Abstract:
Dual propeller propulsion system for water crafts having two propellers (3, 4) fitted in the same axis outside the ends of an underwater housing (2) configured as a gondola so as to facilitate flow, said housing being placed under the hull of the water craft having a driving mechanism lodged in the underwater housing for both propellers (3, 4) to which the energy coming out for both propellers (3, 4) to which the energy coming out of the hull of the water craft is guided by means of a housing shank (18), one of the ends of which is mounted on the hull (24) of the water craft while the other is mounted in the underwater housing (2). Said propulsion system is characterized in that the underwater housing (2) forms part of a control device (20) by means of which the water jet coming out with increased energy from the front propeller (3) in the direction of travel of the water craft is guided with minimal energy loss and optimal irrotational flow to the back propeller (4) placed in the other end of the underwater housing in the direction of travel of the water craft. Both propellers (3, 4) are driven by the driving mechanism placed in the underwater housing in the same direction of rotation and are configured in the area of the respective water jet cross section in such a way that the differing flow energy entering both propellers (3, 4) is optimally used.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention pertains to a hydrojet with a drive unit and a twin propeller, which is driven by the drive unit.  
           [0002]    Such drives have been known with a design wherein the drive unit proper, for example a diesel engine, is arranged within the hull and a transmission, being another part of the drive unit, is located in a gondola under the hull, from which shafts are led out at mutually opposite ends, the shafts being connected to the transmission and, at their outer ends, to one of two propellers shown as identical propellers, rotating in unison with them. Such a solution is described in DE 44 40 738 A1, in which an essential feature is a control device, which is arranged between the two propellers and eliminates the twist in the water after it leaves the propeller that is the front propeller in the direction of travel, so that this water flows to the propeller that is the rear propeller in the direction of travel with a higher energy, but likewise without twist, as it does to the front propeller. The control device is formed by guide blades and a shaft, which connects gondola or the underwater housing to the hull. Such a drive is also described with some additional information in  THE MOTORSHIP,  October 1996, pp. 47, 48: “Double the props: half the problem.” This additional information includes the statement that the underwater housing or the gondola is optimized in terms of flow, without offering further details on what this specifically means. At any rate, it appears from the overall context that the gondola with its design that is favorable in terms of flow is the carrier of the control device, but it is not part of the control device, which means that no statements are made from which an action of the gondola that would functionally support the control device could be inferred. Also, despite the relatively detailed explanation of the drive, no data are presented on the design of the blades of the propeller that is the second propeller in the direction of flow in the sense of the present invention. Thus, the additional data show only that equal propeller size of an additional for improving the effect is possible by a special geometry having been developed for the rear propeller, but what this special geometry is characterized by is not indicated. Drives of this class have also been known in a such a design in which the entire drive is located in the aforementioned gondola. The suitable drive unit in this solution is an electric motor for the propellers at both ends of the gondola, to which electricity is supplied from a power generating unit, which is accommodated in the hull. Such a solution is described in EP 0 590 867 A1.  
           [0003]    A shaft is surrounded by a jacket tube between the drive motor within the hull and its transmission in the gondola in the drive according to the first design, and the electric lines between the power generating unit within the hull and the electric motor in the gondola are surrounded by a jacket tube in the drive according to the second design. This jacket tube is the above-mentioned shaft, which forms the control device together with guide blades. If the jacket tube is associated with the hull rotatably around its longitudinal axis at its top end and nonrotatably carries the gondola at its lower end, it can be associated with a motor operator, which is able to forcibly rotate the jacket tube with the gondola and the propellers associated with the gondola around the longitudinal axis of the jacket tube, so that the direction of discharge from the rear propeller into the free water changes and a rudder twin propeller unit is obtained. In addition, the jacket tube is designed as part of the guide baffles in the first embodiment.  
         SUMMARY OF THE INVENTION  
         [0004]    Against this and other state of the art, which does not, however, offer any further aspects in respect to the present invention, the object of the present invention is to optimize a ship drive such that an efficiency that is optimal according to the current state of knowledge is obtained, and the design and manufacturing technical efforts do not substantially exceed what is associated with the state of the art.  
           [0005]    This object is accomplished according to the present invention by the combination of individual, appropriately selected individual problem solutions, not only to add up but to potentiate the individual advantages into an overall concept that is optimal in its entirety.  
           [0006]    Consequently, the ship drive according to the present invention pertains to a hydrojet with two propellers led out of a gondola outside the hull at its ends, with a drive arranged in the gondola, to which drive energy is supplied from the hull through a jacket tube, one end of which is associated with the hull, and the other end of which is associated with the gondola, wherein the jacket tube is part of a control device, by which the water jet leaving the propeller that is the front propeller in the direction of travel with enriched flow energy at one end of the shaft and of the gondola is made twist-free in order to feed the water jet leaving the front propeller with high energy but with low twist to the propeller that is the rear propeller in the direction of travel of the watercraft, for which purpose both propellers are driven by the drive in the gondola in the same direction of rotation and have essentially the same design in the area of the jet cross section. In the ship drive of this class, the underwater housing or the gondola has an optimized shape, and the propeller that is the rear propeller of the two propellers in the direction of travel of the watercraft has a special geometry according to  Motorship,  Vol. 77, No. 915, October 1996, London, pp. 47, 48: “Double the props: half the problem.” 
           [0007]    In the drive according to the present invention, the two propellers have essentially the same diameter, so that the propeller that is the front propeller in the direction of travel of the watercraft and the propeller that is the rear propeller in the direction of travel of the watercraft both have different blade configurations in the entire diameter range and in the diameter range that is determined by the contraction of the jet as it leaves the front propeller, respectively, while the propeller that is the front propeller in the direction of travel of the watercraft and the propeller that is the rear propeller in the direction of travel of the watercraft have the same blade configuration in an annular area located outside the diameter range determined by the jet contraction. 
       
    
    
       [0008]    These and other features of the present invention appear from the following description of a plurality of exemplary embodiments of the present invention, the exemplary embodiments of the present invention shown in the drawings, and, finally, from the subclaims.  
       DESCRIPTION OF THE DRAWINGS  
       [0009]    In the drawings,  
         [0010]    [0010]FIG. 1 shows a first exemplary embodiment of a hydrojet according to the present invention with one propeller each at the ends of a shaft led out of a gondola-like underwater housing, which has a favorable design with respect to flow, is arranged by means of a housing shaft or foot on the underside of a ship under the ship and accommodates an electric motor, on the shaft or ends of which a propeller each is arranged;  
         [0011]    [0011]FIG. 2 shows a second exemplary embodiment of another design that is advantageous compared with FIG. 1;  
         [0012]    [0012]FIG. 3 shows a third exemplary embodiment, in which a right-angle gear drive, into which drive energy is supplied via a shafting accommodated in a jacket tube or housing shaft from a drive motor, which is arranged inboard and is not shown but may be a usual internal combustion engine, an electric motor or the like, is arranged in the underwater housing;  
         [0013]    [0013]FIG. 4 through FIG. 6 show, in representations which correspond to be above representations, three variants of an additional exemplary embodiment with an electric motor in the underwater housing, to which energy is supplied from an inboard power generator via cables, which are led through the housing shaft; and  
         [0014]    [0014]FIG. 7 shows a twin-propeller design, which is particularly advantageous and is a twin-propeller arrangement which is especially the subject of the present invention and may be used in all the above-mentioned embodiments. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    Description of the Embodiment According to FIG. 1  
         [0016]    The drive comprises essentially an electric motor  1   ∵  in a housing  2   ∵  outside and especially under the hull and two propellers,  3   ∵ ,  4   ∵ , which are driven by the electric motor  1   ∵ . The two propellers are usually of different designs, even though they may have tip circles  5   ∵  of equal diameter as well as a similar wing geometry. They have the same direction of rotation and the same speed of rotation and the flow is directed toward them in the same direction, e.g., according to arrow A ∵ .  
         [0017]    The electric motor  1   ∵  is arranged in the underwater housing  2   ∵  in a watertight manner. The driven shaft  7   ∵  is led out of it on both sides and the shaft is mounted in it rotatably in one of two bearings  8   ∵ ,  9   ∵  of the housing  2   ∵  to the side of the motor. Seals  10   ∵ ,  11   ∵  to the side of the bearings  8   ∵ ,  9   ∵  between the shaft  7   ∵  and the front-side housing walls  2   a   ∵ ,  2   b   ∵  are used for sealing in conjunction with the front surfaces being designed as parts of labyrinth seals. Shaft ends  12   ∵ ,  13   ∵ , each of which carries one of the two propellers  3   ∵ ,  4   ∵ , rotating in unison, are flanged on the shaft  7   ∵  outside the housing  2   ∵ . On the front side, the housing  2   ∵  is joined by hub caps  14   ∵ ,  15   ∵ , wherein a continuous outer contour with head  14   ∵ , which said contour is favorable in terms of flow, is formed in the area of the front propeller  3   ∵ , the middle part in the form of the housing  2   ∵  and the end part  15   ∵  in the area of the rear propeller  4   ∵ . The front walls  14   a   ∵ ,  15   a   ∵  of the hub caps  14   ∵ ,  15   ∵  facing the housing  2   ∵  are second parts of the labyrinth seals  16   ∵ ,  17   ∵ , whose first parts are the aforementioned front surfaces  2   a   ∵ ,  2   b   ∵ . The housing  2   ∵  is held at the hull with a foot  18   ∵ , which is designed as a hollow foot, whose outer contour is part of the control device  19   ∵  between the propellers  3   ∵ ,  4   ∵ , which has additional blades associated with the housing  2   ∵ , of which one blade, which is located diametrically opposite the foot  18   ∵ , is designated by  20   ∵ . On the whole, the blades of the control device  19   ∵  are rigidly associated with the housing  2   ∵ , distributed uniformly around the longitudinal axis of the shaft  7   ∵ .  
         [0018]    On the whole, the propellers  37   ∵ ,  4   ∵  are designed such that the output energy level of the second propeller  4   ∵  is approximately equal to the final energy level of the first propeller  3   ∵  and, in conjunction with the control device  19   ∵ , the output twist of the first propeller  3   ∵  as well as the input twist of the second propeller  4   ∵  are influenced corresponding to the purpose such that only slight energy losses occur at best at the time of the transition of the liquid from the first propeller to the second one.  
         [0019]    The energy is supplied to the electric motor by lines  21   ∵ , which are led to the motor in the foot  18   ∵  and in the housing  2   ∵ , and the inner spaces of the foot  18   ∵  and of the housing  2   ∵  are therefore in connection with one another.  
         [0020]    To make it possible to use the drive not only to generate a thrust in the longitudinal direction of the ship (longitudinal axis of the drive shaft), but also to steer the ship, the entire drive is pivotable around the vertical longitudinal axis  22   ∵  in the middle between the two propellers, optionally all around by 360° due to the corresponding association with the ship and by means of an appropriate pivoting mechanism in the known manner, the axis  22   ∵  being directed at right angles to the axis of rotation of the longitudinal axis  23   ∵  of the shaft.  
         [0021]    Description of the Embodiment According to FIG. 2  
         [0022]    The drive comprises essentially an electric motor  1   ••  in a housing  2   ••  outside and especially under the hull and two propellers  3   •• ,  4   •• , which are driven by the electric motor  1   •• . The two propellers usually have different designs, even though they may have tip circles  5   ••  of equal diameter as well as similar wing geometry. They have the same direction of rotation and the same speed of rotation and the flow is directed toward them in the same direction, e.g., according to the arrow A •• .  
         [0023]    The electric motor  1   ••  is arranged in the underwater housing  2   ••  in a watertight manner. The driven shaft  7   ••  is led out of it on both sides and is mounted in it rotatably in one of two bearings  8   •• ,  9   ••  each of the housing  2   ••  to the side of the motor. Seals  10   •• .  11   ••  to the side of the bearings  8   •• ,  9   ••  between the shaft  7   ••  and the front-side housing walls  2   a   •• ,  2   b   ••  are used for sealing in conjunction with the front surfaces being designed as parts of labyrinth seals. Shaft ends  12   •• ,  13   •• , each of which carries one of the two propellers  3   •• ,  4   •• , rotating in unison, are flanged to the shaft  7   ••  outside the housing  2   •• . On the front side, the housing  2   ••  is joined by hub caps  14   •• ,  15   •• , wherein a continuous outer contour with head  14   •• , which contour is favorable with respect to flow, is formed in the area of the front propeller  3   •• , the middle part in the form of the housing  2   ••  and the end part  15   ••  in the area of the rear propeller  4   •• . The front walls  14   a   •• ,  15   a   ••  of the hub caps  14   •• ,  15   ••  facing the housing  2   ••  are two parts of the labyrinth seals  16   •• ,  17   •• , whose first parts are the aforementioned front surfaces  2   a   •• ,  2   b   •• . The housing  2   ••  is held at the hull with a foot  18   •• , which is designed as a hollow foot, whose outer contour is part of the control device  19   ••  between the propellers  3   •• ,  4   •• , which has additional blades associated with the housing  2   •• , of which one blade, which is located diametrically opposite the foot  18   •• , is designated by  20   •• . On the whole, the blades of the control device  19   ••  are rigidly associated with the housing  2   •• , uniformly distributed around the longitudinal axis of the shaft  7   •• .  
         [0024]    On the whole, the propellers  3   •• ,  4   ••  are designed such that the output energy level of the second propeller  4   ••  is approximately equal to the final energy level of the first propeller  3   ••  and, in conjunction with the control device  19   •• , the output twist of the first propeller  3   ••  as well as the input twist of the second propeller  4   ••  are influenced corresponding to the purpose such that only slight energy losses occur at best at the time of the transition of the liquid from the first propeller to the second one.  
         [0025]    The energy is supplied to the electric motor by lines  21   •• , which are led to the motor in the foot  18   ••  and in the housing  2   •• , and the inner spaces of the foot  18   ••  and of the housing  2   ••  are therefore in connection with one another.  
         [0026]    To make it possible to use the drive not only to generate a thrust in the longitudinal direction of the ship (longitudinal axis of the drive shaft), but also to steer the ship, the entire drive is pivotable around the vertical longitudinal axis  22   ••  in the middle between the two propellers, optionally all around by 360° due to the corresponding association with the ship and by means of an appropriate pivoting mechanism in the known manner, the axis  22   ••  being directed at right angles to the axis of rotation of the longitudinal axis  23   ••  of the shaft.  
         [0027]    The motor  1   ••  is designed as a permanent synchronous motor and thus it is an electric machine with very high power density. Due to the technology of such a motor, it is possible to design the housing  2   ••  between the two propellers hydrodynamically such that a very high efficiency will be reached.  
         [0028]    It is possible in the case of this technology for the foot  18   ••  to be designed as a shaft, so that it will also have an optimal hydrodynamic shape.  
         [0029]    In the lower area located in the proximity of the housing  2   •• , the shaft  18   ••  is designed such that it forms a guide fin pair and thus a control device together with a second, diametrically opposite guide fin  20   •• , so that an optimal flow of water to the propeller  4   •• , which is the second propeller when viewed in the oncoming flow direction A •• , is possible. The guide fins end in the tip circles  5   ••  of equal diameter of the two propellers  3   •• ,  4   •• .  
         [0030]    Due to the combination of the permanent synchronous motor of high power density on a small diameter with the optimal guide means (guide fin pair or control device  20   •• ) as well as with the two propellers  3   •• ,  4   •• , a drive unit is obtained which is characterized by an extremely great improvement in efficiency both electrically and hydrodynamically.  
         [0031]    The design of the motor  1   ••  as a permanent synchronous motor makes it possible to reduce the diameter of the housing  2   ••  by up to 20% compared with other, prior-art motors. The advantages are obvious: Only the smaller masses and more favorable flow conditions and lower flow resistance shall be mentioned.  
         [0032]    Another design according to the present invention pertains to the mounting of the rotor of the permanent motor, which also contains the mounting of the propeller shaft. To reduce or eliminate the displacements and deformations as well as the dynamic loads from the propellers, the rotor, i.e., the drive shaft  7   •• , is connected to the propeller shafts  12   •• ,  13   ••  via disk clutches  23   •• ,  24   •• . As a result, it is possible to obtain a minimal air gap between the stator and the rotor, which means a considerable, additional improvement in efficiency.  
         [0033]    Description of the Exemplary Embodiment According to FIG. 3  
         [0034]    [0034]FIG. 3 shows a ship drive designed as a rudder twin propeller with a drive unit arranged in the hull with vertical drive shaft  1   •  and drive propellers outside the hull. A drive unit comprising a motor and transmission acts on the top end of the vertical drive shaft  1   •  in the known manner, therefore not shown in FIG. 3, in order to set the drive shaft  1   •  into rotation around the longitudinal axis  2   •  at variable speed of rotation. The input bevel gear  3   •  of a right-angle gear drive  3   • ,  4   • , which bevel gear  3   •  is in functional connection with the output bevel gear  4   •  of the right-angle gear drive  3   • ,  4   • , is associated with the lower end of the drive shaft  1   • , rotating in unison. The output bevel gear  4   •  carries a horizontal output shaft  5   •  extending in both directions, rotating in unison with it, with the propellers  6   • ,  7   •  arranged at the free ends of the output shaft, rotating in unison with it. The propellers usually have different designs, even though tip circles  14   •  of equal diameter as well as similar wing geometries may be possible. Due to the joint association with the output shaft  5   • , they have the same direction of rotation and the same speed of rotation and the flow is directed toward them in the same direction, e.g., according to arrow A • .  
         [0035]    The right-angle gear drive  3   • ,  4   •  is surrounded by a housing  9   • , in which the output shaft  5   •  is mounted rotatably by means of two bearings  10   • ,  11   • . This housing  9   •  is carried by a housing pipe  9   a   • , which concentrically surrounds the vertical drive axis  1   •  and is pivotable around its longitudinal axis for the rudder function.  
         [0036]    The underwater part of the drive system may be arranged within a nozzle  12   • .  
         [0037]    In its wake, the front propeller  6   •  generates a rest or aftertwist, which represents lost energy. The wake of the front propeller is admitted to the downstream propeller  7   • , rotating in the same direction. Without a guide means between the two propellers  6   • ,  7   • , the above-mentioned unfavorable wake would lead to intensified cavitation and to an increase in the energy losses.  
         [0038]    To counteract this energy loss, a guide means  8   • , with which the aftertwist of the front propeller  6   •  is directed, is provided between the two propellers  6   • ,  7   • . Lost energy is now recovered by a propulsive force being generated during the flow around the guide means. Furthermore, a pretwist is generated for the downstream propeller  7   • , so that the latter can realize a greater energy gradient. Taking this criterion into account, the second propeller  7   •  preferably has a design different from that of the first propeller  6   • .  
         [0039]    According to FIG. 3, the guide means  8   •  comprises two guide blades  8   a   •  and  8   b   • , wherein one guide blade  8   a   •  is formed by the housing pipe  9   a   •  surrounding the vertical drive shaft  1   • . The second guide blade  8   b   •  is located on the underside  9   b   •  of the housing  9   •  surrounding the horizontal output shaft  5   • , i.e., offset by 180° from the first guide blade. The two guide blades  6   • ,  7   •  form an assembly unit with the overall housing  9   • ,  9   a   • .  
         [0040]    Description of the Exemplary Embodiment According to FIG. 4 Through FIG. 6  
         [0041]    The drive comprises essentially an electric motor  1  in a housing  2  outside and especially under the hull and two propellers  3 ,  4 , which are driven by the electric motor  1 . The two propellers are usually of different design, even though they may have tip circles  5  of equal diameter as well as a similar wing geometry. They have the same direction of rotation and the same speed of rotation and the flow is directed toward them in the same direction, e.g., according to arrow A.  
         [0042]    The electric motor  1  is arranged in the underwater housing  2  in a watertight manner. The driven shaft  7  is led out of it on both sides and the shaft is mounted in it rotatably in one of two bearings  8 ,  9  of the housing  2  to the side of the motor. Seals  10 ,  11  to the side of the bearings  8 ,  9  between the shaft  7  and the front-side housing walls  2   a ,  2   b  are used for sealing in conjunction with the front surfaces being designed as parts of labyrinth seals. Shaft ends  12 ,  13 , each of which carries one of the two propellers  3 ,  4 , rotating in unison, are flanged on the shaft  7  outside the housing  2 . On the front side, the housing  2  is joined by hub caps  14 ,  15 , wherein a continuous outer contour with head  14 , which said contour is favorable in terms of flow, is formed in the area of the front propeller  3 , the middle part in the form of the housing  2  and the end part  15  in the area of the rear propeller  4 . The front walls  14 a,  15 a of the hub caps  14 ,  15  facing the housing  2  are second parts of the labyrinth seals  16 ,  17 , whose first parts are the aforementioned front surfaces  2   a ,  2   b . The housing  2  is held at the hull with a foot  18 , which is designed as a hollow foot, whose outer contour is part of the control device  19  between the propellers  3 ,  4 , which has additional blades associated with the housing  2 , of which one blade, which is located diametrically opposite the foot  18 , is designated by  20 . On the whole, the blades of the control device  19  are rigidly associated with the housing  2 , distributed uniformly around the longitudinal axis of the shaft  7 .  
         [0043]    On the whole, the propellers  3 ,  4  are designed such that the output energy level of the second propeller  4  is approximately equal to the final energy level of the first propeller  3  and, in the control device  19 , the output twist of the first propeller  3  is removed from the input to the second propeller  4 , and thus only slight energy losses occur, at worst, at the time of the transition of the liquid from the first propeller to the second one.  
         [0044]    When leaving the first propeller  3  the water flow has an axial component (“conveyor direction”) and a circumferential component (“twist”). The latter component, i.e. the component in the circumferential direction, is deflected by a guide blade  19  into the axial direction, so that the water flow entering the second propeller  4  has only components in the axial direction, similar to the water flow entering the first propeller  3 .  
         [0045]    The energy is supplied to the electric motor by lines  21 , which are led to the motor in the foot  18  and in the housing  2 , and the inner spaces of the foot  18  and of the housing  2  are therefore in connection with one another.  
         [0046]    To make it possible to use the drive not only to generate a thrust in the longitudinal direction of the ship (longitudinal axis of the drive shaft), but also to steer the ship, the entire drive is pivotable around the vertical longitudinal axis  22  in the middle between the two propellers, optionally all around by 360° due to the corresponding association with the ship and by means of an appropriate pivoting mechanism in the known manner, the axis  22  being directed at right angles to the axis of rotation of the longitudinal axis  23  of the shaft.  
         [0047]    An especially advantageous embodiment of the drive according to the present invention will be described below with reference to FIGS. 5 and 6. The electric motor  1  is advantageously, but not essentially, a permanently excited synchronous motor with permanent magnet rotor  25  and stator laminations  26 . Such motors have are known in the art, so that the electric motor, which is advantageously a permanently excited synchronous motor, as well as other suitable types of electric motors, are not specifically described.  
         [0048]    The use of such a motor in the housing  2 , which has a gondola-like design and is arranged under the hull of the ship  24 , beneath the water surface, for driving the two co-rotating propellers  3 ,  4 , which face the same direction A, has various application-specific advantages, especially in terms of electric efficiency, and it makes it possible to dispense with forced cooling apparatus. In addition, a small overall volume is made possible which in turn makes possible an optimal shape of the underwater housing with respect to resistance, especially a housing with a small maximum diameter. The gondola-shape of the housing  2  performs a guiding function whereby the gondola-shaped housing  2  serves as part of the control device in addition to the guide fins  20  and hollow shaft or foot  18 .  
         [0049]    In order to be able to avoid forced cooling, the outer stator  26  is constructed as a laminated yoke of the electric motor  1 , and is connected tightly and in direct heat contact to the underwater housing  2 . The inner rotor  25  of the motor  1  is rigidly connected via support tube  27  to the rotatable shafts  12 ,  13 . In a similar way, the stator  26  is rigidly supported in the housing  2  via support tube  26 ′ which is made of heat conducting material, and which is firmly connected to the motor  26 , as well as to the housing  2 . In this arrangement there is direct heat transfer from the stator  26  of the motor (motor housing) to the support tube  26 ′ and finally to the underwater housing  2 , from which the heat is dissipated into the surrounding water flow. The underwater housing  2 , the support tube  26 ′ and the motor housing  26  are, thereby, sufficiently cooled.  
         [0050]    The tight connection between support tube  26 ′ and underwater housing  2 , and between stator  26  and support tube  26 ′ respectively, can be accomplished, e.g., by pressing each inner part held at lower temperature into the respective outer part held at a higher temperature. In use, at running temperature, the two parts will then be tightly connected.  
         [0051]    For cooling the bearings  8 ,  9  correspondingly, the bearings  8 ,  9 , which support the shafts  12 ,  13 , are in turn supported on by flanges  8   a ,  9   a  which are made of heat conducting material. The outer flange surfaces  8   b ,  9   b  are tightly connected to the inner surface of the underwater housing  2  and to the motor housing  26 .  
         [0052]    In another design, such a permanently excited synchronous motor is arranged in the gondola-like housing  2  such that the continuous propeller shaft  12 ,  13  and the rotor  25  have a common mounting with the two bearings  8 ,  9 . This is specifically realized such that the permanent rotor  25  is seated on a support tube  27 , which is concentrically surrounded by it and which is associated with the propeller shaft  12 ,  13 , rotating in unison with it, in the proximity of its two ends via one of two annular disk clutches  28 ,  29  each, wherein the disk clutches  28  and  29  as well as the corresponding bearing  8  or  9  are located close to one another at the two shaft ends. Due to the propeller shaft and the electric motor tube having a common mounting, the number of components is minimized and the reliability of the drive unit is increased. Due to the use of the respective disk clutch located in close proximity to the respective plain bearing, a highly accurate centering of the rotor, which is extensively independent from the sag of the propeller shaft, is achieved within the stator. This leads to considerable advantages in terms of the dynamic behavior of the rotor within the machine (e.g., the excitation of structure-borne noise is minimized).  
         [0053]    Likewise as a consequence of the electric motor being designed as a permanently excited synchronous motor  1  (FIGS. 2, 3), integration of the underwater housing shaft  18  (called a “foot” in connection with FIG. 1) within the drive is possible in a particularly advantageous manner. This housing shaft may be made as an especially slender shaft, as a result of which the flow resistance of the unit is considerably reduced. This slender underwater housing shaft  18  has such a cross-sectional profile that an additional untwisting of the wake of the front propeller  3  is achieved in conjunction with a lateral guide fin pair (not shown), which is offset by 90°, the geometry of the housing  2 , and the opposite guide fin  20 , which is offset by 180°. This results in an improvement in efficiency of the drive with two essentially identical co-rotating propellers (in terms of speed of rotation and direction of rotation).  
         [0054]    A parking brake for fixing the propeller shaft  12 ,  13  and thus the assembly unit, whose parts include the propeller shaft, is arranged within the underwater gondola  2  and is designated by  33 .  
         [0055]    Finally, the design according to FIGS. 5, 6 leads to an essential simplification of the underwater assembly efforts.  
         [0056]    Rudder propellers that can be assembled/disassembled with the ship floating are offered by various rudder propeller manufacturers. The corresponding assembly effort is still considerable. The present invention makes possible, especially in the embodiment according to FIGS. 5 and 6, a greatly simplified underwater assembly/disassembly at the underwater housing shaft-carrier cone separation point. The underwater housing shaft is also designated by the reference number  18  in FIG. 6; its top end is located in the plane  24  of the shell of the ship and is connected to the carrier cone  30 . At the top end, the carrier cone is mounted in a steering bearing  31  in the support structure of a ship. This steering bearing  31  has an inner ring  31   a  with an inner toothed ring  31   b,  and this bearing inner ring  31   a  is rigidly associated with the outer circumference of the carrier cone  30 . The outer ring  31   c  cooperates with the inner ring via the rolling bodies and it is rigidly integrated in the support structure of the ship. The pinion (not shown) of a drive (not shown) engages the inner toothed ring of the inner ring of the steering bearing, so that the entire drive can be rotated by 360° around the longitudinal axis  22  for steering the ship.  
         [0057]    The detachable connection between the housing shaft  18  and the carrier cone  30  is symbolized by a flanged connection  32 .  
         [0058]    Common to all embodiments is the combination of the features of claim  1 , according to which the drive is a hydrojet for watercraft, especially ships, which has a drive unit and two propellers driven by same, which said propellers are arranged at the two ends of a gondola-like, streamlined underwater housing outside the underwater housing and are driven by a drive means, which is located within the underwater housing and a common drive shaft acts on the two propellers, wherein the first propeller markedly increases the flow energy of the flow medium and this flow medium is supplied with a high energy content, after eliminating the inevitable aftertwist in a guide means, to the second propeller, which differs from the first propeller in terms of its blading such that the relatively low flow energy in the first propeller is optimally increased, while the relatively high flow energy is increased further in the second propeller; in a special embodiment, described below on the basis of FIG. 2, the second propeller has a central part, which differs from the first propeller in the described manner, and a peripheral part, which is identical to the first propeller to this extent and to which the medium flows in the same manner as to the first propeller.  
         [0059]    Description of the Embodiment According to FIG. 7  
         [0060]    The propeller  3  that is the front propeller in the direction A of the incoming flow has an optimal blading for increasing the energy of the flow medium. The propeller  4  that is the rear propeller in the direction A of the incoming flow has the same blading in this respect in a peripheral area. This peripheral area surrounds a central area, in which the blading differs from that of the front propeller  3  as was described several times above, i.e., it once again increases the energy increased in the first propeller from this energy level after the flow medium leaving the first propeller  3  has been untwisted in the control device  19  and the energy loss caused by the twist has been compensated. The core area and the peripheral area are separated from one another by the contraction surface  100 , i.e., by the jacket surface, which surrounds the flowing fluid after it has left the first propeller  3  and circumscribes a cross section that is markedly smaller than the incoming flow cross section. The flow medium B consequently flows to the second propeller in the peripheral area in the same manner as the flow medium that is characterized by the arrows A flows to the first propeller.