Abstract:
A hydrofoil craft includes a hull having a longitudinal axis, a pylon secured to and extending beneath the hull and a lifting foil secured to the pylon. The lifting foil has an upper surface and a lower surface. The upper surface of the lifting foil is substantially planar and the lower surface of the lifting foil is not coplanar with the upper lifting surface. The lifting foil has a fore portion and an aft portion that are traversed by a longitudinal axis and wherein the longitudinal axis is substantially parallel to the longitudinal axis of the hull and the thickness of the foil is greater at the aft portion than at the fore portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation-in-part of U.S. patent application Ser. No. 10/364,589 filed Feb. 10, 2003 now U.S. Pat. No. 6,948,441. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   n/a 
   FIELD OF THE INVENTION 
   The present invention relates to hydrofoil marine vehicles and more particularly to a hydrofoil configuration to mitigate the effects of wave shock. 
   BACKGROUND OF THE INVENTION 
   The hydrofoil vehicle is analagous to an aircraft, where the wings operate under water. The basic principle of the hydrofoil concept is to lift a craft&#39;s hull out of the water and support it dynamically on the submerged wings, i.e. hydrofoils. The hydrofoils can reduce the effect of waves on the craft and reduce the power required to attain modestly high speeds. As the craft&#39;s speed is increased the water flow over the hydrofoils increase, generating a lifting force and causing the craft to rise. For a given speed the craft will rise until the lifting force produced by the hydrofoils equals the weight of the craft. 
   In a typical arrangement, struts connect the hydrofoils to the craft&#39;s hull, where the struts have sufficient length to support the hull free of the water surface when operating at cruise speeds. As shown in  FIGS. 1   a – 1   c , the basic choices in hydrofoil and strut arrangement are conventional, canard, or tandem. In an example of a conventional arrangement, as shown in  FIG. 1   b , a pair of struts and hydrofoils are positioned fore of the craft&#39;s center of gravity, symmetrical about the craft&#39;s longitudinal centerline, and a single strut and hydrofoil is positioned aft of the craft&#39;s center of gravity along the craft&#39;s longitudinal centerline. In a canard arrangement, as shown in  FIG. 1   c , a single strut and hydrofoil is positioned fore of the craft&#39;s center of gravity along the craft&#39;s longitudinal centerline, and a pair of struts and hydrofoils are positioned aft of the craft&#39;s center of gravity, symmetrical about the craft&#39;s longitudinal centerline. 
   Alternatively, the pairs of struts can include a single hydrofoil, spanning the beam of the craft. Generally, craft are considered conventional or canard if 65% or more of the weight is supported on the fore or the aft foil respectively. 
   In a tandem arrangement, as shown in  FIG. 1   a , pairs of struts and hydrofoils are positioned fore and aft of the craft&#39;s center of gravity and symmetrically about the craft&#39;s longitudinal centerline. Alternatively, the pairs of struts can include a single hydrofoil, spanning the beam of the craft. If the weight is distributed relatively evenly on the fore and aft hydrofoils, the configuration would be described as tandem. 
   The hydrofoil&#39;s configuration on the strut can be divided into two general classifications, fully submerged and surface piercing. Fully submerged hydrofoils are configured to operate at all times under the water surface. The principal and unique operational capability of craft with fully submerged hydrofoils is the ability to uncouple the craft to a substantial degree from the effect of waves. This permits a hydrofoil craft to operate foil borne at high speed in sea conditions normally encountered while maintaining a comfortable motion environment. 
   However, the fully submerged hydrofoil system is not self-stabilizing. Consequently, to maintain a specific height above the water, and a straight and level course in pitch and yaw axes, usually requires an independent control system. The independent control system varies the effective angle of attack of the hydrofoils or adjusts trim tabs or flaps mounted on the foils, changing the lifting force in response to changing conditions of craft speed, weight, and sea conditions. 
   In the surface piercing concept, portions of the hydrofoils are configured to extend through the air/sea interface when foil borne. As speed is increased, the lifting force generated by the water flow over the submerged portion of the hydrofoils increases, causing the craft to rise and the submerged area of the foils to decrease. For a given speed the craft will rise until the lifting force produced by the submerged portion of the hydrofoils equals the weight of the craft. However, because a portion of the surface-piercing hydrofoil is always in contact with the water surface, and therefore the waves, the surface-piercing foil is susceptible to the adverse affect of wave action. The impact of the waves can impart sudden, large forces onto the struts and craft, resulting in an erratic and dangerous motion environment. 
   Additionally, hydrofoil configurations can include a stack foil, or ladder foil, arrangement, where upper foils are used to provide lift at lower speed, initially raising the craft above the waterline. As the craft&#39;s speed is increased, the lower foils produce sufficient lift to support the weight of the craft, further raising the upper foils above the waterline to the cruise height. However, when a wave impacts the craft the upper foil can be instantaneously wetted, producing a sudden increase in lift. The sudden increase in lift produces a jarring impact on the craft, and in some instance can be sufficient enough to instantaneously raise the entire craft, including the main foils, above the waterline. 
   A hydrofoil vehicle is configured to operate at a particular cruise speed. The cruise speed is the speed at which the total lifting force produced by the hydrofoils equals the all up weight of the hydrofoil vehicle. Operating at speeds greater than the cruise speed can cause the hydrofoils to produce excessive lift, resulting in a cyclic skipping action. At speeds less than the cruise speed, when the hydrofoils do not produce sufficient lift to raise vehicle results in the hull crashing into the water. 
   Propulsion systems for hydrofoil vehicles can include both water and air propulsion systems. In an exemplary arrangement of a water propulsion system, a water propeller provides the propulsive force, where a drive shaft operably connects the water propeller to an engine. Alternatively, a water jet can be used to provide the propulsive force, where water is funneled through a water intake into the water jet. The water jet accelerates the water, expelling the water through the outlet creating a propulsive force. Air propulsion systems can include for example, air propeller or jet engines. As shown in U.S. Pat. No. 4,962,718 to Gornstein et al., an air propeller is positioned on the deck of the craft and operatively connected to an engine. 
   SUMMARY OF THE INVENTION 
   The present invention provides a shock mitigation system for hydrofoil marine craft. The shock mitigation system includes a pair of stacked lifting bodies, where an upper lifting body is used to provide initial lift for the craft. As the craft&#39;s speed is increased, the lower lifting body produces sufficient lift to raise the craft and upper lifting body to a specified cruising height. The craft is configured to operate at this selected cruising height and at a maximum wave height, where the wave height is defined as the distance between the crest and trough of a wave. To mitigate the wave effects on the craft when operating at the selected cruise height, the distance between the upper lifting body and the waterline is proportionally related to the maximum wave height to be encountered. When used within the operational parameters, the distance between the upper lifting body and waterline prevents the upper lifting body from becoming wetted and producing sudden increases in lift from wave impact. 
   The hydrofoil marine craft is configured to operate at a selected cruise height above the waterline. This selected cruise height can be maintained by adjusting the thrust output of the propulsion system. To raise the craft to the selected cruise height, the thrust output is increased. Similarly, to lower the craft to the selected cruise height, the thrust output is decreased. 
   Alternatively, the cruise height can be maintained by adjusting the lower lifting body&#39;s angle of attack. An increase in the angle of attack will result in an increase in lift, raising the craft to the selected cruise height. A decrease in the angle of attack will result in a decrease in lift, lowering the craft to the selected cruise height. 
   Advantageously, the above system can also be used to increase or decrease the cruise speed, while maintaining the selected cruise height. For example, a decrease in the angle of attack and an increase in the thrust will result in a higher cruise speed, while maintaining the selected cruise height. Similarly, an increase in the angle of attack and a decrease in the thrust will result in a lower cruise speed, while maintaining the selected cruise height. 
   In an alternative configuration a hydrofoil craft includes a hull having a longitudinal axis, a pylon secured to and extending beneath the hull and a lifting foil secured to the pylon. The lifting foil has an upper surface and a lower surface. The upper surface of the lifting foil is substantially planar and the lower surface of the lifting foil is not coplanar with the upper lifting surface. The lifting foil has a fore portion and an aft portion that are traversed by a longitudinal axis and wherein the longitudinal axis is substantially parallel to the longitudinal axis of the hull and the thickness of the foil is greater at the aft portion than at the fore portion. 
   In yet another configuration for a shock limitation system, a marine craft is configured for operation in water having a known wave height and includes a hull adapted to carry a payload and first and second lifting bodies secured below the hull a predetermined distance, wherein the predetermined distance exceeds the known wave height. The first and second lifting bodies, as well as the hull can be displacement hulls and the first and second lifting bodies can be secured to the hull with struts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
       FIGS. 1   a – 1   c  are prior art hydrofoil configurations of hydrofoil marine craft; 
       FIG. 2  is a side view of the hydrofoil marine craft of the present invention; 
       FIG. 3  is a front view of the hydrofoil marine craft of the present invention; 
       FIG. 4  is a front view of an alternative hydrofoil marine craft configuration of the present invention, including a vertical stabilizer; 
       FIG. 5  is a front view of an alternative hydrofoil marine craft configuration of the present invention, including submerged hydrofoils; 
       FIG. 6  is a front view of an exemplary hydrofoil marine craft including a planing hull configuration of the present invention; 
       FIG. 7  is a flow chart for a variable thrust control system of the present invention; 
       FIG. 8  is a side view of a hydrofoil marine craft including lower hydrofoil with an adjustable angle of attack configuration of the present invention; 
       FIG. 9  is a flow chart for a cruise height control system of the present invention; 
       FIG. 10  is a flow chart for a cruise speed control system of the present invention; 
       FIG. 11  is a sectional view of a foil in accordance with the invention; 
       FIG. 12  is a sectional view of another foil in accordance with the invention; 
       FIG. 13  is a sectional view of yet another foil in accordance with the invention; 
       FIG. 14  illustrates the top surface of a foil showing fences disposed along the span of the foil; 
       FIG. 15  illustrates the top surface of a foil showing an alternate structure for upper surface boundary layer control; 
       FIG. 16  is a view of from the bow of a vessel looking aft and showing foils as set forth in  FIG. 1 ; 
       FIG. 17  illustrates another embodiment of a shock mitigation system. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention advantageously provides a shock mitigation system for hydrofoil marine craft. The shock mitigation system includes a pair of stacked lifting bodies, where an upper lifting body is used to provide initial lift for the craft. As the craft&#39;s speed is increased, the lower lifting body produces sufficient lift to raise the craft and upper lifting body above the waterline, reaching a targeted cruise height. The craft is configured to operate at a selected maximum wave height, where wave height is defined as the distance between the crest and trough of a wave. To mitigate the wave effects on the craft when operating at the cruise height, the distance between the upper lifting body and the waterline is proportionally related to the maximum wave height. When used within the operational parameters, the distance between the upper lifting body and the waterline prevents the upper lifting body from becoming wetted and producing sudden increases in lift from wave impacts. 
   In an exemplary embodiment, as shown in  FIGS. 2 and 3 , the hydrofoil marine craft  10  includes a conventional hydrofoil arrangement, having a pair of lifting bodies positioned fore of the craft&#39;s center of gravity “CG”, symmetrical about the craft&#39;s longitudinal centerline, and lifting bodies positioned aft of the craft&#39;s center of gravity along the craft&#39;s longitudinal centerline. Each of the fore lifting bodies is attached to the craft&#39;s hull  14  with a support structure, which includes a strut  16  and a pylon  18 . The struts  16  are affixed to the craft&#39;s hull  14  and extend laterally outward from the craft  10 . The pylons  18  are affixed to the ends of the struts  16 , opposite the craft  10 , and extend substantially, vertically downward, where the lifting bodies are operably connected to the pylons  18 . The strut  16  can be used to provide increased roll stability to the craft  10 , where the lateral distance that the strut  16  extends is a function of the craft&#39;s  10  specific configuration, depending on the craft&#39;s  10  operational parameters. Alternatively, the pylons  18  can be affixed directly to the hull  14 . The aft lifting bodies are attached to the craft&#39;s hull  14  with a center pylon  20 , where the center pylon  20  is affixed to the hull  14  along the craft&#39;s centerline and the lifting bodies are operably connected to the center pylon  20 . 
   In an exemplary embodiment, as shown in  FIG. 3 , the upper lifting bodies are takeoff foils  22   a  and  22   b  and lower lifting bodies are main foils  24   a  and  24   b . The takeoff foils  22   a  and  22   b  are positioned on the pylons  18  and  20  above the main foils  24   a  and  24   b  and are used to provide lift at lower speeds, initially raising the craft  10  above the waterline “WL”. As the speed of the craft  10  increases to the cruising speed, the main foils  24   a  and  24   b  produce sufficient lift to support the weight of the craft  10 , further raising the craft  10  and takeoff foils  22   a  and  22   b  above the waterline “WL” to the targeted cruising height. The distance between the main foils&#39;  24   a  and  24   b  mid span and the takeoff foils  22   a  and  22   b  is such that at the target cruising height, a distance “WH” is maintained between the lowest sections of the lifting surfaces of the takeoff foils  22   a  and  22   b  and the waterline “WL”. The distance “WH” is an operational parameter, dependent on the selected maximum operational wave height. For example, the distance “WH” is substantially equal to one-half the wave height. 
   The fore main foils  24   a  are surface piercing foils, where at the target cruise height a portion of the fore main foil  24   a  extends through and above the waterline “WL.” The fore main foils  24   a  each include a pair of dihedral foil sections symmetrically attached to the pylon  18  at an angle α from the horizontal axis, where the angle α can be between about 15 degrees and 50 degrees. At the target cruise height, the submerged portion of the fore main foils  24   a  can be from 33% to 80% of the foil&#39;s span length “FS”, and in an embodiment can be about 50% of the main foil&#39;s span length “FS”. 
   The fore takeoff foils  22   a  are dihedral foil sections asymmetrically attached to the pylons  18  at an angle β from the horizontal axis, where the fore takeoff foils  22   a  are directed inward and downward, towards the craft&#39;s  10  center line. The dihedral angle β can be between about 10 degrees and 45 degrees. The distance “WH” is measured from the lower tip of the takeoff foils  22   a  to the water line “WL.” 
   The aft main foils  24   b  are surface piercing foils, where at the target cruise height a portion of the aft main foil  24   b  extends through and above the waterline “WL.” The aft main foils  24   b  include a pair of dihedral foil sections symmetrically attached to the center pylon  20 . The dihedral angle of the aft main foil  24   b  is configured such that the upper most elevation of the aft main foil  24   b  tips matches the upper most elevation of the fore main foil  24   a  tips, and the lowest elevation of the aft main foil  24   b  matches the lowest most elevation of the fore main foil  24   a . At the targeted cruise height, the submerged potion of the aft main foil  24   a  can be from 33% to 80% of the foil&#39;s span length “FS”, and in an embodiment can be about 50% of the main foil&#39;s span length “FS”. 
   The aft takeoff foil  22   b  includes a pair of dihedral foil sections symmetrically attached to the center pylon  20 . The dihedral angle of the aft takeoff foil  22   b  is configured such that the upper most elevation of the aft takeoff foil  22   b  tips matches the upper most elevation of the fore takeoff foil  22   a  tips, and the lowest elevation of the aft takeoff foil  22   b  matches the lowest most elevation of the fore takeoff foils  22   a . The distance “WH” is measured from the lower portion of the interface between the aft takeoff foil  22   b  and the center pylon  20  to the water line “WL.” 
   The shock mitigation system of the present invention maintains the lift equilibrium between the fore and aft main foils  24   a  and  24   b  during wave impact. As shown in  FIG. 3 , at a selected cruise height the waterline “WL” is positioned at about one-half the span of the fore and aft main foils  24   a  and  24   b , where the end tips of the fore and aft main foils  24   a  and  24   b  extend above the waterline “WL”. As such, the lift provided by the submerged portions of the fore and aft main foils  24   a  and  24   b  is in a state of equilibrium. When a wave impacts the craft  10 , additional portions of the fore and aft main foils  24   a  and  24   b  will be temporary submerged, providing an instantaneous increase in lift. To maintain the lift equilibrium between the fore and aft main foils  24   a  and  24   b , the ratio of instantaneous lift provided by the fore and aft main foils  24   a  and  24   b  should be substantially equal to the lift ratio of the fore and aft main foils  24   a  and  24   b  in calm seas. 
   Shock mitigation occurs when a wave washes completely over the main foils  24   a  and  24   b . The normal lift equals the all-up weight when the foils are 50% wetted. When totally wetted, the maximum lift is limited to twice the all-up weight−capping the lift force at +100% of the designed lift. A wave trough can uncover the foil reducing the lift to zero, capping the lift at minus 100%. This shock mitigation to plus or minus 100% is intrinsic to the present invention. 
   Additionally, as show in  FIG. 4 , the fore takeoff foils  22   a  can include a pair of dihedral foil sections symmetrically attached to the pylon  18  at a dihedral angle δ from the horizontal axis, where the angle δ can be between about 10 degrees and 45 degrees. The distance “WH” is measured from the lower portion of the interface between the fore takeoff foils  22   a  and the pylons  18  to the waterline “WL.” 
   In a further exemplary embodiment, at least one vertical stabilizer  26  is affixed to and extends from at least one of the pylons  18  and  20 . As shown in  FIG. 4 , a vertical stabilizer  26  is affixed to and extends from the aft center pylon  20 , where the vertical stabilizer  26  provides additional stability to prevent the craft  10  from yawing. The vertical stabilizer.  26  can additional dampen roll. Alternatively, the vertical stabilizer  26  is retractable, where the vertically stabilizer, for example, is drawn up into the pylons  18  and  20 . 
   As shown in  FIG. 5 , the hydrofoil marine craft  10  can further include a set of submerged foils  28   a  and  28   b . The submerged foils  28   a  and  28   b  are mounted on the pylons  18  and  20  below the main foils  24   a  and  24   b . The submerged foils  28   a  and  28   b  are configured to provide a lifting force such that the submerged foils  28   a  and  28   b  operating cooperatively with the main foils  24   a  and  24   b  to provide the all-up lift at the cruising speed. The submerged foils  28   a  and  28   b  partially uncouple the craft  10  from the effects of the waves, while maintaining the intrinsic stability provided by the surface piercing main foils  24   a  and  24   b.    
   The submerged foils  28   a  and  28   b  are positioned a distance “SH” below the main foils  24   a  and  24   b , where the distance “SH” is at least equal to or greater than “WH.” In an exemplary embodiment, “SH” is substantially equal to four times the chord length of the submerged foils  28   a  and  28   b.    
   In an alternative exemplary embodiment, as shown in  FIG. 6 , the hydrofoil marine craft  10  is a planing craft, where the craft&#39;s hull  14  is a planing hull capable of providing lift at lower speed, acting as an upper lift body  30 . As the craft&#39;s speed is increased, the craft  10  rises to plane, raising a substantial portion of the craft&#39;s hull  14  above the waterline. As the speed is further increased, the lower lifting bodies, main foils  24   a  and  24   b , produce sufficient lift to raise the craft  10  to the target cruise height. The distance “WH” is measured from the lowest point on the hull  14  to the waterline “WL” and is maintained at cruising speed. 
   The hydrofoil marine craft  10  can optionally include a tandem foil arrangement, including pairs of struts and hydrofoils positioned fore and aft of the craft&#39;s center of gravity and symmetrically about the craft&#39;s longitudinal centerline. 
   Alternatively, the hydrofoil marine craft  10  can optionally include a canard hydrofoil arrangement, having lifting bodies positioned fore of the crafts center of gravity along the craft&#39;s longitudinal centerline, and a pair lifting bodies positioned aft of the craft&#39;s center of gravity “CG”, symmetrical about the craft&#39;s longitudinal centerline. 
   The hydrofoil marine craft  10  of the present invention is configured to optimally operate at a cruising height, where a height “WH” is maintained between the waterline “WL” and the upper lifting surfaces. As shown in  FIG. 2 , a propulsion system is provided to power the craft  10 , where the propulsion system includes an engine  32  for providing thrust. As the main foils&#39;  24   a  and  24   b  lift decreases, the height of the craft  10  will decrease, requiring an increase in thrust. As the main foils&#39;  24   a  and  24   b  lift increases, the height of the craft  10  will increase, requiring a decrease in thrust. 
   A height measurement device  36  is included to indicate the craft&#39;s  10  height “CH” above the waterline “WL.” The height measurement device  36  can be a height sensor configured for transmitting and receiving ultra sound waves, radio waves, or laser energy. The height can also be measured by an electromechanical device, electro-optical device, pneumatic-mechanical device, or other height measurement device known in the art. Alternatively, the height can be measured by a device mounted on a main foil  24   a  to detect the waterline “WL” position in relation to the mid span position of the foil  24   a . The height measurement device  36  displays the craft&#39;s  10  height, enabling the operator to increase or decrease the thrust as needed. 
   The hydrofoil marine craft  10  can include a thrust controller  38 . As shown in  FIG. 7 , a flow chart for the thrust controller  38 , the thrust controller  38  is operably connected to the height measurement device  36 , the engine  32 , and the throttle  34 . A filter  37  is interposed between the height measurement device and the thrust controller  38 , where the filter  37  removes noise that can be caused by choppy or rough seas. The thrust controller  38  automatically adjusts the throttle  34 , adjusting the engine&#39;s  32  output, in response to the craft&#39;s  10  height. As the height of the craft  10  decreases, the thrust controller  36  will increase in thrust, raising the craft  10 . Similarly, as the height of the craft  10  increases, the thrust controller  38  decreases the thrust, lowering the craft. The thrust controller  38  optimally maintains the height of the craft  10 , such that the distance “WH” is maintained between the upper lifting surface and the water line “WL.” 
   The height of the craft  10  can be adjusted by changing the lifting forces acting on the main foils  24   a  and  24   b . For example, the lifting forces acting on the main foils  24   a  and  24   b  can be adjusted by changing the angle of attack ω. Increasing the angle of attack ω will increase the lifting forces acting on the main foils  24   a  and  24   b . Decreasing the angle of attack ω will decrease the lifting forces acting on the main foils  24   a  and  24   b.    
   As showing in  FIG. 8 , the main foils  24   a  and  24   b  are pivotally connected to the pylons  18  and  20 , and are rotatable about pivot axis “FP”. The angle of attack (o of the main foils  24   a  and  24   b  is adjusted by rotating the main foils  24   a  and  24   b  about the pivot axis “FP” to the desired angle of attack ω. 
   Alternatively, the pylons  18  and  20  are pivotally connected to the struts  16 , or optionally to craft&#39;s hull  14 , and rotatable about pivot axis “SP”. The angle of attack ω of the main foils  24   a  and  24   b  is adjusted by rotating the pylons  18  and  20  about the pivot axis “SP”, thereby increasing or decreasing the foils&#39; angle of attack ω. Additionally, as the pylons  18  and  20  rotate about the pivot axis “SP”, the angle of attack of the takeoff foils  22   a  and  22   b  will be simultaneously changed with the main foils&#39;  24   a  and  24   b  angle of attack. 
   The main foils  24   a  and  24   b  can also be used to maintain pitch stability of the craft. The angle of attack of the fore main foil  24   a  or aft main foils  24   b  can be individual adjusted to maintain the craft at the appropriate pitch angle. 
   The height of the craft  10  can also be adjusted by simultaneously adjusting the thrust and the foils&#39; angle of attack ω. As shown in  FIG. 9 , a flow chart for the thrust controller  38 , the thrust controller is operably connected to the height indicator  36 , the engine  32 , and system for adjusting the foils&#39; angle of attack  40 . The thrust controller  38  automatically adjusts the engine&#39;s  32  output and foils&#39; angle of attack ω in response to the craft&#39;s  10  height. As the height of the craft  10  decreases, the thrust controller  38  will increase the thrust and/or decrease the foils&#39; angle of attack ω, raising the craft  10 . Similarly, as the height of the craft  10  increases, the thrust controller  38  decreases the thrust and/or increases the foils&#39; angle of attack ω, lowering the craft  10 . The thrust controller  32  optimally maintains the height of the craft  10 , such that the distance “WH” is maintained between the lower lifting surfaces and the water line “WL.” 
   Advantageously, the variable thrust/height control system can also be used to increase or decrease the cruising speed. As shown in  FIG. 10 , the operator can initiate a speed change by changing the angle of attack. The foil control  40  changes the angle attack of all main foils simultaneously. The change in the angel of attack results in an increase or decrease in the lifting force provided by the main foils, causing the waterline “WL” position to change on the main foils. The change in the height of the craft is detected by the height measurement device  36  and is transmitted to the thrust controller  38 . In response, the thrust controller  38  adjusts the engine&#39;s  32  thrust achieving an increase or decrease in the cruising speed, while maintaining the craft at the target cruise height. 
   As shown in  FIGS. 2 and 3 , the propulsion system can include at least one air propeller  42  mounted to the deck  44  of the craft  10 , were the air propeller  42  is operably connected to the engine  32 . Alternatively, the propulsion system can include a water propeller, where a drive shaft is mounted through at least one of the pylons, operatively connecting the water propeller to the engine. Additionally, the propulsion system can be a water jet or a pump jet, and can include more than one air or water propellers. 
   The hydrofoil marine craft  10  further includes a direction control system for turning the hydrofoil marine craft  10 . The direction of the hydrofoil marine craft  10  can be adjusted by selectively changing the lifting forces acting on the hydrofoils causing the hydrofoil marine craft  10  to roll onto a banked turn, such as by creating a lifting force differential between the starboard and port foils. For example, to make a starboard turn, a lifting force differential is created between the starboard foil and port foil, where the port foil has a greater lifting force than the starboard foil. As noted above, the lifting forces acting on the foils can be adjusted by differentially changing the angle of attack of the outboard foils. At a given speed, increasing the foil&#39;s angle of attack will increase the lifting forces action on the foils. Decreasing the angle of attack will decrease the lifting forces acting on the foils. 
   As showing in  FIG. 8 , the main foils  24   a  and  24   b  are pivotally connected to the pylons  18  and  20 , and are rotatable about pivot axis “FP”. The angle of attack ω of the main foils  24   a  and  24   b  are adjusted by rotating the main foils  24   a  and  24   b  about the pivot axis “FP” to the desired angle of attack ω. 
   Alternatively, as shown in  FIG. 8 , the pylons  18  and  20  are pivotally connected to the struts  16 , or optionally to craft&#39;s hull  14 , and rotatable about pivot axis “SP”. The angle of attack ω of the main foils  24   a  and  24   b  is adjusted by rotating the pylons  18  and  20  about the pivot axis “SP”, thereby increasing or decreasing the angle of attack ω. 
   Additionally, the small changes in the differential forces required to achieve a banked turn can by accomplished by adjusting control surfaces on the fore main foils  24   a  as is know in the art. For example, the fore main foils  24   a  can include a set of trim tabs, which when actuated change the fore main foil&#39;s  24   a  lift profile, differentially increasing or decreasing the lifting forces action on the main foils  24   a.    
   Additionally, the vertical stabilizer  26  can be used as a rudder, providing directional control for the hydrofoil marine craft  10 . In an exemplary embodiment, as shown in  FIG. 6 , a pair of vertical stabilizers  26  extends from the fore pylons  18 , and is pivotal about a vertical axis “V.” As the vertical stabilizers  26  are rotated about the vertical axis “V,” the water flow over the vertical stabilizers  26  will cause the hydrofoil marine craft  10  to change directions. As shown in  FIG. 4 , a vertical stabilizer  36  can also pivotally extend from the aft pylon  20 , functioning as a stand-alone rudder or in combination with the fore pylons  18 . 
   In a still further embodiment, the craft&#39;s direction is controllable by directing the thrust. For example, the propulsion system can include a thrust directional controller. 
   The shock mitigation system for hydrofoil marine craft of the present invention has been exemplary described using a mono-hull craft. However, the shock mitigation system can also be applied to multi-hull craft, including catamarans and trimarans. 
   Having explained features and functions of a shock mitigation system and its exemplary components, additional discussion is now provided with respect to alternative foil embodiments set forth in  FIGS. 11–16 . Specifically, although cambered foils can function effectively to act as lifting bodies, other foil configurations are also desirable. For example, a foil can be configured to provide lift for the craft by shaping the foil and/or angling the foil (or a portion thereof) with respect to a reference, such as a motion path, so that it impacts or travels through water at a defined angle or presents a foil face that deflects or pushes the craft upward as it moves forward. This type of foil can be particularly advantageous at speeds ranging from about 50 to 75 knots. 
   An example of such a foil is shown in  FIG. 11 , wherein a foil  42  having a leading edge  44  and a trailing edge  46  is shown in cross-section. In this view it is apparent that the foil is not cambered and that the upper surface  48  is substantially flat. The opposing lower surface  50  diverges from the upper surface  48  increasingly from the leading edge  44  to the trailing edge to provide a deflection surface. The leading edge  48  is shown as being rounded or blunt; however, it can be “pointed” as well. The trailing edge  46  is shown as flat face that is substantially perpendicular to the upper surface  48 ; however, as shown in  FIG. 13 , the trailing edge can include a tapered configuration. 
   Thus, in use, the foil  42  is oriented so that water traveling over the upper surface is not accelerated by the shape or position of the foil to create lift. By contrast, the fluid flowing across the lower surface  50  is pressurized by the impingement of fluid against the lower surface or portion thereof that is presented to the fluid as it traverses the foil before passing behind it, thereby applying a lifting force to the craft. 
   Referring now to  FIG. 12 , a foil  52  is provided having a substantially flat upper surface  54 , a substantially flat lower surface  56  and a positionable element  58  that can be moved as shown by the bidirectional arrow to create an angular difference between the flat lower surface  56  and a selected reference, thereby creating a deflection surface against which a flow a water impinges to create a lifting force for the craft. In addition, the manipulation of the positionable element  58  results in an increase and/or decrease in the thickness of the aft portion of the foil  52 . 
   Yet another feature of the invention is shown in  FIGS. 14 and 15  where the upper surface  60  of a foil section is shown provided with boundary layer control devices to improve laminar flow and to hinder span-wise flow of fluid traversing the upper surface of any foil described hereinabove, but especially cambered foils. For example,  FIG. 14  depicts fences  62  disposed span-wise across the foil; and  FIG. 15  discloses an array of apertures through which high energy fluid can be ejected as represented by the arrows. 
     FIG. 16  depicts a portion of a craft  66  (looking fore to aft) provided with foils  42  as set forth in  FIGS. 11 . By contrast with other configurations, the configuration of  FIG. 16  includes only a singe foil on each pylon  68 . 
   As described above, the system limits vertical lift forces, as well as lateral forces on a craft by separation of the traditional lift generating function of a hull, by using pylon mounted foils, from the cabin, deck, and payload carrying features of the hull. The resultant vertical separation is equal to or greater than the expected operational wave height. Thus, the lift at operational sped is limited to a vertical force equal to the weight of the loaded hull plus a safety factor that might range from 20 to 100 percent of the loaded weight. Lateral forces applied to the craft are limited by the relatively small surface area of the pylons as compared to the freeboard of a conventional monohull. 
   Turning now to  FIG. 17 , yet another configuration is illustrated that mitigates shock by limited vertical and lateral forces. As shown, a catamaran configuration is provided having a first hull  70 , a second hull  72 , and a cargo hull  74  that is positioned above and between the first hull and second hull by struts  76  rather than a substantially hull-length longitudinal support. 
   Unlike the relative proximity of a traditional catamaran deck to the water surface, the cargo hull  74  in the present invention is at a height matched to the operational wave specification. Whereas a traditional catamaran is not severely affected by cargo hull impact with the water or by later forces due to relatively low speeds, speeds above 25 knots can be both punishing and destructive. By contrast, substantially total isolation of the cargo hull  74  from the water surface (and waves) in the present invention, in combination with relatively small freeboards, allows the present craft to travel smoothly at speeds above 50 knots. Should a wave wash over the first and second hulls  70  and  72 , the vertical lift is limited to +1 “G” plus the safety factor. 
   Although the first and second hulls  70  and  72  can have a traditional elongate “V” hull shape and a buoyancy or displacement so that the cargo hull  74  is above water level when the craft is at rest, the first and second hull can also be configured to that the cargo hull is at or near water level at rest with the first and second hulls submerged, wherein the first and second hull are provided with lift or planning surfaces that cause the hulls to rise to the surface or above as the speed of the craft increases. 
   It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.