Patent Publication Number: US-2023150619-A1

Title: Watercraft with battery ballast system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/211,069, filed on Mar. 24, 2021, which claims the benefit of U.S. Provisional Application No. 63/001,305, filed Mar. 28, 2020, the entirety of each of which is hereby incorporated by reference. 
    
    
     FIELD 
     This disclosure relates to watercraft ballast systems, and more specifically, watercraft that use arrays of movable batteries as ballast. 
     DESCRIPTION OF THE RELATED ART 
     Many watercraft use a form of ballast to enhance stability. The ballast is typically some sort of repositionable weight that can be selectively positioned within the vessel to adjust its trim, list, and/or draft. Ballast is important not only to prevent a vessel from capsizing, but also for the safety and comfort of its passengers and the stability of its cargo. Rough seas or shifting of cargo may disturb the vessel&#39;s stability, and the ballast is used to offset their impact on stability. 
     Water is a common form of ballast since it is readily available. A typical ballast water system comprises a series of tanks in the bottom of the vessel hull as well as strainers, pumps, distribution pipes, treatment systems, and discharge systems. The tanks are often segregated so that ballast water may be selectively admitted or expelled from a vessel in a manner that affects the relative ballast water loading fore and aft of the ship&#39;s mid-line along its length axis and/or the relative ballast water loading on the port and starboard sides of the ship&#39;s mid-line along its width axis. In addition to adjusting relative loadings, the total amount of ballast water on board may be increased or decreased, which tends to change the ship&#39;s draft, i.e., the distance from a point on the keel to the waterline. The term “trim” refers to the relative draft at the bow and the stern. When the draft is greater fore than aft, the ship will have a positive “trim by bow” and a negative “trim by stern.” When the draft is greater aft than fore, the ship will have a positive trim by stern and a negative trim by bow. 
     One significant drawback of ballast water systems is that they are a source of invasive species such as zebra mussels, sea lamprey, and spiny water fleas. Invasive plant species such as Eurasian Milfoil may also be introduced. Various international, national and local laws impact whether and to what extent ballast water may be discharged. The International Convention for the Control and Management of Ships&#39; Ballast Water and Sediments requires that ships meet more stringent invasive species standards, which will require the installation of new ballast water management systems (BWMS) within the next five (5) years. It is anticipated that the standards will be cost-prohibitive in many cases. In addition, it is expected that retrofitting efforts will overwhelm current drydock capacity, forcing many vessels into retirement. 
     Ships often carry large numbers of batteries used to power instruments and equipment. The batteries constitute discrete units of mass which may, in theory, be selectively positioned within the vessel to maintain its stability. However, a reliable and automatic means of repositioning them has not been proposed. In addition, batteries cannot be added or removed at sea to increase or decrease the ship&#39;s total ballast weight. Thus, a need has arisen for a battery ballast system. In addition, some existing vessels have an insufficient amount of available space to accommodate the number of batteries required to provide meaningful ballast. Thus, a need has arisen for a watercraft designed to accommodate a battery ballast system. 
     SUMMARY 
     In accordance with a first aspect of the present disclosure, a watercraft is provided which comprises a hull; a propeller operable to propel the watercraft through a body of water; an air motor operative to rotate the propeller; an air storage tank in selective fluid communication with the air motor; an air compressor operable to selectively supply compressed air to the air storage tank; and ballast comprising a plurality of batteries, wherein the batteries in the plurality of batteries are selectively positionable relative to the hull along at least one of a watercraft length axis and a watercraft width axis. In a first embodiment, the batteries are selectively positionable along the watercraft length axis and the watercraft width axis. In the same or other embodiments, the watercraft further comprises a battery ballast system comprising a carriage system and the plurality of batteries, wherein the carriage system comprises a plurality of carriage assemblies, each carriage assembly comprises a plurality of tiers, each tier comprises a pair of tracks, the carriage system further comprising a plurality of battery supports, each battery support engaging a corresponding one of the pairs of tracks and being movable relative to the hull along its corresponding one of the pairs of tracks. 
     In accordance with a second aspect of the present disclosure, a watercraft is provided which comprises a hull; a propeller operable to propel the watercraft through a body of water; an air motor operative to rotate the propeller; an air compressor operable to supply compressed air to the air motor, wherein the watercraft does not include a fossil fuel engine or a fossil fuel tanks; and ballast comprising a plurality of batteries, wherein the batteries in the plurality of batteries are selectively positionable relative to the hull along at least one of a watercraft length axis and a watercraft width axis. In a first embodiment, the batteries are selectively positionable relative to the hull along the watercraft length axis and the watercraft width axis. In the same or other embodiments, the watercraft further comprises a battery ballast system comprising a carriage system and the plurality of batteries, wherein the carriage system comprises a plurality of carriage assemblies, each carriage assembly comprises a plurality of tiers, each tier comprises a pair of tracks, the carriage system further comprising a plurality of battery supports, each battery support engaging a corresponding one of the pairs of tracks and being movable along its corresponding one of the pairs of tracks relative to the hull. 
     In accordance with a third aspect of the present disclosure, a watercraft is provided which comprises a hull; a propeller operable to propel the watercraft through a body of water; a battery ballast system comprising a carriage system and a plurality of batteries, wherein the batteries in the plurality of batteries are selectively positionable relative to the hull along a watercraft length axis and a watercraft width axis. In accordance with a first embodiment, the carriage system comprises a plurality of carriage assemblies, each carriage assembly comprises a plurality of tiers, each tier comprises a pair of tracks, the carriage system further comprising a plurality of battery supports, each battery support engaging a corresponding one of the pairs of tracks and being movable relative to the hull along its corresponding one of the pairs of tracks. In the same or other embodiments, the battery supports are moveable along the watercraft width axis relative to one another. In the same or other embodiments, the carriage assemblies are movable along the watercraft length axis relative to the hull. 
     In accordance with a fourth aspect of the present disclosure, a method of adjusting the trim of a watercraft having a bow and a stern defining a length axis is provided. The method comprises providing a battery ballast system comprising a carriage system and a plurality of batteries, wherein the batteries in the plurality of batteries are selectively positionable relative to the hull along a watercraft length axis and a watercraft width axis; and selectively moving a subset of the plurality of batteries along the length axis relative to the hull. 
     In accordance with a fifth aspect of the present disclosure, a method of adjusting the list of a watercraft having a hull, a port side and a starboard side defining a width axis is provided. The method comprises providing a battery ballast system comprising a plurality of batteries and a plurality of tiers, wherein each tier comprises a plurality of battery supports that are movable relative to the hull along the width axis; and selectively moving a subset of the plurality of batteries along the width axis. 
     In accordance with a sixth aspect of the present disclosure, a method of adjusting the draft of a watercraft, the watercraft comprising a potable water system having an untreated water inlet in fluid communication with a desalination unit, and a fresh water tank in fluid communication with the desalination unit is provided. The method comprises adjusting a volume of fresh water in the fresh water tank. 
     The disclosure will now be described, by way of example, with reference to the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG.  1    is a side elevational view of a watercraft used to illustrate the determination of a ship&#39;s draft and trim; 
         FIG.  2 A  is a rear schematic view of a watercraft in an upright orientation used to illustrate the determination of a watercraft&#39;s list; 
         FIG.  2 B  is a rear schematic view of the watercraft of  FIG.  2 A  in a tilted orientation used to illustrate the determination of the watercraft&#39;s list; 
         FIG.  3 A  is a cross-sectional view of a watercraft comprising a battery ballast system taken along a direction parallel to the watercraft&#39;s length; 
         FIG.  3 B  is a cross-sectional view of the a watercraft comprising a battery ballast system taken along a direction parallel to the watercraft&#39;s height axis. 
         FIG.  4    is a perspective view of a portion of a carriage assembly of the battery ballast system of  FIGS.  3 A and  3 B ; 
         FIG.  5    is an exploded view showing the construction of a battery support attached to the rails of a carriage assembly in the battery ballast system of  FIGS.  3 A and  3 B ; 
         FIG.  6 A  is a schematic depicting a potable water system used in the watercraft of  FIGS.  3 A and  3 B ; 
         FIG.  6 B  is a schematic depicting an exemplary control scheme for using the potable water system of  FIG.  6 A  to control the total ballast of the watercraft; and 
         FIG.  7    is an air and electric propulsion system for a watercraft that has no fossil fuel engines or fossil fuel tanks which is suitable for use in a watercraft with a battery ballast system. 
     
    
    
     DETAILED DESCRIPTION 
     The Figures illustrate examples of a watercraft with a battery ballast system. Based on the foregoing, it is to be generally understood that the nomenclature used herein is simply for convenience and the terms used to describe the invention should be given the broadest meaning by one of ordinary skill in the art. Unless otherwise specified, like numerals refer to like components herein. 
     Referring to  FIG.  1   , a watercraft  10  is depicted. Watercraft  10  comprises a hull  20  which includes a bow  22  and a stern  24 , as well as a keel  26 . A distance between bow  22  and stern  24  defines a length axis L of the watercraft. A rudder  32  projects away from the keel  26  and is used to steer the watercraft  10 . Watercraft  10  comprises at least one propeller that is operable to propel the watercraft  10  through the water. In  FIG.  1    the at least one propeller is propeller  52   a  and propeller  52   b  (not shown in  FIG.  1   ). Propeller  52   a  is spaced apart from the keel  26  and below waterline  34  (when watercraft  10  is in a body of water). A distance along the height axis H from the keel  26  to the waterline  34  defines the watercraft&#39;s draft. 
     As  FIG.  1    indicates, watercraft  10  may have a draft that varies along the length axis. This variation between the draft at the bow  22  and stern  24  is characterized as the watercraft&#39;s trim. The draft is the distance from a portion of the keel to the waterline in a vertical direction, i.e., perpendicular to the waterline and the surface of the water. The difference between the draft at the bow (H F ) and at the stern (H A ) is known as the “trim”. The “trim by bow” is the difference in feet between the draft at the bow and at the stern as shown in equation (1): 
       Trim by Bow=H F −H A    (1)
         wherein, H F =the draft at the bow (ft.)   H A =the draft at the stern (ft.)       

     The “trim by stern” is the difference in feet between the draft at the stern and at the bow, as shown in equation (2): 
       Trim by Stern=H A −H F    (2)
 
     Generally, the convention is to state the trim as a positive number. When H A  is greater than H F , the trim is described as a positive trim by stern, and when H A  is less than H F , the trim is described as a positive trim by bow. When H A  equals H F , the vessel is on an even keel. The trim may be affected by the condition of the body of water, the cargo load, and the ship design. As mentioned previously, ballast is typically adjusted along the length axis of the watercraft  10  to achieve a desired trim. 
       FIG.  2 A- 2 B  provide a schematic representation of the stern  24  of watercraft  10  used to depict the list of the watercraft  10 . In  FIG.  2 A , the watercraft  10  is upright, and its center of gravity and center of buoyancy are collinear with the vertical axis (i.e., an axis perpendicular to the earth, the surface of the water, and the waterline W). Thus, watercraft  10  has a list angle of zero. In  FIG.  2 B  a disturbance has caused the watercraft  10  to tilt toward the starboard direction, and the center of gravity and center of buoyancy are co-linear with a line that defines a list angle θ with the vertical line in  FIG.  2 A . As mentioned previously, the ballast may be adjusted along the width axis of watercraft  10  to adjust the list angle θ. 
     Referring to  FIGS.  3 A and  3 B , a ship  200  is depicted which comprises a hull  201  and a battery ballast system  211 . The battery ballast system  211  is preferably located in a lower deck  210  below the main deck (not shown) along the ship&#39;s height axis H. Lower deck  210  has a starboard bulkhead  202  and a port bulkhead  204  spaced apart along the ship&#39;s width axis. In  FIG.  3 A  battery ballast system  211  is shown below the main deck and cargo hold  206  as well as below a potable water system deck  208  that houses a portion of the ship&#39;s potable water system, an example of which is discussed below with reference to  FIG.  6 A . Purified potable water is located on deck  212 . 
     The battery ballast system  211  comprises a carriage system that includes a plurality of carriage assemblies  214 - 244  ( FIG.  3 B ). Each carriage assembly  214 - 244  comprises a plurality of batteries  292 . The carriage assemblies  214 - 244  are selectively movable along the watercraft&#39;s length axis to adjust the watercraft&#39;s ballast along the length axis. Each carriage assembly  214 - 244  comprises a plurality of tiers arranged along the watercraft&#39;s  10  height axis. Each tier comprises a pair of tracks and a plurality of battery supports, each of which engages and is movable along a corresponding one of the pairs of tracks. 
     Carriage assembly  214  is depicted schematically in  FIG.  3 A . Each of the carriage assemblies  214 - 244  has the same structure in the illustrated embodiments, although different structures may be used. Carriage assembly  214  comprises eight tiers A-H, which are arranged along the watercraft&#39;s height axis H. The same tier naming convention applies to the tiers of carriage assemblies  216 - 244 . The tiers A-H are spaced apart along the height axis in  FIG.  3 A  for ease of viewing, but are actually connected to define an integral carriage assembly  214 . 
     Each tier A-H includes fifty slots which are locations that can accommodate a battery  292 . The slots are arranged along the watercraft&#39;s width axis. The slots  1 - 50  are fixed positions within the carriage assembly. Different batteries  292  may be repositioned to different slots. The batteries  292  are positioned on battery supports (described below) that move along the tracks of the tier to which the battery belongs and along the ship&#39;s width axis. As discussed in greater detail below, the battery supports are generally I-shaped members, the opposite ends of which slidingly engage the rails of the carriage assembly tier to which they belong. In certain examples, each battery and a corresponding battery support (described below) on which the battery is positioned is individually movable along the watercraft&#39;s width axis. However, in other examples, individual batteries are grouped together and move together, such as in groups of five, ten, fifteen, or twenty batteries. Grouping batteries in the matter provides somewhat reduced flexibility in positioning the batteries where desired but simplifies the motor assembly required to move the battery supports. 
     The number of battery supports in a given tier A-H is preferably less than the number of slots  1 - 50  in the tier. Otherwise, the batteries  292  in that tier could not be repositioned along the width axis to alter the list angle θ. ( FIG.  2 B ). All batteries  292  can be moved along the width axis, either toward or away from the starboard and port bulkheads  202 ,  204 . A subset of batteries  292  can be moved from one side of midship to the other side of midship along the width axis (Mw). The number of batteries in the subset depends on the loading of the slots. In the example of  FIG.  3 A , 9 of the 32 batteries in each tier can be moved from one side of midship along the width axis to the other. 
     The number of open slots  293  (slots without battery supports) in a given tier A-H is preferably from about 30% to about 50% of the total slots in the tier, more preferably, from about 35% to about 45% of the total number of slots in the tier, and still more preferably from about 34% to about 38% of the slots in a given tier. In  FIG.  3 A  slots H( 10 )-H( 41 ) of tier H are occupied in carriage assembly  214 , whereas slots H( 1 )-H( 9 ) and H( 42 )-H( 50 ) are unoccupied. 
     As indicated in  FIG.  3 A , the different tiers A-H may be loaded the same or differently with batteries  292 . Tiers A-D have batteries  292  and battery supports in slots  1 - 16  and  35 - 50  only, whereas tiers E-H have batteries and battery supports in slots  10 - 41  only. As an example of how the batteries  292  would be re-positioned to alter the list angle θ, if watercraft  200  had the orientation shown in  FIG.  2 B  for watercraft  10 , moving batteries  292  from the starboard side to the port side of the width axis midship line Mw would tend to restore the watercraft  10  to its upright orientation of  FIG.  2 A . 
     Referring to  FIG.  3 B , the upper tier H of each of carriage assemblies  214 - 244  is shown. The configuration of batteries  292  and slots  1 - 50  in tier H of carriage assembly  214  differs in  FIG.  3 B  relative to  3 A. In  FIG.  3 B  sixteen carriage assemblies  214 - 244  are shown. However, many more may be provided depending on the ship  200 . As the figure indicates, each carriage assembly  214 - 244  is selectively movable along the length axis of the ship relative to hull  201 . As also indicated, the battery  292  loading configurations for the upper tier H of each carriage assembly  214 - 244  may vary along the ship&#39;s length axis. In carriage assemblies  214 - 218  and  220 , slots  1 - 9  and  28 - 50  are occupied with batteries  292 . In carriage assemblies  222 - 228  slots  1 - 32  are occupied. In carriage assemblies  230 - 234 , slots  1 - 22  and  41 - 50  are occupied, and in carriage assemblies  236 - 244  slots  1 - 32  are occupied. 
     The numbers of slots, the percentage of open slots per tier, and the number of carriage assemblies are preferably selected based on the battery dimensions and weights and the desired degree of ballast on each side of the location that is midship along the width axis (M w ) and midship along the length axis (M L ). In certain examples, the distance along the length axis occupied by the carriage assemblies  214 - 244  is about 60% to about 75% of the maximum available distance along the length axis, preferably about 65% to about 70% , and more preferably from about 66% to about 68%. The distance occupied by the carriage assemblies  214 - 244  along the length axis is the distance along the length axis occupied by the carriage assemblies  214 - 244  when they are all placed in abutting engagement with no spaces between them. The maximum available distance is the distance between the maximum fore and aft positions of the two carriages that are closest to the bow and the stern, respectively. In  FIG.  3 B  those carriage assemblies are  214  and  244 . 
     The carriage assemblies  214 - 244  are structured similarly to one another. One carriage assembly  220  is illustrated in  FIG.  4   . Only four tiers A-E and a portion of carriage assembly  220  proximate starboard bulkhead  202  are depicted in  FIG.  4   . Tier E has a parallel set of tracks  262  and  264  which extend along the ship&#39;s  200  width axis and are spaced apart from one another along the ship&#39;s  200  length axis. Four vertical members (not shown) are placed at the ends of the tracks  262  and  264  and are secured to the tracks  262  and  264  as well as to the tracks of all the other tiers in carriage assembly  220  by suitable mechanical fasteners, welding, or other reliable means. Battery support  261  is one of several battery supports in tier E and may also be referred to as a “carriage seat”. Battery support  261  is an I-shaped member that comprises a cross-beam  266  extending between parallel tracks  262  and  264  along the length axis of the ship  200  and end beams  267  and  265  (not shown), which each slidingly engage a respective one of parallel tracks  262  and  264  along the ship&#39;s  200  width axis. End beam  267  includes a vertical section  269  and a horizontal section  273 . End beam  265  (not shown) is structured and engages the corresponding track  264  in a similar fashion. The details of the engagement between the battery supports and tracks are provided in  FIG.  5    and discussed further below. 
     Batteries  292  each sit on a corresponding battery support. In  FIG.  4    tier E slots  45  and  46 - 49  are occupied. Although battery support  261  is visible, when the support  261  is unoccupied, it would typically be removed to allow a greater degree of movement for the other batteries in the tier. 
     Tier D comprises parallel tracks  268  and  270  which extend along the ship&#39;s width axis and are spaced apart along the ship&#39;s  200  length axis. Battery support  271  extends between parallel tracks  268  and  270  along the watercraft&#39;s length axis and comprises cross-beam  272  and end beams  274  and  276 . Each end beam  274  and  276  slidingly engages a respective one of parallel tracks  268  and  270  along the ship&#39;s  200  width axis in the same manner that end beam  267  engages track  262 , as described previously. If the support  271  were occupied, the battery  292  would rest on the cross-beam  272  and the end beams  274  and  276 . 
     Tier C comprises parallel tracks  278  and  280  and battery support  281  (along with additional supports not called out). Parallel tracks  278  and  280  extend along the ship&#39;s  200  width axis and are spaced apart along the ship&#39;s  200  length axis. Battery support  281  is an I-shaped member that comprises a cross-beam  282  which extends between the parallel tracks  278  and  280  along the length axis, and end beams  284  and  286 , each of which slidingly engages a respective one of parallel track  278  and  280  along the ship&#39;s  200  width axis in the same manner that end beam  267  of battery support  261  engages track  262 . In  FIG.  4    slots E( 50 ), D( 50 ), and A( 50 ) are not visible. Carriage assembly  220  preferably slidingly engages a pair of rails spaced apart along the ship&#39;s width axis and extending along the ship&#39;s length axis to move carriage assembly  220  along the ship&#39;s length axis relative to hull  201  as would the other carriage assemblies  214 - 244 . 
     Referring now to  FIG.  5   , an exemplary battery support for the carriage assemblies  214 - 244  described herein is illustrated. In  FIG.  5    the battery support  271  from carriage assembly  220  of  FIG.  4    is illustrated. However, it is understood that in this example, the battery supports for the other carriage assemblies are configured similarly. 
     Each battery support in each tier A-H is movable transversely to the ship&#39;s length axis and along the ship&#39;s width axis relative to the ship&#39;s hull  201 . In the example, battery support  271  includes a cross-beam  272  attached to end beams  274  and  276 , as described previously. End beam  274  comprises an upper horizontal section  312  and a lower vertical section  316 . Two wheels are rotationally mounted on the lower vertical section  316  and are spaced apart along the ship&#39;s  200  width axis direction from one another. Only wheel  320  is visible on lower vertical section  316 . Wheel  320  is positioned between the lower vertical section  316  of end beam  274  and an upward extending vertical section  306  of track  268 . Track  268  also includes a lower lip  302  that projects toward the other track  270  along the length axis of the ship  200 . Wheel  320  rests on the lower lip  302  and rolls along it to allow end member  274  to slidingly engage track  268  along the ship  200  width axis. The other wheel (not shown) attached to the opposite end of the end beam  274  is configured in the same way, 
     End beam  276  also includes an upper horizontal section  314  and a lower vertical section  308 . The track  270  is configured as a mirror image or track  268 . End beam  276  includes two wheels attached to opposite ends of the lower vertical section. Wheel  322  is shown, but the wheel at the other end of the end beam  276  is not shown. Lower lip  304  functions similarly to lower lip  302 . Thus, wheel  322  is positioned between the lower vertical section of the end beam  276  and the upward extending vertical section  308  of track  270  and rides along lower lip  304  of track  270 . Thus, end member  276  has two wheels at opposite ends of end member  276  which roll along lower lip  304  of track  270 , thereby allowing the end member  276  to slidingly engage the track  270  along the width axis of the ship  200 . 
     In certain examples, each battery support in carriage assemblies  214 - 244  is motor-driven along its corresponding pair of tracks. A conventional motor assembly is provided and is operable to move each battery support in a given tier A-H of a given carriage assembly  214 - 244  to a desired slot location. In other examples, the battery supports are connected in groups (such as groups of five, ten, fifteen, or twenty) of battery supports that move together along the ship&#39;s  200  width axis relative to hull  201 . 
     As shown in  FIG.  5   , each battery  292  is positioned and secured to the battery support  271  by, for example, four twist locks  325 ,  327 ,  329 , and  331  at four corners of the battery  292  that can be interlocked with openings  324 ,  326 ,  328 , and  330  (not shown in  FIG.  4   ) positioned in end beams  274 ,  276  of battery support  271  manually or by a remote control. It is to be appreciated that although the battery support  271  has been illustrated with openings  324 ,  326 ,  328 ,  330  for mating the twist locks  325 ,  327 ,  329 , and  331  of the battery support  271 , the battery support  271  can also be provided with twist locks that mate with corresponding openings on the battery  292 . Similar twist locks can also be provided either on the battery supports the batteries for interlocking of abutting battery supports or batteries to each other while the vessel is in motion. 
     In a preferred example of the battery and ballast system  211  described herein, a conventional motor assembly is provided to drive each of the battery supports  271  in each tier A-H along the tracks  268 ,  270 . In addition, a motor assembly control system which comprises a conventional remote control device may also be provided to allow users to operate the ballast system  211  outside the deck on which the battery ballast system  211  is located. The user can thus driveably move batteries  292  to a desired slot in their respective tier. These conventional mechanisms are typically provided in order to achieve proper alignment of the battery supports  271 , within each tier, for storage and retrieval operations. The remotely controlled motor assemblies may be mounted, for example, within a cross-beam  272  of each battery support  271 . 
     Thus, in one example, each individual battery support is separately driveable and the remote motor control is provided with a conventional selection device for separately driving each battery support  271  independently of the other battery supports. When ballast adjustments are required in a particular tier, the user can thus separately drive the individual battery supports to an appropriate slot to affect the vessel&#39;s list and/or trim. In one example, each battery support is assigned a unique identifier and each slot is assigned a unique identifier so that a remote control may be operated to drive a particular battery support to a particular slot. Of course, not all slots will be accessible to all battery supports in a given tier because of the number of battery supports in the tier. For example, each tier may have  32  battery supports identified as S( 1 )-S( 32 ). Within the tiers shown in  FIGS.  3 A and  3 B , the S( 1 ) support could be located in any of slots  1 - 32 . The S( 2 ) battery support could be located in any of slots  2 - 33 , etc. In other words, the number of vacant slots in each tier equals the number of different slots that a given battery support  271  may occupy within that tier. However, the particular slots that a given battery may occupy will depend on the battery&#39;s location relative to other batteries in the same tier. In other examples, the battery supports may be grouped as described previously. They also may be selectively grouped using suitable mechanisms for joining adjacent battery supports  271  together such as an electromagnetic coupling system, an electromotive coupling system or a mechanical coupling system (e.g., a system of hooks connecting adjacent battery supports  271 ). 
     In certain examples, each carriage assembly  214 - 244  is motor driven along its tracks (not shown) and along the length axis of the vessel. The total length of the available area that is unoccupied by carriage assemblies divided by the length (along the watercraft length axis) of each carriage assembly determines how many carriage assembly locations a given carriage assembly may occupy. For example, if the carriage tracks extend  600  feet along the vessel length axis and each carriage assembly has a length of four (4) feet along the vessel length axis, there will effectively be  150  carriage assembly positions along the vessel&#39;s length axis. If 100 carriage assemblies are provided, the total effective length of all carriage assemblies will be 400 feet, leaving 200 feet unoccupied. In that case, each carriage assembly may occupy 50 different carriage assembly locations along the vessel length axis. In other examples, adjacent carriage assemblies may be joined or selectively joined to move as groups along the ship&#39;s length axis relative to hull  201 . 
     In certain examples, the total weight (or mass) of the battery ballast system  211  is from about 20 to 30 percent of the ship&#39;s 200 dead weight tonnage. “Deadweight tonnage” is a measurement of total contents of a ship including cargo, fuel, crew, passengers, food, and water aside from boiler water. In the same or other examples, each slot of the carriage assembly (including battery supports, but not batteries) is from about 15 lbs to about 25 lbs., preferably from about 17 lbs. to about 23 lbs., and more preferably from about 18 lbs., to about 21 lbs. In the same or other examples, batteries 292 weigh from about 100-200 lbs., preferably from about 120 lbs. to about 180 lbs., and more preferably from about 140 lbs. to about 160 lbs. 
     Unlike ballast water systems, battery ballast system  211  cannot add or expel batteries  292  while at sea. Thus, while it can be repositioned along the vessel&#39;s length and width axes, the total amount of battery ballast on ship  200  cannot be varied while the ship  200  is at sea. In one example, the volume of potable water produced by the vessel&#39;s  200  potable water system is varied to effectively provide an additional source of ballast. In certain examples, the ship&#39;s potable water system is used to change the watercraft&#39;s total amount of ballast by changing the total volume of treated water on board such as by expelling treated water overboard or changing the rate of untreated water being fed to the potable water system. 
     Referring to  FIG.  6 A , potable water treatment system  340  is depicted. Potable water treatment system  340  is provided to produce fresh, drinkable water from sea water. The potable water treatment system  340  comprises a desalination unit  341  that includes an evaporator  344  and condenser  342 . The evaporator  344  creates steam from sea water and removes salt and other non-volatile materials. The steam is then condensed to form potable water. 
     Sea water brought in via sea water inlet  345  is pumped by ejector pump  348  into condenser cooling water inlet line  356 . Coil  370  is provided in the condenser  342  to provide additional surface area for heat transfer from condensing steam to the cooling water. The cooling water leaves condenser in discharge stream  372 . A portion of the discharge stream  372  is recycled back to the condenser via recycle stream  366 , and the balance of the discharge stream  372  is discharged overboard in overboard discharge line  364 . 
     The recycle stream  366  enters secondary cooling coil  368  within condenser  342  and provides secondary cooling to evaporating steam. After leaving the cooling coil  368 , the stream is directed to evaporator  344  and becomes evaporator feed steam  374 . Engine jacket water provides the heat of evaporation and enters the evaporator  344  via evaporator heating medium inlet stream  376 . The evaporator heating medium inlet stream  376  enters the evaporator&#39;s heated nest  380  and exits the evaporator  344  via evaporator heating medium outlet steam  378 . The heat from the engine jacket water and the pressure at which the evaporator  344  is operated causes the evaporator feed stream water  374  to evaporate within the evaporator  344  and enter the condenser  342 . The evaporating water (steam) passes through an annular demister  358  and transfers heat to the cooling water in cooling water coils  370  and  368 , causing the steam to condense into condensation trap  360 . Condensed water from condensation trap  360  enters treated water pump suction line  362  and is pumped by treated (fresh) water pump  354  to fresh water tank  346 . In one example, a level controller may also be provided to control the level on the condensation trap  360  and may be cascaded to a condenser fresh water outlet line  355  flow controller  392  ( FIG.  6 B ). Flow controllers may also be provided on the condenser cooling medium recycle line  366  and/or the evaporator heating medium inlet line  376  or outlet line  378 . A variety of different control schemes may be used, but they preferably ensure that the condensate trap  360  does not run dry and that the necessary amount of fresh water is supplied to fresh water tank  346  based on shipboard needs. 
     In a preferred example, a portion of the volume of fresh water in fresh water tank  346  is used as ballast. In accordance with the example no ballast water tanks are provided that are not fluidly coupled to potable water system  340 . As mentioned previously, battery ballast system  211  cannot add or subtract ballast while ship  200  is at sea. If it desirable to increase or decrease the vessel&#39;s draft at both the bow and the stern, merely adjusting the locations of batteries  292  will be insufficient. Thus, in certain examples, the potable water treatment system  340  is sized to allow the volume of water in fresh water tank  346  to be varied to provide a desired amount of total ballast variation, i.e., an amount of fresh water that can be expelled from or added to fresh water tank  346  that corresponds to the maximum ballast weight change that is anticipated. 
     In certain examples, when a decrease in total ballast is required, fresh water from fresh water tank  346  is expelled overboard. The volume of water corresponding to a particular expelled mass of water is shown in equation (3) below: 
         V   E   =M   E   /ρ   (3)
         where, V E =expelled volume (gal.)   M E =expelled mass (lb m )   ρ=density of water (8.35 lb m /gal.)       

     Based on the tank dimensions, the corresponding change in level can be calculated (assuming a cylindrical geometry) as follows: 
       Δ L =(0.5348  V   E )/π D   2    (4)
         where, V E =expelled volume (gal.)   D=Tank diameter (ft.)       

     For a rectangular prism tank, equation (3) still applies, but instead of equation (4), the following equation is used to calculate level changes: 
       Δ L =(0.1337  V   E )/( a·b )   (5)
         where, V E =expelled volume (gal.)   a=tank width (ft.)   b=tank length (ft).       

     When a change in total ballast is required, it can be effected manually or automatically. In a manual implementation, when a ballast increase is required, flow control valve  406  will first be closed if it is open. If the valve  406  is already closed or if closing it does not provide the desired amount of additional ballast, the flow rate of sea water into the potable water treatment system  340  may be increased, for example, by opening flow control valve  351  on the discharge of pump  352  ( FIG.  6 A ) or by increasing the set point of flow controller  353 , which receives a flow measurement signal from flow meter  355 . 
     A variety of suitable control systems may be provided to allow the volume of fresh water in fresh water tank  346  to be varied or expelled based on ballast needs. In one example, a suitable control scheme is provided which is configured to admit or expel a volume of water from tank  346  based on ballast needs while ensuring that the condensate trap  360  does not run dry and while also ensuring that level in tank  346  remains at an acceptable level to operate potable water pump  408  and providing the ship&#39;s fresh water usage needs for cooking, bathing, laundry, etc. via shipboard fresh water line  414 . In one exemplary control scheme, a flow rate of shipboard fresh water in fresh water line  414  is adjusted to control the level of tank  346 , and the overboard discharge line  411  flow rate is adjusted to change the total amount of ship ballast. Fresh water line  414  and overboard discharge line  411  are described in greater detail below. 
     In one implementation, ballast changes are made by varying the flow rate of treated water that is expelled via overboard line  411 . In the same or other implementations, ballast changes are made by varying the flow rate of sea water into the potable water system such as by adjusting the set point of flow controller  353  or opening valve  351  to a desired percentage open. 
     In a further implementation, desired decreases in ballast are made by increasing the flow rate of expelled water in overboard line  411  and then, if necessary, decreasing the sea water inlet flow rate to potable water treatment system  340 . In the same implementation, increases in ballast are made by first decreasing the amount of expelled freshwater in overboard line  411 , and if necessary, increasing the sea water inlet flow rate to potable water treatment system  340 . These adjustments may be made by manually manipulating valves  406  and  351 , by changing the setpoints via their respective flow controllers  404  and  353  or by using a ballast controller such as ballast controller  400 , described further below. 
     In another exemplary control scheme, a ballast controller adjusts the flow rate of overboard discharge line  411  until valve  406  is closed or until the ballast controller is overridden by a level controller that controls the level in tank  346 , at which point the ballast controller adjusts the setpoint of flow controller  353  ( FIG.  6 A ) to adjust the sea water inlet flow rate to potable water system  340 . Because ballast changes will often be discrete, if a ballast decrease is desired, in this example, the ballast controller will first try to increase the flow rate of ballast water in overboard line  411  and will then try to decrease the sea water inlet flow rate to the potable water treatment system  340 . In this example, if a ballast increase is required, the ballast controller will first try to decrease the flow rate of overboard line  411  and will then try to increase the sea water inlet flow rate to potable water treatment system  340  if needed. 
     Referring to  FIG.  6 B , in general the level in tank  346  must be maintained to provide sufficient net positive suction head to pump  408 , which may constrain the extent to which opening or closing valve  406  can be used to effect a desired ballast change. In  FIG.  6 B  an exemplary control system is provided which addresses both the level control of tank  346  and ballast control. The depicted control scheme controls the ship&#39;s total ballast load by adjusting the flow rate of expelled water in overboard line  411 . A desired change in total ballast may be effected in a desired time period and converted to an overboard discharge line  411  flow rate as follows: 
         F   411 =( W   E /ρ)/Δ t    (6)
         F 411 =flow rate in line  411  (gal/hour)   W E =Total desired change in ballast (lbs.)   ρ=density of water (8.35 lb./gal.)   Δt=time interval for changing ballast (hours).       

     Pump  408  pumps fresh, potable water from tank  346  to overboard discharge line  411  and shipboard fresh water line  414 . Overboard discharge line  411  directs fresh water from tank  346  overboard and is used to adjust the total amount of ballast by expelling fresh water overboard when a ballast reduction is needed or throttling back on the amount of water sent overboard when an increase is needed. 
     The overboard discharge line  411  flow rate is controlled by flow controller  404  which adjusts control valve  406  based on a flow rate measured by flow meter  402 . Shipboard fresh water line  414  routes fresh water to showers, bathrooms, laundry, kitchens, and any other areas requiring fresh water. The flow rate of fresh water in shipboard fresh water line  414  is controlled by flow controller  413  which adjusts control valve  412  based on the flow rate measured by shipboard fresh water flow meter  410 . Although not shown, a recycle line may be provided downstream of control valve  412  so that fresh, potable water not demanded by shipboard users can be recycled back to the tank  346  inlet line  355 . 
     A ballast controller  400  is provided and adjusts the overboard discharge line  411  flow rate by resetting the set-point of flow controller  404  to change the total amount of ship ballast in accordance with equations (3) and (4). As indicated in  FIG.  6   , the ballast controller  400  receives a level indication from level transmitter  396  and uses the level indication to determine the current volume and weight of fresh water in tank  346 . Ballast controller  400  receives a user-entered set point that corresponds to a change in the amount of ballast, or a total amount of ballast in tank  346  (the ballast provided by batteries  292  can only be shifted in the vessel and cannot be increased or decreased while at sea), and a time interval during which the ballast change is to be made. If a total ballast set point is entered, ballast controller  400  would calculate the required ballast change to achieve that set point. In either case the ballast controller  400  includes an algorithm that converts a desired change in total ballast weight and a user-entered time frame for making the change into a flow rate of overboard stream  411  in accordance with equation (6). The ballast controller  400  adjusts the set point of flow controller  404  to the determined set-point to direct fresh water overboard via overboard discharge line  411  until the desired amount of total ballast is achieved or until the desired change in ballast is achieved, at which point the ballast controller will re-set the flow controller  404  set-point to zero. The ballast controller  400  may also ramp the setpoint of flow controller  404  gradually to effect a smoother change in ballast. 
     A level controller  398  receives a level indication signal from tank  346  level transmitter  396  and resets the set point of flow controller  413  to maintain a desired level of fresh water in tank  346 . During normal, steady-state operation the overboard discharge line control valve  406  will preferably remain closed to avoid wasting purified water. Thus, level controller  398  will typically adjust the flow rate of shipboard fresh water line  414  by adjusting the set point of flow controller  413  to maintain the desired level in fresh water tank  346 . However, if control valve  412  is fully open and the level in tank  346  continues to rise, level controller  398  will preferably increase the set point of discharge line flow controller  404  to direct fresh water overboard until the tank  346  level reaches it set point. Alternatively or additionally, the level controller  398  may first reset flow controller  353  ( FIG.  6 A ) before resetting the setpoint of flow controller  404  to reduce the amount of seawater coming into the potable water treatment system  340  to stop the level in tank  346  from increasing. The flow of fresh water into tank  346  is controlled by flow controller  392  which adjusts control valve  391  based on the flow rate measured by inlet flow meter  390 . Flow controller  392  is re-set by level controller  394  which controls the level of condensate trap  360 . A level indicator would be provided on condensate trap  360  but is not shown in  FIG.  6 A . As the setpoint of flow controller  353  changes, the level in condensate trap  360  will change, causing condensate tray level controller  394  to adjust the inlet flow rate setpoint of flow controller  392  to stabilize the tank  346  level. 
     In implementations where level controller  398  will override the ballast controller  400  and adjust the set point of flow controller  404  to control the tank  346  level, a high signal selector  403  is provided and selects the higher output signal from among the level controller  398  and the ballast controller  400 . It is preferable that this override function occur only after the shipboard fresh water flow control valve  412  is fully open. Thus, level controller  398  is preferably configured as a split range controller such that for a first part of its output range, say from 0 to 50 percent, it adjusts the shipboard fresh water line  414  flow controller  413  set point and for a second part of its output range, say, from greater than 50 percent to 100 percent, it sends an output signal to the high signal selector  403  to adjust the set point of overboard discharge line  411  flow controller  404  as needed. As suggested above, a three way-split range may be used wherein the level controller opens valve  412  from 0-33 percent, closes valve  351  ( FIG.  6 A ) from 33-66 percent, and then opens valve  406  from 67 to 100 percent of the level controller  398  output signal. The valve adjustments may be directly made or by re-setting the setpoints of flow controllers  353 ,  413 , and  404 . 
     The controllers shown in  FIG.  6 B  may be implemented in software or hardware and may be digital or analog. Appropriate transducers would also be provided to convert electrical signals to pneumatic signals and vice-versa if needed. In one example, the set point of ballast controller  400  is adjusted manually by ship personnel to achieve the desired total amount of ballast on board. However, if a draft measurement device or draft estimating technique is used, an advanced ballast control scheme may also be provided which adjusts the set point of the ballast controller  400  automatically. For example, an advanced control scheme may include a draft controller that allows a user to input a set point for the total amount of draft at one location along the hull or the average of the draft and multiple locations and then re-set the ballast controller set point as needed to achieve the desired draft. 
     In one example of a watercraft with a battery ballast system, the watercraft is devoid of water ballast tanks other than those fresh water tanks that comprise part of the ship&#39;s potable water system. In many existing watercraft, the hull volume consumed by ballast water tanks would leave insufficient room for a battery ballast system with enough batteries to make meaningful ballast adjustments. Thus, in some cases it is preferable that the watercraft  10  be devoid of water ballast tanks, except to the extent such tanks serve the dual purpose of retaining treated, potable water for shipboard use as is the case with tank  346 . In other words, in such cases it is preferable if watercraft  10  is devoid of ballast water tanks that are not fluidly coupled to a fresh, potable water system  340 . 
     In accordance with another example, a water craft is provided with a battery ballast system of the type described herein in which the watercraft is devoid of fossil fuel tanks and fossil fuel engines. Fossil fuel tanks and engines. typically consume a significant amount of shipboard volume and which make it difficult to include a battery ballast system of sufficient size to make meaningful ballast adjustments. In accordance with a further example, a watercraft is provided which comprises a hull, a propeller operable to propel the watercraft through a body of water, an air motor operative to rotate the propeller, an air storage tank in selective fluid communication with the air motor, an air compressor operable to selectively supply compressed air to the air storage tank, and a ballast comprising a plurality of batteries, wherein the batteries in the plurality of batteries are selectively positionable along at least one of a vessel length axis and a watercraft width axis. In one implementation of the further example, the watercraft is devoid of fossil fuel and fossil fuel engines. In accordance with the same or other examples, the watercraft includes a potable water system, including, for example, potable water system  340  type depicted in  FIGS.  6 A-B . 
     Referring to  FIG.  7    an air and electric propulsion system  40  useful for use with a watercraft that includes a battery ballast system of the type described herein is provided. The air and electric propulsion system of  FIG.  7    is sized for a smaller vessel such as watercraft  10  of  FIG.  1   . However, the size and/or number of components may be scaled up as needed depending on the size and weight of the vessel. A vessel comprising an air and electric propulsion system and a battery ballast system will be described with reference to watercraft  10  of  FIG.  1    and air and propulsion system  40  of  FIG.  7   , but it should be understood that the watercraft  10  and air and electric propulsion system  40  can be scaled accordingly to accommodate the battery ballast system  211  of ship  200  and that a ship  200  with a battery ballast system of the type described herein and having the air and electric propulsion system  40  of  FIG.  7   , scaled as appropriate to the size of ship  200 , is expressly contemplated. 
     Propeller  52   a  is operatively connected to a proximal propeller shaft section  48   a  which rotates about its lengthwise axis / to rotate propeller  52   a  within the body of water. The rotation of propeller  52   a  within the water propels the watercraft  10  in a direction defined by the direction of rotation of propeller  52   a,  the geometry of the propeller blades, and the orientation of rudder  32 . 
     In this embodiment, watercraft  10  is not powered by a fossil fuel engine and does not include a fossil fuel engine or fossil fuel tanks. Instead, an air motor is provided which is operative to rotate at least one propeller. Referring to  FIG.  7   , air propulsion system  40  is provided which includes a propeller train  42 , an air supply system  47  and a rechargeable battery system  44 . A control system is also provided. Air supply system  47  includes at least one compressed air storage tank which is in selective fluid communication with the at least one air motor as well as at least one compressor that is operable to selectively supply compressed air to the at least one air storage tank. 
     In  FIG.  7    the at least one propeller used to propel watercraft  10  through the water comprises two propellers  52   a  and  52   b.  Propeller train  42  comprises two parallel propeller systems  43   a  and  43   b.  Each propeller system  43   a  and  43   b  further comprises a respective propeller shaft assembly  46   a  and  46   b  and respective propeller  52   a  and  52   b.  Propeller shaft assembly  46   a  is a multi-segment shaft that comprises a proximal propeller shaft section  48   a  and a distal propeller shaft section  50   a.  The proximal propeller shaft section  48   a  and distal propeller shaft section  50   b  are connected by a coupling  54   a.  The proximal end of the propeller shaft assembly  46   a  is defined by the proximal end of the proximal propeller shaft section  48   a  and is connected to air motor  62   a.  The distal end of propeller shaft assembly  46   a  is defined by the distal end of distal propeller shaft section  50   a  and is connected to propeller  52   a.  Similarly, propeller shaft assembly  46   b  is a multi-segment shaft that comprises a proximal propeller shaft section  48   b  and a distal propeller shaft section  50   b.  The proximal propeller shaft section  48   b  and distal propeller shaft section  50   b  are connected by a coupling  54   b.  The proximal end of the propeller shaft assembly  46   b  is defined by the proximal end of the proximal propeller shaft section  48   b  and is connected to air motor  62   b.  The distal end of propeller shaft assembly  46   b  is defined by the distal end of distal section  50   b  and is connected to propeller  52   b.  Each propeller shaft assembly  46   a  and  46   b  has a length along a length axis l. When its respective air motor  62   a  or  62   b  is activated, each shaft assembly  46   a  and  46   b  rotates about its respective length axis / as indicated by the curved arrows. The shaft rotation causes each respective propeller  52   a  and  52   b  to rotate about its length axis l and move the watercraft  10  through the water. 
     As mentioned above, air motors  62   a  and  62   b  are operable to rotate their respective propeller shaft assembly  46   a  or  46   b  and their respective propeller  52   a  or  52   b.  Air motors take compressed air and allow it to expand to do mechanical work. Air motors may be linear or rotary depending on the type of mechanical work required. In the case of air motors  62   a  and  62   b,  rotary air motors are preferred. The specific rotational frequency of the propeller and horsepower will depend on the weight of the watercraft  10  and the desired speed of travel. In one example, a rotary air motor is used. Suitable, commercially-available, rotary air motors include the 1UP-NRV-15 rotary air motor provided by Gast Manufacturing, Inc. of Benton Harbor, Mich. This motor provides 0.45HP and a torque of 5.25 in-lb at a maximum (no load) rotational speed of 6000 RPM. It also provides a speed of 500 RPM at a maximum torque of 6.0 lb-in. The motor also has a maximum air consumption of 27 cubic feet per minute. The shaft diameter is ⅜ inches, and the air inlet port size is ⅛″ NPT. It is rated for a maximum pressure of 80 psig. In the case of ship  200 , suitable air motors would include Ingersoll Rand KK5B Piston Air Motors which provide at least 29-30 HP and a torque of about 65 lb f -ft at a maximum rotational speed of about 1400 rpm. The motors have a maximum air consumption of about 800-850 standard cubic feet per minute. 
     The air used to run the air motors  62   a  and  62   b  is provided by air supply system  47 . Air supply system  47  comprises air compressor  78  and a plurality of in-line air-storage tanks  80   a,    82   a,    80   b,  and  82   b.  The term “in-line” refers to the fact that each pair of storage tanks ( 80   a / 82   a  and  80   b / 82   b ) is in the flow path from the compressor  78  to the air motors  62   a  and  62   b . The pairs of storage tanks— 80   a / 82   a  on the one hand and  80   b / 82   b  on the other hand—are in parallel with respect to one another, but are each in the flow path from a compressor discharge line ( 108   a  and  108   b,  respectively) to the air motors  62   a  and  62   b.  Put differently, the air storage tanks  80   a,    82   a,    80   b,    82   b  do not supply air motors  62   a  and  62   b  in parallel with the compressor  78 . One or more auxiliary air compressors (not shown) may also be provided to provide supplemental air and ensure that the air motors  62   a  and  62   b  have sufficient air flow rates while at the same time ensuring that the air-storage tanks  80   a,    82   a,    80   b,  and  82   b  can be refilled after reaching a desired state of depletion (e.g., a threshold lower pressure limit). 
     The air compressor  78  discharges to and is in fluid communication with parallel slave air storage tanks  82   a  and  82   b  via compressor discharge lines  108   a  and  108   b.  Each slave air storage tank  82   a  and  82   b  is fluidly coupled to and in fluid communication with a respective master air storage tank  80   a  and  80   b  by a respective pressure drop valve  84   a  and  84   b.  The pressure drop valves  84   a  and  84   b  ensure that the slave air storage tanks  82   a  and  82   b  operate at a higher pressure than their corresponding master air storage tanks  80   a  and  80   b,  ensuring that air flows from the slave air storage tanks  82   a  and  82   b  to their corresponding master air storage tanks  80   a  and  80   b  but not in reverse, such as when the slave air storage tanks  82   a  and  82   b  are being refilled. The extra pressure drop forces the compressor  78  to run at a higher discharge pressure and lower flow rate than it otherwise would, which prevents oversupplying air to the air motors  62   a  and  62   b.  The pressure drop valves  84   a  and  84   b  can be control valves, pressure regulators, check valves, etc. However, in certain examples they are not automatically manipulable to achieve a desired pressure, but rather, just provide a source of pressure drop in the system and adjust the operation of the compressor to a higher discharge pressure regime. In certain examples, the pressure drop across each pressure drop valve is from about 1000 psig to about 4000 psig, preferably from about 1500 psig to about 3500 psig, still more preferably from about 2000 psig to about 3000 psig, and still more preferably from about 2400 psig to about 2600 psig. 
     In preferred examples, the air compressor  78  is run periodically to fill the slave air storage tanks  82   a  and  82   b  until their respective pressures reach a desired maximum pressure (P max  ). Filling slave air storage tanks  82   a  and  82   b  will also cause master air storage tanks  80   a  and  80   b  to fill with air. Such periodic refilling operations are carried out when the pressure in the slave air storage tanks  82   a  and  82   b  reaches a predefined lower limit (P min ). A low pressure switch may be installed on the slave air storage tanks  82   a  and  82   b  to determine when the predefined lower pressure limit P min  has been reached. Alternatively, hardware or firmware in the control unit  69  may use pressure signals provided from pressure sensors in slave air storage tanks  82   a  and  82   b  to determine if the pressures have fallen below P min . Among other benefits, periodic (as opposed to continuous) operation of the compressor  78  allows watercraft  10  to run more quietly for long stretches of time (e.g., when the compressor is off). In certain examples, P min  is no less than about 1500 psig, preferably not less than about 1700 psig, and more preferably not less than about 1900 psig. In the same or other examples, P min  is no more than about 2500 psig, preferably not less than about 2200 psig, and more preferably not less than about 2100 psig. 
     The in-line slave air storage tanks  82   a  and  82   b  are preferably maintained at an operating pressure that is above a first specified threshold value, which is a pre-defined lower limit (P min ) and below a second specified threshold value, which is a pre-defined upper limit (P max ). The predefined lower limit P min  is preferably high enough to ensure that a desired air flow rate to the air motors  62   a  and  62   b  can be maintained at a desired air inlet pressure at the air motors  62   a  and  62   b.  Rotary air motors  62   a  and  62   b  have characteristic curves that relate the speed of rotation of the motor to the air motor inlet pressure and volumetric flow rate. The in-line air storage tanks  80   a / 80   b  and  82   a / 82   b  ensure that the desired combination of volumetric air flow rate and air motor inlet pressure can be maintained so that the desired speed of propeller rotation can be achieved. Also, the tanks  80   a / 80   b  and  82   a / 82   b  are preferably pre-filled to the maximum desired tank pressure (P max  ) before a trip. As a result, the compressor  78  may run only periodically. However, when compressor  78  is running, it is preferred that the compressor discharge flow rate (mass of air) exceeds the rate of consumption by air motors  62   a  and  62   b  so that the tanks  80   a,    80   b  and  82   a,    82   b  are replenished. Nevertheless, even during refilling operations, the air motors  62   a  and  62   b  may periodically consume more air than the compressor  78  provides as long as on average the air motors  62   a  and  62   b  consume less air than is being provided by compressor  78 . Thus, the in-line air storage tanks  80   a,    80   b,    82   a,    82   b  provide greater flexibility in adjusting the speed of the boat by providing surge volumes and reserve volumes of air. 
     In certain examples, the desired maximum slave tank  82   a,    82   b  air pressure P max  is at least about 3000 psig, preferably at least about 4000 psig, and more preferably at least about 4200 psig. P max  is preferably no greater than about 6000 psig, preferably no greater than about 5000 psig, and more preferably not greater than about 4600 psig. In the same or other examples, the volume of each slave tank  82   a,    82   b  and master tank  80   a  and  80   b  is at least about  350  cubic feet, preferably at least about 380 cubic feet, and more preferably at least about 440 cubic feet, and the volume is no more than about 530 cubic feet, preferably no more than about 500 cubic feet, and more preferably no more than about 450 cubic feet. One exemplary type of air storage tank useful as master tanks  80   a,    80   b  and slave tanks  82   a,    82   b  is the NUVT4500 storage tank supplied by Nuvair of Oxnard, Calif. The tank has a maximum service pressure of 4500 psig, and an internal storage volume of 437 cubic feet. In one example where the watercraft is a ship  200 , the volume of each slave tank  82   a,    82   b  and master tank  80   a  and  80   b  is sized to provide the desired maximum ship speed at the maximum expected ship weight based on the weight of the ship, the selected air motors, and the maximum expected cargo load, as well as based on any non-cargo items that affect the ship&#39;s weight. 
     The air compressor  78  takes air from the atmosphere and compresses it to a pressure sufficient to supply the master and slave tanks  80   a / 80   b  and  82   a / 82   b  until the slave air storage tanks  82   a  and  82   b  reach their desired maximum pressure (P max  ) during a refilling operation. A high pressure switch may be provided to determine when P max  has been reached. The switch may be a hardware switch installed on each slave air storage tank  82   a  and  82   b  or a software or firmware switch in a controller within power distribution panel  88  which receives pressure sensor signals from sensors installed on the slave air storage tanks  82   a,    82   b.  In either configuration, the controller uses an input signal or signals to determine whether to turn off the compressor  78  motor. In the case of multiple slave air storage tanks  82   a,    82   b,  the compressor  78  may be turned off when either slave tank  82   a,    82   b  reaches P max . Alternatively, the compressor  78  may remain on until both slave air storage tanks  82   a  and  82   b  reach P max . However, the former approach is preferred as it prevents overfilling the slave air storage tanks  82   a,    82   b  if one of the pressure sensors or switches fails. Suitable commercially available air compressors include the Bauer Model No. 100 air compressor which has a maximum air discharge pressure of about 5000 psig. In the case of ship  200 , suitable air compressors would preferably be selected based on the maximum desired motor power. 
     Compressor  78  discharges compressed air to slave air storage tank  82   a  via compressor discharge line  108   a  and to slave tank  82   b  via compressor discharge line  108   b.  In some examples, the air compressor  78  can supply air at a mass flow rate in excess of the rate of consumption of air by the air motors  62   a  and  62   b  at their maximum speed of operation and at the maximum desired compressor discharge pressure. In that case, as the slave air storage tanks  82   a  and  82   b  are being refilled (when their pressures hit the desired low pressure limit P min ), the rate at which compressed air is added to the slave air storage tanks  82   a  and  82   b  by compressor  78  will exceed the rate at which air is consumed by the air motors  62   a  and  62   b  so that the amount of air in the master  80   a / 80   b  and slave  82   a / 82   b  tanks will increase until the slave air storage tank  82   a  and  82   b  pressures read the desired upper limit P max . 
     The slave air storage tanks  82   a,    82   b  are maintained at a pressure that varies between a first selected value (the predefined minimum pressure (P min )) and a second selected value (the predefined maximum pressure (P max )). If air is flowing to the air motors  62   a  and  62   b,  the pressure in the master air storage tanks  80   a  and  80   b  will be less than the pressure in the slave air storage tanks  82   a  and  82   b.  The air pressure in the slave  82   a,    82   b  and master  80   a,    80   b  tanks will be significantly higher than the pressure required at the air motors  62   a  and  62   b  because it is desirable to maximize the amount of air with which the master tanks  80   a / 80   b  and slave tanks  82   a / 82   b  are pre-filled while still regulating the air flow rate to air motors  62   a  and  62   b  so that the watercraft  10  speed may be controlled. In order to regulate the air flow rate to the air motors  62   a  and  62   b,  the pressure must be reduced significantly from the pressure in storage tanks  80   a / 80   b  and  82   a / 82   b.  In the first instance, pressure drop valves  84   a  and  84   b  drop the air pressure significantly. In addition, however, pressure regulators  86   a  and  86   b  (fixed or adjustable valves that drop the air pressure) are provided downstream of the master air storage tanks  80   a  and  80   b . Master air storage tank discharge line  110   a  is connected to regulator  86   a  and master air storage tank discharge line  110   b  is connected to regulator  86   b.  The regulators  86   a  and  86   b  control the inlet air pressure to pneumatic control unit  69 . In certain examples, the regulators  86   a  and  86   b  control the control unit  69  inlet pressure to from about 80 psig to about 120 psig, preferably from about 90 to about 110 psig, and more preferably from about 95 to about 105 psig. In one specific example, 100 psig is used. 
     The pneumatic control unit  69  includes compressed air discharge lines  68  and  70 . The air pressure supplied to air motors  62   a  and  62   b  via discharge lines  68  and  70  is adjustable using throttle  72 . Compressed air discharge line  68  is a forward line that is connected, preferably in parallel, to air motor forward rotation inlet port  64   a  of air motor  62   a  and air motor forward rotation inlet port  64   b  of air motor  62   b.  Compressed air discharge line  70  is a reverse line that is connected, preferably in parallel, to air motor reverse rotation inlet ports  66   a  and  66   b  of air motor  62   b  One or more internal air control valves within control unit  69  adjust the air pressure in discharge lines  68  and  70  based on the throttle  72  position. The throttle  72  includes two levers which can be manipulated to cause the watercraft  10  to go forward and in reverse by causing air to be selectively supplied from forward line  70  or reverse line  68  (i.e., the throttle  72  is operable to adjust the air flow rate and propeller rotational direction). Supplying air to the air motor forward rotation inlet ports  64   a  and  64   b  causes gears in air motors  62   a  and  62   b  to rotate in a first direction, which in turn causes propellers  52   a  and  52   b  to rotate in a first direction about the propeller shaft length axes l, propelling the watercraft  10  forward. Supplying air to air motor reverse rotation air inlet ports  66   a  and  66   b  causes gears in air motors  62   a  and  62   b  to rotate in a second direction, which in turn causes propellers  52   a  and  52   b  to rotate in a second direction about the propeller shaft length axes l, propelling watercraft  10  in reverse. The levers on throttle  72  are manipulable to rotate the propellers  52   a  and  52   b  in forward and reverse from a speed of zero to the maximum rate of rotation of the air motors  62   a  and  62   b.  In one example, the supply pressure to the air motors  62   a  and  62   b  ranges from 0 to 100 psig, which corresponds to a propeller rotational frequency of from 0 to about 400 rpm. 
     Throttle  72  includes wires  98   a  and  98   b  and/or suitable electronic components which send a control signal to the control unit  69  to cause control unit  69  to adjust the controller discharge pressure in lines  68  and  70  via internal air control valves. Thus, the master air storage tanks  80   a  and  80   b  are in fluid communication with the air motors  62   a  and  62   b  via the pressure regulators  86   a  and  86   b  and the air control valves in the control unit  69 . In certain examples, the compressed air pressure in compressed air discharge lines  68  and  70  ranges from 0 to about 100 psig. 
     Control unit  69  is also operatively connected to indicators  74  and  76 . Indicators  74  and  76  provide a visual indication of the frequency of rotation of each propeller  52   a  and  52   b  (e.g., RPM) based on appropriate instruments connected to the propeller shaft assemblies  46   a  and  46   b  or the air motors  62   a  and  62   b.  Indicator lines  100   a  and  100   b  provide electrical signals necessary to operate the indicators  74  and  76  and are in electrical communication with air motors  62   a  and  62   b  or other devices used to indicate the speed of rotation of the shaft assemblies  46   a  and  46   b.    
     Air compressor  78  (and an auxiliary compressor, if provided) is preferably capable of being powered by battery power. A plurality of batteries  92   a,    92   b,    94   a,  and  94   b  are provided to supply electrical energy necessary to operate air compressor  78 . The positive terminals of batteries  92   a  and  94   a  are connected to a power distribution panel  88  via electrical connection lines  102   a  and  102   b,  respectively, and the negative terminals of batteries  92   a  and  94   a  are connected to ground. The positive terminals of batteries  92   b  and  94   b  are connected to power distribution panel  88  via electrical connection lines  103   a  and  103   b,  and the negative terminals of batteries  92   b  and  94   b  are connected to ground. The power distribution panel  88  is connected to a positive terminal of the air compressor  78  electric motor via connection  113   a  and to a negative terminal of the air compressor  78  electric motor via connection  113   b.  The power distribution panel  88  selects one from among the four batteries  92   a,    94   a,    92   b,    94   b  at a time to supply power to compressor  78 . 
     The batteries  92   a,    94   a,    92   b,    94   b  are preferably rechargeable and are each preferably capable of supplying the energy needed to cyclically operate compressor  78 . Suitable examples include lithium iron phosphate batteries. The batteries  92   a,    94   a,    92   b,    94   b  are preferably selected to provide a voltage compatible with the requirements of the compressor  78  motor and a capacity sufficient to ensure that electric power is sufficient to allow watercraft  10  to remain at sea for a desired period at a desired speed without recharging. In one example, four (4) size 8D lithium iron phosphate batteries supplied by RELi 3 ON® of Fort Mill, S.C. are used. The batteries  92   a,    94   a,    92   b,    94   b  are connected to a recharging panel  90  via recharging lines  104   a ,  104   b,    106   a,  and  106   b.  Recharging panel  90  is connected to a plug  96  for connecting recharging panel  90  to a dock power source. When watercraft  10  is in port, plug  96  may be connected to a power source to recharge batteries  92   a,    94   a,    92   b,  and  94   b.  As indicated previously, in the case of ship  200 ,  512  batteries are shown. The particular size, weight, and energy capacity of the batteries may be selected based on the weight of ship  200 , the expected cargo load, the desired maximum draft, and the expected electrical load to run the ship&#39;s electrical systems as well as based on the expected variations in list and trim that the battery ballast system  211  is expected to encounter. Exemplary masses of individual batteries  292 , include masses of at least 40 lb m , at least 60 lb m ., and at least 80 lb m , and at the same time masses of not more than 200 lb m ., not more than 175 lb m ., and not more than 150 lb m . 
     In certain examples, the kinetic energy of the rotating propeller shaft assemblies  46   a  and  46   b  is converted to electrical energy for use by other electrically-powered systems onboard watercraft  10 . In one implementation, alternators  58   a,    58   b,    60   a,    60   b  are connected to each shaft assembly  46   a  and  46   b  and convert a portion of the rotating shaft kinetic energy to electrical energy. The electrical current supplied by the alternators  58   a,    58   b,    60   a,    60   b  is then supplied to the power distribution panel  88 . The power distribution panel  88  can then supply the current to recharge accessory batteries used to run lights, horns, radios, etc. 
     In certain implementations, propulsion system  40  is used to retrofit a watercraft  10 , from which an existing fossil fuel engine and fossil fuel tanks have been removed. In certain implementations, the components forming the propulsion system  40  allow watercraft  10  to remain at sea longer than the watercraft  10  with the fossil fuel engine and fuel tanks while weighing significantly less than the removed fossil fuel tanks and engines, fossil fuel, and engine. In certain examples, additional batteries such as batteries  92   a,    94   a,    92   b,  and  94   b  may be installed and used both as ballast and as a source of additional electricity, allowing watercraft  10  to remain at sea even longer. In such cases, each battery  92   a,    94   a,    92   b  and  94   b  is preferably selectively positionable along one or both of a watercraft length axis and a watercraft width axis. 
     In a preferred example, a large number of batteries  92   a,    94   a,    92   b,  and  94   b  are provided, and each battery serves as one of the ballast batteries  292  in  3 A- 3 B and  4 - 5 . In one example, each battery  92   a,    94   a,    92 ,  94   b  is a standard truck battery. In accordance with the preferred example, the ballast system  211  is designed to selectively electrically connect any number of the batteries  92   a,    94   a,    92   b,  and  94   b  to power distribution panel  88  and a power grid that is operatively connected to compressor  78  and any other battery-powered components so that any combination of batteries  92   a,    94   a,    92   b,  and  94   b  may be used. In such cases, carriage assemblies such as carriage assemblies  214 - 244  are provided and are designed with conductive pathways so that when any given slot is occupied by a battery  92   a,    94   a,    92   b,    94   b,  that battery can be selectively connected to the power grid and power distribution panel  88  to provide power to whatever accessories or equipment need battery power. 
     A method of operating watercraft  10  will now be described. Watercraft  10  is initially docked. Compressed air storage tanks  80   a / 80   b  and  82   a / 82   b  are filled with air until the slave air storage tanks  82   a  and  82   b  reach their desired maximum pressure P max . As air motors  62   a  and  62   b  are initially off, the master tanks  80   a  and  80   b  will be at the same pressure as their respective slave tanks  82   a  and  82   b.  In the case of NUVT4500 tanks, the maximum pressure is the service pressure of 4500 psig. At this point, pressure regulators  86   a  and  86   b  are set to supply a desired air pressure (e.g., 100 psig) to control unit  69  supply lines  112   a  and  112   b.  However, internal valves in control unit  69  are closed and supply no air to the air motors  62   a  and  62   b  (e.g. 0 psig). Batteries  92   a,    94   a,    92   b,    94   b  are fully charged. After unmooring the watercraft  10 , throttle  72  is actuated to transmit air pressure via forward rotation line  68  to air motor forward rotation input ports  64   a  and  64   b,  with the position of the throttle corresponding to both the pressure in forward rotation line  70  and the rotational frequency of propellers  52   a  and  52   b.  Batteries  92   a,    92   b,    94   a ,  94   b  are aligned along the length and width axes of watercraft  10  to provide the desired trim and list at the start of the journey. Fresh water tank  346  also preferably has an amount of water which, when combined with the battery and carriage assembly weights, provides an initial desired amount of total ballast. 
     After the journey has progressed for a period of time, the air pressure in slave air storage tanks  82   a  and  82   b  drops to a first selected value, the desired minimum pressure P min . At this point, a controller in the power distribution panel  88  electrically connects one of the batteries  92   a,    94   a,    92   b,    94   b  to an electric motor that drives compressor  78  and/or activates the electric motor that runs compressor  78 . Compressor  78  intakes and compresses ambient air, causing it to flow to the slave air storage tanks  82   a  and  82   b  and then into the master air storage tanks  80   a  and  80   b.  Alternatively, the regulators  86   a  and  86   b  can be configured and/or controlled to allow only one tank pair  80   a / 82   a  or  80   b / 82   b  to be used at any one time. Once the pressure in the slave air storage tanks  82   a  and  82   b  reaches a second selected value, the maximum desired pressure P max , the compressor  78  is turned off (such as by discontinuing the supply of electric power from power distribution panel  88 ). If the pressures in slave air storage tanks  82   a  and  82   b  are different, the system may be configured to turn off compressor  78  when either slave tank  82   a  or  82   b  reaches the maximum desired pressure P max . While the system could be configured to keep the compressor  78  running until both slave tanks  82   a,    82   b  reach P max , it is preferred to turn the compressor  78  off when one of them reaches P max  to prevent overfilling if one of the pressure sensors or switches fails. 
     This process of cycling the compressor  78  on and off as the pressure drops and rises in the slave tanks  82   a,    82   b  is repeated. Eventually, the currently operative battery from among batteries  92   a,    94   a,    92   b,    94   b  drops to a potential difference that is low enough to cause the controller in the power distribution panel  88  to place another one of the batteries  92   a,    94   a,    92   b ,  94   b  in electrical communication with the motor in compressor  78 . Moreover, during the entire journey, no fossil fuels are consumed and no carbon dioxide, carbon monoxide, water, NOx, SOx or other pollutants are emitted. 
     If it is desired to adjust the trim of the watercraft  10 , one or more of the batteries  92   a ,  92   b,    94   a,    94   b  may be moved along the length axis of the watercraft  10 . If it is desired to adjust the list of the watercraft  10 , one or more of the batteries  92   a,    92   b,    94   a,    94   b  may be moved along the width axis of the watercraft  10 . In examples in which the watercraft  10  is a larger ship such as ship  200 , additional batteries would be provided in the manner described previously for ballast batteries  292  of  FIGS.  3 A- 3 B,  4 , and  5   . If the overall draft of watercraft  10  needs to be reduced, ballast controller  400  or any of the other techniques previously described may be used to expel potable water overboard via overboard line  411 . Conversely, if more draft is required, the flow rate of sea water into the potable water treatment system  340  may be increased by increasing the setpoint of flow controller  353  or using any of the other techniques previously described to increase the level in tank  346 . 
     EXAMPLE 1 
     A 1972 Luhrs Sport Fishing Boat weighing approximately 19,000 lbs. is provided. The boat includes two Chrysler 318 cc engines. Including the reverse and reduction gears, the engines weigh approximately 900 lbs. each. Two 75 gallon gas tanks are also included, which collectively weigh about 250 lbs. empty. 150 gallons of gasoline weighs approximately 1,100 lbs. Thus, the total weight of the gasoline engines, gas tanks, and gasoline is about 3150 lbs. The boat is retrofitted with a propulsion system in accordance with propulsion system  40  of  FIG.  2   . 
     The Chrysler engines, the gas tanks, and the gas are removed from the vessel. Four Nuvair NUVT4500 compressed air storage tanks are installed in the vessel, each of which has an empty weight of about 145.5 lbs. 
     Two GAST 1UP-NRV-15 rotary air motors are installed as shown in  FIG.  2   . One commercially available main compressor weighing about 800 lbs. and two commercially available auxiliary compressors weighing about 400 lbs. each are also installed. The compressors are selected to have a maximum discharge pressure of about 4500 psig and to supply a flow rate or air to both air tanks  80   a,    82   a,    80   b,  and  82   b  which exceeds the amount of air consumed by air motors  62   a  and  62   b  when watercraft  10  is at a cruising speed of 15-18 miles per hour. The weight of each motor  62   a  and  62   b  is approximately 25 lbs. Twelve RELi 3 ON® lithium iron phosphate 12V, size 8D batteries weighing approximately 83 lbs each are installed. The boat has an existing control panel and power distribution panel which are rewired and outfitted with pneumatic lines for use with air motors. 
     The retrofitted components weigh about 220 lbs more than the removed components. However, prior to retrofitting, when watercraft  10  is cruising at a speed of about 15-18 miles per hour, it consumes about 7 gallons of gasoline per hour, which will exhaust the full 150 gallon fuel supply in about 21.4 hours. In contrast, each of the 12 lithium iron phosphate batteries is estimated to be able to run the main and auxiliary compressors for 72 hours continuously, even though in operation, the compressors will only be run periodically (i.e., when the slave tank  82   a ,  82   b  pressures fall below P min ). With 12 lithium iron phosphate batteries of the type described above, even if the main and auxiliary compressors were operating continuously, the air motors could be operated continuously for about 36 days (874 hours) while moving watercraft  10  at a speed of about 15-18 miles per hour through the water. Thus, air propulsion systems in accordance with the present disclosure provide the ability to stay at sea for more than 30 times as long as a fossil fuel engine and fuel system sized for the same watercraft. 
     If only one of the twelve (12) lithium iron phosphate batteries were used, watercraft  10  could still remain at sea more than three times as long with the air propulsion system of the present disclosure than with the replaced fossil fuel system and the retrofitted watercraft  10  would weigh over 650 lbs. less than the original watercraft. Thus, it has surprisingly been discovered that not only can air propulsion systems built in accordance with the present disclosure avoid the burning of fossil fuels, but they can allow the watercraft to remain at sea far longer than fossil fuel engines. 
     It has also been discovered that adding lithium iron phosphate batteries also helps maintain the list and trim of the watercraft  10 . In accordance with this example, the lithium iron phosphate batteries are selectively positionable along the length and width axes of boat, preferably using a carriage system similar in design and smaller in size to ballast system  211  of  FIGS.  3 A-B ,  4  and  5 . If the watercraft  10  shows a positive trim by stern ( FIG.  1   ), one or more of the  12  lithium iron phosphate batteries would be moved along the watercraft&#39;s length axis toward the bow to reduce the trim by stern. Conversely, if the watercraft  10  shows a negative trim by stern, one or more of the lithium iron batteries would be moved along the length axis toward the stern to increase the trim by stern. 
     Referring to  FIG.  2 B , in the case of a list angle that is positive in the clockwise direction when viewing the stern of watercraft  10  in a direction toward the bow of watercraft  10 , one or more of the lithium iron phosphate batteries would be moved along the width axis of the watercraft  10  toward the port side of the watercraft  10 . Conversely, if the watercraft  10  has a negative list angle in the clockwise direction when viewing the stern of watercraft  10  in a direction toward the bow, one or more of the lithium iron phosphate batteries would be moved along the watercraft  10  width axis toward the starboard side of the watercraft  10 . 
     EXAMPLE 2 
     An example of a large ship having a 112 foot beam with a battery ballast system like battery ballast system  211  of  FIGS.  3 A- 3 B  will now be provided. 100 carriage assemblies similar to carriage assemblies  214 - 244  are provided and located in the lower deck  210 . Each carriage assembly has eight (8) tiers arranged along the ship&#39;s height axis H. Each battery support in each carriage assembly (e.g., battery support  271 ) has a length along the ship&#39;s length axis of 4 feet, a width along the ship&#39;s width axis of two (2) feet, and is spaced apart from is vertically adjacent neighbors by two (2) feet. 100 feet of the ship&#39;s 112 foot width is available for carriage assemblies. Thus, there are 100/2=50 slots (e.g., H( 1 )-H( 50 )) comprising each tier of each carriage assembly. Each tier has 32 batteries and battery supports occupying 32 of the 50 slots. Each battery weighs 150 pounds, and the average weight per slot (accounting for the fact that 18 slots do not have a battery support  271  in them) is 20 pounds. Thus, the battery weight per tier of each carriage assembly is 150 lbs.×32 batteries/tier=49,800 lbs./tier. The slot weight per tier (without batteries) is 20 lbs./slot×50 slots/tier=1,000 lbs./tier. Thus, the weight of each tier including batteries is 50,800 lbs. or 25.4 tons. 
     Each carriage assembly has eight (8) tiers, bringing the total weight per carriage assembly to 8 tiers/carriage assembly (25.4 tons/tier)=203.2 tons/carriage assembly. The total weight of the entire battery ballast system is then 100 carriage assemblies×203.2 tons/carriage assembly=20,320 tons. A potable water system is also provided and includes a fresh water tank having a rectangular prism shape with a length of 200 feet and a cross-section of 50 feet by 14 feet, yielding a volume of 145,600 cu. ft. The weight of potable water for such a tank is 145,600 cu. ft.×62.4 lbs./cu. ft.=4542 tons. 
     In certain examples, the potable water tank is designed to provide an amount of ballast water capacity beyond that which is needed to satisfy the expected maximum consumption of potable water on the ship. As explained previously, the battery ballast can be used to adjust the ship&#39;s list and trim, but batteries cannot be selectively added or expelled from a ship at sea. In one example, the potable water tank is sized to hold the maximum required volume of potable water required for shipboard consumption over a specified period of time and to ensure that the ship&#39;s waterline does not vary by more than a desired amount when the cargo loading varies between the minimum and maximum expected load. Based on known relationships between the gravitational force on the ship (i.e., the weight expressed as a force), the buoyancy force exerted by the body of water, and the maximum desired variation in the waterline, a maximum allowable change in the ship&#39;s mass can be calculated. This variation will correspond to a maximum change in the mass and volume of ballast water held in the potable water tank and the cargo weight. If it is desired for the ship to handle greater swings in cargo mass while staying within the maximum desired waterline variation, additional potable water tank capacity may be provided so that the mass of the potable water allocated to ballast is adjusted accordingly. For example, if the ship&#39;s maximum waterline variation is 20 feet, a corresponding change in total ship mass may be calculated which corresponds to that water-line variation. That maximum weight variation may be allocated as follows: 
       Δ M   T   =ΔM   C   +ΔM   B    (7)
         wherein, ΔM T =Total change in ship mass corresponding to maximum allowable waterline height variation (lb m  or kg)   ΔM C =maximum expected variation in cargo mass (lb m  or kg); and   ΔM B =maximum variation in mass of ballast (lb m  or kg).       

     Because the mass of the battery ballast will not change at sea, ΔM B  may be used to calculate the incremental potable tank volume required to accommodate the maximum desired cargo and waterline variations using equation (3), above. 
     The present invention has been described with reference to certain exemplary embodiments thereof. However, it will be readily apparent to those skilled in the art that it is possible to embody the invention in specific forms other than those of the exemplary embodiments described above. This may be done without departing from the spirit of the invention. The exemplary embodiments are merely illustrative and should not be considered restrictive in any way. The scope of the invention is defined by the appended claims and their equivalents, rather than by the preceding description.