Abstract:
A toy airboat ( 10 ) having a negative trim element ( 38 ) and an adjustable positive trim element ( 44 ) for selectively controlling the performance of the craft in varying environmental conditions such as wind speeds and wave heights. The negative trim element may be smoothly curved protrusions ( 40 ) formed integral to the hull ( 12 ) in order to avoid snagging debris in water and to allow operation over land without damage to the hull. The protrusions may be symmetrically located on opposed sides of a centerline (CL) of the craft approximately midway between the centerline and a perimeter ( 16 ) of the hull. The adjustable positive trim element may be a bolt ( 46 ) threaded into a nut ( 48 ) affixed to a deck ( 14 ) of the craft. The nut extends to make contact with the hull to impart a desired degree of convex curvature to the otherwise generally flat bottom of the hull. The bolt may be attached to the hull with a push/pull mechanism ( 54 ) to allow the nut alternatively to impart a desired degree of concave curvature to the hull.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to the field of toy watercraft, and particularly to toy airboats. 
   BACKGROUND OF THE INVENTION 
   Radio controlled toy boats are well known in the hobby industry. Toy boat designs typically mimic full size boat designs, including V-hull inboard and outboard boats, tunnel hulls boats, “cigar” racing boats, and air cushion hover craft vehicles. 
   It is well known to adjust the trim angle of a boat to regulate its ride and performance. The terms trim and trim angle refer to the horizontal pitch of the boat relative to the horizontal surface of the water. Many devices are known for adjusting trim angle, for example U.S. Pat. No. 4,458,622 describes a full size boat having a variable hull configuration, and U.S. Pat. No. 3,589,058 describes a toy boat having an adjustable stabilizing weight system. 
   Airboats, also known as swamp boats, are specially designed craft having aircraft style propeller or fan propulsion and a flat bottom for operation in very shallow water, weed-infested water, and over land or mud. While the flat bottom design is useful for relatively low speeds in multi-terrain environments, such as swamps, it has the disadvantage of being relatively unstable at high speeds and especially in high-speed turns. Airboats are also susceptible to degraded performance under adverse environmental conditions such as high wind or high waves. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in following description in view of the drawings that show: 
       FIG. 1  is a perspective view of one embodiment of a toy airboat incorporating the present invention and illustrated without an engine. 
       FIG. 2  is a side view of the airboat of  FIG. 1  including its engine and illustrating positive and negative trim elements in the hull. 
       FIG. 3  is a rear view of the airboat of  FIG. 1  illustrating the negative trim element. 
       FIG. 4  is a partial cross-sectional view of the airboat of  FIG. 1  illustrating the adjustable positive trim element. 
       FIG. 5  is a partial cross-sectional view of another embodiment of a toy airboat illustrating an adjustable positive/negative trim element. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In spite of the similarity of the designs of toy boats and full size boats, the performance of toy boats can be significantly different than their full size counterparts due to the importance of scaling factors. For example, the power-to-weight ratio in a toy boat may be significantly higher than in its full size counterpart, and environmental conditions such as wind speed and wave height may have a proportionately greater affect on a toy boat. Full size airboats are generally constructed of metal or are reinforced with metal and are relatively heavy vehicles with a modest power-to-weight ratio, such as 0.1 horsepower/pound in one example. Toy airboats may be made of plastic and may have a much higher power-to-weight ratio, such as 0.3 horsepower/pound in one example. As a result, toy airboats can reach speeds of over 40 miles per hour, which may equate to a scale speed of several hundreds of miles per hour in a full size airboat. Furthermore, small fluctuations in wave height and wind speed have a proportionately much greater impact on the performance of a toy airboat than on a full size airboat due to the toy&#39;s small size and light weight. 
   Toy airboat  10  of  FIGS. 1-3  includes innovative features that provide improved performance by addressing both the unique design aspects of airboats generally and the significant scaling factors that limit the performance of prior art toy airboats. Airboat  10  includes hull  12 , which may be made of a vacuum formed plastic and reinforced with a lightweight foam resin in selected areas, for example. In many regards, the hull  12  is similar to that of a full size airboat, with important exceptions to be discussed more fully below. A deck  14  is sealed to the hull  12  about its perimeter  16  to define a trapped airspace providing floatation for the craft. An engine stand  18  is attached to the deck  14  proximate the stern  20  of the airboat  10 . A model aircraft engine  22  (illustrated as installed only in  FIG. 2 , with the craft shown without engine in  FIGS. 1 and 3 ) as known in the art may be mounted to the engine stand  18 . The engine stand  18  also provides a location below the engine  22  for mounting of a fuel tank  24  (shown only in  FIG. 3 ) for gas-powered engines. The propeller  26  of the engine  22  is surrounded by a protective cage  28 , which may be formed of a wire mesh and tube steel in order to provide safe operation with minimal weight and air drag. Other embodiments may utilize a different type of propulsion, such as a ducted fan engine that may not require a separate protective cage  28 . At least one air rudder  30  is attached to the engine stand  18  by hinges  32 . The airboat  10  is controlled remotely with a radio control system  34  as is known in the hobby arts. Remote control of both the rudder  30  and the engine  22  are provided to allow an operator to control both the direction and speed of the craft. The radio control system  34  components are housed in a compartment  36  having a removable watertight lid  37  (illustrated as being clear plastic) providing access to the radio control system  34 . When two air rudders  30  are used, they may be connected together to be controlled by a single servo-actuator of the radio control system  34 . 
   The hull  12  of the airboat  10  is illustrated in the side view of  FIG. 2  and the rear view of  FIG. 3 . The hull  12  includes a negative trim element  38  located proximate the stern  20  of the craft. In the illustrated embodiment, the negative trim element  38  is integrally formed into the hull  12  as two separate protrusions  40  extending somewhat below the plane of a main body of the bottom of the hull, as may be appreciated by viewing both  FIGS. 2 and 3 . In this embodiment, the negative trim element  38  provides fixed amount of negative trim for the craft. Other embodiments may have the negative trim element being formed separately from the hull, such as when using external, remotely adjustable trim tabs for providing a variable amount of negative trim to the craft. Advantageously, the illustrated trim element  38  has a generally smooth surface shape that provides the desired lifting force while minimizing drag and allowing the airboat  10  to operate in water without snagging debris and on land without damage to the hull  12 . The negative trim element  38  provides a fixed amount of negative trim during operation of the airboat  10 , i.e. it constantly urges the stern  20  of the craft upward and the bow  42  of the craft downward. Negative trim is necessary for preventing the bow  42  of airboat  10  from bouncing excessively at high speeds or under high wave or high headwind conditions. 
   Negative trim element  38  may be purposefully designed to provide more negative trim than is necessary for optimal operation of the airboat  10 . With too much negative trim, the toy airboat  10  would plow through the water, creating a large wake, and reducing its speed and maneuverability. To counteract this excessive amount of negative trim, an adjustable positive trim element  44  is provided to allow the curvature of the generally flat bottom of the hull  12  forward of the negative trim element  38  to be selectively changed, and in particular, to be selectively displaced into a convex shape.  FIG. 2  illustrates the generally flat bottom of the hull in two alternative conditions: a flat condition illustrated with a solid line and a more convex shape illustrated with dashed line  45 . As illustrated, the term convex is used herein to describe the condition where a portion of the generally flat hull  12  is bowed downward away from the deck  14  beyond its generally flat planar position. The term concave is used herein to describe the condition where a portion of the generally flat hull  12  is bowed upward toward the deck  14  beyond its generally flat planar position. The amount of positive trim imposed by the positive trim element  44  during operation of the airboat  10  is generally proportional to the degree of convex curvature imposed on the hull  12 . By providing an adjustable degree of positive trim, the balance between the positive and negative trim imposed by the positive trim element  44  and the negative trim element  38 , respectively, may be selected and controlled by the user to provide optimal performance under a wide range of boat configurations and environmental conditions. For example, as wave heights and/or wind conditions increase, a more negative balance between the trim elements may be selected to provide increased stability and protection against excessive bouncing and loss of control or flip-over of the craft. When wave heights and/or wind conditions decrease, a less negative balance between the trim elements may be selected to provide smoother operation and to maximize speed. Such control is very advantageous for a toy airboat  10 , since even small changes in environmental conditions can have a very large affect on the performance of the craft, and because the operator of the craft will want to maximize the craft speed under a variety of conditions. Adjustment of the trim balance may also be desired when making changes to the airboat  10 , such as when placing accessories on-board that may change the center of gravity (CG) of the craft. 
     FIG. 4  illustrates one embodiment of the adjustable positive trim element  44  mounted between the hull  12  and the deck  14 . A threaded bolt  46  is adjustably engaged with a threaded nut  48 , such as a T-nut, attached to the deck  14 . When the bolt  46  is screwed into the nut  48 , it engages the hull  12  directly or indirectly through a protective reinforcing structure  50  attached to the hull  12 . As the bolt  46  is advanced farther into the nut  48 , it applies force against the hull  12  and functions to impart a selectively increasing convex shape to the hull  12 . The position of the positive trim element  44  in the hull  12  is forward of the negative trim element  38  and preferably along a centerline (CL) of the craft parallel to its direction of movement through the water. In one embodiment, the positive trim element  44  is located proximate the location of the center of gravity of the craft along the centerline. The operator of the airboat  10  manipulates the adjustable positive trim element  44  of  FIG. 4  manually by turning bolt  46  with a screwdriver or wrench. Other embodiments of positive trim elements may allow for remote adjustment of the degree of positive trim during operation of the craft via the radio control system  34 . 
   The design of the negative trim element  38  may vary for various applications. In the embodiment of  FIG. 3 , the two protrusions  40  are symmetrically located about the centerline of the craft and are each somewhat inboard from the perimeter  16  of the hull  12 , such as approximately centered between the centerline and the respective portion of the hull perimeter  16 . This arrangement maintains a negative trim effect even when the airboat  10  tilts during turns, thereby ensuring positive control even during high speed turns. While tilting of a full size airboat is undesirable because of the danger of tipping, it is quite desirable in a toy airboat  10  because it generates excitement for the operator and allows for a maximum speed of operation. By placing the protrusions  40  closer to the perimeter  16  of the hull  12 , they would function to reduce the amount of slide and bounce during a turn, which provides somewhat more control but also may give the toy craft a less exciting and less realistic performance. 
   Note that the size of the protrusions  40  and the magnitude of the convex shape imposed by the positive trim element  44  are somewhat exaggerated in the figures for purposes of illustration. In one embodiment, a toy airboat  10  of the present invention has an overall hull length of about 26 inches, a full hull depth of about 2 inches, is powered by a 0.46 cubic inch 2-cycle gas model airplane engine that generates about 2.2 horsepower at full throttle, weighs about 8 pounds, and can achieve smooth water speeds of about 40 miles per hour. For that particular embodiment, the present inventor has successfully used smoothly curved negative trim protrusions  40  that extend only about 0.25 inch below the plane of the main body of the bottom of the hull with a positive trim element that provides up to a maximum of about 0.25 inch of convex displacement as measured from the generally flat plane of the bottom of the hull to the point of maximum downward displacement. 
     FIG. 5  illustrates an adjustable positive/negative trim element  52  that may be used in lieu of the adjustable positive trim element  44  in other embodiments. The adjustable positive/negative trim element  52  includes many of the same structures as the adjustable positive trim element  44 , and such structures are numbered consistently between  FIGS. 4 and 5 . The key difference between these two figures is that the bolt  46  is connected to the hull  12  by a push/pull mechanism  54  that allows the bolt  46  to impart both pushing and pulling forces on the hull  12  to create either a positive (convex) or a negative (concave) trim effect. The push/pull mechanism  54  is illustrated in  FIG. 5  as a ball and socket arrangement, although other mechanisms may be used. This embodiment allows a toy airboat to be configured with less fixed negative trim in the negative trim element  38  while still being able to accommodate the most extreme conditions. Even when a positive/negative trim element  52  is used, it is still desirable to include a negative trim element  38  in order to provide improved control during high speed turns as discussed above. 
   While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.