High deflection hydrofoils and swim fins

A method for providing a propulsion hydrofoil, comprising:

• (a) providing said hydrofoil with a blade member connected to a predetermined body, said blade member having an attacking surface, a lee surface, outer side edges, a root portion near said predetermined body and a free end portion spaced from said predetermined body, said blade member having a predetermined length between said root portion and said free end portion, said blade member having a longitudinal midpoint between said root portion and said free end portion, said blade member having a first half blade portion between said root portion and said longitudinal midpoint and a second half portion between said longitudinal midpoint and said free end portion, said blade member having sufficient flexibility to bow between said outer side edges to form a longitudinal channel shaped contour having, said longitudinal channel shaped contour extends from said free end portion toward said root portion to base of said longitudinal channel shaped contour, said base being located a predetermined distance from said predetermined body, said longitudinal channel shaped contour having a predetermined longitudinal dimension between said free end portion and said base;
• (b) providing said first half blade portion of said blade member with sufficient flexibility to experience a predetermined lengthwise deflection from a predetermined neutral orientation to a predetermined reduced lengthwise angle of attack around a transverse axis during use, said transverse axis being located within said first half portion of said blade member;
• (c) providing said blade member with sufficient spring-like tension during said predetermined lengthwise deflection so as to permit said blade member to experience a significantly strong snapping motion from said predetermined lengthwise deflection toward said predetermined neutral position;
• (d) controlling the build up of longitudinally directed compression forces within said blade member sufficiently to permit said predetermined longitudinal dimension of said channel shaped contour to extend over a majority of said predetermined length of said blade member as said channel shaped contour experiences said predetermined lengthwise deflection to said predetermined reduced lengthwise angle of attack during use;
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• (e) providing at least one elongated stiffening member connected to said blade member, said at least one elongated stiffening member having an upper surface notched portion and a lower surface notched portion, said upper surface notched portion having a predetermined upper notched size, said lower surface notched portion having a predetermined lower notched size, said predetermined lower notched size being different than said predetermined upper notched size.