PURPOSE: To improve the cooling property without enlarging a cooling fin or providing a large cooling fan by guiding the pressure air for floating to the cooling fin of an air-cooled engine via a duct.

CONSTITUTION: A skirt 2 is fitted around a hull 1, one end of a cooling duct 3 is opened in the skirt 2, and the other end is fitted to the cooling air intake port 5 of an air-cooled engine 4. The fan 7 in a duct 6 is rotated by the air- cooled engine 4, the pressure air is fed to a cushion chamber 8 from the skirt 2. The pressure air in the skirt 2 is fed to the cooling fin 9 of the air-cooled engine 4 through the cooling air intake port 5 from the cooling duct 3 to cool the engine 4. The cooling property can be improved without enlarging the cooling fin 9 or providing the large cooling fan.