PURPOSE: To improve propeller efficiency by forming a cover with many adjacent segments kept in contact with each other, forming each segment into a blade section having a tip and a tail edge and connected with inner and outer surfaces, and adapting the theory of the blade section.

CONSTITUTION: An inside surface 8 and an outside surface 9 are connected to form a horizontally flat segment 3 having a NASA cross section LS 1 with a blade section 7 continuously bent in the longitudinal direction, and many segments 3 are connected to form a circular cover 13. Blades 10 of a propeller are located at the narrowest inner diameter of the cover 13, and their tips are made compatible with the inside surface 8. When the blade section theory is adapted to the design of a nozzle cross section, nozzles 13 have a high lift coefficient and a low resistance coefficient suitable for turbulence, and propulsion efficiency is improved.