FIELD 
       [0001]    This technical disclosure relates to techniques for reducing drag on surfaces of vehicles that are in contact with water, including surface vehicles and underwater vehicles such as unmanned underwater vehicles (UUVs) and autonomous underwater vehicles (AUVs), using a polymer solution. 
       BACKGROUND 
       [0002]    Various approaches to reducing drag on underwater vehicles using a polymer solution that is introduced into the boundary layer are described in U.S. Pat. Nos. 3,382,831, 3,392,693, and 4,186,679. 
       SUMMARY 
       [0003]    This disclosure describes systems and methods of reducing drag on outer or exterior surfaces of vehicles that are in direct, intimate contact with water using a polymer solution that reduces the drag on the surfaces as the vehicles travel through the water. The systems and methods described herein can be utilized on any vehicle surface that is in contact with the water. The vehicles can be surface vehicles or underwater vehicles including manned underwater vehicles, or unmanned vehicles such as AUVs and UUVs. In the case of an underwater vehicle, the underwater vehicle can be completely submerged in water during use, or only partially submerged in water during use. 
         [0004]    The techniques described herein introduce a polymer solution into the boundary layer of the water flowing past a surface of the vehicle. The polymer solution is mixed within the vehicle and introduced into the boundary layer passively with minimal or no usage of electrical energy. For example, an electrically driven pump is not used to distribute the polymer or the polymer solution within the vehicle or to force the polymer solution from the vehicle into the boundary layer. 
         [0005]    In some vehicles, such as AUVs or UUVs, the range, endurance, and envelope of operation are limited by the amount of storage volume available for propulsion energy for a given level of vehicle drag. Therefore, energy storage density, energy conversion efficiency, and vehicle drag are key performance parameters. The techniques for reducing drag using passive mixing and ejection of the polymer solution described herein minimize energy consumption, thereby maximizing electrical energy consumption efficiency during operation of the vehicle, including during high-speed, low angle of attack operations. In contrast, active pumping system components such as described in U.S. Pat. Nos. 3,392,693 and 4,186,679 reduce drag at the expense of added energy consumption and increased weight. 
         [0006]    In one embodiment described herein, the polymer can be in liquid form in the vehicle and stored in one or more flexible bladders that are located in the vehicle. The bladders can be exposed to ambient pressure which acts on the bladder(s). One or more force applying mechanisms act on the bladder(s) to force the liquid polymer from the bladder(s). One or more metering valves can be used to control the amount of the liquid polymer that flows from the bladder(s) to a mixing chamber where the liquid polymer mixes with water to create a polymer solution that is then passively ejected into the boundary layer due to a pressure differential existing between one or more water inlets and the outlet(s) for the polymer solution. 
         [0007]    In another embodiment described herein, the polymer can be in the form of a water-soluble solid body. The solid polymer body can be disposed within the vehicle at a location so that the solid polymer body is exposed to water in which the vehicle is disposed that is passively circulated, under forced convection, around, over, and/or through channels in the solid polymer body. The water contacting the solid polymer body dissolves some of the polymer which mixes with the water to form the polymer solution that is then passively ejected into the boundary layer due to a pressure differential existing between one or more water inlets and the outlet(s) for the polymer solution. In other embodiments, the water-soluble solid polymer body can be disposed outside the vehicle where the solid polymer body is directly exposed to the water in which the vehicle is disposed. The solid polymer body, whether mounted in the vehicle interior or on the vehicle exterior, can be made in a shape that will simplify fabrication and packaging within or on the vehicle. 
         [0008]    The polymer solution ejection techniques described herein can be utilized on any outer surface(s) of the vehicle that is subject to drag resulting from the surface being in contact with the water as the vehicle travels through the water. Examples of surfaces on which the polymer solution ejection can be utilized include, but are not limited to, an exterior surface of the hull of the vehicle, a surface on a fixed or actuatable control fin connected to the hull, a surface on a duct (for example surrounding a propeller) connected to the hull, a surface on a nose cone or tail cone of the hull, a surface of a propeller of a propulsion mechanism that propels the vehicle through the water, and other appendages of the vehicle that are in contact with the water. 
     
    
     
       DRAWINGS 
         [0009]      FIG. 1  illustrates an example of a vehicle in the form of an underwater vehicle that uses a polymer solution to reduce drag on the hull of the underwater vehicle. 
           [0010]      FIG. 2  illustrates a portion of an interior of the vehicle of  FIG. 1  with a system for ejecting polymer solution for drag reduction. 
           [0011]      FIG. 2A  is a perspective view of the front end of the vehicle of  FIGS. 1 and 2 . 
           [0012]      FIG. 3  is a schematic depiction of another embodiment of a system for ejecting polymer solution for drag reduction. 
           [0013]      FIG. 4  is a schematic depiction of another embodiment of a system for ejecting polymer solution for drag reduction. 
           [0014]      FIG. 5  is a schematic depiction of polymer solution ejection for drag reduction on vehicle surfaces other than or in addition to the hull. 
           [0015]      FIG. 6  illustrates a front portion of a vehicle in the form an underwater vehicle that uses a water-soluble solid polymer to create a polymer solution for drag reduction on the vehicle, with a portion of the front portion broken away to illustrate an interior space. 
           [0016]      FIGS. 7A and 7B  illustrate different embodiments of the water-soluble solid polymer. 
           [0017]      FIGS. 8A and 8B  illustrate different mounting locations for the water-soluble solid polymer within the vehicle. 
           [0018]      FIGS. 9A and 9B  illustrate operation of one example of a mechanism that can be used to meter the inlet flow of water for creating the polymer solution. 
           [0019]      FIG. 10  is a schematic depiction of ejecting polymer solution created from the water-soluble solid polymer for drag reduction on vehicle surfaces other than or in addition to the hull. 
           [0020]      FIG. 11  depicts another embodiment where a water-soluble solid polymer is mounted directly on a surface of a vehicle so that the solid polymer is directly exposed to the water flowing past the surface. 
           [0021]      FIG. 12  is a cross-sectional view taken along line  12 - 12  of  FIG. 11  showing the water-soluble solid polymer applied to the leading edge of a fin of the vehicle. 
           [0022]      FIG. 13  is a partial perspective view of a duct used on the vehicle of  FIG. 11  where the water-soluble solid polymer is applied to the leading edge of the duct. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Unless otherwise defined or indicated to the contrary in the description or claims, a vehicle as used herein is intended to mean any type of surface or submersible vehicle that has at least one surface thereof that is in direct, intimate contact with water as the vehicle travels through the water. The vehicle can be completely submerged in water during use in the case of a submersible, or the vehicle can be a surface vehicle with only portions of the vehicle in contact with the water during use. Examples of vehicles on which the concepts described herein can be used include, but are not limited to, manned or unmanned surface vehicles such as military, commercial or recreational vessels or boats, unmanned submersible vehicles such as UUVs, AUVs, and torpedoes, and manned submersible vehicles. 
         [0024]    Unless otherwise defined or indicated to the contrary in the description or claims, the techniques described herein can be utilized on any outer or exterior vehicle surface that is in direct contact with the water during use of the vehicle and where drag is created on the surface by the water as the vehicle moves through the water. Examples of vehicle surfaces on which the concepts described herein can be used include, but are not limited to, an exterior surface of the hull of the vehicle, a surface on a fixed or actuatable control fin connected to the hull, a surface on a duct, for example surrounding a propeller, connected to the hull, a surface on a nose cone or tail cone of the hull, a surface of a propeller of a propulsion mechanism that propels the vehicle through the water, and other appendages of the vehicle that are in contact with the water. 
         [0025]    The vehicles described herein can operate in seawater, freshwater or brackish water. Therefore, the polymer solution described herein can be formed from a polymer mixed together with seawater, freshwater or brackish water (hereinafter referred to collectively as “water”). 
         [0026]      FIG. 1  illustrates a vehicle  10  in the form of an underwater vehicle that has a generally cylindrical hull  12 , a front end  14  of the hull  12  that is hydrodynamically-shaped, for example bullet-shaped, to enhance the hydrodynamic efficiency of the vehicle  10 , and a rear end  16  that can include a propulsion mechanism  18 , for example a ducted propeller (best seen in  FIG. 5 ), for propelling the vehicle  10  through the water. The vehicle  10  may also include other appendages such as fixed or steerable control fins  20  (best seen in  FIG. 5 ) for directional control of the vehicle  10  through the water. 
         [0027]    As shown in  FIGS. 1 and 2 , a polymer solution  21  is passively ejected from the vehicle  10  into the boundary layer  22  that develops around the vehicle  10  as the vehicle moves through the water to reduce skin friction drag on the vehicle  10 . In the example illustrated in  FIGS. 1 and 2 , the polymer solution  21  is ejected through outlets  24  circumferentially spaced around the circumference of the hull  12  to reduce the drag on the hull  12 . In some embodiments, the polymer solution  21  can be ejected around substantially the entire circumference of the hull  12 , and the amount of polymer solution  21  that is ejected is substantially continuous around the circumference. In other embodiments discussed below, the polymer solution  21  can be ejected at select locations of the hull  12  that is less than the entire circumference and/or the amount of the polymer solution  21  ejected can vary depending upon location around the circumference. Further, in addition to or separately from ejecting the polymer solution  21  to reduce drag on the hull  12 , polymer solution  21  can be ejected at other locations of the vehicle  10 , such as from the duct of the propulsion mechanism  18  and/or from one or more of the fins  20  or other surfaces of the vehicle  10  as discussed further below. 
         [0028]    Referring to  FIG. 2 , an example of a portion of an interior of the vehicle  10  that includes a system  30  for creating and ejecting the polymer solution  21  is illustrated. In this example, the vehicle  10  is illustrated as including a plurality of axially spaced, circumferential hull ribs  32  that provide structural rigidity to the hull  12 . The space between the hull ribs  32  is exposed to the ambient pressure of the water that the vehicle  10  is operating in. 
         [0029]    The system  30  includes at least one bladder  34  or storage chamber that stores a liquid polymer therein, a mixing chamber  36  where the polymer solution  21  is created, at least one fluid passage  38  fluidly connecting the bladder  34  to the mixing chamber  36 , a metering valve  40  in the fluid passage  38 , at least one water inlet  42 , and at least one of the polymer solution outlets  24 . 
         [0030]    The bladder  34  can be positioned in the space between two of the hull ribs  32  which helps to utilize space in the vehicle  10  that may otherwise not be utilized. The bladder  34  stores the polymer in liquid form that will be used to generate the polymer solution  21 . The polymer stored in the bladder  34  can be any polymer that is suitable for use in the polymer solution  21  to reduce drag, such as a high molecular weight water-soluble polymer. Examples of polymers that can be used are disclosed in U.S. Pat. No. 4,186,679, the entire contents of which are incorporated herein by reference. 
         [0031]    The bladder  34  can be a circumferential ring structure that extends circumferentially around the entire interior circumference of the vehicle  10 . The bladder  34  is flexible and compressible, and is located in a flooded area of the vehicle  10  that is exposed to ambient water pressure, and is “pressure balanced”. A stored energy force applying mechanism  44  acts on the bladder  34  to provide pressurization of the liquid polymer above ambient pressure. The force applying mechanism  44  can be any stored energy force applying device(s) including, but not limited to, one or more springs, one or more bellows, one or more diaphragms, a pressurized gas device, thermo-chemical (exo-thermic) reactants that can produce heat and pressure, and combinations thereof. The mechanism  44  will apply a force to the bladder  34  to push the liquid polymer out of the bladder  34  through the fluid passage  38  to the metering valve  40  providing positive flow of liquid polymer, independent of depth, from the bladder  34  to the mixing chamber  36 . 
         [0032]    In some embodiments, a plurality of the bladders  34  can be provided.  FIG. 2  shows the use of two of the bladders  34 . As discussed further below, the bladders  34  can be serially connected to one another so that liquid polymer from one bladder  34  flows into the other bladder  34  and then into the fluid passage  38 . Alternatively, as also discussed further below, the bladders  34  can each be separately fluidly connected to the mixing chamber  36 . The use of more than one bladder  34  increases the amount of liquid polymer that can be stored on the vehicle  10 . In addition, one type of liquid polymer having particular drag reduction properties can be stored in one of the bladders, while a different type of liquid polymer with different drag reduction properties can be stored in another one of the bladders. 
         [0033]    The fluid passage  38  leads from the bladder  34  to direct the liquid polymer into the mixing chamber  36 . The flow of the liquid polymer through the fluid passage  38  is controlled by the metering valve  40  to control the amount of the liquid polymer flowing into the mixing chamber  36 . The metering valve  40  can be the only component of the system  30  that utilizes electrical energy and can be controlled by a controller (not shown) in the vehicle  10  to regulate the amount of the liquid polymer flowing to the mixing chamber  36 . 
         [0034]    The one or more water inlets  42  allow water to enter the interior of the vehicle  10  and flow to the mixing chamber  36  for creating the polymer solution  21  with the liquid polymer, with the polymer solution  21  then flowing out through the outlets  24 . The water inlet(s)  42  is an opening or plurality of openings that can be located at the front end  14  or nose of the hull  12  at any location that is suitable for allowing ingress of water. In one example, the water inlet(s)  42  is located at a location on the front end  14  where increased pressure due to stagnation of the water develops. In one embodiment, a plurality of the water inlets  42  are provided (as shown in  FIG. 2A ) that are circumferentially equally spaced from one another about the circumference of the front end  14 . Each inlet  42  is individually fluidly connected to the mixing chamber  36  so that water entering each inlet  42  flows into the mixing chamber  36 . Alternatively, each inlet  42  can communicate with an inlet water manifold (not shown) that in turn is in fluid communication with the mixing chamber  36  to direct water from the manifold into the mixing chamber  36 . 
         [0035]    In one embodiment, the water inlets  42  are located on the vehicle  10  where the pressure is higher than the pressure at the outlets  24 . As a result, the water can continuously flow into and through the vehicle  10  via the mixing chamber  36  and out the outlets  24 . In other embodiments, water flow into the water inlets  42  and through the vehicle  10  can be controlled via a suitable flow control mechanism such as one similar to the flow control mechanism discussed below with respect to  FIGS. 9A and 9B . 
         [0036]    In the embodiment illustrated in  FIG. 2 , the mixing chamber  36  is a circumferential ring structure that extends circumferentially around the entire interior circumference of the vehicle  10 . The mixing chamber  36  receives the liquid polymer from the bladder  34  and the water from the inlets  42 , and mixes the two to form the polymer solution  21 . To aid in mixing, a static mixer  46  can be disposed in the mixing chamber  36  to help achieve mixing of the water and the liquid polymer to create the polymer solution  21 . As shown in  FIG. 2 , the fluid passage  38  can intersect a water passage  48  that directs the water from the inlet  42  into the mixing chamber  36  just upstream of the mixing chamber  36 . The water flowing through the water passage  48  creates a venturi effect on the liquid polymer in the fluid passage  38 , helping to draw the liquid polymer through the fluid passage  38  and into the mixing chamber  36 . 
         [0037]    This “direct injection mixing” and passive intake and ejection of entwined flows of water and liquid polymer utilizes pressure gradients that are developed between the high stagnation pressure at the nose where the inlets  42  are located and the low pressure on the hull sides at the outlets  24 . Precise delivery of water and liquid polymer will be achieved by employing fluidic metering via the metering valve  40  to start/stop/re-start the flow of liquid polymer, and control of the ratio of water to liquid polymer, ensuring optimal dispersion into the boundary layer, to conserve polymer while providing drag reduction over select portions of the vehicle&#39;s  10  mission. 
         [0038]    The polymer solution  21  generated in the mixing chamber  36  is then passively ejected through one or more outlets formed in a surface of the vehicle  10 . Passive ejection as used herein refers to ejection of the polymer solution  21  without the use of an electrically driven pump. In the illustrated embodiment, the polymer solution  21  is passively ejected due to a pressure differential between the inlets  42  and the outlets  24 , the pressure differential between the inlets  42  and the outlet  24  being sufficient to result in the polymer solution  21  passively flowing out of the outlets  24 . In one embodiment, when the metering valve  40  is closed to prevent flow of liquid polymer to the mixing chamber  36 , the water can still flow through the vehicle  10  from the inlets  42  and out of the outlets  24  unless a flow control mechanism is provided for the water as described above. In another embodiment, the polymer solution  21  may be passively ejected as a result of hydrostatic pressure acting on one or more of the bladders  34  that could result in passive ejection of the polymer solution  21  into, for example, higher pressure areas such as the front end  14  and/or lower pressure areas such as a side area of the hull  12  that contains the outlets  24 . 
         [0039]    In the example illustrated in  FIG. 2 , the polymer solution  21  can be ejected through the plurality of outlets  24 . In other embodiments, the polymer solution  21  can be ejected through outlets formed in other surfaces, for example one or more outlets  50  formed in one or more of the fins  20  and/or one or more outlets  52  formed in an interior surface and/or exterior surface of the duct of the propulsion mechanism  18  as shown in  FIG. 5 . 
         [0040]    As indicated in  FIG. 2 , in the case of discharge of the polymer solution  21  around the hull  12 , the water inlets  42  are positioned forwardly of the outlets  24 . In addition, the outlets  24  are positioned rearwardly of the mixing chamber  36 . The inlets  42  and the outlets  24  can be positioned at any locations relative to one another where a pressure differential is achieved between the two to permit the passive ejection of the polymer solution  21  so that the polymer solution  21  is not pumped by an electrically driven pump, and both the creation and discharge of the polymer solution  21  can be accomplished with reduced use of electrical energy. 
         [0041]      FIG. 3  schematically depicts an embodiment where three of the bladders  34  described above are provided each of which is acted upon by stored energy force applying mechanisms (not shown) as described above. Each of the bladders  34  can be directly connected (i.e. connected in parallel) to the mixing chamber  36  by one of the fluid passages  38  with its own metering valve  40 . Each metering valve  40  controls the flow of the liquid polymer from its associated bladder  34  into the mixing chamber  36 . Alternatively, as illustrated in dashed lines in  FIG. 3 , the bladders  34  can be serially connected to one another by flow passages  54  so that, for example, liquid polymer from the bladder  34  on the right in  FIG. 3  can flow into the center bladder  34  which then flows into the bladder  34  on the left, and the liquid polymer then flows through the fluid passage  38  into the mixing chamber  36 . 
         [0042]      FIG. 4  schematically depicts an embodiment where, instead of a single, circumferentially continuous mixing chamber  36 , the mixing chamber  36  is divided into zones  36   a ,  36   b ,  36   c ,  36   d . The zones  36   a - d  can be physically separated or spaced from one another, or the zones  36   a - d  can be fluidly separated sections of a single mixing chamber. Although four zones  36   a - d  are illustrated, a smaller or larger number of zones can be provided. In this example, two of the bladders  34  are serially connected by the fluid passage  54 , and the flow passage  38  is divided into four separate passages  38   a ,  38   b ,  38   c ,  38   d  connected to respective ones of the mixing chamber zones  36   a - d . Each of the bladders  34  can be acted upon by stored energy force applying mechanisms (not shown) as described above. Flow of the liquid polymer through each passage  38   a - d  is controlled by the metering valves  40 . Alternatively, the metering valves  40  in the flow passages  38   b - d  are optional (as indicated by the dashed lines) with the metering valve  40  in the flow passage  38  controlling the flow to all of the zones  36   a - d.    
         [0043]    In another alternative configuration shown in dashed lines in  FIG. 4 , the mixing chamber zones  36   a - d  can be supplied with liquid polymer from separate ones of the bladders  34  each of which is acted upon by stored energy force applying mechanisms (not shown) as described above. For example, one of the bladders  34  can supply liquid polymer to the mixing chamber zones  36   a - b  while the other bladder  34  can supply liquid polymer to the mixing chamber zones  36   c - d.    
         [0044]    One advantage of using mixing chamber zones is that the discharge of the polymer solution  21  can be controlled to select portions of the surface. For example, with some angles of attack of the vehicle  10 , the resulting drag that is produced on surfaces of the vehicle  10  can vary significantly based on location on the surface. Therefore, polymer solution  21  or more polymer solution  21  can be ejected to areas of high drag, while no polymer solution  21  or less polymer solution  21  can be ejected to areas of lower drag. 
         [0045]    Returning to  FIG. 2 , an example operation of drag reduction on the hull  12  of the vehicle  10  will be described. Drag reduction on other surfaces of the vehicle  10  operates in a similar manner, for example as illustrated in  FIG. 5 . When the metering valve  40  is closed, water flows continuously through the vehicle  10  from the inlets  42  and out the outlets  24 . When drag reduction is desired, the metering valve  40  is opened to achieve the desired amount of liquid polymer flow. In one embodiment, the opening degree of the metering valve  40  can be controlled based on the speed of the vehicle  10 , for example the metering valve  40  can increase the amount liquid polymer flow as the speed of the vehicle  10  increases. The liquid polymer is forced from the bladder(s)  34  and through the fluid passage  38  by the force applying mechanism(s)  44 . At the same time, water flowing through the water passage  48  helps draw the liquid polymer through the passage  38 , with the water and the liquid polymer then flowing into the mixing chamber  36  where the water and the liquid polymer are then mixed together to form the polymer solution  21 . Thereafter, the resulting polymer solution  21  is then passively ejected through the outlets  24  into the boundary layer  22  resulting in a reduction in the drag on the exterior surface of the hull  12 . 
         [0046]    With reference now to  FIGS. 6-10 , an embodiment of drag reduction on a hull  90  of a vehicle  100 , for example an underwater vehicle, is illustrated. In this embodiment, instead of a liquid polymer, the polymer solution is created using a water-soluble solid polymer body  102 . 
         [0047]    A front end portion  104  of the vehicle  100  is illustrated in  FIG. 6 . The front end portion  104  defines an interior cavity  106  between an exterior hull  108  and an interior pressure hull  109  (illustrated in  FIGS. 8A and 8B ). In one example, the cavity  106  can be circumferentially continuous around a central longitudinal axis A-A of the vehicle  100 . The cavity  106  is located outside of the interior pressure hull  109  or boundary of the vehicle  100  so that the cavity  106  is subject to the ambient water pressure. In use, the cavity  106  forms the mixing chamber in which a polymer solution  121  is generated. 
         [0048]    One or more water inlets  110  are formed in the exterior hull  108  through which water can flow into the cavity  106  as indicated by the arrow W i  in  FIG. 6 . The water inlet(s)  110  is an opening or plurality of openings that can be located at the front end portion  104  or nose of the hull  90  at any location that is suitable for allowing ingress of water. In one embodiment, the water inlet(s)  110  is located at a location on the front end portion  104  where increased pressure due to stagnation of the water develops. In one embodiment, a plurality of water inlets  110  are provided (as shown in  FIG. 6 ) that are circumferentially equally spaced from one another about the circumference of the front end  104 . Each inlet  110  is individually fluidly connected to the cavity  106  so that water entering each inlet  110  flows into the cavity  106 . Alternatively, each inlet  110  can communicate with an inlet water manifold (not shown) that in turn is in fluid communication with the cavity  106  to direct water from the manifold into the cavity  106 . 
         [0049]    The vehicle  100  further includes one or more polymer solution outlets  112  in a surface, such as in the hull  90 , through which the polymer solution  121  is passively ejected as indicated by the arrow S o . In the example illustrated in  FIG. 6 , a plurality of the outlets  112  are provided in the hull  90  at a location lower in pressure than the pressure at the water inlets  110  to create the pressure differential. In other embodiments, the polymer solution  121  can be ejected through one or more outlets formed in other surfaces, for example one or more outlets  114  formed in one or more of the fins  20  and/or one or more outlets  116  formed in an interior surface and/or exterior surface of the duct of the propulsion mechanism  18  as shown in  FIG. 10 . 
         [0050]    Returning to  FIG. 6 , in the case of discharge of the polymer solution  121  around the hull  90 , the water inlets  110  are positioned forwardly of the outlets  112 . In addition, the outlets  112  are positioned rearwardly of the majority of the cavity  106 , and are radially outward of the cavity  106  and the polymer body  102 . Therefore, the flow of water through the cavity  106  and the resulting flow of the polymer solution  121  within the cavity  106  has little or no reverse or forward component back toward the front end of the vehicle  100 . 
         [0051]    The solid polymer body  102  is disposed within the cavity  106 . The body  102  is positioned such that water that flows into the cavity  106  flows over and past the body  102  in contact therewith, causing a portion of the polymer in the body  102  to dissolve into the water to create the polymer solution  121 . Examples of polymers that can be used are solid forms of the polymers disclosed in U.S. Pat. No. 4,186,679. 
         [0052]    In one embodiment, the body  102  is a circumferentially continuous, ring-shaped solid body of polymer that extends circumferentially around the longitudinal axis A-A.  FIGS. 7A and 7B  provide perspective views of example forms of the body  102 , with only portions, for example approximately one quarter, of the body  102  shown. However, the body  102  need not be circumferentially continuous, but can instead be circumferentially interrupted, for example formed in quarter or near quarter sections and the sections can abut one another or be spaced apart from one another. In one embodiment, even if the body  102  is not circumferentially continuous, the body  102  nonetheless occupies a majority of the circumference to provide adequate amounts of the polymer. 
         [0053]    The body  102  can also be provided with a geometry that increases the surface area thereof that is exposed to the water, as well as increase the length of the flow path past the body  102  and help to mix the water and the dissolved polymer into the water. For example,  FIG. 7A  illustrates the body  102  as being formed with integral fins or ribs  120  that can be generally straight and parallel to the longitudinal axis A-A or straight and disposed at an angle to the axis A-A.  FIG. 7B  illustrates the body  102  as being formed with integral fins or ribs  122  that are curved or generally helical to introduce a swirl into the flow of water past the body  102 . Many other examples of body  102  shapes and features are possible. 
         [0054]    The polymer body  102  can also be fixed within the cavity  106  in any suitable manner. For example, the body  102  can be mounted on a mount structure (not shown) disposed in the cavity  106 .  FIG. 8A  illustrates an example where the solid body  102  is mounted to an exterior surface of the interior pressure hull  109 .  FIG. 8B  illustrates an example where the solid body  102  is mounted to an interior surface of the exterior hull  108 .  FIG. 8B  also illustrates the body  102  as being formed in two separate pieces  102   a ,  102   b , with the piece  102   a  being mounted to the interior surface of the hull  108  forwardly of the inlets  110 , and the piece  102   b  being mounted to the interior surface of the hull  108  between the inlet  110  and the outlets  112 . 
         [0055]    Returning to  FIG. 6 , the water can be allowed to flow continuously into the cavity  106  through the water inlets  110  and out of the outlets  112 . Alternatively, a flow control mechanism can be provided to control the flow of water through the water inlets  110  and thus through the vehicle  100 . For example, with reference to  FIGS. 9A and 9B , a rotatable inlet cover  130  can be rotatably disposed on hull  108  at the front end  104 . The inlet cover  130  can include a plurality of openings  132  extending therethrough. The inlet cover  130  can be circumferentially rotated relative to the hull  108 , manually or using a suitable mechanical drive mechanism, between a first position (shown in  FIG. 9A ) where the openings  132  are aligned with the water inlets  110  to permit maximum water entry into the cavity  106  and a second position (shown in  FIG. 9B ) where the openings  132  are not aligned with the water inlets  110  thereby preventing the inflow of water. Other types of flow control mechanisms can be used, including mechanisms that are actuated axially instead of being rotated. Further, a similar type of flow control mechanism can be utilized on the outlets  112  in order to control the flow of the polymer solution  121  through the outlets  112 . In addition, a similar type of flow control mechanism can be used in the embodiments discussed above in  FIGS. 1-5 . 
         [0056]    With reference to  FIG. 6 , an example operation of drag reduction on the hull  90  of the vehicle  100  will be described. Drag reduction on other surfaces of the vehicle  100  operates in a similar manner, for example as illustrated in  FIG. 10 . This example will also assume that a flow control mechanism such as the mechanism  130  described in  FIGS. 9A and 9B  is utilized. Assuming that the flow control mechanism is initially closed, when drag reduction is desired, the flow control mechanism is actuated to the first, open position allowing water to flow into the cavity  106  through the inlets  110 . The water flows past the body  102  dissolving some of the solid polymer which mixes with the water to create the polymer solution  121  in the cavity  106 . Thereafter, the polymer solution  121  is then passively ejected through the outlets  112  into the boundary layer resulting in a reduction in the drag on the exterior surface of the hull  90 . With this construction, due to the pressure differential created by the positioning of the inlets  110  and the outlets  112 , the polymer solution  121  is passively ejected and is not pumped by an electrically driven pump, and the creation and discharge of the polymer solution  121  can be accomplished without the use of electrical energy. 
         [0057]    With reference now to  FIGS. 11-13 , another embodiment of drag reduction on surfaces of a vehicle  150 , for example an underwater vehicle, is illustrated. In this embodiment, a water-soluble solid polymer body  152  is molded into a shape that is removably affixed directly onto an exterior portion of a surface of the vehicle  150  that is directly exposed to the water. Therefore, the polymer body  152  is directly exposed to the flow of water past the surface, and the polymer of the body dissolves directly into the water. The polymer body  152  can be removably affixed to any desired surface of the vehicle  150  that is in contact with the water. In one embodiment, the polymer body  152  can be designed to attach to the surface to form at least a portion of a leading edge of the surface. 
         [0058]    For example, with reference to  FIG. 12 , a cross-sectional view of one of the fins  20  of the vehicle  150  of  FIG. 11  is illustrated. The polymer body  152  is mounted at a front end  154  of the fin  20  thereby forming the leading edge of the fin  20 .  FIG. 13  is a perspective view of a front portion of the duct of the propulsion mechanism  18  where the polymer body  152  is mounted at a front end of the duct thereby forming a leading edge of the duct. However, the polymer body  152  can be mounted to locations on the surfaces other than leading edges, as long as the polymer body  152  is in direct contact with the water to create the polymer solution, and the polymer solution ends up in the boundary layer to reduce drag. 
         [0059]    With reference to  FIG. 12 , the polymer body  152  can include a support base  156  that is used to removably affix the polymer body  152  to the front end  154 , and a water-soluble solid polymer  158  attached to the support base  156 . The support base  156  can be a rigid structure such as a metal or plastic plate that can be removably attached to the fin  20  using removable fastening means such as mechanical fasteners like screws or bolts, or an adhesive. The water-soluble solid polymer  158  can be secured to the base  156 , for example by being molded directly onto the base  156  or being adhered to the base  156 . Examples of polymers that can be used are solid forms of the polymers disclosed in U.S. Pat. No. 4,186,679. Removably affixing the polymer body  152  to the fin  20  or other surface permits replacement of the polymer body  152  once the polymer  158  dissolves over time. In other embodiments, the polymer body  152  can be non-removably affixed to the fin  20  or other surface. 
         [0060]    In embodiments where the polymer body  152  forms some or all of a surface, such as forming the leading edge as shown in  FIGS. 12 and 13 , the water-soluble solid polymer  158  attached to the base  156  is hydrodynamically-shaped to maintain the hydrodynamic efficiency of the surface. For example, as shown in  FIG. 12 , the solid polymer  158  can be bullet-shaped to match what would be the shape of the leading edge of the fin  20  if the polymer body  152  were not present. 
         [0061]    In the embodiments described herein, the outlets  24 ,  112  are not limited to being rearward of the water inlets  42 ,  110 . In some embodiments, the outlets  24 ,  112  could be positioned forwardly of the water inlets  42 ,  110 , or some of the outlets  24 ,  112  can be positioned forwardly of the water inlets  42 ,  110  while some of the outlets  24 ,  112  are positioned rearwardly of the water inlets  42 ,  110 . Many other configurations are possible as long as the polymer solution can be passively ejected. 
         [0062]    The examples disclosed in this application are to be considered in all respects as illustrative and not limitative. The scope of the invention is indicated by the appended claims rather than by the foregoing description; and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.
Systems and methods of reducing drag on outer surfaces of vehicles that are in contact with water using a polymer solution that reduces the drag on the outer surfaces of the vehicles as the vehicles travel through water. A polymer solution is passively ejected into the boundary layer of the water flowing past the outer surface of the vehicle. The polymer solution is mixed and introduced into the boundary layer passively with minimal or no usage of electrical energy. The passive mixing and ejection of the polymer solution minimizes energy consumption, thereby maximizing electrical energy consumption efficiency during operation of the vehicle.