Abstract:
An efficient double-ended watercraft is disclosed that comprises four low-Froude number hulls: two forward, two aft, two on the port side, and two on the starboard. Each hull comprises an independent engine that drives a propeller shaft and propeller that is located amidships. The propeller shafts on the starboard are collinear, as are the propeller shafts on the port. The propellers on the starboard are near each other and counter rotate, as do the propellers on the port. The propellers are variable-pitch propellers. When the ferry is changes from moving forward to moving in reverse and from reverse to forward, the propellers on the starboard exchange pitch. This enables the ferry to move as efficiently in reverse as it does when forward. The propellers on the port also exchange pitch when changing from moving forward to moving in reverse and from reverse to forward.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to naval architecture in general, and, more particularly, to double-ended watercraft.  
       BACKGROUND OF THE INVENTION  
       [0002]     Ferries, roll-on/roll-off and other “double-ended” watercraft (i.e., watercraft that often travel in both forward and reverse) require propulsion systems that are efficient for travel in both forward and reverse. In accordance with one technique in the prior art, propulsors (e.g., waterjets, propellers, etc.) on azipods are located at both ends of a double-ended vessel and rotate around a vertical axis so that they point in the desired direction of travel. This technique is disadvantageous, however, because the equipment used to hold and rotate the azipods is heavy, bulky, expensive, and prone to malfunction.  
         [0003]     Therefore, the need exists for an improved double-ended watercraft design that avoids this disadvantage.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention is particularly well-suited with double-ended watercraft and avoids some of the costs and disadvantages associated with double-ended watercraft in the prior art. Although the illustrative embodiment of the present invention is a double-ended watercraft, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention in other watercraft and machines, such as aircraft, windmills, and axial impellers for ventilation and other air handling applications such as wind tunnels.  
         [0005]     The illustrative embodiment is a double-ended ferry that comprises four high-Froude number hulls: two forward, two aft, two on the port side, and two on the starboard. The ferry&#39;s hulls exhibits longitudinal and lateral symmetry and provide equally efficient propulsion in both forward and reverse.  
         [0006]     Each hull comprises an independent engine and gearbox that drives a propeller shaft and propeller that is located amidships. Mounting the propellers amidships is advantageous because it reduces the propellers vulnerability to ice, grounding in shallow water, and other impediments.  
         [0007]     The propeller shafts on the starboard are collinear, as are the propeller shafts on the port. The propellers on the starboard oppose each other and counter rotate, as do the propellers on the port, and form a counter-rotating propeller system. This affords the advantages of a counter-rotating propeller system without the disadvantage of counter-rotating propeller systems in the prior art (e.g., counter-rotating concentric shafts, complex gear boxes, etc.). Having two independent engines/gearboxes/propeller shafts/shafts on both the port and starboard also provides redundancy, which makes the craft fault-tolerant.  
         [0008]     In accordance with the illustrative embodiment, the propellers are variable-pitch propellers. When the ferry changes from moving forward to moving in reverse and from reverse to forward, the pitch and the direction of rotation of each propeller changes. This enables the ferry to move as efficiently in reverse as it does when forward.  
         [0009]     The illustrative embodiment comprises: a first hull; a second hull; a first propeller shaft that extends from the first hull towards the second hull; and a second propeller shaft that extends from the second hull towards the first hull. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  depicts an isometric drawing of the salient components of the illustrative embodiment of the present invention.  
         [0011]      FIG. 2  depicts a side view of the salient components of the illustrative embodiment of the present invention.  
         [0012]      FIG. 3  depicts a plan view drawing—taken at elevation YY—of ferry  100 , as depicted in  FIG. 2 .  
         [0013]      FIG. 4  depicts variable-pitch propeller  213 -x- 1  at six different pitches (for a given diameter).  
         [0014]      FIG. 5  depicts a plan-view diagram that focuses on the pitch and direction of rotation of propellers  213 -x- 1  and  213 -x- 2  when ferry  100  is moving forward (i.e., bow first).  
         [0015]      FIG. 6  depicts a plan-view diagram that focuses on the pitch and direction of rotation of propellers  213 -x- 1  and  213 -x- 2 .  
     
    
     DETAILED DESCRIPTION  
       [0016]      FIG. 1  depicts an isometric drawing and  FIG. 2  depicts a side view drawing of the salient components of the illustrative embodiment of the present invention, ferry  100 .  FIG. 3  depicts a plan view drawing—taken at elevation Z=0—of ferry  100 , as depicted in  FIG. 2 .  
         [0017]     Ferry  100  comprises: 
        superstructure  101 ,     four (4) hulls: hull  111 - 1 - 1 , hull  111 - 2 - 1 , hull  111 - 1 - 2 , and hull  111 - 2 - 2 ;     four (4) engines and gearboxes: engine/gearbox  211 - 1 - 1 , engine/gearbox  211 - 2 - 1 , engine/gearbox  211 - 1 - 2 , and engine/gearbox  211 - 2 - 2 ,     four (4) propeller shafts: propeller shaft  212 - 1 - 1 , propeller shaft  212 - 2 - 1 , propeller shaft  212 - 1 - 2 , and propeller shaft  212 - 2 - 2 , and     four (4) propellers: propeller  213 - 1 - 1 , propeller  213 - 2 - 1 , propeller  213 - 1 - 2 , and propeller  213 - 2 - 2 , 
 
 interconnected as depicted in  FIGS. 1, 2 , and  3 . Although the illustrative embodiment comprises four sets of hulls/engines/gearboxes/propeller shafts/propellers, it will be clear to those skilled in the art, after reading this disclosure, how to make and use embodiments of the present invention that comprises two or more sets of hulls/engines/gearboxes/propeller shafts/propellers. 
       
 
         [0023]     Ferry  100  is a double-ended watercraft that is capable of loading and unloading passengers, cargo, and vehicles from the bow as easily as from the stern, and of being driven bow first as easily and efficiently as being driven stern first. Except for the pitch of the propellers, ferry  100  exhibits longitudinal symmetry (i.e., is symmetrical about plane Y=0, as shown in  FIGS. 2 and 3 ), and lateral symmetry (i.e., is symmetrical about plane X=0, as shown in  FIG. 3 ).  
         [0024]     Because of the longitudinal symmetry, the designation of bow and stern are arbitrary. In accordance with the illustrative embodiment, the end of ferry  100  nearest hulls  111 - 1 - 1  and  111 - 2 - 1  is designated as the bow.  
         [0025]     Hulls  111 -x- 1  (wherein x is chosen from the set of integers {1, 2}), engines/gearboxes  211 -x- 1 , propeller shafts  212 -x- 1 , and propellers  213 -x- 1  are associated with the bow of ferry  100 , while hulls  111 -x- 2 , engines/gearboxes  211 -x- 2 , propeller shafts  212 -x- 2 , and propellers  213 -x- 2  are associated with the stern of ferry  100 . Similarly, hulls  111 - 1 -x, engines/gearboxes  211 - 1 -x, propeller shafts  212 - 1 -x, and propellers  213 - 1 -x are on the starboard of ferry  100 , while hulls  111 - 2 -x, engines/gearboxes  211 - 2 -x, propeller shafts  212 - 2 -x, and propellers  213 - 2 -x are on the port.  
         [0026]     Superstructure  100  is a free-standing composite metal structure that houses the passengers, cargo, crew, and equipment for piloting ferry  100 . Superstructure  100  rides above waterline  201  (shown in  FIG. 2 ) atop the four hulls and provides the structural support necessary to keep the relative position and orientation of the hulls fixed. As shown in  FIG. 2 , the lower portion of hulls  111 - 1 - 1 ,  111 - 2 - 1 ,  111 - 1 - 2 , and  111 - 2 - 2  are completely submerged. It will be clear to those skilled in the art how to make and use superstructure  100 .  
         [0027]     Each of the four hulls comprises an independently-controlled engine/gearbox that turns a propeller shaft that extends from that hull towards the other hull on the same side of ferry  100 . For example, propeller shaft  212 - 2 - 1  extends from hull  111 - 2 - 1  towards hull  111 - 2 - 2 , and conversely propeller shaft  212 - 2 - 2  extends from hull  111 - 2 - 2  towards hull  111 - 2 - 1 . As shown in  FIGS. 2 and 3 , propeller shaft  212 - 1 - 1  is collinear with propeller shaft  212 - 1 - 2  along line  202 - 1 , and propeller shaft  212 - 2 - 1  is collinear with propeller shaft  212 - 2 - 2  along line  202 - 2 .  
         [0028]     In normal operation, propeller shaft  212 - 1 - 1  counter-rotates with respect to propeller shaft  212 - 1 - 2 , and propeller shaft  212 - 2 - 1  counter-rotates with respect to propeller shaft  212 - 2 - 2 . In particular, when propeller shaft  212 - 2 - 1  turns with a rate of rotation of +ω, propeller shaft  212 - 2 - 2  turns with a rate of rotation of −ω (i.e., the two shafts turn at the same number of revolutions per minute but in opposite directions).  
         [0029]     Each of propellers  213 - 1 - 1 ,  213 - 2 - 1 ,  213 - 1 - 2 , and  213 - 2 - 2  is a variable-pitch propeller whose blades can be changed from −90° to +180°, in well-known fashion.  FIG. 4  depicts variable-pitch propeller  213 -x- 1  at six different pitches (for a given diameter). When the pitch of a propeller is set to −75° and the direction of rotation is +ω, then the direction of movement is towards the left. In contrast, when the pitch the propeller is set to +75° and the direction of rotation is −ω, then the direction of movement is towards the right. It will be clear to those skilled in the art how to make and use variable-pitch propellers.  
         [0030]      FIG. 5  depicts a plan-view diagram that focuses on the pitch and direction of rotation of propellers  213 -x- 1  and  213 -x- 2  when ferry  100  is moving forward (i.e., bow first), and  FIG. 6  depicts a plan-view diagram that focuses on the pitch and direction of rotation of propellers  213 -x- 1  and  213 -x- 2  when ferry  100  is moving in reverse (i.e., stern first). Table 1 depicts the rate and direction of rotation, and blade pitch for each of the four propellers on ferry  100  when ferry  100  is moving forward.  
                                 TABLE 1                           Rate and Direction of Rotation and Pitch for Propellers       When Moving Forward.                    Rate and               Propeller   Direction of Rotation   Pitch                       213-1-1   +ω S     φ L             213-1-2   −ω S     φ T             213-2-1   −ω P     φ L             213-2-2   +ω P     φ T                          
 The value ω S  represents the rate of rotation on the starboard, and the value ω P  represents the rate of rotation on the port. The two values are opposite when ferry  100  is traveling in a straight line—forward or in reverse—but the values are different when ferry  100  is turning at very slow speed. It will be clear to those skilled in the art, after reading this specification, how to determine the appropriate values for ω s  and ω p  in any circumstance. 
 
         [0031]     The value φ L  represents the pitch of a propeller when it is the leading propeller (i.e., the first propeller in the flow), and the value φ T  represents the pitch of a propeller when it is the trailing propeller (i.e., the second propeller in the flow). It should be understood that the designations of leading propeller and trailing propeller are not permanent, but are only in relation to the direction that ferry  100  is traveling. In accordance with the illustrative embodiment, 
 
φ T =−φ L    (Eq. 1) 
 
 In some alternative embodiments of the present invention, the pitch of the trailing propeller, φ T , is slightly different than the pitch of the leading propeller, φ L , 
 
φ T ≈−φ L    (Eq. 2) 
 
 because a set of counter-rotating propellers is most efficient when the pitch of trailing propeller is slightly different than the pitch of the leading propeller. In either case, it will be clear to those skilled in the art, however, how to make and use embodiments of the present invention in which the propellers have the same or different pitch. 
 
         [0032]     Table 2 depicts the rate and direction of rotation, and blade pitch for each of the four propellers on ferry  100  when ferry  100  is moving in reverse.  
                                 TABLE 2                           Rate and Direction of Rotation and Pitch for Propellers       When Moving In Reverse.                    Rate and               Propeller   Direction of Rotation   Pitch                       213-1-1   −ω S     φ T             213-1-2   +ω S     φ L             213-2-1   +ω P     φ T             213-2-2   −ω P     φ L                        
 
         [0033]     In some alternative embodiments of the present invention, the watercraft only moves in one direction, rather than in both forwards and reverse. In theses cases, each of propellers  213 - 1 - 1 ,  213 - 2 - 1 ,  213 - 1 - 2 , and  213 - 2 - 2  can be a fixed-pitch propeller, wherein the pitch of the leading propeller is fixed at (DL and the pitch of the trailing propeller is fixed at φ T .  
         [0034]     It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. For example, in this Specification, numerous specific details are provided in order to provide a thorough description and understanding of the illustrative embodiments of the present invention. Those skilled in the art will recognize, however, that the invention can be practiced without one or more of those details, or with other methods, materials, components, etc.  
         [0035]     Furthermore, in some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the illustrative embodiments. It is understood that the various embodiments shown in the Figures are illustrative, and are not necessarily drawn to scale. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the present invention, but not necessarily all embodiments. Consequently, the appearances of the phrase “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout the Specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.