Patent Publication Number: US-2021188413-A1

Title: Outboard motor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on Japanese Patent Application No. 2019-230353 filed on Dec. 20, 2019, the contents of which are incorporated herein by way of reference. 
     BACKGROUND 
     The present disclosure relates to an outboard motor including a structure that lubricates a gear connecting shafts and a bearing rotatably supporting the shafts. 
     The outboard motor includes an engine (internal combustion engine, electric motor, and the like), a propeller shaft, a drive shaft that transmits the power of the engine to the propeller shaft, a gear device that connects the drive shaft and the propeller shaft, and a shift device that switches gears of the gear device to switch a rotation direction of the propeller shaft. Further, the propeller shaft is accommodated in a lower case provided at a lower portion of the outboard motor. Further, a shift chamber and a gear chamber are provided in the lower case so as to be adjacent to each other in a front-back direction, the shift device is accommodated in the shift chamber, and the gear device is accommodated in the gear chamber. In addition, in the lower case, a drive shaft insertion hole through which a lower end side portion of the drive shaft extending in an up-down direction from the engine to the gear device is inserted is provided. 
     In the lower case of the outboard motor, an oil circulation mechanism that circulates oil for lubricating and cooling a bearing that rotatably supports the propeller shaft, a bearing that rotatably supports the drive shaft, the gears of the gear device, a clutch of the shift device and the like is provided. That is, the oil for lubricating and cooling the bearings, the gears, the clutch and the like is stored in the gear chamber and the shift chamber. In addition, in the lower case, passages through which the oil flows are separately secured between the gear chamber and a lower end side portion of the drive shaft insertion hole, between an upper end side portion of the drive shaft insertion hole and the shift chamber, between the shift chamber and the gear chamber and the like. When the outboard motor is in operation, basically, the oil circulates through the passages in the order of the gear chamber, the drive shaft insertion hole, the shift chamber, and the gear chamber so as to lubricate and cool the bearings, the gears, the clutch and the like. 
     Patent Document 1 below describes an example of an outboard motor equipped with such an oil circulation mechanism.
     Patent Document 1: JP-A-2018-177166   

     SUMMARY 
     The temperature of the oil rises due to frictional heat generated from the bearings or gears. If the temperature of the oil becomes high, the oil deteriorates prematurely, and therefore, it is necessary to cool the oil. 
     Usually, the oil is cooled by the lower case while circulating in the lower case. However, depending on the type of the outboard motor, such as a large outboard motor or a high-power outboard motor, an amount of heat generated from the bearings or gears may be large, and the temperature of the oil may be greatly increased. For such an outboard motor, it is possible to reduce the temperature rise of the oil by forming a bypass passage of the oil in the lower case and increasing a circulation path of the oil. That is, by increasing the circulation path of the oil, the oil circulation can be improved, and therefore, it is possible to improve the oil cooling efficiency. By increasing the oil cooling efficiency in this way, the oil can be sufficiently cooled even when the temperature of the oil greatly rises. 
     Specifically, a bearing that rotatably supports a front end side portion of the propeller shaft is arranged between the shift chamber and the gear chamber. Normally, the oil in the shift chamber moves into the gear chamber through a gap (the gap between an inner ring and an outer ring of bearing) in the bearing. However, in order to improve the oil cooling efficiency, a flow rate of the oil moving from the shift chamber to the gear chamber is insufficient only by passing the oil through the gap in the bearing. Therefore, a bypass passage is formed in the partition between the shift chamber and the gear chamber, which is located above the bearing, and the oil is passed through both the bypass passage and the gap in the bearing. As a result, the flow rate of the oil moving from the shift chamber to the gear chamber can be increased. 
     However, such a structure has the following problems. 
     An inlet of the bypass passage opens into the shift chamber and an outlet of the bypass passage opens into the gear chamber. The bypass passage is inclined so that the inlet thereof is located higher than the outlet thereof. When the outboard motor is in operation, the oil in the shift chamber flows into the bypass passage and flows by gravity through the bypass passage toward the gear chamber. On the other hand, when the outboard motor is operating, the gears of the gear device rotate in the gear chamber, and the gears stir the oil stored in the gear chamber. A part of the stirred oil flows into the bypass passage from the outlet of the bypass passage and tries to flow backward in the bypass passage against gravity. Alternatively, a part of the stirred oil stays near the outlet of the bypass passage and closes the outlet of the bypass passage. In such a case, the oil flowing in the bypass passage from the shift chamber toward the gear chamber collides with the oil that tries to flow backward in the bypass passage or the oil staying near the outlet of the bypass passage. As a result, the oil may not smoothly flow from the shift chamber to the gear chamber via the bypass passage. That is, even if the bypass passage is formed in the partition between the shift chamber and the gear chamber, which is located above the bearing, in order to improve the oil cooling efficiency, the flow rate of the oil moving from the shift chamber to the gear chamber cannot be sufficiently increased. 
     The present disclosure has been made in view of the above-described problems, and an object of the present disclosure is to provide an outboard motor in which oil can be smoothly moved from a shift chamber to a gear chamber through an oil passage formed between the shift chamber and the gear chamber, such that oil circulation can be improved and oil cooling efficiency can be increased. 
     In order to solve the above problems, according to a first aspect of an outboard motor of the present disclosure, an outboard motor includes an engine; a propeller shaft that is provided below the engine and extends in a front-back direction; a drive shaft that extends in an up-down direction between the engine and the propeller shaft, and is configured to transmit power of the engine to the propeller shaft; a gear device that includes a drive gear configured to rotate with an axis of the drive shaft as a rotation axis, a forward gear configured to rotate with an axis of the propeller shaft as a rotation axis and transmit rotation of the drive shaft to the propeller shaft to rotate the propeller shaft in a positive direction, and a reverse gear configured to rotate with the axis of the propeller shaft as a rotation axis and transmit the rotation of the drive shaft to the propeller shaft to rotate the propeller shaft in a reverse direction; a shift device that is configured to select an either the forward gear or the reverse gear as a gear for transmitting the rotation of the drive gear to the propeller shaft, and set a rotation direction of the propeller shaft; and a case that accommodates the gear device and the shift device. A gear chamber accommodating the gear device, a shift chamber accommodating the shift device, and an oil circulation mechanism configured to circulate oil in the gear chamber and the shift chamber are provided in the case. The oil circulation mechanism includes an oil passage that communicates with the gear chamber and the shift chamber with each other, and through which the oil flows from the shift chamber to the gear chamber. A circular recessed portion includes an opening portion having a circular shape, accommodates the drive gear, and is formed on an upper wall surface of the gear chamber. A rectangular recessed portion includes an opening portion having a rectangular shape, communicates with the circular recessed portion, and is formed on a portion of the upper wall surface of the gear chamber adjacent to the circular recessed portion. The rectangular recessed portion includes a first side wall surface facing a side surface of the circular recessed portion, a second side wall surface located on a reverse side of a rotation direction of the drive gear with respect to the first side wall surface, and a third side wall surface located on a side of the rotation direction of the drive gear with respect to the first side wall surface. An outlet of the oil passage is arranged on the first side wall surface at a position closer to the second side wall surface than to the third side wall surface. 
     According to a second aspect of an outboard motor of the present disclosure, an outboard motor includes an engine; a propeller shaft that is provided below the engine and extends in a front-back direction; a drive shaft that extends in an up-down direction between the engine and the propeller shaft, and is configured to transmit power of the engine to the propeller shaft; a gear device that includes a drive gear configured to rotate with an axis of the drive shaft as a rotation axis, a forward gear configured to rotate with an axis of the propeller shaft as a rotation axis and transmit rotation of the drive shaft to the propeller shaft to rotate the propeller shaft in a positive direction, and a reverse gear configured to rotate with the axis of the propeller shaft as a rotation axis and transmit the rotation of the drive shaft to the propeller shaft to rotate the propeller shaft in a reverse direction; a shift device that is configured to select an either the forward gear or the reverse gear as a gear for transmitting the rotation of the drive gear to the propeller shaft and set a rotation direction of the propeller shaft; and a case that accommodates the gear device and the shift device. A gear chamber accommodating the gear device, a shift chamber accommodating the shift device, and an oil circulation mechanism configured to circulate oil in the gear chamber and the shift chamber are provided in the case. The oil circulation mechanism includes an oil passage that communicates with the gear chamber and the shift chamber with each other, and through which the oil flows from the shift chamber to the gear chamber. A circular recessed portion includes an opening portion having a substantially circular shape, accommodates the drive gear, and is formed on an upper wall surface of the gear chamber. A rectangular recessed portion includes an opening portion having a substantially rectangular shape, communicates with the circular recessed portion, and is formed on a portion of the upper wall surface of the gear chamber adjacent to the circular recessed portion. The rectangular recessed portion includes a first side wall surface facing a side surface of the circular recessed portion, a second side wall surface located on a reverse side of a rotation direction of the drive gear with respect to the first side wall surface, and a third side wall surface located on a side of the rotation direction of the drive gear with respect to the first side wall surface. When the upper wall surface of the gear chamber is viewed from below, assuming that a point at which an edge portion of the circular recessed portion intersects with an edge portion of the second side wall surface is a point P, a circle passing through an outermost peripheral portion of the drive gear is a circle Q, and a tangent to the circle Q passing through the point P and being located on the reverse side of the rotation direction of the drive gear with respect to the point P is a straight line T, at least a part of an outlet of the oil passage is located on the first side wall surface at the reverse side of the rotation direction of the drive gear than the straight line T. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an explanatory view showing an outboard motor according to an embodiment of the present disclosure. 
         FIG. 2  is a sectional view showing a lower unit of the outboard motor according to the embodiment of the present disclosure. 
         FIG. 3  is an enlarged sectional view showing a portion in which a bypass oil passage is formed and a peripheral portion thereof in the lower unit of the outboard motor according to the embodiment of the present disclosure. 
         FIG. 4  is an explanatory view showing a state in which a drive gear and an upper portion of a gear chamber where an outlet of the bypass oil passage is located are viewed from below in the lower unit of the outboard motor according to the embodiment of the present disclosure. 
         FIG. 5A  is an explanatory view showing how difficult it is for oil to hit the outlet of the bypass oil passage in the outboard motor according to the embodiment of the present disclosure. 
         FIG. 5B  is an explanatory view showing how easy it is for oil to hit the outlet of the bypass oil passage in the outboard motor according to a comparative example. 
         FIG. 6  is an explanatory view showing a state in which a drive gear and an upper portion of a gear chamber where an outlet of a bypass oil passage is located are viewed from below in a lower unit of an outboard motor according to another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLIFIED EMBODIMENTS 
     First Embodiment 
     An outboard motor according to a first embodiment of the present disclosure includes an engine (internal combustion engine, electric motor, and the like), a propeller shaft that is provided below the engine and extends in a front-back direction, a drive shaft that extends in an up-down direction between the engine and the propeller shaft and is configured to transmit power of the engine to the propeller shaft, a gear device that is configured to transmit rotation of the drive shaft to the propeller shaft, a shift device that is configured to switch a rotation direction of the propeller shaft, and a case that accommodates the gear device and the shift device. 
     In the outboard motor according to the first embodiment of the present disclosure, the gear device includes a drive gear configured to rotate with an axis of the drive shaft as a rotation axis, a forward gear configured to rotate with an axis of the propeller shaft as a rotation axis and transmit the rotation of the drive shaft to the propeller shaft to rotate the propeller shaft in a positive direction, and a reverse gear configured to rotate with the axis of the propeller shaft as a rotation axis and transmit the rotation of the drive shaft to the propeller shaft to rotate the propeller shaft in a reverse direction. The shift device has, for example, a clutch, and selects, between the forward gear and the reverse gear, a gear that transmits the rotation of the drive gear to the propeller shaft by switching the clutch, so as to set a rotation direction of the propeller shaft. 
     In the outboard motor according to the first embodiment of the present disclosure, a gear chamber accommodating the gear device and a shift chamber accommodating the shift device are provided in the case. Further, for example, the gear chamber and the shift chamber store oil that lubricates and cools the drive gear, the forward gear, the reverse gear, the clutch, a bearing that rotatably supports the propeller shaft, and the like. Further, an oil circulation mechanism for circulating the oil in the gear chamber and the shift chamber is provided in the case. Further, the oil circulation mechanism includes an oil passage that communicates with the gear chamber and the shift chamber with each other and through which the oil flows from the shift chamber to the gear chamber. 
     In the outboard motor according to the first embodiment of the present disclosure, on an upper wall surface of the gear chamber, a circular recessed portion having a substantially circular opening portion and accommodating the drive gear is formed. Further, a rectangular recessed portion having a substantially rectangular opening portion and communicating with the circular recessed portion is formed in a portion of the upper wall surface of the gear chamber adjacent to the circular recessed portion. Further, the rectangular recessed portion includes a first side wall surface facing the circular recessed portion, a second side wall surface located on a reverse side of a rotation direction of the drive gear with respect to the first side wall surface, and a third side wall surface located on a side of the rotation direction of the drive gear with respect to the first side wall surface. Further, both the opening portion of the circular recessed portion and the opening portion of the rectangular recessed portion face the gear chamber. 
     In the outboard motor according to the first embodiment of the present disclosure, an inlet of the oil passage opens into the shift chamber. Further, an outlet of the oil passage opens into the gear chamber. Specifically, the outlet of the oil passage is formed on the first side wall surface of the rectangular recessed portion. Further, the outlet of the oil passage is arranged on the first side wall surface of the rectangular recessed portion at a position closer to the second side wall surface of the rectangular recessed portion than to the third side wall surface of the rectangular recessed portion. Further, for example, the inlet of the oil passage is arranged at a position higher than the outlet of the oil passage, and the oil passage is inclined downward from the inlet to the outlet. The oil flowed into the oil passage moves by gravity toward the outlet of the oil passage. 
     When the outboard motor is in operation, the oil circulates in the gear chamber and the shift chamber via the oil circulation mechanism. During the oil circulation process, the oil moves from the shift chamber to the gear chamber through the oil passage. Further, when the outboard motor is in operation, the drive shaft rotates by driving of the engine, the drive gear rotates accordingly, and the forward gear and the reverse gear rotate accordingly. At this time, when the forward gear is selected as the gear that transmits the rotation of the drive gear to the propeller shaft by the shift device, the propeller shaft rotates in the positive direction. On the other hand, when the reverse gear is selected as the gear that transmits the rotation of the drive gear to the propeller shaft by the shift device, the propeller shaft rotates in the reverse direction. Further, when the outboard motor is in operation, the oil stored in the gear chamber is stirred by the rotating drive gear, forward gear, and reverse gear. A complex flow of the oil is formed in the gear chamber by the stirring of the oil, and the complex flow includes the following forms of flow. 
     That is, the oil stored in the gear chamber is pumped up by teeth of the rotating forward gear and the rotating reverse gear. The pumped oil hits the rotating drive gear. The oil hit the drive gear temporarily accumulates between the teeth of the drive gear, and then is pushed out toward the side of the rotation direction of the drive gear by the teeth of the rotating drive gear substantially along tangents to a circle passing through the outermost peripheral portion of the drive gear. 
     When the direction in which the oil is pushed out toward the side of the rotation direction of the drive gear by the teeth of the rotating drive gear substantially along the tangents to the circle passing through the outermost peripheral portion of the drive gear is referred to as an “oil push-out direction”, the oil pushed out by the teeth of the rotating drive gear easily hits, among the side wall surface of the circular recessed portion around the drive gear, and side wall surfaces of the rectangular recessed portion, portions that are opposed to the oil push-out direction, and does not easily hit portions that are not opposed to the oil push-out direction. 
     Among the side wall surface of the circular recessed portion around the drive gear, and side wall surfaces of the rectangular recessed portion, the portions that are opposed to the oil push-out direction are the side wall surface of the circular recessed portion, the third side wall surface of the rectangular recessed portion, and a portion on the first side wall surface of the rectangular recessed portion which is closer to the third side wall surface than to the second side wall surface. Further, among the side wall surface of the circular recessed portion around the drive gear, and the side wall surfaces of the rectangular recessed portion, the portions that are not opposed to the oil push-out direction are the second side wall surface of the rectangular recessed portion, and a portion on the first side wall surface of the rectangular recessed portion which is closer to the second side wall surface than to the third side wall surface. Therefore, the oil pushed out by the teeth of the rotating drive gear easily hits the side wall surface of the circular recessed portion, the third side wall surface of the rectangular recessed portion, and the portion on the first side wall surface of the rectangular recessed portion which is closer to the third side wall surface than to the second side wall surface. On the other hand, the oil pushed out by the teeth of the rotating drive gear do not easily hit the second side wall surface of the rectangular recessed portion, and the portion on the first side wall surface of the rectangular recessed portion which is closer to the second side wall surface than to the third side wall surface. 
     The outlet of the oil passage is arranged on the first side wall surface of the rectangular recessed portion at a position closer to the second side wall surface than to the third side wall surface. Therefore, the oil pushed out by the teeth of the rotating drive gear does not easily hit the outlet of the oil passage. 
     The outlet of the oil passage is arranged on the first side wall surface of the rectangular recessed portion at a position where the oil pushed out from the teeth of the drive gear does not easily hit, so that the oil pushed out from the drive gear is prevented from flowing into the oil passage from the outlet of the oil passage and trying to flow backward in the oil passage. Further, the oil pushed out from the drive gear is prevented from staying near the outlet of the oil passage. Accordingly, it is possible to prevent the oil that is about to move from the shift chamber to the gear chamber through the oil passage from colliding with the oil that tries to flow backward in the oil passage, and it is possible to prevent the oil that is about to move from the shift chamber to the gear chamber through the oil passage from colliding with the oil staying near the outlet of the oil passage. As a result, the oil smoothly moves from the shift chamber to the gear chamber through the oil passage. By smoothing the movement of the oil in this way, it is possible to improve the circulation of the oil and increase the oil cooling efficiency. 
     Second Embodiment 
     An outboard motor according to a second embodiment of the present disclosure includes an engine, a propeller shaft, a drive shaft, a gear device, a shift device, and a case accommodating the gear device and the shift device, similarly to the outboard motor according to the first embodiment of the present disclosure. Further, the gear device in the outboard motor according to the second embodiment of the present disclosure includes a drive gear, a forward gear, and a reverse gear similarly to the gear device in the outboard motor according to the first embodiment of the present disclosure. Further, the shift device in the outboard motor according to the second embodiment of the present disclosure is configured similarly to the shift device in the outboard motor according to the first embodiment of the present disclosure. 
     In the outboard motor according to the second embodiment of the present disclosure, a gear chamber accommodating the gear device and a shift chamber accommodating the shift device are provided in the case. Further, for example, the gear chamber and the shift chamber store oil that lubricates and cools the drive gear, the forward gear, the reverse gear, the clutch, a bearing that rotatably supports the propeller shaft, and the like. Further, an oil circulation mechanism for circulating the oil in the gear chamber and the shift chamber is provided in the case. Further, the oil circulation mechanism includes an oil passage that communicates with the gear chamber and the shift chamber with each other and through which the oil flows from the shift chamber to the gear chamber. 
     In the outboard motor according to the second embodiment of the present disclosure, a circular recessed portion and a rectangular recessed portion are formed on an upper wall surface of the gear chamber, similarly to the outboard motor according to the first embodiment of the present disclosure. Further, the rectangular recessed portion in the outboard motor according to the second embodiment of the present disclosure has a first side wall surface, a second side wall surface and a third side wall surface, similarly to the rectangular recessed portion in the outboard motor according to the first embodiment of the present disclosure. 
     In the outboard motor according to the second embodiment of the present disclosure, an inlet of the oil passage opens into the shift chamber. Further, an outlet of the oil passage opens into the gear chamber. Specifically, the outlet of the oil passage is formed on the first side wall surface of the rectangular recessed portion. Further, when the upper wall surface of the gear chamber is viewed from below, assuming that a point at which an edge portion of the circular recessed portion intersects with an edge portion of the second side wall surface of the rectangular recessed portion is a point P, a circle passing through an outermost peripheral portion of the drive gear is a circle Q, and a tangent to the circle Q passing through the point P and being located on the reverse side of the rotation direction of the drive gear with respect to the point P is a straight line T, at least a part of the outlet of the bypass oil passage is located on the first side wall surface of the rectangular recessed portion at the reverse side of the rotation direction of the drive gear than the straight line T. Further, for example, the inlet of the oil passage is arranged at a position higher than the outlet of the oil passage, and the oil passage is inclined downward from the inlet to the outlet. The oil flowed into the oil passage moves by gravity toward the outlet of the oil passage. 
     When the outboard motor is in operation, the oil circulates in the gear chamber and shift chamber via the oil circulation mechanism, and during the oil circulation process, the oil moves from the shift chamber to the gear chamber through the oil passage. Further, when the outboard motor is in operation, the drive shaft, the forward gear and the reverse gear rotate, and the propeller shaft rotates in the positive direction or in the reverse direction, as in the case of the outboard motor according to the first embodiment of the present disclosure. Further, when the outboard motor is in operation, the oil stored in the gear chamber is stirred by the rotating drive gear, forward gear, and reverse gear. A complex flow of the oil is formed in the gear chamber by the stirring of the oil, and the complex flow includes the following forms of flow as in the case of the outboard motor according to the first embodiment of the present disclosure. That is, the oil stored in the gear chamber is pumped up by teeth of the rotating forward gear and the rotating reverse gear, and the pumped oil hits the rotating drive gear. The oil hit the drive gear temporarily accumulates between the teeth of the drive gear, and then is pushed out toward the side of the rotation direction of the drive gear by the teeth of the rotating drive gear substantially along tangents to a circle passing through the outermost peripheral portion of the drive gear. 
     When the direction in which the oil is pushed out toward the side of the rotation direction of the drive gear by the teeth of the rotating drive gear substantially along the tangents to the circle passing through the outermost peripheral portion of the drive gear is referred to as an “oil push-out direction”, the oil pushed out by the teeth of the rotating drive gear easily hits, among the side wall surface of the circular recessed portion around the drive gear, and side wall surfaces of the rectangular recessed portion, portions that are opposed to the oil push-out direction, and does not easily hit portions that are not opposed to the oil push-out direction. 
     Among the side wall surface of the circular recessed portion around the drive gear, and the side wall surfaces of the rectangular recessed portion, the portions that are not opposed to the oil push-out direction are the second side wall surface of the rectangular recessed portion, and a portion of the first side wall surface of the rectangular recessed portion that is on the reverse side of the rotation direction of the drive gear than the straight line T. Therefore, the oil pushed out by the teeth of the rotating drive gear does not easily hit the second side wall surface of the rectangular recessed portion, and the portion of the first side wall surface of the rectangular recessed portion that is on the reverse side of the rotation direction of the drive gear than the straight line T. 
     At least a part of the outlet of the bypass oil passage is located on the reverse side of the rotation direction of the drive gear than the straight line T on the first side wall surface of the rectangular recessed portion. Therefore, the oil pushed out by the teeth of the rotating drive gear does not easily hit the outlet of the oil passage. Accordingly, it is possible to prevent the oil pushed out from the drive gear from flowing into the oil passage from the outlet of the oil passage and trying to flow backward in the oil passage. Further, the oil pushed out from the drive gear is prevented from staying near the outlet of the oil passage. Therefore, it is possible to prevent the oil that is about to move from the shift chamber to the gear chamber through the oil passage from colliding with the oil that tries to flow backward in the oil passage, and it is possible to prevent the oil that is about to move from the shift chamber to the gear chamber through the oil passage from colliding with the oil staying near the outlet of the oil passage. As a result, the oil smoothly moves from the shift chamber to the gear chamber through the oil passage. In this way, the circulation of the oil can be improved and the oil cooling efficiency can be improved. 
     An embodiment of an outboard motor according to the present disclosure will be described with reference to  FIGS. 1 to 6 . In the following description, when describing directions of up (U), down (D), front (F), back (B), left (L), and right (R) regarding a structure, an operation, and the like of the outboard motor, arrows drawn in the lower right of  FIGS. 1 to 6  are followed. 
     (Outboard Motor) 
       FIG. 1  shows an outboard motor  1  according to an embodiment of the present disclosure. In  FIG. 1 , an upper unit  2  that forms an upper portion of the outboard motor  1  is provided with, for example, a 4-cycle multi-cylinder engine  11 . The engine  11  is arranged such that an axial center of a crankshaft  12  extends in an up-down direction. The engine  11  is supported by an engine base and is covered with an upper cover  13  and a lower cover  14 . 
     A drive shaft  15  and a middle case  16  that covers the drive shaft  15  are provided in a middle unit  3  that forms an intermediate portion in the up-down direction of the outboard motor  1 . The drive shaft  15  is a shaft that transmits the power of the engine  11  provided at the upper portion of the outboard motor  1  to a propeller shaft  24  provided at the lower portion of the outboard motor  1 . The drive shaft  15  extends in the up-down direction between the engine  11  and the propeller shaft  24 , and an upper end portion of the drive shaft  15  is connected to the crankshaft  12  of the engine  11  via a gear and the like. Further, a lower end portion of the drive shaft  15  is connected to the propeller shaft  24  via a gear device  31 . 
     The middle unit  3  is provided with a clamp bracket  18  and a swivel bracket  19  for supporting the outboard motor  1  on a boat hull  71 . The clamp bracket  18  is fixed to a stern plate  72  of the boat hull  71 , and the swivel bracket  19  is connected to the clamp bracket  18  via a tilt shaft  20 . The swivel bracket  19  also supports a steering shaft. Although the illustration is omitted, the steering shaft extends in the up-down direction, and both end portions thereof are connected to a front portion of the middle unit  3  via a connection bracket, and the swivel bracket  19  rotatably supports the steering shaft. With this structure, the outboard motor  1  can be rotated in the horizontal direction with respect to the boat hull  71 . Further, the swivel bracket  19  can be rotated in the up-down direction with respect to the clamp bracket  18  via the tilt shaft  20 . As a result, the outboard motor  1  can be tilted in the up-down direction with respect to the boat hull  71 . 
       FIG. 2  shows a cross section of the lower unit  4  that forms the lower portion of the outboard motor  1 , taken along a plane extending in the up-down direction and a front-back direction and passing through the center of the lower unit  4  in a left-right direction. In  FIG. 2 , the lower unit  4  is provided with a propeller  23  that forms a propulsive force of the boat hull, the propeller shaft  24  that forms a rotation axis of the propeller  23 , the gear device  31  that transmits the rotation of the drive shaft  15  to the propeller shaft  24 , and a shift device  36  that switches a rotation direction of the propeller shaft  24 . 
     The lower unit  4  is provided with a lower case  41  that accommodates a lower end side portion of the drive shaft  15 , the propeller shaft  24 , the gear device  31 , and the shift device  36 . In the lower case  41 , there are provided a drive shaft insertion portion  42  through which the lower end side portion of the drive shaft  15  is inserted, a propeller shaft insertion portion  43  through which the propeller shaft  24  is inserted, a gear chamber  45  accommodating the gear device  31 , and a shift chamber  46  accommodating a lower end side portion of a shift rod  38  in the shift device  36  and a shift mechanism  39 . The drive shaft insertion portion  42  extends in the up-down direction from an upper portion of the lower case  41  toward the lower side. The propeller shaft insertion portion  43  extends in the front-back direction at a lower portion of the lower case  41 . The gear chamber  45  is located below the drive shaft insertion portion  42 , and is arranged in a front portion of the propeller shaft insertion portion  43 . The shift chamber  46  is arranged in front of the drive shaft insertion portion  42  and the gear chamber  45 . 
     The lower end side portion of the drive shaft  15  is rotatably supported in the drive shaft insertion portion  42  via bearings  25  and  26  provided on an upper end side and a lower end side of the drive shaft insertion portion  42 , respectively. 
     The propeller shaft  24  extends in the front-back direction, and the propeller  23  is connected to a back end portion of the propeller shaft  24 . Further, a front portion of the propeller shaft  24  reaches the inside of the gear chamber  45 . The propeller shaft  24  is rotatably supported in the propeller shaft insertion portion  43  via bearings  27 ,  28  and the like provided in the propeller shaft insertion portion  43 . 
     The gear device  31  has a drive gear  32 , a forward gear  33 , and a reverse gear  34 . The drive gear  32 , the forward gear  33 , and the reverse gear  34  are all bevel gears. In the present embodiment, helical bevel gears are used as these gears, but bevel gears whose teeth extend linearly in a radial direction can also be used as these gears. 
     The drive gear  32  is fixed to the lower end portion of the drive shaft  15 , and is arranged in an upper portion of the gear chamber  45 . The drive gear  32  rotates with the drive shaft  15  with an axis of the drive shaft  15  as a rotation axis. 
     The forward gear  33  is provided on an outer peripheral side of the propeller shaft  24  in a state of being separated from the propeller shaft  24 , and is arranged at a front portion in the gear chamber  45 . The forward gear  33  usually meshes with the drive gear  32 , and rotates with an axis of the propeller shaft  24  as a rotation axis as the drive gear  32  rotates. 
     The reverse gear  34  is provided on the outer peripheral side of the propeller shaft  24  in a state of being separated from the propeller shaft  24 , and is arranged at a back portion in the gear chamber  45 . The reverse gear  34  usually meshes with the drive gear  32 , and rotates with the axis of the propeller shaft  24  as a rotation axis as the drive gear  32  rotates. 
     The shift device  36  selects a gear that transmits the rotation of the drive gear  32  to the propeller shaft  24  among the forward gear  33  and the reverse gear  34  according to the operation of a lever of a remote controller (hereinafter, abbreviated as “remote control”) connected to the outboard motor  1  or a shift lever provided on the outboard motor  1 , so as to set the rotation direction of the propeller shaft  24 . The shift device  36  has a clutch  37 , the shift rod  38 , and the shift mechanism  39 . 
     The clutch  37  is arranged in the gear chamber  45  between the forward gear  33  and the reverse gear  34 , integrally rotates with the propeller shaft  24 , and is slidable in the front-back direction with respect to the propeller shaft  24 . 
     The shift rod  38  is arranged in front of the drive shaft  15  and extends in the up-down direction. When the outboard motor  1  has a remote control, an electric actuator that is remotely controlled by operating a lever of the remote control is provided on an upper portion of the middle unit  3 , and an upper end portion of the shift rod  38  is connected to the electric actuator. When the outboard motor  1  does not have a remote control, the upper end portion of the shift rod  38  is connected to the shift lever provided on the middle unit  3 . Further, a lower end portion of the shift rod  38  is connected to the shift mechanism  39  in the shift chamber  46 . 
     The shift mechanism  39  is a mechanism for sliding the clutch  37  in the front-back direction according to the rotation of the shift rod  38 , and is arranged in a lower portion of the shift chamber  46 . 
     When the lever of the remote control or the shift lever is switched to a forward position, the shift rod  38  accordingly rotates in one direction around the axis thereof, and the shift mechanism  39  accordingly slides the clutch  37  forward. As a result, the clutch  37  is connected to the forward gear  33 , the rotation of the drive shaft  15  is transmitted to the propeller shaft  24  via the forward gear  33  and the clutch  37 , and the propeller shaft  24  rotates in a positive direction. On the other hand, when the lever of the remote control or the shift lever is switched to a reverse position, the shift rod  38  accordingly rotates in the other direction around the axis thereof, and the shift mechanism  39  accordingly slides the clutch  37  backward. As a result, the clutch  37  is connected to the reverse gear  34 , the rotation of the drive shaft  15  is transmitted to the propeller shaft  24  via the reverse gear  34  and the clutch  37 , and the propeller shaft  24  rotates in a reverse direction. When the lever of the remote control or shift lever is switched to a neutral position, the clutch  37  is not connected to either the forward gear  33  or the reverse gear  34 , and the rotation of the drive shaft  15  is not transmitted to the propeller shaft  24 . 
     (Oil Circulation Mechanism) 
     In  FIG. 2 , the lower case  41  is provided with an oil circulation mechanism. The oil circulation mechanism is a mechanism for circulating oil (lubricating oil) in the gear chamber  45 , the drive shaft insertion portion  42 , and the shift chamber  46 . The oil is stored in the gear chamber  45  and the shift chamber  46 , and the oil circulation mechanism supplies the oil to the bearings  25  and  26  that support the drive shaft  15 , the bearings  27  and  28  that support the propeller shaft  24 , the drive gear  32 , the forward gear  33 , the reverse gear  34 , the clutch  37 , and the like so as to lubricate and cool them. 
     The oil circulation mechanism includes a lower oil passage  51  that communicates the gear chamber  45  with a lower end portion of the drive shaft insertion portion  42 , a spiral recessed groove  52  formed on an outer peripheral surface of the lower end side portion of the drive shaft  15 , an upper oil passage  53  that communicates with the drive shaft insertion portion  42  and the shift chamber  46  in the upper portion of the lower case  41 , and a bypass oil passage  54  that communicates with the shift chamber  46  and the gear chamber  45  in the lower portion of the lower case  41 . The bypass oil passage  54  is a specific example of “oil passage” in the claims. 
     The lower oil passage  51  is a recessed groove or a notch formed in a part of an inner peripheral surface of the lower end portion of the drive shaft insertion portion  42 . 
     The lower end side portion of the drive shaft  15  in which the spiral recessed groove  52  is formed functions as a screw pump. 
     The upper oil passage  53  is a hole formed in a wall portion that separates the drive shaft insertion portion  42  and the shift chamber  46  in the upper portion of the lower case  41 . The upper oil passage  53  is configured such that an inlet thereof opening in the drive shaft insertion portion  42  is arranged at a position higher than an outlet thereof opening in the shift chamber  46 , and the upper oil passage  53  is inclined downward from the drive shaft insertion portion  42  toward the shift chamber  46 . 
     The bypass oil passage  54  is a hole formed in a wall portion that separates the shift chamber  46  and the gear chamber  45  in the lower portion of the lower case  41 . The bypass oil passage  54  is configured such that an inlet  54 A thereof opening in the shift chamber  46  is arranged at a position higher than an outlet  54 B thereof opening in the gear chamber  45 , and the bypass oil passage  54  is inclined downward from the shift chamber  46  toward the gear chamber  45  (see  FIG. 3 ). 
     When the outboard motor  1  is in operation, the drive shaft  15  is rotated by the drive of the engine  11 , and the drive gear  32 , the forward gear  33 , and the reverse gear  34  are rotated. When the clutch  37  is connected to the forward gear  33 , the propeller shaft  24  rotates in the positive direction, and when the clutch  37  is connected to the reverse gear  34 , the propeller shaft  24  rotates in the reverse direction. The oil stored in the gear chamber  45  is stirred by the rotation of the drive gear  32 , the forward gear  33 , and the reverse gear  34 . As a result, the drive gear  32 , the forward gear  33 , the reverse gear  34 , the clutch  37 , the bearing  28 , and the like are lubricated and cooled. Further, as the oil in the gear chamber  45  is stirred in this way, a part of the oil flows into the drive shaft insertion portion  42  through the lower oil passage  51 . The bearing  26  is lubricated and cooled by the oil flowing into the drive shaft insertion portion  42 . 
     The oil flowed into the drive shaft insertion portion  42  is carried to an upper side inside the drive shaft insertion portion  42  by the lower end side portion (recessed groove  52 ) of the drive shaft  15  that functions as a screw pump. The oil carried to the upper side in the drive shaft insertion portion  42  moves to an upper end portion in the drive shaft insertion portion  42  through a gap between an outer ring and an inner ring of the bearing  25  and the like. At this time, the bearing  25  is lubricated and cooled. 
     The oil that has moved to the upper end portion of the drive shaft insertion portion  42  flows into the upper oil passage  53 , flows down in the upper oil passage  53  due to gravity, and flows into the shift chamber  46 . The oil that has flowed into the shift chamber  46  flows down to the lower portion inside the shift chamber  46 . The oil that has flowed down to the lower portion inside the shift chamber  46  flows into the gear chamber  45  through a gap between an outer ring and an inner ring of the bearing  27 . At this time, the bearing  27  is lubricated and cooled. 
     A part of the oil that has flowed into the shift chamber  46  flows into the bypass oil passage  54  on the way of flowing down to the lower portion inside the shift chamber  46 , flows down in the bypass oil passage  54  by gravity, and flows into the upper portion of the gear chamber  45 . The drive gear  32  and the forward gear  33  are further lubricated and cooled by the oil that has flowed into the upper portion of the gear chamber  45  through the bypass oil passage  54 . 
     In this way, when the outboard motor  1  is in operation, the oil stored in the gear chamber  45  and the shift chamber  46  is circulated in the order of the gear chamber  45 , the drive shaft insertion portion  42 , the shift chamber  46 , and the gear chamber  45  by the oil circulation mechanism. 
     The oil removes heat from the bearings  25  to  28 , the drive gear  32 , the forward gear  33 , the reverse gear  34 , the clutch  37 , and other members that generate heat, and as a result, the temperature of the oil rises. However, the oil is cooled by the lower case  41  while passing through the upper oil passage  53  and the bypass oil passage  54 , for example. That is, since an outer surface of the lower case  41  is in contact with seawater and the like, the lower case  41  has a low temperature as a whole. When the oil passes through a circulation path of the oil away from the heat-generating members, the heat of the oil moves to the lower case  41 , and the temperature of the oil is lowered. 
     (Structure Near Outlet of Bypass Oil Passage) 
     Here, the structure near the outlet of the bypass oil passage  54  in the oil circulation mechanism of the outboard motor  1  will be described with reference to  FIGS. 3 and 4 .  FIG. 3  is an enlarged view showing a portion in which the bypass oil passage  54  is formed and a peripheral portion thereof in the lower unit  4  in  FIG. 2 .  FIG. 4  shows a state in which the drive gear  32  and the upper portion of the gear chamber  45  where the outlet of the bypass oil passage  54  is located are viewed from below. In  FIG. 4 , the propeller shaft  24 , the bearings  27  and  28 , the forward gear  33 , the reverse gear  34 , and the clutch  37  are removed for convenience of description. In addition, not that since the drive gear  32  and the like are viewed from below the outboard motor  1  in  FIG. 4 , the left and right are reversed as compared to the case where the drive gear  32  and the like is viewed from above the outboard motor  1 . 
     As shown in  FIG. 3 , the gear chamber  45  has an upper wall surface  45 A located on the upper portion of the gear chamber  45  and fronting the inside of the gear chamber  45 . On the upper wall surface  45 A, a circular recessed portion  55  having a shape recessed upward and opened downward is formed. As shown in  FIG. 4 , the circular recessed portion  55  has a substantially circular opening portion, and a back side portion of the drive gear  32  where teeth  32 A are not formed is accommodated in the circular recessed portion  55 . A diameter of the opening portion of the circular recessed portion  55  is larger than a diameter of the drive gear  32 . The center O of the drive gear  32  coincides with the center of the opening portion of the circular recessed portion  55 . In addition, the drive gear  32  is separated from a side wall surface (inner peripheral surface) and an upper wall surface (bottom surface) of the circular recessed portion  55 . 
     As shown in  FIG. 3 , at a portion of the upper wall surface  45 A of the gear chamber  45  adjacent to the circular recessed portion  55 , a rectangular recessed portion  56  having a shape recessed upward and opened downward is formed. The rectangular recessed portion  56  is located in front of the circular recessed portion  55 . Further, the rectangular recessed portion  56  communicates with the circular recessed portion  55 . 
     As shown in  FIG. 4 , the rectangular recessed portion  56  has a substantially rectangular opening portion. Further, the rectangular recessed portion  56  has a front wall surface  56 A as a first side wall surface, a left wall surface  56 B as a second side wall surface, and a right wall surface  56 C as a third side wall surface. Since a back portion of the rectangular recessed portion  56  communicates with the circular recessed portion  55 , the rectangular recessed portion  56  does not have a back wall surface. 
     In the rectangular recessed portion  56 , the front wall surface  56 A faces the circular recessed portion  55 . The drive gear  32  rotates in the direction of arrow K in  FIG. 4  when viewed from below. In this case, the left wall surface  56 B is located on a reverse side of the rotation direction of the drive gear  32  with respect to the front wall surface  56 A, and the right wall surface  56 C is located on a side of the rotation direction of the drive gear  32  with respect to the front wall surface  56 A. 
     The outlet  54 B of the bypass oil passage  54  is formed on the front wall surface  56 A of the rectangular recessed portion  56 . Further, the outlet  54 B of the bypass oil passage  54  is arranged on the front wall surface  56 A of the rectangular recessed portion  56  at a position closer to the left wall surface  56 B than to the right wall surface  56 C. Further, in  FIG. 4 , when the upper wall surface  45 A of the gear chamber  45  is viewed from below, assuming that a point at which an edge portion of the circular recessed portion  55  intersects with an edge portion of the left wall surface  56 B of the rectangular recessed portion  56  is a point P, a circle passing through an outermost peripheral portion of the drive gear  32  is a circle Q, and a tangent to the circle Q passing through the point P and being located on the reverse side of the rotation direction of the drive gear  32  with respect to the point P is a straight line T, at least a part of the outlet  54 B of the bypass oil passage  54  is located on the front wall surface  56 A of the rectangular recessed portion  56  at the reverse side of the rotation direction of the drive gear  32  than the straight line T. In the present embodiment, the edge portion of the circular recessed portion  55  corresponds to a boundary line between the upper wall surface  45 A of the gear chamber  45  and the side wall surface of the circular recessed portion  55 , and the edge portion of the left wall surface  56 B of the rectangular recessed portion  56  corresponds to a boundary line between the upper wall surface  45 A of the gear chamber  45  and the left wall surface  56 B of the rectangular recessed portion  56 . 
     In the left-right direction, the position of the outlet  54 B of the bypass oil passage  54  coincides with the position of the center O of the drive gear  32 . On the other hand, the position of the rectangular recessed portion  56  is displaced rightward with respect to the position of the center O of the drive gear  32 , and the left wall surface  56 B is closer to the outlet  54 B of the bypass oil passage  54  than the right wall surface  56 C. 
     The rectangular recessed portion  56  is a working seat on which a drilling tool such as a drill is installed when performing processing for forming the bypass oil passage  54  in the lower case  41  during manufacturing of the outboard motor  1 . 
     In the embodiment of the present disclosure, the outlet  54 B of the bypass oil passage  54  is arranged at a position closer to the left wall surface  56 B than to the right wall surface  56 C on the front wall surface  56 A of the rectangular recessed portion  56 , or at least a part of the outlet  54 B of the bypass oil passage  54  is located on the reverse side of the rotation direction of the drive gear  32  than the straight line T on the front wall surface  56 A of the rectangular recessed portion  56 . With such a configuration, the oil pushed out by the teeth  32 A of the drive gear  32  rotating in the direction of the arrow K is unlikely to hit the outlet  54 B of the bypass oil passage  54 . 
     This principle will be described with reference to  FIGS. 3, 5A and 5B .  FIG. 5A  schematically shows a state in which the drive gear  32  and the upper portion of the gear chamber  45  where the outlet  54 B of the bypass oil passage  54  is located are viewed from below in the outboard motor  1  according to the embodiment of the present disclosure.  FIG. 5B  schematically shows a state in which the drive gear  32  and the upper portion of the gear chamber  45  where the outlet  54 B of the bypass oil passage  54  is located are viewed from below in an outboard motor according to a comparative example. 
     As compared with the embodiment of the present disclosure shown in  FIG. 5A , in the comparative example shown in  FIG. 5B , a left wall surface  86 B of a rectangular recessed portion  86  is separated from the outlet  54 B of the bypass oil passage  54 , and in the left-right direction, a distance between the outlet  54 B of the bypass oil passage  54  and the left wall surface  86 B is equal to a distance between the outlet  54 B of the bypass oil passage  54  and a right wall surface  86 C. That is, in the outboard motor according to the comparative example, the rectangular recessed portion  86  is expanded leftward, and as a result, the outlet  54 B of the bypass oil passage  54  is located at the center of a front wall surface  86 A of the rectangular recessed portion  86  in the left-right direction. Except for this point, the outboard motor according to the comparative example has the same configuration as the outboard motor  1  according to the embodiment of the present disclosure. 
     When the outboard motor  1  is in operation, the oil in the gear chamber  45  is stirred by the rotation of the drive gear  32 , the forward gear  33 , and the reverse gear  34 . A complex flow of the oil is formed in the gear chamber  45  by the stirring of the oil, and the complex flow includes the following forms of flow. 
     That is, the oil stored in the gear chamber  45  is pumped up by the teeth of the rotating forward gear  33  and reverse gear  34  respectively, as shown by arrow A in  FIG. 3 . The pumped oil hits the rotating drive gear  32 . The oil hit the drive gear  32  temporarily accumulates between the teeth  32 A of the drive gear  32 , and then is pushed out toward the side of the rotation direction of the drive gear  32  as shown by arrow B in  FIGS. 5A and 5B  by the teeth  32 A of the rotating drive gear  32  substantially along tangents to the circle Q passing through the outermost peripheral portion of the drive gear  32 . 
     When the direction in which the oil is pushed out toward the side of the rotation direction of the drive gear  32  by the teeth  32 A of the rotating drive gear  32  substantially along the tangents to the circle Q passing through the outermost peripheral portion of the drive gear  32  is referred to as an “oil push-out direction”, the oil pushed out by the teeth  32 A of the rotating drive gear  32  easily hits, among the side wall surface of the circular recessed portion  55  around the drive gear  32 , and the front wall surface  56 A ( 86 A), the left wall surface  56 B ( 86 B), and the right wall surface  56 C ( 86 C) of the rectangular recessed portion  56  ( 86 ), portions that are opposed to the oil push-out direction, and does not easily hit portions that are not opposed to the oil push-out direction. 
     In  FIGS. 5A and 5B , among the side wall surface of the circular recessed portion  55  around the drive gear  32 , and the front wall surface  56 A ( 86 A), the left wall surface  56 B ( 86 B), and the right wall surface  56 C ( 86 C) of the rectangular recessed portion  56  ( 86 ), the portions that are opposed to the oil push-out direction are hatched. As shown in  FIGS. 5A and 5B , in both the embodiment of the present disclosure and the comparative example, the side wall surface of the circular recessed portion  55  and the right wall surface  56 C ( 86 C) of the rectangular recessed portion  56  ( 86 ) are the portions that are opposed to the oil push-out direction. Therefore, the oil pushed out by the teeth  32 A of the rotating drive gear  32  easily hits the side wall surface of the circular recessed portion  55  and the right wall surface  56 C ( 86 C) of the rectangular recessed portion  56  ( 86 ). On the other hand, in both the example of the present disclosure and the comparative example, the left wall surface  56 B ( 86 B) of the rectangular recessed portion  56  ( 86 ) is the portion that is not opposed to the oil push-out direction. Therefore, the oil pushed out by the teeth  32 A of the rotating drive gear  32  does not easily hit the left wall surface  56 B ( 86 B) of the rectangular recessed portion  56  ( 86 ). 
     As shown in  FIGS. 5A and 5B , in both the embodiment of the present disclosure and the comparative example, a portion of the front wall surface  56 A ( 86 A) of the rectangular recessed portion  56  ( 86 ) that is closer to the right wall surface  56 C ( 86 C) than to the left wall surface  56 B ( 86 B), or a portion of the front wall surface  56 A ( 86 A) that is on the side of the rotation direction of the drive gear  32  than the straight line T is the portion that is opposed to the oil push-out direction. Therefore, the oil pushed out by the teeth  32 A of the rotating drive gear  32  easily hits the portion of the front wall surface  56 A ( 86 A) of the rectangular recessed portion  56  ( 86 ) that is closer to the right wall surface  56 C ( 86 C) than to the left wall surface  56 B ( 86 B), or the portion of the front wall surface  56 A ( 86 A) that is on the side of the rotation direction of the drive gear  32  than the straight line T. On the other hand, in both the embodiment of the present disclosure and the comparative example, a portion of the front wall surface  56 A ( 86 A) of the rectangular recessed portion  56  ( 86 ) that is closer to the left wall surface  56 B ( 86 B) than to the right wall surface  56 C ( 86 C), or a portion of the front wall surface  56 A ( 86 A) that is on the reverse side of the rotation direction of the drive gear  32  than the straight line T is the portion that is not opposed to the oil push-out direction. Therefore, the oil pushed out by the teeth  32 A of the rotating drive gear  32  does not easily hit the portion of the front wall surface  56 A ( 86 A) of the rectangular recessed portion  56  ( 86 ) that is closer to the left wall surface  56 B ( 86 B) than to the right wall surface  56 C ( 86 C), or the portion of the front wall surface  56 A ( 86 A) that is on the reverse side of the rotation direction of the drive gear  32  than the straight line T. 
     In the embodiment of the present disclosure, as shown in  FIG. 5A , the outlet  54 B of the bypass oil passage  54  is arranged at the position closer to the left wall surface  56 B than to the right wall surface  56 C on the front wall surface  56 A of the rectangular recessed portion  56 , or at least a part of the outlet  54 B of the bypass oil passage  54  is located at the portion on the reverse side of the rotation direction of the drive gear  32  than the straight line T on the front wall surface  56 A. Therefore, the oil pushed out by the teeth  32 A of the rotating drive gear  32  does not easily hit the outlet  54 B of the bypass oil passage  54  in the embodiment of the present disclosure. 
     On the other hand, in the comparative example, as shown in  FIG. 5B , the outlet  54 B of the bypass oil passage  54  is arranged at the center of the front wall surface  86 A of the rectangular recessed portion  86  in the left-right direction, or the entire outlet  54 B of the bypass oil passage  54  is located at a portion on the side of the rotation direction of the drive gear  32  than the straight line T on the front wall surface  86 A. Therefore, the oil pushed out by the teeth  32 A of the rotating drive gear  32  easily hits the outlet  54 B of the bypass oil passage  54  in the comparative example. 
     When the outboard motor  1  is in operation, the oil that has flown down into the shift chamber  46  flows into the bypass oil passage  54  and flows down in the bypass oil passage  54  due to gravity in the direction of arrow C in  FIGS. 3, 5A and 5B  toward the gear chamber  45 . In the embodiment of the present disclosure, the oil pushed out by the teeth  32 A of the rotating drive gear  32  does not easily hit the outlet  54 B of the bypass oil passage  54 , so that the stirred oil in the gear chamber  45  is prevented from flowing into the bypass oil passage  54  from the outlet  54 B of the bypass oil passage  54  and trying to flow backward in the bypass oil passage  54  against gravity. Further, the stirred oil in the gear chamber  45  is prevented from staying near the outlet  54 B of the bypass oil passage  54 . Therefore, it is possible to prevent the oil flowing in the bypass oil passage  54  from the shift chamber  46  toward the gear chamber  45  from colliding with the oil that tries to flow backward from the gear chamber  45  toward the shift chamber  46  in the bypass oil passage  54 , and it is possible to prevent the oil flowing in the bypass oil passage  54  from the shift chamber  46  toward the gear chamber  45  from colliding with the oil staying near the outlet  54 B of the bypass oil passage  54 . As a result, the oil can be smoothly moved from the shift chamber  46  to the gear chamber  45  via the bypass oil passage  54 . 
     On the other hand, in the comparative example, the oil pushed out by the teeth  32 A of the rotating drive gear  32  easily hits the outlet  54 B of the bypass oil passage  54 . For this reason, the phenomenon that the stirred oil in the gear chamber  45  flows into the bypass oil passage  54  from the outlet  54 B of the bypass oil passage  54  and tries to flow backward in the bypass oil passage  54  against gravity, or the phenomenon that the stirred oil in the gear chamber  45  stays near the outlet  54 B of the bypass oil passage  54  is likely to occur. When such a phenomenon occurs, the oil flowing in the bypass oil passage  54  from the shift chamber  46  toward the gear chamber  45  frequently collides with the oil that tries to flow backward from the gear chamber  45  toward the shift chamber  46  in the bypass oil passage  54 , or with the oil staying near the outlet  54 B of the bypass oil passage  54 . As a result, the oil cannot be smoothly moved from the shift chamber  46  to the gear chamber  45  via the bypass oil passage  54 . 
     As described above, according to the outboard motor  1  according to the embodiment of the present disclosure, the outlet  54 B of the bypass oil passage  54  is arranged at the position closer to the left wall surface  56 B than to the right wall surface  56 C on the front wall surface  56 A of the rectangular recessed portion  56 , or at least a part of the outlet  54 B of the bypass oil passage  54  is located at the portion on the reverse side of the rotation direction of the drive gear  32  than the straight line T on the front wall surface  56 A of the rectangular recessed portion  56 , so that the oil can be smoothly moved from the shift chamber  46  to the gear chamber  45  via the bypass oil passage  54  when the outboard motor  1  is in operation. Accordingly, the circulation of the oil can be improved, the oil cooling efficiency can be enhanced, and the life of the oil can be prolonged. Further, by improving the circulation of the oil, the lubricating effect of the oil on the bearings  25  to  28 , the drive gear  32 , the forward gear  33 , the reverse gear  34 , the clutch  37 , and the like can be enhanced. Further, by enhancing the oil cooling efficiency, the cooling effect of the oil on the bearings  25  to  28 , the drive gear  32 , the forward gear  33 , the reverse gear  34 , the clutch  37 , and the like can be enhanced. Further, according to the outboard motor  1  of the embodiment of the present disclosure, the oil can be smoothly moved from the shift chamber  46  to the gear chamber  45  without adding a pump and the like for pumping the oil from the shift chamber  46  to the gear chamber  45 . Therefore, it is possible to improve the oil cooling efficiency without increasing the number of parts or increasing the weight or cost of the outboard motor  1  due to the increase in parts. 
     In the outboard motor  1  according to the embodiment of the present disclosure, it is preferable that half or more of the outlet  54 B of the bypass oil passage  54  is located on the reverse side of the rotation direction of the drive gear  32  than the straight line T on the front wall surface  56 A of the rectangular recessed portion  56 . Accordingly, it becomes more difficult for the oil pushed out by the teeth  32 A of the rotating drive gear  32  to hit the outlet  54 B of the bypass oil passage  54 , so that the smoothness of the movement of the oil from the shift chamber  46  to the gear chamber  45  via the bypass oil passage  54  can be reliably enhanced. 
     In the outboard motor  1  according to the embodiment of the present disclosure, the outlet  54 B of the bypass oil passage  54  and the rectangular recessed portion  56  are arranged above the portion where the drive gear  32  and the forward gear  33  mesh with each other. With this configuration, the bypass oil passage  54  can be shortened, and the oil can be moved more smoothly from the shift chamber  46  to the gear chamber  45  via the bypass oil passage  54 . 
     In the outboard motor  1  according to the embodiment of the present disclosure, the rectangular recessed portion  56  is a working seat for the processing of forming the bypass oil passage  54 . That is, in the outboard motor  1  according to the embodiment of the present disclosure, by using the working seat for the processing of forming the bypass oil passage  54 , the smooth movement of the oil from the shift chamber  46  to the gear chamber  45  via the bypass oil passage  54  is realized. Therefore, according to the embodiment of the present disclosure, the outboard motor  1  having excellent oil cooling performance and the like can be manufactured without increasing the number of manufacturing steps as compared with an outboard motor in related art. 
     In the above embodiment, as shown in  FIG. 4 , an example is given in which the position of the outlet  54 B of the bypass oil passage  54  coincides with the position of the center O of the drive gear  32  in the left-right direction, and the position of the rectangular recessed portion  56  is displaced rightward with respect to the position of the center O of the drive gear  32 . However, the present disclosure is not limited to this. For example, as shown in  FIG. 6 , in the left-right direction, a position of a rectangular recessed portion  66  may coincide with the position of the center O of the drive gear  32 , a position of an outlet  64 B of a bypass oil passage  64  may be displaced leftward with respect to the position of the center O of the drive gear  32 , and the outlet  64 B of the bypass oil passage  64  may be arranged on a front wall surface  66 A of the rectangular recessed portion  66  at a position closer to a left wall surface  66 B than to a right wall surface  66 C. 
     In the above embodiment, the case where the drive gear  32  rotates in the direction of arrow K in  FIG. 4  is described as an example, but the rotation direction of the drive gear  32  may be a direction reverse to the direction of arrow K. In this case, the outlet  54 B of the bypass oil passage  54  is arranged on the front wall surface  56 A of the rectangular recessed portion  56  at a position closer to the right wall surface  56 C than to the left wall surface  56 B. 
     In the above embodiment, the oil circulation mechanism having the lower oil passage  51 , the recessed groove  52 , the upper oil passage  53 , and the bypass oil passage  54  is described as an example, but the number, arrangement and shape of the passages other than the bypass oil passage  54  are not limited in the oil circulation mechanism. Also, with respect to the bypass oil passage  54 , a plurality of bypass oil passages  54  may be provided, and the outlet  54 B of the bypass oil passage  54  and the rectangular recessed portion  56  may be arranged at a position that is not in front of the circular recessed portion  55 . 
     In the above embodiment, the case where the rectangular recessed portion  56  is a working seat is described as an example, but the rectangular recessed portion  56  may not be a working seat. 
     The present disclosure can be modified as appropriate without departing from the scope or spirit of the disclosure which can be read from the claims and the entire specification, and the outboard motor accompanying such a change is also included in the technical concept of the present disclosure.