Abstract:
A waterboard with externally adjustable stiffness includes a stringer assembly having a rotatable beam to modulate the stiffness of the beam in a selected direction to impart a desired stiffness to the waterboard.

Description:
RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119(e) to U.S. provisional application No. 61/075,659, filed Jun. 25, 2008, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to waterboards and stiffening elements thereof. 
     2. Discussion of the Background 
     Sports boards composed of a preformed, preshaped, generally planar foam core with a slick bottom skin are very popular for use on water, snow, grass, ice or other surfaces. One type of sports board is a waterboard such as bodyboard or surf board and is employed in the water, more particularly for wave surfing. Generally, waterboards are made of semi-rigid foam core, typically with polystyrene foam, polyethylene foam or polypropylene foam, and have polyethylene foam sheets laminated to the top and side surfaces of the foam core, and have a bottom surface composed of a polymeric film material such as polyethylene or Surlyn® to provide a low-friction surface. 
     During wave riding, a user may bend the board and turn on the water. The board typically restores to a neutral position after bending. The recovery of the original shape is referred as the ‘memory’ of the foam core. Polypropylene foam cores have better memory characteristics than other foam core materials. Therefore, a polypropylene foam core is typically used for high end performance waterboards due to its resiliency, rigidity and light weight. 
     Typically, waterboards are ridden in a prone position, with one arm extending forward for gripping the nose of the board and the other arm positioned in a trailing manner for gripping the front portion of the side edge of the board. With the arms and hands thus positioned, the rider can push or pull against the engaged front or side edges to bend or twist the board to increase friction and drag on selected parts of the board, which helps the rider in redirecting the board. It is generally desirable to have a bodyboard with low flexibility (i.e., high stiffness) in the rearward portion of the board and higher flexibility in the forward portion of the board. This combination provides stiff support for the rider&#39;s body on the rearward portion of the board while allowing the rider to maneuver the board as described above. 
     A variety of stringers and stiffening methods have been described in the prior art. U.S. Pat. No. 6,036,560 (the &#39;560 patent) discloses an encapsulated two-part stringer rod having a stiff portion in the body and tail of the bodyboard and a less stiff portion toward the nose of the bodyboard. The flexible front nose area provides greater maneuverability for the bodyboard. The &#39;560 patent discloses an elongated stringer element comprising a stiff rear portion fabricated from fiberglass or graphite resin-impregnated material and a flexible front portion fabricated from a polyethylene material. the stringer is generally longitudinally arranged within the foam core material of the board and extends substantially from the tail end toward the front end. 
     U.S. Pat. No. 7,347,754 (the &#39;754 patent) also discloses a two part encapsulated stringer providing greater stiffness in the body of the bodyboard and less stiffness in the nose of the bodyboard. The amount of stiffness imparted to the body is determined by a fiberglass tube and the amount of flexibility imparted to the nose is determined by a helical coil or spring. 
     The disadvantage of using an encapsulated stringer is that once a particular stiffness profile is selected at the time of manufacture, it cannot be changed. Riders vary in weight and strength and wave riding skills, so the optimum level of flexibility varies from rider to rider. It would be desirable, therefore, to provide a waterboard with externally adjustable stiffening element(s) configured to provide variable resistance to flex. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the invention relate to a bodyboard with an externally adjustable flexibility. In particular, a bodyboard with an externally adjustable, variable stiffness stringer element is provided. The present invention incorporates a rotatable beam in lieu of a helical spring or a solid plastic rod used by the prior art. The rotatable beam significantly improves the ease of adjustment and the range of flexibility adjustment. 
     In embodiments having features of the invention, a waterboard having externally adjustable stiffness includes a generally elongated foam core having a forward nose and a rearward tail and a longitudinally disposed channel within. In one embodiment, the channel may have a generally cylindrical cross-section having an approximately uniform diameter, an opening at the tail and terminating within in forward portion of the foam core. In other embodiments, the channel may have an elliptical or polygonal cross-section and may also have a non-uniform cross-section. The waterboard further includes an adjustable flex stringer assembly disposed substantially within the channel, the adjustable flex stringer assembly including: a housing having a cylindrical bore, configured to create a friction or interference fit with the channel and occupying a rearward portion of the channel; a stringer comprising a cylindrical shank disposed within the cylindrical bore of the housing and a beam element disposed within approximately a forward third of the channel; an end cap engaged with the cylindrical shank and configured to rotate the stringer under an application of torque, wherein the stiffness of the beam element in a direction normal to a surface of the waterboard is modulated between a minimum stiffness and a maximum stiffness. 
     In embodiments having features of the invention, a waterboard having externally adjustable stiffness includes a generally elongated foam core having a forward nose and a rearward tail and a longitudinally disposed channel within. An adjustable flex stringer assembly is disposed substantially within the channel, the adjustable flex stringer assembly including: a stringer comprising a shank portion and a beam portion, said shank portion being disposed rearward of said beam portion when said adjustable flex stringer assembly is in position within said channel, said beam portion including a beam element disposed within approximately a forward third of the channel; and an end cap engaged with the shank and configured to rotate the stringer under an application of torque, wherein the stiffness of the beam element in a direction normal to a surface of the waterboard is modulated between a minimum stiffness and a maximum stiffness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view of a waterboard illustrating an adjustable flex stringer according to one embodiment of the invention; 
         FIG. 2A  is an exploded view of an adjustable flex stringer assembly according to one embodiment of the invention; 
         FIGS. 2B and 2C  are cross-sectional views of the adjustable flex stringer of  FIG. 2A  in minimum and maximum stiffness configurations; 
         FIGS. 3A-3C  illustrate a stringer according to one embodiment of the invention; 
         FIG. 4A  illustrates an adjustable flex stringer in a minimum stiffness configuration according to one embodiment of the invention; 
         FIG. 4B  illustrates an adjustable flex stringer in a maximum stiffness configuration according to one embodiment of the invention; 
         FIG. 5  is a cross-sectional perspective view of a waterboard illustrating an adjustable flex stringer according to one embodiment of the invention; 
         FIG. 6  illustrates a flex control mechanism according to one embodiment of the invention; 
         FIGS. 7A-7F  illustrate a stringer retaining control mechanism according to one embodiment of the invention; 
         FIG. 8  illustrates a waterboard according to one embodiment of the invention; and 
         FIG. 9  is a cross-sectional view of  FIG. 8  illustrating an adjustable flex stringer assembly according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     An adjustable stiffness stringer for a waterboard is described. In the following description, numerous specific details are set forth such as examples of specific methods, materials, components, etc. in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that these specific details need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid unnecessarily obscuring the present invention. Embodiments of the invention are directed to an adjustable flex waterboard which includes a preformed, preshaped board such as a bodyboard or a surfboard, having a generally planar form with top and bottom surfaces, a nose end, a tail end and two opposing side rail surfaces which may extend from one end to the other end of the board. The board may include a low density closed-cell thermoplastic foam core such as polystyrene, polyethylene, polypropylene foam material or the like. A low-friction thermoplastic polymer film material may be laminated to the bottom surface of the board and the upper and lower rail surfaces and the top surface may be covered by a closed-cell foam material having a higher density than the foam core. The board includes a stringer assembly that may be externally adjusted to alter the flexibility of the waterboard. 
       FIG. 1  is a cross-sectional view of a waterboard  100  including an adjustable flex stringer assembly  200  according to one embodiment of the invention. Waterboard  100  includes a low-density foam core  101 , which may be molded or bored out to provide a channel for the insertion of the adjustable flex stringer assembly  200 . In one embodiment, the channel may have a uniform circular cross-section over its length. As noted above, waterboard  100  may include a low-friction thermoplastic polymer film  102  on its bottom surface and a closed-cell foam  103  on its top and side rail surfaces as described above, although embodiments of the invention are not so limited and may utilize any type of materials and manufacturing processes as are known in the art. 
     The adjustable flex stringer assembly  200  includes a stiff housing element  104  of fiberglass or rigid plastic having a cylindrical internal bore. In one embodiment, as illustrated in  FIG. 2 , housing element  104  may have a circular cross-section with a diameter equal to or greater than the diameter of the channel in foam core  101  such that housing  104  has a friction fit or interference fit with the channel and is prevented from rotating thereby. In other embodiments, housing  104  may have a non-cylindrical cross-section that matches a cross-section of the channel such that housing  104  cannot be rotated within the channel. Housing  104  is configured to accept a cylindrical shank of a stringer element  105  with a sliding fit or friction fit such that the shank of stringer element  105  may be rotated within the cylindrical bore of housing element  104  with the application of suitable torque. Stringer element  105  may be fabricated (e.g., molded or machined) as a single piece of a flexible plastic such as nylon, delrin or other suitable material. The shank of stringer element  105  may include a counterbore  106 , which may have a slotted, or irregular, or hexagonal or other polygonal shape, or any suitable shape configured to accept an Allen wrench, star wrench or other type of tool capable of rotating stringer element  105  within the cylindrical bore of housing  104 . It will be appreciated that the stiffness of the stringer assembly over the length of the housing element  104  will be determined by the stiffness of the housing element  104  and not by the stiffness of the shank of stringer element  105 . 
     Stringer element  105  also includes a beam segment  107  forward of the shank and terminated at its ends by disks  108  and  109 . Disk  108  is located between the shank and beam element  107  and provides a stop to prevent stringer element  105  from sliding rearward with respect to housing element  104 . Disk  109  is located at the forward end of beam segment  107  and bears against the bottom surface of the channel in foam core  101 . In one embodiment, anti-buckling foam pieces  110  with approximately semicircular cross-sections may be placed on each side of beam element  107  as described in greater detail below. Anti-buckling foam pieces  110  may be the same material as foam core  101  or different material; in embodiments, anti-buckling foam pieces  110  may be made from low density foam. The diameters of disks  108  and  109 , and the combined diameter of beam element  107  and foam pieces  110 , may be less than the diameter of the channel in the foam core  101  such that beam element  107 , foam pieces  110  and disks  108 ,  109  may rotate freely within the channel when a torque is applied to the shank of stringer element  105 . In one embodiment, stringer assembly  200  may include an end cap  111  configured to retain stringer assembly  200  within waterboard  100 . For example, end cap  111  may have a serrated or saw-toothed outer surface (not shown) as is known in the art to irreversibly engage foam core  101 . Alternatively, end cap  111  may be glued or otherwise bonded to foam core  101  and surfaces  103 . As illustrated in  FIG. 1 , end cap  111  may also include an outer flange to limit the penetration of the end cap into foam core  101  and an inner flange to retain housing  104 . In embodiments, for example, end cap  111  may be made from low density polyethylene (LDPE) or other suitable material. 
       FIG. 2A  is an exploded view of stringer assembly  200  illustrating all of the elements described above. From the foregoing description, it will be seen that the beam element  107  can be viewed as a cantilevered beam secured at disk  108  by housing element  104 , which may be rotated within foam core  101  via the application of torque to the shank of stringer element  105 . Beam element  107  has non-uniform stiffness in a direction normal to the top surface of waterboard  100  as a function of the rotational orientation of beam element  107  within the foam core  101 . Stringer element  105  may be made from, for example, nylon or delrin or other suitable material. 
     The stiffness of a beam is defined as the ratio of an applied force to the deflection of the beam in the direction of the force.  FIGS. 2B and 2C  are cross-sectional views of beam element  107  looking toward disk  109 . As illustrated in  FIGS. 2B and 2C , beam element  107  has an approximately rectangular cross-section with broad dimension α and narrow dimension β. When beam element  107  is rotated such that the broad dimension α is horizontal, as illustrated in  FIG. 2B , the stiffness of the beam will be minimized in the direction of an applied force F. When beam element  107  is rotated such that the broad dimension α is vertical, then the stiffness of the beam will be maximized in the direction of the applied force F. Thus, the stiffness of a beam having features of the invention, such as beam element  107 , is greater with respect to an applied force that is parallel to a broad dimension α (e.g., as illustrated in  FIG. 2C ), and the stiffness of a beam having features of the invention, such as beam element  107 , is lesser with respect to an applied force that is perpendicular to a broad dimension α (e.g., as illustrated in  FIG. 2B ). Stiffness values between the minimum value and the maximum value may be obtained at intermediate angular orientations between horizontal and vertical. Anti-buckling foam pieces  110  prevent beam element  107  from buckling sideways or twisting when beam element  107  is in a non-minimum stiffness orientation. 
       FIGS. 3A-3C  illustrate an embodiment of a stringer element  205  that eliminates the need for anti-buckling foam pieces.  FIG. 3A  is a perspective view and  FIGS. 3B and 3C  illustrate minimum and maximum stiffness orientations respectively (assuming the same orientation of waterboard  100  illustrated in  FIG. 1 ). As illustrated in  FIGS. 3A-3C , stringer element  205  has a cylindrical shank  212  and a plurality of intermediate disk elements  206  spaced along a beam element  207  between end disk elements  208  and  209 , respectively. Intermediate disk elements  206  and end disks  208  and  209  have a diameter small enough to allow beam element  207  to rotate freely within a cylindrical channel in foam core  101  and large enough to prevent beam element  207  from buckling under an applied stress as described above. In other respects, the operation and characteristics of stringer element  205  may be equivalent to those of stringer element  105 . As illustrated in  FIG. 3B , beam element  207  may also have a taper that provides a variation in stiffness along its length and a tenon  210  at its opposite end to engage a stiffness control mechanism as described below. 
     In embodiments, as illustrated in  FIGS. 3A and 3B , a stringer element  205  may have a stringer length  325  which may be, for example, between about 20 inches and about 50 inches; or may be, for example, between about 25 inches and about 40 inches; or may be, for example, between about 30 inches and about 35 inches; and may be, for example, about 33 inches. In embodiments, a beam element  207  may have, for example, a length  327  of between about 5 inches and about 15 inches; or may have a length  327  of between about 8 inches and about 12 inches; or may have a length  327  of about 10.7 inches. 
     In embodiments, as illustrated in  FIGS. 3A and 3B , a stringer element  205  may have a thickness  315  along a cylindrical shank portion  212 ; in embodiments, a shank thickness  315  may be, for example, between about 0.3 inches and about 1 inches, or between about 0.5 inches and about 0.9 inch, or may be between about 0.6 inches and about 0.8 inches, and may be, for example, about 0.68 inches. In embodiments, as illustrated in  FIGS. 3A and 3B , an end disk element  208  may have a thickness  317  that may be, for example, between about 0.4 inches and about 1.2 inches, or between about 0.6 and about 1 inch, or may be, for example, about 0.8 inches. A beam element  207  may have a width  321  that may be, for example, between about 0.4 inches and about 1.2 inches, or between about 0.6 and about 1 inch, or may be, for example, about 0.8 inches. An intermediate disk element  206  may have a disk span  323  that may be, for example, between about 0.05 inches and about 0.2 inches, or between about 0.07 inches and about 0.1 inch, or may be, for example, about 0.9 inches. 
     In embodiments, as illustrated in  FIGS. 3A and 3B , a stringer element  205  may have a tenon  210  having a thickness  311  and a length  313 ; in embodiments, a tenon thickness  311  may be, for example, between about 0.2 and about 0.6 inch, or between about 0.3 and about 0.5 inch, and may be, for example, about 0.4 inches. In embodiments, a tenon length  313  may be, for example, between about 0.3 and about 0.8 inch, or between about 0.4 and about 0.6 inch, and may be, for example, about 0.5 inches. 
       FIG. 4A  is a cross-sectional perspective view of a waterboard  300  showing stringer element  205  with beam segment  207  in a minimum stiffness orientation.  FIG. 4B  is a cross-sectional perspective view of waterboard  300  showing stringer element  205  with beam segment  207  in a maximum stiffness orientation. 
       FIG. 5  is a cross-sectional perspective view of waterboard  300  illustrating an adjustable flex stringer assembly  400  according to one embodiment of the invention. Stringer assembly  400  includes a rigid housing element  204 , which may be functionally and structurally similar to housing element  104  described above. Stringer assembly  400  also includes a stringer element  205  having a beam element  207  as described above and a cylindrical shank  212  (not visible in  FIG. 5 ) engaged with housing  204 . In one embodiment, as illustrated in  FIG. 5 , stringer assembly  400  may be retained within waterboard  300  by an end cap  211  as described below. 
       FIG. 6  is a partial cross-sectional view of waterboard  300  illustrating a stiffness control mechanism for stringer assembly  400  in one embodiment. As illustrated by the cutaway of housing  204  in  FIG. 6 , tenon  210  in shank  212  is engaged with a matching mortise in end cap  211 . End cap  211  has a circular flange  213  which is captured by a matching channel feature in waterboard  300  and which allows end cap  211  and stringer element  205  to rotate under an applied torque. End cap  211  may also have a cylindrical body with a diameter smaller than circular flange  213 , but large enough to have a fiction fit with a matching circular channel in waterboard  300  such that end cap  211  and stringer element  205  do not rotate in the absence of an applied torque. Finally, end cap  211  may also include a counter bore  214 , which may be a polygonal counter bore  214 , or may be a non-polygonal counter bore  214 , as described above to accept a wrench or other torque applying tool to rotate stringer element  205  to adjust the stiffness of stringer assembly  400  within waterboard  300 . 
       FIGS. 7A-7F  illustrate the details of end cap  211  according to one embodiment of the invention. In the embodiment shown in  FIGS. 7A-7F , end cap  211  is a circularly symmetrical end cap  211 . It will be understood that an end cap  211  need not be circularly symmetrical, but that, in embodiments, an end cap  211  may have triangular, square, rectangular, or other polygonal features, or may have irregular features, and may or may not be symmetrical. As illustrated in  FIG. 7C , an end cap  211  may have, for example, a length  717  of between about 0.5 inches to about 3.5 inches, or between about 1 inches to about 2.8 inches, or, in embodiments, may have a length  717  of about 2.2 inches. As illustrated in  FIG. 7C , an end cap  211  may have a portion of smaller width and a portion of larger width; for example, a smaller width portion may have a length  715  of between about 0.5 inches to about 2.5 inches, or between about 1 inches to about 2 inches, or, in embodiments, may have a length  715  of about 1.7 inches. An end cap  211  may have a larger width portion with a length  719 , for example, of between about 0.2 inches to about 1 inches, or of between about 0.3 inches and about 0.8 inches, or, in embodiments, may have a length  719  of about 0.5 inches. 
     As illustrated in  FIG. 7D , an end cap  211  may have, for example, a width  711  of between about 0.5 inches to about 1.5 inches, or between about 0.9 inches to about 1.3 inches, or, in embodiments, may have a width  711  of about 1.25 inches. A polygonal counter bore  214  may have a width  713 , for example, of between about 0.1 and about 0.9 inches, or of between about 0.33 and about 0.5 inches, or, in embodiments, of about 0.38 inches. 
     As illustrated in  FIG. 7E , an end cap  211  may have a mortise  707  configured to receive tenon  210  within a slot with a width  729 ; in embodiments, a width  729  may be between about 0.2 inches to about 0.6 inches, or may be between about 0.3 inches and about 0.5 inches, or, for example, a width  729  may be about 0.42 inches. A mortise  707  configured to receive a tenon  210  may have square or flat inner walls, or may, as illustrated in  FIGS. 7A and 7E , for example, may have rounded walls. A rounded wall as illustrated in  FIG. 7E  may have a radius of curvature  727  of, for example, between about 0.2 inches and about 0.5 inches, or of between about 0.3 inches and about 0.4 inches, or, for example, may have a radius of curvature  727  of about 0.35 inches. 
     As illustrated in  FIG. 7F , an end cap  211  may have a mortise  707  configured to receive a tenon portion  210 , and, for example, within a cavity having a length  725  of between about 0.6 inches to about 1.8 inches, or of between about 0.8 inches and about 1.6 inches, or, for example, of about 1.4 inches. As illustrated in  FIG. 7F , a counter bore  214  may have a depth  723 , in embodiments, of between about 0.2 inches to about 0.8 inches, or of between about 0.3 inches and about 0.6 inches, or, for example, have a depth  723  of about 0.5 inches. In embodiments, as illustrated in  FIG. 7F , an end cap  211  may have a width  721  of between about 0.5 inches to about 1.2 inches, or of between about 0.7 inches and about 1 inches, or, for example, of about 0.82 inches. 
       FIG. 8  is a top view of waterboard  300  according to one embodiment of the invention. In embodiments, a waterboard  300  may have a length  811 , for example, of between about 20 inches to about 60 inches, or of between about 30 inches and about 50 inches, or, for example, of about 40 inches. In embodiments, a waterboard  300  may have a width  813 , for example, of between about 10 inches to about 30 inches, or of between about 15 inches and about 25 inches, or, for example, of about 22 inches. 
       FIG. 9  is a cross-section of waterboard  300  through line  9 - 9  of  FIG. 8 . As illustrated in  FIGS. 8 and 9 , waterboard  300  may be approximately 40 inches long and approximately 20 inches wide. Stringer element  205  may be approximately 30 inches long, with approximately ⅔ of its length comprising shank  212  and ⅓ of its length comprising beam element  207 . Stringer element  205  may be positioned within waterboard  300  such that disk  209  is located approximately 6 inches from the nose of waterboard  300 . 
     As illustrated in  FIG. 9 , a waterboard  300  may have a first length  911  relating to an end cap  211 , where first length  911  may be between about 1.5 inches and about 3 inches; in embodiments, a first length  911  may be about 2.2 inches. A waterboard  300  may have a second length  919  relating to shank  212 ; a second length  919 , for example, may be between about 10 inches to about 30 inches, or between about 15 inches and about 25 inches, and may be, for example, about 21 inches. A waterboard  300  may have a third length  921  relating to beam element  207 ; a third length  921 , for example, may be between about 5 inches to about 25 inches, or may be between about 8 inches and about 15 inches, and may be, for example, about 11 inches. A waterboard  300  may have a fourth length  923  as indicated in  FIG. 9 ; a fourth length  923 , for example, may be between about 3 inches to about 25 inches, or between about 4 inches and about 10 inches, and may be, for example, about 6.1 inches. A stringer element  205  may have widths  913 ,  915 ,  917 ,  925 ,  927 , and  929 ; in embodiments, for example, widths  913 ,  915 ,  917 ,  925 ,  927 , and  929  may be between about 0.1 inches and about 1.5 inches; or may be between about 0.15 inches and about 1 inch; or may be between about 0.2 inches and about 0.9 inches. In embodiments, for example, a width  913  may be about 0.8 inches; a width  915  may be about 0.7 inches; a width  917  may be about 0.68 inches; a width  925  may be about 0.55 inches; a width  927  may be about 0.41 inches; and a width  929  may be about 0.2 inches. 
     Embodiments of the invention described above include a single, longitudinally disposed adjustable flex stringer assembly. However, embodiments of the invention are not so limited. For example, two or more adjustable flex stringers may be disposed within the body of the waterboard and may be oriented at angles such that their respective endcaps and points of adjustment are located on the sides of the waterboard or at the leading edges of the waterboard. Other configurations not so limited are also contemplated to be within the scope of the invention. 
     Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.