US Patent 6926569 Fin with a blade having adjustable closed area

A fin includes a blade, which is lengthwise split into a plurality of sub-blades, so that the sub-blades are separated from one another from an outer end of the blade to a near central point of the blade; and at least one zipper, which is located between two adjacent sub-blades and adapted to zip up for the adjacent sub-blades to provide a closed area. The closed area provided by the adjacent sub-blades is adjustable by zipping up the zipper by different length to thereby regulate a propulsive force that may be produced by the blade of the fin.