Abstract:
An outboard motor has an engine, a propeller drivable by the engine, and a drive shaft for transmitting a driving force from the engine to the propeller. A casing assembly supports the engine and rotatably supports and accommodates the drive shaft. A cover structure defines at least part of an engine space accommodating therein the engine. The cover structure has a resin cover body comprised of at least first and second cover members detachably connected to one another, and a frame assembly integrally connected to an inner surface of the first cover member for reinforcement threof. The first cover member has an opened flange receiving a seal of the second cover member. The frame assembly extends along a contour of the opened flange.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an outboard motor and, more particularly, an improvement in a cover structure and a drain structure in an engine space of the outboard motor. 
   2. Background Information 
   A body of an outboard motor is conventionally formed from an aluminum alloy. In recent years, however, a demand has grown for a new engine-housing cover made from resin for reduction in weight and cost. A combination of a synthetic resin lower cover divided generally into left and right halves and a synthetic resin upper cover demountably mounted on the lower cover, for example, has been put to practical use. 
   In the cover, the engine space is made as large as possible for easy engine mounting operation and maintenance. Resin molded parts, however, have smaller rigidity as compared to aluminum alloy parts. It is especially required to provide a resin lower-half cover supporting a demountable cover with sufficient rigidity. 
   For securing the rigidity of a resin cover, a cover with a ribbed configuration is presented, for example, in Japanese Patent Laid Open Publication No. HEI-6-234393. This cover structure avoids the influence of sink marks due to ribs on the shell member consisting of resin parts. 
   However, the above cover, whose ribbed configuration needs to be thickened, requires verification by trial that sink marks do not appear on the exterior of the resin cover, taking time to be commercialized and having disadvantages in quantity production and cost. 
   When the ribs of the above ribbed resin cover cooperate with another body part to be fitted thereto to constitute a bottom portion constituting part of a partition wall for the engine space, the existence of webs requires another consideration to the withdrawal direction from the mold or prevents grooves from being made much deep in view of the draft for the ribs, resulting in a limited degree of freedom in design. 
   Thus desired is an outboard motor with an external cover such as an engine cover of a resin cover which has secured rigidity without being made thick, facilitating the production, and having a cost benefit. 
   An outboard motor allowing for maximum prevention of entering of water from the outside into the engine space which houses the engine is also known. This outboard motor is configured to quickly drain water entering from the outside. 
   For drainage, a small opening has been conventionally provided, for example. Provision of such a small opening, however, contrarily leads to entering of water from outside. 
   A “Drain Device for Outboard Motor” having a drain opening for discharging water entered into the engine space, and provided with a check valve for preventing entry of water from the drain opening into the engine space has been presented, for example (Japanese Patent Laid-Open Publication No. SHO-55-83696). 
   This drain device, however, has a cost disadvantage due to the provision of the check valve. A drain portion formed by cutting off part of a resilient sealing member fitted onto a flange provided below the engine has a vertical passage structure with a simple opening shape. Entry of water from the outside is thus prevented only by the shutting-off function of the check valve. 
   In this context, an outboard motor capable of preventing extraneous water from entering the engine space of the outboard motor and of smoothly draining entered extraneous water with a simple structure is desired. 
   SUMMARY OF THE INVENTION 
   According to an aspect of the present invention, there is provided an outboard motor which comprises: an engine; a propeller to be driven by the engine; a drive shaft for transmitting a driving force from the engine to the propeller; casing means for supporting the engine and rotatably housing the drive shaft; and a cover forming at least a part of an engine space housing the engine, which cover comprises: a resin cover body; and a resin frame assembly integrally fixed to inside surfaces of the cover body. 
   Since the cover body of the outboard motor is made from resin and the resin frame assembly is integrally fixed to the inside of the cover body, sink marks conventionally occurring in molding in which ribs are integrally provided to the inside of a resin cover body do not occur, and the need for any measures against sink marks is eliminated, and there is no influence on the appearance of the resin cover body. There is thus provided a resin cover of an outboard motor in good appearance, which uses a cover reinforcement structure for the cover while having no influence on the appearance, resulting in a good appearance of a main part constituting the appearance of the outboard motor. 
   The resin cover body constituting the external cover of the outboard motor has increased rigidity with the resin frame assembly integrally provided on the inside surfaces, securing required rigidity even when an opening or the like for maintenance is made larger. The secured or increased rigidity is realized by integrating the frame assembly to the inside surfaces by welding or the like after the cover is molded, which facilitates production of a resin cover having high rigidity and providing good appearance to an outboard motor, allowing reduction in weight and providing a cost benefit. 
   Preferably, the cover body preferably has holding portions on the inside surfaces for fixing the frame assembly thereto, so as to facilitate the fixing of the resin frame assembly and eliminate any influence on the appearance of the resin cover. 
   In a preferred form, the cover body comprises a lower cover and an upper cover demountably mounted on the lower cover, which lower cover has flanges serving both as seals for the upper cover and as fixing portions for the frame assembly. Thus, the sealing and fixing functions can be performed by a single part, resulting in a simplified structure of fixing portions of the frame assembly and a simplified structure of the cover body provided with the reinforcing frame assembly. 
   Desirably, frame members constituting parts of the frame assembly function as a flange at the bottom periphery of the engine space. A single component can thus have multi-functions, resulting in a simplified structure. 
   Preferably, frame members constituting parts of the frame assembly contact or engage a peripheral portion of a mounting case substantially constituting the bottom of the engine space, thereby constituting the bottom of the engine space and defining the engine space within the exterior cover. Thus, no additional components for separating the engine space are required. The multifunctional components and the contact or engagement of the periphery of the mounting case to or with the frame members allow the separation of the inside of the cover body above and below to define the engine space, resulting in a further simplified structure and facilitated production. 
   It is preferred that the frame assembly provided on the inside of the cover body comprises lateral frame members provided in upper and lower positions and extending in a rearward-and-forward direction, and a plurality of vertical frame members connecting the upper and lower lateral frame members. The rigidity of the frame-assembly is thus increased, resulting in a resin cover structure of high rigidity even with the resin frame assembly integrally fixed to the resin cover. 
   Preferably, the cover has a cover body forming the engine space housing the engine, and drain passages extending from the inside of the cover body to the outside of the cover body; which drain passages have inlets facing the engine space and outlets facing the outside of the engine space; which inlets and outlets are provided in positions distanced vertically and also distanced laterally of the outboard motor. 
   The external water inlets and outlets are thus in a labyrinthine arrangement in which water entering in the direction opposite to the drainage direction, that is, external water entering through the outlets into the drain passages strikes the ceilings of the drain passages because the inlets are not located immediately above the outlets, attenuated in energy by change of direction, and hardly enters the engine space. The simple structure can thus prevent the entering of external water into the engine space of the outboard motor and drain entering external water smoothly. 
   In a preferred form, the cover body is a resin cover and has hollow frame members integrally fixed to the inside surfaces of the cover body after the cover body is molded. The drain passages may be partly formed in hollow portions of the frame members. Making the reinforcing frame members provided on the inside of the resin cover hollow and using the hollow portions as drain passages facilitate the formation of drain passages, allowing the formation of drain passages in the engine space using the reinforcing members also serving as drains. The inlets and outlets may be formed in the tops and bottoms of the hollow frame members in positions shifted in parallel relationships, allowing the formation of drain passages having a simple structure easily produced and good functions. 
   The cover body may be an undercover provided below an engine cover, defining at least a part of the engine space. In other words, the cover of this invention comprises an undercover constituting an external cover of the outboard motor, disposed below an engine cover. The hollow frame members are provided to the undercover, thereby to form the drain passages. Extraneous water entered the engine space is thus discharged from a lower portion of the undercover. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will be described in detail below, by way of example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a side view, partly in cross section, of an outboard motor according to the present invention; 
       FIG. 2  is a plan view of the outboard motor with an undercover and an engine shown in cross section with an engine cover removed; 
       FIG. 3  is an exploded perspective view illustrating the undercover consisting of a left lower cover and a right lower cover and a mounting case; 
       FIG. 4  is a plan view of the left lower cover of  FIG. 3 ; 
       FIG. 5  is a side view as seen from inside of the left lower cover shown in  FIG. 3 ; 
       FIG. 6  is a partial sectional view from the rear of the left lower cover shown in  FIG. 3 ; 
       FIG. 7  is a cross-sectional view taken along line  7 — 7  of  FIG. 5 , illustrating a fixed state of an upper lateral frame member; 
       FIG. 8  is a cross-sectional view taken along line  8 — 8  of  FIG. 5 , illustrating a fixed state of a lower lateral frame member; 
       FIG. 9  is a plan view of the right lower cover of  FIG. 3 ; 
       FIG. 10  is a side view from inside of the right lower cover of  FIG. 3 ; 
       FIG. 11  is a partial sectional view from the rear of the right lower cover shown in  FIG. 3 ; 
       FIG. 12  is a top plan view of the left and right lower covers joined together; 
       FIG. 13  is a perspective view illustrating joining rear portions of the left and right lower covers; 
       FIG. 14  is a plan view illustrating in enlargement the rear of the undercover with the left and right lower covers joined together; 
       FIG. 15  is a perspective view of the rear of the undercover, illustrating mounting a lid for closing an opening; and 
       FIG. 16  is a cross-sectional view illustrating left and right lower lateral frame members provided with drain passages. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , reference character Fr represents forward in a direction of propulsion and Rr rearward in a direction of propulsion. 
   An outboard motor  1  has casing means for supporting an engine and covering means for covering the engine, defining an engine space. 
   An engine  2  used in the outboard motor  1  in this embodiment is a vertical engine with a crank shaft  2   a  vertically oriented, a multi-cylinder engine having a plurality of cylinders  2   b  arranged one above the other with their axis lines horizontally oriented. 
   A piston  2   c  is fitted into each of the cylinders  2   b  of the engine  2 . A cylinder block  2   d  is disposed in a middle region of the outboard motor  1  in rearward and forward directions. 
   At the rear of the cylinder block  2   d , a cylinder head  2   e  is provided, at the rear of which a cylinder head cover  2   f  is provided. A crankcase  2   g  is disposed forward of the cylinder block  2   d.    
   The engine  2  is supported at the bottom on a mounting case  4  via a pump body  3 . An oil case  5  is provided at the underside of the mounting case  4 , hanging down. In other words, the oil case  5  is fixed to the undersurface of the mounting case  4 , extending downward. 
   An exhaust passage  5   b  is extended downward near an oil pan  5   a  of the oil case  5 . The exhaust passage  5   b  is connected to a connecting hole  5   b  formed in the mounting case  4   b  for communication. Reference numeral  4   a  denotes a water jacket. 
   Exhaust air is guided from combustion chambers of the engine  2  through the cylinder head  2   e , an exhaust manifold  2   h  (see FIG.  2 ), an exhaust guide  6 , an exhaust passage  4   b  in the mounting case  4  and the exhaust passage  5   b  to the inside of an extension case  13  to be described below. 
   A suction tube  3   b  extends downward from the undersurface of the pump body  3  into the oil pan  5   a , having a strainer  3   a  at the lower end located in a lower portion inside the oil pan  5   a.    
   The crankshaft  2   a  is located in a forward portion of the entire outboard motor  1 , the lower end of which vertically passes through the pump body  3  and connected to the upper end of a drive shaft  7  provided vertically. 
   The drive shaft  7  passes through a through hole  4   c  provided in a forward portion of the mounting case  4 , passing between the oil pan  5   a  and a peripheral wall  5   c  of the oil case  5  and extending downward for driving an output shaft  9   a  via a transmission mechanism  8 . 
   A propeller  9  is connected to the output shaft  9   a . The engine  2  drives the drive shaft  7  to drive the propeller  9  via the transmission mechanism  8 , thereby to provide the outboard motor  1  with a propulsion power. 
   A covering means  10  constituting a part of the shell member of the outboard motor  1  covers upper, side and lower portions of the engine  2 . The covering means  10  consists of a cover structure having a first cover member comprised of a cap-shaped engine cover  11  oriented downward for covering an upper portion of the engine  2  above a vertically intermediate portion thereof, and a second cover member comprised of an undercover  20  for enclosing a lower portion of the engine  2 , the pump body  3 , the mounting case  4  and the oil case  5 . 
   The undercover  20 , as shown in  FIG. 3 , consists of two cover portions, a left lower cover  21  and a right lower cover  31  as left and right cover members. 
   The engine space  12  is defined by the engine cover  11  and an upper portion of the undercover  20 . In other words, the engine space  12  is formed by an upper portion separated from a lower portion by the mounting case  4  as will be described below. The engine space  12  is located in an upper portion of the outboard motor  1 . The mounting case  4  corresponds to the bottom of the engine space  12  as will be described below. 
   As described above, the oil case  5  is fixed to the underside of the mounting case  4 . The extension case  13  is mounted to the underside of the oil case  5 . The extension case  13  is made from an aluminum alloy. A gear case  14  is connected below the extension case  13 . The gear case  14  houses a lower portion of the drive shaft  7 , the transmission mechanism  8  and the output shaft  9   a.    
   The undercover  20  externally covers the junction between the mounting case  4  and the oil case  5 . A lower portion of the undercover  20  extends downward. 
   The outboard motor  1  swings laterally about a swivel shaft  15   a . The swivel shaft  15   a  is vertically provided between a forward portion of the undercover  20  and a front portion of the extension case  13 . The outboard motor  1  also swings vertically about a tilt shaft  16   a . The tilt shaft  16   a  is provided at a stern bracket  16  connected to a swivel case  15 . 
   The mounting case  4 , oil case  5 , extension case  13  and gear case  14  constitute a casing assembly or casing means for supporting the engine and for rotatably supporting and accommodating the drive shaft. 
     FIG. 2  illustrates, in part in cross section, the undercover  20  and the engine  2  with the engine cover  11  as an upper cover of the outboard motor  1  removed. 
   The undercover  20  consists of a left lower cover  21  and a right lower cover  31  which constitute left and right cover components. The left lower cover  21  and the right lower cover  31  when viewed from top have symmetrical wing-shaped cross sections with generally intermediate portions  21   a  and  31   a  bulging left and right. 
   As shown in  FIG. 3 , front and rear portions  21   b  and  21   c  of an upper half portion  21   g  of the left lower cover  21  and front and rear portions  31   b  and  31   c  of an upper half portion  31   g  of the right lower cover  31  extend in forward and rearward directions of the outboard motor  1 . The front portions  21   b  and  31   b  of the left and right lower covers  21  and  31 , at which the swivel case  15  and the stern bracket  16  are disposed, are hollowed in a rectangular shape toward the rear of the outboard motor  1 . 
   As shown in  FIG. 2 , a cable support bracket  17  is provided between the front of the crankcase  2   g  of the engine  2  and the fronts of the left and right lower covers  21  and  31 . 
   An exhaust passage  2   i  of the cylinder head  2   e  is connected to the exhaust manifold  2   h  for communication. An intake passage  2   j  is provided in a position opposite to the exhaust passage  2   i . An intake manifold connected to the intake passage  2   j  for communication is not shown in the figure. Reference numeral  2   k  denotes a sparking plug. 
     FIG. 3  is an exploded perspective view of the left and right lower covers  21  and  31  and the mounting case  4 . 
   The left and right lower covers  21  and  31  are made from resin such as fiber-glass reinforced plastic (polypropylene). 
   Rear portions  21   d  and  31   d  of the left and right lower covers  21  and  31  have rear edges  21   e  and  31   e  opposed to one another. The rear edges  21   e  and  31   e  are butt-joined as shown in  FIG. 12 , serving as joint edges. 
   Lower half portions  21   f  and  31   f  of the left and right lower covers  21  and  31  are smaller in width in rearward and forward directions of the outboard motor (see  FIG. 1 ) than the upper half portions  21   g  and  31   g . A front portion  21   g ′ of the upper half portion  21   g  of the left lower cover  21  is greater in vertical width than a front portion  31   g ′ of the upper half portion  31   g  of the right lower cover  31 . A front portion of a top portion  21   h  of the upper half portion  21   g  of the left lower cover  21  is flash with the top portion  21   h , constituting a part of the top portion  21   h.    
   The bottom edge of the front portion  31   g ′ of the upper half portion  31   g  is the same in height as the bottom edge of the front portion  21   g ′ of the upper half portion  21   g . However, an upper portion of the front portion  31   g ′ of the upper half portion  31   g  is cut off, so that a top edge  31   g - 1  of the front portion  31   g ′ is lower in height than the top edge of the front portion  21   g′.    
   The left and right lower covers  21  and  31  together form an elliptical tube elongated in rearward and forward directions and vertically split in halves with a front portion hollowed sharply and with a rear portion constricted gently. The rear portions  21   d  and  31   d  of the left and right lower covers  21  and  31  are butt-joined at the rear edges  21   e  and  31   e . At the front, opposite front edges  21   i  and  31   i  of the lower half portions  21   f  and  31   f  are butt-joined. 
   With the left and right lower covers  21  and  31  joined together, the cable support bracket  17  is disposed between the front portions  21   g ′ and  31   g ′ of the upper half portions  21   g  and  31   g.    
   On the front portion  31   g ′ as a protrusion protruded forward of the right lower cover  31 , a separate cover  18  is superimposed for connection. A front end portion of the separate cover  18  is formed with a notch  18   a  of a shape symmetrically corresponding to a notch  31   j  formed in a front end face of the front portion  31   g . The notches  31   j  and  18   g  form a through hole for holding a cable with a grommet not shown. 
   The mounting case  4  has a connecting opening  4   d  for connecting the oil pan  5   a  (see  FIG. 1 ) thereto and a plurality of through holes  4   c.    
   An outside wall of the mounting case  4  is provided at its periphery with a flange  4   e  protruded outward, forming a part of the bottom of the engine space  12 . The flange  4   e  is shaped to have a lower rear portion and a higher front portion, inclined downward from the front portion to the rear portion. 
   A sealing member  19  is fitted onto the flange  4   e  of the mounting case  4 , surrounding the outer periphery. The sealing member  19  contacts or engages part of a reinforcing frame to be described below provided on the inside surfaces of the left and right lower covers  21  and  31 . 
   The flange  4   e  of the mounting case  4  and the frame partly contacting or engaging the flange  4   e  constitute a partition wall for separating the engine space  12 . 
   Now, the configuration of the left lower cover  21  will be described with reference to  FIGS. 4  to  8 . 
   The top portion  21   h  of the left lower cover  21  has a flange  21   h - 1 . The flange  21   h - 1  protrudes inward, formed along the longitudinal direction of the top portion  21   h . The top portion  21   h  is also integrally formed with a rib edge  21   h - 2  protruding upward to engage an inner edge of the cap-shaped engine cover  11  opening downward. 
   Reinforcing frame members  22 ,  23 ,  25 ,  26 ,  27 ,  28  and  29  are integrally fixed to the inside surface of the left lower cover  21 . Each frame member is softer than the left lower cover  21  for moldability and has a small rectangular section, for the formation of which a material of high strength such as polypropylene is selected. 
   To the underside of the flange  21   h - 1  on the inside surface of the left lower cover  21 , the upper lateral frame member  22  is fixed, extending longitudinally as shown in  FIGS. 5 and 7 . 
   The upper lateral frame member  22  is generally horizontally disposed, corresponding to the undersurface of the engine cover  11 . The upper lateral frame member  22  has a hollow tube shape with a hollow cross section as shown in  FIG. 7 , a generally square shape in this embodiment. 
   After the left lower cover  21  is molded, the upper lateral frame member  22  is fixed along the inside surface of the lower cover  21  by welding or the like, integrated with the left lower cover  21 . 
   An exemplary fixing method of the upper lateral frame member  22  will be described with reference to  FIG. 7. A  top surface  22   a  of the upper lateral frame member  22  is made to abut against a bottom surface  21   h - 3  of the top portion  21   h  including the flange  21   h - 1 . 
   Protruding stripes  22   c ,  22   c  are provided at the top and bottom of an outside surface  22   b  of the upper lateral frame member  22 . The protruding stripes  22   c ,  22   c  abut against an inside surface  21   k  of the cover  21 . The protruding stripes  22   c ,  22   c  are vibration-welded to the inside surface below the top portion  21   h.    
   As shown in  FIG. 5 , the lower lateral frame member  23  similar to the upper lateral frame member  22  is also fixed to a vertically intermediate portion of the inside surface of the left lower cover  21  in rearward and forward directions of the outboard motor  1  (see FIG.  1 ). 
   The lower lateral frame member  23  has a rear portion  23   d  lower in level, a front portion  23   f  higher in level, and an intermediate portion  23   e  inclined upward to the front. 
   The lower lateral frame member  23  protrudes inward by a greater amount than the upper lateral frame member  22  as shown in FIG.  4 . Specifically, the intermediate portion  23   e  protrudes inward by a greater amount as compared with the rear portion  23   d , and the front portion  23   f  protrudes by a smaller amount than the rear portion  23   d.    
   It is important in the lower lateral frame member  23  that, as shown in  FIGS. 5 ,  6  and  8 , an inner edge  23   a  is provided with a C-shaped hollow groove  24  opening inward. 
   The hollow groove  24  is formed with a large upward inclination at the front portion  23   f  of the lower lateral frame member  23  while being extended rearward more gently than the rear portion  23   d  of the lower lateral frame member  23 , corresponding to a rear portion of the mounting case  4 . 
   A flange  23   g  projecting inward is formed below the front portion  23   f  of the lower lateral frame member  23 . The flange  23   g  supports a front undersurface of the mounting case  4 . 
   The lower lateral frame member  23  is fixed in the same manner as the upper lateral frame member  22 . As shown in  FIG. 8 , a shelf protrusion  21   m  is provided at a holding portion on the inside surface  21   k  of the left lower cover  21 , on which the lower lateral frame member  23  is placed. Protruding stripes  23   c ,  23   c  are provided on an outside surface  23   b  of the lower lateral frame member  23 . The protruding stripes  23   c ,  23   c  are vibration-welded to the inside surface  21   k.    
   Between the rear portions, intermediate portions and front portions of the upper and lower lateral frame members  22  and  23 , the vertical frame members  25 ,  26 ,  27  and  28  are provided as shown in  FIGS. 3 and 5 . The upper and lower lateral frame members  22  and  23  and the vertical frame members  25 ,  26 ,  27  and  28  constitute a left frame assembly. 
   The vertical frame members  25  to  28  are also fixed to the inside surface  21   k  of the left lower cover  21  by welding or the like, vertically connecting the upper and lower lateral frame members  22  and  23  for an increased frame rigidity. The vertical frame members  25  to  28  have hollow shapes like the upper and lower lateral frame members  22  and  23 . As shown in  FIG. 8 , the vertical frame members  25  to  28  may be partly or at the front configured to communicate with the upper and lower lateral frame members  22  and  23 . The embodiment shown in  FIG. 8  illustrates the lateral frame members  22  and  23  and the vertical frame member  26  configured to communicate with one another. 
     FIG. 8  illustrates a state in which the sealing member  19  fitted onto an engaging rim  4   d  protruded on the periphery of the flange  4   e  of the mounting case  4   e  is fitted into the hollow groove  24  provided in the inner edge  23   a  of the lower lateral frame member  23  to retain the mounting case  4 . 
   As shown in  FIGS. 3 and 5 , a sub frame member  29 , an intermediate lateral frame member, is provided on a rear inner portion of the left lower cover  21 , extending from a generally vertically intermediate portion of the vertical frame member  25  to the rear edge  21   e  of the left lower cover  21 . 
   At the rear ends of the upper lateral frame member  22 , lateral intermediate sub frame member  29  and lower lateral frame member  23 , joint portions  122   a ,  129   a  and  123   a  having connecting holes  122   b ,  129   b  and  123   b  are integrally provided as shown in FIG.  6 . 
   The left lower cover  21  has, at the rear portion  21   c  as shown in  FIG. 6 , a hollow cut portion  21   n  opening inward to the joint edge  21   e  of the upper half portion  21   g . The cut portion  21   n  constitutes, as shown in  FIG. 15 , part of a maintenance opening  50  formed by joining the left lower cover  21  and the right lower cover  31  together. A vertically-elongated bearing surface  21   p  is formed around the periphery of the hollow cut portion  21   n  in a surrounding manner. The bearing surface  21   p  has a plurality of mounting holes  21   q.    
   Now, the configuration of the right lower cover  31  will be described with reference to  FIGS. 9  to  11 . 
   The right lower cover  31  has a structure substantially identical to that of the left lower cover  21  shown in  FIGS. 4  to  8 , and the description will be made only about the components. 
   In those figures, reference numeral  31   h - 1  denotes a flange,  31   h - 2  a rib edge,  32  an upper lateral frame member,  33  a lower lateral frame member,  34  a hollow groove, and  35 ,  36  and  38  vertical frame members. The upper and lower lateral frame members  32  and  33  and the vertical frame members  35 ,  36  and  38  constitute a right frame assembly. Reference numeral  39  denotes a sub frame member which is hollowed like the vertical frame members  35 ,  36  and  38  and is fixed to an inside surface  31   k  of the right lower cover  31  by welding or the like. 
   Reference numeral  31   e  denotes a joint edge to be butt-joined to the joint edge  21   e  (see  FIG. 3 ) of the left lower cover  21 , and  31   i  a joint edge at the front. 
   Reference numerals  132   a ,  139   a  and  133   a  denote joint portions for connecting the left lower cover  21  shown in FIG.  3  and the right lower cover  31  by bolts, configured the same as those of the left lower cover  21 . Reference numerals  132   b ,  139   b  and  133   b  denote mounting holes. Reference numeral  3  in denotes a hollow cut portion for forming the opening  50  shown in  FIG. 15 ,  31   p  a bearing surface, and  31   q  mounting holes. 
     FIG. 12  illustrates the undercover  20  with the left and right lower covers  21  and  31  joined together. In the figure, the cable support bracket  17  is not shown for ease of understanding. 
   The left and right lower covers  21  and  31  are connected by bolts by a method to be described below with the joint edges  21   e  and  31   e  at the rear of the left and right lower covers  21  and  31  butt-joined, the joint edges  21   i  and  31   i  at the front butt-joined, and the joint portions  122   a ,  132   a ,  129   a ,  139   a ,  123   a  and  133   a  superimposed back and front in the embodiment shown in the figure. 
   The left and right lower covers  21  and  31  are connected in that manner to form the undercover  20 . The lower lateral frame members  23  and  33  continuously form a flange in a loop. The hollow grooves  24  and  34  formed in the lower lateral frame members  23  and  33  are also formed in a loop. The sealing member  19  around the periphery of the mounting case  4  (see  FIG. 3 ) is engaged with the hollow grooves  24  and  34 , thereby to form a flange portion around the periphery of the mounting case  4  with the lower lateral frame members  23  and  33 . The lower lateral frame members  23  and  33  and the mounting case  4  form the bottom of the engine space  12 . 
     FIGS. 13 and 14  illustrate joining the rear portions of the left and right lower covers  21  and  31 . 
   To join the rear portions of the left and right lower covers  21  and  31  by butt-joining the joint edges  21   e  and  31   e  of the rear edges of the left and right lower covers  21  and  31 , the joint portions  122   a ,  132   a ,  129   a ,  139   a ,  123   a  and  133   a  extended out at the ends of the upper lateral frame member  22 , upper lateral frame member  32 , sub frame member  29 , sub frame member  39 , lower lateral frame member  23  and lower lateral frame member  33 , respectively, are superimposed back and front with the joint edges  21   e  and  31   e  butt-joined. 
   Bolts  40  are inserted through the respective mounting holes  122   b  and  132   b , mounting holes  129   b  and  139   b  and mounting holes  123   b  and  133   b  of the superimposed joint portions  122   a ,  132   a ,  129   a ,  139   a ,  123   a  and  133   a , so as to fasten and join the connecting portions  122   a  and  132   a , connecting portions  129   a  and  139   a  and connecting portions  123   a  and  133   a  with the bolts  40 . 
   For the bolt joining, nuts, for example, are embedded around the mounting holes  132   b ,  139   b  and  133   b  to screw the bolts  40  into weld nuts. 
     FIG. 15  illustrates mounting a lid  41  for closing the opening  50  to the rear portion of the undercover  20 . 
   On a rear surface  20   a  of the undercover  20 , a vertically-elongated generally-rectangular mounting bearing surface D is formed by joining together the mounting bearing surfaces  21   p  and  31   p  formed on the rear portions of the left and right lower covers  21  and  31 . The joint edges  21   e  and  31   e  are vertically located at the transverse center of the mounting bearing surface D, and three joints A, B and C appear. The mounting holes  21   q  and  31   q  are provided in the mounting bearing surface D. 
   The rectangular opening  50  is formed by the hollow cut portions  21   n  and  31   n  in an upper portion of the mounting bearing surface D. With the opening  50  opened, maintenance such as the inspection, cleaning and replacement of sparking plugs at the engine cylinder head  2   e , for example, is done by inserting tools through the opening  50 . 
   In  FIG. 15 , the lid  41  closes the opening  50  and water-tightly seals the joint edge portion. The lid  41  is made from resin, synthetic rubber, rubber or the like. The lid  41  has a plane in a plate shape covering the mounting bearing surface D from the rear surface. A lower half portion  41   a  is provided at the four corners on the inner surface with a plurality of protrusions  42  to be press-fitted into the mounting holes  21   q  and  31   q  of the mounting bearing surface D for engagement. An upper half  41   b  is provided on the inner surface with a rectangular-frame-shaped sealing portion  44  having around its periphery a sealing groove  44   a  to be press-fitted into the opening  50  to engage the inner peripheral edge of the opening  50 . 
   A hinge  45  is provided between the upper half portion  41   b  and the lower half portion  41   a  of the lid  41 . A plurality of ribs  43  of protruding stripes distanced vertically from one another is provided on the outer surface of the lower half portion  41   a.    
   The lid  41  is retained by press-fitting the protrusions  42  formed on the lower half portion  41   a  into the mounting holes  21   q  and  31   q  of the mounting bearing surface D. The sealing groove  44   a  of the sealing portion  44  engages the inner peripheral edge of the opening  50  by fitting the sealing portion  44  on the upper half portion  41   b  into the opening  50 . 
   Pulling the upper half portion  41   b  of the lid  41  rearward disengages the sealing portion  44  from the opening  50 . The upper half portion  41   b  bends and tilts rearward on the hinge  45 , allowing access to the opening  50 . By contrast, raising the upper half portion  41   b  to press-fit the sealing portion  44  into the opening  50  closes the opening  50 . 
     FIG. 16  illustrates the configurations of drain passages according to the present invention. 
   The embodiment shown in the figure illustrates an example of using the left and right lower lateral frame members  23  and  33  of the hollow frame members as drain passages. 
   Hollow passages  61  and  71  are formed in through the left and right lower lateral frame members  23  and  33  except the joint portions  123   a  and  133   a  (see FIGS.  5  and  10 ), extended to portions below the joint portions  123   a  and  133   a  with opposite ends  23   h  and  33   h  butt-joined. 
   Extraneous water inlets  62  and  72  are respectively formed in top surface portions  23   i  and  33   i  of the left and right lower lateral frame members  23  and  33 . Extraneous water outlets  63  and  73  are respectively formed in bottom surface portions  23   j  and  33   j  of the frame members  23  and  33 . The outlets and inlets  62 ,  63 ,  72 ,  73  communicate with the passages  61  and  71  inside the frame members  23  and  33 . 
   The extraneous water inlets  62  and  72  are formed in positions laterally distanced from the butt-joined ends  23   h  and  33   h . The extraneous water outlets  63  and  73  are formed in positions shifted inward from the upper inlets  62  and  72 , that is, positions in the vicinities of the butt-joined ends  23   h  and  33   h . In short, the upper inlets  62  and  72  and the lower outlets  63  and  73  are formed in distant positions in parallel relationships. 
   In  FIG. 12 , the extraneous water inlets  62  and  72  are shown on the left and right of the joint portions  123   a  and  133   a  of the left and right lower lateral frame members  23  and  33 . In  FIG. 14 , the extraneous water inlets  62  and  72  are shown by solid lines on the left and right of the joint portions  123   a  and  133   a  of the lower lateral frame members  23  and  33 , and the outlets  63  and  73  are shown by broken lines in laterally closer portions. 
   Extraneous water entering the engine space  12  thus comes down along the inner wall of the undercover  20 . 
   The lower lateral frame members  23  and  33  formed on the inside of the undercover  20  located in the engine space  12  are higher at the front (Fr side) and lower at the rear (Rr side) as shown in  FIGS. 5 and 10 . Extraneous water entering the engine space  12  thus comes, even at the forefront, for example, downward as shown by arrow {circle around ( 1 )} and flows rearward along the slope inclined downward to the rear of the lower lateral frame members  23  and  33 . The water finally collects in the direction of the lowest joint portions  123   a  and  133   a  of the lower lateral frame members  23  and  33  as shown by arrow {circle around ( 2 )} and flows from those portions into the drain passages  61  and  71  through the inlets  62  and  72  which are provided in those portions as drainage holes. 
   As shown in  FIG. 2 , the top surfaces of the lower lateral frame members  23  and  33  and the top surface of the flange  4   e  of the mounting case  4  constitute a terrace for guiding extraneous water. As indicated by reference numerals  60  and  70  in  FIGS. 2 ,  12  and  14 , the top surfaces of the lower lateral frame members  23  and  33  have large areas and are inclined downward to the rear as described above. Extraneous water entering the engine space  12  shown in  FIG. 1  is thus effectively collected to the drain passage inlets  62  and  72 . 
   As shown in  FIG. 16 , extraneous water entering the engine space  12  flows downward and rearward of the engine  2  as described above and flows into the drain passage  61  in the frame member  23  through the inlet  62  as shown by arrow {circle around ( 3 )}. 
   The inflow water is discharged downward of the undercover  20  through the outlet  63  displaced in parallel from the inlet  62 . As shown in the figure, the drain passages  61  and  71  including joint portions of the lower lateral frame members  23  and  33  are slightly inclined downward at the laterally (transversely) central portion of the undercover  20  so as to lower the opposite ends  23   h  and  33   h . As a result, the outlets  63  and  73  are inclined in the directions of the outlets  63  and  73 , located in lower levels, so that water flowing through the inlets  62  and  72  into the drain passages  61  and  71  is smoothly discharged. 
   With  FIG. 16 , the water collection and drainage have been described with respect to the right drain passage  61 . The same holds true for the left drain passage  71 . 
   As shown in the left drain passage  71  in  FIG. 16 , extraneous Water entering from below the outboard motor  1  due to waves enters through the outlet  73  opening downward into the drain passage  71  as shown by arrow {circle around ( 5 )}. 
   The water entering the drain passage  71  is prevented from reaching the inlet  72  because the inlet  72  is provided in a position distanced in parallel from the outlet  73 , and strikes a ceiling portion  33   k  of the drain passage  71 . As a result, the power of the inflow water is attenuated. The water entering the passage  71  thus comes down. The extraneous water entering the drain passage  71  through the lower outlet  73  is thus prevented from entering the undercover  20 , that is, the engine space  12  through the inlet  72  opening upward. 
   The description has been made with the figure about the entering of extraneous water through the left outlet  73 . The same holds true for the entering of extraneous water through the right outlet  63 . 
   The present disclosure relates to the subject matters of Japanese Patent Applications No. 2002-209990 and No. 2002-210014, both filed Jul. 18, 2002, the disclosures of which are expressly incorporated herein by reference in their entireties.