Hovercrafts are vehicles that travel across land or water just above a cushion of air. Generally, a skirt extends down from the periphery of the vehicle to the land or water in order to entrap the supporting cushion of air. Depending on the size of the vehicle and/or its intended application, it may be necessary to provide a stability system that reduces the effects of roll and pitch forces. For example, a longitudinally extending keel system can be attached to the underside of the vehicle hull along the vehicle's center line. Ideally, the keel stays in contact with the water's surface to prevent air on one side thereof from moving to the other side thereof. In this way, differential pressure cells are created to provide roll stability. Similarly, a laterally extending keel system can be used to provide pitch stability.
One such keel system is utilized by the U.S. Navy in its LCAC (landing craft, air cushion) vehicle. As shown in the plan view of FIG. 1, the LCAC has a segmented longitudinal keel system 20 and a segmented lateral keel system 50 contained within the periphery of a hovercraft's skirt (not shown). Longitudinal keel system 20 extends along the direction of craft motion which is indicated by arrow 100. Lateral keel system 50 is transverse to longitudinal keel system 20 and consists of a plurality of inflated cones 52.
One of cones 52 of lateral keel system 50 is shown in a side view in FIG. 2 which is a view taken along line 2--2 of FIG. 1. The side view in FIG. 2 depicts a triangle (of height h and base dimension b) that is essentially positioned parallel to craft motion 100. The portion of cone 52 parallel to the load forces determines the cone's ability to resist the buckling forces associated with craft motion 100. This is best understood by examining the relationship used to determine bending stress .sigma..sub.bending, i.e., the force that causes buckling of a cone 52 subjected to craft motion 100. Bending stress .sigma..sub.bending can be defined mathematically as ##EQU1## where M is the bending moment (i.e., the applied force at the cone's tip or apex times the height h of the cone), c is the distance from the outermost surface of a cone to its central axis (i.e., in this case, one half of the cone's width w as indicated in FIG. 1), and I is the moment of inertia. The moment of inertia I for a triangle is defined as ##EQU2## Since .sigma..sub.bending and I are inversely proportional, the b and h values of each cone 52 must be sufficient to keep .sigma..sub.bending from exceeding a threshold at which the (inflated) cone will buckle.
The existing longitudinal keel system is shown in greater detail in isolation in FIG. 3 and in cross-section in FIG. 4 which is taken along line 4--4 of FIG. 3. The longitudinal keel system is attached to the underside of the LCAC's hull which is shown in part and referenced by numeral 10. The longitudinal keel system consists of upper inflatable bladder sections 20A, 20B and 20C (extending fore to aft) attached to the underside of hull 10. Each upper inflatable bladder section 20A, 20B and 20C is trough-shaped and is inflated or pressurized by the craft's lift/fan system (not shown). Attached to each section 20A, 20B and 20C is a corresponding lower trough-shaped inflatable bladder section 22A, 22B and 22C. Inflation of sections 22A, 22B and 22C is accomplished by the provision of a plurality of inflation holes 24 (see FIG. 4) passing between the upper and lower inflatable lower bladder sections 20A and 22A. Thus, inflation of bladder sections 22A, 22B and 22C is accomplished when air passes through these holes. Port and starboard longitudinally extending blades 26 and 28 are attached to the lower portion of sections 22A, 22B and 22C. Blades 26 and 28 are flexible and oppose one another to define a channel therebetween. Holes 30 between blades 26 and 28 in the lower portion of lower inflatable bladder sections 22A, 22B and 22C provide an air exhaust and allow for the removal of water that might seep into the inflated sections while blades 26 and 28 cooperate to prevent water from entering holes 30.
Although accomplishing its intended function of roll stability, the longitudinal keel system shown and described above has been susceptible to environmental abrasion and impact damage that propagates forward and aft from the point of damage. Since the aft-most lower inflatable bladder section 22C is the section most likely to be damaged, such damage tends to propagate forward to section 22B. Damage greatly affects the craft's roll stability and is further costly to repair owing to the location of the keel system.