Patent Publication Number: US-2022234705-A1

Title: Marine vessel and marine propulsion unit

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2021-008738 filed on Jan. 22, 2021. The entire contents of this application are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to marine vessels and marine propulsion units. 
     2. Description of the Related Art 
     Conventionally, as a propulsion unit for a marine vessel, a jet pump that propels the marine vessel such as a water jet propulsion boat is known. For example, Japanese Laid-Open Patent Publication (kokai) No. 2013-107596 has disclosed a propulsion unit that rotationally drives an impeller of a jet pump by means of an electric motor. 
     However, in the case of driving a rotating shaft of the impeller by one motor as disclosed in Japanese Laid-Open Patent Publication (kokai) No. 2013-107596, a large output is required for the motor used. Therefore, it is conceivable to mount a high-power (large output) motor. However, in order to arrange a large-sized motor, if the position of an output shaft of the motor is higher than the bottom of the marine vessel (hereinafter referred to as “a vessel bottom”), the propulsion efficiency will decrease. Therefore, in the case of driving the jet pump by one large-sized motor, there is a problem that the degree of freedom in the layout of the marine vessel is lowered. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide marine vessels and marine propulsion units that are able to increase the degree of freedom in the layout of the marine vessels. 
     According to a preferred embodiment of the present invention, a marine vessel includes a hull, a jet pump including an impeller, a plurality of motors, and a transmission to transmit outputs of the plurality of motors to the impeller of the jet pump. 
     According to another preferred embodiment of the present invention, a marine propulsion unit includes a jet pump including an impeller, a plurality of motors, and a transmission to transmit outputs of the plurality of motors to the impeller of the jet pump. 
     According to preferred embodiments of the present invention, outputs of a plurality of motors are transmitted to the impeller of the jet pump by a transmission. As a result, it is possible to increase the degree of freedom in the layout of the marine vessel. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view of a marine vessel, to which a marine propulsion unit according to a first preferred embodiment of the present invention is applied. 
         FIG. 2  is a block diagram of a maneuvering system mounted on the marine vessel. 
         FIG. 3  is a longitudinal section view of a second propulsion unit. 
         FIG. 4  is a longitudinal section view of a second propulsion unit according to a second preferred embodiment of the present invention. 
         FIG. 5  is a longitudinal section view of a second propulsion unit according to a third preferred embodiment of the present invention. 
         FIG. 6  is a longitudinal section view of a main portion of a second propulsion unit according to a fourth preferred embodiment of the present invention. 
         FIG. 7  is a schematic view that shows a spatial relationship between a jet pump and a plurality of electric motors. 
         FIG. 8  is a schematic view that shows a spatial relationship between the jet pump and a plurality of electric motors. 
         FIG. 9  is a schematic view of a marine vessel according to modified preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. 
     First, a first preferred embodiment of the present invention will be described.  FIG. 1  is a schematic plan view of a marine vessel, to which a marine propulsion unit according to the first preferred embodiment of the present invention is applied. In  FIG. 1 , a portion of a marine vessel  11  is shown in an exposed view. The marine vessel  11  includes a hull  12 , and a deck  13  disposed on an upper portion of the hull  12 . The marine vessel  11  is, for example, a water jet propulsion boat. 
     In the following description, as shown in  FIG. 1 , front, rear, left, and right directions refer to front, rear, left, and right directions of the hull  12 , respectively. The right-and-left direction is defined with reference to the hull  12  being viewed from the rear. A vertical direction is a direction perpendicular to the front-and-rear direction and the right-and-left direction. Further, the vertical direction is a direction perpendicular to an upper surface of the deck  13 . 
     The marine vessel  11  includes a plurality of propulsion units  14  and  15  to propel the hull  12 , a steering handle  17 , and an output adjusting unit  18  (e.g., throttle). The steering handle  17  is operated by a vessel operator to steer the marine vessel  11 . The output adjusting unit  18  includes a lever, etc., and is operated by the vessel operator to adjust a thrust force and perform switching of traveling directions. The steering handle  17  and the output adjusting unit  18  are disposed in a maneuvering seat provided on the deck  13 . 
     The plurality of propulsion units  14  and  15  are mounted on a rear portion of the hull  12 . Each of two first propulsion units  14  uses an engine  34  (see  FIG. 2 ) as a power source. Further, each of two second propulsion units  15  uses two or more electric motors (see  FIG. 2 ) as the power source. All of the first propulsion units  14  and the second propulsion units  15  are jet propulsion units. The propulsion units  14  and  15  are independent of each other. 
     A pair of the first propulsion units  14  are disposed symmetrically with respect to a vertical plane (a hull center C 1 ) passing through a bow and the center of a stern. Further, a pair of the second propulsion units  15  are disposed at locations farther from the hull center C 1  than the pair of the first propulsion units  14  in a width direction of the hull  12 , and are disposed symmetrically with respect to the hull center C 1 . 
       FIG. 2  is a block diagram of a maneuvering system mounted on the marine vessel  11 . As components mainly related to maneuvering, in addition to the steering handle  17  and the output adjusting unit  18  that are described above, the maneuvering system includes a controller  30 , a display unit  39 , a setting operation unit  29 , a plurality of engines  34 , a sensor group  36 , an actuator group  37 , and a plurality of inverters  35 . A plurality of electric motors M 1  and M 2  are included in each of the second propulsion units  15 . That is, each of the second propulsion units  15  includes the electric motors M 1  and M 2 . An inverter  35  is provided for each of the electric motors M 1  and M 2 . 
     The sensor group  36  includes a steering angle sensor, a lever position sensor, a hull speed sensor, a hull acceleration sensor, a posture sensor, an engine speed sensor, and the like (none are shown). The actuator group  37  includes actuators that drive deflectors (not shown) provided within the first propulsion units  14 . The deflectors are components to change a direction of a jet flow to the left or right. 
     The controller  30  includes a CPU (Central Processing Unit)  31 , a ROM (Read Only Memory)  32 , a RAM (Random Access Memory)  33 , and a timer (not shown). The ROM  32  stores control programs. The CPU  31  performs various kinds of control processes by executing the control programs, which are stored in the ROM  32 , in the RAM  33 . The RAM  33  provides a work area for the CPU  31  to execute the control programs. 
     The display unit  39  displays various kinds of information. The setting operation unit  29  includes an operator to perform operations related to the maneuvering, a setting operator to perform various kinds of settings, and an input operator to input various kinds of instructions (none are shown). Various kinds of detection results obtained by the sensor group  36  are supplied to the controller  30 . 
     In the sensor group  36 , the hull speed sensor and the hull acceleration sensor detect a speed and an acceleration of navigation of the marine vessel  11  (the hull  12 ), respectively. The posture sensor includes, for example, a gyro sensor, a magnetic azimuth sensor, etc. The engine speed sensor detects the number of rotations per unit time of the engine  34 . The steering angle sensor detects a turning angle of the steering wheel  17 . The lever position sensor detects a shift position of the output adjusting unit  18 . 
     The first propulsion unit  14  may include an engine ECU (Electronic Control Unit), and the second propulsion unit  15  may include a motor ECU. In this case, the controller  30  functions as a main ECU and controls the engine ECU and the motor ECU. 
     The output adjusting unit  18  is movable in an F region, an N region, and an R region. The N region is provided between the F region and the R region. The F region is a region that makes the marine vessel  11  go forward, and the R region is a region that makes the marine vessel  11  go rearward. 
     The controller  30  propels the marine vessel  11  by at least either of the first propulsion units  14  and the second propulsion units  15 . The vessel operator is able to select an operation mode by operating the setting operation unit  29 . The operation modes include a manual mode. The manual mode includes an engine mode in which the marine vessel  11  is propelled only by the pair of the first propulsion units  14 , an electric mode in which the marine vessel  11  is propelled only by the pair of the second propulsion units  15 , and an assist mode in which the first propulsion units  14  and the second propulsion units  15  cooperate to propel the marine vessel  11 . The electric mode will be mainly described. 
     In the electric mode, when the vessel operator instructs the marine vessel  11  to go forward straight, the steering handle  17  is operated to a straight-ahead position, and the output adjusting unit  18  is located in the F region. In this state, the controller  30  controls the electric motors M 1  and M 2  within each of the second propulsion units  15  so that the magnitudes of outputs of the two second propulsion units  15  match. When the vessel operator turns the marine vessel  11  while instructing it to go forward, the steering handle  17  is steered, and the output adjusting unit  18  is located in the F region. In this state, the controller  30  controls the electric motors M 1  and M 2  within each of the second propulsion units  15  so that the magnitudes of the outputs of the two second propulsion units  15  are different from each other. 
     In the case of instructing the marine vessel  11  to go rearward, the output adjusting unit  18  is located in the R region, and rotation directions of the electric motors M 1  and M 2  are reversed with respect to the go forward case described above. Further, in the case of rotating the marine vessel  11  at the same point, the rotation directions of the electric motors M 1  and M 2  within one of the second propulsion units  15  and the rotation directions of the electric motors M 1  and M 2  within another of the second propulsion units  15  may be reversed. Moreover, the second propulsion units  15  may also be provided with deflectors to change the direction of the jet flow to the left or right. In that case, the controller  30  controls the deflectors to have a posture that a water jetting direction is tilted to the left or right with respect to the front-and-rear direction in a plan view. 
     Next, the detailed configurations of the second propulsion units  15  will be described. Since the configurations of the two second propulsion units  15  are the same except that they are symmetrical in the right-and-left direction, one of the second propulsion units  15  will be described. 
       FIG. 3  is a longitudinal section view of the second propulsion unit  15 . The second propulsion unit  15  mainly includes a jet pump  28  and a transmission unit  100 . The jet pump  28  of the second propulsion unit  15  is disposed on the outside of the hull  12 . Specifically, the jet pump  28  is accommodated in an accommodation portion  12   a , which is formed on the outside of the bottom of the rear portion of the hull  12 . However, the transmission unit  100  of the second propulsion unit  15  is mainly disposed inside the hull  12 . The accommodation portion  12   a  is recessed upward from the vessel bottom. 
     The jet pump  28  includes a duct  41 . The second propulsion unit  15  is mounted on the hull  12  by fixing a front portion of the duct  41  to the hull  12  with a plurality of bolts, for example. The jet pump  28  is driven by the electric motors M 1  and M 2 , sucks in water from the vessel bottom, and jets the sucked in water rearward. The jet pump  28  has a streamlined housing  46  extending in the front-and-rear direction and a flow path  40  defined by flow forming members. The jet pump  28  includes an impeller  44  and a stationary blade  45  disposed in the flow path  40 , as well as a grid-like screen  49  to prevent foreign matter from entering the flow path  40 . The flow forming members include the duct  41  that defines a water suction port  48 , a cylindrical moving blade housing portion that surrounds the impeller  44 , a tubular stationary blade housing portion that surrounds the stationary blade  45 , and a nozzle portion that defines an injection port  47 . 
     The second propulsion unit  15  includes a drive shaft  43  as an element of the transmission unit  100 . The drive shaft  43  is disposed in the front-and-rear direction so as to extend insides and outside of the hull  12 , and transmits rotations of the electric motors M 1  and M 2  to the impeller  44 . The water suction port  48  opens downward at the vessel bottom. The injection port  47  opens rearward behind the water suction port  48 . The flow path  40  connects the water suction port  48  and the injection port  47 . The flow path  40  extends rearward from the water suction port  48  diagonally upward. 
     The impeller  44  includes a plurality of vanes (a moving blade) that are disposed around a rotating shaft line A 1  extending in the front-and-rear direction. Similarly, the stationary blade  45  includes a plurality of vanes that are disposed around the rotating shaft line A 1  behind the impeller  44 . The stationary blade  45  is disposed around the housing  46 . The impeller  44  is connected to the drive shaft  43 . The drive shaft  43  is also a rotating shaft of the impeller  44 . Therefore, the impeller  44  is rotatable around the rotating shaft line A 1  with respect to the flow path  40 . On the other hand, the stationary blade  45  is fixed to the housing  46  and the stationary blade housing portion, and does not rotate with respect to the flow path  40 . 
     The drive shaft  43  is pivotally supported on a shaft support  42  by the duct  41 . The shaft support  42  includes a bearing and a seal. Further, the drive shaft  43  penetrates a through hole  12   b  of the hull  12  in front of the shaft support  42 . The through hole  12   b  includes a seal. Therefore, the drive shaft  43  is rotatable about the rotating shaft line A 1 . 
     The transmission unit  100  includes the drive shaft  43 , the electric motors M 1  and M 2 , drive gears G 1  and G 2 , and driven gears  51  and  52 . The drive gears G 1  and G 2  are connected and fixed to output shafts M 1   a  and M 2   a  of the electric motors M 1  and M 2 , respectively. The driven gears  51  and  52  are driven units corresponding to the electric motors M 1  and M 2 , respectively. The driven gears  51  and  52  are connected to the drive shaft  43  at different locations in a direction of the rotating shaft line A 1  (an axial direction of the rotating shaft of the impeller  44 ). That is, the driven gears  51  and  52  are disposed in series in the rotating shaft line A 1  direction. The driven gears  51  and  52  and the drive shaft  43  rotate integrally. The drive gears G 1  and G 2  as drive units mesh with the driven gears  51  and  52 , respectively. Outputs of the electric motors M 1  and M 2  are transmitted to the driven gears  51  and  52  via the drive gears G 1  and G 2 , respectively. Therefore, the drive shaft  43  is rotationally driven by the electric motors M 1  and M 2 . The maximum outputs of the electric motors M 1  and M 2  according to the standard are equal or substantially equal. 
     The electric motors M 1  and M 2  (the output shafts M 1   a  and M 2   a ) are able to rotate in a forward rotation direction and a reverse rotation direction. When the electric motors M 1  and M 2  rotate in the forward rotation direction (for example, in a clockwise direction when viewed from the rear), the impeller  44  also rotates in the forward rotation direction. As a result, water is sucked into the flow path  40  from the water suction port  48 , and the sucked in water is sent from the impeller  44  to the stationary blade  45 . The stationary blade  45  reduces the torsion of the water flow caused by the rotation of the impeller  44  and regulates the water flow. Then, the rectified water is jetted rearward from the injection port  47 . As a result, a jet of water is formed, and a thrust force in a go forward direction is generated with respect to the hull  12 . On the other hand, when the electric motors M 1  and M 2  rotate in the reverse rotation direction, the impeller  44  also rotates in the reverse rotation direction. Therefore, water is sucked into the flow path  40  from the injection port  47 , and the sucked in water is jetted forward from the water suction port  48  diagonally downward. As a result, a thrust force in a go rearward direction is generated with respect to the hull  12 . As described above, the second propulsion unit  15  is configured so that the direction of the thrust force is able to be changed by switching the rotation direction of the impeller  44 . 
     In such a configuration, the controller  30  controls the electric motors M 1  and M 2  based on the shift position of the output adjusting unit  18  detected by the lever position sensor. The controller  30  determines the rotation directions of the electric motors M 1  and M 2  depending on whether the shift position of the output adjusting unit  18  belongs to the F region or the R region. Further, the controller  30  determines an indicated speed according to the shift position (an operation amount) of the output adjusting unit  18 , and controls rotational speeds of the electric motors M 1  and M 2  by using the inverter  35  and according to the indicated speed. Since the rotational speeds of the electric motors M 1  and M 2  are variable, the output of the second propulsion unit  15  is easily adjusted. The controller  30  uses the inverter  35  to perform synchronous control so that the rotations of the electric motors M 1  and M 2  are synchronized with each other. This is because if the rotational speeds of the electric motors M 1  and M 2  are different, the slower electric motor becomes a resistance to the rotation drive, and the drive efficiency will decrease. Since the outputs of the electric motors M 1  and M 2  are equal or substantially equal to each other, the operation efficiency of each motor is high. 
     Moreover, the electric motor that actually operates among the electric motors M 1  and M 2  may be determined according to the indicated speed. For example, in the case that the indicated speed is equal to or less than a predetermined speed, only one of the electric motors M 1  and M 2  may be operated. In this case, it may be configured that a mechanical connection between the drive gear G 1  and the driven gear  51  and a mechanical connection between the drive gear G 2  and the driven gear  52  is able to be released. Alternatively, it may be configured that even in the case that the electric motors M 1  and M 2  are stopped, the drive gears G 1  and G 2  or the output shafts M 1   a  and M 2   a  are able to idle. In this way, by selectively operating some of the electric motors among the plurality of electric motors, it is easy to adjust the output of the second propulsion unit  15 . 
     According to the first preferred embodiment, since the outputs of the plurality of electric motors M 1  and M 2  are transmitted to the impeller  44  by the transmission unit  100 , it is possible to increase the degree of freedom in the layout by driving the jet pump by the plurality of the motors. For example, it becomes easy to design the drive shaft  43  to be close to the vessel bottom. Generally, a large-sized motor has a high development cost and a high cost of the motor itself. However, in the first preferred embodiment, since the output is obtained by a plurality of small-sized electric motors, it is possible to use versatile and inexpensive motors, and as a result, it is possible to minimize the cost. Further, even in the case that some electric motors break down, it is still possible to operate the jet pump  28 . 
     Further, since the electric motors M 1  and M 2  are disposed in the inside of the hull  12 , it is easy to ensure the waterproofness of the electric motors M 1  and M 2 . 
     Furthermore, the driven gears  51  and  52  are disposed in series at different locations in the rotating shaft line A 1  direction, and correspondingly, the drive gears G 1  and G 2  are also disposed at different locations in the rotating shaft line A 1  direction. As a result, it becomes easy to dispose the plurality of electric motors at different locations in the rotating shaft line A 1  direction, and the layout is further eased. 
     Further, since respective rotational speeds of the plurality of electric motors are controlled based on the indicated speed, it becomes easy to adjust the output of the second propulsion unit  15 . Moreover, in the case of selectively operating some of the electric motors among the plurality of electric motors based on the indicated speed, it also becomes easy to adjust the output of the second propulsion unit  15 . 
     Next, a second preferred embodiment of the present invention will be described.  FIG. 4  is a longitudinal section view of a second propulsion unit  15  according to the second preferred embodiment of the present invention. This second propulsion unit  15  includes a transmission unit  100 - 2 . In the first preferred embodiment, the two driven gears  51  and  52  are disposed at different locations in the rotating shaft line A 1  direction. On the other hand, in the second preferred embodiment, only one driven gear is provided. Further, the drive gears G 1  and G 2  are disposed at the same locations in the rotating shaft line A 1  direction. 
     That is, in the transmission unit  100 - 2 , the drive gears G 1  and G 2  are meshed with one driven gear  51  at different locations in the circumferential direction of the one driven gear  51 . The locations of the electric motors M 1  and M 2  in the rotating shaft line A 1  direction are the same. Therefore, the drive gears G 1  and G 2  are disposed in parallel, and correspondingly, the electric motors M 1  and M 2  are also disposed in parallel. Other configurations and controls are the same as those in the first preferred embodiment. 
     According to the second preferred embodiment, it is possible to obtain the same effects as that of the first preferred embodiment with respect to increasing the degree of freedom in the layout by driving the jet pump by the plurality of the motors. 
     Further, it becomes easy to dispose the plurality of electric motors at a common location in the rotating shaft line A 1  direction, and the layout is further eased. The second preferred embodiment is especially useful when there is insufficient space in the front-and-rear direction. 
     Next, a third preferred embodiment of the present invention will be described.  FIG. 5  is a longitudinal section view of a second propulsion unit  15  according to the third preferred embodiment of the present invention. This second propulsion unit  15  includes a transmission unit  100 - 3 . In the first preferred embodiment and the second preferred embodiment, the electric motors M 1  and M 2  are disposed in the inside of the hull. On the other hand, in the third preferred embodiment, the electric motors M 1  and M 2  are disposed on the outside of the hull. 
     That is, the main portions of the electric motors M 1  and M 2 , and the main portion of the transmission unit  100 - 3  are disposed between the duct  41  and the hull  12  in the accommodation portion  12   a . The electric motor M 1  is fixed to the duct  41  via a stay  53 . Further, the electric motor M 2  is fixed to the duct  41  via a stay  54 . In the accommodation portion  12   a , the drive gears G 1  and G 2  are meshed with one driven gear  51 . The locations of the electric motors M 1  and M 2  in the rotating shaft line A 1  direction are the same. Therefore, the drive gears G 1  and G 2  are disposed in parallel, and correspondingly, the electric motors M 1  and M 2  are also disposed in parallel. Other configurations and controls are the same as those in the first preferred embodiment. 
     Further, electric power and control signals are supplied to the electric motors M 1  and M 2  via a wire  59 . The wire  59  penetrates a through hole  12   c  of the hull  12 . The through hole  12   c  includes a seal. 
     According to the third preferred embodiment, it is possible to obtain the same effects as that of the first preferred embodiment with respect to increasing the degree of freedom in the layout by driving the jet pump by the plurality of the motors. 
     Further, since the electric motors M 1  and M 2  are disposed on the outside of the hull  12 , the electric motors M 1  and M 2  are easily cooled by water. Therefore, it is not necessary to provide a cooling mechanism for the electric motors M 1  and M 2 . 
     Moreover, if a space is provided in the accommodation portion  12   a , similar to the first preferred embodiment, two driven gears may be disposed in series at different locations in the rotating shaft line A 1  direction, and correspondingly, the drive gears G 1  and G 2  may also be disposed at different locations in the rotating shaft line A 1  direction. 
     Next, a fourth preferred embodiment of the present invention will be described.  FIG. 6  is a longitudinal section view of the main portion of a second propulsion unit  15  according to the fourth preferred embodiment of the present invention. This second propulsion unit  15  includes a jet pump  28  and a transmission unit  100 - 4 . In the fourth preferred embodiment, the electric motors M 1  and M 2  are disposed on the outside of the hull. Further, the drive shaft  43  is eliminated, and the transmission unit  100 - 4  is disposed around (mainly, the upper side of) the impeller  44  and the stationary blade  45 . 
     As shown in  FIG. 6 , the jet pump  28  includes a duct  57 , and the duct  57  is fixed to the hull  12 . A rim  58  is disposed within the duct  57 . The rim  58  is supported by the duct  57  via two thrust bearings  55  and two radial bearings  56 . The rim  58  holds the impeller  44  and the stationary blade  45  on the inner circumference thereof. The rim  58  rotates integrally with the impeller  44  about a rotating shaft line corresponding to the rotating shaft line A 1  ( FIG. 3 ). The rim  58  includes a first gear G 3  (the driven unit) on the outer circumference portion. A second gear G 4  is disposed in a gap of the duct  57 . The second gear G 4  is meshed with the first gear G 3 , and is driven by the first gear G 3  to rotate about a rotating shaft line A 3 . The drive gears G 1  and G 2  are meshed with one second gear G 4  at different locations in the circumferential direction of the one second gear G 4 . Respective outputs of the electric motors M 1  and M 2  are transmitted to the first gear G 3  via the drive gears G 1  and G 2 , and the second gear G 4 , and the rim  58  is rotationally driven. 
     According to the fourth preferred embodiment, it is possible to obtain the same effects as that of the first preferred embodiment with respect to increasing the degree of freedom in the layout by driving the jet pump by the plurality of the motors. 
     Further, since the rim  58  is rotationally driven by the electric motors M 1  and M 2 , the impeller  44  on the inner circumference of the rim  58  is rotated, and as a result, the drive shaft becomes unnecessary. 
     Moreover, the drive gears G 1  and G 2  may be directly meshed with the first gear G 3  without providing the second gear G 4 . Therefore, the drive gears G 1  and G 2  may be drive units that directly or indirectly transmit driving forces of the electric motors M 1  and M 2  to the driven unit such as the first gear G 3 . 
     In each of the above-described preferred embodiments, a plurality of electric motors are provided, and may be three or more. Other preferred locations of the plurality of electric motors with respect to a rotation center (the rotating shaft line A 1 ) of the impeller  44  will be described with reference to  FIGS. 7 and 8 . The arrangements shown in  FIGS. 7 and 8  can be applied to any one of the first to fourth preferred embodiments described above. 
       FIGS. 7 and 8  are schematic views that show a spatial relationship between the jet pump  28  and the plurality of electric motors. The driven gear and the drive gear are not shown in  FIGS. 7 and 8 . The output shafts (not shown) of a plurality of electric motors M rotate about their respective rotation centers A 2 . Moreover, in the description of  FIGS. 7 and 8 , the location of each electric motor M in the vertical direction and the horizontal direction is defined with respect to the location of the rotation center A 2 . 
     First, in the preferred embodiment shown in  FIG. 7 , two electric motors M are disposed above a horizontal plane L 1  passing through the rotating shaft line A 1 , and two electric motors M are disposed below the horizontal plane L 1  passing through the rotating shaft line A 1 . In particular, the plurality of electric motors M are disposed at equal or substantially equal intervals around the rotation center (the rotating shaft line A 1 ) of the impeller  44 . As a result, radial forces received by the drive shaft  43  (or the rim  58 ) from the plurality of electric motors M are canceled out and become zero or close to zero. Therefore, the eccentricity of the rotation of the impeller  44  is reduced, and the impeller  44  rotates stably. 
     Moreover, from the viewpoint of stable rotation of the impeller  44 , the arrangement of the plurality of electric motors M is not limited to equal or substantially equal intervals. For example, in the case that the number of the electric motors M is an even number, there may be a plurality of pairs of electric motors M disposed diagonally across the rotation center of the impeller  44 . 
     Next, in the preferred embodiment shown in  FIG. 8 , all of the plurality of electric motors M are closely disposed at locations higher than the rotation center (the rotating shaft line A 1 ) of the impeller  44 . That is, all of the electric motors M are located above the horizontal plane L 1 . This facilitates the design of disposing the jet pump  28  as low as possible. This makes it easier for the jet pump  28  to be immersed in water, which contributes to increasing the propulsion efficiency. 
     Moreover, in the above-described preferred embodiments, the driven gears  51  and  52 , and the first gear G 3  are exemplified as the driven units that rotate integrally with the impeller  44 , and the drive gears G 1  and G 2  are exemplified as the drive units that transmit the outputs of the electric motors to the driven unit. However, the mechanism to transmit the driving forces of the electric motors is not limited to gears, and for example, a belt or the like may be used. 
     Furthermore, in each of the above-described preferred embodiments, the marine vessel  11  is a hybrid type marine vessel provided with the propulsion units  14  and  15 . However, it is not essential to provide the first propulsion unit  14 , and only the second propulsion unit  15  may be provided. For example, as in a modified preferred embodiment shown in  FIG. 9 , the present invention can be applied to a PWC (Personal Watercraft) that has only the second propulsion unit  15 , which uses an electric motor as the power source, without having an engine. Therefore, the present invention can be applied to electric water motorcycles, electric underwater motorcycles, and even kayaks. 
       FIG. 9  is a schematic view of a marine vessel  11  according to the modified preferred embodiment. This marine vessel  11  is a saddle riding type PWC that is equipped with a saddle type seat  62 . The vessel operator sits down and operates a handle  61 . Although the marine vessel  11  includes one second propulsion unit  15 , the marine vessel  11  may include a plurality of the second propulsion units  15 . Moreover, as the second propulsion unit  15 , any one of the above-described first to fourth preferred embodiments may be applied. Further, the present invention can also be applied to a standing riding type PWC as shown in FIG. 19B of Japanese Laid-Open Patent Publication (kokai) No. 2013-107596. 
     Although the present invention has been described in detail based on the preferred embodiments above, the present invention is not limited to these specific preferred embodiments, and various preferred embodiments within the scope of the gist of the present invention are also included in the present invention. Some of the above-described preferred embodiments may be combined as appropriate. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.