Abstract:
A mechanism for cleaning out an inlet grate of a water jet propulsion system. The mechanism includes an actuator and a wiper. The actuator penetrates and is supported by a through-hull housing installed in an opening in the hull. The actuator is linearly displaceable relative to the through-hull housing by manipulation of a handle or other operator input device or system. An inlet grate comprising a plurality of cantilever tines is pivotably mounted to and supported by the through-hull housing. A weed wiper is connected to the end of the actuator remote from the end that is actuated by the boat operator. When the actuator is displaced downward, the weed wiper moves downward along a linear path. At the point in the wiper displacement where the wiper bar engages the tines of the inlet grate, the downwardly displacing weed wiper causes the grate to pivot downward. During this action, the wiper slides across the grate and pushes any weeds or other debris entangled on the grate toward the ends of the tines. Because weeds usually wrap and ball around the inlet grate tines, the weeds will slide off easily when pushed or dragged by the sweeping wiper bar.

Description:
FIELD OF THE INVENTION 
     This invention generally relates to water jet apparatus which are mounted to the hull of a boat or other watercraft. In particular, the invention relates to mechanisms for cleaning out an inlet grate of a water jet apparatus which has become clogged with weeds or other debris. 
     BACKGROUND OF THE INVENTION 
     It is known to propel a boat or other watercraft using a water jet apparatus mounted to the hull, with the powerhead being placed inside (inboard) the hull. The drive shaft of the water jet apparatus is coupled to the output shaft of the inboard motor. The impeller is mounted on the drive shaft and housed in a jet propulsion duct. 
     To facilitate use of water jet-propelled boats in shallow water, it is known to mount the water jet apparatus at an elevation such that the apparatus does not project below the bottom of the boat hull. In one type of design, part of the duct of the water jet apparatus is installed inside the hull while the remaining part penetrates the transom and extends beyond the rear of the hull. An inlet housing of the duct has a horizontal opening and an inlet ramp for guiding water into the housing where the impeller resides. The horizontal opening of the inlet housing is mounted in a hole in the bottom or near the bottom of the hull. A midportion of the duct penetrates a hole in the transom. A water jet stream is discharged out a steering nozzle pivotably mounted to the distal end of the duct. 
     When operating a water jet-propelled boat in shallow water, it is possible to ingest seaweed and other debris into the duct when water is being drawn into the water jet inlet. To prevent seaweed and other debris from entering the water jet duct and possibly ensnarling or damaging the rotating impeller, a grate or screen is typically placed across the inlet opening. During continuous use of a water jet-propelled watercraft in shallow, weed-infested water, floating weeds can accumulate on and become entangled with the inlet grate to such a degree that the inlet grate becomes clogged. In particular, in the case where the inlet grate comprises an array of mutually parallel tines, the suction created by the impeller causes weeds and other debris to wrap around the tines of the grate and slide rearwardly along the tines. The buildup of weeds, if allowed to continue unabated, can ultimately form a dense mass that reduces the intake of water through the inlet opening and into the water jet. Reduced water flow can cause the jet pump to stall and the boat to stop moving. 
     The result is that the boat operator must unclog the inlet opening by removing the dense mass of entangled weeds from the tines of the inlet grate. However, removing entangled weeds from the inlet grate can be very difficult. This task can also be unpleasant if a person has to enter the water and submerge under the boat to remove the weeds from the grate. Moreover, even if the entangled weeds are successfully removed, when boat operation is resumed and the boat operator attempts to leave the weeded area, the inlet grate may become plugged after only a short distance, repeating the same clean-out problem. 
     One method which has been tried to eliminate this problem uses a cantilevered grate that is attached to the top of the inlet housing through a pivot pin. When the grate becomes clogged with weeds, a lever is activated inside the boat that allows the grate to rotate down away from the inlet. The operator then drives the boat forward and the weeds are supposed to slide off the ends of the inlet grate tines. The inlet grate is then rotated back into place. 
     Another attempt at a solution to the problem of clogged inlet grates was disclosed in U.S. Pat. No. 5,577,941. In that disclosure, the inlet grate comprises a plurality of cantilever tines which extend rearwardly across the water intake and have suspended aft end tips spaced from the aft end of the water intake. This spacing enables rearward sliding of weeds and other debris along and then off of the cantilever tines without clogging. U.S. Pat. No. 5,577,941 characterizes this anti-clogging feature as being “automatic”, with no additional mechanism being needed for unclogging. 
     U.S. Pat. No. 5,876,258 purports to be an improvement over the teaching of U.S. Pat. No. 5,577,941. In particular, U.S. Pat. No. 5,876,258 states that the inlet grate of U.S. Pat. No. 5,577,941 is problematic because the size of the gap between the end tips of the tines and the aft end of the weed grate plate limits the size and amount of weeds that can pass through the gap and through the water jet. Thus, large clumps of weeds which have a thickness greater than the gap between the tines and the aft end of the inlet grate can cause clogging. Instead, U.S. Pat. No. 5,876,258 proposes an inlet grate comprising a plurality of cantilever tines each joined to a pivot rod. The cantilever tines extend across the inlet opening to prevent debris from entering the water jet. A spring member is mounted between the cantilever tines and a mounting frame such that the spring member provides an outward rotational bias force against the rotatable cantilever tines. During operation of the watercraft, if a mass of weeds becomes clogged in the cantilever tines, the upward and inward suction force of the water jet causes the cantilever tines to rotate upward against the opposing rotational bias force of the spring member. This self-activating mechanism allows the weeds to slide off of the ends of the cantilever tines and be ingested by the water jet. This has the disadvantage that large volumes of weeds will plug up the water jet at the impeller, making clean-out even more difficult than with a fixed-grate system. Also, when the inlet grate is open, hard debris, such as oysters, stones and sticks, can enter the water jet, causing damage to the impeller. 
     Another grate clean-out device was disclosed in U.S. Pat. No. 6,083,063. This device comprises a cantilevered inlet grate which is pivotably mounted to the top of an inlet housing. The rotation of a lever inside the boat causes the wiper bar to wipe across the top of the tines, which in turn causes the grate to rotate down while causing the weeds to slide off the ends of the tines. 
     There is a need for an inlet grate clean-out mechanism which can be reliably actuated by a boat operator when conditions require without ingesting weeds or debris into the water jet and which has simplified mechanics. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an operator-actuated inlet grate clean-out mechanism which can be actuated as often as required and which has simple mechanics. By linearly displacing an actuator, e.g., by depressing a sliding rod, located in the boat stern, a person can operate a weed wiper which is pivotably coupled to the end of the actuator. The weed wiper is arranged to push weeds off of a pivotable cantilever-tine inlet grate as the wiper travels along the tines toward their tips. As the wiper sweeps along the inlet grate tines, it bears against the inlet grate and causes the grate to swing downward. Because the weeds wrap and ball around the inlet grate, they also slide off easily when pushed or dragged by the sweeping wiper bar even in large quantities. The invention enables fast and easy weed removal, without the necessity of the boat operator entering the water. 
     One aspect of the invention is directed to a mechanism for cleaning out an inlet grate which extends across an inlet of a duct of a water jet propulsion system. The mechanism comprises an actuator penetrating the duct and a wiper supported by the actuator and disposed across the tines of the inlet grate. The actuator is linearly displaceable. The wiper bears against at least one tine during linear displacement of the actuator from a first position to a second position. The inlet grate pivots from a first angular position to a second angular position as the actuator displaces from the first position to the second position. 
     Another aspect of the invention is directed to a boat hull comprising: an inlet ramp defining a channel; an inlet grate comprising a plurality of cantilever tines and pivotably mounted to the inlet ramp with its tines extending at least partly across said channel; an actuator which penetrates the inlet ramp and is linearly displaceable; and a wiper supported by the actuator and disposed across the tines. The wiper bears against the tines and causes the inlet grate to pivot, at the same time wiping the tines, as previously described. 
     In accordance with other aspects of the invention, an inlet grate and a grate clean-out actuator are both coupled to a through-hull housing to form a module or assembly which can be installed in an opening of an inlet ramp formed in the hull. The invention encompasses both a module housing a pivotable inlet grate and a grate clean-out actuator, and a boat hull having such a module installed in an opening in the hull. 
     In accordance with the preferred embodiment, the actuator penetrates and is supported by a through-hull housing installed in an opening in the hull. The actuator is linearly displaceable relative to the through-hull housing by manipulation of a handle or other operator input device or system. An inlet grate comprising a plurality of cantilever tines is pivotably mounted to and supported by the through-hull housing. A weed wiper is connected to the end of the actuator remote from the end that is actuated by the boat operator. When the actuator is displaced downward, the weed wiper moves downward along a linear path. At the point in the wiper displacement where the wiper bar engages the tines of the inlet grate, the downwardly displacing weed wiper causes the grate to pivot downward. During this action, the wiper slides across the grate and pushes any weeds or other debris entangled on the grate toward the ends of the tines. Because weeds usually wrap and ball around the inlet grate tines, the weeds will slide off easily when pushed or dragged by the sweeping wiper bar. 
     In accordance with another preferred embodiment, a device is provided which comprises a cantilevered inlet grate mounted on a single-direction pivot with spring return; a weed removal bar; and an actuation rod. The design intent of this grate is to combine the passive weed shedding of a cantilevered grate with an active weed removal system. During normal passive operation, the majority of weeds caught by the grate will be drawn by fluid forces towards the free ends of the tines and slide off. During this phase the grate tines are stationary, forced against the pivot stop by the water flow through the pump inlet. The active mode of operation is used when the inlet grate becomes severely entangled with weeds and pump performance is degraded. The weeds are removed by depressing the actuator rod, which causes the weed removal bar to move in a linear motion and come into contact with the inlet grate, causing it to pivot downward and opening up the gap between the free ends of the tines and the inlet lip. The bar slides down the grate, pushing the entangled weeds off the ends of the tines. Releasing the spring-loaded actuation rod automatically causes all components to retract back into their original positions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic showing a sectional view of a water jet apparatus incorporating a weed grate clean-out system in accordance with a prior art teaching. 
     FIG. 2 is a schematic showing operation of the weed grate clean-out system depicted in FIG. 1, with the running position depicted by solid lines and the clean-out position depicted by dashed lines. 
     FIG. 3 is a schematic showing a bottom view of the weed grate clean-out system depicted in FIG.  1 . 
     FIG. 4 is a schematic showing a sectional view of a portion of the stern of a boat incorporating a weed grate clean-out system in accordance with the preferred embodiment of the present invention. The passive clean-out positions of the moving components are indicated by solid lines, while the active clean-out positions of the moving components are indicated by dashed lines. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A known mechanism for cleaning out the inlet grate of a water jet propulsion system is depicted in FIGS. 1-3, taken from U.S. Pat. No. 6,083,063. These drawings disclose the basic structure components of a water jet propulsion system and the basic structure of an inlet grate having cantilevered tines. With this background, the description of the preferred embodiment, made with reference to FIG. 4 later, will be more readily understandable. 
     FIG. 1 shows a portion of a boat hull  2  having a cavity in which an inlet portion of a water jet apparatus  4  is installed. The inlet portion may comprise a separate component such as an inlet housing or may comprise the forward section of a water jet housing which houses the impeller. FIG. 1 shows a single housing  6  which houses an impeller  8  having a plurality of blades, a stator  10  having a plurality of stator vanes, and a tail cone  12 . The interior surfaces of housing  6  form a duct which defines a water tunnel or channel  7 . 
     FIG. 1 also shows a steering nozzle  14  which is pivotably mounted to the housing  6  at the outlet of the housing, the steering nozzle being pivotable about a vertical axis defined by the centerline of a pair of pivot pins  16  to enable steering the boat. For the purpose of illustration, one type of gear drive train  18  is depicted for coupling to an inboard motor (not shown). In the exemplary drive train shown in FIG. 1, a horizontal drive shaft  20  is coupled to an output shaft (not shown) of the inboard motor. The drive shaft  20  penetrates the transom  22  and extends into the upper gear housing  24 . A bevel drive gear  26  is mounted to the end of drive shaft  20 . The teeth of bevel drive gear  26  mesh with a bevel gear  28  mounted to the end of a vertical drive shaft  30 , which penetrates the water jet housing  6 . The opposite end of the vertical drive shaft  30  has a bevel gear  32  mounted thereto. The bevel gear  32  in turn meshes with a bevel gear  34  mounted on an impeller shaft  36  which has a portion extending forward of the vertical shaft  30 . The impeller (i.e., rotor)  8  is mounted on the forward end of the impeller shaft  36  and may have a conventional structure. 
     The impeller shaft  36  is supported by bearings (not shown) arranged inside a hollow hub  40  of stator  10  having a streamlined exterior surface. The stator hub  40  is surrounded by housing  6 . The interior surface of housing  6  is streamlined, so that the opposing surfaces of the housing  6  and hub  40  define a circumferential passageway through which the impelled water flows. The circumferential passageway forms part of the water tunnel  7 . The housing  6  and hub  40  are preferably connected by a plurality of stator vanes  42 . 
     In accordance with the exemplary drive train described above, an inboard engine provides a torque which drives the impeller shaft  36  to rotate via drive shafts  20  and  30 , and gears  26 ,  28 ,  32 , and  34 . Rotation of the impeller shaft  36  in turn causes the impeller  8  to rotate. During rotation, the angled blades of the impeller  8  impel water in the aft direction through the circumferential passageway between housing  6  and hub  40 . The stator vanes  42  function to redirect the swirling flow out of the impeller and eliminate swirl. The water exits the steering nozzle  14  as a downstream jet. 
     In accordance with the system depicted in FIGS. 1-3, weeds and other debris are prevented from entering the water tunnel  7  by an inlet grate  44  which is pivotably mounted to the housing  6 . The inlet grate  44  comprises a base  46  and a plurality of spaced cantilever tines  48 . The opposing ends of base  46  are pivotably mounted in the housing  6 . The inlet grate is pivotable about a pivot axis in a downward direction. During normal operation of the water jet apparatus, the inlet grate is in a so-called “running” or passive position, which is indicated by solid lines in FIG.  2 . During a clean-out operation, the inlet grate is pivoted downward to a so-called “clean-out” or active position indicated by dashed lines in FIG.  2 . The housing has recesses  50 , best seen in FIG. 3, which each receive a tip of a respective cantilever tine when the inlet grate is in the running position. A spring  52  (shown only in FIG. 2) holds the inlet grate in the passive position and urges the inlet grate  44  to return from the active position to the passive position. The spring  52  can be anchored to the housing. 
     The weed grate clean-out system depicted in FIGS. 1-3 comprises a wiper for wiping the tines  48  in a lengthwise direction while simultaneously causing the inlet grate  44  to pivot downward. The wiper comprises a wiper bar  54  disposed across the tines  48  and movable between first and second positions by actuation of a wiper arm  56 , which is pivotably mounted to the housing  6  by a pivot pin  58 . When the inlet grate  44  is in the running position, the wiper arm  56  is retracted into a recess  60  formed in housing  6 . The retracted position of the wiper arm  56  is indicated by solid lines in FIG.  2 . In response to operator actuation, the wiper arm rotates to an extended position indicated by dashed lines in FIG.  2 . The wiper arm can be actuated to rotate by any conventional mechanical or electromechanical means. FIG. 1 shows a push-pull cable  62  which is slidable inside one or more tubes  64  affixed to the boat and which penetrates the hull  2  and the housing  6 . One end of cable  62  is connected to a handle (not shown) manipulated by the boat operator. The other end of cable  62  is pivotably coupled to the wiper arm  56  to facilitate adjustment of the angular position of the cable end relative to the wiper arm during displacement of the former and pivoting of the latter. The wiper bar  54  bears against at least one of the cantilever tines  48  during movement of the wiper bar from a first position, when the wiper arm  58  is retracted, to a second position, when the wiper arm is extended. The wiper bar is pivotably mounted to the wiper arm. The wiper incorporates a return member  55  which is mechanically linked to the wiper bar  54  (the linkage is not shown in FIG. 1) and disposed on the underside of the inlet grate such that the return member  55  pushes the inlet grate up as the wiper bar is returned to the running position. 
     The first and second positions of a pivotable wiper bar  54  are depicted in FIG. 3 by solid and dashed lines respectively. Preferably the wiper bar has a cross section such that the surface which bears against the inlet grate is relatively flat so that the wiper bar will slide, not roll, along the length of the tines. The wiper bar  54  sweeps across the tines  48  from the first position to the second position as the wiper arm  56  is actuated to rotate from its retracted position to its extended position (shown in FIG.  2 ). As the wiper bar sweeps across the tines, it pushes or drags clumps of debris which might be ensnarled or accumulated on the tines toward the tips of the tines. At the same time, in the passive or running position of the inlet grate, the tines  48  are disposed inside the arc which the wiper bar  54  will travel during extension of the wiper arm  56 . In response to the interference presented by the tines with which the wiper bar is in contact, the wiper bar will push the inlet grate out of the path of the wiper bar, causing the inlet grate to pivot downward toward the active or clean-out position indicated by dashed lines in FIG.  2 . In the clean-out position, the tips of the tines are separated from the aft edge of the inlet opening by a distance sufficient to allow clumps of weeds or other debris to be pushed of off the inlet grate by the wiper bar. After the inlet grate has been unclogged, the boat operator actuates the wiper arm to rotate from its extended position to its retracted position, during which the spring  52  urges the inlet grate toward the running position. 
     The preferred embodiment of the present invention differs from the system shown in FIGS. 1-3 in that the actuator is linearly displaceable instead of being pivotable. Furthermore, in the passive grate position, the ends of the cantilevered tines do not contact the lower lip of the housing of the water jet propulsion unit, but rather there is a gap which leaves the tine ends free for weeds or other debris to slide off. Another difference lies in the fact that the inlet grate in accordance with the preferred embodiment is pivotably mounted to the hull, i.e., to a through-hull housing installed in an opening in the hull, rather than to the housing of the water jet propulsion unit. The linearly displaceable actuator penetrates and is supported by the same through-hull housing. It should be further appreciated that the water jet propulsion unit and the through-hull housing can be designed to enable the impeller drive shaft to pass through the through-hull housing to the inboard motor. In the latter case, the actuator and the wiper bar must be situated so that there is clearance for the drive shaft. 
     The preferred embodiment of the invention is depicted in FIG.  4 . In accordance with this embodiment, an inlet ramp is formed during molding of the boat hull. The inlet ramp comprises a pair of opposing sidewalls  66  (only one of which is visible in FIG. 4) which increase in height continuously from a starting point on the hull bottom to the respective points where the sidewalls join the transom  22 . The top edges of the opposing sidewalls  66  are connected by a ramp ceiling  68 , which curves continuously upward. The sidewalls and ceiling form part of the molded hull bottom and define an inlet channel  70 . Optionally, the junctures connecting the sidewalls to the ceiling may be formed as rounded corners. 
     A mounting adapter  72  in the form of a flanged ring having a rounded lower lip  74  is mounted to the rear face of the transom  22 . The bottom edges of the inlet ramp and the forward tip of the lower lip  74  define an inlet opening for entry of ambient water into the inlet channel  70 . The mounting adapter  72  is mounted to the transom  22  by fasteners (not shown). The water jet propulsion unit is in turn mounted to the mounting adapter  72  in cantilever fashion in a well-known manner. For example, the water jet propulsion unit may comprise an impeller housing  76  connected to the mounting adapter, a stator housing (not shown in FIG. 4) connected to the impeller housing, a discharge nozzle (not shown in FIG. 4) connected to the stator housing, and a steering nozzle (not shown in FIG. 4) pivotably mounted to the discharge nozzle. The entire assembly extends from the mounting adapter in cantilever fashion. The outlet of the discharge nozzle is in flow communication with the inlet opening via the inlet ramp, the mounting adapter, the impeller housing and the stator housing. All of these components, communicating with each other in series, form a duct. 
     As seen in FIG. 4, the preferred embodiment of the invention comprises a module or assembly  80  installed in an opening in the hull bottom, i.e., in the ceiling  68  of the inlet ramp. The assembly comprises a housing  82 , an inlet grate  84 , an actuator  86  and a wiper  88 . Any conventional means may be used to install the module  80  in the opening in the inlet ramp ceiling. For example, the housing  82  may be provided with a peripheral mounting flange  96  which bolts to a matching mounting flange  98  formed along the periphery of the opening in the hull. Alternatively, a separate mounting plate could be used to clamp the housing  82  to the inlet ramp ceiling. 
     The housing  82  has a first recess  90  for receiving a base portion of the inlet grate  84 , a second recess  92  for receiving the wiper, and a linear bore  94  communicating with the second recess for passage of the actuator  86 . The linear bore  94  extends upward through a boss  100  formed as an integral part of the housing  82 . 
     In the preferred embodiment, the actuator  86  comprises a rod  102 , which slides in and is guided by the linear bore  94 , and a handle  104 . The actuator  86  slides downward from a fully up position to a fully down position when a person in the stern of the boat depresses a handle  104  connected to the top end of the rod  102 . The wiper  88  preferably takes the form of a bar pivotably mounted to the lower end of the rod  102 . In the passive position, the rod  102  is fully up and the wiper bar  88  resides in the recess  92  of housing  82 . In the active position, the rod  102  is displaced downward and the wiper bar  88  extends into the channel  70 . During operation of the water jet propulsion unit, the wiper bar  88  resides in the recess  92  so that the wiper bar does not obstruct water flow through the inlet ramp channel  70 . The actuator is spring-loaded with a return compression spring  106  so that the wiper bar  88  retracts automatically into recess  92  when the handle  104  is released. 
     To aid sliding of the rod  102  in the linear bore  94 , grease may be applied to the rod surface. An O-ring  114  is seated in an annular recess formed in the boss  100  to wipe the rod surface, thereby acting as a dam against upward escape of grease. A double-seal arrangement is installed in an annular recess which communicates with the second recess  92 . The lower seal  116  is designed to prevent the entry of water from the inlet channel  70  into the linear bore  94 , while the upper seal  118  wipes the rod surface, again to block the grease from escaping. 
     In the preferred embodiment, the inlet grate  84  comprises a base  108  and a plurality of spaced mutually parallel cantilever tines  110 . As seen in FIG. 4, the length of the tines  110  is such that there is a gap between the ends of the tines and the lower lip  74  of the mounting adapter  72 . This gap allows weeds or other debris to slide off the tines even when the inlet grate is in the passive position. Preferably the tines are tapered along their length, with the taper being dramatically increased near the free ends of the tines. The opposing ends of base  108  are pivotably mounted in the recess  90  of housing  82 . The inlet grate  84  is urged toward the passive position by a spring  112 . The passive position is defined by a stop surface formed as part of housing  82 , which stop surface blocks further pivoting of the inlet grate in the return direction. 
     In accordance with the preferred embodiment, the actuator is linearly displaceable between up and down positions, while the inlet grate is pivotable between first and second angular positions, depicted in FIG. 4 by solid and dashed lines respectively. As the actuator  86  is moved downward from the fully up position indicated by solid lines in FIG. 4, the wiper bar  88  eventually engages at least one tine  110 . This engagement causes the wiper bar to pivot during the next increment in downward displacement of the actuator, until a planar forward face of the wiper bar  88  lies flat against the upper edges of the tines  110 . Thereafter, during further downward displacement of the actuator  86 , the wiper bar  88  will both bear against and slide along the upper edges of the tines  110 , causing the inlet grate to pivot from the first angular position (indicated by solid lines in FIG. 4) to the second angular position (shown by dashed lines in FIG.  4 ). If weeds and/or other debris are entangled on the distal sections of the tines, the wiping action of the wiper bar will push the entangled matter off of the free ends of the tines. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. For example, it will be apparent to a person skilled in the art that the cantilever tines of the inlet grate could be independently pivotable instead of being connected to a common pivotable base. Also, means other than a push rod can be used to actuate the clean-out system disclosed above. For example, clean-out could be actuated by the boat operator using a wheel connected to a jack screw, which is in turn mechanically coupled to the wiper. Alternatively, electromechanical means could be used to actuate rotation of the wiper, in which case clean-out could be actuated by depression of a pushbutton on a control panel. In addition, many modifications may be made to adapt a particular situation to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. 
     As used in the claims, the term “duct” means any structure which defines a channel having an inlet and an outlet. The structure may comprise a single cast piece or an assembly of components. In the preferred embodiment disclosed above, the duct comprises three main components: an inlet ramp formed in the hull bottom, a through-hull housing for the inlet grate and the actuator that is installed in an opening in the hull, and a housing (or series of housings) of a water jet propulsion unit attached to the hull. However, the invention encompasses, for example, forming the inlet ramp and the grate/actuator housing as integral parts of the hull, or forming the inlet ramp and the grate/actuator housing integrally with the housing (or inlet housing) of the water jet propulsion unit.