Abstract:
Presented is an improved waterjet propulsors that has application to air lubricated ships since it solves problems related to air ingestion into the waterjet inlets. This is done by disposing the waterjet inlets into side members of the ship disposed either side of the ship&#39;s artificially pressurized air layer(s). The waterjet inlets are best located, at least in their majority, lower than and aft of the ship&#39;s artificially pressurized air layer(s) and the side members of the ship are, in their preferred embodiment, extended aft of and, at least in their majority, below and aft portion of the artificially pressurized air layer. The improved waterjet propulsor can see improvements in its efficiency by absorption of ship boundary layer water. Further, air fences can be employed to restrict air from entering the waterjet inlets from either the ship&#39;s air layer(s) or water surfaces external to the ship.

Description:
CROSS REFERENCE TO OTHER APPLICATIONS  
       [0001]    This application is a continuation-in-part to Applicant&#39;s earlier filed application Ser. No. 09/790,086 filed Feb. 21, 2001 now abandoned. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Waterjet propulsors as applied to marine surface vessels have been available for many years. Some of their obvious advantages over exposed propellers include that the waterjet has no exposed rotor, has low underwater noise signature, offers more even engine loadings, and, when used with a flush hull water inlet, provides shallow boat draft. They are currently making some significant inroads when applied to marine surface vessels, such as high-speed ferries and patrol craft, which cruise mostly at high speeds and therefore can put up with the generally poor performance of the waterjet at low speeds. They are also used in small PWCs (Personnel Water Craft) where their poor performance in such small sizes compared to exposed propellers is accepted since exposed propellers are totally unacceptable due to safety considerations.  
           [0003]    There has been some interest in applying waterjets to submarines, torpedoes, and boats whose main hulls are supported by underwater hulls or underwater hull portions. An example of a boat that is supported by underwater hull(s) is the SWATH (Small Waterplane Twin Hull). The SWATH configuration has twin underwater torpedo shaped hulls that are disposed below and to either side of an above water hull. Connection is made by strut-like members. The advantage being a much more comfortable ride in rough seas due to the small waterplane area offered by the struts. Other variations of this same theme include the SLICE that has four under hulls, a variation that has a single underwater hull with hydrofoils either side for stability, and another variation with three underwater hulls. So the possibilities for an above water hull supported by one or more underwater hulls are many. The underwater supporting hulls of the SWATH or its related types are generally cigar or torpedo shaped.  
           [0004]    In general, the waterjet has not been able to compete from an efficiency standpoint with the open water propeller for any of the just mentioned underwater hull types. There have also been attempts to utilize waterjets on large displacement type hulls. All of these waterjet installations have been successful from an operational standpoint but not from an efficiency standpoint when compared with the open water exposed propeller.  
           [0005]    The Kort Nozzle, first introduced in the 1930&#39;s, yields an efficiency advantage over the open water propeller at low boat speeds. It applies a simple ringed nozzle around the periphery of an underwater propeller. By use of carefully designed angled airfoil shapes to the nozzle ring it is possible for the Kort Nozzle to actually gain thrust from external forces acting on the nozzle. A well designed Kort Nozzle shows noticeable performance gains over a standard underwater propeller at speeds up to about 16-20 knots. Beyond those speeds, the drag of the nozzle itself rules out use of the Kort Nozzle from an efficiency standpoint. Kort Nozzle are widely applied to tug boats and other low speed mostly work boats. For purposes of this application, low speed is defined as boat speeds up to and including 20 knots and high speeds as boat speeds of over 20 knots.  
           [0006]    The waterjet propulsor is severely outclassed from an efficiency standpoint, compared to the exposed propeller, at low to moderate, up to about 25 knot, speeds. The waterjet has made inroads when applied to higher speed surface vessels where its inlet water is taken from below the hull and its discharge is through the transom or stern. The reason for much of its efficiency shortcomings has to do with its inlet performance. A well-designed waterjet system can have a rotor efficiency of 93 percent, flow straightening stator vane efficiency of 92 percent, and discharge nozzle efficiency of 98 percent. That comes to an overall pump efficiency of 84 percent. However, its average inlet pressure recovery efficiency will probably only be in the 70 percent area. Consequently, the best overall efficiency that can be expected from such a conventional waterjet while running at its most efficient performance point at high boat speeds is about 59 percent. The major reason that waterjet inlet efficiency or inlet pressure recovery is so poor is because of distortion in the inlet flow. The high velocity incoming water in a typical flush with the hull waterjet inlet piles up over the lower half of the inlet duct. Due to this distorted flow, the rotor generally sees recoveries of 90 percent or more of boat freestream dynamic head over its lower half and as low as 50-60 percent over its upper half.  
           [0007]    In contrast, the instant invention not only provides a uniform inlet flow to its enclosed rotor but also has augmented thrust by way of a carefully shaped after section on its outer surfaces. It does all of this without the high drag at high speeds associated the Kort Nozzle since the instant invention has essentially zero external drag water inlets. It can be applied to all manner of craft but has particular important application to vessels with underwater hulls and/or significant underwater hull portions. The latter includes displacement hulls that ride deeply in the water.  
           [0008]    Another aspect of the instant invention is that it is especially conceived to take advantage of the added performance seen from taking in the boundary layer adjacent to the hull forward of its water inlet. This is particularly relevant for long hulls where the boundary layer next to the hull can be very large. For example, a 650 foot displacement without boundary layer unit of about 30 percent. The reason for this is because the with boundary layer unit has added more momentum to the fluid passing through it than the without boundary layer unit. This additional momentum well overcomes the greater flow rate of the without boundary layer unit in this example.  
           [0009]    With that said, it is obvious that any flush inlet waterjet applied to a boat&#39;s hull will see some boundary layer ingestion. Again, the effect is almost negligible on small high speed craft due their thin boundary layers. The instant invention is especially superior in its application to larger craft where it is particularly conceived to take in the boundary layer next to the underwater hull portion upstream of its water inlet(s). This is enhanced by use of circular arc shaped or especially curvilinear shaped hull sections upstream of the invention&#39;s water inlets. Boundary layer control fences can also be applied.  
           [0010]    Application to air cushioned vessels is also possible through judicious use of especially designed air cushion underwater sidewall sections that house the instant invention propulsor(s). Air fences may be applied to keep air out of the inlets here. Rieben, U.S. Pat. No. 5,679,037; Lang, U.S. Pat. No. 3,623,444; Sinko et al, U.S. Pat. No. 6,152,791; Walters, U.S. Pat. No. 4,936,237; and Sinko et al, U.S. Pat. No. 6,203,388 all show surface ships propelled by two or more waterjet propulsors. None of these ships have an artificially pressurized gas layer disposed in their underside as does the instant invention.  
           [0011]    Lang, U.S. Pat. No. 3,205,846; Wisclicenus, U.S. Pat. No. 3,575,127; Lehmann, U.S. Pat. No. 3,182,623; and Fawkes, U.S. Pat. No. 2,351,750 all show submerged rather than surface vehicles and all utilize propulsors located at or near the centerlines of their hulls. These patents differ from the instant invention in that they: 1) are submerged bodies, 2) have only one propulsor at or near their hull&#39;s centerlines, and 3) have no air layer disposed between their waterjet propulsors.  
           [0012]    Korganoff, U.S. Pat. No. 3,302,602 offers a vehicle that is intended to operate below, at, and above the sea surface. It has one or more pressurized circular air cushions disposed in its underside but only has a single waterjet propulsor on centerline for water propulsion. As such, Korganoffs waterjet propulsor is more akin to Rieben, Lang, Sinko et al, and Walters.  
           [0013]    There are also what are known as Surface Effect Ships (SESs) that have catamaran like sidehulls with a blower pressurized air cushion disposed between the sidehulls. There are normally large flexible type seals that run transversely between the sidehulls fore and aft to seal in the pressurized air cushion. These SESs can be propelled by waterjets located in the sidehulls. In every instance known to Applicant, such waterjet installations have a rectangular waterjet inlet that is flush with the bottom of the sidehull or, in some cases, a water inlet that drops down below the sidehull with the waterjet remaining in the sidehull. The reason that the high drag drop down waterjet inlet is sometimes used is to try to eliminate or at least reduce the amount of air from the air cushion that enters the waterjet inlet. Since conventional waterjets are pump-like pressurizing systems, they are not tolerant of aerated water. The instant invention avoids the air ingestion problem by having inlets that are outboard away from the air cushion(s) and are also normally in rounded or curvilinear hull surfaces that air does not adhere to. Applicant also offers air fences on one or both sides of his inlets to prevent air from reaching the waterjet inlets. All of this is in addition to the instant invention&#39;s use of specially configured waterjet inlets and housings to enhance performance of the inventive waterjets as applied to air cushioned or air layered marine surface vehicles.  
           [0014]    In summary, the instant invention offers greatly improved waterjet thrust values at all boat speeds. In its preferred embodiment, critical shaping of the outside portions of its housings augments its thrust with little or no increase in external drag. As a further enhancement to its rotor&#39;s performance, the instant invention virtually eliminates flow distortion at the rotor inlet. Shaping of its water inlet and of hull sections upstream of the water inlet also enhance performance especially when applied to large vessels with thick boundary layers. These features are described in detail in the following sections.  
         SUMMARY OF THE INVENTION  
         [0015]    With the foregoing in mind, it is the principal object of the preferred embodiment of the instant invention to provide a new waterjet propulsor that has particular application to air lubricated ships where it is so configured as to have little or no air ingestion from the ship&#39;s artificially pressurized air layer(s).  
           [0016]    It is a related object of the invention that the water inlet(s) for the new waterjet propulsor be disposed, at least in their majority, to the sides of the ship&#39;s artificially pressurized air layer(s).  
           [0017]    It is a further related object of the invention that the waterjet inlet(s) be disposed, at least in their majority, aft of the ship&#39;s artificially pressurized air layer(s).  
           [0018]    It is a further related object of the invention that the new waterjet propulsor be disposed in aft and underside extensions of the ship.  
           [0019]    It is a further related object of the invention the air layered ship have a displacement hull design forward such that its bow is lower than the artificially pressurized air layer(s) in its undersides.  
           [0020]    It an object of the invention that it utilize water forces in a forward direction that act on its outer housing to create a forward thrust.  
           [0021]    It is another object of the invention that the outside surfaces of the housings of the instant invention taper aft from proximal the instant invention&#39;s water inlet at an angle of less than 25 degrees.  
           [0022]    It is yet another object of the invention that the outside surfaces of the housings of the instant invention taper aft from proximal the instant invention&#39;s water inlet at an angle of less than 18 degrees.  
           [0023]    It is yet another object of the invention that the outside surfaces of the housings of the instant invention taper aft from proximal the instant invention&#39;s water inlet at an angle of less than 12 degrees.  
           [0024]    It is a further object of the invention that its water inlets have very low drag.  
           [0025]    It is a related object of the invention that, in its preferred embodiment, its water inlets be essentially flush with the outer surfaces of the underwater hull portion disposed forward of its water inlets.  
           [0026]    It is a further object of the invention that its water inlets may extend into the freestream so long as such water inlets are designed to be of minimum drag.  
           [0027]    It is yet another object of the invention that the underwater hull disposed forward of the inlet be, at least in part and as seen in vertical transverse planes of underwater hull portion, curvilinear in shape.  
           [0028]    It is a directly related object of the invention that the curvilinear shape, either whole or in part, extend over a majority of 180 degrees of a periphery of the underwater hull portion.  
           [0029]    It is a further related object of the invention that the curvilinear shape be at least in part made up of circular-arc sections.  
           [0030]    It is yet another object of the invention that its waterjet inlet be, as seen in a vertical transverse plane of the underwater hull, at least in part curvilinear in shape.  
           [0031]    It is a directly related object of the invention that its waterjet inlet be, as seen in a vertical transverse plane of the underwater hull, at least in part, made up of circular-arc shaped sections.  
           [0032]    It is another object of the invention that its water inlet, when the instant invention waterjet propulsor is propelling a hull forward, absorb portions of the hull&#39;s boundary layer to thereby enhance its thrust performance.  
           [0033]    It is a directly related object of the invention that the amount of boundary layer absorbed by the instant invention&#39;s water inlet, on average, be less than fifteen percent of the thickness of the boundary layer forward of and adjacent to the water inlet.  
           [0034]    It is a further directly related object of the invention that the amount of boundary layer absorbed by the instant invention&#39;s water inlet, on average, be less than twenty-five percent of the thickness of the boundary layer forward of and adjacent to the water inlet.  
           [0035]    It is yet a further directly related object of the invention that the amount of boundary layer absorbed by the instant invention&#39;s water inlet, on average, be less than fifty percent of the thickness of the boundary layer forward of and adjacent to the water inlet.  
           [0036]    It is a further object of the invention that boundary layer fences can be utilized to control the boundary layer adjacent to the water inlet of the instant invention.  
           [0037]    It is a further object of the invention that the waterjet inlets supply water to the rotor that has minimal distortion.  
           [0038]    It is yet another object of the invention that there be supporting vanes that connect the hull to the aft housing of the instant invention waterjet propulsor.  
           [0039]    It is a directly related object of the invention that such supporting vanes be, at least in part, airfoil shaped.  
           [0040]    It is a directly related object of the invention that the aft housing be in mechanical communication with stator vanes.  
           [0041]    It is another directly related object of the invention that the stator vanes support a bearing that in turn aligns the rotor with the housings.  
           [0042]    It is a further related object of the invention that the aft housing either attach to include a discharge nozzle.  
           [0043]    It is yet another object of the invention that it can be applied to air cushioned marine craft.  
           [0044]    It is a further object of the invention it that one or more air fences can be applied to restrict air from entering the instant invention&#39;s water inlet. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0045]    [0045]FIG. 1 presents a centerline cross-section of a prior art Kort Nozzle. In its normal application, the Kort Nozzle is disposed below the hull of a surface ship such as a low speed tugboat. There is no structure, other than its drive shaft which may include a right angle gear, forward of the Kort Nozzle.  
         [0046]    [0046]FIG. 2 is an enlarged view of a partial cross-section of the lower part of the prior art Kort Nozzle presented in FIG. 1. Note the forces acting on the inner and outer surfaces of the nozzle ring. The effect of these forces is explained under the following section titled “DETAILED DESCRIPTION”.  
         [0047]    [0047]FIG. 3 is a centerline cross-section of a preferred embodiment of the instant invention This shows a drive motor, rotor, stator vanes, nozzle, and water inlets. It also depicts a boundary layer adjacent to the hull forward of the water inlets. This ingestion of the boundary layer actually enhances the efficiency of the instant invention propulsor. However, in the case shown here the boundary layer is relatively thin in respect to the size of the inlet so that all of the boundary layer is absorbed. This is a typical case for a smaller shorter hull that does not buildup much boundary layer thickness. The thicker boundary layers of longer hulls combined with relatively narrower water inlets have a much greater positive effect on efficiency.  
         [0048]    [0048]FIG. 4 is a cross-section, as taken through line  4 - 4  of FIG. 3, that illustrates a transverse plane forward of the instant invention waterjet propulsor water accelerating rotor. The support vanes forward of the rotor are normally airfoil shaped to reduce drag.  
         [0049]    [0049]FIG. 5 presents a side view of a SWATH that is propelled by two of the instant invention waterjet propulsors. Note the water inlets in the SWATH&#39;s underwater hulls. The SWATH&#39;s underwater hulls are actually like two torpedoes and are connected to an above water hull by thin wave-slicing struts. It is realized that submarines or torpedoes would actually function similar to the underwater hull portions of a SWATH. It is to be realized that there are other types of craft similar to the SWATH in concept with underwater hull(s) supporting an above water hull. These craft can have from one to several underwater hulls.  
         [0050]    [0050]FIG. 6 is a stem view of the SWATH of FIG. 5.  
         [0051]    [0051]FIG. 7 shows an alternative application of the instant invention waterjet propulsor. In this application, the waterjet propulsors are installed in an aft section of a large displacement hull. Realize that the instant invention is actually applicable to all hull types.  
         [0052]    [0052]FIG. 8 is a stern view of the displacement vessel shown in FIG. 7.  
         [0053]    [0053]FIG. 9 is a profile view of a prior art type large displacement hull such as an oceangoing freighter hull.  
         [0054]    [0054]FIG. 10 shows, in its lower half, a view, as taken through line  10 - 10  of FIG. 9, of the prior art large displacement hull of FIG. 9. FIG. 10 also shows, in its upper half, a cross-section, as taken through line  10 - 10  of FIG. 9, the development of the boundary layer on a typical large hull. Note the distortion and breakup of the boundary layer as it approaches the propeller going aft.  
         [0055]    [0055]FIG. 11 is a profile view of a similar large displacement hull as presented in FIG. 9 but with the instant invention waterjet propulsor incorporated into its aft portion.  
         [0056]    [0056]FIG. 12 presents, in its lower half, a view, as taken through line  12 - 12  of FIG. 11, of the hull of FIG. 11. FIG. 12 also illustrates, in its upper half, a cross-section, as taken through line  12 - 12  of FIG. 11, the boundary layer on this large hull as it moves forward through the water. Note that a relatively small amount of the boundary layer close to the hull forward of the water inlet is actually taken into the inlet. This insures that much of the water taken into the inlet has been accelerated by being dragged up to at least a portion of hull speed. The result is that the velocity change across the instant invention waterjet&#39;s rotor is high resulting in a large increase in momentum and hence an enhanced efficiency.  
         [0057]    [0057]FIG. 13 is a similar profile view of a hull as presented in FIG. 11 but in this case of an air-cushioned ship such as some of Applicant&#39;s air cushioned ship patent applications.  
         [0058]    [0058]FIG. 14 is a bottom plan view of the air-cushioned hull presented in FIG. 13. Note the air/boundary layer fences either side of the water inlets.  
         [0059]    [0059]FIG. 15 is a cross-sectional view, as taken through line  15 - 15  of FIG. 14, that shows a blower for pressurizing the air cushion and air cushion of this air cushioned hull.  
         [0060]    [0060]FIG. 16 is a midship cross-section, as taken through line  16 - 16  of FIG. 14, that shows the air cushion and the preferred curvilinear shaped sidehulls of this air cushioned hull. Note the air cushion location and the boundary layers around each sidehull.  
         [0061]    [0061]FIG. 17 presents a cross-section, as taken through line  17 - 17  of FIG. 14, that shows the water inlets of the instant invention. Note that the inlets are within the confines of the boundary layer here. Note also the air/boundary layer fences disposed either side of the water inlets. Note also the curvilinear shape, as seen in vertical transverse planes of the hull, of the inlet shape and of the hulls in the area of the water inlets. Ideally speaking, these would be circular-arc shaped. 
     
    
     DETAILED DESCRIPTION  
       [0062]    [0062]FIG. 1 presents a centerline cross-sectional view of the prior art Kort Nozzle  33 . This is simply a rotor in the form of a propeller  34  in a nozzle ring  39  disposed below the hull of a boat (not shown). Also shown are the propeller drive shaft  36 , inlet water velocity arrows  31 , discharge jet water velocity arrows  32 , external water velocity arrows  30 , and a horizontal transverse centerline plane  40 . The Kort Nozzle is noted for generating more thrust than an open water propeller at low boat speeds. The reasons for this are explained under the discussion of FIG. 2 that follows.  
         [0063]    [0063]FIG. 2 presents an enlarged view of the lower portion of the Kort Nozzle ring  39  cross-section that was shown in FIG. 1. Note that this nozzle ring  39  is actually at least partially airfoil shaped and that its outer or lower surface A is angled upward from forward to aft in this illustration. The water flow, as depicted by inlet water velocity arrow  31 , entering the inside of the nozzle ring  39  that is being acted upon by the propeller  34  is traveling at a much greater velocity (V B )—normally about one and one half times—than the outside velocity (V A ).  
         [0064]    The vector force arrows shown in FIG. 2 are defined as follows: P A - N  is the external static pressure normal to surface A, P A - V  is the external static pressure vertical to surface A, and P A - F  is the internal static pressure force normal to internal surface B, PB-R is the internal static force acting rearward. It has been found by test and application of Kort Nozzles over the years that the favored value for angle is generally about nine degrees.  
         [0065]    By Bernoulli&#39;s equation, neglecting minor elevation considerations total pressure is made up of static pressure and dynamic pressure. Dynamic pressure is a function of velocity squared so even minor velocity differences make for big changes in dynamic and hence static pressure. As such, there is more static pressure on the outer surface (A) than the inner surface (B) of the nozzle ring. Since the outer surface (A) is angled outward there is a forward force on outer surface (A). This is calculated by simply multiplying the external surface area of A by the external static pressure forward (P A - F ). Importantly, most of the inner surface (B) is parallel to the water flow. There is no forward force on the parallel portion of surface B. Therefore, there is a resultant positive forward force on the entire nozzle ring  39  that accounts for at least most of the higher efficiencies of the Kort Nozzle compared to a standard open water propeller at low boat speeds.  
         [0066]    The positive forward force acting on the Kort Nozzle ring  39  coupled with the increased propeller efficiency resulting from the ducted propeller effect makes for improved performance compared to an open water propeller up to vessel speeds of, say, 18 knots or so. At that speed the drag of the nozzle ring penalizes the Kort Nozzle resulting in the open water propeller being favored.  
         [0067]    [0067]FIG. 3 presents a centerline cross-section of a preferred embodiment of the instant invention augmented thrust waterjet propulsor  50 . Some of the items shown are the water inlet  53 , inlet guide vanes  38 , rotor  34 , stator vanes  35 , nozzle  45 , drive shaft  36 , drive engine  48 , and bearings  49 . Note that the inlet guide vanes  38  are, in their preferred embodiment, at least partially airfoil shaped to reduce drag.  
         [0068]    An underwater hull  51 , which would preferably have a torpedo-like shape, is shown disposed forward of the instant invention waterjet propulsor  50 . This arrangement is the preferred configuration to give maximum performance by the instant invention waterjet propulsor  50 . Note that the boundary layer that the underwater hull  51  has accelerated is defined by the boundary layer outline  52 . The boundary layer velocity profiles  42  are also shown. In this case of a relatively short hull, the boundary layer is thin and is taken in its entirety into the water inlet  53 . Note that the boundary layer here has a much less effect on performance than is the case for a large long hull where the boundary layer is very thick.  
         [0069]    It is a most important feature of the instant invention to realize that its housings  50  are acted on by similar water pressure forces that were described for the Kort Nozzle in FIG. 2. However, very importantly, the instant invention waterjet propulsor  50  has carefully designed water inlets  53  that have essentially zero drag. As such, the instant invention waterjet propulsor  50  functions very efficiently at all vessel speeds. It is to be noted that, while the waterjet inlets  53  shown here are essentially flush with the submerged hull  51  forward, other inlet shapes including partial ram type water inlets (not shown) are quite feasible. It is possible to design partial ram type water inlets that have zero or very little drag also. The zero to low drag inlet feature of the instant invention is clearly able to absorb the boundary layer next to the underwater hulls  51  forward of it which further enhances its propulsive efficiencies.  
         [0070]    [0070]FIG. 4 is a transverse plane cross-section, as taken through line  4 - 4  of FIG. 3, that shows inlet guide vanes  38  that also support the aft end of the instant invention waterjet propulsor  50 . The inlet guide vanes  38  are preferably at least partially airfoil shaped to minimize drag.  
         [0071]    [0071]FIG. 5 presents a profile view of a SWATH vessel  46 . The waterline  44  intersects the SWATH hull&#39;s supporting struts  54 . In this illustration, the instant invention waterjet propulsor  50  has been installed proximal the aft end of the SWATH&#39;s supporting underwater hulls  51 . Note the water inlets  53 . It is easy to realize such an installation of the instant invention waterjet propulsor  50  on a submarine hull, torpedo, or other at least partially underwater hull forms by seeing the underwater hull  51  portion of FIG. 5. Note that the SWATH is just one of a family of marine craft that have underwater hulls supporting an above water hull by means of struts and the instant invention waterjet propulsor  50  can be applied to any of those hull forms.  
         [0072]    [0072]FIG. 6 is a stem view of the SWATH vessel  46  presented in FIG. 5.  
         [0073]    [0073]FIG. 7 shows a dual installation of the instant invention waterjet propulsor  50  in a surface vessel  47 . In this case the instant invention waterjet propulsors  50  are installed at least partially circular shaped underwater hulls  51  that extend aft of the main hull of a surface vessel  47 . Therefore, performance of the instant invention waterjet propulsor  50  is similar to that described for the SWATH in the preceding paragraphs. Again, note the water inlets  53 . In the preferred embodiment of the invention, the waterjet inlet(s)  53  encircles, as seen in a vertical transverse plane of the submerged hull  51  parallel to the cross-section plane seen in FIG. 4, in total, as a sum of its openings, a majority of 180 degrees of the periphery of the submerged hull. Actually, a waterjet inlet(s) that, as a total of its openings, encircles a majority of 360 degrees of the periphery of the underwater hull  51  is preferred as that gives most even rotor inlet water flow characteristics and hence best overall propulsive coefficients.  
         [0074]    [0074]FIG. 8 presents a stem view of the surface vessel  47  shown in FIG. 7 that shows two of the instant invention&#39;s waterjet propulsors  50  installed.  
         [0075]    [0075]FIG. 9 is a profile view of a prior art type large displacement hull  47  such as a large freighter&#39;s hull. The hull sheer  57  and rudder  58  are noted.  
         [0076]    [0076]FIG. 10 shows, in its lower half, a view, as taken through line  10 - 10  of FIG. 9, of a prior art large displacement hull  47  of FIG. 9. FIG. 10 also shows, in its upper half, a cross-section, as taken through line  10 - 10  of FIG. 9, the development of the boundary layer  52  on such a large hull. The boundary velocity profiles  42  are also depicted. It is important to note the breakup of the boundary layer and its mixing with freestream water as the flow approaches the propeller  34 . Because of this turbulence and mixing of the exposed propeller&#39;s inlet flow, there is little or none of the beneficial performance effects due to a controlled boundary layer ingestion that is realized by the instant invention waterjet propulsor.  
         [0077]    [0077]FIG. 11 is a profile view of a similar large displacement hull  47  as presented in FIG. 9 but with the instant invention waterjet propulsor  50  incorporated into the hull&#39;s aft portion.  
         [0078]    [0078]FIG. 12 presents, in its lower half, a view, as taken through line  12 - 12  of FIG. 11, of the hull  47  of FIG. 11. FIG. 12 also illustrates, in its upper half, a cross-section, as taken through line  12 - 12  of FIG. 11, the boundary layer  52  on this large hull as it moves forward through the water. Note that a relatively small amount of the boundary layer close to the hull forward of the water inlet is actually taken into the water inlet  53 . This insures that much of the water taken into the water inlet  53  has been accelerated by being dragged up to at least a portion of hull speed. The result is that the velocity change across the instant invention waterjet&#39;s rotor is high resulting in a large increase in momentum and hence an enhanced efficiency.  
         [0079]    The thickness and other characteristics of the boundary layer are dependent upon the waterline length of the ship, ship&#39;s speed, and other factors. The waterjet inlet is purposely designed to take in, on average, less than all of the boundary layer. Examination of the boundary layer velocity profiles  42  gives an idea of why this is best. The boundary layer velocity profile  42  closest to the water inlet  53  obviously shows that more of the boundary layer water close to the hull  47  is at or approaching hull speed. The boundary layer  52  furthest from the hull is little effected by the movement of the hull  47  and is therefore essentially at freestream conditions. As such, the waterjet inlet  53  and rotor  34  flow pumping characteristics are best matched to take in reduced amounts of the boundary layer  52  next to the hull  47 . By way of definition for this application, it is established that a preferred water inlet  53  boundary layer thickness intake condition is to be less than fifteen percent for large vessels, less than 25 percent for mid-sized vessels, and less than fifty percent for smaller vessels. The actual hull sizes are not defined here as they are dependent upon hull speed, hull size and shape, water conditions, and other factors.  
         [0080]    [0080]FIG. 13 is a profile view of a hull of an air-lubricated ship  59  that is propelled by the instant invention waterjet propulsor  37 . FIGS. 13 through 17 present the preferred embodiment of the instant invention. Note the application of a foil shaped member  55  that is disposed to act as an air fence to reduce the possibility of air being drawn down to the water inlet  53 . This normally foil shaped structure  55  also acts to direct the boundary layer flow to the water inlet  53 . It can also be referred to as a boundary layer fence.  
         [0081]    [0081]FIG. 14 presents a bottom plan view of the air-lubricated ship  59  of FIG. 13. Shown are a blower gas discharge  63 , gas flow arrows  60 , and the air or gas layer  56 . Note the location of the waterjet inlets  53  and the air/boundary layer fences  55  that are disposed to restrict the absorption of air into the waterjet inlet  53 .  
         [0082]    [0082]FIG. 15 is a cross-sectional view, as taken through line  15 - 15  of FIG. 14, that shows a blower  61 , blower drive engine  62 , waterlines  44 , gas or air layer  56 , and gas layer recess aft seal  61 . It is very important to note the waterline  44  forward of the bow here that shows that the bow is deeper than the waterline  44  below the air layer  56  that is aft of the bow. Thus the hull can be noted as having a generally displacement hull shape forward with a rising waterline  44  below the air layer  56  aft of the bow since the air layer  56  itself is rising. The water inlets  53 , as shown in FIGS. 13 and 14, are disposed, at least in their majority, aft of an to the sides of the air layer in this preferred embodiment case. The intention of all of this is to restrict air from the air layer  56  from entering the waterjet inlet(s)  53 .  
         [0083]    [0083]FIG. 16 is a cross-section, as taken through a vertical transverse plane of the hull  47  defined by section line  16 - 16  of FIG. 14, that shows a typical midship section of the air-layered hull of FIG. 14. Note the curvilinear shape, circular-arc shapes are preferred, of the underwater hulls  51 . This is actually a very low drag design for very large air-layered hulls. These, at least in part, curvilinear shapes also lend themselves best to incorporation of the instant invention waterjet propulsors in their aft portions.  
         [0084]    [0084]FIG. 17 presents a cross-section, as taken through a vertical transverse plane of the hull as defined by section line  17 - 17  of FIG. 14, that shows the water inlets  53 , air/boundary layer fences  55 , and boundary layer  52 . It is best to have the curvilinear shape of the water inlet  53  extend over a majority of 180 degrees of its periphery with extension to a majority of 360 degrees of its periphery even better. Note that the underwater hull surfaces  64  proximal to and forward of the water inlets  53  should also follow the same criteria for best recovery of inlet water especially when boundary layer water is to be recovered properly.  
         [0085]    While the invention has been described in connection with a preferred and several alternative embodiments, it will be understood that there is no intention to thereby limit the invention. On the contrary, there is intended to be covered all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims which are the sole definition of the invention.