Foil

A hydrofoil for use submerged in a liquid medium, where relative motion between every point of the hydrofoil and the liquid medium produces a flow across the hydrofoil and defines a direction of undisturbed flow in every point of said hydrofoil, said hydrofoil extending from a base to a tip and having a leading edge and a trailing edge, wherein said flow is in a direction from said leading edge to said trailing edge, and said hydrofoil having a pressure side and a suction side, wherein said flow around both said sides generates a pressure difference between said sides and produces a lift force perpendicular to the direction of said undisturbed flow and directed from said pressure side to said suction side;

• said hydrofoil having a main line extending from said base to said tip, wherein said main line is the geometrical locus of centres of effort of sections of said hydrofoil by sectioning surfaces and said main line lies on a surface created by all mean lines of said sections and locations of maximum thickness of said sections define a maximum thickness line, wherein each of said sectioning surfaces passes through one endpoint on said leading edge and through one endpoint on said trailing edge, and said sectioning surfaces belong to a class of surfaces which includes planes used when said hydrofoil is engaged in a progressive motion and which includes rotational surfaces meant as surfaces of revolution described by said hydrofoil, said rotational surfaces being used whenever said hydrofoil is engaged in a rotational motion, said sectioning surfaces being shaped such that said sections are lead between said endpoints approximately in line with streamlines of said flow;
• said hydrofoil defining a chord surface formed by chords of said sections, wherein each chord joins said endpoints along said sectioning surface to define a chord length;
• said hydrofoil having an angle .alpha., said hydrofoil having at least one of an angle .beta. and an angle .gamma., and said hydrofoil having an angle .delta., said angles .alpha., .beta., .gamma. and .delta. being respectively angles of sweep of said leading edge, said main line, said maximum thickness line and said trailing edge, where each of said angles of sweep is measured in relation to a direction perpendicular to the direction of said undisturbed flow and tangent to said chord surface of said hydrofoil at that point of said chord surface which is nearest to that point of one of said edges and said lines in which a corresponding one of said angles of sweep is measured, each of said angles of sweep being determined in at least one point of the corresponding one of said lines and said edges, wherein each of said angles of sweep has a positive value for those locations where the corresponding one of said lines and said edges is swept forward;
• said hydrofoil, whenever said hydrofoil belongs to one of groups which include round ended hydrofoils and sharp ended hydrofoils which groups both characterize by said hydrofoil having a single point on said hydrofoil tip being furthermost from said hydrofoil base, for said hydrofoil belonging to one of groups which include round ended hydrofoils and sharp ended hydrofoils being defined an angle .epsilon. which is the absolute value of a difference between said angle .alpha. and said angle .delta., which whenever said hydrofoil is sharp ended are for the purpose of determining said angle .epsilon. measured at said hydrofoil tip, and whenever said hydrofoil is round ended said angle .epsilon. is measured between the tangents to said leading edge and to said trailing edge at points which are 7% of the length of said hydrofoil distant from that point of said hydrofoil tip, which is furthermost from said hydrofoil base;
• this invention comprising said hydrofoil being orientable to the direction of said undisturbed flow such that;
• a:
• in at least one point along said leading edge said angle .alpha. is positive and greater than or equal to 10.degree.;
• b:
• and in at least one point along said hydrofoil, at least one of said angle .beta. and said angle .gamma. is positive and greater than or equal to 14.degree.;
• c:
• and the chord length at the hydrofoil tip is not greater than 40% of a maximum chord length of said hydrofoil;
• d:
• and whenever said hydrofoil belongs to one of groups which include round ended hydrofoils and sharp ended hydrofoils, said angle .epsilon. is smaller than the difference of 90.degree. minus an angle .alpha..sub.t, said angle .alpha..sub.t having the value of angle .alpha.at the hydrofoil tip measured at that point of said leading edge which is used to determine said angle .epsilon..