16 f and a spring 16 h extending between the fixed part 16 d and the moving part 16 e to push the moving part 16 e away from the fixed part 16 d the second locking members 16 c attached to the upper cover 14 are inserted in the guide holes 16 b to join the upper cover 14 to the lower cover 13 then the moving parts 16 e is guided by and moved in the guide holes 16 b to position the upper cover 13 in place on the lower cover 13 and to join the upper cover 14 to the lower cover 13 a gap between the lower cover 13 and the upper cover 14 is sealed by a sealing member 17 the vertical size of the gap can be adjusted by adjusting the respective positions of the support parts 16 d 1 relative to the corresponding moving parts 16 e by turning the nuts 16 g referring to fig1 and 2 a flywheel 18 is mounted on the lower end part of the crankshaft 7 namely the output shaft of the internal combustion engine e and a drive shaft 19 is coupled with the lower end part of the crankshaft 7 the drive shaft 19 driven for rotation by the internal combustion engine e extends downward through the mount case 10 and the extension case 11 into the gear case 12 the drive shaft 19 is interlocked with a propeller shaft 21 by a reversing mechanism 20 held in the gear case 12 the power.