Abstract:
A power steering assist system for a watercraft includes a hydraulically actuated steering cylinder assembly and a helm. The helm has a high pressure port being coupled to a fluid pressure source, a return port coupled to a reservoir, and a metering port coupled to a second chamber of the steering cylinder. A control valve assembly in the helm is switchable between at least first and second states to alternatively couple a metering element in the helm to the high pressure and return ports of the helm, respectively, hence alternatively permitting pressurized fluid to flow into the metering port from the metering element to steer the watercraft in a first direction and from the metering port into the metering element to steer the watercraft in a second direction. The system is simple, compact, reliable, and still usable in watercraft having multiple engines and/or multiple helms.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to marine steering systems and, more particularly, relates to a power assist steering system for a boat or other watercraft. Specifically, the invention relates to a steering system that incorporates an operator controlled helm and a separate hydraulic steering cylinder that is controlled by the helm in a master/slave fashion to steer the watercraft. 
     2. Discussion of the Related Art 
     In a conventional marine steering system, a watercraft such as a boat is steered by pivoting a rudder and/or outboard motor on the stern of the watercraft about a vertical steering axis upon steering actuation by an operator stationed at the helm. One typical steering system for a boat having a hull-mounted motor comprises a steering cable extending between the steering helm and the motor so that steering at the helm actuates the cable to pivot the motor about the steering axis. The cable typically comprises a push-pull cable having a reciprocatable inner core slidable in a protective, flexible outer sheath or housing. One end of the cable is connected to the steering helm, and the other end is connected to a tiller arm coupled to the motor or rudder. When the wheel is turned at the helm, the cable is actuated by a push-pull movement of the inner core, thereby pivoting the tiller arm. These systems work reasonably well on small boats, but the steering forces required for pivoting the tiller arm increase progressively with system size to the point that many larger boats can be steered manually only with great difficulty, if at all. 
     In order to reduce the forces required to steer a watercraft, it is well-known with marine outboard drives, particularly those employing large displacements, to employ a hydraulic power steering assist system for assisting the operator in steering the boat. The typical hydraulic power steering assist system includes a hydraulic cylinder that is connected to a tiller arm or other steered mechanism and that is energized in response to operator control to actuate the steered mechanism. Specifically, a helm-responsive controller is coupled to a hydraulic cylinder assembly that, in turn, is coupled to the steered mechanism, either directly or via an intervening push-pull cable. When the steering wheel is turned one way or the other, hydraulic fluid is pumped from the steering helm to one end or the other of the cylinder assembly to pivot the motor one way or the other. 
     A power steering assist system that is generally of the type described above is described in U.S. Pat. No. 5,603,279 (the &#39;279 patent). The system described in the &#39;279 patent comprises a hydraulic cylinder-piston assembly and a helm. The cylinder-piston assembly has a reciprocally mounted piston and first and second chambers in the cylinder on opposite sides of the piston. The steering cylinder has a balanced piston. In fact, as with most systems of this general type, a rod extends through both ends of the steering cylinder making for a longer assembly. The helm includes two separate cylinder assemblies that are divided into four separate internal chambers by a stepped flanged piston. One of the cylinder assemblies forms a master cylinder that is actuated directly by a control valve assembly under power supplied from the pressure source. The portion of the piston in this part of the assembly is stepped so as to form an unbalanced cylinder in the helm. The second cylinder assembly comprises a slave cylinder divided into third and fourth chambers by an annular flange on an extension of the piston. The third and fourth chambers are coupled to respective chambers of a steering cylinder. The control valve assembly is actuatable to regulate the flow of hydraulic fluid into and out of the second chamber to drive the piston and, thereby, vary the volumes of the third and fourth chambers and driving the steering piston one way or the other within the steering cylinder to effect a steering operation. The actuator of the valve assembly comprises a rotatable valve body that has first and second valves mounted in it. A rotatable input member (e.g., a steering shaft or extension thereof), actuable upon steering at the helm, is operably connected to the valve actuator. Thus, steering at the helm actuates the valve actuator to regulate the flow of pressurized hydraulic fluid through the cylinder, thereby driving the piston in one direction or the other depending upon the steering direction. 
     The system disclosed in the &#39;279 patent, while effective, exhibits several drawbacks and disadvantages. For instance, because its helm has four chambers and, in effect, two pistons, it requires a great many seals. The helm is also relatively large (both axially and radially). In fact, it is so large that it must be formed from a casting rather than machined components. It is therefore difficult to mount on the back of the dashes of many smaller boats. Several of the hydraulic fittings on the helm also are necessarily located on the periphery of the helm rather than on the rear end, rendering it difficult to access those fittings after the helm is installed behind the dash. 
     Moreover, in the system disclosed in the &#39;279 patent, only part of the system (namely, the first and second chambers of the helm) is pressurized directly by the pressure source. The remainder of the system (namely, the third and fourth chambers of the helm and both chambers of the steering cylinder) is pressurized indirectly via translation of the slave portion of the piston. Air in the lines of that portion of the system can lead to noticeable “looseness” or play of the cylinders. 
     Many of the problems associated with the &#39;279 patent were addressed and overcome in co-pending and commonly assigned application Ser. No. 09/967,792 (the &#39;792 application), filed Sep. 28, 2001, now U.S. Pat. No. 6,524,147 The system disclosed in the &#39;792 application includes a hydraulically actuated unbalanced steering cylinder assembly, a pressure source, and a helm that is spaced from the steering wheel assembly, typically within the dash. The helm includes a helm cylinder having a slave chamber fluidically coupled to a second chamber in the steering cylinder, a high pressure port fluidically coupled to the outlet of the pressure source and to a first chamber in the steering cylinder, and a return port fluidically coupled to a vent. A helm piston is slidably mounted in the helm cylinder, and a control valve assembly is movable between at least first and second positions to alternatively couple a control chamber in the helm cylinder to the high pressure and return ports, respectively. The resultant system is considerably simpler and more compact than that disclosed in the &#39;279 patent. It also is pressurized directly by a single source and, therefore, does not exhibit the looseness experienced by some other systems. In fact, it is extremely well configured for use in a relatively small, single engine watercraft. However, it is not easily adaptable to a multiengine watercraft having a separate steering cylinder for each rudder. It also is not usable with watercrafts having multiple helms. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention, a power steering assist system for a watercraft includes a hydraulically actuated steering cylinder assembly and a helm. The steering cylinder assembly is configured for connection to a steered mechanism of the watercraft. It includes a steering cylinder, a steering piston that is mounted in the steering cylinder to define first and second chambers on opposite sides thereof, and a rod that is affixed to the steering piston. Either rod or the steering cylinder is movable relative to the other and is configured for connection to the steered mechanism. The helm, which is spaced from the steering cylinder assembly, has high pressure, return, and metering ports formed therein. The high pressure port is coupled to a fluid pressure source, the return port is coupled to a reservoir, and the metering port is coupled to the second chamber of the steering cylinder. The helm includes a metering element having at least first and second ports, the second port being coupled to the metering port in the helm, and a control valve assembly that is coupled to the metering element and that is switchable between at least first and second states to alternatively couple the first port in the metering element to the high pressure and return ports of the helm, respectively, thereby alternatively permitting pressurized fluid to flow into the metering port from the metering element and from the metering port into the metering element. The control valve assembly may also be switchable to a third, neutral state in which the first port of the metering element is isolated from both of the high pressure and return ports. 
     In a preferred embodiment, the control valve assembly comprises first and second two-way/two-position valves that are configured to be actuated by an operator manipulated steering mechanism (such as steering wheel) such that 1) both the first and second valves remain closed when the steering mechanism remains stationary, 2) movement of the steering mechanism in a first direction opens the first valve while leaving the second valve closed, and 3) movement of the steering mechanism in a second direction opens the second valve while leaving the first valve closed. In this case the control valve assembly preferably comprises a valve actuator and a valve body, the valve body 1) being rotatably coupled to the metering element and to the steering mechanism, 2) housing the first and second valves, 3) having a first passage formed therein that couples the high pressure port to the first port of the metering element, and 4) having a second passage formed therein that couples the first port of the metering element to the return port. The valve actuator is movable relative to the valve body between first, second, and third positions thereof corresponding to the first, second, and third states of the valve assembly. 
     The system preferably additionally includes a relief valve assembly that allows the system to be operated manually in the event of pressure source failure. The relief valve assembly may include a two-way/two-position pilot-operated valve that allows manual operation of the system if the pressure source is inoperative. Due at least in part to the incorporation of the relief valve assembly into the system, the metering element is coupled to the control valve assembly such that the metering element is rotated manually by the control valve assembly so as to act as a pump in the event of pressure source failure. 
     In accordance with another aspect of the invention, a method of steering a watercraft includes placing a pressure source in fluid communication with a high pressure port of a helm casing of a helm and a first chamber in a hydraulic steering cylinder located remote from the helm casing, the first chamber being separated from a second chamber by a steering piston, and a driven member being formed by one of the steering cylinder and the rod and being coupled to a steered mechanism of the watercraft. Then, in response to movement of a steering mechanism of the watercraft in a first direction from an at-rest position thereof, the system causes a metering element in the helm casing to rotate in a first direction and deliver fluid to the second chamber in the steering cylinder, thereby causing the driven member to move in a first direction. Conversely, in response to movement of the steering mechanism in a second direction from the neutral position, the system causes the metering element to rotate in a second direction to permit hydraulic fluid to flow into the metering element from the second chamber in the steering cylinder, thereby causing the driven member to move in a second direction opposite the first direction. 
     In a preferred embodiment, the metering element has a first port and has a second port in fluid communication with the second chamber in the steering cylinder. In this system, when the steering mechanism is in the at-rest position, a control valve assembly of the helm is switched to a first state isolating the first port of the metering element from the pressure source and from vent. When the steering mechanism moves in the first direction from the at-rest position, the control valve assembly switches to a second state fluidically coupling the first port in the metering element to the pressure source. Conversely, when the steering mechanism moves in the second direction, the valve assembly switches to a third position fluidically coupling the first port in the metering element to vent. 
     In order to facilitate mounting of the helm to the dash of the watercraft, the helm has only three ports (namely, a slave port that is fluidically connected to the second chamber in the steering cylinder, the high pressure port, and the return port), and all three ports are all located on a rear axial end of the helm cylinder. The helm cylinder also is very compact. 
    
    
     These and other advantages and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred exemplary embodiments of the invention are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
     FIG. 1 is a schematic top plan view of a boat incorporating a power steering assist system constructed in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is somewhat schematic perspective view of the power steering assist system of FIG. 1; 
     FIG. 3 is an elevation view of a portion of a dash of the boat of FIG. 1, showing a steering wheel and a helm of the power steering assist system mounted on the dash; 
     FIG. 4 is a hydraulic circuit schematic of the power steering assist system; 
     FIG. 5 is a side sectional elevation view of the power steering assist system, illustrating the system in a first operational state thereof; 
     FIG. 6 is an end elevation view of a front end of a steering cylinder of the system; 
     FIG. 7 is an end elevation view of a rear end of a valve assembly of the system; 
     FIG. 8 corresponds to FIG.  5  and illustrates the system in a second operational state thereof; and 
     FIG. 9 corresponds to FIG.  5  and illustrates the system in a third operational state thereof. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings and initially to FIG. 1, a boat  12  incorporates a power steering assist system  10  (hereafter simply “power steering system”) constructed in accordance with a preferred embodiment of the present invention. The boat  12  includes a hull  14  having a bow  16  and a stern  18 , an outboard motor  20  mounted on the stern  18 , and a cowling or dash  22  extending laterally across the hull  14  near the bow  16 . As is conventional, the motor  20  is mounted on the boat  12  by a pivoting mount assembly (not shown) that permits the motor  20  to be pivoted about a vertical axis to cause a rudder formed on or by the motor  20  to steer the boat  12 . The motor  20  could alternatively be a non-pivoting inboard or outboard motor, and the boat  12  could be steered by one or more rudders movable separately from the motor  20 . 
     Referring now to FIGS. 1-2, the steering system  10  for the boat  12  includes a tiller arm  24  coupled to the motor  20  and forming the boat&#39;s steered mechanism, a helm  26  including a steering wheel  28  serving as the boat&#39;s steering mechanism, a pressure source  30 , and a steering cylinder assembly  32 . The present embodiment contains no mechanical linkage connecting the helm  26  to the steering cylinder assembly  32 . Both assemblies  26  and  32  are pressurized by a single power source. The helm  26  is mounted through the dash  22  and is actuated by the steering wheel  28 . The steering cylinder assembly  32  is actuated by the helm  26  to move the tiller arm  24  and pivot the motor  20  on its mount under power supplied by the pressure source  30 . In order to minimize the size and weight of the components that are mounted behind the dash  22 , the steering cylinder assembly  32  is located remote from the helm  26 , possibly adjacent the motor  20  as illustrated, or on the motor, so as to be connectable directly to the tiller arm  24 . Alternatively, the steering cylinder assembly  32  could be mounted at some other location on the boat  12  and connected to the tiller arm  24  by a push-pull cable or the like. Multiple steering cylinders could be provided in a multiple engine watercraft and connected to the helm  26  in a parallel fashion. The helm  26  is connected to the pressure source  30  by a high pressure line  34  and a return line  36 . It is also connected to the steering cylinder assembly  32  by the high pressure line  34  and a slave line  38 . 
     The fluid pressure source  30  could comprise any structure or assembly capable of generating hydraulic pressure and of transmitting it to the helm  26  and the steering cylinder assembly  32 . It also can be located virtually anywhere on the boat  12 . In the illustrated embodiment, the fluid pressure source  30  includes a pump  40  and a reservoir  42 , best seen in the assembly illustrated in FIG.  2 . The pump  40  has an inlet  44  connected to an outlet of the reservoir  42  and has an outlet  46  connected to or, as in the illustrated embodiment, forming the pressurized outlet of the pump assembly  30 . An accumulator (not shown) could be provided between the pump outlet  46  and the helm  26 , if desired. The reservoir  42  has an inlet  48  connected to or, as in the illustrated embodiment, forming the unpressurized inlet of the pressure source  30 . 
     Referring to FIGS. 2,  4 ,  5 , and  6  the steering cylinder assembly  32  comprises a hydraulically actuated, unbalanced steering cylinder assembly operatively coupled to the helm  26 , the pump outlet  46 , and the tiller arm  24 . “Unbalanced” as used herein means that the cylinder assembly&#39;s piston has different effective surface areas on opposite sides thereof such that equal fluid pressures on both sides of the piston generate an intensification effect on the side of the piston having a greater effective surface area and drive the piston to move towards the side of the cylinder facing the side of the piston having a smaller effective surface area. The steering cylinder assembly  32  includes a steering cylinder  50 , a steering piston  52  mounted in the steering cylinder to form first and second chambers  54 ,  56  on opposite sides of the steering piston  52 , and a rod  57  connected to the steering piston  52 . A first port  58  opens into the first chamber  54  for connection to the high pressure line  34  in a check valve  59 . A second port  60  opens into the second chamber  56  for connecting to the metering line  38 . The steering cylinder  50  of this embodiment is stationary and is mounted on the stern  18  of the hull  14  by a suitable bracket  62 . The rod  57  extends axially through a rod end  64  of the steering cylinder  50  (disposed opposite a cylinder end  66 ) and terminates at a free end that is coupled to the tiller arm  24 . The unbalanced condition of the assembly  32  therefore is created by virtue of the attachment of the rod  57  to the steering piston  52  and the consequent reduction in piston surface area exposed to fluid pressure in the first chamber  54 . Alternatively, the rod  57  could extend completely through the steering cylinder  50  and could be affixed to a stationary support, in which case the steering cylinder  50  would be coupled to the tiller arm  24  and would reciprocate relative to the stationary piston  52 . In this case, the unbalanced condition of the assembly  32  would be achieved by other measures, e.g., by making one end of the steering rod  57  diametrically smaller than the other. 
     Referring to FIG. 3, the helm  26  is mounted through the dash  22 . It includes the steering wheel  28 , a steering shaft  68  extending forwardly from the dash  22 , and a helm casing  70  located behind the dash  22 . The helm casing  70  is relatively compact, having a body  72  and a cap  74  screwed onto the front end of the body  72 . The back end of the cap  74  is mounted on the front surface of the dash  22  by bolts  76 . The body  72  is cylindrical, having a rear axial end  78  (FIG.  5 ), a front axial end  80 , and an outer radial periphery  82 . It is very narrow, having a diameter of no more than 3¼″. The body  72  also is relatively short, having a total length of no more than about 3″ to 3½″. The entire helm casing  70 , including the body  72  and the cap  74 , is no longer than 6″ to 7″. Mounting behind the dash  22  is facilitated by the fact that the helm casing  70  has only a limited number of fittings (three in the preferred embodiment), and all of those fittings extend from the relatively easily-accessible rear axial end  80  of the helm casing  70 . The helm  26  therefore is considerably smaller than the helm disclosed in the &#39;279 patent and easier to mount to the dash. It is also considerably lighter, weighing 6 to 7 pounds less than the commercial version of the helm disclosed in the &#39;279 patent. The helm cylinder also need not be formed from a casting. 
     The hydraulic circuitry contained within the pressure source  30 , the helm  26 , and the steering cylinder assembly  32  will now be described with reference to FIG.  4 . The helm casing  70  has a high pressure port  84  connected to the high pressure line  34 , a metering port  86  connected to the metering line  38 , and a return port  88  connected to the return line  36 . Located within the helm casing  70  (FIG. 3) are a valve body  90  having a control valve assembly, a metering device  92 , and a relief valve assembly including a relief valve  186 , a pilot-operated valve  188 , and a make-up valve  196 . A control chamber  100  is formed in helm casing  70  (FIG. 3) between the metering device  92  and the valve body  90 . 
     The control valve assembly includes first and second normally closed two-way/two-position valves. Still referring to FIG. 4, the first valve is a supply valve  102  having an inlet port  104  coupled to the high pressure port  84  and having an output port  106  coupled to the control chamber  100 . The second valve is a return valve  108  having an inlet port  110  coupled to the control chamber  100  and an outlet port  112  connected to the return port  88  via the valves  186  and  196 . Both valves  102  and  108  are coupled to a common actuator  114  (preferably one acted upon by the steering shaft  68  (FIG.  3 )), such that movement of the actuator in a first direction opens one of the valves  102  or  108  while leaving the other valve closed, and movement of the actuator in a second direction opens the other valve  108  or  102  while leaving the one valve closed. A suitable actuator is described below in conjunction with FIG.  5 . 
     It can thus be seen that the first chamber  54  of the steering cylinder  50  will always be at a pressure P 1  that is the same pressure as the pump outlet pressure. The control chamber  100  of the helm casing  70  (FIG. 3) and the second chamber  56  of the steering cylinder  50  will all be at a second pressure P 2  when no load is applied to the rod  57 . The pressure P 2  will, depending upon the operational state of the valve assembly and the direction of load applied to rod  57 , vary from a low of essentially 0 psi relative to the atmosphere to a high of P 1  (typically on the order of 1000 psi). 
     Referring now to FIG. 5, the physical structure of a helm assembly incorporating the hydraulics of FIG. 4 can be seen to include a helm casing  70  that supports, from rear to front end, the steering shaft  68 , the valve actuator  114 , the valve body  90 , and the metering device  92 . The steering shaft  68  protrudes outwardly from an opening  120  in the rear end  78  of the casing  70 . The valve actuator  114  and valve body  90  are housed in an interior chamber  122  of the casing  70 . The front end  80  of the helm casing  70  is formed from a casing  200  of the metering device  92 , which is attached to the remainder of the helm casing  70  by bolts  126 . The valve body  90  is coupled to a rotary input (not shown) of the metering device  92  so that the metering element and valve body rotate together as a unit. 
     Referring to FIGS. 5 and 6, the steering shaft  68  is sealed to the opening  120  via an O-ring seal  128 . The front end of the steering shaft  68  is stepped to present a rectangular protrusion  130 . An actuator pin  134  extends forwardly from the protrusion  130  and into the valve actuator  114 . As best seen in FIG. 6, the actuator pin  134  is located eccentrically on the protrusion  130  so as to revolve about the axis of rotation of the steering shaft  68  upon steering shaft rotation, thereby driving the actuator  114  to move radially relative to the valve body  90  when the steering shaft  68  rotates relative to the valve body  90 . 
     Referring to FIGS. 5 and 7, the valve body  90  comprises a tubular metal structure having a body portion  136 , a rear end  138 , and a front shaft portion  140 . The body portion  136  is sealed against an inner surface of the helm casing  70  by O-rings  139  to form (1) a vent chamber  142  in front of the valve body  90  and (2) the control chamber  100  between the valve body  90  and the metering device  92 . The rear end cap  138  is rotatably bound in the helm casing  70  by a thrust bearing  144 . The thrust bearing  144  bears the load imposed on the system by pressure in the control chamber  100 . The end cap  138  also has a rectangular central opening  146  formed in it that receives the protrusion  130  on the end of the steering shaft  68  in a manner that permits limited relative rotational movement between the protrusion  130  and the periphery of the opening  146 . Finally, the shaft portion  140  extends forwardly from the front end of the body portion  136  and is connected to the metering device  92  such that the valve body  90  and the operated component (metering element  202 ) of the metering device  92  rotate as a unit. Still referring to FIG. 5, the supply and return valves  102  and  108  are housed in cross-bores formed in the rear end of the body portion  136  of the valve body  90 . The valves  102  and  108  cooperate with supply and vent passages  148  and  150  in the body portion  136  so as to permit a control passage  152  in the body portion  136  to be selectively connected the high pressure port  84  and to the vent chamber  142  by suitable switching of the valves  102  and  108 . Specifically, the supply passage  148  has an helm coupled to the high pressure port  84  of the valve casing  70  by an inlet passage  154  extending through the metering device  92  and has an outlet coupled to the inlet  104  of the supply valve  102 . The vent passage  150  has an inlet communicating with the outlet  112  of the return valve  108  and an outlet opening into the vent chamber  142 . The control passage  152  extends axially from the control chamber  100  to the valve assembly in fluid communication with the outlet  106  of the supply valve  102  and the inlet  110  of the return valve  108 . 
     Still referring to FIG. 5, the supply valve  102  seats toward one end of the supply passage  148 . It includes a ball-type valve element biased towards its seat by a spring  156 . Conversely, the return valve  108  seats towards the other side of the vent passage  150 . It also includes a ball valve element biased toward its seat by a spring  158 . 
     The valve actuator  114  is coupled to the steering shaft  68  so as to move radially through a limited stroke with respect to the valve body  90  upon relative rotational movement between the steering shaft  68  and the valve body  90 . Specifically, with reference to FIGS. 5 and 7, the valve actuator  114  comprises a shaft  160  mounted in the rear end of the body portion  136  of the valve body  90 . A slot  162  is cut in the center of the shaft  160  for receiving the actuator pin  134 . In addition, first and second actuator support tabs  164  and  166  are bolted to opposed peripheral surfaces of the shaft  160  and extend forwardly from the front end of the shaft  160 . First and second actuator pins  168  and  170  are threaded into bores in the respective tabs  164  and  166 . The bases of the pins  168  and  170  are spaced from one another by a distance that is greater than the diameter of the valve body  90 , thereby forming a radial clearance between each of the bases and the outer periphery of the valve body  90  when the valves  102  and  108  are closed. This arrangement permits adjustment of the at-rest clearance of the actuator pins  168  and  170  and the stroke of the pins. Hence, when the valve actuator  114  moves radially relative to the valve body  90  in a first direction, the pin  168  engages the ball-type valve element forming the supply valve  102  to open the supply valve (compare FIG. 5 to FIG.  8 ). Similarly, when the valve actuator  114  moves relative to the valve body in the second direction, the pin  170  engages the ball-type valve element forming the return valve  108  to open the return valve (compare FIG.  5  and FIG.  9 ). 
     Still referring to FIG. 5, the vent chamber  142  is connected to the return port  88  in the valve casing by a drain passage  180  extending generally axially through the outer portion of the helm casing  70 . The drain passage  180  includes first and second branches  182  and  184  having first and second valves  186  and  188  mounted therein. Each of the valves  186  and  188  is biased into its closed position by a respective return spring  190 ,  192 . The valve  186  is a spring-loaded relief valve. The valve  188  is a pilot actuated valve that normally held in its open position by fluid pressure in a branch  194  of the inlet passage  154  so as to permit unrestricted flow into the branch  184  of the drain passage  180  from the vent chamber  142 . However, upon pump failure, the valve  188  closes under the force of the biasing spring  192  so as to isolate the vent chamber  142  from the drain passage  180 , whereupon the vent chamber  142  drains to the reservoir  42  only when the pressure therein rises to a pressure high enough to overcome the biasing force of the spring  190  and open the relief valve  186 . Finally, make-up fluid for manual operation is provided via a spring loaded check valve  196  located in a make-up passage  198  extending from the drain passage  180  to the control chamber  100 . 
     The metering device  92  may comprise any commercially available metering pump. Still referring to FIG. 5, a suitable metering device includes a stationary annular casing  200  forming the rear end of the helm casing  70  and a central rotatable metering element  202  coupled to the shaft portion  140  of the valve body  90 . A first port  204  of the metering element  202  opens into the control chamber  100 . Passages  206  and  208  are formed axially through the casing  200  for receiving tubular extensions of the inlet passage  154  and drain passage  180 , respectively. The metering element  202  typically (but not necessarily) will comprise a so-called metering gear. Depending on the operational status of the system, the metering device  92  may operate as a valve (controlling fluid flow between the control chamber  100  and the metering port  86 ) or as a pump (pumping fluid to or from the metering port  86 ). A metering device having these characteristics is available, e.g., from Eaton Corporation, under the brand name Char-Lynn. 
     The operation of the power assist steering system will now be described with the assumption that the components are in the position illustrated in FIG.  5  and the steering wheel  28  and steering shaft  68  are stationary. The valve actuator  114  is balanced with respect to the valve body  90  at this time, and both the supply and return valves  102  and  108  are closed to block flow into or out of the control chamber  100 . Initial rotation of the steering shaft  68  in either direction drives the valve actuator  114  to move radially relative to the valve body  90  until one of the actuator pins opens the associated valve. Hence, counterclockwise rotation of the steering shaft  68  drives the actuator pin  168  to the position illustrated in FIG. 8 to open the supply valve  102 . Pressurized fluid from the first chamber  54  in the steering cylinder  52  and the pump  40  flows through the high pressure port  84  of the helm casing, through the supply passage  148 , through the open supply valve  102 , and into the control chamber  100 . Fluid in the control chamber  100  then flows through the metering device  92  and into the second chamber  56  of the steering cylinder  32 , thereby driving the piston  52  of the steering cylinder  32  to move to the right as illustrated in FIG.  8 . Fluid flow through the metering device  92  drives the metering gear  202  to rotate, thereby driving the valve body  90  to rotate counterclockwise. When the operator stops turning the steering shaft  68 , the metering element  202  of the metering body  90  relative to the valve actuator  114  until the supply valve  102  is reseated to terminate fluid flow through the metering device  92  from the control chamber and, accordingly, to terminate steering cylinder piston movement. 
     Conversely, when the steering shaft  68  is rotated clockwise, the valve actuator pin  170  opens the return valve  108  as seen in FIG.  9 . Fluid is therefore free to flow from the second chamber  56  of the steering cylinder  32 , through the metering port  86  of the helm casing  70 , through the metering device  92 , through the control chamber  100 , through the return valve  108 , and into the vent chamber  142 . Because the pilot operated valve  188  is open at this time under pilot pressure in the supply passage branch  194 , fluid in the vent chamber  142  is free to flow through the valve  188  and the second branch  184  of the vent passage, out of the return port  88 , and to the reservoir  42 . The pressure differential across the piston  52  resulting from fluid flow from the second chamber  56  in the steering cylinder  32  drives the steering piston  52  to the left at this time to alter the steering angle of the watercraft. Fluid flow through the metering device  92  under these conditions also drives the metering element  202  to drive the valve body  90  to rotate the valve body in the same direction as the steering shaft  68 , i.e., clockwise. When steering shaft rotation ceases, the metering element  202  will continue to rotate for a brief period of time until the valve body  90  moves relative to the actuator  114  sufficiently to reseat the return valve  108 . At this time, fluid flow out of the second chamber  56  of the steering cylinder  32  terminates, arresting further movement of the steering piston  52 . 
     In the event of pressure source failure, the relief valve assembly operates to permit the helm  26  to be operated manually. Specifically, if the steering shaft  68  is rotated clockwise under these conditions, the actuator pin  170  will open the return valve  108  as discussed above. Continued steering shaft rotation will cause the rectangular protrusion  130  on the end of the steering shaft  68  to contact the periphery of the opening  146  in the rear end cap  138 , at which point the steering shaft  68  will drive the valve body  90  to rotate. The valve body will, in turn, drive the metering element  202  to rotate. The metering device  92  now acts as a pump and draws fluid out of the second chamber  56  of the steering cylinder  32  and into the control chamber  100 . Fluid in the control chamber  100  then flows through the return valve  108  and into the vent chamber  142 . However, because the inlet passage  154  is now unpressurized, the valve  188  is closed, and fluid flow out of the vent chamber  142  is blocked until the pressure therein rises to a level that sufficiently high to unseat the relief valve  186 . When the fluid pressure in the vent chamber  142  reaches this level, the supply valve  102  also opens to allow fluid flow past the supply valve  102 , backwards through the supply passage  148 , out of the high pressure port  84 , and into the first chamber  54  of the steering cylinder  32 . The resultant pressure differential across the piston  52  drives the piston to the left. Because of the volume differential between the first and second chambers  54  and  56  of the steering cylinder  32 , and because the volume of the second chamber  56  is larger than the volume of the first chamber  54 , the first chamber  54  is incapable of receiving all of the fluid flowing out of the second chamber  56 . The excess fluid instead flows past the relief valve  186  and back to the reservoir  42  through the drain passage  180 . 
     Conversely, if the pump  40  fails and the steering shaft  68  is rotated counterclockwise, steering shaft rotation serves to first open the supply valve  102  and then drive the valve body  90  to rotate counterclockwise to drive the metering element  202  to rotate counterclockwise. Counterclockwise rotation of the metering element  202  pumps fluid from the control chamber  100  to the second chamber  56  of the steering cylinder  32 . Simultaneously, fluid will be forced out of the first chamber  54  of the steering cylinder  32 , through the high pressure port  84 , the inlet and supply passages  154  and  148 , the open supply valve  102 , and into the control chamber  100 . The resulting pressure differential drives the steering cylinder piston  52  to the right to effect a steering operation in the opposite direction. Because volume of the first chamber  54  of the steering cylinder  32  is smaller than the volume of the second chamber  56 , a negative pressure is generated in the control chamber  100  during this process. That negative pressure lifts the valve  196  off its seat to permit make-up fluid to be drawn into the control chamber  100  from the reservoir  42 , the drain passage  180 , and the make-up passage  198 . 
     Many changes and modifications could be made to the invention without departing from the spirit thereof. Some of these changes are discussed above. Other changes will become apparent from the appended claims.