The 27 examples in  FIG. 10  show all of the possible combinations of 10, 0, and −10 for all three of the commands received from the joystick  70 . In these hypothetical examples shown in the table, the X dimension for the joystick represents a left-right selection, the Y dimension represents a forward-reverse selection and the M dimension represents a rotation of the handle  70  about is central axis  86  as discussed above in conjunction with  FIGS. 6B and 6C . The results illustrated in the table of  FIG. 10  show that movement of the second marine propulsion device  12 , with the first marine propulsion device  11  fixed in a forward direction, allows the controller  106  to achieve all of the commanded movements selected by the operator of the marine vessel. 
     With continued reference to  FIGS. 1-4 ,  5 A- 5 E,  6 A- 6 C, and  7 - 10 , it can be seen that a preferred embodiment of the present invention provides for a method for controlling the movement of a marine vessel which comprises the steps of attaching a first sterndrive unit  11  to the marine vessel  10  for rotation about a first steering axis  21 , attaching a second sterndrive unit  12  to the marine vessel  10  for rotation about a second steering axis  22 , connecting a first steering actuator  121  to the first sterndrive unit  11 , connecting a second steering actuator  122  to the second sterndrive unit  12 , connecting a third steering actuator  123  between the first and second sterndrive units,  11  and  12 , with the third steering actuator  123  being selectively configurable in a first state wherein the first and second sterndrive units,  11  and  12 , are rigidly attached together for synchronous rotation about their respective steering axes,  21  and  22 , and a second state wherein the first and second sterndrive units are movable relative to each other, providing a steering wheel  110  which is configured to receive vessel movement commands from an operator of the marine vessel, providing a joystick  70  which is configured to receive vessel maneuver commands from the operator of the marine vessel, providing a controller  106  connected in signal communication with the steering wheel  110  and the joystick  70 , receiving a set of vessel maneuver commands (e.g. X, Y, and M in  FIG. 10 ) from the joystick  70 , calculating a first thrust magnitude (e.g. thrust port in  FIG. 10 ) for the first sterndrive unit  11 , calculating a second thrust magnitude (e.g. thrust starboard in  FIG. 10 ) for the second sterndrive unit  12 , and calculating an angular position (e.g. theta in  FIG. 10 ) of the second sterndrive unit  12  which will achieve a maneuver of the marine vessel  10  which accomplishes the vessel maneuver commands, causing the second sterndrive unit  10  to move to the angular position θ, causing the first drive sterndrive unit  11  to generate the first thrust magnitude  17 , and causing the second sterndrive unit  12  to generate the second thrust magnitude  18 . 
     With continued reference to all of the figures, a preferred embodiment of the present invention can further comprise the steps of using the first and second steering actuators,  121  and  122 , to respond to the vessel movement commands. In addition, it can further comprise the step of causing the first sterndrive unit  11  to move to a fixed position, as illustrated in  FIG. 8 , wherein the fixed position disposes a propeller shaft of the first sterndrive unit  11  in parallel association with a central axis  32  of the marine vessel  10  extending from a transom  14  to a bow  36  of the marine vessel  10 . In a preferred embodiment of the present invention, the vessel maneuver commands comprise a left-right command, a forward-reverse command, and a rotate command. 
     Although the present invention has been described with particularly specificity and illustrated to show a preferred embodiment, it should be understood that alternative embodiments are also within its scope.