STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to waterjet propulsion, more particularly to the steering and reversing of waterjet propulsion systems for marine vessels such as ships. 
     Marine waterjet propulsion has increasingly gained acceptance in recent years, and has begun to challenge the long-established dominance of screw propellers. A waterjet-propelled craft is known to be capable of affording a superior maneuvering capability. 
     In fact, waterjet propulsion offers several advantages over conventional screw propellers, including the following: simplification of mechanical arrangement by eliminating reverse gears, a complicated mechanical device to change propeller pitch, and long propulsion shafting; flexibility of machinery arrangement and placement of machinery in the hull; improved maneuverability, especially the ability to turn at zero forward speed; and, elimination of external rudders, shafting and propellers, thereby improving shallow water operation. 
     However, several disadvantages are known to be associated with existing waterjet craft, which conventionally effectuate jet discharge in the air. A significant disadvantage is low propulsive efficiency at speeds less than about 25 knots. As a consequence, existing waterjets have been principally applied to high speed vessels at speeds between approximately 35 to 70 knots. Such waterjet designs suffer from poor performance at off-design speeds. 
     Peterson et al. U.S. Pat. No. 5,476,401 issued Dec. 19 1995, incorporated herein by reference, and Dai et al. U.S. Pat. No. 5,439,402 issued Aug. 8, 1995, incorporated herein by reference, disclose improvements pertaining to a conventionally abovewater waterjet system insofar as providing good hydrodynamic performance (e.g., high propulsive efficiency and good cavitation performance) both at low speeds and at high speeds. 
     The present invention is especially motivated by the need, discerned by the present inventors, to successfully effectuate waterjet propulsion underwater (into a water medium)—rather than abovewater (into an air medium), as is conventionally done. Jets on existing waterjet craft are typically discharged into the air. The present inventors recognize the benefits which a naval ship or commercial ship could enjoy by circumventing certain problems normally associated with waterjet discharge into the air. Notably, discharging the jets underwater would eliminate the resultant noise from the jets plunging into the sea, and would increase propulsive efficiency. 
     Nevertheless, conventional steering and backing systems for waterjet craft are effective for the familiar abovewater mode of jet discharge, but would be unsuitable for the unfamiliar underwater mode of jet discharge. Conventional waterjet steering/backing systems include outlet nozzles for receiving the accelerated flow from the pumps and discharging the jets in a rearward direction above the waterline. The steering/backing of the craft is typically accomplished by deflecting the jets using rotating steering sleeves or buckets. Hence, according to common practice, steering and backing systems use rotating sleeves to vector the jets for maneuvering. These types of devices would experience severe drag penalties in water. Moreover, the bulky sleeves would trigger severe cavitation. 
     Therefore, there is a need for a waterjet steering/backing system which is suitable for a waterjet craft having one or more jets discharged underwater. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide a steering and reversing system for a waterjet-propelled craft. 
     It is another object of the present invention to provide a steering and reversing system for a waterjet-propelled craft wherein a jet, or a plurality of jets, are discharged underwater. 
     A further object of the present invention is to provide, for an underwater-discharge waterjet-propelled craft, a steering and reversing system which is characterized by efficiency in terms of steering and maneuvering capability. 
     Another object of the present invention is to provide, for an underwater-discharge waterjet-propelled craft, a steering and reversing system which is relatively uncomplicated and economical. 
     The present invention provides a relatively simple waterjet steering system which is characterized by efficient capabilities in terms of steering and maneuvering. In particular, the present invention advantageously affords minimum drag and cavitation-free operation in straight course and during course-keeping maneuvering. 
     In accordance with typical embodiments of the present invention, a waterjet exit assembly comprises an upper horizontal wall, a lower horizontal wall and a plurality of approximately equidistantly spaced vertical walls. The upper horizontal wall, the lower horizontal wall and the vertical walls form a plurality of adjacent channels for the waterjet. The waterjet exit assembly also comprises a plurality of horizontally pivotable vertical flaps, wherein each vertical wall includes a vertical flap. Each vertical flap is capable of deflecting to selected dispositions with respect to its corresponding vertical wall. 
     According to typical inventive practice, the vertical walls include two lateral vertical walls and at least one medial vertical wall. Each medial vertical wall includes a vertical flap which is a divisible vertical flap, which is divisible into two horizontally pivotable demiflaps. Each demiflap is capable of deflecting to selected dispositions with respect to its corresponding medial vertical wall. Many inventive embodiments are characterized by two channels, three vertical walls and three flaps. 
     Generally, the present invention is implemented in association with waterjet means characterized by the capability of accelerating water and discharging the accelerated water in a manner suitable or adaptable for propelling a marine vessel. According to typical inventive practice, the waterjet means will comprise a water-accelerative mechanism of a kind which includes at least one pump; nevertheless, the present invention can be used in association with any kind of waterjet means having the requisite capability. 
     According to frequent practice of this invention, water is introduced into two adjacent waterjet pumps, and exits via two respective accelerated flow discharge nozzles. The accelerated flow is then controlled by the inventive steering-and-backing apparatus, which includes two adjacent flow-straightening chambers. Each flow-straightening chamber includes its own sidewall flap. Also, both flow-straightening chambers share a longitudinally divisible (splitable) medial sidewall flap. The three sidewall flaps are used for jet direction vectoring, thereby effectuating steering. 
     Depending on the inventive embodiment, backing is effectuated according to either of two inventive methodologies. According to some inventive embodiments, a set of rotating buckets is implemented in the transom stern for flow reversing. According to other inventive embodiments, bottom wall flaps are implemented for flow reversing; each flow-straightening chamber includes a bottom wall flap. Some inventive embodiments can be provided, in the alternative, with both flow reversing capabilities. 
     Featured by this invention is the provision of sidewall flaps which serve to vector the jet direction for steering and maneuvering. The sidewall flaps are parts of the dual flow-straightening exit chamber assembly. The two lateral sidewall flaps are each a part of one such flow exit chamber, while the single medial sidewall flap is shared by both flow exit chambers and is capable of splitting into two medial semi-flaps. Each lateral sidewall flap is embedded in a lateral sidewall. The medial sidewall flap is embedded in the medial sidewall. 
     Sidewall flap deflection is inventively effectuated without leading edge protrusion into the flow, thereby avoiding cavitation. Many inventive embodiments provide a flexible seal at the front part of each sidewall flap to avoid gap cavitation. The inventive steering system thus succeeds in vectoring the jets without causing cavitation during the course-keeping maneuvering. 
     Further featured by the present invention is the provision of two alternative backing methodologies, each of which is cooperative with the inventive steering methodology. In order to reverse the direction of the discharged jets for purposes of effecting backing operations, this invention can utilize: (i) a deployable bucket; or, (ii) two bottom wall flaps, each of which is part of a respective flow-straightening chamber. 
     Many inventive embodiments provide a hydrodynamically designed and integrated hull-and-waterjet propulsion system. Such inventive embodiments feature an integrated stem which incorporates a pump drive with steering and reversing devices. The inventively integrated hull-and-steering system beneficially affords minimum drag and cavitation-free course-keeping operation. 
     As mentioned hereinabove, for certain marine applications (e.g., naval applications) it is desirable that the waterjet be discharged underwater. The fluid density of water is about 900 times greater than that of air. The present invention is particularly efficacious in the context of underwater-discharge waterjet propulsion systems. 
     By comparison, conventional steering systems implementing rotating sleeves, used extensively in existing waterjet craft, generally work adequately in air but would experience tremendous drag or resistance in water; in addition, the bulky rotating sleeves would produce severe cavitation in water. If a conventional steering system were used for a waterjet craft with the jets discharged underwater, the craft would suffer drag and possibly cavitation on the steering devices at high speeds; the craft would have poor hydrodynamic performance. 
     The multifarious applicability of the present invention admits of transportation in any water or marine environment, locality or milieu. The terms “water” and “marine,” as used herein, are synonymous, and pertain to any body of water, natural or man-made, including but not limited to oceans, seas, gulfs, lakes, harbors, canals, rivers, straits, etc. 
     Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein like numbers indicate the same or similar components, and wherein: 
     FIG. 1 is a diagrammatic side elevation view of an embodiment of the present invention, showing one waterjet pump, its corresponding exit nozzle and its corresponding flow-straightening exit chamber. 
     FIG. 2 is a diagrammatic rear perspective view, partially cutaway, of the inventive embodiment depicted in FIG. 1, showing both waterjet pumps and their corresponding flow-straightening exit chambers. 
     FIG. 3 is a diagrammatic top plan view, partially cutaway, of an inventive embodiment such as depicted in FIG. 1, showing the flow-straightening exit chambers and their corresponding steering flaps, particularly illustrating operation of an inventive steering system wherein the marine vessel is navigating in straight course. 
     FIG. 4 is diagrammatic top plan view (similar to the view in FIG. 3) of the inventive embodiment shown in FIG. 3, particularly illustrating operation of an inventive steering system wherein the marine vessel is navigating in a turn. 
     FIG. 5 is a graphical representation of the calculated flow streamline distributions, wherein the three sidewall flaps are deflected by ten degrees, and the flap length equals six feet. 
     FIG. 6 is a graphical representation of the calculated pressure distributions, wherein the three sidewall flaps are deflected by ten degrees, and the flap length equals six feet. 
     FIG. 7 is a graphical representation (similar to the graphical representation in FIG. 6) of the calculated pressure distributions, wherein the three sidewall flaps are deflected by ten degrees, and the flap length equals twelve feet. 
     FIG. 8 is a graphical representation (similar to the graphical representations in FIG.  6  and FIG. 7) of the calculated pressure distributions, wherein the three sidewall flaps are deflected by five degrees, and the flap length equals six feet. 
     FIG. 9 is a partial, enlarged diagrammatic top plan view of an inventive sidewall such as represented in FIG.  3  and FIG. 4, showing the leading edge portion of the inventive sidewall flap, particularly illustrating deflection of the inventive sidewall flap. 
     FIG. 10 is a diagrammatic top plan view (similar to the views in FIG.  3  and FIG. 4) of the inventive embodiment shown in FIG.  3  and FIG. 4, showing the flow-straightening exit chambers and their corresponding backing (bottom) flaps, particularly illustrating operation of an inventive backing system wherein (in conjuction with opening of the bottom flaps as shown in FIG. 11) the steering (sidewall) flaps are closed for reversing flow to the forward direction, thereby effecting reversal of the marine vessel. 
     FIG. 11 is diagrammatic side elevation view of the inventive embodiment as shown operating in FIG. 10, particularly illustrating operation of the inventive backing system wherein (in conjuction with closure of the sidewall flaps as shown in FIG. 10) the bottom flaps are opened for reversing flow to the forward direction, thereby effecting reversal of the marine vessel. 
     FIG. 12 is diagrammatic rear perspective view, partially cutaway, of an inventive embodiment, different from that shown operating in FIG.  10  and FIG. 11, showing the flow-straightening exit chambers and a bucket, particularly illustrating operation of an inventive backing system wherein the bucket is deployed for reversing flow to the forward direction, thereby effecting reversal of the marine vessel. 
     FIG. 13 is diagrammatic bottom plan view, partially cutaway, of the inventive embodiment shown operating in FIG. 12, particularly illustrating operation of the inventive backing system wherein the bucket is deployed for effecting straight backing of the marine vessel. 
     FIG. 14 is diagrammatic bottom plan view (similar to the view in FIG. 13) of the inventive embodiment shown operating in FIG. 12, particularly illustrating operation of the inventive backing system wherein the bucket is deployed for effecting maneuvering backing of the marine vessel. 
     FIG. 15 is diagrammatic rear perspective view (similar to the view in FIG. 12) of the inventive embodiment shown operating in FIG. 12, particularly illustrating operation of the inventive backing system wherein the bucket is retracted deployed for effecting forward motion of the marine vessel. 
     FIG. 16 is a diagrammatic top plan view, partially cutaway, of an inventive embodiment characterized by three flow-straightening exit chambers and four sidewalls (including four steering flaps corresponding therewith). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG.  1  and FIG. 2, juxtaposed are a pair of adjacent waterjet pumps  20   a  and  20   b . Waterjet pump  20   a  includes a flush inlet  22   a  and an outlet (exit) nozzle  24   a ; waterjet pump  20   b  includes a flush inlet  22   b  and an outlet (exit) nozzle  24   b . The unaccelerated water w enters waterjet pumps  20   a  and  20   b  through the corresponding flush inlets  22   a  and  22   b . The accelerated water f exits from waterjet pumps  20   a  and  20   b  through the corresponding outlet nozzles  24   a  and  24   b.    
     The accelerated water flow f enters inventive flow exit assembly  26 . Inventive flow exit assembly  26  describes a box-like rectilinear configuration which is approximately equally divided by medial sidewall  28  into two adjacent, approximately equivalent “flow-straightening” flow exit chambers  30   a  and  30   b . Flow exit chamber  30   a  and flow exit chamber  30   b  serve to “straighten” the flow received, respectively, from outlet nozzle  24   a  (of waterjet pump  20   a ) and outlet nozzle  24   b  (of waterjet pump  20   b ). Chamber  30   a  is associated with waterjet pump  20   a ; chamber  30   b  is associated with waterjet pump  20   b . Because FIG. 1 presents a side view, or a mirror-imaged opposite-side view, only one waterjet pump and one chamber are visible in this figure. 
     Inventive flow exit assembly  26  includes the following “walls,” each of which is approximately or substantially planar: medial sidewall  28 , topwall  32 , bottomwall  34 , lateral sidewall  36   a  and lateral sidewall  36   b . Topwall  32  and bottomwall  34  are approximately parallel and approximately congruent. Medial sidewall  28 , lateral sidewall  36   a  and lateral sidewall  36   b  are approximately parallel and approximately congruent. 
     Topwall  32  includes topwall portions  33   a  and  33   b . Bottomwall  34  includes bottomwall portions  35   a  and  35   b . Flow exit chamber  30   a  is bounded by lateral sidewall  36   a , medial sidewall  28 , topwall portion  33   a  and bottomwall portion  35   a . Flow exit chamber  30   b  is bounded by lateral sidewall  36   b , medial sidewall  28 , topwall portion  33   b  and bottomwall portion  35   b . Thus, inventive flow exit assembly  26  includes medial sidewall  28 , topwall portion  33   a , bottomwall portion  35   a , sidewall  36   a , topwall portion  33   b , bottomwall portion  35   b  and sidewall  36   b.    
     Chambers  30   a  and  30   b  are each characterized by a rectangular tubular shape formed by a horizontal topwall, a horizontal bottomwall and two vertical sidewalls. Chambers  30   a  and  30   b  share medial sidewall  28 . In addition to sharing medial sidewall  28  with chamber  30   b , chamber  30   a  also has topwall portion  30   a , bottomwall portion  35   a  and lateral sidewall  36   a . In addition to sharing medial sidewall  28  with chamber  30   a , chamber  30   b  also has topwall portion  33   b , bottomwall portion  35   b  and lateral sidewall  36   b.    
     Inventive flow exit assembly  26  approximately defines a rectangular parallelepiped which is longitudinally split by vertical medial sidewall  28 , approximately down the middle, into two approximately identically shaped chambers  30   a  and  30   b . Still with reference to FIG.  1  and FIG.  2  and especially with reference to FIG.  3  and FIG. 4, chamber  30   a  has an interior chamber width w Ca , and chamber  30   b  has an interior chamber width w Cb , wherein w Ca  is approximately equal to w Cb . Inventive flow exit assembly  26  has a total exit assembly width w T  which approximately equals the sum of w Ca  plus w Cb  plus the sum of the widths of sidewalls  36   a ,  36   b  and  28 . 
     When the bottomwall flaps are in undeflected condition (or are not included) such as shown in FIG. 1, topwall  32  and bottomwall  34  are approximately congruent and are approximately parallel. Topwall portion  33   a , topwall portion  33   b , bottomwall portion  35   a  and bottomwall portion  35   b  are approximately congruent. Topwall portion  33   a  and topwall portion  33   b  lie approximately in the same plane. Bottomwall portion  35   a  and bottomwall portion  35   b  lie approximately in the same plane. Topwall portion  33   a  and bottomwall portion  35   a  are approximately parallel. Topwall portion  33   b  and bottomwall portion  35   b  are approximately parallel. When the sidewall flaps and flap subsections are in undeflected condition such as shown in FIG. 3, sidewall  28 , sidewall  36   a  and sidewall  36   b  are approximately congruent and are approximately parallel. 
     Still with reference to FIG.  1  through FIG. 4, lateral sidewall  36   a  includes, as integral parts thereof, lateral sidewall non-flap section  78   a  and lateral sidewall flap  38   a . Lateral sidewall  36   b  includes, as integral parts thereof, lateral sidewall non-flap section  78   b  and lateral sidewall flap  38   b . Medial sidewall  28  includes, as integral parts thereof, medial sidewall non-flap section  80  and medial sidewall flap  40 . 
     The “flap” sections of lateral sidewall  36   a , lateral sidewall  36   b  and medial sidewall  28 , respectively, are lateral sidewall flap  38   a , lateral sidewall flap  38   b  and medial sidewall flap  40 . The “non-flap” sections of lateral sidewall  36   a , lateral sidewall  36   b  and medial sidewall  28 , respectively, are lateral sidewall non-flap section  78   a , medial sidewall non-flap section  80 , and lateral sidewall non-flap section  78   b . As indicated by dashed line m and as later discussed herein in relation to FIG. 10, medial sidewall flap  40  is longtitudinally divisible into two approximately equal flap subsections  42   a  and  42   b . The marine vessel&#39;s longitudinal axis of symmetry is indicated by dashed line M. As shown in FIG. 3, when medial sidewall  28  is in an undeflected condition, dashed lines m and M lie approximately in the same vertical plane. 
     Lateral sidewall flap  38   a  is horizontally pivotable (at least about plus-or-minus thirty degrees as shown in FIG. 4 by angle β a  and bidirectional arrow p a ) about vertical pivot  44   a . Lateral sidewall flap  38   b  is horizontally pivotable (at least about plus-or-minus thirty degrees as shown in FIG. 4 by angle β b  and bidirectional arrow p b ) about vertical pivot  44   b . Medial sidewall flap  40  is horizontally pivotable (at least about plus-or-minus thirty degrees as shown in FIG. 4 by angle β m  and bidirectional arrow p m ) about vertical pivot  46 . Vertical pivots  46 ,  44   a  and  44   b  each turn about a central axis c such as shown in FIG.  9 . 
     The two chambers  30   a  and  30   b  are hence separated by medial sidewall  28  whereby lateral sidewalls  36   a  and  36   b  are approximately parallel to medial sidewall  28  and to each other. Chambers  30   a  and  30   b  are used to smooth out vortex generated by pumps  20   a  and  20   b , respectively. Chambers  30   a  and  30   b  receive the water from pumps  20   a  and  20   b , respectively, at chamber inlet ends  48   a  and  48   b . Chambers  30   a  and  30   b  discharge the water at chamber outlet ends  50   a  and  50   b , respectively. The water is thus discharged by chambers  30   a  and  30   b  underwater at the transom stern  54  such as shown in FIG.  2 . 
     As shown in FIG. 2, according to generally preferred inventive practice, the inventive steering/maneuvering system is integrated into the ship hull to ensure a minimum ship resistance. Topwall  32  (which includes topwall portions  33   a  and  33   b ) is made an integral part of an area of hull bottom  53  in the vicinity of transom stern  54 . Sidewall flap trailing edges  58 ,  60   a  and  60   b , topwall  32 , bottomwall  34  and transom stern  54  are shown to all be approximately even or coextensive. According to the integrated arrangement characteristic of many inventive embodiments, pumps  20   a  and  20   b  smoothly transition into chambers  30   a  and  30   b ; the pump outlet nozzles  24   a  and  24   b  effectively merge or blend into the chamber inlet ends  48   a  and  48   b , respectively, the conversion occurring approximately at the location indicated by dashed line t shown in FIG.  1 . 
     It can be considered that there are at least two basic approaches to practicing the present invention. According to a first approach, a marine vessel (and, in particular, its propulsion system) is originally designed (“on the drawing board”) as being inclusive of the present invention, and is then constructed in accordance therewith. This “integral” approach, wherein the inventive apparatus is “built into” the marine vessel as part thereof (e.g., made an integral part of the hull) during fabrication of the marine vessel, will frequently yield cost savings and greater hydrodynamic efficiency, in comparison with the second approach. 
     According to the second approach, an existing marine vessel (and, in particular, its propulsion system) is adapted to being inclusive of the present invention. Generally, the existing marine vessel must be changed in one or more respects in order to accommodate or comport with the inventive apparatus. This “retrofit” approach, wherein the inventive apparatus is attached to, mounted on or otherwise coupled with the marine vessel so as to appropriately engage the waterjet mechanism, will normally be expected to entail a degree of structural alteration of the existing marine vessel construction (e.g., of the hull and/or the waterjet pumping mechanism); frequently, this structural alteration will include that of the waterjet mechanism (e.g., the waterjet pump or pumps), especially the outlet ends thereof, so as to be rendered harmonious with the inventive apparatus, especially the inlet ends thereof. 
     Regardless of how the inventive apparatus is caused to be associated with a marine vessel, it is generally an inventive objective to optimize (or at least account for) the hydrodynamic and propulsive effects and ramifications of the inventive arrangement in the context of the marine vessel. The inventive practitioner should appropriately regard the hydrodynamic “flow lines” and contours of the marine vessel as a whole, and particularly of the configuration involving the engagement of the inventive apparatus with the waterjet apparatus. Generally speaking, hydrodynamic efficiency of the marine vessel will be more readily attained or advanced when the inventive apparatus is incorporated into the marine vessel as part of the original design of the marine vessel (such as illustrated in FIG.  1  and FIG.  2 ), as compared with when the inventive apparatus is retrofittedly incorporated into the marine vessel. 
     Outlet nozzles  24   a  and  24   b  are shown in FIG.  1  and FIG. 2 to be diametrically or perimetrically equivalent to, and indistinguishably (or virtually indistinguishably) consolidated with, chamber inlet ends  48   a  and  48   b ; this inventive configuration is more typically associated with embodiments wherein the marine vessel&#39;s propulsion system is designed from the start with the present invention in mind. When the inventive apparatus is retrofitted with respect to an existing waterjet mechanism, the configuration of the waterjet mechanism in combination with the inventive apparatus may perhaps be more often typified by a lesser degree of geometric regularity in the vicinity of the junctions between the waterjet outlet nozzles and the inventive flow chambers&#39; inlet ends. 
     Inventive practice will generally dictate that, in order to suitably couple the waterjet mechanism with the inventive apparatus, the waterjet&#39;s outlet nozzles “fit” (i.e., be geometrically congruous with) the inventive flow chamber(s)&#39;s inlet ends. Regardless of whether the inventive apparatus is design-integratively or retrofittedly provided, connective compatibility will usually be characterized by relative apertural sizes wherein the cross-sectional areas of the flow chambers&#39; inlet ends are greater than or equal to the cross-sectional areas of the waterjet outlet nozzles, and wherein the former are encompassing of the latter. In other words, the waterjet outlet nozzles should be diametrically or perimetrically agreeable with the flow chambers&#39; inlet ends, and should be rendered in appropriate engagement therewith. 
     Sidewall flap trailing edge  58 , sidewall flap trailing edge  60   a , sidewall flap trailing edge  60   b , topwall  32  and bottomwall  34 , though preferably being approximately even or coextensive with respect to each other, need not be approximately even or coextensive with respect to transom stern  54 . Generally in inventive practice, the chambers can be even/coextensive with respect to the transom/stern (such as exemplified in FIG. 1 by a chamber  30   a  having flap trailing edge  60   a ), or recessed with respect to the transom/stem (such as exemplified in FIG. 1 by a chamber  30   a  having a flap trailing edge  60   a ′), or protrusive with respect to the transom/stem (such as exemplified in FIG. 1 by chamber  30   a  having a flap trailing edge  6   a ″). 
     The spatial relation of the inventive apparatus&#39;s aft end with respect to the marine vessel&#39;s stem is a factor to be considered in the overall design of the invention for a particular use, especially in terms of hydrodynamics and propulsion. For instance, whether the aft end of the inventive apparatus is even with the stem, or whether and how much (e.g., at what distance) the aft end of the inventive apparatus is forward of or aft of or even with the stem, should figure in the evaluation of the inventive apparatus&#39;s desired configuration in terms of lengths, angles, etc. 
     Shown in FIG. 3, FIG. 4, FIG.  10  and FIG. 11 are two kinds of inventive maneuvering devices, viz.: (i) sidewall flaps  40 ,  38   a  and  38   b , used for steering; and, (ii) bottomwall flaps  56   a  and  56   b , used for backing. Every such flap is made an integral part of a corresponding wall of inventive flow exit assembly  26 : Medial sidewall flap  40  is imbedded in medial sidewall  28 ; lateral sidewall flap  38   a  is imbedded in lateral sidewall  36   a ; lateral sidewall flap  38   b  is imbedded in lateral sidewall  36   b ; bottomwall flap  56   a  is imbedded in bottomwall  34   a ; bottomwall flap  56   b  is imbedded in bottomwall  34   b.    
     It is reemphasized that, rather than implementing bottomwall flaps imbedded in the bottom walls of the flow exit chambers, such as bottomwall flaps  56   a  and  56   b , the present invention can effectuate the backing aspect of maneuverability via an alternative methodology using a convexo-concave structure such as one known as a “bucket.” According to the inventive “bucket” methodology, a deployable bucket is stored in the transom stern. Hence, some inventive embodiments do not include bottomwall flaps for backing purposes, but instead utilize a bucket-type structure such as further discussed hereinbelow in relation to FIG.  12  through FIG.  15 . Some inventive embodiments may include bottomwall flaps and additionally include a bucket-type structure, as well as the associated means for effectuating either mode of backing. 
     The back ends of sidewalls  28 ,  36   a  and  36   b  are not blunt trailing edges; rather, the back ends of sidewall flaps  40 ,  38   a  and  38   b  are curved to avoid flow separation and to reduce ship resistance. Sidewall flaps  40 ,  38   a  and  38   b  have curvilinear trailing edges  58 ,  60   a  and  60   b , respectively. Lateral trailing edges  60   a  and  60   b  each describe a partially curved, partially straight shape. 
     Lateral trailing edge  60   a  defines an arcuate exterior surface portion  64   a  of sidewall flap  38   a , in combination with a truncating linear interior surface portion  66   a  of sidewall flap  38   a ; similarly, lateral trailing edge  60   b  defines an arcuate exterior surface portion  64   b  of sidewall flap  38   b , in combination with a truncating linear interior surface portion  66   b  of sidewall flap  38   b.    
     Medial trailing edge  58  describes a substantially curved shape. Medial trailing edge  58  defines the combination of an arcuate surface portion  68   a  of medial sidewall flap  40  and an arcuate surface portion  68   b  of medial sidewall flap  40 . Dashed line m represents the junction between medial sidewall flap subsection (e.g., half-section)  42   a  and medial sidewall flap subsection (e.g., half-section)  42   b.    
     As later discussed herein in relation to FIG. 10, medial sidewall flap subsections  42   a  and  42   b  are capable of separating for effectuating closure of the sidewall flaps (for purposes of reversing flow in order to achieve backing of the marine vessel); when this occurs, it can be considered that medial sidewall flap subsection  42   a  has medial subsection trailing edge  59   a , and that medial sidewall flap subsection  42   b  has medial subsection trailing edge  59   b . Medial subsection trailing edge  59   a  defines the combination of arcuate surface portion  68   a  with a truncating linear surface portion  70   a ; similarly, medial subsection trailing edge  59   b  defines the combination of arcuate surface portion  68   b  with a truncating linear surface portion  7   b.    
     As viewed in FIG. 3, FIG.  4  and FIG. 10, the flow f is from left to right. The ship speed is designated V S . The jet speed (the “jet” or water flow inside a chamber  30   a  or  30   b ) is designated V J . Typically, the jet speed V J  is approximately 20% to 60% higher than the ship speed V S . The steering devices (sidewall flaps  40 ,  38   a  and  38   b ) and the backing devices (bottomwall flaps  56   a  and  56   b ) are imbedded in the flow exit chambers  30   a  and  30   b.    
     Still referring to FIG.  3  and especially referring to FIG. 4, sidewall flaps  40 ,  38   a  and  38   b  have rotatable medial leading edge  72 , rotatable lateral leading edge  74   a  and rotatable lateral leading edge  74   b , respectively. The axis of rotation is centric to each leading edge of the corresponding sidewall flap. The ordinarily skilled artisan is well acquainted with the mechanics pertaining to the rotation of ship rudders; similar mechanics are inventively implemented to rotate the sidewall flaps. A rotating device is conventionally used to deflect a ship rudder, and variations thereof are well known in the marine industry. 
     While the marine craft is operating in a straight course, sidewall flaps  40 ,  38   a  and  38   b  remain even or nearly even with their respective sidewalls (sidewalls  28 ,  36   a  and  36   b , respectively) and bottomwall flaps  56   a  and  56   b  remain even or nearly even with their respective bottomwall portions (bottomwall portions  34   a  and  34   b , respectively, as shown in FIG.  2 ). Flap angle β is the angle formed by each sidewall flap with its respective sidewall. Every sidewall flap deflects at approximately the same flap angle β at approximately the same time (i.e., approximately synchronously). That is, as shown in FIG. 4, flap deflection angle β a  approximately equals flap deflection angle β b . 
     In typical inventive practice, during normal straight course-keeping, every flap angle β shifts constantly, simultaneously (i.e., approximately synchronously) and equally (i.e, approximately parallelly) within an approximate range of plus or minus 5 or 6 degrees (i.e., in a range between no more than about five or six degrees to one side of the sidewall and no more than about five or six degrees to the opposite side of the sidewall). In this manner, sidewall flaps  40 ,  38   a  and  38   b  and bottomwall flaps  56   a  and  56   b  are flushly or nearly flushly disposed in their respective sidewalls, without significant protrusion to the flow; hence, practically no additional or unwanted drag is incurred in association with the present invention. 
     With particular reference to FIG. 4, when the waterjet craft is called for turning maneuver, sidewall flaps  38   a ,  38   b  and  40  are each deflected to approximately the same flap angle β and approximately synchronously, thereby vectoring the jet. It is readily appreciated, in consideration of FIG. 4, that the sidewall flaps will be rotated to flap angle β in the direction opposite that shown, in order that the craft be turned in the other direction. 
     Typical inventive practice provides that, when effectuating sharp (wide) turns, every flap angle β shifts simultaneously (i.e., approximately synchronously) and equally (i.e., approximately parallelly) to an equivalent deflective position (with respect to its corresponding sidewall) which is up to a maximum value of about 20 or 30 degrees (i.e., between about twenty or thirty degrees to one side of the sidewall). Hence, generally during inventive practice involving normal navigation (whether effecting straight course-keeping, moderate turns or extreme turns), it can be expected that every flap angle β will shift virtually identically and concurrently within an approximate range of plus or minus 30 degrees (i.e., in a range between no more than about thirty degrees to one side of the sidewall and no more than about thirty degrees to the opposite side of the sidewall). Therefore, according to typical inventive embodiments, the sidewall flaps will have the capability of deflecting at a plus-and-minus angle β of at least thirty degrees. 
     The value of the angle β which is inventively effectuated for achieving particular maneuvers varies among inventive applications, depending upon factors including (i) the size and weight of the marine vessel and (ii) the configuration of the inventive waterjet apparatus. 
     Reference now being made to FIG. 5, shown are the streamline distributions along the surfaces of the sidewalls and the sidewall flaps, wherein all three sidewall flaps are deflected by ten degrees. This 10° flap angle is selected merely as exemplary. 
     FIG. 5 provides a spatial representation, in top plan perspective, of lateral sidewall  36   a  (including lateral sidewall flap  38   a ), medial sidewall  28  (including medial sidewall flap  40 ) and lateral sidewall  36   b  (including lateral sidewall flap  38   b ). The distance in the fore-and-aft direction, commencing at the extreme fore end of the sidewall (designated X=0), is represented as the horizontal axis of graph, and is designated “X.” The distance in the port-and-starboard direction is represented as the vertical axis of graph, and is designated “Y.” 
     It is seen in FIG. 5 that each sidewall flap has a flap length l F  equal to about six feet, that each sidewall non-flap section has a non-flap length l N  equal to about eighteen feet, and that each sidewall flap has a flap deflection angle β equal to about ten degrees. In accordance with typical inventive embodiments, every sidewall flap has approximately the same flap length l F , and every sidewall non-flap section has approximately the same non-flap length l N . Thus as shown in FIG.  3  through FIG. 5, flap length l Fa , flap length l Fb  and flap length l Fm  are approximately equal. Similarly, non-flap length l Na , non-flap length l Nb  and non-flap length l Nm  are approximately equal. Inventive flow exit assembly  26  has a total exit assembly length l T  which approximately equals the sum of l F  plus l N . 
     As clearly illustrated by FIG. 5, the jet streamline distributions are effectively deflected by the sidewall flaps. As discussed hereinbelow, the significant flow entrainment outside the jet area greatly enhances the steering capability of the present invention. 
     With reference to FIG. 6, FIG.  7  and FIG. 8, shown in each figure is the computed pressure distribution along the sidewalls and the sidewall flaps. A graphical display of pressure distributions, such as shown in each figure, provides a very useful mathematical tool. The graph can reveal the degree of cavitation susceptibility of a steering system. Moreover, the graph exhibits the physics of side force-producing mechanics among the various steering components. 
     In each of FIG.  6  through FIG. 8, the computed pressure (“minimum pressure coefficient” or “suction pressure peak”) is represented as the vertical axis of the graph, and is designated “-C p .” The distance in the fore-and-aft direction, commencing at the extreme fore end of the sidewall (designated X=0), is represented as the horizontal axis of graph, and is designated “X.” 
     The David Taylor Navier Stokes (DTNS) code has been used in the present calculations. There are many Navier Stokes codes available in the marine and aerospace industries. In the light of this disclosure, it is well understood by the ordinarily skilled artisan that any Navier Stokes code capable of solving the fluid problem with adequate numerical accuracy can be used. By integrating the pressure distributions along the sidewalls and the sidewall flaps, the side force due to flap deflection can be computed. By adjusting the flap deflection angle, the required side force can be generated. By integrating the pressure distributions along the flap, the required torque to rotate the flap can be determined. 
     As shown in each of FIG.  6  through FIG. 8, suction pressure peaks are generated at the flap leading edge. The suction pressure peak -C p  is often termed the “minimum pressure coefficient” in the marine industry. By comparing the minimum pressure coefficient -C p  and the cavitation number, cavitation inception speed and cavitation inception flap angle of a steering system can be evaluated. A cavitation-free steering system is obtained if the minimum pressure coefficient is less than the cavitation number. The definition of cavitation number is well known in the marine industry, and no further discussion thereof need be given herein. If the minimum pressure coefficient is greater than the cavitation number, the design parameters as discussed hereinbelow must be adjusted. 
     Generally, according to the present invention&#39;s design process, design parameters including the following can be adjusted to obtain the desired side force: (i) sidewall flap length, such as flap length l F  shown in FIG.  3  through FIG. 5; (ii) sidewall flap deflection angle, such as flap deflection angle β shown in FIG.  4  and FIG. 5; (iii) jet exit area; (iv) the static water head in the jet exit location; and, (v) jet velocity with respect to the ship speed. The torque to rotate the flap can be obtained by integrating the pressure distributions around the flap. 
     The effects of design parameters on side force and pressure distributions are illustrated by comparing the graph shown in FIG. 6 with the graphs shown in FIG.  7  and FIG.  8 . Consideration of FIG. 6 versus FIG. 7 reveals the effect of flap length l F . Consideration of FIG. 6 versus FIG. 8 reveals the effect of flap angle β. 
     FIG. 7 vis-a-vis′ FIG. 6 manifests how the change in flap length affects the pressure distributions. The flap length l F  shown in FIG. 7 is twice the flap length l F  shown in FIG.  6 . The side force is found to be nineteen percent (19%) higher in FIG. 7 than in FIG.  6 . The suction pressure peak which is shown in FIG. 6 is found in FIG. 7 (wherein the flap length is doubled) to be slightly reduced. 
     FIG. 8 vis-a-vis′ FIG. 6 manifests how the change in flap angle affects the pressure distributions. The design parameters used in FIG. 8 are the same as in FIG. 6 except that flap angle β is decreased by half, to five degrees (5°). The side force shown in FIG. 8 is approximately fifty-four percent (54%) of the side force shown in FIG.  6 . The suction pressure peak shown in FIG. 8 is substantially lower than the suction pressure peak shown in FIG.  6 . 
     Inventive procedures similar to those described herein in relation to FIG.  6  through FIG. 8 can be effectuated so as to vary other design parameters, such as jet velocity and jet exit area. Based on the inventive design process, the required side force and cavitation-free operation can be evaluated, and a desirable steering system can be obtained. However, the inventive design parameters can have a significant effect on design of the pump. Accordingly, the inventive design methodology generally requires close interaction with the pump design. 
     Results from numerical calculations demonstrate that, because of the flow entrainment outside of the jet area, the present invention can produce a side force which is substantially higher than the side force which is produced by the jet momentum alone. For example, the side force produced by a side wall flap deflection angle β of ten degrees (10°), as given in FIG. 7, is found to be ninety-two percent (92%) higher than the side force produced by the jet momentum associated with the sleeves used in a conventional waterjet steering system. This represents an important advantage of the present invention. 
     Another advantage of the present invention is that the magnitude of the suction pressure peak in the mid-flap is greatly alleviated due to the neighboring flaps. The inventive steering system having multi-flaps thus can operate at higher speeds without cavitation than can a steering system having a conventional rudder. 
     A naval ship with a high-speed capability greater than thirty (30) knots is not uncommon. At such a high speed, any physical protrusion from the steering system in the flow can easily cause cavitation to occur. To circumvent this problem, the sidewall flaps in accordance with the present invention are specially designed. 
     Referring again to FIG.  3  and FIG.  4  and also referring to FIG. 9, medial sidewall flap  40  has rotatable medial leading edge  72 ; lateral sidewall flap  38   a  has rotatable lateral leading edge  74   a ; and lateral sidewall flap  38   b  has rotatable lateral leading edge  74   b . Medial sidewall non-flap section  80  has cavity-like medial trailing edge  82 ; lateral sidewall non-flap section  78   a  has cavity-like lateral trailing edge  84   a ; and lateral sidewall non-flap section  78   b  has cavity-like lateral trailing edge  84   b . Flap length l F  is approximately the length measurement from medial leading edge  72  to medial trailing edge  58  (flap length l Fm ), which is approximately the length measurement from lateral leading edge  74   a  to lateral trailing edge  60   a  (flap length l Fa ), which is approximately the length measurement from lateral leading edge  74   b  to lateral trailing edge  60   b  (flap length l Fb ). 
     FIG. 9 provides an enlarged depiction of the conjunction of the rotatable leading edge portion of a sidewall flap with the cavity-like trailing edge portion of its associated (upstream) sidewall non-flap section. The rotatable flap leading edge and the cavity-like trailing edge engage in a kind of “ball-and-socket” configuration; as shown in ball-and-socket joint mechanism  86 , the rotatable flap leading edge is the “ball” to the cavity-like trailing edge “socket.” The flap leading edge shown in FIG. 9 is representative of any of the three flap leading edges shown in FIG.  3  and FIG. 4, viz.,  72 ,  74   a  or  74   b ; similarly, the cavity-like trailing edge shown in FIG. 9 is representative of any of the three non-flap trailing edges shown in FIG.  3  and FIG. 4, viz.,  82 ,  84   a  or  84   b.    
     As shown in FIG. 9, the flap leading edge approximately defines the shape of a circular arc. The non-flap trailing edge also approximately defines the shape of a circular arc. In the context of ball-and-socket joint mechanism  86 , the arcuate shape of the non-flap trailing edge concentrically comports with the arcuate shape of the flap leading edge. The axis of rotation is the central axis c of the circular arcs (approximate semicircles) defined by both the flap leading edge and the non-flap trailing edge. 
     Gap  88  is interposed between the circularly arcuate flap leading edge and the circularly arcuate non-flap trailing edge. Gap  88  is filled by bearings (such as bearing  90 ) and gasket  92  so that no flow proceeds through gap  88  when the flap is deflected. The seal afforded by bearings  90  and gasket  92  provides a smooth flow transition from the upstream sidewall non-flap section to the flap. Consequently, the flap geometry around the flap&#39;s leading edge is smooth and continuous. Such a continuous surface, even with the flap deflected, will further reduce the magnitude of the suction pressure peak as shown in FIG. 6, FIG.  7  and FIG.  8 . Accordingly, inventive practive further improves cavitation-free operation through efficacious implementation of fluid-tight seals and circular arcs in the junctional design of the flap and its associated non-flap. 
     Reference is now made to FIG. 10, which provides a top view of a reversing flow system in accordance with the present invention. Medial sidewall flap  40  is separable (splittable) into sidewall flap subsections  42   a  and  42   b . Chambers  30   a  and  30   b  are shown to be closed by sidewall flap  38   a , sidewall flap  38   b  and medial flap  40  (in particular, via the division, as if cleaved by a vertical planar cleaver, of medial flap  40  into flap subsections  42   a  and  42   b ). 
     Such closure of chambers  30   a  and  30   b , generally for purposes of effecting backward motion of the marine vessel, is inventively accomplished as follows: Lateral sidewall flap  38   a  (rotatable about a central axis c such as shown in FIG. 9) deflects inward (i.e., toward medial sidewall flap  40 ) at flap deflection angle β a ; lateral sidewall flap  38   b  (rotatable about a central axis c such as shown in FIG. 9) deflects inward (i.e., toward medial sidewall flap  40 ) at flap deflection angle β b ; medial sidewall flap subsection  42   a  (rotatable about a central axis c such as shown in FIG. 9) deflects outward (i.e., toward lateral sidewall flap  38   a ) at flap deflection angle β ma ; medial sidewall flap subsection  42   b  (rotatable about a central axis c such as shown in FIG. 9) deflects outward (i.e., toward lateral sidewall flap  38   b ) at flap deflection angle β mb . 
     Flap angles β a  and β b  are the angles formed by each sidewall flap with its respective sidewall. Flap angles β ma  and β mb  are the angles formed by each sidewall flap subsection with its respective sidewall. According to many inventive embodiments, when effectuating closure, every sidewall flap and sidewall flap subsection deflects at an approximately equal angle β, wherein a first half of the flaps and a first half of the flap subsections are approximately parallel to each other, and wherein a second half of the flaps and a second half of the flap subsections are approximately parallel to each other; the flaps and flap subsections of the first half are obliquely disposed with respect to the flaps and flap subsections of the second half. 
     As shown in FIG. 10, lateral sidewall flap  38   a  obliquely meets medial sidewall flap subsection  42   a  at juncture  94   a  and is approximately parallel to medial sidewall flap subsection  42   b . Similarly, lateral sidewall flap  38   b  obliquely meets medial sidewall flap subsection  42   b  at juncture  94   b  and is approximately parallel to medial sidewall flap subsection  42   a . As shown in FIG. 3, sidewall flaps  38   a ,  38   b  and  40  are approximately parallel to each other when in an undeflected condition. Therefore, as shown in FIG. 10, flap deflection angle β a  approximately equals flap deflection angle β ma , and flap deflection angle β b  approximately equals flap deflection angle β mb . 
     Moreover, many inventive embodiments effectuate closure wherein every flap and flap subsection deflects at about the same angle β. Hence, as shown in FIG. 10, flap deflection angle β a , flap deflection angle β ma  flap deflection angle β b  and flap deflection angle β mb  are all approximately equal. During closure, the value of flap deflection angle β depends upon the value of sidewall flap length l F  and the value of chamber width w C . In accordance with the majority of inventive embodiments, it is assumed that every sidewall flap has about the same flap length l F , and that every flow chamber has about the same chamber width w C . For instance, if the length l F  of each sidewall flap is half of each chamber width w C , a flap deflection angle β of thirty degrees (30°) blocks the flow completely. 
     Reference is now made to FIG. 11, which provides a side view of the inventive reversing flow system shown in FIG.  10 . “Twin” bottomwall flaps  56   a  and  56   b  are opened, as shown in FIG. 11, for ship reversing—i.e., to reverse the water flow for backing of the marine vessel. Flap deflection angle α is the angle formed by each bottomwall flap with bottomwall  34 . Angle α shown in FIG. 11 can be considered to be representative of either bottomwall flap  56   a  or bottomwall flap  56   b . Although not explicitly depicted in the figures, it is readily understood that each of bottomwall flaps  56   a  and  56   b  has associated therewith its own flap deflection angle α; in other words, it is readily envisioned that bottomwall flap  56   a  has associated therewith a flap deflection angle α a , and bottomwall flap  56   b  has associated therewith a flap deflection angle α b . 
     Bottomwall flap  56   a  has a bottomwall flap trailing edge  98   a . Bottomwall flap  56   b  has a bottomwall flap trailing edge  98   b . It is readily appreciated, in the light of this disclosure, that sealed rotatability of bottomwall flaps  56   a  and  56   b  can be inventively practiced in a manner similar to that described hereinabove in relation to FIG.  9 . Similarly as shown in FIG. 9, each bottomwall flap trailing edge  98  approximately defines the shape of a circular arc. Each arcuate bottomwall flap trailing edge  98  engages a compatible arcuate edge portion provided in bottomwall  34  as included in a kind of pivotable ball-and-socket joint mechanism such as mechanism  86  shown in FIG.  9 . The axis of rotation is the center (similar to center c shown in FIG. 9) of the circular arcs. Again, the ordinarily skilled artisan is familiar with methodologies, known in pertinent marine arts, suitable for incorporating into the inventive practice of the fluid-tight bottomwall flap rotational mechanism. 
     The degree of downward rotation (i.,e., bottomwall flap deflection angle α) depends upon the marine craft&#39;s operational requirements, particularly with regard to stopping distance. If possible, the deflection angle α of the bottom flaps should be small to enhance the breaking force. On the other hand, if the bottomwall flap angle α is too small, the flow may be choked and the backing capability thereby degraded. Accordingly, the designing of the backing system in accordance with the present invention generally involves a “tradeoff” between these two competing considerations. 
     For typical inventive embodiments, each bottomwall flap will defect downward to a maximum angle α in the range between about thirty degrees (30°) and about forty-five degrees (45°). A value within this range of values (thirty to forty-five degrees) for bottomwall flap deflection angle α will provide optimum efficiency for most inventive embodiments. 
     Frequently according to inventive practice, when effectuating backing (reversing) maneuvers, every bottomwall flap shifts downward from bottomwall  34  simultaneously (i.e., approximately synchronously) and equally (i.e., approximately parallelly) to an equivalent deflective angle α position beneath bottomwall  34 . However, some inventive embodiments provide for a backing system wherein at least two bottomwall flaps are capable of being adjusted individually, thereby deflecting independently to different angles α. This inventive approach, wherein at least two bottomwall flaps can independently deflect to disparate angles α, serves to afford additional maneuvering capability during the backing of the marine vessel. 
     Referring again to FIG. 10, it is seen that, in the horizontal plane approximately defined by bottomwall  34 , each of bottomwall flaps  56   a  and  56   b  is obliquely angled with respect to the sidewalls, that is, with respect to the incoming flow. This inventive feature, wherein each bottomwall flap is characterized by an oblique angle θ, is especially useful when implemented in conjunction with the independent deflection of two or more bottomwall flaps to different angles α, and thereby serves to enhance the marine vessel&#39;s maneuvering capability during the backing thereof 
     As shown in FIG. 10, bottomwall flaps  56   a  and  56   b  are each approximately rectangular, each having a trailing edge ( 98   a  and  98   b , respectively), a leading edge ( 99   a  and  99   b , respectively) and two side edges ( 97   al  and  97   am , and  97   bl  and  97   bm , respectively). Bottomwall flap  56   a  (more specifically, side edge  97   al  with respect to lateral sidewall  36   a , and side edge  97   am  with respect to medial sidewall  28 ) forms an angle θ a  with respect to lateral sidewall  36   a  and medial sidewall  28 ; similarly, bottomwall flap  56   b  (more specifically, side edge  97   bl  with respect to lateral sidewall  36   b , and side edge  97   bm  with respect to medial sidewall  28 ) forms an angle θ b  with respect to lateral sidewall  36   b  and medial sidewall  28 . 
     For typical inventive embodiments, each bottomwall flap will be situated at an oblique dispositional angle θ which is in the range between about twenty degrees (20°) and about thirty degrees (30°). A value within this range of values (twenty to thirty degrees) for bottomwall flap oblique dispositional angle θ will provide optimum efficiency for most inventive embodiments. 
     With regard to independent control of flaps, it is appreciated by analogy that this invention can provide for a steering system wherein at least two sidewall flaps are capable of being adjusted individually, thereby deflecting independently to different angles β; however, such capacity in association with the sidewall flaps generally lacks the operational utility afforded by such capacity in association with the bottomwall flaps. 
     With reference to FIG.  12  through FIG. 15, another inventive backing system utilizes one or more buckets  100  to reverse the water flow. The views shown in FIG.  13  and FIG. 14 are from below the marine vessel and looking up. As discussed previously herein, existing waterjet steering and maneuvering systems are employed in the air. In contrast, the present invention executes steering and backing maneuvering underwater. 
     As shown in FIG. 12, a large bucket  100  is lowered into the discharge stream to enable reverse maneuvering. A member such as arm  101  is used to selectively raise, lower and rotate (in a horizontal plane) bucket  100 . Arm  100  is connected to a mechanical device (not shown) for imparting movement thereto. In the light of this disclosure, the ordinarily skilled artisan is capable of devising a mechanical scheme suitable for moving and positioning bucket  100  in accordance with the present invention. 
     Bucket  100  has a width w B  which is greater than the total width w T  of inventive flow exit assembly  26 , thus completely covering flow exit chambers  30   a  and  30   b  at outlet (discharge) ends  50   a  and  50   b , respectively. Bucket  100  has a height h B  which is roughly equal to the height h of the inventive exit assembly  26  (or, in other words, the height h of each of chambers  30   a  and  30   b ). 
     In non-maneuvering (straight) backing operation, the concave inside of bucket  100  faces the water flow discharge so as to be approximately parallel to chamber outlet ends  50   a  and  50   b  and to the marine vessel&#39;s transom stern  102 , as shown in FIG.  12  and FIG.  13 . Water elevation line e (shown in FIG.  12  and FIG. 15) falls slightly above topwall  32  of inventive flow exit assembly  26 . As indicated in FIG. 13 by navigational directional arrow d, the marine vessel is going approximately straight backward. 
     The inside of bucket  100  includes two adjacent approximately congruous scoop-like portions, scoop  104   a  and scoop  104   b . Scoops  104   a  and  104   b  catch the discharged flow f from chamber outlet (discharge) ends  50   a  and  50   b , respectively, and turn the discharged flow f around so that the discharged flow f exits bucket  100  along the outboard sides  106   a  and  106   b  of bucket  100 , and in the reverse direction d. The change in momentum of the discharge jets imparts a reverse thrust on the marine vessel (e.g., ship). 
     Bucket  100  also enables maneuvering control while the marine vessel is going in reverse. Bucket  100  is rotatable in the horizontal plane, as shown in FIG.  14 . Rotating bucket  100 , or tilting bucket  100  to the side, chokes the flow on one side (chamber outlet end  50   a , as shown in FIG. 14) and opens the flow on the other side (chamber outlet end  50   b , as shown in FIG.  14 ). The greater discharge on one side of the marine vessel causes asymmetry in the discharge f, and reverse thrust becomes obliquely angled with respect to the marine vessel&#39;s axis M; that is, the greater reverse flow in association with scoop  104   b  produces a turning moment. 
     The angled reverse thrust causes the marine vessel to travel backward at a corresponding angle. In this manner, maneuvering control is provided while the marine vessel is reversing. As indicated in FIG. 14 by navigational directional arrow d, the marine vessel is going backward at an oblique angle with respect to the marine vessel&#39;s longitudinal axis M. 
     As shown in FIG. 15, during forward motion of the marine vessel wherein backing is not required, bucket  100  is retracted (raised out of the water) into a shelter-like structure  107 , which is attached to or integral with the transom stern  102  of the marine vessel. Thus disassociated via retraction, bucket  100  exerts no influence on the forward motion d of the marine vessel. Flow f which is discharged from flow exit chambers  30   a  and  30   b  is unimpeded, and hence accelerates in a generally backward direction. 
     Now referring to FIG. 16, the principles of the present invention are applicable to any plural number of flow exit chambers  30 . Here, inventive flow exit assembly  26  has three flow exit chambers (viz.,  30   a ,  30   b  and  30   m ) and four sidewalls (viz., lateral sidewall  36   a , lateral sidewall  36   b , medial sidewall  28   a  and medial sidewall  28   b ). Lateral sidewall  36   a  includes lateral sidewall flap  38   a . Lateral sidewall  36   b  includes lateral sidewall flap  38   b . Medial sidewall  28   a  includes medial sidewall flap  40   a . Medial sidewall  28   b  includes medial sidewall flap  40   b . Medial sidewall flap  40   a  and medial sidewall  40   b  are each longitudinally splittable into a medial flap subsection; medial sidewall flap  40   a  includes medial sidewall flap subsections  42   aa  and  42   ab , and medial sidewall flap  40   b  includes medial sidewall flap subsections  42   ba  and  42   bb.    
     Accordingly, an inventive flow exit assembly  26  having any plural number of flow exit chambers  30  can be inventively practiced. In the light of the instant disclosure, the ordinarily skilled artisan will be capable of.practicing the present invention in relation to any plural number of flow exit chambers  30 . Any number (singular or plural) of waterjet pumps  20  can be implemented in association with inventive flow exit assembly  26 . It may be preferable for some inventive embodiments to utilize at least two waterjet pumps  20 , wherein each waterjet pump is associated with at least one flow exit chamber  30 ; in the event of failure of a waterjet pump  20 , at least one functional waterjet pump  20  remains. 
     Certain general principles obtain with regard to practice of the present invention, particularly as pertains to the number of flow exit chambers  30 . Generally, an inventive flow exit assembly  26  characterized by “N” number of flow exit chambers  30  will also be characterized by the following: “N+1” number of total sidewalls, viz., medial sidewalls  28  plus lateral sidewalls  36 ; two lateral sidewalls  36 ; “N−1” number of medial sidewalls  28 ; and, “2(N−1)” number of medial sidewall flap subsections  42 . In addition, oblique dispositional angles θ of bottomwall flaps  56  will be “balanced,” similarly as exemplified in FIG. 16, wherein bottomwall flaps  56   a  and  56   b  are approximately equally and oppositely disposed at angles θ a  and θ b respectively, while bottomwall flap  56   m  is disposed at an angle θ m  which approximately equals zero. 
     Other embodiments of this invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. Various omissions, modifications and changes to the principles described may be made by one skilled in the art without departing from the true scope and spirit of the invention which is indicated by the following claims.
The inventive apparatus exercises control of the water flow which is discharged from a marine waterjet propulsor. Typical inventive embodiments comprise plural horizontal and plural vertical blade-like structures which together describe an open-ended, box-like rectilinear configuration characterized by at least two adjacent channels. Every vertical blade-like structure includes, at its aft end, a "steering" flap which is pivotable about a vertical axis. Some inventive embodiments advance marine craft reversing by implementing one or more bucket-like devices behind the inventive apparatus. According to many inventive embodiments, every vertical blade-like structure (other than the two laterally extreme vertical blade-like structures) has a steering flap attributed with lengthwise severability (as if severed by a vertical plane) into two sub-flaps, each sub-flap itself being pivotable about the same vertical axis; in cooperation therewith, the marine craft is reversed by implementing plural "backing" flaps (included in the bottom-side horizontal blade-like structure or structures) which are each pivotable about a horizontal axis. The inventively coordinated movements and dispositions of the flaps, sub-flaps and/or bucket-like devices effect steerable and/or reversible maneuverability of the marine craft. Utilization of the present invention is especially efficacious in the context of waterjet propulsion systems wherein the water flow is discharged underwater; underwater-discharge marine waterjet propulsion, practically unknown but theoretically recognized for its potential benefits, is rendered a real and viable option by the present invention.