Abstract:
The present invention relates to a surfboard fin system which includes a generally horizontal hydrofoil intersecting or connecting with one or more vertical fins. The generally horizontal hydrofoil of the invention is designed using principles of hydrodynamics, that is, the forces generated by the flow of water over a hydrofoil or wing, to create a force that pulls downward on the fin system and, in turn, downward on the tail end of the water planing hull or surfboard.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention is related to water planing hulls, such as surfboards, and the design of hydrodynamic fins to stabilize the performance of the planing hull as it moves through the water.  
       BACKGROUND OF THE INVENTION  
       [0002]     Surfboards may generally be described as water planing hulls and are constructed of various lengths, width, shapes and thicknesses. Surfboards have a forward tip which is connected by generally symmetrically shaped sides to the rearward tail. Surfboards currently available in the marketplace typically have a large vertical fin located underneath and near the tail of the surfboard. The large fin is usually located on the center line of the surfboard. Some surfboards have additional smaller fins, which are offset at equal distances from the center line. Current fin designs may result in undesirable horizontal or lateral stability of the entire surfboard. Moreover, fin designs typically do not affect the vertical or pitch stability of the surfboard. Thus, the desire for enhanced stability is an objective of surfboard designers.  
       SUMMARY OF THE INVENTION  
       [0003]     Briefly, the present invention relates to a fin which includes a generally horizontal hydrofoil intersecting or connecting with a typical vertical fin. The generally horizontal hydrofoil of the invention is designed using principles of hydrodynamics, that is, the forces generated by the flow of water over a hydrofoil or wing, to create a force that pulls downward on the fin system and, in turn, downward on the tail end of the water planing hull or surfboard. The effect of the downward force on the tail is counterbalanced by an upward force on the planing hull forward of the tail. The upward force acts to stabilize the planing hull by the effects of the positive upward force of the lifting or rising of the middle and forward section of the planing hull.  
         [0004]     The improved fin design results in a surfboard with a forward section that will rise higher out of the water due to the tail being pulled downwardly from the surface of the water by the negative force generated by hydrodynamic flow over the hydrofoil. The negative force is generated by the generally inverted wing foil shape of the hydrofoil. Thus a generally vertical fin provides lateral stability and directional control, and generally horizontal sections or vector components thereof on fins or foils attached to the vertical section provide vertical stability and negative lift. For purpose of the invention “horizontal” describes the hydrofoil attached to the vertical section or fin, and does not require that the foil be limited to any particular angle as compared to the vertical section or fin. Thus, the foil may include or generate both horizontal as well as vertical vector forces on the generally horizontal board.  
         [0005]     Thus, it is an object of the invention to provide a surfboard construction having enhanced stability.  
         [0006]     Another object of the invention is to provide a surfboard construction which employs stabilizing fin elements or foils on the underside of the surfboard.  
         [0007]     A further feature of the invention is to provide a surfboard construction which employs a stabilizing fin construction which provides a downward vector force on the board.  
         [0008]     These and other objects, advantages and features will be set forth in the detailed description which follows.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     In the detailed description which follows, reference will be made to the drawing comprised of the following figures:  
         [0010]      FIG. 1  is a side elevation view an embodiment of the fin system of the invention attached to the tail portion of a surfboard;  
         [0011]      FIG. 2  is a side elevation view of the fin or foil construction of  FIG. 1 ;  
         [0012]      FIG. 3  is a cross-section of the hydrofoil of the fin system of  FIG. 1 ;  
         [0013]      FIG. 4  is a top plan view of the fin system of  FIG. 1  depicting a rectangular shaped hydrofoil;  
         [0014]      FIG. 5  is a top plan view of a fin system having two vertical fins and a rectangular-shaped hydrofoil;  
         [0015]      FIG. 6  is a top plan view of a fin system with two generally parallel vertical fins, a rectangular-shaped center hydrofoil section and two tapered rearward-swept hydrofoil sections;  
         [0016]      FIG. 7  is a top plan view of a fin system with a tapered, rearward-swept hydrofoil;  
         [0017]      FIG. 8  is a top view of a fin system, with a tapered, forward-swept hydrofoil;  
         [0018]      FIG. 9  is a top plan view of a fin system with a rectangular-shaped, rearward-swept hydrofoil;  
         [0019]      FIG. 10  is a front plan view of the fin system of  FIG. 1 ;  
         [0020]      FIG. 11  is a front plan view of a fin system with the hydrofoil attached to the vertical fin with a dihedral angle;  
         [0021]      FIG. 12  is a front plan view of a fin system with winglets;  
         [0022]      FIG. 13  is a front view of a fin system with a hydrofoil attached to the vertical fin with an anhedral angle and with winglets on the ends of the hydrofoils;  
         [0023]      FIG. 14  is a front plan view of a fin system with a hydrofoil attached to the vertical fin with a dihedral angle and with winglets on the ends of the hydrofoils;  
         [0024]      FIG. 15  is a front plan view of a fin system with two vertical fins and a rectangular-shaped hydrofoil; and  
         [0025]      FIG. 16  is an isometric view of the fin system of  FIG. 1 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     A surfboard with a fin system of the invention typically includes at least one vertical fin  21 , to provide lateral stability and directional control, and a horizontal section or foil  22  attached to the vertical fin  21  to provided vertical stability and negative lift. Vertical stability and negative lift are generated by the inverted wing shape of the horizontal fin or hydrofoil. The term “horizontal” is intended to broadly describe a hydrofoil  22  attached to the vertical fin, and does not require that the foil be limited to any particular angle as compared to the vertical fin  21  or the plane of the surfboard. Moreover, the generally horizontal fin or fin construction which provides which provides negative lift, need not be attached to a vertical fin. For example, the negative lift fin constructs described may be positioned on the underside of a surfboard by means of a rod or post independent from a vertical fin.  
         [0027]     The vertical fin  21  may be of any shape, and generally has a proximal end  24  attachable to a surfboard and a distal end  23  defining a vertical fin tip  23 . In addition to the inverted wing shape, the hydrofoil typically has an arcuate leading edge  31 , an upper or first surface  33  nearer to the proximal or surfboard attachment end  24  of the vertical fin  21 , a lower or second surface  32  nearer to the distal end or vertical fin tip  23 , and a trailing edge  34 . The hydrofoil has a root end  41 , which is the end that is attached to the vertical fin  21 , and a tip end  42  opposite the root end  41 .  
         [0028]     The inverted wing shape is most easily determined by examining the cross-sectional profile of the hydrofoil such as depicted in  FIG. 3 . A chord line  35  of the inverted wing or foil  22  is the straight line between the leading edge  31  and the trailing edge  34  of the foil  22 . The mean camber line  36  is the mid-point of the thickness  37  of the hydrofoil  22  at an infinite number of points moving across the foil  22  from the leading edge  31  to the trailing edge  34 , parallel to the root end of the foil  22 . The camber  38  of the foil  22  is greatest distance between the chord line and the mean camber line. In the inverted foil  22  in cross-section, the distance from the lower surface  32  to the mean camber line  36  is predominately less than the distance from the lower surface  32  to the chord line  35 . At some points in the cross-section of the inverted foil  22 , the distance from the lower surface to the mean camber line may be equal to, or in some designs, greater than the distance from the lower surface to the chord line  35 . However, overall, the distance from the lower surface  32  to the mean camber line  36  is less than the distance from the lower surface  32  to the chord line  35  in order to provide negative lift.  
         [0029]     The foil  22  typically is characterized by an overall distance from the lower surface  32  to the mean camber line  36 , which is less than the overall distance from the lower surface  32  to the chord line  35 , and thus will have a lower surface area that is greater than the upper surface area. As the foil  22  is passed through the water, there is typically laminar flow over the lower  32  and upper  33  surfaces of the hydrofoil. The longer path of water traveling over the lower surface  32 , as compared to the upper surface  33 , creates what can be described as “negative lift.” The negative lift pulls the hydrofoil  22  and thus the surfboard  12  downward, toward the tip of the vertical fin  21 . Because the hydrofoil  22  is attached to the vertical fin  21  at the root end  41  of the hydrofoil  22 , the downward force is transferred through the vertical fin  21  to the surfboard  12 . The transfer of the downward force through the vertical fin  21  causes the tail of the surfboard  12  to be pulled downward as the surfboard  12  moves forward in the water.  
         [0030]     The inverted wing shape of the hydrofoil  22  generates downward force as the hydrofoil  22  is moved through the water at an angle of attack that is generally parallel to the forward movement of the surfboard. That is, because of the shape of the hydrofoil  22 , the foil  22  need not be angled upward, from leading edge  31  to trailing edge  34 , to generate the downward force. Although not required to generate the downward force, the angle of attack of the hydrofoil  22  may be adjusted to supplement or negate in part the downward force generated by the shape of the hydrofoil  22  itself.  
         [0031]     Utilizing the inverted wing cross-sectional profile, the hydrofoil  22  may be formed in many different shapes. The hydrofoil  22  may have a relatively constant chord length  35  from the root end  41  to the tip end  42 , forming a rectangular shape when viewed from above [ FIG. 4 ]. In such an embodiment, there may be slight variations at the root end  41 , as the hydrofoil  22  is flared for attachment to the vertical fin  21 , and at the tip end  42 , as the corners are rounded. However, over the majority of the foil, the chord length  35  remains the same. Alternatively, the foil  22  may be tapered from root end  41  to tip end  42  [ FIG. 7 ]. That is, the chord length  35  is reduced or increased or varied in whole or in part from root end  41  to tip end  42 . Other configurations may have the foil swept forward in relation to the forward movement of the fin through the water [ FIG. 8 ]. In the embodiment of  FIG. 8 , the leading edge  31  of the foil  22  at the tip end  42  is forward of the leading edge  31  of the foil  22  at the root end  41 . Conversely, the foil  22  may be swept rearward, with the leading edge  31  of the foil  22  at the root edge  41  forward of the leading edge  31  of the foil  22  at the tip  42 , as the fin system moves through the water [ FIGS. 7 and 9 ]. In either forward-swept or rearward swept embodiment, the foil  22  may be tapered or have a constant chord length.  
         [0032]     In addition to the being swept forward and rearward, tapered or rectangular, the sections of the hydrofoil  22  on each side of the vertical fin  21  may be angled. If the hydrofoils  22  are angled upward from root end  41  to tip end  42  symmetrically on either side of the vertical fin  21 , they are configured with a dihedral angle  61  [ FIGS. 11 and 14 ]. Conversely, if the hydrofoils  22  are angled downward from root end  41  to tip end  42  symmetrically on either side of the vertical fin  21 , they are configured with an anhedral angle  62  [ FIG. 13 ]. The hydrofoils  22  may be attached to the vertical fin  21  at any point along the entire length and depth of the vertical fin  21  or independently of the fin  21 . Embodiments within the scope of the invention may have many of these combined attributes, such as a tapered, rearward-swept hydrofoil with a dihedral angle. Winglets  51  may also be used at the tip end  42  of the hydrofoil  22  to improve the lift characteristics of the foil  22  and to reduce drag [ FIGS. 12, 13   14 ].  
         [0033]     In another embodiment, two vertical fins  21  are employed, with a hydrofoil  23  section between the two vertical fins  21 , as well as hydrofoils  22  extending outwardly from each vertical fin  21 . The center hydrofoil  23  is generally rectangular in shape, while the outward-extending foils  22  may be rectangular or tapered, unswept, swept forward or swept rearward. The described fin construction or system is typically positioned at or near the tail of a surfboard. However, it may be positioned at other places along the median longitudinal axis of the board. It may be placed on the axis or parallel thereto and spaced therefrom.  
         [0034]     Variations and modifications of the foregoing are within the scope of the present invention. It should be understood that the invention disclosed and defined herein extends to the individual features and all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein comprise modes known for practicing the invention and will enable others skilled in the art to utilize the invention. The claims are to be construed to include alternative embodiments to the extent permitted by the prior art. The invention is therefore limited only by the following claims and equivalents thereof.