3 10 extending from the lower foil member seating surface 2 11 and along the foil axis is formed on the rear end 3 2 portion of the lower foil members 3 upper foil connection protrusion 2 10 described above is placed into the lower foil connection recess 3 10 a plurality of lower foil connection holes 3 9 are formed along the width of the lower foil connection recesses 3 10 cross section of the lower foil connection recesses 3 10 is in the form of a u201c u u201d in accordance with the form of the upper foil connection protrusions 2 10 wherein said cross section can also comprise a u201c dovetail u201d form or an arcuate form such as a semi elliptical form underwater foil shown in fig1 and 15 is formed when upper foil members 2 and lower foil members 3 are secured to each other to achieve this as shown in fig2 the connection protrusion 3 7 of a lower foil member 3 is inserted into the respective upper foil connection recess 2 7 by sliding thereof axially in the meantime the upper foil connection protrusion 2 10 is slid into the lower foil connection recess 3 10.