Abstract:
The invention relates to high-speed sailing craft, in particular, high-speed sailing craft that have one or more substantially horizontal airfoils for lifting the craft up from the water and one or more substantially vertical, rotating foils that are generally at least partially submerged in the water for providing tracking or steering. By lifting the craft out of the water, the airfoil allows the craft to travel faster, by reducing the friction of the water on the hull(s) and/or float(s). By rotating, the generally at least partially submerged portions of the vertical foils reduce the friction on the vertical foils. In combination, airfoils and rotating tracking/steering foils have the combined effect of reducing the friction of water on the craft, and improving the speed of the craft.

Description:
RELATED APPLICATION  
       [0001]     This application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 60/716,168, filed Sep. 12, 2005, entitled “High Speed Sailing Craft,” attorney docket number XYP-002PR, incorporated herein by reference. 
     
    
     FIELD  
       [0002]     The present invention is directed generally to the field of water craft.  
       BACKGROUND AND OBJECTIVES OF THE INVENTION  
       [0003]     Generally, the force of the wind on a sail can be resolved into a force that drives a boat forward and, a force that causes the boat to slide sideways and heel (tip to the side). In a typical sailboat, the boat is restrained from sliding sideways by one or more foil(s) in the water (e.g., keel(s), centerboard(s), rudder(s)), and the tipping moment is counteracted by such things as a heavy weight on the bottom of a keel, the weight of the crew leaning out to windward, or for example, in the case of a catamaran, the weight of the windward hull and the buoyancy of the leeward hull.  
         [0004]     The speed of a sailboat typically is constrained by drag, particularly drag due to dynamic displacement of water by (a) the hull, steering foils (e.g., a rudder) and tracking foils (e.g., keel, centerboard), and (b) skin friction on the hull and foils. Displacement drag results from the energy required to move water out of the way of the moving hull. Friction drag results from the water sticking to the hull as a result of molecular attraction. The loss of energy through drag decreases the energy available to push the boat forward. Generally, the amount of force needed to overcome these drags increases as the square of the speed, which tends to put a practical limit on possible speed.  
         [0005]     To increase forward driving force, techniques such as increasing sail area are needed, but increasing the sail area can result in a variety of problems such as decreased control and increased tipping moment, which causes the boat to heel over too far. Also, as the speed of a sailboat increases, it becomes more difficult to balance the large driving forces and large drags so constant adjustment of sails and boat direction is required.  
         [0006]     Recent advances in sailboat design have resulted in the development and construction of sailboats which can travel at much higher speeds than sailboats many years ago. For example, it is now common for high-speed sailboats to go faster than the wind, in some cases twice or as much as almost three times as fast. Such increase in performance has been largely due to changes in design which use better sails, reduce weight, make use of planing hulls that raise the boat out of the water, hydrofoils, novel sail configurations, or some combination of these.  
         [0007]     Some of the advances in boat design are described in the books “The Physics of Sailing Explained” by Bryon Anderson, Sheridan House, 2003, ISBN 1-57409-170-0 and “Aero-hydrodyanmics of Sailing” by C. A. Marchaj, Dodd, Mead &amp; Company, 1979, ISBN 0-396-07739-0.  
         [0008]     With such technologies, some modem high speed sailboats can occasionally achieve high speeds, but their maximum speeds have been limited to somewhat less than 50 knots (approx. 57 mph). In 2004, for example, the speed sailing record for water borne-sailing craft was set at 46.85 knots (53.95 mph) by a windsurfer. Prior to that it was held for eleven years by an Australian boat, the Yellow Pages Endeavour (YPE), (http://innovoile.free.fr/YPE_e.html); at 46.52 knots (53.57 mph). This modest increase in recent years illustrates the difficulties of increasing speed.  
         [0009]     One of the early contributions to increased speed was the refinement of the planing board, which is designed to lift water craft up out of the water. Water skis, surf boards and windsurfers are examples of water craft which use planing boards. Also some modem high speed sailboats such as the Hobie Trifoiler (http://www.hobiecat.com/sailing/models_trifoiler.html ) use planing boards which are similar to water skis.  
         [0010]     In addition, very wide sailboats with multiple hulls, such as the YPE, have improved balancing of the tipping moment of the sail with the righting moment of the weight of the crew, allowing larger sail areas. Also, improvements have been made with such innovations as stiff sails, which avoid driving force loss due to sail twist; solid sails similar to airplane wings which are more aerodynamically efficient than soft sails; steering and tracking foils having shapes that result in reduced drag; and taller sails for better wind attack angles. Nevertheless, significant limitations due to displacement drag and skin friction drag have remained. In addition, with current designs, at very high speeds, cavitation on the steering, tracking foils and hydrofoils, if employed, reduce control because the cavitating foils lose contact with the water. Moreover, many high-speed sailboats are designed so they can only operate with the wind coming from one side of the boat, and are therefore not suitable for purely recreational use.  
         [0011]     Jean Margail of France, the leader of the Water.Resist speed sailing team (http://foxxaero.com/indsail — 013.html), has noted that “everybody knows that for speed sailing the craft should be out of the water . . . hydrofoils generate too much drag—max speed 45 knots.” He published a design on his web site that appears to use airfoils (e.g., wings) to lift the boat up from the water, but the fixed steering and tracking foils that he proposes remain submerged, which still limit the speed at which his proposed sailboat can travel because of the displacement and skin friction drags on those vertical foils.  
         [0012]     One of the objectives of the technology described here is to reduce displacement and skin friction drags described above. A further objective is to provide more stable control of a boat that is lifted out of the water, by reducing cavitation, which results in loss of control. A further objective is to enable high speed sailboats to operate more like conventional sailboats so a larger part of the sailing public can enjoy high-speed sailing.  
       SUMMARY  
       [0013]     In general, in one aspect, the disclosed technology relates to high-speed sailing craft. In one aspect the high-speed sailing craft has one or more airfoils. The airfoil may be shaped, for example like a wing, such as an airplane wing. The airfoil may extend from the craft or be integral to the craft, for example, may be designed into a deck, or used to connect portions of the craft. The airfoil is preferably approximately (and relative to vertical foils) horizontal, although generally at a slight angle, and is configured for lifting the craft up from the water, such that in operation of a portion of, and in some cases, much of, the craft can be out of the water.  
         [0014]     The craft also has one or more rotating foils. Just as one example, the rotating foils can be, or can be mounted on, wheels, that are configured to rotate according to the direction of travel of the boat. In one embodiment, a rotating foil(s) typically is partially submerged in the water for providing tracking and/or steering. The rotating foil is preferably approximately (and relative to horizontal foils) vertical. For tracking, the top of the rotating foil preferably rotates in line with the direction of travel, such that the rotation of the bottom of the rotating foil is approximately stationary with respect to the water as the boat travels. To provide steering, one or more rotating foils can also be configured to pivot, such that the foil rotates in a direction that is at an angle from the direction from the front to the back of the craft, thereby providing a force that will direct other than straight ahead. This is analogous to the pivot of a rudder or the wheels of a car that are controlled by a steering wheel.  
         [0015]     By lifting the craft up from the water, the airfoil allows the craft to travel faster by reducing the displacement drag and friction drag of the water on hull(s) and/or float(s). The rotating action of tracking and/or steering foil(s) reduces the friction on them, also resulting in reduced drag and reduced cavitation. In combination, one or more airfoils and one or more rotating tracking and/or steering foils have the combined effect of significantly reducing the forces holding back and impeding control of a water craft, thereby improving speed and performance.  
         [0016]     In general, in another aspect, the disclosed technology relates to a sailing craft that can attain high speeds by using one or more airfoils alone or in combination with one or more planing boards to lift the board and boards out of the water. Rotating foils are used in combination with the boards. In increased-speed operation, the craft is lifted almost entirely out of the water. As the boards and foils for steering and tracking rise, displacement and skin friction drags are greatly reduced. In addition, the steering and tracking foils needed for control are free to turn, further reducing skin friction.  
         [0017]     In some such embodiments, at rest and at very low speeds, planing board buoyancy is the primary source of lift, but at higher speeds, the boards plane (lift up in the water), and at yet higher speeds, aerodynamic lift from the airfoils provides lift, and the planing boards lift out of the water.  
         [0018]     The rotating steering and tracking foils can be implemented as wheels. One exemplary implementation, for example, uses three varieties of wheels. Wheels near the front of the boat include a body, a tread and a peripherally mounted foil for tracking. These wheels protrude through planing boards such that their bottoms are below the boards&#39; bottom surfaces. At top speed, only the foils and a small part of the wheel body of the wheels are submerged.  
         [0019]     In one implementation, these wheels are spring-loaded such that the wheels descend downward as the airfoils lift the boat and raise it from the water. The descending motion also is coordinated with control of an aileron on the adjacent foils such that lift is reduced. This action prevents the boat from becoming airborne and keeps the tracking foil submerged so it can perform its function.  
         [0020]     For example, in one embodiment, the wheel is attached to an arm that activates a mechanism to adjust aileron position. When enough lift has been provided to lift the wheel bodies mostly out of the water, leaving the bottoms of the foils submerged, any further lift causes ailerons to reduce lift and prevent the boat from becoming airborne. Other embodiments use electronic sensors and aileron position motors to accomplish this function. In other embodiments, wind speed is sensed, and when the wind exceeds the level at which the boat would become airborne, the ailerons are raised to reduce lift. In another embodiment, a water level sensor is used, alone or in combination with the above, to control the ailerons.  
         [0021]     In one embodiment, the wheel(s) in the aft section of the boat have only the wheel body and tread, with no foil, but are connected to a lift-reducing apparatus which is configured to coordinate with the nearby aileron.  
         [0022]     In one embodiment, in a third type of wheel, only the foil portion is used, the body and tread are omitted, and the wheel is used as a rudder. The foils on the aft wheels can be omitted so that they do not interfere with rudder function.  
         [0023]     In one embodiment, the craft includes a combination of a planing board, an airfoil and a rotating wheel at one or more locations. The wheel and planing board comprise an assembly referred to as a pod.  
         [0024]     In one embodiment, the boat is designed generally as a rectangle, similar to a catamaran. However, instead of a starboard and a port hull, at each corner of the catamaran, is a pod, such that there are two pods on the starboard side of the boat that replace the right hull of a traditional catamaran, and likewise two pods on the port side replace the left hull of a traditional catamaran. Airfoils are attached to the center of the boat and extend out to the pods. Wheels are mounted in the planing boards and their foils, if any, protrude below the bottoms of the boards.  
         [0025]     In one embodiment, planing hulls are not used, and the boat floats on its buoyant wheels.  
         [0026]     In one embodiment, the boat is in the form of a trimaran with two pods to starboard of a center hull and two to port. In such a configuration, the planing boards in the pods may be omitted with a planing board added to the center of the boat. This embodiment can result in a boat which is lighter and simpler to construct since the boat is not held up at the ends of the airfoils while at rest or in light winds since the pods at the ends of the airfoils contribute little to floatation.  
         [0027]     In some embodiments, the pods and/or wheels are spring loaded to adjust to waves. This reduces the drag experienced by the planing hull plowing into a wave.  
         [0028]     In some embodiments, there are multiple wheels in each pod.  
         [0029]     In some embodiments, the tread of the wheels are honeycomb structures bonded to the wheel on one side and open to the water on the other. As the wheel revolves, the honeycomb pockets become submerged, trapping air in each one. At rest or at low speeds, the buoyancy of the boat is enhanced by these air pockets, and the wetted surface of the boat is reduced, and the tread has less tendency to contribute to loss of energy caused by throwing water into the air at the back of the wheel. Essentially the honeycomb decouples the boat from the water.  
         [0030]     In some embodiments, a water level sensor is mounted in each pod to determine how much the pod has been lifted by its associated planing board, air foil and wheel. This sensor is used to adjust the position of an aileron on one or more the associated airfoils to control the amount of lift provided by the airfoil and to prevent the foil from coming completely out of the water. The adjustment can be automatic so that at high speeds the planing board is above the water surface and only a small portion of the wheel tread and body is submerged, leaving only the foil portion of the steering/tracking foil completely submerged. The adjustment could also (or instead) be controlled by the crew.  
         [0031]     In some embodiments, a trampoline deck is used and configured so that the crew can move out toward or beyond the windward pods countering the tipping moment of the sail. In other embodiments, the leeward airfoils are adjusted to increase lift to balance sail tipping moment and in still further embodiments the windward foils are adjusted to decrease lift or produce reverse lift to balance sail tipping moment. In some embodiments, the deck is a net so that its interference with air circulation necessary to produce lift is minimized.  
         [0032]     In some embodiments, the aft and forward airfoils are separately controlled to keep the boat level fore to aft. This is particularly useful when the boat is on a run (with the wind behind it). In this situation, some boats can be prone to tip over bow first. Control of the boat angle can prevent such a forward pitch.  
         [0033]     In some embodiments a wind shield is mounted on each pod to reduce drag due to wind similar to aerodynamic designs used in land vehicles (e.g., cars).  
         [0034]     In one embodiment, in which there are no ailerons on the airfoils, the boat is prevented from becoming airborne by adjusting the trim of the sail so that as the lift-off speed is approached, the pulling force of the sail is reduced. The trim may be automatically adjusted in response to a water level sensor or may be adjusted by the crew. In other embodiments in which there are no ailerons on airfoils, the angle of the airfoils may be adjusted to control lift.  
         [0035]     In one embodiment, the boat is operated by a single skipper and in others by a crew of two of more.  
         [0036]     In one embodiment, the boat is small, similar in size to a windsurfer, and the skipper operates the boat in a standing position and trims the sail by rocking the sail forward and aft as in a windsurfer. In this embodiment there is no rudder.  
         [0037]     The boat can be any sort of sailing craft, with any sort of configuration of sails and propulsion. In some embodiments there are multiple masts and sails. In some embodiments, the boat is equipped with a jib and/or a spinnaker.  
         [0038]     In one embodiment, an engine or other propulsion is used to accelerate the boat to the speed necessary for the airfoils to lift all but the wheel foils out of the water, thereby taking advantage of the resulting reduced displacement and skin friction drags. Once the necessary lift has been achieved, the engine is turned off.  
         [0039]     In some embodiments, the mast is taller than would typically be found in a similar-sized sailboat, reducing leakage of air off the top of the sail and decreasing the angle of attack of the boat to the apparent wind. The taller mast is facilitated by the availability of using the airfoils to counteract tipping moment. Masts twice the length of the boat or more are possible.  
         [0040]     In one embodiment, steering is controlled by a tail fin, like an airplane&#39;s. The tail fin can be used instead or in combination with a rudder.  
         [0041]     In one embodiment, the pods are arranged in a triangular configuration rather than a rectangle. Preferably, in such embodiment, the frame of the craft may be triangular. Generally in such embodiments the rudder will be at the point of the triangle, either forward or aft, and the wheel at that location will have a body and tread as well as a rudder foil.  
         [0042]     In some embodiments one or more pods pivot, or turn, like the wheels of a car, to provide a steering function. In some embodiments, just the wheels, not the pods, are configured to pivot, or turn, to provide steering. In a further embodiments, only forward pods or wheels pivot, only aft pods or wheels pivot, or pods or wheels in forward and aft positions pivot.  
         [0043]     In one embodiment, the pods are wide and are in the form of airfoils. In such an embodiment, the pods act both as planing boards at low speeds and airfoils at high speeds.  
         [0044]     In one embodiment the airfoil is designed to produce a stalling action at a desired maximum speed to assist in preventing the boat from becoming completely airborne. For example, as wind speed increases, airfoils with large lift angles will produce turbulence in the air flow pattern, which reduces lift. The angle of airfoils also may be adjustable to control lift.  
         [0045]     In various embodiments, commercially available boats may be modified, or designs for commercially available boats may be modified to include embodiments of the invention. In one exemplary embodiment, an A Class catamaran is modified to provide a lower cost version of the invention. An A Class catamaran is a light (e.g., 165 pound), fast catamaran, typically with a 30 foot mast, which can be sailed single or double-handed. In one embodiment, the hulls of such a boat are separated further than normal, the trampoline is tilted back, and in some configurations it is cambered to provide greater lift. In addition, airfoils may be placed in front of the mast to provide lift. In some such embodiments, the usual daggerboards and rudders may be replaced by rotating foils. The result is a boat that may lift substantially off the water in true wind speeds as low as 10-15 knots and may reach very high forward speeds in high winds. It should be understood that this embodiment is exemplary, and other boats may be modified in an analogous manner.  
         [0046]     In various embodiments, two or more airfoils may be used, e.g., a port-side airfoil or set of airfoils and a starboard-side airfoil or set of airfoils. The lift generated by each of the airfoils may be adjusted during operation, for example but not limited to by using ailerons, adjusting the airfoil angle, or by another technique, so that the airfoils can be used help control the angle of the boat to avoid tip-over. In this way, a boat may travel at speeds that might otherwise cause tip-over. For example, in a boat in which a skipper is hiking out, airfoils (and/or ailerons) on one or both sides may be adjusted to help hold the windward side down, and so control tipping. The adjustment of airfoils (and/or ailerons) may be automatic, for example but not limited to based on distance from water surface and/or boat angle and/or based on the weight distribution within the boat. Adjustments may be made manually instead of or in addition to automatic adjustment.  
         [0047]     In some embodiments, a sailing craft may include a frame, a sail connected to the frame, a port side airfoil for providing lift, and a starboard side airfoil for providing lift. The lift provided by the port side airfoil and the lift provided by the starboard side airfoil may be adjusted automatically to control tipping. The lift may be adjusted for example but not limited to by adjusting airfoil angle, adjusting ailerons, or some other method. In some implementations, there may be a set of two, three or more port side airfoils and/or a set of two, three or more starboard side airfoils. There may be additional airfoils not associated with one side or the other.  
         [0048]     As mentioned, the principles described here can be used in any suitable craft or configuration. For example, in one embodiment, a craft has a proa configuration in which the mast is on either the starboard or port side of the boat rather than in the middle. As another example, another embodiment has catamaran hulls instead of or in combination with the pods described above. In yet another embodiment, the craft is a trimaran, and all three·hulls lift out of the water at high speeds, and one or more of the hulls have rotating foils built-in or affixed thereto.  
         [0049]     In general, in one aspect, the invention uses the teachings of co-pending International PCT Application No. PCT/US2004/012241, entitled “Sailing Craft with Wheels,” which is incorporated herein by reference in its entirety. The technology described here, such as the use of airfoils and planing boards can be used effectively with the foils and crafts described therein. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0050]      FIG. 1  shows a three dimensional view of an illustrative embodiment of the invention.  
         [0051]      FIG. 2   a  shows a wheel with a peripheral foil such as might be used in the embodiment illustrated in  FIG. 1  in the forward two pods.  
         [0052]      FIG. 2   b  is a side view of a tracking wheel and  FIG. 2   c  is a cross section of a this wheel.  
         [0053]      FIG. 2   d  is an isometric view of a wheel such as would be used in the aft portion of the illustrative embodiment. Here the foil is omitted as it might interfere with the operation of the rudder.  
         [0054]      FIG. 2   e  shows a rotating foil used for steering.  
         [0055]      FIG. 3   a  shows the forward airfoil of the illustrative embodiment.  
         [0056]      FIG. 3   b  shows a top view of the forward airfoil illustrative embodiment with its associated pods while  FIG. 3   c  is the side view with the wheel in its lifted and unlifted positions.  
         [0057]      FIG. 3   d  is a top view of a forward airfoil for an ultra high speed embodiment in which the planing board is on the bottom of the central torsion box and the planing boards are omitted from the pods.  FIG. 4   e  is the side view.  
         [0058]      FIG. 3   f  illustrated a forward airfoil with the wings swept forward.  
         [0059]      FIG. 4   a  is a top view of a pod.  
         [0060]      FIG. 4   b  shows a side view when the airfoil has lifted the boat to the level which leaves only the wheel&#39;s foil submerged as would occur at top speed.  
         [0061]      FIG. 4   e  shows the position of the wheel in the pod and the position of the aileron with the boat as rest or a very low speed.  
         [0062]      FIG. 4   c  and  4   d  are the end views corresponding to  4   b  and  4   e .  
         [0063]      FIG. 5   a  is a top view of the rudder assembly of the illustrative embodiment while  5   b  is the side view with the boat at rest or at low speed.  
         [0064]      FIGS. 6   a ,  6   b ,  6   c  and  6   d  are vector diagrams showing motion of the boat for various true wind directions.  
         [0065]      FIG. 7   a  and  7   b  illustrate a pod in which the planing board can pivot to accommodate waves.  
         [0066]      FIG. 8  shows the action of spring loaded airfoils to accommodate waves.  
         [0067]      FIG. 9  describes the velocities of points on the rotating foil relative to the water.  
         [0068]      FIG. 10  shows an example of a commercially available boat modified to include embodiments of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0069]     Referring to  FIG. 1 , in one illustrative embodiment, a boat  100  has four pods  101 , a sail  102  and associated equipment, and a frame  103  with its associated equipment.  
         [0070]     In this embodiment, the sail  102  is attached to a mast  104  by conventional means. The mast  104  is held in place by a fore stay  105 , a starboard shroud  106  and a port shroud (mostly hidden behind the sail). The skipper controls the trim of the sail  102  using the main sheet  117  which is attached to the boom  118  and passes through the block  119 . This may be a conventional sail  102  and rigging, or may be a sail that is specially configured for operation of this boat.  
         [0071]     The skipper controls the boat from the deck, which in this embodiment is a trampoline  112 . The skipper steers the boat using a tiller extension  1   13  attached to a tiller  114  which in turn is attached to a tiller post  1   5 . This can be a conventional tiller and assembly. As in a conventional craft, the boat is steered by tiller operation that results in turning the rudder  121 . In this boat, however, the rudder is a freely rotating disk, such as a wheel as described below with reference to  FIG. 2   e.    
         [0072]     Forward and aft airfoils  109 ,  110  are attached to the frame  103 . These are equipped with ailerons  111 , which are adjusted to control the amount of lift provided by the airfoils.  
         [0073]     Each pod  101  includes a freely rotating wheel  107 , also with reference to  FIGS. 2   a  and  2   d , and a planing board  108 . The pods are covered by shields  122  to reduce aerodynamic drag. The pods are attached to the airfoils  109 ,  110  by posts  120 .  
         [0074]     A central structural member  116  of the frame runs fore to aft, and in this embodiment is in the form of a torsion box so that the pressure on the sail results in minimal twist to the boat, keeping the pods at approximately the same level with respect to the frame each other. In the preferred embodiment, this structure is approximately one foot square in cross section. It is fabricated from aluminum extrusions, but could instead be manufactured as a tube of composite material. Torsion boxes, for example, were widely used in early biplanes to prevent the wings from twisting. The supports used in them can be made from struts and cables.  
         [0075]     In the absence of wind, the boat floats on the planing boards  108 , such that the outer portions of the wheels  414  ( FIG. 4 ) protrude below the bottoms of the boards. In light winds, the boat operates in a similar manner to an ordinary sailboat. The wind generates forces which cause the boat to move forward, and the drag holding the boat back comes primarily from the energy taken to displace water as the planing boards move through it. This is referred to as a displacement mode of operation.  
         [0076]     In moderate winds the planing boards  108  will move more rapidly, and it becomes more difficult to move the water out of the way fast enough, so drag increases. This is called the forced mode. As the wind increases further, the planing boards  108  will begin to plane, i.e., rise up, and the boat enters a planing mode. In this mode the drag continues to increase with increased velocity by not at the same rate as in the forced mode because less and less water is displaced. For most boats, the displacement drag generally increases as the square of velocity in the displacement and planing modes and as the cube of velocity or higher in the forced mode. A second form of drag, skin friction drag, begins to significantly add to drag as the boat accelerates. This drag also generally increases as the square of velocity. Since the wetted surface area of the boat in the planing mode is less than in the displacement or forced modes, planing somewhat mitigates the increase in skin friction drag.  
         [0077]     As the boat continues to go faster, the airfoils provide lift, which raises the boat with its planing boards out of the water, thereby dramatically reducing both displacement drag, because less volume of water needs to be displaced, and skin friction drag because (i) the planing board is out of the water (ii)the bottom of the wheel is almost stationary relative to the surface of the water and (iii) the tread  205  ( FIG. 2   c ) is decoupled from the water by air trapped in the honeycomb pattern when the wheel is partially submerged.  
         [0078]     The lift of an airfoil varies as the square of wind velocity. Since displacement drag varies as the volume of water displaced by the hull which varies with depth of immersion, and skin friction varies as the wetted area which also varies as the depth of immersion, the lift of the airfoils counters some of the typical drag increases that result from increased velocity. This decrease in drag due to the airfoils will greatly facilitate the boat&#39;s acceleration compared to ordinary boats.  
         [0079]     As speed increases, the boat will lift until only portions of the wheels and the rudder wheel are immersed, and drag from the water is reduced to its minimum. This is the ideal running elevation. If the airfoils were to continue to increase lift, the boat might become completely airborne, and the wind will push it sideways and backwards, allowing the boat to descend back into the water but at a lower velocity.  
         [0080]     In one embodiment control of the airfoils is provided by a mechanism which senses the depth of the tread in the water. In this embodiment the tread does not fully lift out of the water as described below and illustrated in  FIGS. 4   b  and  4   c . However, since almost all the tread is above the water surface, wheel displacement drag and skin friction drag are extremely low compared to conventional boats.  
         [0081]     As wind speed increases, drag due to the airfoils increases. However, due to the lower density and viscosity of air compared to water, this drag increases at a much slower rate than the decreases in drag due to the planing board and wheel treads being lifted out of the water.  
       Wheels and Rotating Foils  
       [0082]     Referring to  FIG. 2   a , a three-dimensional view of a design of an exemplary wheel depicts an axle  201 , a body  202  with a tread  205  ( FIG. 2   c ) and a foil  203 . The body includes the core  204  ( FIG. 2   c ), tread  205  ( FIG. 2   c ) and axle  201 . In the illustrative embodiment, the foil  203  is not included on the aft wheels, as shown in  FIG. 2   d.  In  FIG. 1  the foil is shown  107  as protruding through the top of a forward pod aerodynamic shell, but as there is no foil used in the aft pods, no foil is shown.  
         [0083]     Referring briefly to  FIG. 2   c , in one embodiment, the tread  205  is attached to the outside of each cylinder of the body and consists of a honeycomb or other suitable structure bonded to a cylinder on one side and open to the water on the other. The core  204  of the wheel body is made from foam or other light weight material. In other embodiments the core may be hollow.  
         [0084]     Referring again to  FIG. 2   a , in an illustrative embodiment, the diameter of the foil  203  is about 36 inches and its thickness is approximately ⅛ inch. Any suitable material, for example, a composite material such as fiberglass re-enforced resin or stainless steel can be used. The foil protrudes six inches beyond the diameter of the body  202 . It must be sufficiently strong and stiff to withstand the sideways pressure exerted when the boat is underway.  
         [0085]     The core of the wheel may be formed using lightweight foam such as polystyrene or polystyrene foam sheets, such as those manufactured by the Owens Corning Company of Midland Michigan, available in standard two inch thickness, and which can be purchased in lumber supply stores. The sheets may be cut to the needed diameters and bonded together using Resorcinol glue, which can be obtained from Allerd &amp; Associates 2 South St., Auburn N.Y. 13021. In one embodiment, on the periphery of each cylindrical section of the wheel a layer of honeycomb  205  such as that shown in  FIG. 2   c  is bonded to the foam body using epoxy resin such as can be obtained from West System Inc. of Bay City, Mich. Honeycomb is available, for example, from Plascore Inc. of Zeeland Mich. The unbonded side of the core is left open so that air pockets are formed when the wheel is immersed and the amount of wetted surface area on the tread is kept low. Other suitable structures can also be used.  
         [0086]     The axle  201  can be made from any suitable material, and in the illustrative implementation is made from stainless steel. Preferably, in general, the metal materials used are preferably corrosion resistant or plated to prevent attack by salt water. Aluminum boat parts, for example, can be anodized.  
       Wheel Mounts and Planing Boards  
       [0087]     Referring to  FIGS. 4   a ,  4   b ,  4   c ,  4   d  and  4   e , the assembly of the wheel in the pod is shown with the pod&#39;s wind shield removed so that the inside of the pod can be seen.  FIG. 4   a  is a top view,  FIG. 4   b  a side view and  FIG. 4   c  a view from the back. These are two dimensional drawings using standard mechanical drawing protocols.  
         [0088]     Referring now to both  FIGS. 2   a  and  4   a , the axle  201  of a wheel is held by mounting brackets  411  at each end. Each bracket contains a set of ball bearings to allow smooth rotation of the wheel and a set of thrust bearings to accommodate sidewise thrust. The bearings can be commercially available standard stainless steel bearings, available, for example, from AFC Bearings, 11 E. 44 th  St., Suite 700, NY, N.Y. 10017. The mounting brackets  411  ride on shafts  412  which are attached to long L-brackets  405  which are bonded or bolted to the planing board  402 . The L-brackets  405  can be anodized aluminum or a composite material such as fiberglass and epoxy resin.  
         [0089]     In an illustrative embodiment, the L-brackets  405  are attached to the airfoil  406  by posts  408 . The wheel mounting is strengthened by braces  409  that form a torsion box to enhance the stability of the assembly. Referring to  FIG. 4   b , L-brackets  405  are mounted on the airfoil to provide anchors for the posts  408 . The posts  408  can be extruded aluminum tubing, available from Speed D Metals located in Willoughby Ohio or other commercial sources of extrusions, or can be pultrusion extruded composites from Ten Com LTD, Holland, Ohio or other commercial sources. Stainless steel may also be used for braces and brackets.  
         [0090]     An aerodynamic shell  122  ( FIG. 1 ), omitted from  FIG. 4  drawings for clarity, can be made from composite sheet material and can be formed using standard molding techniques such as described in the book “Fiberglass Composite Materials” by Forbes Aird, HP Books,1996, ISBN-1-55788-239-8. Flexible composite sheet stock cut and bent into shape can also be used. The aerodynamic shell is mounted to the planing board around its periphery using epoxy resin or rivets. In some embodiments the aerodynamic shell is omitted because it adds weight. It might not reduce aerodynamic drag by a substantial amount but may improve the appearance of the boat in commercial applications.  
       Planing Board  
       [0091]     Referring to  FIG. 4   a,  an exemplary planing board  402  is constructed of plastic foam. Its shape, density and thickness are similar to surf board and windsurfer boards. These boards can be custom manufactured. Custom surf board suppliers, such as Johnny Rice Custom Surfboards, Santa Cruz Calif., produce boards  402  of this type. In an illustrative embodiment, the planing boards are approximately 3 inches thick, 28 inches wide and 76 inches long at the bottom. A cutout is made in the middle of the hull for the wheel to protrude down into the water. Grooves toward the stem can allow water thrown off by the wheels to exit the pod.  
         [0092]     Information on hydrodynamic planing can be found in the book “High Performance Sailing” by Frank Bethwaite, International Marine,1993, ISBN 0-07-05799-0.  
       Airfoil  
       [0093]     Referring to  FIG. 3   a,  an airfoil follows conventional design practices for wings on light aircraft. These techniques are described, for example, in the book “Understanding Aircraft Composite Construction ” by Zeke Smith, Aeronaut Press, 1996, ISBN 0-9642828-1-X or “Composite Construction for Homebuilt Aircraft: The Basic Handbook of Composite Aircraft Aerodynamics, Construction, Maintenance and Repair Plus, How-to and Design Information” by Jack Lambie, 1996. In addition information on airfoil design can be found in “Foundations of Aerodynamics Bases of Aerodynamic Design” by Arnold Kuethe and Chuen-Yan Chow, John Wiley &amp; Sons, 1998, ISBN 0-471012919-4 and “Theory of Wind Sections” by Ira Abbott and Albert Doenhoff, Dover Publications, 1949, Standard Book Number 486-60586-8. Also software on airfoil design is available from DaVinci Technologies, LLC, in Laurel, Md. The basics of aerodynamic design can be found in “Introduction to Aerodynamics” by Gale Craig, Regenerative Press, 2002, ISBN 0-9646806-3-7.  
         [0094]     It sometimes comes as a surprise to some people that a boat can go faster than the wind because often people view a boat as being pushed from behind by the wind. In reality, a boat is propelled by the difference in air pressure between the air on the front of the sail and on the back of the sail. As the pressure in front of the sail is less than that behind it, the boat will move forward.  
         [0095]     If the wind is coming directed from the front of a sailboat, the sailboat will be driven backwards. However, if the wind is coming largely from the front, e.g., at an angle of 45 degrees from the front, if the flow of air along the front of the sail is faster then than behind it, the air pressure in front of the sail will be less than behind it, and the boat will be pushed forward. The keel and rudder are needed to prevent the boat from going sideways. This situation by itself places no limit on how fast the boat can go. Ice boats, for example, have been known to go as much as five times as fast as the wind.  
         [0096]     The direction of the wind experienced by the sail, called the apparent wind, is combination of the true wind, i.e., the direction sensed by a person standing on the shore and the wind generated by the motion of the boat, i.e., the wind experienced by the skipper of a motor boat traveling on a windless day. In technical terms this combination is the vector sum of the two winds, the true wind and the boat speed wind. For instance if the true wind is coming directly from the side of a sailboat, and the boat is traveling forward at same speed as the true wind, the two winds will have equal speeds but will be a right angles to each other and the apparent wind, i.e., the wind experienced by the sail will be coming at an angle of 45 degrees to the forward direction of the boat. This is the direction of the apparent wind. The angle of the sail with respect to the boat, normally called the trim of the sail, is adjusted to apparent wind to maximize the boat speed. (For a conventional boat a typical relationship between boat speed and wind speed as a function of the angle between the boat direction and the wind is illustrated on page 71 of the book by Marchaj previously mentioned.)  
         [0097]     For high speed sailboats, the relationships between the true wind, the boat speed wind and the apparent wind and the significance of these relationships to the performance of the airfoil can be better understood by referring to  FIGS. 6   a ,  6   b ,  6   c,  and  6   d.    
         [0098]     Referring to  FIGS. 6   a ,  6   b ,  6   c , and  6   d,  the dark arrows  601   a ,  601   b ,  601   c ,and  601   d  represent vectors for the true wind. The arrow&#39;s length represents the speed of the wind, and its angle its direction. The arrows with long dashes  602   a ,  602   b ,  602   c , and  602   d  represent the vectors for the wind generated by the forward motion of the boat, while the arrows representing the vectors for the apparent wind  603   a ,  603   b ,  603   c  and  603   d  have shorter dashes. The apparent wind is the wind which operates the sail. For each true wind example in each of  FIGS. 6   a ,  6   b ,  6   c , and  6   d,  diagrams of boats  604   a ,  604   b ,  604   c , and  604   d  are shown along with their sails  605   a ,  605   b ,  605   c , and  605   d  and direction of travel  606   a ,  606   b ,  606   c , and  606   d.  The angles between the apparent wind and the boat direction  607   a  and  607   b  are also shown, and for the illustrative examples in  FIG. 6   a  and  FIG. 6   b  are both 20 degrees.  
         [0099]     The  FIGS. 6   a - 6   d  are illustrative of a boat with a tall sail. A boat equipped with a tall sail can sail closer to the wind, i.e., more nearly into the wind, than conventional boats, for example as described on page 199 of Bethwaite&#39;s book, previously mentioned.  
         [0100]     In the example of  FIG. 6   a , a boat is driven by the true wind  601  a coming from behind and to starboard. With this geometry, the boat speed is shown as 2.3 times the true wind speed. Note that the angle between the apparent wind  603   a  and the boat direction  607   a  is 20 degrees.  
         [0101]     In the example of  FIG. 6   b  the true wind  601   b  is coming from the front of the boat. The apparent wind is still 20 degrees off the boat direction, but the boat speed is slower. As shown, the boat speed is 12% faster than the wind.  
         [0102]     In the example of  FIG. 6   c,  when the true wind  601   c  comes directly toward the boat, the boat will go backward  606   c.  This is an undesirable situation which occasionally occurs with amateur sailors.  
         [0103]     In the example of  FIG. 6   d,  the true wind  601   d  comes from directly behind. In this situation, the boat goes more slowly than the wind.  
         [0104]     As illustrated in these figures, for a large variety of true wind angles, for example in  601   a  and  601   b,  the angle that the apparent wind makes with the sail is the same if the skipper uses ideal sail trim.  
         [0105]     Then airfoil responds best to wind coming head on to the boat, but a 20 degree angle such as that shown in  FIGS. 6   a  and  6   b  is acceptable to obtain lift. If the true wind is coming largely from the side, the airfoil still can provide substantial lift since the apparent wind is from the front. But when the wind is directly from the back, the apparent wind direction is from the back of the boat so an airfoil is ineffective. Likewise, if the apparent wind is from the side, an airfoil will not generate lift. Very high speed sailboats cause the apparent wind to normally approach the boat mostly from the front, so for most true wind directions, the airfoils will generate lift.  
         [0106]     Referring to  FIG. 3   a , in the illustrative embodiment, the airfoil  301  is 18′ 4″ tip to tip and 5′ 1¾″ wide. In this embodiment, if the boat travels at twice the rate of the actual wind speed, the sum of the lifts of both the forward and aft airfoils will be approximately 130 lbs at an actual wind of 7.5 mph. Using for this example an approximate boat weight of 500 lbs, this will not be enough wind to raise the planing boards completely out of the water. But, at an apparent wind speed of 15 mph, the total lift is approximately 500 pounds, which can cause the boat to ride with primarily only the wheel foils submerged. As wind speed increases, the ailerons  302  are raised to reduce lift so that the boat does not become completely airborne.  
         [0107]     When the boat is moving in very light winds, the skipper would operate the boat from the center of the trampoline. As wind increases, he may move out to windward to counter the tipping moment of the sail and keep the boat level. Eventually, as wind continues to increase, he cannot go any further out, and the ailerons on the windward side can be raised to decrease lift and assist in countering the tipping moment. In one embodiment, this happens automatically: when the windward pods are lifted out of the water leaving only the foils submerged, this initiates an automatic aileron-raising mechanism, such as that described with reference to  FIG. 4 .  
         [0108]     In this embodiment, the trailing edge of the airfoil  301  with the aileron in its normal position is aimed about 12 degrees downward. Other embodiments with larger and smaller angles are possible, and since lift is directly proportional to the sine of the angle, larger angles can facilitate narrower airfoils. Lift is also directly proportional to airfoil length and width, and is proportional to the square of wind velocity.  
         [0109]      FIG. 3   b  shows a top view of the forward airfoil  301  for the illustrative embodiment, and depicts a torsion box  303 , pods  304  and the location of the wheels  305 .  
         [0110]      FIG. 3   c  is a side view of this assembly which shows the position of the wheels for the highest  306  and lowest  307  lift positions and their associated water levels  308 ,  309 .  
         [0111]     Wind impinging on the boat is rarely directly from the front of the boat because such a wind would merely drive the boat backwards. As previously described, under ideal operating conditions, the wind is about 20 degrees off the direction of the boat. Thus, unlike the airfoils of an airplane, the wind does not come head-on. The wind directions  310 ,  311  for starboard and port tacks are shown in  FIG. 3   b.    
         [0112]     In the illustrative embodiment the torsion box may interfere with air flow. This latter effect can be mitigated by using an embodiment such as shown in  FIG. 3   d.  Here the torsion box  313  is nearer the water, leaving a gap between it and the airfoil. A planing board  314  is mounted on the bottom of the torsion box and the planing boards may be omitted from the pods. As a result, the weight of the boat is reduced, and the boat is no longer held at the end of the wings which simplifies the design of the airfoils. As illustrated in  FIG. 3   e,  the wheel which operates the automatic lift mechanism is the same as for the illustrative embodiment.  FIG. 3   e  also shows the airfoil tipped slightly upward in front to enhance lift due to higher a angle of attack.  
         [0113]     In general, the airfoils are positioned as low as convenient to the water so that the airfoils operate in the ground effect region. In this mode of operation, speed is enhanced because induced drag from the vortices generated at the wingtips is less.  
         [0114]      FIG. 3   f  shows an airfoil configuration in which the wings sweep forward by the angle of best performance between wind direction and boat direction. If the wind is approaching somewhat from starboard, the leeward airfoil lift will be enhanced at the expense of the windward airfoil. Since it is desirable for the leeward lift to be larger in any case to counter tipping moment, the sweep wing approach can improve overall performance. In the figure, the direction of the wind  312  is shown for starboard tack.  
       Pod and Aileron Control Assemblies  
       [0115]     Referring again to  FIG. 4   a,  the interior of a pod is shown from the top. In this view both the aerodynamic shell of the pod and the airfoil have been deleted to show the parts inside the pod. The bottom surface  401  of the planing hull is adhered to the buoyant part of the board  402  using epoxy resin which is available from Fibre Glast Developments Corporation in Brookville, Ohio. The bottom surface  401  is a fiberglass and resin sheet bent at the bow for appropriate entry into waves. The sheet can be formed using pre-preg fiberglass sheet which can be obtained from Adhesive Prepregs for Composites Manufacturers, Plainfield Conn., or it can be made in a mold using techniques which are described in the book, “Fiberglass and Composite Materials” by Forbes Aird referred to above. The buoyant part is a foam sheet, made, for example, of polystyrene foam, such as that manufactured by Owens-Coming Company of Toledo, Ohio.  
         [0116]     A wheel  403  protrudes into the water through an opening  404  in the planing board.  
         [0117]     The wheel  403  has an axle  410 ,  201  ( FIG. 2   a ) that extends into bearing housings  411  that ride on rods  412 . Springs  413  press the bearing housings  411  down. The movement of the wheels  403  is illustrated in  FIGS. 4   c  and  4   d,  which show the interior of the pod from the back.  FIG. 4   c  shows the wheel in its lowest position, which corresponds to a high wind situation in which the airfoils have lifted the boat so that only the foil  414  and a small part of the wheel body are in the water. The dotted line  415  delineates the level of the water.  
         [0118]     In light air, the wheel can go up to the position shown in  FIG. 4   d.  These figures also show braces  409  used to strengthen the assembly and the mounted airfoil  406 .  
         [0119]     The operation of the illustrative embodiment&#39;s automatic system used to reduce lift as wind reaches or exceeds an acceptable limit is shown in  FIGS. 4   b  and  4   e.  As illustrated in  4   e,  a rod  416  is attached to one of the bearing housings  411 . The rod can pivot at the bearing housing. The rod passes through a bushing  417  which is mounted on a brace  418  which in turn is mounted on a cross brace  419  ( FIG. 4   a ) between the aft two posts. The rod also passes through a bushing  420  on the tip the aileron  421 . As the boat lifts, and the water level gets further below the airfoil, the springs depresses the wheel, and the rod moves from the position shown in  FIG. 4   e  to that shown in  FIG. 4   b . This moves the tip of the aileron upward, which reduces lift, since the amount of lift is dependent on the angle at which air leaves the tip of the aileron.  
         [0120]     Using this arrangement, the skipper of the boat does not have to control the four ailerons. In other embodiments, the skipper can control the ailerons manually or in combination with an automatic system. For example, the height of the wheel can be measured using a displacement sensor that generates a signal that is used to control a motor that moves an aileron. Arrangements for moving ailerons are common in airplanes.  
         [0121]     In some embodiments, the aileron can be raised far enough so that air leaving the aileron is aimed upward. This is useful if it is desired to use reverse lift on the windward side of the boat to compensate for the tipping moment of the sail. Again the action can be automatic, since the tipping action will cause the windward wheels to lift and the leeward wheels to lower. The aileron adjustment mechanism described above decreases and perhaps reverses the lift of the windward airfoils, while the lift of the leeward airfoils is increased. Thus the overall effect will be to keep the boat more level.  
         [0122]     As an added benefit, if a lifted pod encounters a wave, its wheel will rise due to the buoyancy of the wheel, tipping the aileron down thereby increasing lift and assisting the boat to ride over the wave. Likewise at a wave trough, the lift will decrease forcing the boat down into the trough.  
         [0123]     In embodiments which use airfoil/pod assemblies spring loaded at the central torsion box, the ability of the boat adjust to waves and ride at the appropriate height above the water is further enhanced. Such an embodiment is illustrated in  FIGS. 8 . This embodiment uses the planing board under the torsion box as illustrated in  FIG. 3   d  and  3   e.  These figures show the action of the forward airfoils. Aft airfoils operate similarly.  
         [0124]     The airfoils mount on posts  804  and attached to hinges  805  which include a pivot point  806 . Springs  807  hold the airfoils in place.  
         [0125]     In other embodiments accommodation to waves can be implemented as shown in  FIG. 7   a  and  7   b.  These figures show a pivoting planing board in a pod. The pivoting action is provided so that the pod rides up and down waves more easily rather plowing through them. The airfoil  701  is held by struts  702  to the planing board pivot point  703  on which the wheel  704  is held. If the water level  705  angle changes as a wave is encountered the planing board  706  will tip to adjust to it.  
       Rudder and Tiller Assembly  
       [0126]      FIG. 5   a  shows a top view of the illustrative embodiment&#39;s rudder assembly. A side view is shown in  FIG. 5   b.  The rudder  501  is a rotating foil. The rudder  501  is attached to a small diameter rim  502  and hub assembly  503 , such as used in tricycles. The assembly  503  shown has a hub with ball bearings, spokes and a rim, and can be obtained from retail bicycle stores. The rudder is attached to the rim  502  using bolts and/or adhesives. The rudder is a thin composite disk made from fiberglass and resin. Alternatively, the disk may be fabricated from stainless steel. The rudder is held in place by a fork  504  similar to that used in a bicycle. The top part of the fork is a rod  505  as in a bicycle which passes through two braces  506  which are attached to the torsion box  507  with four braces  508 . The fork assembly operates like a bicycle&#39;s except that instead of handle bars a conventional pulley wheel  509  is mounted on the top. A cable  510  connects the pulley wheel  509  to a second pulley wheel  512  which is mounted on the torsion box  507 . Such cables and pulley wheels can be obtained from hardware and industrial suppliers.  
         [0127]     The tiller  512  is attached to the second pulley  511 , and a tiller extension  513  is connected to the tiller. This assembly can be obtained from marine supply retailers such as Layline in Rayleigh N.C. The steering assembly thus operates as in normal sailboats such that as the tiller is pulled to starboard, the boat turns to port. As a rotating foil, this rudder has the advantage described above of decreased drag in the direction of travel.  
       Foil Skin Friction and Cavitation  
       [0128]     The advantages of rotating foils are further illustrated with reference to  FIG. 9 , which depicts a wheel  902  submerged up to the water line  904 . The boat is traveling to the right in the figure as shown by the arrow  905 . Since the boat is traveling with respect to the water, the axle  903  of the wheel  902  is traveling at the same speed and in the same direction as the boat. The parts of the foil  901  and wheel body  906  which are below the water line experience skin friction drag when the boat moves. Since the drag is at a distance from the hub and the axle and the axle rides on ball bearings, the wheel will turn much like a car&#39;s wheels rotate as a car moves forward on pavement. The rate of rotation will be constant if the boat&#39;s speed in constant. In this situation the periphery of the wheel  907  will be approximately stationary with respect to the water at its lowest point and will have a speed of twice the boat speed at its top  908 . At all other points it will have a forward component and vertical component either down or up depending on whether the point is ahead of the axle or behind it.  
         [0129]     The speeds of various points on the wheel&#39;s foil are depicted using vectors. The vectors represent the velocity of the boat  905   a ,  905   b ,  905   c  and  905   d  as shown by the solid arrow at points a, b, c, and d in the vector diagrams. The vectors representing the velocities  909   a ,  909   b ,  909   c  and  909   d  due to rotation at points a, b, c, and d are the heavy dotted line with the small dashes. The resultant vectors representing the actual speeds and directions of the points are represented by the fine dotted lines  910   a ,  910   c ,  910   d  with the long dashes.  
         [0130]     At point b, the resultant vector has a value of zero, indicating that point is approximately stationary with respect to the water. At a and c, the result vector has speed about 90% of the boat speed for the embodiment shown. At d the speed is about 57% of boat speed. On average, the points on the submerged portion of the foil will be traveling at about half the boat speed. As a result, the onset of serious cavitation occurs at a much higher boat speed than if the foil were being dragged forward through the water, as happens in conventional boats. Furthermore the lowest part of foil, which performs most of the function of the foil, travels at the lowest speed relative to the water.  
         [0131]      FIG. 9  assumes that the bottom of the foil is stationary with respect to the water. This is only an approximation because some water will adhere to the foil and will be thrown off by centrifugal force. The energy required for this may diminish wheel rotational speed, but only by a small amount.  
         [0132]     Referring to  FIG. 10 , embodiments of the invention may involve modifications to commercially available boats to add features described here, for example, components designed to provide lift, and rotating foils, to reduce surface friction, and allow a boat so modified to travel faster. It should be understood, however, that just the airfoils, or just the rotating foils may be used, alone, or in combination with other features described here.  
         [0133]     An illustrative example boat shown in the figure is an A Class catamaran (“Acat”) that has been modified to include features of the invention. The figure shows the boat going up-wind (beating) on starboard tack in a high wind.  
         [0134]     An Acat typically has hulls that are approximately 18 feet long, separated by 8 feet, and a mast which is 30 feet tall, with a total sail area of 150 square feet. A Class catamarans may be obtained, for example, from Performance Catamarans, 1800 East Boarchard Ave., Santa Ana Calif. 92705.  
         [0135]     As shown in the figure, a modified Acat has hulls,  1001  and  1018 , that are separated by approximately 12 feet, four feet more than normal. Additional separation increases the sizes of the trampoline  1002  and airfoils  1003 ,  1005  that may be used for lift and may allow the boat to become substantially airborne at lower wind speeds.  
         [0136]     As is typical, a trampoline  1002  is mounted between the two hulls  1001 ,  1018 . To provide lift in this modification, however, the trampoline  1002  is tilted upward in front and cambered (curved) in a manner that enhances lift (e.g., in the shape of an airplane wing, or other suitable shape). Thus, in this exemplary implementation, the trampoline also may be referred to as an airfoil, in that it serves both the functions of a trampoline and an airfoil. The trampoline also may be made of any other suitable material, for example but not limited to a solid material such as foam or sailcloth.  
         [0137]     Two additional airfoils,  1003  and  1005 , are mounted in front of the mast  1004 . In some embodiments, these airfoils are made from sailcloth, but these also may be made of any suitable material, including but not limited to rubber or foam. If made of sailcloth, the airfoils  1002 ,  1003 , and  1005  may be adjusted to control lift, for example manually and/or automatically, and it should be understood that various techniques may be used to control the angle of the airfoils  1002 ,  1003 , and  1005 . It should be understood that various embodiments may include only the modified trampoline, additional airfoils, or both.  
         [0138]     In this exemplary implementation, a rotating foil  1006  is in a position that may typically hold a dagger board in an Acat. In this exemplary implementation, a rotating foil is also used for rudder  1007 . As shown, the top portion of the foil may be covered by a shield  1017  to prevent a sailor from disturbing the foil. For example, the skipper of the boat may operate the boat from this area in strong winds. The rotating rudder  1007  is held by forked bracket  1008 , similar to that used to hold the front wheel of a bicycle. The forked bracket is held in place by a fixed bracket  1009  affixed to the hull  1001 . The rudder is turned by a tiller assembly  1010 .  
         [0139]     The boat&#39;s sail  1011  is attached to the mast  1004  and the boom  1012  in a manner typical for sailboats. The trim of the sail is adjusted by a main sheet  1013 , also in typical fashion. The hulls are held in place by rods  1014 ,  1015  and  1016 .  
         [0140]     Each of the forward airfoils,  1003  and  1005 , is mounted on a rectangular frame  1018 . The sailcloth airfoils are loosely affixed to the forward and aft parts of the frames so they assume an upward camber if the frame is tilted up and a downward camber if it is tilted down. The fames are attached to brackets  1019 . The brackets contain a pivot point  1020  at the position in the airfoil representing the center of lift for the airfoil. At the front edge of an airfoil&#39;s frame a bracket  1021  is attached with a small water ski  1022  on its bottom. A spring at the pivot point  1020  of the bracket causes slight pressure on the frame to cause the ski to push down slightly into the water.  
         [0141]     In the absence of wind both airfoils  1003  and  1005  assume a tilt shown by the port airfoil  1003 . As the wind increases, all three of the airfoils,  1002 ,  1003  and  1004  contribute to lifting the boat out of the water thereby reducing displacement and frictional drag. As the boat rises the lowering skis cause the tilt of the airfoil frame to lessen, thereby reducing lift and diminishing the tendency of the boat to become airborne in a puff.  
         [0142]     As wind speed continues to increase, the weight of the skipper hiking out on the windward hull may be insufficient to hold the windward hull down on the water. In this condition, the starboard hull  1001  is lifted out of the water sufficiently far that it is desirable to reduce the lift on the starboard side of the boat so that the boat does not tip over. The port hull  10018  is either at the surface of the water or its bottom is slightly submerged.  
         [0143]     As shown in  FIG. 10  when the windward hull  1001  lifts out of the water the windward ski  1022  descends downward staying on the surface of the water, and the windward airfoil  1005  assumes a downward tilt. The upward tilt of the leeward airfoil  1003  and the downward tilt of the windward airfoil  1005  act to right the boat assisting the hiking skipper to keep the boat at the desired angle.  
         [0144]     In some embodiments adjustment of the tilts of the forward airfoils  1003  and  1005  could be done by the crew, but since the skipper needs to trim the sail and steer the boat, a second crew member would be needed and his extra weight would slow the boat down.  
         [0145]     A boat as configured in this exemplary figure may not achieve the speeds of an embodiment such as that shown in  FIG. 1 ; however, it may be less expensive to construct as many of the parts are used in boats already in production.