Abstract:
A pump jet apparatus having a double-walled stator housing containing an annular passage through which exhaust gas can flow. Gas enters the annulus through an exhaust gas inlet or port formed in the outer stator shell at the top of the stator housing, flows in two streams around the annular passage formed between the inner and outer stator shells, and exits the stator housing through exhaust outlets or ports formed in the outer stator shell near the bottom of the stator housing. The streams of exhaust gas and impelled water flowing through the stator housing of the pump jet are kept separate by the inner stator shell.

Description:
FIELD OF THE INVENTION 
     This invention generally relates to pump jets used with outboard motors or in inboard/outboard or stern drive units of boats and other vehicles. In particular, the invention relates to pump jets in which exhaust gas from the motor is discharged into the water stream surrounding the pump jet. 
     BACKGROUND OF THE INVENTION 
     In conventional outboard motors, a propeller is driven by a powerhead to propel a boat through the water. Essentially all modern motors inject the exhaust gas stream under water in order to reduce noise of the engine. However, the injected stream of exhaust gas can occupy a space, causing drag. 
     Prior to the 1970s most outboard motors injected the exhaust gas from a powerhead through a downstream channel to an exhaust gas outlet  14 . The exhaust is injected from the exhaust gas outlet into the water at a location downstream from the propeller. This type of motor will be referred to herein as a downstream exhaust motor. 
     During the 1970s, many outboard motors were changed over to a configuration in which gas from the powerhead was exhausted through a hollow hub in the propellor (provided for that purpose). The reason for the change over to an “exhaust through hub” (ETH) motor was the drag caused by the exhaust. It is known that the gear case causes drag. By locating the exhaust stream concentrically behind the gear case, the drag of the exhaust can be canceled out by the drag of the gear case. Manufacturers received an added benefit when the ETH configuration was used, namely, they were able to increase efficiency by using a larger-diameter gear case, larger crown gears, and thus slower-turning, more efficient propellers without increasing drag. 
     Another type of conventional outboard motor has an axial-flow pump jet system driven by the powerhead. In a pump jet system, an impeller or rotor is mounted (e.g., spline fitted) directly on the propeller output shaft in place of the propeller. There are typically no modifications to the drive train, cooling or sealing components. A ducted housing surrounds the rotor. Such a system has the advantages of reducing hazards to swimmers in the vicinity of the motor, protecting the rotating elements from interference with and damage by foreign objects in the water, and improving the efficiency and performance of the propulsion system. Another benefit inherent with the pump jet is a directed jet of water that results in greater steering response. 
     An example of this kind of pump jet installed on a downstream exhaust motor is shown in FIG. 1. A bladed rotary impeller or rotor is positioned below an anti-ventilation plate  12  and rearward of a lower unit housing  10 . The rotor comprises a plurality of blades  18  extending radially outward from an outer rotor hub  19 , the latter being is attached to a rearwardly projecting propeller shaft  16  for rotation therewith. A housing or shroud  21  having a front section or rotor housing  20  and a rear section or stator housing  22  houses the bladed rotary impeller. The rotor housing  20  is part of a one-piece rotor housing assembly, which also comprises a plurality of inlet vanes  63  and an inlet vane hub  70 . Each inlet vane  63  is joined at one end to the inlet vane hub  70  and at the other end to the rotor housing  20 . The inlet vanes direct water flow into the blades  18  of the rotor. The inlet vanes also block debris, sea creatures or human limbs from contacting the rotating blades of the rotor. A bearing support  26  engages the rear end of the propeller shaft  16 . The stator vanes  30 , which are present to neutralize the swirl from the impeller, also serve to attach the bearing support  26  to the stator housing  22 . At the rear end of the anti-ventilation plate  12  is a downwardly projecting exhaust gas outlet  14  which directs the exhaust gas into a channel  24  formed in the upper surface of the stator housing  22 . 
     Referring to FIG. 2, a pump jet  44  is mounted on an outboard motor  32 . The outboard motor  32  comprises a powerhead  34  and a leg  36 . The outboard motor  32  also includes conventional anti-ventilation plate  12  and lower unit housing  10 . The outboard motor  32  is preferably attached to a boat  40  or other marine vehicle or watercraft by an appropriate mounting bracket  38 , which attaches to the transom of the boat hull. 
     During operation of the motor  32 , an exhaust gas stream  110  flows downwardly from the powerhead  34  through an exhaust duct  50  positioned in the central portion of the outboard motor. The exhaust gas stream is injected in a rearward direction from the exhaust gas outlet into the water at a location downstream of the squeeze point P and above the stator housing  22 . 
     In normal operation of a downstream exhaust motor having an attached pump jet as shown in FIG. 2, flow streamlines  102  follow the shape of the lower unit housing  10 . Streamlines  104  behind the lower unit housing  10  follow the surface of the rotor housing  20  and stator housing  22 . At the maximum diameter of the pump jet between the top of the pump jet surface and the bottom surface of the anti-ventilation plate  12  is a so-called “squeeze point” P. Streamlines  106  down-stream of the squeeze point P and near the surface of the pump jet try to follow the conical surface of the pump housing and streamlines  108  near the anti-ventilation plate  12  try to remain parallel thereto. During the operation of this downstream exhaust motor, drag is created downstream of the squeeze point P. 
     FIG. 3 diagrammatically illustrates a prior art pump jet apparatus in which an exhaust gas stream  110  flows downwardly from the powerhead  34  through an exhaust duct  62  positioned in the central portion of the outboard motor. The exhaust gas is channelled in a rearward direction from the exhaust duct  62  to an exhaust channel  42 . The exhaust gas flows from the exhaust channel  42  above the stator housing  22  to exit the outboard motor  32 . An exhaust extension duct  46  is positioned above the stator housing  22  and is coupled to the exhaust channel  42  for discharging the exhaust gas rearwardly of the squeeze point P. The rear end of the exhaust extension duct  46  flares outwardly for controlling the size of the exhaust gas stream. The angle of the flare of the exhaust extension duct  46  can be increased or decreased to control the expansion of the exhaust gas stream. A trough  48  is formed in the upper surface of the stator housing  22  below the exhaust extension duct  46  to receive the exhaust gas. The trough  48  allows a portion of the exhaust stream to be concealed behind the pump jet housing, whereby an improved flow of the exhaust gas stream is achieved and drag is reduced. 
     Since the exhaust streams of the prior art propulsion systems shown in FIGS.  1 - 3  are released near the water surface, the level of exhaust noise is relatively high. For pump jets to be viable on recreational watercraft, the level of exhaust noise needs to be reduced. 
     One current approach to this problem is to distribute the exhaust flow among several hollow stator vanes, which discharge the gas at relatively high velocity through several openings distributed circumferentially around the stator housing. The procedure is effective in reducing exhaust noise, but requires the use of rotating gas seals and hollow stator vanes. Such an “exhaust through vane” (ETV) configuration is depicted in FIG.  4 . The stator housing  52  is part of a one-piece stator housing assembly, which also comprises a plurality of stator vanes  54  and a stator hub  56 . Each stator vane  54  is joined at one end to the stator hub  56  and at the other end to the stator housing  52 . The stator vanes  54  convert rotational energy imparted to the water flow by the rotor blades into axial flow energy at the outlet of the stator housing  52 . One or more of the stator vanes  54  is hollow. Similarly, an internal cavity in the stator hub  56  forms a plenum cavity  58 , which is in flow communication with each hollow stator vane. The exhaust gas from the powerhead  34  flows downwardly through an exhaust channel  60 . The lower end of the exhaust channel  60  is in flow communication with a hub exhaust channel  62  which channels the exhaust stream rearward through the hub. The hub exhaust channel  62  is an annular space, which is bounded internally by the propeller shaft bearing housing  64  and the inner rotor hub  66 , and externally by the wall of the gear case  68 , the inlet vane hub  70  and the outer rotor hub  72 . Rotating gas seals (not shown) must be installed between the outer rotor hub  72  and the stator hub  56  to prevent exhaust gas from leaking into the water jet stream inside the pump jet housing. The exhaust stream flows from the hub exhaust channel  62  to the plenum cavity  58  in stator hub  56 , and then into the hollow stator vanes  54  which communicate with the plenum cavity. The exhaust stream in each hollow stator vane flows the length of the stator vane and discharges from a respective exhaust port or outlet  74  into the water stream surrounding the stator housing  52 . 
     In ETV pump jets, the hollow stator vanes need to be large in order to provide adequate flow area for exhaust gas. But stator vanes that are too large or too numerous can begin to present significant blockage area to the water stream. 
     Thus, there is a need for a pump jet apparatus which requires neither hollow stator vanes nor rotating gas seals. 
     SUMMARY OF THE INVENTION 
     The present invention is a pump jet apparatus for use with marine engines mounted on boats or other watercraft, which apparatus does not include either hollow stator vanes (or hollow struts) or rotating gas seals. As used herein, the term “marine engines” includes, but is not limited to, outboard motors and inboard/outboard or stern drive units. 
     In accordance with the preferred embodiments, the pump jet apparatus comprises a double-walled stator housing containing an annular passage through which exhaust gas can flow. In the following written description, the two walls of the double-walled stator housing will be respectively referred to as the inner and outer stator shells. Gas enters the annulus through an exhaust gas inlet or port formed in the outer stator shell at the top of the stator housing, flows in two streams around the annular passage formed between the inner and outer stator shells, and exits the stator housing through exhaust outlets or ports formed in the outer stator shell near the bottom of the stator housing. The streams of exhaust gas and impelled water flowing through the stator housing of the pump jet are kept separate by the inner stator shell. Preferably, the exhaust outlets are circular, although the invention is not limited to the use of circular holes for exhaust outlets. For example, the exhaust outlets can be elliptical. 
     In accordance with a further preferred embodiment of the invention, exhaust outlet ducts are attached to the external surface of the outer stator shell. [The term “exhaust outlet duct” is adopted to distinguish the ducts attached to the stator housing from the exhaust ducts  50  and  62  depicted in FIGS.  1 - 4 .] Each exhaust outlet duct is positioned to be in flow communication with a respective exhaust gas outlet in the outer stator shell and are configured to block “bushing out” of the exhaust gas stream flowing out of the exhaust outlets. The exhaust outlet ducts may be attached by welding or brazing, by fastening (e.g., using bolts or screws), or by any other conventional attachment means. As used herein, the term “exhaust outlet duct” is not a tubular channel, which is the normal sense in which the term is “duct” is used, but rather is a portion of a duct which acts as a shield to allow the exhaust gases to discharge from the exhaust outlets free of interaction with the water stream external to the stator housing. The outlet of each exhaust outlet duct is defined by the trailing edge of the duct portion and the opposing external surface of the outer stator shell. 
     Preferably, each exhaust outlet duct comprises a curved piece of sheet material, e.g., metal, having a three-dimensional curved edge which abuts the external surface of the outer stator shell along a contour which partly surrounds a corresponding exhaust outlet, and having an arc-shaped or eyebrow-shaped trailing edge which preferably lies in a plane perpendicular to the central axis of the pump jet. Preferably, the duct material is a portion of a circular cylindrical surface and lies substantially parallel to the pump jet central axis. However, the duct portions need not be sections of a circular cylinder. Other shapes may be used to decrease the cross-sectional area of the outlet formed by the outer stator shell and the trailing edge of each duct portion. 
     In the case where the exhaust outlet ducts are portions of a circular cylinder, exhaust gases exiting the exhaust outlets are redirected by the inner surfaces of the ducts to flow in parallel with the direction of pump jet motion. In addition, the ducts provide a cross-sectional area for the exhaust gas stream which increases from adjacent the exhaust outlet to the duct outlet formed by the outer stator shell and the trailing edge of the exhaust outlet duct. The result will be an exhaust gas stream which exits the exhaust outlet duct parallel to and at a velocity equal to or less than that of the water stream flowing along the outer surface of the exhaust outlet duct during forward motion of the pump jet (provided that the eyebrow-shaped ducts are properly sized). It is expected that the exhaust outlet ducts will achieve improved performance over the entire pump jet speed range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially sectioned side elevation view of a prior art downstream exhaust pump jet. 
     FIG. 2 is a schematic depicting a side elevation view of a prior art downstream exhaust motor with a pump jet. 
     FIG. 3 is a schematic depicting a side elevation view of a prior art downstream exhaust motor with a pump jet and having an exhaust stream discharged rearward of the squeeze point. 
     FIG. 4 is a partially sectioned side elevation view of a prior art ETV pump jet having exhaust streams discharged through at least two stator vanes. 
     FIG. 5 is a partially sectioned side elevation view of a pump jet fitted with a double-walled stator housing in accordance with one preferred embodiment of the invention. 
     FIG. 6 is a sectional view of the pump jet shown in FIG. 5, the section being taken along section line A—A denoted in FIG.  5 . 
     FIG. 7 is a partially sectioned side elevational view of a pump jet fitted with a double-walled stator housing having exhaust outlet ducts in accordance with another preferred embodiment of the invention. 
     FIG. 8 is a sectional view of the pump jet shown in FIG. 7, the section being taken along section line A—A denoted in FIG.  7 . Section line B—B in FIG. 8 denotes the section taken in FIG.  7 . 
     FIG. 9 is a rear view of a double-walled stator housing having a partial exhaust outlet skirt in accordance with yet another preferred embodiment of the invention. The exhaust extension duct is shown in section to reveal the opening at the top of the annular passage. 
     FIG. 10 is a bottom view of the outer stator shell of the double-walled stator housing with partial skirt shown in FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One preferred embodiment of the invention is depicted in FIGS. 5 and 6. The pump jet housing comprises a rotor housing  20  and a double-walled stator housing  80 . The rotor assembly inside the rotor housing  20  may have the structure shown in FIG. 1, the structure shown in FIG. 4 or any other functionally equivalent structure. The present invention does not lie in the structure of the rotor assembly. Nor does it lie in the structure of the marine engine to which the pump jet apparatus is coupled. In particular, the invention has application with outboard motors (such as the outboard motor  32  shown in FIGS. 2 and 3) and in inboard/outboard or stern drive units (not shown) for watercraft and other vehicles. A propulsor of a stern drive unit is typically mounted to the stern or transom of a boat hull via a transom mount assembly or bracket. The shaft on which the pump jet rotor is mounted is driven to rotate by an engine mounted inside the boat via conventional gear assemblies mounted outside the boat. In addition, for outboard motor applications, lower unit housing  10 , skeg  78 , gear case  68 , and anti-ventilation plate  12 , shown in FIG. 5, may have conventional structures. Also a steering nozzle and a reverse gate may be mounted on the stator housing in conventional fashion. 
     Referring again to FIG. 5, the rotor housing  20 , which has an inlet  33  for the intake of water, forms the upstream portion of the shroud which fully encloses the pump jet. The rearward portion of the shroud comprises the double-walled stator housing  80  which has an outlet  82  for the water propelled rearward by the rotor blades. The double-walled stator housing  80  preferably comprises two parts: an inner stator shell  84  and an outer stator shell  86 . However, a person skilled in the art will recognize that the double-walled stator may alternatively comprise a monolithic piece or more than two pieces. The inner stator shell  84  is a slight modification of stator housings of current design, the latter preferably being shaped as a surface of revolution having an axis of symmetry coaxial with the pump jet centerline  88 . The inner stator shell  84  has an upstream edge which form fits with the downstream edge of the rotor housing  20 . The outer stator shell  86  is preferably configured to slide onto inner stator shell  84  like a boot slides onto a leg, and can be fastened in place with screws, longer but similar to, those currently used to attach the conventional one-piece stator housing to the rotor housing. 
     Installation of a pump jet in accordance with the preferred embodiments comprises the following steps: (1) attach the rotor housing  20  to the anti-ventilation plate  12  and skeg  78 ; (2) install the rotor on the propellor shaft (not shown in FIGS.  5  and  6 ); and (3) attach the rotor housing  20 , the inner stator shell  84  and the outer stator shell together by means of screws (not shown). The inner stator shell  84  has a generally conical portion which decreases in internal diameter in the downstream direction. The minimum internal diameter of inner stator shell  84  is preferably located at the outlet  82 . 
     In accordance with the embodiment depicted in FIGS. 5 and 6, the inner stator shell  84  is part of an assembly which also comprises a plurality of stator vanes  54  and a stator hub  56 . Each stator vane  54  is joined at one end to the stator hub  56  and at the other end to the inner stator shell  84 . The stator vanes convert rotational energy imparted to the water flow by the rotor blades into axial flow energy at the stator housing outlet  82 . 
     The outer stator shell  86 , on the other hand, is preferably integrally formed with an exhaust extension duct  90 . Preferably, the outer stator shell  86  is shaped as a surface of revolution having an axis of symmetry coaxial with the pump jet centerline  88 , i.e., coaxial with the axis of symmetry of the inner stator shell  84 . The circular upstream edge  92  of the outer stator shell  86  is dimensioned to seat on a shoulder machined into the external surface of an upstream portion of the inner stator shell  84 . The circular downstream edge  94  of the outer stator shell  86  is dimensioned to seat on the external surface of a downstream portion of the inner stator shell  84 . Between edges  92  and  94  of the outer stator shell  86 , the internal diameter of the outer stator shell  86  is greater than the outer diameter of the inner stator shell  84  by a gap dimension which increases to a maximum at a point between edges  92  and  94 , thereby forming a generally annular passage  96  having inner and outer diameters (and height) which vary in a longitudinal direction (i.e., parallel to the pump jet centerline axis  88 ). The annular passage  96  surrounds a mid-portion of the closed inner stator shell  84 . The top of the annular passage  96  is in flow communication with the exhaust extension duct  90  via an opening  98  formed in the outer stator shell  86 . Opening  98  is preferably circular. The lower half of the annular passage  96  is in flow communication with the space external to the outer stator shell  86  via one or more exhaust outlets  99 , also formed in the outer stator shell  86 . Although only one exhaust outlet  99  is visible in FIG. 5, three can be seen in FIG.  6 . Each of the exhaust outlets  99  is preferably a circular opening. In addition, the exhaust outlets  99  are preferably distributed at equal angular intervals along a portion of the circumference of the outer stator shell  86 , as best seen in FIG.  6 . 
     The exhaust extension duct  90  preferably has a rectangular cross section at its upstream end (to match rectangular outlet  42 ), as best seen in FIG. 6, but gradually changing to a semi-circular cross section downstream of the anti-ventilation plate. The exhaust extension duct  90  is open at its upstream edge, the latter being attached to and in flow communication with the downstream edge of the exhaust channel  42 . Both the exhaust extension duct  90  and the exhaust channel  42  can be attached to the underside of the anti-ventilation plate  12 . The exhaust gas stream from the marine engine flows from the exhaust channel  42  into the exhaust extension duct  90  and then into the annular passage  96  via the opening  98 . The exhaust gas stream then divides—one half flowing clockwise in the right-hand half of the annular passage  96 , as seen in FIG. 6, and the other half flowing counterclockwise in the left-hand half of the annular passage  96 . Finally, in the preferred embodiment of FIG. 5, the exhaust gas exits the stator housing through three round exhaust outlets  99 . [The person skilled in the art will appreciate, however, that fewer or more than three exhaust outlets can be used. The present invention is not limited to a particular number of exhaust outlets.] The cross-sectional area of the exhaust extension duct  90 , the diameter of opening  98 , the total cross-sectional area of the two branches of the annular passage  96 , and the diameter of the exhaust outlets  99  can be designed such that the most constricted point of the entire flow path is the cross section of the split path around the annular passage  96 . 
     The double-walled stator housing shown in FIGS. 5 and 6 can be designed to eject exhaust gas at a velocity in the neighborhood of the velocity of the water flowing past the pump when the boat is moving at top speed. Further, the gas is being ejected into the water at a lower depth than is the case for a comparable propeller-driven design. Thus, the invention reduces the noise produced by the marine engine exhaust gas stream. However, the propeller-driven design has one advantage: not only is there a good match between the water stream velocity and the velocity of the ejected gas stream; there is a perfect match between the vector directions of the two flowing streams. In contrast, the embodiment shown in FIGS. 5 and 6 ejects exhaust gas into the water stream surrounding the stator housing  80  at a vector direction almost at right angles to the direction of water flow. Without further structural modification of the pump jet shown in FIGS. 5 and 6, the exhaust gas stream will “bush out” and present a significant added frontal area to the water stream, producing added drag. 
     There is a way to deflect the flowing stream of exhaust gas so it flows parallel with the stream of water, however. A preferred embodiment for accomplishing the foregoing is illustrated in FIGS. 7 and 8. In this example, four round exhaust outlets  99  are provided in the lower half of the annular passage  96 . The four exhaust outlets  99  are preferably distributed at equal angular intervals along a portion of the circumference of the outer stator shell  86 , as best seen in FIG.  8 . 
     In accordance with the preferred embodiment shown in FIGS. 7 and 8, exhaust outlet ducts  100  are attached to the external surface of the stator housing  60 . Each exhaust outlet duct  100  is positioned to overlie a respective exhaust gas outlet  99 . The exhaust outlet ducts  100  may be attached by welding or brazing, by fastening (e.g., using bolts or screws), or by any other conventional attachment means. Preferably, each exhaust outlet duct  100  comprises a curved piece of sheet material, preferably metal, having a three-dimensional curved edge which abuts the external surface of the outer stator shell  86  and is joined thereto (e.g., by tack welding) along a contour which partly surrounds the corresponding exhaust outlet  99 ; and having an arc-shaped or eyebrow-shaped trailing edge (best seen in FIG. 8) which preferably lies in a plane perpendicular to the axis of the pump jet. Preferably, the duct material is a concave segment of a cylindrical (e.g., circular cylindrical) surface and lies substantially parallel to the pump jet central axis  88 . For example, each exhaust outlet duct  100  can be a piece cut from aluminum tubing having a circular cross section. In this case, exhaust gases exiting the exhaust outlets will be redirected by the inner surfaces of the ducts to flow in parallel with the pump jet axis, i.e., in parallel with the direction of pump jet motion. Thus, the exhaust outlet ducts function as walls to block “bushing out” of the exhaust gas stream being discharged from the exhaust outlets. In addition, the ducts provide a cross-sectional area for the exhaust gas stream which increases from a point adjacent the exhaust outlet to the duct outlet formed by the external surface of the outer stator shell  86  and the trailing edge of the exhaust outlet duct. The cross-sectional area of the exhaust extension duct  90 , the diameter of opening  98 , the total cross-sectional area of the two branches of the annular passage  96 , the diameter of the exhaust outlets  99  and the radius of curvature of the exhaust outlet ducts  100  can be designed such that gas emerging from the four exhaust outlet ducts  100  would show a reasonably close velocity match to that of the water stream both in magnitude and in vector direction. The result will be an exhaust gas stream which exits the exhaust outlet duct parallel to and at a velocity equal to or less than that of the water stream flowing along the outer surface of the exhaust outlet duct during forward motion of the pump jet. 
     Selection of the appropriate dimensions to achieve an approximate match of gas velocity and water velocity (a velocity match) requires the designer to make reasonable estimates of the volume rate of exhaust gas being discharged by the engine and the speed at which the motor will be traveling. The gas exit velocity equals the volume rate of discharge in cubic feet divided by the total eyebrow exit area in square feet. 
     If a stator housing having eyebrow-shaped exhaust outlet ducts as shown in FIGS. 7 and 8 were to be tested in a water tunnel without gas flow, one would expect that the “chopped-off” trailing edge of each eyebrow-shaped duct would produce additional drag (hereinafter “base drag”). However, when gas flow through the hollow stator vanes is established—with the gas flow velocity equal to or slightly less than the water stream velocity—the base drag vanishes. Thus, the placement of eyebrow-shaped exhaust outlet ducts  100  over the exhaust outlets  99  eliminates both the directional mismatch and (with properly sized eyebrow-shaped ducts) the velocity mismatch. 
     A pump jet like that shown in FIG. 5 and 6, operating near full speed, introduces exhaust gas into the flowing water stream with a minimum of commotion. The exhaust stream exits the pump at an angle, but should quickly turn and merge with the water, slowly rising to the surface. The resulting noise level should be much lower than that from prior art pump jets, where the exhaust gas emerges forcefully, at a higher velocity than the water, and near the surface. 
     A pump jet like that shown in FIGS. 7 and 8 should be even quieter, because the exhaust streams from the eyebrow exhaust outlet ducts gently merge with the water stream external to the stator housing. 
     Instead of providing a respective exhaust outlet duct for each exhaust outlet, a single wall or partial skirt  112  can be placed over the exhaust outlets, as depicted in FIGS. 9 and 10. As also shown in FIG. 9, the inner and outer stator shells  84  and  86  are attached to the rotor housing by means of a plurality of circumferentially distributed screws  114 . In addition, FIG. 10 shows that the centers of exhaust outlets  99  need not all be aligned in a radial plane. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. For example, one could readily conceive of a double-walled stator housing in which the inlet and outlet of the outer stator shell communicate via only a semicircular passage, corresponding in structure to one of the two branches of the annular passage disclosed hereinabove. In addition, many modifications may be made to adapt a particular situation to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. 
     As used in the claims, the term “marine engines” includes both inboard and outboard motors.