Abstract:
A towed personal watercraft apparatus includes a submergible unit and a platform. The submergible unit comprises at least two independent vertical stabilizers and one or more hydrofoil units mounted to at least one of the vertical stabilizers. One or more of the hydrofoil units is pivotally attached to the vertical stabilizers. Pivotal attachment can include a torsion spring or roller bearing to allow the hydrofoil units to pivot or rotate about the pitch axis of the platform, thereby improving stability of the towed personal watercraft. Rotation of the hydrofoil units can be limited for functional concerns and performance. For example, rotation of the hydrofoils about the pivot can be allowed in a first direction to prevent a nose dive, while being restricted in a second direction to facilitate a launch of the watercraft out of the water. The platform includes a user mount, such as foot holds or equivalents.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
       [0001]    The following non-provisional application follows provisional application 60/894,782, filed Mar. 14, 2007, Attorney Docket No. 350932-0106, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a marine apparatus. Specifically, the present invention relates to a towed personal watercraft apparatus. 
       BACKGROUND OF THE INVENTION 
       [0003]    Standard rigid towed personal watercraft, designed for standing or kneeling, such as waters skis, water toboggans, ski boards, and wake boards, ride on the surface of the water, and as a result, are subjected to high drag forces and surface roughness. Towed personal watercraft drag forces can be reduced and surface roughness issues mitigated by reducing or eliminating contact with the water&#39;s surface. To this end, a variety of towed personal watercraft have been fitted with lift elements, commonly called hydrofoils, to allow the body of the towed personal watercraft to ride above the water&#39;s surface, reducing drag and the effects of surface roughness. 
         [0004]    Hydrofoil design for towed personal watercraft differs from hydrofoil design for larger, self-propelled watercraft. In earlier models, stability in the critical pitch axis in larger watercraft was maintained by a set of bow and stern hydrofoils that were delta-shaped or U-shaped. Rotations of the watercraft in the pitch axis were countered by an increase in lift force generated by the newly submerged hydrofoil segments. The delta-shaped or U-shaped hydrofoil, designed to lift self-propelled watercraft that weighed hundreds or thousands of pounds, was more than adequate in countering pitch deflections caused by passengers shifting weight. This is not as true for a towed, personal watercraft, where the weight of the passenger, or rider, is much greater than the watercraft itself. 
         [0005]    Delta-shaped or U-shaped hydrofoils designed for self-propelled watercraft have given way to adjustable T-shaped hydrofoils that function similar to an airplane&#39;s wing. As the density of water is much greater than that or air, the pitch stability for the T-shaped hydrofoil is much more sensitive than pitch stability for an airplane&#39;s wing, and as such the angle of attack of each hydrofoil must be constantly adjusted by computer control. Computer controlled towed personal watercraft are obviously not a practical option for towed personal watercraft. 
         [0006]    Several single-hydrofoil personal watercraft have been developed, for towed and wind-powered watercraft. Typically, these watercraft require tremendous skill and balance. None are stable; that is, none have a restoring force to counteract pitch deflections in the body of the towed personal watercraft. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention fulfills needs present in the art by providing a towed personal watercraft apparatus where drag forces are reduced, surface roughness issues are mitigated, and pitch instabilities found in the present art are practically eliminated. To accomplish each of these objectives simultaneously, a torsion spring-mounted forward hydrofoil assembly, herein referred to as a canard assembly, in an embodiment of the present invention provides a restoring lift force and torque to counteract pitch deflections in the body of the towed personal watercraft. 
         [0008]    A towed personal watercraft apparatus comprises a platform, one or more vertical stabilizers, and one or more hydrofoils attached to the one or more vertical stabilizers. The platform comprises a platform top surface, a platform bottom surface, and a platform major axis, wherein the platform top plane and the platform bottom plane are substantially parallel. The platform major axis is aligned approximately parallel to the largest platform dimension of the platform top surface and the platform bottom surface. 
         [0009]    In some embodiments, each of the one or more vertical stabilizers comprises a pair of two substantially parallel stabilizer surfaces, wherein the platform major axis lies in a plane substantially parallel to each pair of stabilizer side surfaces associated with the one or more vertical stabilizers, and each set of the two vertical stabilizer side planes associated with the one or more vertical stabilizers is substantially perpendicular to the platform top plane and the platform bottom plane. 
         [0010]    A first subset of the one or more hydrofoils are attached to the one or more vertical stabilizers pivotally. Pivotal attachment can include one of a spring and a bearing. A second subset of the one or more hydrofoils are rigidly attached to the one or more vertical stabilizers. One of a spring and a bearing provide for rotation of the first subset of the one or more hydrofoils about a pitch axis substantially perpendicular to each pair of stabilizer surfaces associated with the one or more vertical stabilizers. The spring can be one of a torsion spring, a coil spring, a leaf spring, a resilient material, and combinations of these. 
         [0011]    A rotation of the first subset of the one or more hydrofoils is limited by one or more of an upper stop and a lower stop. In some embodiments, the upper stop and the lower stop are rigidly affixed to the one or more vertical stabilizers. The rotation angle between the first subset of the one or more hydrofoils and the platform major axis is limited to a range of between about −5 degrees and +90 degrees. 
         [0012]    A rotation of the first subset of the one or more hydrofoils approximately aligns the first subset of the one or more hydrofoils to a fluid flow vector in a reference frame centered at the one of a spring and bearing. Providing for rotation of the first subset of the one or more hydrofoils stabilizes the platform about the pitch axis. 
         [0013]    Pitch stability is a critical design issue. Rigidly mounting one or more hydrofoils forward of the platform center of mass results in pitch instability. That is, pitching a front end of the platform up urges the hydrofoil to a greater angle of attack, increasing the forward lift, further increasing the pitch. Similarly, pitching down a front end of the platform urges the hydrofoil to a reduced angle of attack, reducing lift and increasing drag, further reducing pitch. Pitch attitude transients can be introduced by weight shifts of an operator or water turbulence. 
         [0014]    A canard assembly comprises a subset of the first subset of the one or more hydrofoils free to rotate, which are herein designated as rotatable hydrofoils, the one of a spring and a bearing, and the upper and lower stops, wherein each of the elements of the canard assembly reside forward of the platform center of mass. To address pitch instabilities, the rotatable hydrofoils of the canard assembly in the present invention are designed to align to the fluid flow vector. In some embodiments, this rotation is also based on torques about the pivot due to a resilient member, such as a torsion spring, storing energy in the torsion spring. The torsion spring is rigidly affixed to a vertical stabilizer; the vertical stabilizer is rigidly attached to the platform of the towed personal watercraft. 
         [0015]    The rotatable hydrofoils are provided a nearly constant angle of attack by the canard assembly. This is in sharp contrast with the one or more hydrofoils rigidly attached to the one or more vertical stabilizers, where the angle of attack is fixed to the pitch of the platform. 
         [0016]    The center of lift of the rotatable hydrofoils lies on or near a pivot axis of the canard assembly; the pivot axis of the canard assembly is approximately collinear with a rotation axis of the one of a spring and a bearing. Nominally, the lift and drag forces imparted to the rotatable hydrofoils by the fluid streamlines produce a lift force along the canard assembly pivot axis with little of no torsion spring deflection. The term fluid streamline refers to the fluid flow across a hydrofoil surface; the term fluid flow vector is reserved for the relative velocity component of the fluid flow with respect to the platform. 
         [0017]    Pitch transients in the platform of the towed personal watercraft do not rotate the canard assembly with respect to a fluid flow vector. Thus, the hydraulic force dampens canard assembly rotation for any appreciable towed velocity. A vertical velocity component associated with a pitch rate momentarily changes the rotatable hydrofoils effective angle of attack, providing a short lift force pulse that resists the pitch transient just before the rotatable hydrofoils is able to realign with the new fluid flow vector. When the vertical velocity component is removed, the torsion spring remains deflected by the angular difference in the pitch of the platform of the towed personal watercraft and the fluid flow vector. The torsion spring provides a restoring torque to counteract the pitch transient in the platform of the towed personal watercraft. As a result, the towed personal watercraft in the present invention can ride above the water&#39;s surface, reducing drag and the effects of surface roughness, without suffering the pitch instabilities of similar towed personal watercraft in the present art. 
         [0018]    In some embodiments, a lift force is greater for a third subset of one or more aft hydrofoils than for a fourth subset of one or more forward hydrofoils. This arrangement is beneficial for embodiments in which most of the operator&#39;s weight or force is imparted on the aft portion of the platform. 
         [0019]    In some embodiments, each of the one or more hydrofoils comprise a pair of hydrofoils substantially symmetric about a plane parallel to the pair of vertical stabilizer surfaces. One or more of the pair of hydrofoils may rotate independently. 
         [0020]    In some embodiments, each of the one or more hydrofoils comprises a lateral termination features. The lateral termination feature can be approximately cylindrical with a radius greater than approximately two times a distance between the vertical stabilizer side planes. The lateral termination feature reduces a formation of air bubbles on or near the one or more hydrofoils and reduces a turbulence at or near the one or more hydrofoils. 
         [0021]    The one or more vertical stabilizers minimize a resistance to fluid flow and stabilizes the platform about a pitch axis substantially parallel to the platform major axis. 
         [0022]    The platform further comprises an operator interface. The operator interface is one of a foot hold, a toe hold, a hand hold, and a seat. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The accompanying drawings, which are incorporated herein and from and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. 
           [0024]      FIGS. 1A-1D  show a schematic of an embodiment of a towed personal watercraft, in accordance with the present invention. 
           [0025]      FIG. 2  shows a top/rear perspective of an embodiment of the towed personal watercraft, shown in  FIGS. 1A-1D , in accordance with the present invention. 
           [0026]      FIG. 3  shows a schematic of an embodiment of a canard assembly of the towed personal watercraft, in accordance with the present invention. 
           [0027]      FIG. 4A  shows an embodiment of the towed personal watercraft pitched downward, with no limit on the rotation of an exemplary rotatable hydrofoil, in accordance with the present invention. 
           [0028]      FIG. 4B  shows an embodiment of the towed personal watercraft pitched upward, with a limit on the rotation of an exemplary rotatable hydrofoil, in accordance with the present invention. 
           [0029]      FIG. 5  shows a dimensioned side view of one embodiment of the towed personal watercraft, in accordance with the present invention. 
           [0030]      FIG. 6  shows a dimensioned top view of one embodiment of the towed personal watercraft, in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    It should be appreciated that the particular implementations shown and described herein are examples of the present invention and are not intended to otherwise limit the scope of the present invention in any way. Further, the techniques are suitable for applications in marine systems, hydraulics systems, aeronautic and aerospace systems, wind tunnel systems, or any other application. 
         [0032]    The present invention is an improved, towed personal watercraft apparatus. A canard assembly comprising a forward pair of rotatable hydrofoils pivotally attached to a vertical stabilizer, the vertical stabilizer itself rigidly affixed to the platform of the towed personal watercraft, is configured to provide a restoring lift force or torque to counteract pitch transients of the platform resulting from operator weight shifts or water turbulence. In a representative embodiment, a rotation axis of a torsion spring pivotally attached to the rotatable hydrofoils is positioned at or near the center of lift of the canard assembly. 
         [0033]      FIGS. 1A-1D  show a schematic of an embodiment of a towed personal watercraft ( 100 ), in accordance with the present invention. The towed personal watercraft is subdivided into the following two units for descriptive purposes: a submergible unit ( 101 ), which provides lift, and a platform ( 151 ), which allows for an operator to ride the towed personal watercraft ( 100 ). 
         [0034]    The submergible unit ( 101 ) comprises a canard assembly ( 110 ), which is attached to a forward vertical stabilizer ( 120 ). In some embodiments, a forward hydrofoil ( 121 ) is rigidly affixed to the forward vertical stabilizer ( 120 ) at or near the bottom of the forward vertical stabilizer ( 120 ). The submergible unit further comprises an aft vertical stabilizer ( 130 ), which is attended by an aft hydrofoil ( 131 ) rigidly affixed to the aft vertical stabilizer ( 130 ) at or near the bottom of the aft vertical stabilizer ( 131 ). 
         [0035]    Submergible unit ( 101 ) forward and aft vertical stabilizers ( 120 ,  130 ) provide for minimum resistance to water flow in an intended direction and provide for lateral stability, resisting roll, rotation of the platform ( 151 ) about its major axis. In another embodiment, the forward hydrofoil ( 121 ) is not present. 
         [0036]    In the exemplary embodiment, both of the forward hydrofoils ( 121 ) and aft hydrofoil ( 131 ) are comprised of two symmetric sections, extending laterally and slightly aft of their respective forward and aft vertical stabilizers ( 120 ,  130 ), substantially parallel to the platform ( 151 ). In another embodiment, at least one of the forward hydrofoil ( 121 ) and aft hydrofoil ( 131 ) are not parallel to the platform ( 151 ), but rather extend downward, forming an inverted ‘V’. The inverted ‘V’ configuration allows for a lift force surface area to vary as the towed personal watercraft submersion changes. Each of the two symmetric sections for both the forward hydrofoil ( 121 ) and aft hydrofoil ( 131 ) comprise a lateral termination feature ( 140 ), a cylindrical extension with radius greater than twice the width of the hydrofoil in which they are mounted. The lateral termination feature ( 140 ) minimizes the formation of air bubbles on the hydrofoil surfaces and reduces turbulence near the hydrofoil surfaces, allowing for greater lift. 
         [0037]    Both the forward and aft hydrofoils ( 121 ,  131 ) provide lateral stability and lift to the platform ( 151 ). The surface area of the aft hydrofoil ( 131 ) is greater than that of the forward hydrofoil ( 121 ), generating greater lift and stability than the forward portion of the platform ( 151 ). The aft portion of the platform ( 151 ) supported the majority of the operator&#39;s weight. The aft hydrofoil ( 131 ) affixed to the bottom of the aft vertical stabilizer ( 130 ) is a greater distance from the platform ( 151 ) than the forward hydrofoil ( 121 ) affixed to the bottom of the forward vertical stabilizer ( 120 ). 
         [0038]    Both of the submergible unit ( 101 ) forward and aft vertical stabilizers ( 120 , 130 ) are rigidly affixed to a towed personal watercraft body, or ski ( 152 ), an element of the platform ( 151 ). The ski ( 152 ) comprises a ski top surface ( 152   a ) and a parallel ski bottom surface ( 152   b ). The ski top surface ( 152   a ) and ski bottom surface ( 152   b ) are joined at their respective boundary edges by one or more substantially curved surfaces. 
         [0039]    Additionally, the ski ( 152 ) comprises an operator interface, such as a series of operator holds. In the present embodiment, the ski comprises a set of foot holds ( 153 ,  154 ). The ski top surface ( 152   a ) supports an operator; the foot holds ( 153 ,  154 ) allow for operator control. The ski bottom surface ( 152   b ) provides an initial water interface. The ski bottom surface ( 152   b ) releases from the water at a speed sufficient for submerged unit ( 101 ) elements  110 ,  121 , and  131  to generate the necessary lift to overcome the weight of the operator, the weight of the platform ( 151 ), the surface tension of the water surface, and the weight of the submerged unit ( 101 ) less the buoyant force of the submerged unit ( 101 ). 
         [0040]    In some embodiments, the forward portions of both top ski surface ( 152   a ) and bottom ski surface ( 152   b ) curve upward, remaining parallel, away from the horizontal plane in which the majority of the ski ( 152 ) lays. The aft portions of both top ski surface ( 152   a ) and bottom ski surface ( 152   b ) also curve upward, but to a significantly smaller degree than the forward portion. The remaining portions of the top ski surface ( 152   a ) and bottom ski surface ( 152   b ) do not deviate significantly from the aforementioned horizontal plane, with the exception of a series of parallel grooves ( 155 ), each approximately six inches in length, notched into the most aft portion of the bottom ski surface, parallel to the major axis of the ski ( 152 ). The parallel grooves ( 155 ) provide control at the start of a tow. 
         [0041]    In some embodiments, the towed personal watercraft ( 100 ) is itself buoyant. The submerged unit ( 101 ) is neutrally buoyant or not buoyant. A buoyant submerged unit would make it very difficult for the operator to balance underwater prior to starting a tow. In another embodiment, weight is added to the forward and aft vertical stabilizers ( 120 ,  130 ) for ballast to resist an additional lift force that accompanies an increase in platform velocity, raising the vertical stabilizers above the water&#39;s surface. In a further embodiment, the canard assembly ( 110 ) can be adjusted such that a symmetric pair of rotatable hydrofoils are nominally set to a higher angle of attack and allowed to rotate to a lower angle of attack at increased platform velocities. In this way, the additional lift force that accompanies an increase in platform velocity for a fixed hydrofoil is reduced. 
         [0042]    Suitable materials for each element of the submerged unit ( 101 ) include, but are not limited to: composite materials such as weighted fiberglass and weighted carbon-fiber epoxy, metals such as aluminum and stainless steel, polymers, and combinations of these materials. The platform ( 151 ) is buoyant. Suitable materials for each element of the platform ( 151 ) excluding foot holds ( 153 ,  154 ) include, but are not limited to: wood, epoxy, carbon-fiber epoxy, metals, foam, plastics, and combinations of these materials. Suitable materials for each foot hold ( 153 ,  154 ) include, but are not limited to: rubber, leather, foam, plastics, and combinations of these materials. 
         [0043]      FIG. 2  shows a top/rear perspective of an embodiment of a towed personal watercraft ( 200 ), in accordance with the present invention. As with  FIGS. 1A-1D , the towed personal watercraft is subdivided into the following two units for descriptive purposes: the submerged unit ( 101 ), which provides lift, and the platform ( 151 ), which allows for an operator to ride the towed personal watercraft ( 100 ).  FIG. 2  numbering and description follow the numbering and description given previously for  FIGS. 1A-1D . 
         [0044]      FIG. 3  shows a schematic of one embodiment of the canard assembly ( 110 ) of the towed personal watercraft, in accordance with the present invention. The canard assembly ( 110 ) includes a symmetric pair of rotatable hydrofoils ( 221 ), a torsion spring ( 211 ) that attaches the rotatable hydrofoils ( 221 ) to the forward vertical stabilizer ( 120 ), and an upper and lower canard stop ( 212 ,  213 ), rigidly affixed to the forward vertical stabilizer ( 120 ), to limit rotation of the rotatable hydrofoils ( 221 ). 
         [0045]    Nominally, with a level platform ( 151 ), the lift and drag forces imparted to the canard assembly ( 110 ) by the fluid streamlines produce a lift force at a point at near a canard assembly pivot axis, which is approximately collinear with a rotation axis of the torsion spring ( 211 ), with little of no torsion spring deflection. The canard assembly ( 110 ) is designed to rotate based on the hydraulic forces of the fluid flow over the rotatable hydrofoils ( 221 ) and the torsion spring ( 211 ) attaching the rotatable hydrofoils ( 221 ) to the forward vertical stabilizer ( 120 ) rigidly affixed to the platform ( 151 ). 
         [0046]    Pitch transients in the platform ( 151 ) do not rotate the canard assembly ( 110 ) with respect to the fluid flow vector; the hydraulic force of the streamlines damps the canard assembly ( 110 ) rotation for any appreciable velocity. The vertical velocity component associated with the pitch rate changes the rotatable hydrofoils ( 221 ) effective angle of attack, momentarily providing a lift force that resists the pitch transient before the canard assembly ( 110 ) realigns with the new fluid flow vector. When the vertical velocity component is removed, the torsion spring ( 211 ) remains deflected by the angular difference in the pitch of the body of the towed personal watercraft and the fluid flow vector. As a result, the torsion spring ( 211 ) is deflected by the angular difference in the pitch of the platform ( 151 ) and the fluid flow vector. The torsion spring ( 211 ) provides a restoring torque to counteract the pitch transient in the platform ( 151 ), returning the platform ( 151 ) and torsion spring ( 211 ) to their respective nominal positions. 
         [0047]    Upper and lower canard stops ( 212 ,  213 ) are designed to allow the rotatable hydrofoils ( 221 ) to rotate from an elevation angle of 90 degrees, the upper stop ( 212 ) limit, down to an elevation angle of −5 degrees, the lower stop ( 213 ) limit. The upper stop ( 212 ) limit of 90 degrees allows the rotatable hydrofoils ( 221 ) to follow the fluid streamlines in a steep dive and to provide a momentary restoring lift through the rotatable hydrofoils ( 221 ) and a restoring torque through the torsion spring ( 211 ). The lower stop ( 213 ) limit of −5 degrees allows the rotatable hydrofoils ( 221 ) to follow the platform pitch, which increases the rotatable hydrofoils ( 221 ) angle of attack, providing positive feedback for lifts in jumps and stunt execution. 
         [0048]    In the present embodiment, the canard assembly ( 110 ) symmetric pair of rotatable hydrofoils ( 221 ) rotate in tandem. In another embodiment, the pair of rotatable hydrofoils ( 221 ) that comprise the canard assembly ( 110 ) rotate independently, each attached to the forward vertical stabilizer ( 120 ) with independent torsion springs ( 211 ). 
         [0049]    In another embodiment, the canard assembly ( 110 ) is mounted to the forward vertical stabilizer in a roller bearing housing, free to rotate with no torsion spring ( 211 ) restoring torque. In this embodiment, the lift force will be momentarily increased or reduced as a function of platform ( 151 ) vertical velocity, which effectively changes the angle of attack of the rotatable hydrofoils ( 221 ). Following a pitch or vertical velocity transient, the rotatable hydrofoils ( 221 ) will return to their nominal angle of attack, following the fluid flow vector, without imparting a restoring torque. The canard assembly ( 110 ), without the torsion spring ( 211 ), is marginally stable. The canard assembly ( 110 ) without the torsion spring ( 211 ) is not as stable as with the torsion spring ( 211 ), as it does not produce a restoring torque. 
         [0050]    The towed personal watercraft in the preferred embodiment is stable, and as such, can ride above the water&#39;s surface, reducing drag and the effects of surface roughness, without suffering the pitch instabilities of similar towed personal watercraft in the present art. 
         [0051]      FIG. 4A  shows an embodiment of the towed personal watercraft ( 100 ) pitched downward, with no limit on the rotation on an exemplary rotatable hydrofoil, in accordance with the present invention. The rotatable hydrofoils ( 221 ) align with the fluid flow vector such that the angular spring deflection is approximately equal to the negative pitch. The angle of attack remains approximately constant, approximately equal to the angle of attack in a level pitch state. 
         [0052]      FIG. 4B  shows an embodiment of the towed personal watercraft pitched upward, with a limit on the rotation on an exemplary rotatable hydrofoil, in accordance with the present invention. The rotatable hydrofoils ( 221 ) start to align with the fluid flow vector, but are limited by the lower canard stop. In an embodiment, the lower canard stop can be set to limit the rotatable hydrofoils ( 221 ) to an angle of approximately −5 degrees. The resulting angle of attack is the pitch angle minus plus the spring deflection, −5 degrees in the present embodiment, plus the angle of attack in a level pitch state. The lower canard stop allows for a higher angle of attack in pitch up maneuvers, giving extra lift for jumps and tricks. 
         [0053]      FIGS. 5 and 6  show dimensioned side and top views of one embodiment of the towed personal watercraft, in accordance with the present invention, and as such require no further explanation. One or more of the dimensions can be varied for performance enhancement, as a function of the operator size, weight, and style. 
         [0054]    While the invention has been described in connection with the specific embodiments thereof, it will be understood that it is capable of further modification. Furthermore, this application is intended to cover any variations, uses, or adaptations of the invention, including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains, and as fall within the scope of the appended claims. 
         [0055]    All publications, patents, and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference.