Abstract:
A marine vessel used for loading and offloading vehicles and cargo that has both excellent seakeeping and beaching properties. The vessel is provided with azimuthing or steerable thrusters below the waterline at the bow and stern of the ship, with the stern end thrusters being retractable into the hull during beaching operations. Because the vessel is beached “stern first” the bow may be designed with better wave breaking and seakeeping features than are on prior landing craft designs.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    This invention relates to a marine vessel, and more particularly, to a landing craft that uses fixed and retractable azimuthing drives in combination with an improved bow shape to obtain both excellent seakeeping and beaching abilities.  
           [0003]    2. Description of Prior Art  
           [0004]    Throughout the history of modern military operations, there has been a requirement to transfer men and materials from ships to a beachhead. Without docking facilities, such transfers are difficult to accomplish. In the twentieth century, particularly during the Second World War, specialized landing craft were developed to facilitate such transfers. Unfortunately, such craft were limited to transporting a relatively small group of men, one or two vehicles, or an inadequate amount of cargo, due to their limited size and cargo weight capacity. Larger marine vessels were limited for such operations due to their inability to effectively and safely “beach” themselves to unload cargo and then return to normal seakeeping operations.  
           [0005]    Some of the aforementioned problems where addressed with the development of so-called “roll-on, roll-off” vessels. Used in both military and commercial applications, vehicles and other cargo are loaded onto a marine vessel, typically by way of a ramp on the bow or stern of the ship. The vehicles or cargo are then unload by way of the same ramp used for loading. An obvious disadvantage of using one ramp is that vehicles must be “rolled-on” in reverse, such that they can be driven off the vessel in a forward orientation so that they can be immediately used.  
           [0006]    When loading/unloading ramps are provided at both the bow and the stern of the ship, such vessel still suffer from several disadvantages, particularly if used during beaching operations. In such prior art vessels, vehicle, cargo, etc. were loaded at the stern of the ship. Beaching operations were conducted with the bow portion of the ship being beached, and the materials offloaded by a ramp or similar means at the bow. However, the use of ramps and other offloading systems located on the ship&#39;s bow affected the seakeeping ability of the ship as design compromises must be made to the hull design which affect the ability to sail through the water and break waves. Furthermore, due to the compromised designs of such hulls, the bow ramps would suffer damage from the force of wave impacts, beaching and general sailing operations.  
           [0007]    Prior art landing craft are also destabilized by the surf slamming into the flat stern of the ship when beached. When extricating from beach, such craft must operate in reverse—i.e., they drive stern first into the surf, operating its propellers or other propulsion system in reverse. Operation of the propulsion system in this manner is inefficient and taxing on the mechanical systems. Once clear of the shallow draft of the beach area, such ships must perform a broad turn to point the bow of the ship forward, towards the sea, for normal seakeeping operations. This is difficult in rough surf and can result in loss of vessels during combat operations.  
           [0008]    It is conventionally known in the art that steerable or azimuthing drives may be used to orient marine vessels in different directions. U.S. Pat. No. 5,522,335 to Veronesi et al. describes a combined azimuthing and tunnel auxiliary thruster which can be retracted inside of a boat hull if hazards are present, the contents of which are hereby incorporated by reference. However, there is no discussion in Veronesi et al. of utilizing the disclosed thrusters in combination with a vessel configuration which is appropriate for both seagoing and beaching operations.  
           [0009]    U.S. Pat. No. 5,660,131 to Gulling, et al. describes an icebreaking attachment for use with a parent vessel. This attachment, when connected to the parent vessel, provides an ice knife at the bow for increased icebreaking ability and protection. The icebreaking attachment also provides fully rotatable z-axis drives to steer the parent vessel, or the icebreaking attachment alone when detached, during icebreaking operations. While incorporating conventionally known z-axis drives to increase maneuverability , the disclosure of Gulling, et al. does not contribute to the problems associated with prior art landing craft.  
           [0010]    Another representative example of a marine vessel utilizing steerable drives is U.S. Pat. No. 5,966,520 to Arpiainen, et al. which discusses an icebreaking method and icebreaker. The icebreaker of Arpiainen, et al. uses a relatively narrow hull and first and second propulsion mechanisms to propel the icebreaker through an ice field in a direction at a substantial angle to the keel line of the ship to open a wide opening in the ice. Again, while the use of steerable drives is discussed, the apparatus and method of Arpiainen does not disclose how such drives, or the hull configuration of the ship, can be used to accomplish beaching operations while maintaining good seakeeping abilities.  
           [0011]    It is therefore an object of the present invention to provide a marine vessel that is capable of engaging in beaching operations and maintain good seakeeping abilities while doing so and when traveling in open water.  
           [0012]    It is a further object of the present invention to provide a marine vessel that is capable of moving in multiple directions including traveling bow forward as well as stem forward.  
           [0013]    It is yet another object of the present invention to provide a marine vessel which cargo can be loaded and unloaded quickly and easily.  
           [0014]    An additional object of the present invention is to provide an agile craft capable of turning in a short radius and handling well in surf and landing operations.  
         SUMMARY OF INVENTION  
         [0015]    The marine vessel described herein has the ability to travel in multiple directions, particularly stern forward, during landing operations on a beach or other unimproved water/land interface. The high maneuverability of the disclosed marine vessel is accomplished through the use of conventional steerable or azimuthing drives mounted below the waterline on the bow and stern portions of the craft.  
           [0016]    Unlike prior landing craft, which beach bow first, the ship disclosed herein uses the combination of fixed and retractable azimuthing drives to beach the vessel stem first. The stem azimuthing drives are adapted to retract into the hull of the ship such that they are not damaged by the shallow waters in the beaching area.  
           [0017]    The first advantage of this system is it allows a “drive-through” cargo deck. Vehicles, such as tanks, trucks, and humvees, are driven into the cargo area at a loading depot from the bow. The front of the vehicles will then face towards the stem of ship. Once the ship is beached at the designated landing zone, the vehicles can drive off the ship, right onto the beach in a forward gear. By unloading vehicles and other cargo astern of the vessel, the bow of the ship can be designed for excellent seakeeping and wave breaking ability since it is not used for beaching. This feature is also effective for getting on and off the beach since the effects of high surf near the beach are minimized because the efficient, wave breaking bow of the vessel is pointed towards the surf. This eliminates the problem with past landing craft and logistic support vehicles that have had problems getting on and off the beach due to the flat stern slamming into the surf or being pushed by the surf while beaching.  
           [0018]    The azimuthing drive propulsion system adds two advantages over traditional drive systems. First, the bow azimuthing drives provide the capability of full bollard thrust in both directions and are therefore extremely effective for powering off a beach. Second, with at least three drives operating (bow and stern), the vessel has full dynamic position capability while at sea. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    Specific embodiments of the invention have been chosen for purposes of illustration and description, and are shown in the accompanying drawing, forming a part of the specification wherein:  
         [0020]    [0020]FIG. 1 shows a side view of a symmetric vessel according to the invention  
         [0021]    [0021]FIG. 2 shows a front, or bow, profile view of a vessel according to the invention.  
         [0022]    [0022]FIG. 3 shows a rear, or stern, profile view of a vessel according to the invention.  
         [0023]    [0023]FIG. 4 shows a cut-away view of the stem end of the vessel according to the invention, illustrating the stern azimuthing thrusters in operational and retracted positions.  
         [0024]    [0024]FIG. 5 shows a perspective view of the bottom of the vessel according to the invention.  
         [0025]    FIGS.  6 A-B show top cut-away views of an alternative embodiment of the vessel according to the invention containing clamshell doors on bow of the vessel.  
         [0026]    FIGS.  7 A-C show the beaching procedure of the vessel according to the invention.  
         [0027]    FIGS.  7 D-E show extracting the vessel according to the invention from a beach.  
         [0028]    [0028]FIG. 8 illustrates a panoramic view of beaching operations utilizing the vessel according to the invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0029]    Referring to FIG. 1, vessel  10  generally contains a boat hull  30  having a bow portion  12 , a stem portion  14  and a keel  26 . Located at the bow  12  is a bow ramp  16 . The bow ramp  16  is generally used to load vehicles or other cargo onto the vessel  10 . Located at the stem  14  is a stem ramp  18  that is generally used to offload vehicles or other cargo, particularly during beaching operations. Below the waterline of the vessel  10  are two sets of azimuthing or steerable thruster drives  20  and  22 . At least one drive  20  is located on the bow portion  12  of the vessel. Similarly, at least one drive  22  is located on the stem portion  14  of the vessel. In the preferred embodiment of the invention, vessel  10  contains two bow thrusters  20  and two stern thrusters  22 . However, as shown in FIGS. 2 and 3, one stem thruster  22  may be used in combination with two bow thrusters  20 .  
         [0030]    Referring to FIGS. 1 and 2, the bow portion  12  of the vessel  10  incorporates, to the extent possible, characteristics of a well designed sea-going vessel. The bow portion shape is ship-like (i.e., not flattened to accommodate a beaching ramp), having a deep forefoot and fine “V” sections on the port and starboard sides of the bow. The bow end  12  is also fitted with a bulb  24  to reduce calm water resistance. The bow ramp  16  and deck  28  are r above the waterline unlike prior art landing craft. This allows an improved ship-like shape of the bow and serves to prevent damage to the bow ramp  16  during seakeeping operations. As a result of this design, the bow portion  12  is optimized for a high-sea-state environments resulting in improved controllability and performance of the vessel  10 . Further, when extracting the ship from a beach, the bow portion  12  is facing seaward to reduce wave impacts and the vessel  10  does not have to be turned around to return to sea.  
         [0031]    Referring to FIGS. 1 and 3, the stern portion  14  of the vessel  10  incorporates features that are designed to enhance the movement of vehicles and other cargo onto a beach. The deck  28  is close to the waterline, which serves to reduce ramp  18  length and break angle, or the angle between the deck and the beach. As is shown in FIGS. 1 and 3, the stem portion bottom shape is flat and broad, improving the stability of the vessel  10  when beached.  
         [0032]    In the preferred embodiment of the invention, the azimuthing drives  20  and  22  are powered by DC electric motors  30 , as shown in FIG. 4 The azimuthing drives are conventionally known in the art and may be obtained from several producers such as Rolls-Royce (United Kingdom) and John Crane-Lips (United Kingdom). The stem end thrusters  22  can be retracted into the vessel hull  30  such that they are above the keel  26 . This operation is shown in shadow in FIG. 4. Any typical means for retracting the stem thrusters  22  into the hull  30  may be used, such as hydraulic lifts or mechanical lifts. This allows efficient beaching operations without damaging the stern thrusters  22  on the beach/sea floor. The stem thrusters  22  are also fitted with fixed pitched propellers, which are optimized for open water speeds. In contrast, the bow end thrusters  20  are non-retractable. In the preferred embodiment, the bow thrusters  20  are also fitted with fixed pitch propellers which are mounted in nozzles optimized for maximum bollard (zero speed of advance) thrust. In open-sea or transit conditions, 60% of the power is preferably directed to the stem end thrusters  22  while the other 40% of power is directed to the bow thrusters  20 . In the preferred embodiment of the invention, this is done because the stern end thrusters  22  have a higher propulsive efficiency and the bow end thrusters are limited in diameter and therefore would be prone to cavitation if more power was applied. Because, by their very nature, azimuthing thrusters have complete rotation about a vertical axis, the direction of the vessel is preferably changed by movement and/or different directional alignment of the bow and stern thrusters. This configuration is shown in FIG. 5. which shows the bow thrusters  20  and stern thrusters  22  at different angles. However, as is known in the art, the vessel may also be turned through the use of variable power to the thrusters when all are facing the same direction, i.e., all are pointing to the stern of the ship.  
         [0033]    In general, the waterline of the vessel is fairly fixed, particularly during open sea operations. The ship also has an operating draft, defined herein as the distance between the keel  26  and the sea floor. The operating draft generally varies, depending on the distance between the sea floor and the top of the water. Quite obviously, when closer to the beach, the operating draft is reduced. To further decrease or increase the operating draft during beaching operations, the vessel of the instant invention contains an adjustable ballast and means to adjust such ballast. As is know in the art, tanks or other storage areas are located throughout the ship, such that the operating draft of the ship can be lowered or raised by adding or removing ballast, typically sea water, from the ballast tanks. The adjustable ballast system assists extraction of the vessel from the beach upon completion of unloading operations by allowing the stern portion  14  of the vessel, and to lesser extent the bow  12 , to float off of the beach.  
         [0034]    In another embodiment of the invention, shown in FIGS.6A and 6B, the bow portion of the ship  12  may be additionally fitted with clamshell doors  34  ahead of or proximal to the bow ramp  16 . Preferably, the clamshell doors  34  are split and open vertically, i.e., to the port and starboard sides of the ship, as shown in FIG. 6A. FIG. 6B illustrates the clamshell doors  34  in the closed position. When at sea, the vessel  10  will certainly experience repeated wave impacts to the bow ramp  16  which will slow the vessel and may ultimately damage the bow ramp. The addition of clamshell doors will protect the ramp and provide an even truer ship-shaped bow  12 .  
         [0035]    The operation of the previously disclosed structures in connection with beaching operations is shown in FIGS.  7 A-E and FIG. 8. In FIGS.  7 A-E, the heavy arrow, in all cases, points to the direction of the beach. Referring now to FIG. 7A, the vessel  10  is in open water  36  approaching the beach  38  (not shown) with the bow  12  pointed toward the beach. Bow thrusters  20  and stem thrusters  22  are pointed toward the stem portion  14  of the vessel Once the vessel reaches a predetermined distance from the beach, the thrusters  20  and  22  are turned, such that the stem  14  is pointed toward the beach, as shown in FIG. 7B. The stem thruster  22  is retracted into the hull  30  while the bow thruster  20  is pointed towards the bow  12  (i.e., the open sea). The vessel  10  is shown beached in FIG. 7C. The stem  14  of the vessel is on the beach  38  while the bow is still floating in the water  36  with the bow thruster  20  inactive. The stem ramp  18  is lowered upon the beach  38  for cargo unloading. FIG. 8 illustrates a beached vessel  10  on unloading tanks  40  upon the beach  38  by way of stem ramp. A second vessel  110  is shown in the water  36 .  
         [0036]    When the beaching operations are complete, the vessel  10  is extracted from the beach. As shown in FIG. 7D, the stem ramp  18  is retracted and the bow thruster  20  is oriented toward the beach. By removing ballast and operating the bow thruster  20 , the ship is extracted from the beach. Once the vessel is sufficiently clear of the beach, the stem thruster  22  deploys from the hull  30 , and in combination with the bow thruster  20  propels the vessel away from the beach and toward open water.  
         [0037]    In addition to the structures, sequences, and uses immediately described above, it will be apparent to those skilled in the art that other modifications and variations can be made the method of the instant invention without diverging from the scope, spirit, or teaching of the invention. Therefore, it is the intention of the inventors that the description of instant invention should be considered illustrative and the invention is to be limited only as specified in the claims and equivalents thereto.