Patent Publication Number: US-7901258-B1

Title: Hydraulic system for a marine outboard engine

Description:
CROSS-REFERENCE 
     This application claims priority to U.S. Provisional Patent Application No. 60/991,370, filed Nov. 30, 2007, the entirety of which is enclosed herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to hydraulic systems used on marine outboard engines. 
     BACKGROUND OF THE INVENTION 
     Marine outboard engines have various systems that are necessary for their operation, or at least necessary to facilitate and/or improve their operation. A steering system is used to steer the outboard engine. A tilt and trim system is to adjust the vertical orientation of the outboard engine. A throttle system is used to control the flow of air to the engine of the outboard engine to control the power generated by the engine. A shifting system is used to shift the direction of rotation of a propeller of the outboard engine. A variable pitch propeller system is sometimes used to change the pitch of the propeller blades of the propeller. 
     Most of today&#39;s marine outboard engines have two or more of the above systems. Actuation of these systems can be done in different ways such as electrically (with electric motors or solenoids) or mechanically (with linkages or push-pull cables). Another way of actuating these systems is through the use of hydraulic actuators. When using hydraulic actuators, each hydraulically actuated system includes an a hydraulic fluid reservoir, a pump for pumping hydraulic fluid, at least one hydraulic actuator, and at least one valve for controlling the actuation of the actuator, and hoses for connecting all of these components together. 
     As would be understood, when multiple systems are hydraulically actuated, the assembly of the systems becomes complex due to the great number of hydraulic parts and the relatively limited space provided in outboard engines. Also, the increased complexity and number of parts increases the likelihood of failure in one of the hydraulically actuated systems. Finally, hydraulic components, such as pumps, are relatively expensive, therefore using multiple hydraulic systems significantly increases the cost of manufacturing outboard engines. 
     Therefore, there is a need for outboard engine having multiple hydraulically actuated systems that ameliorates at least some of the above inconveniences. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to ameliorate at least some of the inconveniences present in the prior art. 
     It is also an object of the present invention to provide an outboard engine having at least three hydraulically actuated systems, and having a hydraulic system having a pump for supplying hydraulic fluid to the hydraulic actuators of the three hydraulically actuated systems via at least one valve. 
     It is another object of the invention to provide a method of prioritizing the actuation of the hydraulically actuated systems of the above outboard engine in case of reduction of fluid pressure in the hydraulic systems. 
     In one aspect, the invention provides a marine outboard engine having an upper engine cover, a lower engine cover disposed vertically below the upper motor cover, a swivel bracket operatively connected to at least one of the upper and lower engine covers, a stern bracket connected to the swivel bracket, an engine disposed at least part in the upper motor cover, and a driveshaft disposed generally vertically in the lower engine cover. The driveshaft has a first end and a second end. The first end of the driveshaft is operatively connected to the engine. A gear case connected to the lower engine cover. A propeller shaft is disposed at least in part in the gear case generally perpendicular to the driveshaft. The propeller shaft is operatively connected to the second end of the driveshaft. A propeller is connected to the propeller shaft. The propeller has a hub and a plurality of propeller blades disposed on the hub. The plurality of propeller blades are rotatable relative to the hub to adjust a pitch of the plurality of propeller blades. A hydraulic system includes a hydraulic steering actuator operatively connected to the swivel bracket for steering the marine outboard engine about a generally vertical steering axis, a hydraulic trim actuator operatively connected to the swivel bracket for trimming the marine outboard engine about a generally horizontal trim axis, a hydraulic propeller pitch actuator operatively connected to the plurality of propeller blades for adjusting the pitch of the plurality of propeller blades, at least one valve having at least one inlet, a first outlet, a second outlet, and a third outlet, the first outlet fluidly communicating with the hydraulic steering actuator, the second outlet fluidly communicating with the hydraulic trim actuator, and the third outlet fluidly communicating with the hydraulic propeller pitch actuator, a pump fluidly communicating with the at least one inlet of the at least one valve for pumping hydraulic fluid to the at least one valve, and a reservoir for storing hydraulic fluid, the reservoir fluidly communicating with the pump for supplying hydraulic fluid to the pump. A pressure sensor is associated with the hydraulic system for sensing a pressure of the hydraulic fluid in the hydraulic system. A control unit electronically communicates with the pressure sensor and with the at least one valve. The control unit controls the at least one valve to control a flow of hydraulic fluid through each of the first, second, and third outlets based at least in part on a signal received from the pressure sensor. 
     In a further aspect, the marine outboard engine has an accumulator chamber fluidly communicating with the pump and the at least one inlet of the at least one valve. The accumulator chamber is downstream of the pump and upstream of the at least one inlet of the at least one valve. 
     In an additional aspect, the at least one valve is a priority valve. 
     In a further aspect, the hydraulic system further includes a first valve fluidly communicating with the first outlet of the priority valve and the hydraulic steering actuator for controlling the flow of hydraulic fluid to the hydraulic steering actuator, a second valve fluidly communicating with the second outlet of the priority valve and the hydraulic trim actuator for controlling the flow of hydraulic fluid to the hydraulic trim actuator, and a third valve fluidly communicating with the third outlet of the priority valve and the hydraulic propeller pitch actuator for controlling the flow of hydraulic fluid to the hydraulic propeller pitch actuator. 
     In an additional aspect, the at least one valve has a fourth outlet. The hydraulic system further includes a fourth hydraulic actuator fluidly communicating with the fourth outlet of the at least one valve. The fourth hydraulic actuator being one of a hydraulic throttle actuator and a hydraulic shift actuator. The control unit controls the at least one valve to control a flow of hydraulic fluid through the fourth outlet based at least in part on a signal received from the pressure sensor. 
     In a further aspect, the fourth hydraulic actuator is the hydraulic throttle actuator. 
     In an additional aspect, the at least one valve is a priority valve. 
     In a further aspect, the hydraulic system further includes a first valve fluidly communicating with the first outlet of the priority valve and the hydraulic steering actuator for controlling the flow of hydraulic fluid to the hydraulic steering actuator, a second valve fluidly communicating with the second outlet of the priority valve and the hydraulic trim actuator for controlling the flow of hydraulic fluid to the hydraulic trim actuator, a third valve fluidly communicating with the third outlet of the priority valve and the hydraulic propeller pitch actuator for controlling the flow of hydraulic fluid to the hydraulic propeller pitch actuator, and a fourth valve fluidly communicating with the fourth outlet of the priority valve and the fourth hydraulic actuator for controlling the flow of hydraulic fluid to the fourth hydraulic actuator. 
     In an additional aspect, the fourth hydraulic actuator is the hydraulic throttle actuator. 
     In a further aspect, the hydraulic system further includes a manifold fluidly communicating with the pump. The at least one valve includes a first valve, a second valve, and a third valve. The first valve has a first inlet fluidly communicating with the manifold and has the first outlet. The second valve has a second inlet fluidly communicating with the manifold and has the second outlet. The third valve has a third inlet fluidly communicating with the manifold and has the third outlet. 
     In an additional aspect, the at least one valve has a fourth outlet. The hydraulic system further includes a fourth hydraulic actuator fluidly communicating with the fourth outlet of the at least one valve. The at least one valve further includes a fourth valve. The fourth valve has a fourth inlet fluidly communicating with the manifold and has the fourth outlet. 
     In another aspect, the invention provides a method of controlling a hydraulic system of a marine outboard engine. The hydraulic system includes a pump, at least one valve fluidly communicating with the pump, and first, second, and third hydraulic actuators fluidly communicating with the at least one valve. The marine outboard engine includes a pressure sensor associated with the hydraulic system for sensing a pressure of the hydraulic fluid in the hydraulic system. The method comprises receiving a pressure input from the pressure sensor; receiving first, second, and third inputs associated with the first, second, and third hydraulic actuators respectively; prioritizing actuation of the first, second, and third hydraulic actuators based at least in part on the pressure input and the first, second, and third inputs, such that the second hydraulic actuator has a higher priority than the third hydraulic actuator, and the first hydraulic actuator has a higher priority than the second hydraulic actuator; and controlling the at least one valve for controlling a flow of hydraulic fluid from the at least one valve to the first, second, and third hydraulic actuators based at least in part on the prioritizing. 
     In a further aspect, controlling the at least one valve based at least in part on the prioritizing includes: a) if the first input indicates a desired actuation of the first hydraulic actuator, causing the at least one valve to provide hydraulic fluid to the first hydraulic actuator and actuating the first hydraulic actuator according to the first input; b) if the second input indicates a desired actuation of the second hydraulic actuator, causing the at least one valve to provide hydraulic fluid to the second hydraulic actuator and actuating the second hydraulic actuator according to the second input; and c) if the third input indicates a desired actuation of the third hydraulic actuator, causing the at least one valve to provide hydraulic fluid to the third hydraulic actuator and actuating the third hydraulic actuator according to the third input; wherein b) occurs only when the pressure in the hydraulic system is sufficient to carry out a) and b); and wherein c) occurs only when the pressure in the hydraulic system is sufficient to carry out a), b) and c). 
     In an additional aspect, the first hydraulic actuator is a hydraulic steering actuator, the second hydraulic actuator is a hydraulic throttle actuator, and the third hydraulic actuator is a hydraulic propeller pitch actuator. 
     In a further aspect, when the second input indicates a desired actuation of the hydraulic throttle actuator and when the pressure in the hydraulic system is insufficient, an opening of a throttle valve is reduced. The throttle valve fluidly communicates with an engine of the outboard engine. 
     In an additional aspect, when the second input indicates a desired actuation of the hydraulic throttle actuator and when the pressure in the hydraulic system is insufficient, a pitch of a plurality of propeller blades of a propeller of the marine outboard engine is reduced. 
     In a further aspect, the first hydraulic actuator is a hydraulic steering actuator, the second hydraulic actuator is a hydraulic throttle actuator, and the third hydraulic actuator is a hydraulic propeller pitch actuator. 
     In an additional aspect, the hydraulic system further includes a fourth hydraulic actuator fluidly communicating with the at least one valve. The method further comprises prioritizing actuation of the fourth hydraulic actuator based at least in part on the pressure input and the first, second, third and fourth inputs, such that the third hydraulic actuator has a higher priority than the fourth hydraulic actuator; and controlling the at least one valve for controlling a flow of hydraulic fluid from the at least one valve to the fourth hydraulic actuator based at least in part on the prioritizing. 
     In a further aspect, controlling the at least one valve based at least in part on the prioritizing includes: a) if the first input indicates a desired actuation of the first hydraulic actuator, causing the at least one valve to provide hydraulic fluid to the first hydraulic actuator and actuating the first hydraulic actuator according to the first input; b) if the second input indicates a desired actuation of the second hydraulic actuator, causing the at least one valve to provide hydraulic fluid to the second hydraulic actuator and actuating the second hydraulic actuator according to the second input; c) if the third input indicates a desired actuation of the third hydraulic actuator, causing the at least one valve to provide hydraulic fluid to the third hydraulic actuator and actuating the third hydraulic actuator according to the third input; and d) if the fourth input indicates a desired actuation of the fourth hydraulic actuator, causing the at least one valve to provide hydraulic fluid to the fourth hydraulic actuator and actuating the fourth hydraulic actuator according to the fourth input; wherein b) occurs only when the pressure in the hydraulic system is sufficient to carry out a) and b); wherein c) occurs only when the pressure in the hydraulic system is sufficient to carry out a), b) and c); and wherein d) occurs only when the pressure in the hydraulic system is sufficient to carry out a), b), c), and d). 
     In an additional aspect, the first hydraulic actuator is a hydraulic steering actuator, the second hydraulic actuator is a hydraulic throttle actuator, the third hydraulic actuator is a hydraulic propeller pitch actuator, and the fourth hydraulic actuator is a hydraulic trim actuator. 
     In yet another aspect, the invention provides a method of controlling a hydraulic system of a marine outboard engine. The hydraulic system includes a pump, at least one valve having at least one inlet, a first outlet, a second outlet, and a third outlet, the at least one inlet fluidly communicating with the pump, a first hydraulic actuator fluidly communicating with the first outlet, a second hydraulic actuator fluidly communicating with the second outlet, and a third hydraulic actuator fluidly communicating with the third outlet. The marine outboard engine includes a pressure sensor associated with the hydraulic system for sensing a pressure of the hydraulic fluid in the hydraulic system. The method comprises sensing the pressure of the hydraulic fluid; causing the at least one valve to close the third outlet to prevent hydraulic fluid to flow to the third hydraulic actuator when the pressure is below a first predetermined pressure; and causing the at least one valve to close the second outlet to prevent hydraulic fluid to flow to the second hydraulic actuator when the pressure is below a second predetermined pressure, the second predetermined pressure being lower than the first predetermined pressure. 
     In a further aspect, the first hydraulic actuator is a hydraulic steering actuator, the second hydraulic actuator is a hydraulic trim actuator, and the third hydraulic actuator is a hydraulic propeller pitch actuator. 
     In an additional aspect, the at least one valve has a fourth outlet. The hydraulic system has a fourth hydraulic actuator fluidly communicating with the fourth outlet. The method further comprises causing the at least one valve to close the fourth outlet to prevent hydraulic fluid to flow to the fourth hydraulic actuator when the pressure is below a third predetermined pressure, the third predetermined pressure being greater than the first predetermined pressure. 
     In a further aspect, the first hydraulic actuator is a hydraulic steering actuator, the second hydraulic actuator is a hydraulic throttle actuator, the third hydraulic actuator is a hydraulic trim actuator, and the fourth hydraulic actuator is one of a hydraulic propeller pitch actuator and a hydraulic shift actuator. 
     For purposes of this application, description of the spatial orientation of the various elements described herein is being made relative to a position of the marine outboard engine where the driveshaft is in a vertical orientation. It should be understood that should the orientation of the marine outboard engine change, such as when the marine outboard engine is trimmed or tilted, the description of the spatial orientation of the various elements should still be understood with respect to the orientation of the driveshaft representing the vertical orientation. 
     Embodiments of the present invention each have at least one of the above-mentioned objects and/or aspects, but do not necessarily have all of them. It should be understood that some aspects of the present invention that have resulted from attempting to attain the above-mentioned objects may not satisfy these objects and/or may satisfy other objects not specifically recited herein. 
     Additional and/or alternative features, aspects, and advantages of embodiments of the present invention will become apparent from the following description, the accompanying drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, as well as other aspects and further features thereof, reference is made to the following description which is to be used in conjunction with the accompanying drawings, where: 
         FIG. 1  is left side elevation view of a marine outboard engine in accordance with the present invention; 
         FIG. 2  is schematic illustration of a hydraulic steering system for an outboard engine; 
         FIG. 3  is a schematic illustration of a hydraulic variable pitch propeller system for an outboard engine; 
         FIG. 4  is a schematic illustration of a hydraulic tilt and trim system for an outboard engine; 
         FIG. 5  is a schematic illustration of a hydraulic throttle system for an outboard engine; 
         FIG. 6  is a schematic illustration of a hydraulic shifting system for an outboard engine; 
         FIG. 7A  is a schematic illustration of a first embodiment of a hydraulic system of the outboard engine of  FIG. 1 ; 
         FIG. 7B  is a schematic illustration of a second embodiment of a hydraulic system of the outboard engine of  FIG. 1 ; 
         FIG. 8  is a schematic illustration of a third embodiment of a hydraulic system of the outboard engine of  FIG. 1 ; 
         FIG. 9  is a schematic illustration of an electrical system associated with the hydraulic system of the outboard engine of  FIG. 1 ; 
         FIG. 10  is a logic diagram of a general method of controlling the hydraulic system of the outboard engine of  FIG. 1 ; 
         FIG. 11  is a logic diagram of a first embodiment of a detailed method of controlling the hydraulic system of the outboard engine of  FIG. 1 ; and 
         FIG. 12  is a logic diagram of a second embodiment of a detailed method of controlling the hydraulic system of the outboard engine of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the figures,  FIG. 1  is a side view of a marine outboard engine  40  having a cowling  42 . The cowling  42  surrounds and protects an engine  44 , shown schematically. Engine  44  is a conventional two-stroke internal combustion engine, such as an in-line two-stroke, two-cylinder engine. It is contemplated that other types of engines could be used, such as a V-type, four-stroke engine. An exhaust system  46 , shown schematically, is connected to the engine  44  and is also surrounded by the cowling  42 . 
     The engine  44  is coupled to a vertically oriented driveshaft  48 . The driveshaft  48  is coupled to a drive mechanism  50 , which includes a transmission  52  and a propeller  54 , described in greater detail below, mounted on a propeller shaft  56 . The propeller shaft  56  is generally perpendicular to the driveshaft  48 . Other known components of an engine assembly are included within the cowling  42 , such as a starter motor and an alternator. As it is believed that these components would be readily recognized by one of ordinary skill in the art, further explanation and description of these components will not be provided herein. 
     A stern bracket  58  is connected to the cowling  42  via a swivel bracket  59  for mounting the outboard engine  40  to a watercraft. The stern bracket  58  can take various forms, the details of which are conventionally known. The swivel bracket  59  houses a steering shaft (not shown) of the outboard engine  40  which defines a vertical steering axis  95 . 
     A steering arm  60  is operatively connected to the swivel bracket  59  to allow steering of the outboard engine  40  about vertical steering axis  95 , as described in greater detail below. 
     The cowling  42  includes several primary components, including an upper motor cover  62  with a top cap  64 , and a lower motor cover  66 . A lowermost portion, commonly called the gear case  68 , is attached to the lower motor cover  66 . The upper motor cover  62  preferably encloses the top portion of the engine  44 . The lower motor cover  66  surrounds the remainder of the engine  44  and the exhaust system  46 . The gear case  68  encloses the transmission  52  and supports the drive mechanism  50 , in a known manner. The propeller shaft  56  extends from the gear case  68  and supports the propeller  54 . 
     The upper motor cover  62  and the lower motor cover  66  are made of sheet material, preferably plastic, but could also be metal, composite or the like. The lower motor cover  66  and/or other components of the cowling  42  can be formed as a single piece or as several pieces. For example, the lower motor cover  66  can be formed as two lateral pieces that mate along a vertical joint. The lower motor cover  66 , which is also made of sheet material, is preferably made of composite, but could also be plastic or metal. One suitable composite is fiberglass. 
     A lower edge  70  of the upper motor cover  62  mates in a sealing relationship with an upper edge  72  of the lower motor cover  66 . A seal  74  is disposed between the lower edge  70  of the upper motor cover  62  and the upper edge  72  of the lower motor cover  66  to form a watertight connection. 
     A locking mechanism  76  is provided on at least one of the sides of the cowling  42 . Preferably, locking mechanisms  76  are provided on each side of the cowling  10 . 
     The upper motor cover  62  is formed with two parts, but could also be a single cover. As seen in  FIG. 1 , the upper motor cover  62  includes an air intake portion  78  formed as a recessed portion on the rear of the cowling  42 . The air intake portion  78  is configured to prevent water from entering the interior of the cowling  42  and reaching the engine  44 . Such a configuration can include a tortuous path. The top cap  64  fits over the upper motor cover  62  in a sealing relationship and preferably defines a portion of the air intake portion  78 . Alternatively, the air intake portion  78  can be wholly formed in the upper motor cover  62  or even the lower motor cover  66 . 
     The marine outboard engine  40  includes a plurality of hydraulically actuated systems. Examples of such systems are described below. It should be understood that the marine outboard engine does not necessarily have all of the hydraulically actuated systems described below. 
     One hydraulically actuated system is a hydraulic steering system  100  shown in  FIG. 2 . The hydraulic steering system  100  includes a hydraulic steering actuator  102  in the form of a linear hydraulic actuator having one end pivotally connected to the steering arm  60  and another end pivotally connected to the stern bracket  58 . By lengthening or shortening the hydraulic steering actuator  102 , the steering arm  60  rotates about the steering axis  95  thus causing the marine outboard engine  40  to be steered about that same axis  95 . It should be understood that the hydraulic steering system  100  described above is only one possible embodiment of a hydraulic steering system and that other systems are contemplated. For example, the hydraulic steering actuator  102  could be in the form of a rotary or helicoidal hydraulic actuator. U.S. Pat. No. 5,330,375, issued Jul. 19, 1994, the entirety of which is incorporated herein by reference, discloses another example of a hydraulic steering system. 
     Another hydraulically actuated system is a hydraulic variable pitch propeller system  104  shown in  FIG. 3 . The propeller  54  has a hub  106  and a plurality of propeller blades  108  disposed on the hub  106 . The plurality of propeller blades  108  are rotatable relative to the hub  106  about blade pivots  110 . Each blade pivot  110  has a pin  112  disposed at the end thereof that is offset from a center of rotation of the blade pivot  110 . The hydraulic variable pitch propeller system  104  includes a hydraulic propeller pitch actuator  114  in the form of a liner hydraulic actuator. The hydraulic propeller pitch actuator  114  includes a housing  116  having a piston  118  that is movable linearly therein. A shaft  120  connected to the piston  118  extends through an end of the housing  116 . Hoses  122  (or conduits integrally formed in the gear case  68 ) connected to the housing on either side of the piston  118  are used to deliver and remove hydraulic fluid inside the housing  116  to cause the piston  118 , and therefore the shaft  120 , to move as would be understood by those skilled in the art. The end of the shaft  120  has an adapter  124  thereon. The adapter  124  has slots (not shown) to receive the pins  112 . Thus, moving the shaft  120  causes the pins  112  to move inside the slots of the adapter  124 , which in turn, causes the blades  108  to turn relative to the hub  106 . Rotating the blades  108  in this way is known as adjusting the pitch of the propeller blades  108 . The hydraulic propeller pitch actuator  114  can optionally be provided with a spring  124  inside the housing  116 . The spring  126  biases the piston  118  towards a position of where the blades  108  have a relatively small pitch. It is also contemplated that water pressure on the blades  108  (when the propeller  54  is turning) could move the blades  108  to a position where the blades  108  have a relatively small pitch. It should be understood that the hydraulic variable pitch propeller system  104  described above is only one possible embodiment of a hydraulic variable pitch propeller system and that other systems are contemplated. U.S. Pat. No. 4,028,004, issued Jun. 7, 1977, the entirety of which is incorporated herein by reference, discloses an example of a hydraulic variable pitch propeller system. 
     Another hydraulically actuated system is a hydraulic tilt and trim system  128  shown in  FIG. 4 . The hydraulic tilt and trim system  128  includes a hydraulic tilt and trim actuator  130  in the form of a linear hydraulic actuator having one end pivotally connected to the swivel bracket  59  and another end pivotally connected to the stern bracket  58 . By lengthening or shortening the hydraulic tilt and trim actuator  130 , the swivel bracket  59  rotates about a horizontal tilt/trim axis  132  thus causing the marine outboard engine  40  to be tilted or trimmed about that same axis  132 . Trimming the outboard engine  40  means rotating the outboard engine  40  about the axis  132  so as to adjust the angle of the hull of the boat to which the outboard engine  40  is attached relative to the waterline. Tilting the outboard engine  40  means rotating the outboard engine about the axis  132  such that the outboard  40  is lifted out of the water. It should be understood that the hydraulic tilt and trim system  128  described above is only one possible embodiment of a hydraulic tilt and trim system and that other systems are contemplated. For example, the hydraulic tilt and trim actuator  130  could be in the form of a rotary or helicoidal hydraulic actuator. U.S. Pat. No. 4,925,411, issued May 15, 1990, the entirety of which is incorporated herein by reference, discloses an example of a hydraulic tilt and trim system. It is also contemplated that the hydraulic tilt and trim actuator  130  would only provide the trimming function and that a second hydraulic actuator would provide the tilting function. 
     Yet another hydraulically actuated system is a hydraulic throttle system  134  shown in  FIG. 5 . The hydraulic throttle system  134  includes a throttle body  136  and a hydraulic throttle actuator  138  in the form of a liner hydraulic actuator. The throttle body  136  fluidly communicates with the intake of the engine  44  to control the flow of air to the engine  44  and therefore the speed of the engine  44 . The throttle body  136  has a throttle valve  140  therein that can be opened or closed by rotating it about axis  142 . A throttle arm  142  is connected to the throttle valve  140  such that rotating the throttle arm  142  rotates the throttle valve  140 . The hydraulic throttle actuator  138  includes a housing  144  having a piston  146  that is movable linearly therein. A shaft  148  connected to the piston  146  extends through an end of the housing  144 . Hoses  150  connected to the housing on either side of the piston  146  are used to deliver and remove hydraulic fluid inside the housing  144  to cause the piston  146 , and therefore the shaft  148 , to move as would be understood by those skilled in the art. The end of the shaft  148  is connect to the throttle arm  142 . Thus, moving the shaft  148  causes the throttle arm  142  to rotate, which in turn, causes the throttle valve  140  to rotate. The hydraulic throttle actuator  138  can be provided with a spring  152  inside the housing  144 . The spring  152  biases the piston  146  towards a closed position to reduce a speed of the engine  44 . It should be understood that the hydraulic throttle system  134  described above is only one possible embodiment of a hydraulic system and that other systems are contemplated. For example, the hydraulic throttle actuator  138  could be in the form of a rotary hydraulic actuator connected directly to the throttle valve  140 . 
     Another hydraulically actuated system is a hydraulic shifting system  154  shown in  FIG. 6 . The hydraulic shifting system  154  includes a hydraulic shift actuator  156  in the form of a linear hydraulic actuator similar to the other linear actuators described above. A shaft  158  of the hydraulic shift actuator  156  is connected to one end of a linkage  160  that is pivotable about axis  162 . The other end of the linkage  160  is connected to a shift rod  164  of the outboard engine  40 . A lower end of the shift rod  164  is operatively connected to the transmission  52  to select between a forward gear and a reverse gear (not shown) to change the direction of rotation of the propeller  54 . Thus, moving the shaft  158  causes the shift rod  164  to move up or down to select one of the forward gear and the reverse gear. It should be understood that the hydraulic shifting system  154  described above is only one possible embodiment of a hydraulic shifting system and that other systems are contemplated. For example, the hydraulic shift actuator  156  could be connected directly to the end of the shift rod  164  and be coaxial therewith such that the shifting rod  164  is raised and lowered directly by the hydraulic shift actuator  156 . U.S. Pat. No. 4,698,035, issued Oct. 6, 1987, the entirety of which is incorporated herein by reference, discloses another example of a hydraulic shifting system. 
     The marine outboard engine  40  has at least three of the above hydraulically actuated systems. In one embodiment, the marine outboard engine  40  has the hydraulic steering system  100 , the hydraulic variable pitch propeller system  104 , and the hydraulic tilt and trim system  128 . In another embodiment, the marine outboard engine  40  has the hydraulic steering system  100 , the hydraulic variable pitch propeller system  104 , the hydraulic tilt and trim system  128 , and one of the hydraulic throttle system  134  and the hydraulic shifting system  154 . In yet another embodiment, the marine outboard engine  40  has the hydraulic steering system  100 , the hydraulic variable pitch propeller system  104 , and the hydraulic throttle system  134 . In a further embodiment the marine outboard engine  40  has the hydraulic steering system  100 , the hydraulic variable pitch propeller system  104 , the hydraulic tilt and trim system  128 , and the hydraulic throttle system  134 . In yet another embodiment, the marine outboard engine  40  has all five hydraulically actuated systems described above. Other embodiments of an outboard engine  40  having at least three above hydraulically actuated systems are also contemplated. 
     Turning now to  FIG. 7A , a first embodiment of a hydraulic system  170  of the outboard engine  40  will be described. In this embodiment, the marine outboard engine  40  is provided with the hydraulic steering system  100 , the hydraulic tilt and trim system  128 , the hydraulic variable pitch propeller system  104 , and the hydraulic throttle system  134 . As such, the hydraulic system  170  includes the hydraulic steering actuator  102 , the hydraulic tilt and trim actuator  130 , the hydraulic propeller pitch actuator  114 , and the hydraulic throttle actuator  138 . It should be understood that if the marine outboard engine  40  was provided with another combination of the hydraulically actuated systems described above, that their respective hydraulic actuators would be included in the hydraulic system  170 . 
     The hydraulic system  170  includes a reservoir  172  for storing hydraulic fluid. A pump  174  is connected to the reservoir  172  for pumping hydraulic fluid from the reservoir  172  to the rest of the hydraulic system  170 . The pump  174  can be a mechanical pump driven by the engine  44  or an electrical pump which receives electrical power generated by the electrical system of the engine  44 . From the pump  174 , the hydraulic fluid flow to an accumulator chamber  176 . It is contemplated that another pump could be provided to pump hydraulic fluid from the reservoir  172  to the accumulator chamber  176  at the same time as the pump  174  or under certain conditions. For example, if the pump  174  is a mechanical pump driven by the engine  44  and the other pump is an electrical pump, then the electrical pump could be used to compensate for the reduction in fluid flow from the pump  174  that occurs at low engine speeds. The accumulator chamber  176 , as the name suggests, accumulates a certain volume of hydraulic fluid such that if there is a reduction in the supply of hydraulic fluid by the pump  174 , the accumulator chamber  176  can provide additional hydraulic fluid to make up for the reduction for a certain amount of time. A pressure release valve  177  is located downstream of the accumulator chamber  176  to return hydraulic fluid back to the reservoir  172  when pressure inside the hydraulic system  170  exceeds a predetermined amount, such as when demands for hydraulic fluid by the hydraulically actuated systems is low. 
     A pressure sensor  178  senses the pressure of the hydraulic fluid in the accumulator chamber  176  and sends a signal indicative of the pressure to a control unit  180 . Although it is preferred that the pressure sensor  178  senses the pressure of the hydraulic fluid directly, it is contemplated that other types of sensors could sense some other element which, through calculations, can provide a value of the pressure of the hydraulic pressure. In such cases, even though the pressure is not sensed directly, the sensor would still be considered a pressure sensor  178 . 
     The hydraulic system  170  also has a priority valve  182  having one inlet  184 , and four outlets  186 ,  188 ,  190 ,  192 . The inlet  184  is connected to the pressure release valve  177  to receive hydraulic fluid therefrom. The outlet  186  is connected to a valve  194 , and the valve  194  is connected to the hydraulic steering actuator  102 . The outlet  188  is connected to a valve  196 , and the valve  196  is connected to the hydraulic tilt and trim actuator  130 . The outlet  190  is connected to a valve  198 , and the valve  198  is connected to the hydraulic propeller pitch actuator  114 . The outlet  192  is connected to a valve  200 , and the valve  200  is connected to the hydraulic throttle actuator  138 . 
     The control unit  180  is electronically connected to the priority valve  182  and to the valves  194 ,  196 ,  198 , and  200 . Based at least on the pressure input received from the pressure sensor  178 , and as described in greater detail below, the control unit  180  prioritizes the actuation of the actuators  102 ,  130 ,  114 , and  138  and controls the priority valve  182  to regulate the flow of hydraulic fluid through the outlets  186 ,  188 ,  190 , and  192  or to close one or more of the outlets  186 ,  188 ,  190 , and  192  as the case may be. The reason for doing this is that there may be a reduction in hydraulic fluid pressure in the hydraulic system  170  due to a reduction in the output of the pump  174  or a leak in the hydraulic system  170 . In the case of a mechanically actuated pump  174 , the output of the pump  174  is proportional to the speed of operation of the engine  44 , so at low engine speeds, the output of the pump  174  is reduced. In the case of an electrically actuated pump  174 , the electrical system of the engine  44  generates less power at low engine speeds which could also result in a reduction of the output of the pump  174 . Regardless of the reason for the reduction in hydraulic fluid pressure, the control unit  180  prioritizes the hydraulically actuated systems which are more essential to the proper operation of the outboard engine  44 , and as such will control the priority valve  182  to provide hydraulic fluid to these systems as described in greater detail below. In the hydraulic system  170 , the hydraulic steering actuator  102  has the highest priority, followed by the hydraulic throttle actuator  138 , then the hydraulic propeller pitch actuator  114 , and finally the hydraulic tilt and trim actuator  130 . 
     The control unit  180  also controls the valves  194 ,  196 ,  198 , and  200  based on the inputs described below with respect to  FIG. 9  and other factors such as the speed of the engine  44  so as to actuate the actuators  102 ,  130 ,  114 , and  138  in the direction necessary to control their respective systems as requested. These other factors (such as the speed of the engine  44 ) are provided to the control unit  180  by an engine control unit (ECU)  179 . The ECU  179  controls the various systems of the engine  44 , such as fuel injection, and ignition. The ECU  179  receives inputs from a plurality of sensors  181 , such as an engine speed sensor, a vehicle speed sensor, and a temperature sensor, sends this information to the control unit  180  as required for the control of the priority valve  182  and valves  194 ,  196 ,  198 ,  200 . The control unit  182  also sends a feedback signal to the ECU  179 . 
     Hydraulic fluid that is removed from the actuator  102 ,  130 ,  114 , and  138  as they are actuated is returned to the reservoir  172 . 
     Turning now to  FIG. 7B , a second embodiment of the hydraulic system  170  of  FIG. 7A  will be described. This embodiment has the same components as hydraulic system  170  of  FIG. 7A , except that the priority valve  182  has been replaced by a manifold  183 . For this reason, like components have been labelled with the same reference number as in  FIG. 7A  and will not be described further. In this embodiment, the manifold  183  has one inlet  184 , and four outlets  186 ,  188 ,  190 ,  192  to distribute the hydraulic fluid. The inlet  184  is connected to the pressure release valve  177  to receive hydraulic fluid therefrom. The outlets  186 ,  188 ,  190 ,  192 , are connected to the valves  194 ,  196 ,  198 , and  200  respectively. Based at least on the pressure input received from the pressure sensor  178 , and as described in greater detail below, the control unit  180  prioritizes the actuation of the actuators  102 ,  130 ,  114 , and  138  and controls the valves  194 ,  196 ,  198 , and  200  to regulate the flow of hydraulic fluid to the actuators  102 ,  130 ,  114 , and  138  accordingly. 
     Turning now to  FIG. 8 , a third embodiment of a hydraulic system  170 ′ will be described. The hydraulic system  170 ′ has the same components as hydraulic system  170  of  FIG. 7A , except that it does not have valves  194 ,  196 ,  198 , and  200 . For this reason, like components have been labelled with the same reference number as in  FIG. 7A  and will not be described further. In hydraulic system  170 ′, the functions of valves  194 ,  196 ,  198 , and  200  have been integrated in the priority valve  182 ′. For this reason, priority valve  182 ′ has eight outlets  186 ,  186 ′,  188 ,  188 ′,  190 ,  190 ′,  192 ,  192 ′. The outlets  186 ,  186 ′ are connected to the hydraulic steering actuator  102 . The outlets  188 ,  188 ′ are connected to the hydraulic tilt and trim actuator  130 . The outlets  190 ,  190 ′ are connected to the hydraulic propeller pitch actuator  114 . The outlets  192 ,  192 ′ are connected to the hydraulic throttle actuator  138 . It is contemplated that only one outlet could be provided such that a single connection exists between an actuator and the priority valve  182 ′. For example, if the hydraulic throttle actuator  138  is provided with the spring  152  as shown in  FIG. 3 , hydraulic fluid only needs to be provided on one side of the piston  146  to overcome the bias of the spring  152  to move the shaft  148  in one direction, and the spring  152  is used to move the shaft  148  in the other direction. Since the priority valve  182 ′ integrates the functions of valves  194 ,  196 ,  198 , and  200 , the control unit  180 ′ prioritizes the actuation of the actuators  102 ,  130 ,  114 , and  138  and controls the priority valve  182 ′ to regulate the flow of hydraulic fluid through the outlets  186 ,  186 ′,  188 ,  188 ′,  190 ,  190 ′,  192 , and  192 ′ or to close one or more of the outlets  186 ,  186 ′,  188 ,  188 ′,  190 ,  190 ′,  192 , and  192 ′, as the case may be, based at least on the pressure input from the pressure sensor  178 , and the control unit  180 ′ also regulates the flow through the outlets  186 ,  186 ′,  188 ,  188 ′,  190 ,  190 ′,  192 , and  192 ′ based on the inputs described below with respect to  FIG. 9  and other factors such as the speed of the engine  44  so as to actuate the actuators  102 ,  130 ,  114 , and  138  in the direction necessary to control their respective systems as requested. 
     Turning now to  FIG. 9 , a portion of the electrical system  202  of the outboard engine  44  and associated elements will be described. The system  202  shown in  FIG. 9  is for a marine outboard engine  40  having all five hydraulically actuated systems  100 ,  104 ,  128 ,  134 , and  154  described above. It should be understood that if the outboard engine  40  is not provided with all of the hydraulically actuated system  100 ,  104 ,  128 ,  134 , and  154 , then the components corresponding to the missing systems would also not be provided in the system  202 . As indicated above, the control unit  180  receives an input signal indicative of hydraulic fluid pressure from a pressure sensor  178 , and controls the priority valve  182 , and in the case of hydraulic system  170 , the control unit  180  also controls valves  194 ,  196 ,  198 ,  200 . The ECU  179  and sensors  181  are also provide as described above. A driver of the boat (not shown) onto which the outboard engine  40  is mounted controls the steering of the outboard engine  40  by using a steering wheel  204  provided in the boat. A steering position sensor  206  senses a position of the steering wheel  204  and sends a steering input signal to the control unit  180 , which in turn controls the actuation of the hydraulic steering actuator  102  as desired. Similarly, the driver of the boat controls the speed of the engine  44  by using a throttle lever  208  provided in the boat. A throttle lever position sensor  210  senses a position of the throttle lever  208  and sends a throttle input signal to the control unit  180 , which in turn controls the actuation of the hydraulic throttle actuator  138  as desired. The driver of the boat may control the pitch of the propeller blades  108  by pressing propeller pitch buttons  212  (increase pitch button/decrease pitch button). The propeller pitch buttons  212  send corresponding propeller pitch input signals to the control unit  180 , which in turn controls the actuation of the hydraulic propeller pitch actuator  114 . It is contemplated that the propeller pitch button  212  could be replaced by a propeller pitch lever associated with a propeller pitch lever position sensor which would send the input signal to the control unit  180 . It is also contemplated that the propeller pitch could be controlled automatically by the control unit  180  based on inputs such as engine speed and throttle position, in which case no propeller pitch buttons  212  would be provided. The driver of the boat controls the tilt and trim of the outboard engine  40  by pressing tilt and trim buttons  214  (tilt up/tilt down/trim up/trim down). The tilt and trim buttons  214  send corresponding tilt and trim input signals to the control unit  180 , which in turn controls the actuation of the hydraulic tilt and trim actuator  130 . It is contemplated that the tilt and trim button  214  could be replaced by a tilt and trim lever associated with a tilt and trim lever position sensor which would send the input signal to the control unit  180 . It is also contemplated that the trim could be controlled automatically by the control unit  180  based on inputs such as engine speed and throttle position, in which case no trim buttons  214  would be provided, only tilt buttons  214 . The driver of the boat controls the transmission  52  by using a shift lever  216  provided in the boat. A shift lever position sensor  218  senses a position of the shift lever  216  and sends a shift input signal to the control unit  180 , which in turn controls the actuation of the hydraulic shift actuator  138  as desired. It is contemplated that the shift lever  216  and shift lever position sensor  218  could be replaced by shift buttons that would send shift input signals directly to the control unit  180 . It is contemplated that the various input signals could first be sent to the ECU  179  which then relays the input signals to the control unit  180 . It is also contemplated that the ECU  179  could integrate the functions of the control unit  180 . 
     Turning now to  FIG. 10 , a general method  220  of controlling the hydraulic system  170  of the outboard engine  40  will be described. At step  222 , the control unit  180  receives a pressure input from the pressure sensor  178 . Then, at step  224  the control unit  180  receives the various relevant hydraulic systems inputs described above with respect to  FIG. 9 . The control unit  180  then prioritizes the actuation of the hydraulic actuators at step  226  based on the inputs received at step  222  and  224 . To do this, the control unit first determines which hydraulic actuators are desired to be actuated. Of these, the control unit  180  determines their priority of actuation. In a hydraulic system having three hydraulically actuated system, and therefore three hydraulic actuators, one system (i.e. the first) is given a higher priority than the other two (i.e. the second and third), and another has the lowest priority (i.e. the third). If all three systems are desired to be actuated, then at step  226  they would be prioritized according to that order (i.e. first, second, and third consecutively). However, if one of the systems is not desired to be actuated, then this system is taken out of the prioritizing. For example, if the second system is not desired to be actuated, then the control unit  180  would still prioritize the first system as the first to be actuated, but the third system would be prioritized as second to be actuated, since the second system does not need to be actuated. Also at step  226 , the control unit  226 , still based on the inputs of steps  222  and  224  determines if there is enough hydraulic fluid pressure to actuate the hydraulic actuators of having the lower priorities as desired after having actuated the hydraulic actuators having the higher priorities. If not they are removed from the prioritization. For example, if first, second, and third hydraulic actuators are desired to be actuated and are given first, second, and third priority respectively, then if the control unit  180  determines that there is not enough hydraulic fluid pressure to actuate all three, it will remove the third actuator from the prioritization, and only keep the first and second actuators in the prioritization if the control unit  180  determines that the hydraulic fluid pressure is sufficient to actuate the first and second actuators. Finally, at step  228 , the control unit  180  controls the priority valve  182  based on the prioritization made at step  226 . For example, if at step  226  it is determined that there is only sufficient pressure to actuate two out of three hydraulic actuators even though all three are desired to be actuated, then the outlet of the priority valve  182  corresponding to the actuator that is not to be actuated will be closed, and the outlets corresponding to the two actuators that are to be actuated will be regulated to control the flow of hydraulic fluid to the two actuators based on the desired actuation of these two actuators. It should be noted that for hydraulic systems which do not have a priority valve  182 , but only valves like in the system illustrated in  FIG. 7B , that the method of  FIG. 10  would be carried out in the same way except that the control unit  180  would control the valve corresponding to the relevant actuator instead of the priority valve  182 . It is also contemplated that when the control unit  180  determines that the hydraulic fluid pressure is insufficient to actuate all actuators, that the control unit  180  could actuate the actuators in pulses.  FIGS. 11 and 12  illustrate two more detailed methods  230  and  260  of achieving the general method of  FIG. 10 . 
       FIG. 11  is a first embodiment of a detailed method  230  of controlling a hydraulic system of the outboard engine  40 . In the method  230 , the outboard engine  40  is provided with the hydraulic steering system  100 , the hydraulic throttle system  134 , the hydraulic variable pitch propeller system  104 , and the hydraulic tilt and trim system  128  described above. Therefore the hydraulic system is the hydraulic system  170  illustrated in  FIG. 8 . The steps of method  230  shown in  FIG. 11  will therefore be explained with reference to  FIG. 8 . It should be understood that the method  230  could be carried out similarly with other combinations of hydraulically actuated system. For the systems described above, the steering system  100  has the highest priority, followed by the throttle system  134 , the variable pitch propeller system  104 , and finally the tilt and trim system  128 . This is the preferred order of priority for these systems, since if the hydraulic fluid pressure becomes too low to operate all four systems, not operating the tilt and trim system  128 , or the variable pitch propeller system  104  would still result in an adequate operation of the outboard engine  40 . It should be understood that different combinations of hydraulically actuated systems would result in a different preferred order of priority. 
     At step  232 , the control unit  180 ′ receives a pressure input from the pressure sensor  178 . Then at step  234 , the control unit  180 ′ determines if an input received from the steering position sensor  206  is indicative of a desired change in steering of the outboard engine  40 . If so, then at step  236 , the control unit  180 ′ regulates the priority valve  182 ′ such that the outlets  186 ,  186 ′ of the priority valve  182 ′ provide hydraulic fluid to the hydraulic steering actuator  102  to actuate it according to the input from the steering position sensor  206 . Since the steering system  100  has the highest priority, the steering actuator  102  gets actuated regardless of the hydraulic fluid pressure. 
     If after carrying out step  236  or if no change in steering is desired at step  234 , as the case may be, the control unit  180 ′ determines if an input received from the throttle lever position sensor  210  is indicative of a desired change in position of the throttle valve  140  is desired at step  238 . If so, then at step  240 , the control unit  180 ′ determines if the hydraulic fluid pressure is sufficient to move the throttle valve  140  as desired. If the steering actuator  102  was actuated at step  236  and the hydraulic fluid pressure was only sufficient to accomplish this actuation, the control unit  180 ′ moves to step  242  or  244  and none of the other actuators ( 138 ,  114 ,  130 ) will be actuated regardless of whether such an actuation was desired or not. At step  242 , the opening of the throttle valve  140  is reduced by causing the spring  152  to move the shaft  148  so as to close the throttle valve  140 , thus releasing hydraulic fluid from the hydraulic throttle actuator  138 . Step  242  is provided since if hydraulic fluid pressure is insufficient to actuate the throttle actuator  138 , it is likely that there is a problem in the hydraulic system  170  (such as a leak) in which case a reduction in the speed of the engine  44  is advisable. Alternatively, if it is determined at step  240  it is determined that the hydraulic fluid pressure is insufficient, the control unit  180 ′ could move to step  244 . At step  244 , the pitch of the propeller blades  108  is reduced by causing the spring  136  to move the shaft  120  so as to reduce the pitch of the propeller blades  108 , thus releasing hydraulic fluid from the hydraulic propeller pitch actuator  114 . By reducing the pitch of the propeller blades  108 , the load on the engine  44  is also decreased which results in the speed of the engine  44  increasing. By increasing the speed of the engine  44 , the pump  174  will operate faster (whether because the engine  44  is going faster for a mechanical pump, or because more electrical power is generated by the engine  44  for an electrical pump), which may provide sufficient hydraulic fluid pressure to actuated the hydraulic throttle actuator  138  as desired when the method  230  is repeated. From step  242  or  244 , the control unit  180 ′ returns to step  232  and the method  230  starts again. It is contemplated that if it is determined at step  240  that the hydraulic fluid pressure is insufficient, that the control unit  180 ′ could return directly to step  232 . If however at step  240 , it is determined that the hydraulic fluid pressure is sufficient to actuate the hydraulic throttle actuator  138  as desired, then at step  246 , the control unit  180 ′ regulates the priority valve  182 ′ such that the outlets  192 ,  192 ′ of the priority valve  182 ′ provide hydraulic fluid to the hydraulic throttle actuator  138  to actuate it according to the input from the throttle lever position sensor  210 . 
     If after carrying out step  246  or if no change in throttle opening is desired at step  238 , as the case may be, the control unit  180 ′ determines if an input received from the propeller pitch buttons  212  is indicative of a desired change in the pitch of the propeller blades  108  is desired at step  248 . If so, then at step  250 , the control unit  180 ′ determines if the hydraulic fluid pressure is sufficient to move the propeller blades  108  as desired. If one or both of the steering actuator  102  and the throttle actuator  138  were actuated at steps  236  and  246  and the hydraulic fluid pressure was only sufficient to accomplish this/these actuation (s), the control unit  180 ′ moves back to step  232  and none of the other actuators ( 114 ,  130 ) will be actuated regardless of whether such an actuation was desired or not. If however at step  250 , it is determined that the hydraulic fluid pressure is sufficient to actuate the hydraulic propeller pitch actuator  114  as desired, then at step  252 , the control unit  180 ′ regulates the priority valve  182 ′ such that the outlets  190 ,  190 ′ of the priority valve  182 ′ provide hydraulic fluid to the hydraulic propeller pitch actuator  114  to actuate it according to the input from the propeller pitch buttons  212 . 
     If after carrying out step  252  or if no change in propeller pitch opening is desired at step  248 , as the case may be, the control unit  180 ′ determines if an input received from the tilt and trim buttons  214  is indicative of a desired change in the tilt or trim of the outboard engine  40  is desired at step  254 . If so, then at step  256 , the control unit  180 ′ determines if the hydraulic fluid pressure is sufficient to move the propeller blades  108  as desired. If one or more of the steering actuator  102 , the throttle actuator  138 , and the propeller pitch actuator  11  were actuated at steps  236 ,  246 , and  252  and the hydraulic fluid pressure was only sufficient to accomplish this/these actuation (s), the control unit  180 ′ moves back to step  232  and the tilt and trim actuator  130  will not be actuated regardless of whether such an actuation was desired. If however at step  256 , it is determined that the hydraulic fluid pressure is sufficient to actuate the hydraulic tilt and trim actuator  130  as desired, then at step  258 , the control unit  180 ′ regulates the priority valve  182 ′ such that the outlets  188 ,  188 ′ of the priority valve  182 ′ provide hydraulic fluid to the hydraulic tilt and trim actuator  114  to actuate it according to the input from the tilt and trim buttons  214 . From step  258 , or if no change in tilt or trim is desired at step  254 , as the case may be, the control unit  180 ′ returns to step  232  and the method  230  is repeated. 
     It should be noted that for hydraulic systems which do not have a priority valve  182 , but only valves like in the system illustrated in  FIG. 7B , that the method of  FIG. 11  would be carried out in the same way except that the control unit  180  would control the valve corresponding to the relevant actuator instead of the priority valve  182 . 
       FIG. 12  is a second embodiment of a detailed method  260  of controlling a hydraulic system of the outboard engine  40 . In the method  260 , the outboard engine  40  is provided with the hydraulic steering system  100 , the hydraulic throttle system  134 , the hydraulic tilt and trim system  128 , and one of the hydraulic variable pitch propeller system  104  and the hydraulic shifting system  154  described above. It should be understood that the method  260  could be carried out similarly with other combinations of hydraulically actuated system. For the systems described above, the steering system  100  has the highest priority, followed by the throttle system  134 , the tilt and trim system  128 , and finally the one of the variable pitch propeller system  104  and the shifting system  154 . It should be understood that different combinations of hydraulically actuated systems would result in a different preferred order of priority. In this hydraulic system, a first outlet of the priority valve  182  is connected to the hydraulic steering actuator  102 , a second outlet of the priority valve  182  is connected to the hydraulic throttle actuator  128 , a third outlet of the priority valve  182  is connected to the hydraulic tilt and trim actuator  130 , and a fourth outlet of the priority valve  182  is connected to the one of the hydraulic propeller pitch actuator  114  and the hydraulic shift actuator  156 . 
     At step  262 , the control unit  180  receives a pressure input from the pressure sensor  178 . Then at step  264 , the control unit  180  determines if the hydraulic fluid pressure P is less than a first predetermined pressure P 1 . If not, then the control unit  180  returns to step  262  and the method  260  is repeated. If P is less than P 1 , then at step  266 , the control unit  180  causes the fourth outlet of the priority valve  182  to be closed and the one of the hydraulic propeller pitch actuator  114  and the hydraulic shift actuator  156  can no longer be hydraulically actuated. From step  266 , the control unit  180  moves to step  268 . At step  268 , the control unit  180  determines if P is less than a second predetermined pressure P 2 , which is less than P 1 . If not, then the control unit  180  returns to step  262  and the method  260  is repeated. If P is less than P 2 , then at step  270 , the control unit  180  causes the third outlet of the priority valve  182  to be closed and the hydraulic tilt and trim actuator  130  can no longer be hydraulically actuated. From step  270 , the control unit  180  moves to step  272 . At step  272 , the control unit  180  determines if P is less than a third predetermined pressure P 3 , which is less than P 2 . If not, then the control unit  180  returns to step  262  and the method  260  is repeated. If P is less than P 3 , then at step  274 , the control unit  180  causes the second outlet of the priority valve  182  to be closed and the hydraulic throttle actuator  138  can no longer be hydraulically actuated. It should be noted that regardless of the hydraulic fluid pressure, the hydraulic steering actuator  102  can be actuated since it has the highest priority. 
     It should be understood, that upon repeating the method  260  the pressure P increases, any outlets that were previously closed but for which the pressure P is now high enough to actuate their corresponding actuators would be reopened. For example, if the first time the method  260  is performed, P is less than P 3 , then the fourth, third, and second outlets were closed, but upon repeating the method  260  P is now greater than P 1 , then the fourth, third, and second outlets will be reopened. 
     It should be noted that for hydraulic systems which do not have a priority valve  182 , but only valves like in the system illustrated in  FIG. 7B , that the method of  FIG. 12  would be carried out in the same way except that the control unit  180  would control the valve corresponding to the relevant actuator instead of the priority valve  182 . 
     Modifications and improvements to the above-described embodiments of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the present invention is therefore intended to be limited solely by the scope of the appended claims.