Abstract:
A propeller blade device used in environments where a propeller system is subjected to blade erosion and wear and may also be subjected to an axial force from water. The propeller blade includes a blade tip mounted on its outer end using a mounting device such as nuts and bolts to form a junction there between. The tip may easily be replaced when worn or damaged without removing the blade. The tip is designed to fracture upon impact with an axial force before transmitting the force radially inward to the propeller blade.

Description:
BACKGROUND 
       [0001]    The present invention relates to above deck propulsors for marine hovercraft applications. More particularly, the invention relates to a blade system that reduces the time and cost of repairing propeller blades damaged in use. 
         [0002]    Hovercraft vehicles employ blowers for vehicle lift and a propeller and motor to drive forward. The blowers are directed downwardly on to sandy beaches at times, causing salt water and sand laden air to rise up into the propeller. Also, blade tip erosion due to airborne sand causes damage to the propeller blades, resulting in frequent repair and replacement of the blades. 
         [0003]    A protective layer for the blade tips to protect them from erosion has not worked well because it is difficult to attach and tends to create an aerodynamic disruption in flow over the blade. Propeller blade tips are typically thin, leaving little room to hold fasteners. 
         [0004]    A second problem in the use of above deck propulsors in marine hovercraft applications is known as the “green water” event. This occurs when a wave washes over the deck and is ingested by the propeller. This causes extensive damage to the propeller blades as well as to supporting structure. 
         [0005]    Repair procedures for damaged propeller blades in these applications is costly and time consuming. 
       SUMMARY 
       [0006]    A blade for use with marine hovercraft propulsor applications and other water craft using a propeller system including a blade having a thin sacrificial tip attached to the base blade assembly. The blade assembly has a base section that is thick enough to contain the fasteners that hold the tip on the blade. The tip is fastened so that the propeller operates as intended, but the tip can break off when subjected to forces that have, in the past, adversely affected the propellers. In the event of a “green water” event, the tip functions as a fuse, breaking away from the blade base to prevent or minimize transmission of extreme impact loads from the blade to the blade retention and propeller supporting structure. Because the tip is located on the outboard end of the propeller and it is thus subjected to the greatest amount of environmental wear. This arrangement minimizes damage to the rest of the propeller. 
         [0007]    The blade and tip assembly allows for quick and inexpensive repair of damaged tips when the tip is attached to the blade in a manner that allows the tip to break off when subjected to an amount of stress that is less than that amount that would damage the blade or the blade retention and propeller supporting structure. Because the blade is a larger structure than the tip, a plurality of bolts are formed on the end of the blade that will have the tip attached. Corresponding bolt holes are formed on the end of the tip. The bolt holes may be threaded or may include a nut at the side of the hole facing away from the bolts. Other fastening devices may also be used. In all instances, the tip itself or the means fastening the tip to the blade allows the tip to break off outboard of it&#39;s bolted connection from the blade under a force less than would damage the blade. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a perspective view of a hovercraft. 
           [0009]      FIG. 2  is an enlarged view of a portion of the hovercraft in  FIG. 1 . 
           [0010]      FIG. 3  is a view of a blade and tip for use on the propellers of the hovercraft of  FIG. 1 . 
           [0011]      FIG. 4  is a view of the other side of the blade and tip of  FIG.3 . 
           [0012]      FIG. 5  is a view of the blade portion of the device in  FIG. 3 . 
           [0013]      FIG. 6  is a view of the tip portion of the device in  FIG. 3 . 
           [0014]      FIG. 7  is an enlarged sectional view of the attachment region of the blade and tip of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    As seen in  FIG. 1 , a hovercraft  10  includes craft body  11  that has the lifting blowers and motors under the body, not shown, in a conventional manner. Hovercraft  10  also includes two propeller systems  13  and  15  that drive the hovercraft over water, beaches and other land surfaces. This particular hovercraft  10  is exemplary of the type of crafts where the propellers are subject to potential damage. 
         [0016]      FIG. 2  shows an enlarged view of propeller system  15 , with a plurality of propeller blades  17 . The propellers  17  are in protective cage  19  that is necessarily open enough to function to propel the craft cage  19  is mounted via strut  18 . It is clear that propellers  17  are exposed and can be subjected to debris such as sand, sand filled water, seaweed, and other flotsam and jetsam encountered in operating the crafts. The greatest force on propellers  17  is in the axial direction  20  of propeller hub  21 . Propellers  17  have an inboard end  17   a  attached to propeller hub  21  and an outboard end  17   b,  which is more exposed to the environment, particularly in the direction of axial direction  20 . 
         [0017]      FIG. 3  is an enlarged view of one blade  17  that has been fitted with a blade tip  23 , and has been attached by bolts  25  to form a junction  27  between blade  17  and tip  23 . Other means to join tip  23  to blade  17  are also used. Examples of other means are tongue-in-groove assemblies, dovetail assemblies adhesives, screws, straps and bands. Junction  27  combines the strength of blade  17  in part and the strength of tip  23  in part, and is therefore stronger than tip  23  itself. While the relative lengths of blade  17  and tip  23  will depend on the specific propeller system, normally tip  23  will have a length of 10% to 40% of the length of blade  17 . 
         [0018]    In addition, tip  23  is not intended to be as strong as blade  17  in ability to resist a sudden increase in axial force such as a “green water” event where water washes into the cage  19  in the direction of axis  20 . Generally, the blade load during operation of a hovercraft is about 2,500 pounds (1137 kg.) in air, based on a four blade system. More blades will reduce the load per blade. When water contacts a blade or set of blades, such as in a “green water” event, the load increases by a factor of 800 or more, which is enough to break the blade. 
         [0019]    In order to have a reference for the direction that propeller  17  extends, tip  23  is located on the outboard side of j unction  27  and the blade  17  is on the inboard side of junction  27 . Inboard and outboard are terms used to describe the radial direction toward and away from axis  20 . 
         [0020]    In most situations, it is possible to locate the point on a propeller blade at which the most wear takes place. The length of blade  17  and tip  23  are designed so that the junction  27  is inboard of the region of greatest wear. Thus when the tip  23  is damaged or eroded, it can readily be replaced and a new tip  23  can be installed using ordinary tools and in a short period of time. It is no longer necessary to replace the entire propeller blade  17 , thus saving time and money. 
         [0021]    Propellers such as those used in hovercrafts are made from a wide variety of materials, and all of them are within the scope of this invention. Blade  17  and tip  23  may be made from composites of fiberglass, carbon, or aramid fibers, as well as combinations of these fibers. Also, plastics such as reinforced resins, thermoplastics may be used. Metals such as alloys of aluminum, steel, nickel or titanium and these metals hardened by heat treatments are also materials that are within the scope of this invention. It is also contemplated that the propellers, either blade  17  or tip  23  or both, may be coated with coatings providing high erosion and wear resistant properties. The use of ceramic coatings or of ceramic materials in the manufacture of the blade tip for erosion and wear resistance are also within the scope of this invention. 
         [0022]      FIG. 4  illustrates the other side of the blade  17  and tip  23  of  FIG. 3 . Nuts  29  have been attached to bolts  25  at junction  27 . As can be seen, it is a simple matter to disengage nuts  29  from bolts  25  to remove and replace tip  23 . Again, other fastening means may be used with equal effectiveness because the function of the fastening means is to hold the blade  17  and tip  23  together. Under stress, the tip  17  will break at a point outboard of the junction  27 . 
         [0023]      FIG. 5  illustrates blade  17  with bolts  25  before attachment of a tip. Blade  17  has a shelf  31  inboard of bolts  25 . Shelf  31  is sized and shaped to allow a tip to nest on blade  17 . The right angle shelf  31  makes with blade end  33  provides two surfaces for supporting the mating at junction  27 . Similarly,  FIG. 6  illustrates tip  23  with holes  35  that hold nuts  29 . Tip  23  also has a shelf  37  that mates with tip end  39  to form a right angle. 
         [0024]    Attachment of tip  23  to blade  17  provides for junction  27  that is stronger than tip  23 . A sudden increase in axial bending force such as from the previously described “green water” event will cause tip  23  to break or bend outboard of junction  27 . 
         [0025]    While it is contemplated that tip  23  will be shaped to function aerodynamically as a propeller tip, means to further insure that tip  23  breaks before any damage to propeller blade  17  occurs can be provided. It is desirable that tip  23  fracture under sudden axial load prior to transmitting the load radially inboard to the blade. Effectively, the blade  17  and junction  27  have a higher flex strength in the axial direction  20  than at least a portion of tip  23 . Tip  23  can have a score line  41  or a region  43  of tip  23  can be made thinner than the rest of tip  23 . Use of either score line  41  or thin region  43 , or both, is within the scope of this invention. 
         [0026]      FIG. 7  is a cross section of the assembly with tip  23  attached to blade  17  via bolt  25  and nut  29 . 
         [0027]    In operation, the hovercraft functions in a normal manner, encountering the environment such that the tips of the propellers incur the greatest wear and, should it be necessary, the tips will break or bend before any damage is done to the rest of the propeller or to the mounting assembly including the propeller hub. 
         [0028]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.