Submarine Cable Control by Use of Variable Specific Gravity and Diameter Cables and/or External Forces for Cables Used with Cable-Propelled Marine Vessels

Abstract

Cables for propulsion and/or guidance of cable ferries are disclosed which permit their use on longer ferry routes. One embodiment provides the use of cables for propulsion and/or guidance of cable ferries where the unit weight, density, specific gravity and/or diameter of the cable is variable over the length of the cable. According to one embodiment, heavier or denser cable is used adjacent to the terminals, while lighter or less dense cable is used for a middle section. Where sections have positive buoyancy, the cable may be tethered to one or more anchors. Where sections have negative buoyancy, the cable may be tethered to one or more floats.

Claims

1. A cable wherein the unit weight, specific gravity, density or diameter of said cable is variable over the length of the cable, wherein said cable is used for propulsion and/or guidance of a cable-propelled marine vessel across a body of water between first and second terminal locations on opposing shores of said body of water, wherein said cable is configured to be fixed at either end thereof adjacent said first and second terminal locations and to pass through one or more pulleys, guides or drive wheels on said marine vessel, whereby when so configured the unit weight, specific gravity, density or diameter of said cable is selected at a location along the length of the cable between said first and second terminal locations according to the desired buoyancy of said cable at said location.

2. The cable of claim 1 wherein the unit weight, specific gravity, density or diameter of said cable is greater in sections of said cable closer to each said terminal location than in portions of said cable further from each said terminal location.

3. The cable of claim 1 wherein a combination of any one or more of the unit weight, specific gravity, density or diameter of said cable varies over the length of said cable.

4. The cable of claim 1 wherein the unit weight of said cable varies over the length of said cable.

5. The cable of claim 1 wherein the specific gravity of said cable varies over the length of said cable.

6. The cable of claim 1 wherein the density of said cable varies over the length of said cable.

7. The cable of claim 1 wherein the diameter of said cable varies over the length of said cable.

8. The cable of claim 1 wherein said cable comprises sections of wire rope cable at either end and a central section of synthetic rope.

9. The cable of claim 1 wherein said cable comprises a section of cable having negative buoyancy and a section of cable having positive buoyancy.

10. The cable of claim 1 wherein sections of said cable having positive buoyancy are tethered to one or more anchors by flexible connectors.

11. The cable of claim 1 wherein sections of said cable having negative buoyancy are tethered to one or more floats.

12. The cable of claim 10 wherein said flexible connectors are elastic.

13. A cable system for propulsion and/or guidance of a cable-propelled marine vessel between terminals, comprising: i) a cable according to claim 1 which comprises a cable section with positive buoyancy; and ii) one or more anchors connected to said cable at spaced locations by flexible connectors.

14. The cable system of claim 13 wherein said flexible connectors are elastic.

15. A cable system for propulsion and/or guidance of a cable-propelled marine vessel between terminals, comprising: i) a cable according to claim 1 which comprises a cable section with negative buoyancy; and ii) one or more flotation devices connected to said cable at spaced locations by flexible connectors.

16. The cable system of claim 15 wherein said flexible connectors are elastic.

17. A cable system for propulsion and/or guidance of a cable-propelled marine vessel between terminals, comprising: i) a cable according to claim 1 which comprises a section having positive buoyancy and a section having negative buoyancy; and ii) one or more anchors or flotation devices connected to said cable at spaced locations by flexible connectors.

18. The cable system of claim 17 wherein said flexible connectors are elastic.

19. A method of controlling the position or dynamics of a cable used for propelling or guiding a cable propelled or guided marine vessel over the path of said vessel on a maritime crossing of a body of water between two points on said path comprising the steps of: i) determining a selected path over said maritime crossing between said two points; ii) selecting a cable according to claim 1 for said propulsion and/or guidance to meet the characteristics of said crossing.

20. A method of controlling the position or dynamics of a cable used for propelling or guiding a cable propelled or guided marine vessel over the path of said vessel on a maritime crossing of a body of water between two points on said path comprising the steps of: i) determining a selected path over said maritime crossing between said two points; ii) selecting a cable according to claim 1 for said propulsion and/or guidance to meet the characteristics of said crossing; iii) selecting a combination and location of force-imposing elements for connection to said cable at spaced locations along the length of said cable to meet the desired characteristics of said crossing.

21. The method of claim 20 wherein said force-imposing elements are selected from the group consisting of: i) flotation devices connected to said cable at spaced locations by elastic connecting elements; ii) anchoring devices connected to said cable at spaced locations by elastic connecting elements; iii) elastic connecting devices connecting said cable to the bottom of said body of water; and iv) a combination of the foregoing elements.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.

[0013] FIG. 1 is a side cross-sectional view of a prior art cable ferry showing the undersea profile of the cable with the ferry at dock.

[0014] FIG. 2 is a side cross-sectional view of a prior art cable ferry showing the undersea profile of the cable with the ferry in transit.

[0015] FIG. 3 is a side cross-sectional view of a cable ferry using a VSGD cable according to one embodiment of the invention, showing the undersea profile of the cable with the ferry at dock.

[0016] FIG. 4 is a side cross-sectional view of a cable ferry using a cable according to one embodiment of the invention, showing the undersea profile of the cable with the ferry in transit.

[0017] FIG. 5 is a side cross-sectional view of a cable ferry using a cable according to a second embodiment of the invention, showing the undersea profile of the cable with the ferry in transit.

[0018] FIG. 6 is a top view of a cable ferry using a cable according to a third embodiment of the invention, partly in perspective to show the undersea profile of the cable with the ferry docked.

[0019] FIG. 7 is a partial side cross-sectional view of the cable ferry shown in FIG. 6.

[0020] FIG. 8 is a partial side cross-sectional view of a cable ferry cable according to a fourth embodiment of the invention.

[0021] FIG. 9 is a partial side cross-sectional view of the cable ferry shown in FIG. 8 with the ferry in transit.

[0022] FIG. 10 is a side cross-sectional view of a cable ferry using a cable according to a fifth embodiment of the invention with the ferry docked.

[0023] FIG. 11 is a side cross-sectional view of a cable ferry using a cable according to a sixth embodiment of the invention with the ferry docked.

[0024] FIG. 12 through 15 are plots of the catenary curves formed by a cable according to a first embodiment where the ferry is docked, and where the ferry is in transit.

[0025] FIG. 16 is a side cross-sectional view of a cable ferry using a cable according to an embodiment of the invention used to protect sensitive underwater habitat.

[0026] FIG. 17 is a top view of the embodiment of the cable ferry shown in FIG. 16.

[0027] FIG. 18 is a side cross-sectional view of a cable ferry using a cable according to an embodiment of the invention to avoid underwater infrastructure.

[0028] FIG. 19 is a side cross-sectional view of a cable ferry using a cable according to an embodiment of the invention to avoid underwater features which might cause cable wear or damage.

[0029] FIG. 20 is a side cross-sectional view of a cable ferry using a cable according to an embodiment of the invention to allow passage of large ships over the cable.

[0030] FIG. 21 is a top view of a cable ferry using a cable according to one embodiment of the invention in strong currents as compared to the use of a standard cable.

[0031] FIG. 22 is a top view of a cable ferry using a cable according to one embodiment of the invention where two floating cables are tethered to anchors and showing the cable ferry in transit stretching the cables up and inward toward the cable ferry.

[0032] FIG. 23 is a side cross-sectional view of a cable ferry using a standard cable to illustrate the occurrence of chafing points where the cable contacts the bottom to support its weight.

DESCRIPTION

[0033] Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.

Terms

[0034] Cable ferry means any ferry, ship, barge, tug or any other vessel that is guided by or uses cables, ropes, chains or any other type of line(s) for guidance and/or propulsion.

[0035] Cable shipping means any ferry, ship, barge, tug or any other vessel that is guided by or uses cables, ropes, chains or any other type of line(s) for guidance and/or propulsion.

[0036] Cable control means the control of the positioning of the cable and its dynamic qualities.

[0037] Specific weight (also referred to as the unit weight) is the weight per unit volume of a material. The density of a material is the mass per unit volume, while the specific or unit weight is the force of gravity on the unit mass of the material.

[0038] Specific gravity is the ratio of the density of a substance to the density of a reference substance, namely the density of water at 4 degrees Celsius which has a specific gravity of 1; equivalently, it is the ratio of the mass of a substance to the mass of a reference substance for the same given volume.

[0039] Tether means a line attached to the cable on one end and with the other end attached or anchored to the bottom, to a weight or a float.

[0040] VSGD is an acronym for Variable Specific Gravity and Diameter and means that the unit weight, specific gravity, density and/or diameter of the cable is variable over the length of the cable.

[0041] In the drawings a thick black line is used to designate VSGD cable sections that vary in unit weight, specific gravity, density, and/or diameter from the rest of the cable.

[0042] FIGS. 1 and 2 illustrate in side view a typical prior art cable ferry configuration. Cable ferry 10 crosses body of water 12 between docks 14, 16. Ferry 10 is guided by, and uses for propulsion, cable 18 which is secured at either end to the dock or shoreline. Cable 18 may be a single cable or multiple parallel cables. Cable 18 may for example be made of stranded steel wire rope with 6 or 8 main strands wrapped around an Independent Wire Rope Core with a synthetic plastic coating or wrapping, with a diameter in the 1 to 1 size range. Typically, one end of the cable 18 is attached to a buried deadman anchor with the other end being attached to a tensioning winch. For part of the crossing, the cable may lie on the seabed and be pulled up into one end of the deck of the ferry 10 through a friction drive bullwheel(s) or idler(s), then pass back overboard at the other end of the deck of the ferry 10.

[0043] In FIG. 1 the ferry 10 is shown docked at dock 14. Cable 18 therefore, if not lying on the seabed, forms a catenary curve, which is the curve a hanging flexible wire or chain assumes when supported at its ends and acted upon by a uniform gravitational force. In FIG. 2, the ferry 10 is in transit between docks 14 and 16. Cable 18 forms two catenary curves 22, 24 whose relative sizes change as the ferry 10 moves across the surface 20 of the water until a single catenary curve is again formed when ferry 10 reaches dock 16. For the reasons noted above, the use of cables for longer ferry routes has not been practical, so cables have up until now been used only for ferries travelling a short distance from shore to shore. Changes of direction for a cable ferry has also not been possible. Changes of direction for a cable ferry would be desirable for many reasons, including allowing for more routes for such ferries and additional terminal locations to be considered.

[0044] FIGS. 3 and 4 illustrate a side view of a first embodiment in which cable 18 is formed of cable sections 26, 28, 30 which are of varying specific gravity and/or diameter (VSGD) that are spliced, tied, or linked together along the length of the cable 18. The cable could also be especially manufactured resulting in varying density or diameter without any splices, knots or other attachments. Sections 26 and 30 may be of heavier, standard wire rope cable, whereas the central section is of lower specific gravity or density which is of neutral buoyancy, such as a synthetic rope cable. For example the sections 26, 30 may be formed of 1.5 diameter wire rope, while the central section 28 is formed of 1.5 diameter synthetic rope cable. This allows the ability to decrease the tensile forces required to provide the desired tension that will guide and propel the ferry without approaching the breaking point. A lighter cable, typically toward the center of the span, allows the tensile strength to be diverted from holding its own dead weight to be devoted instead to withstanding the forces exerted on the cable by the ferry or on the cable itself by current while providing an adequate safety factor. Using materials with varying specific gravity, the depth of the cable across the cable span can be designed to have the desirable qualities of cable weight and depth near the terminals by using heavier cable in that section while using a more buoyant material in the center of the span.

[0045] Modern synthetic ropes with breaking loads equal to or exceeding that of the same diameter steel wire rope are now readily available. Rope manufacturers such as Samson Ropes and Cortland Ropes offer cables made up of a number of different fibers to give the finished cable the requisite strength, weight, abrasion resistance and cyclic bending fatigue characteristics required for the intended application. Cables can be custom manufactured with varying diameters, either as a continuous rope or using splices, and in almost any length. For the cable ferry application, there are the advantages disclosed herein that result from having a cable that has a variable buoyancy, whether neutral, positive or negative buoyancy, and also is able to take repetitive cyclic bending as the cable passes over the drive bullwheel and the deck guide sheaves.

[0046] Buoyancy of the cable section 28 can be increased by incorporating into the cable large masses of low density material such as cellular rubber or closed cell expanded polyethylene, such as disclosed in U.S. Pat. Nos. 2,403,693 and 2,818,905, or by incorporating filament braid flotation members as disclosed in U.S. Pat. No. 3,155,768 in connection with a buoyant electrical cable. U.S. Pat. No. 3,806,568 provides a method of manufacturing a continuous cable which has a uniform diameter and sections of different strength and weight in sea water, without splicing different cable sections. Other VSGD cables can be made from synthetic rope or steel incorporating the desired characteristics.

Example

[0047] A notional cable ferry route, with a length of 7600 feet in very deep water was studied. For the notional ferry route it was assumed that the cable will allow a deep sea vessel drawing up to 35 feet to sail over it except when the ferry is in passage. Therefore it is necessary for the portion of the cable in the area of the ship passage to be no shallower than 80 feet from the surface when the ferry is not in passage. Given the end forces on the cable in such a configuration, it is clear that a 2 diameter conventional wire rope cable is unsuitable for a route of that length and depth, applying a safety factor of 5 to the breaking force of the cable. The nominal breaking load for a 2 wire rope is 360,000 lbs which, after applying the factor of safety of 5 translates to a maximum allowable force of 79,000 lbs. The end force is generated by the physical weight of the cable, the initial tension in the cable induced by the tensioning winch and the resistance of the water on the hull of the ferry as it pulls itself along the cable. The hull resistance is proportional to the load (number of vehicles) on the ferry and its speed through the water. It is not difficult to conclude that the terminal anchor axial and vertical forces and the problems imposed on the ferry crew and equipment to handle the heavy weights involved when handling long cables make it impractical to consider steel wire ropes for the drive and guide cables for the proposed route.

[0048] Use of a cable having a constant cross section along its entire length but using synthetic rope cable for a middle section of the cable was therefore examined. Synthetic cables can also be manufactured with varying density and diameters along their length and without the need for splicing. Calculations were done to determine the shape of the cable catenary having ends with a unit weight the same as a 2 diameter steel wire rope (7.4 lbs/ft) and with the longer middle section of 2 diameter synthetic rope with a unit weight of less than 1 lb/ft. The 2 diameter cable displaces a volume of water that weighs more than 1 lb. and therefore the middle section of the cable will arch upwardly towards the surface, being restrained only by the heavy ends. Using a 690 ft length of 2 steel wire rope the cable found equilibrium with the lighter mid-section when the lower end of the heavy section was submerged at 190 ft and the top of the upward arc of the synthetic mid-section was 85 ft below the surface. This is illustrated where the ferry is docked, with same and exaggerated vertical scales, in FIGS. 12 and 13. This is illustrated where the ferry is in transit, with same and exaggerated vertical scales in FIGS. 14 and 15. Preferably a combination of heavier (denser) synthetic rope cable end sections and a lighter synthetic rope middle section that will be in equilibrium at 80 to 100 ft below the surface can also be used. While 690 feet was considered for the length of the heavier end sections, a different length would apply for different configurations, whether 500 feet or some other length depending on the particular ferry route considered.

[0049] Therefore the solution was considered to use a dense, high specific gravity section of cable in the waters approaching each terminal and then using a much lighter cable to connect the ends of the heavy cables, and thus provide a continuous set of drive and guide cables for the ferry. As the drive cable must come into physical contact with the friction-drive bullwheel and idler, the transitions between the heavier end sections of the cable and the lighter centre section have to be smooth and symmetrical. The transition may take the form of a splice, with appropriate further adjustment to the allowable safe working loads for the cables. The technology surrounding synthetic ropes is now sufficiently developed that the applicable safety factor multiplicands are available, or could soon be calculated, and a suitable splice manufactured and tested. A preferred option is a continuous synthetic rope cable with heavier ends and less dense middle sections.

[0050] This design has a very beneficial effect on the reaction forces at the terminal anchors and the depth of the catenary loop. Several of the synthetic fibers have unit weights that are less than the weight of water that their volumes displaces and they will generate an upward buoyant force. It was concluded that using a lighter mid-section to the cable is feasible, permitting the consideration for cable ferry routes having much greater distances between ferry terminals without the cables becoming too heavy and unworkable.

[0051] Some of the modern synthetic ropes have comparable mechanical strength properties to the same diameter steel wire rope while being considerably lighter. Samson, Amsteel Blue Dyneema 2 diameter synthetic rope has a breaking load of 396,000 lbs. Applying a factor of safety of 5 would give a safe working load for this synthetic rope of 79,000 lbsthe same value as the 2 diameter steel wire rope. The elastic stretch of the Dyneema synthetic rope at 20% of breaking load is less than 1 percent which is comparable with the elasticity in the steel wire rope. There is a significant difference in the unit weight of the two cables. The steel cable weighs 7.3 lbs/ft while the synthetic cable weighs only 0.91 lbs/ft. The volume of water displaced by each linear foot of 2 dia. cable which has a cross section area of 3.142 sq. in. is 37.7 cu ins per linear foot. The synthetic cable under consideration generates 1.39 lbs of upward buoyancy and the cable will float. When the geometric forces at the two cable end anchors with the lighter synthetic cable are calculated, using a 7600 foot spacing between the cable end terminal anchors and the same Factor of Safety of 5, it is found that that while the droop before taking into account the buoyancy forces is the same as for the steel wire, the resultant forces acting on each anchor is reduced to 8,600 lbs. When the buoyancy of the cable is taken into consideration the cable floats on the surface and the forces at the anchors are considerably lower.

[0052] With reference to FIG. 5, use of the disclosed VSGD cable also allows design changes in the approach and egress angles B, A of the cable, fore and aft of the ferry, as the ferry brings the cable to the surface in propelling itself along the route and as it approaches the terminal. A thicker cable, which is greater in diameter, has more frictional force resisting it being drawn to the surface. Varying the cable diameter makes it possible to alter the dynamic interaction of the ferry with the cable. A larger diameter acts in harmony with the specific gravity of the cable with the result that the cable approach angle can be maximized. The angle of egress A of the cable from the ferry (FIG. 5) is smaller than the angle of approach B of the cable to the ferry. That is because the cable exits the ship under less tension than the cable entering the ship which is being pulled upon by the ship. The cable diameter can be increased in order to increase the approach angle B of the cable entering the ferry. A steeper approach angle B makes it more likely the cable will avoid contact with other vessel traffic that might cross the route while the ferry is in transit. Alternatively, decreasing the cable diameter can be employed in sections of a route where there is a strong current to decrease the current's effect on the cable.

[0053] FIG. 5 illustrates a side view of a variation on the first embodiment in FIGS. 3 and 4 using a cable of greater diameter, such as 3. Again the cable 18 is formed of cable sections 26, 28, 30 which are of varying specific gravity and/or diameter (VSGD) that are spliced, tied, or linked together along the length of the cable 18. With the greater diameter cable, as shown in FIG. 5, there is a steeper angle of approach B to the ferry in transit as compared to the 1.5 cable as shown by angle C due to the increased friction on the 3 cable by the water as it is drawn up.

[0054] FIGS. 6 and 7 illustrate respectively a top view and a side view of a third embodiment in which combinations of floats and anchors, tethered to the cable are used in conjunction with a VSGD cable. Anchors or weights 32 lie on the sea floor 36 and are attached to the cable 18 by tethers 38. Similarly floats or buoys 34 float on the water surface 20 and are attached to cable 18 by tethers 38. The locations where the floats, anchors and weights 32, 34 are connected to the cable 18 are selected to achieve a desired cable profile. For example in FIGS. 6 and 7, sections 40 of cable 18 are buoyant, sections 42 are sinking, and section 44 may be variable specific gravity.

[0055] In the embodiments shown in FIG. 8 through 11, cable control is achieved by applying an external force to a uniform cable which is not a VSGD cable. Whereas by incorporating a VSGD cable, cable control, namely the positioning of the cable and its dynamic qualities, is achieved through the physical qualities of the cable itself, in this embodiment, cable control is achieved by using external forces through weights, floats or an attachment to the bottom. In FIG. 8 a side view shows that the entire cable 18 may be buoyant, and held by elastic, stretchable tethers 38 below the surface 20 at a selected depth when the ferry is docked. FIG. 9 shows another side view of the configuration in FIG. 8 when the ferry 10 is in transit.

[0056] The foregoing cable tethering method permits both the depth profile and direction of the cable to be selected as shown in FIGS. 6 and 7. The method may be used by cable ferries that have the driving sheaves mounted outboard of the ferry, typically on either side. By using a system of fairleads, idlers or guards, the tether 38 between the cable and the anchor or weight 32 can be directed and kept away from going through the drive sheaves, such as by using a method such as used on longline commercial fishing vessels. Similar technology can be used to direct the tether 38 of floats 34 attached to the cable away from the drive sheaves. Notches may be machined into the outside sheave into which the tether would be directed just before it approaches the sheave. Alternately, an attachment similar to that used by a ski chairlift on the cable to prevent the tether to the attached weight from following the main line through the drive sheaves may be used.

[0057] Weights 32 can be used to draw the cable 18 nearer to the bottom but without touching the bottom, as shown in FIG. 11. When the weight 32 touches on bottom 36 the cable 18 is not drawn down any further. The floats 34 (as shown in FIG. 7) can be used to mark the position of a navigation route across the cable or to establish cable depth. Buoys may be used as the float 34 to both act on the cable and to serve to mark the cable for navigational or other purposes.

[0058] Floats 34 may be preferable in shallow water where bottom tethers 38 to anchors 32 are not suitable due to the limitations that shallow water places on the tether length to anchors with the corresponding limit to the elasticity of those tethers. See also FIG. 10.

[0059] As shown in FIG. 7, a system of anchors 32 and buoys 34 thus control the cable 18 in the longitudinal and vertical, (xy), sense and anchors 32 placed at a location spaced laterally from the main cable line will orient it on the horizontal plane (z). The system will be designed and put in place and then tensioned to take a desired profile in the xyz axes to allow other marine traffic various routes across the cable line where the cable is located sufficiently below the surface, and which may traverse many miles of water, as shown in FIG. 6. When the ferry is in transit it brings the main cable 18 to the surface which is allowed by the elasticity of the tethers 38 anchored to the bottom. The cable then sinks, or is drawn down by the tethers 38 to the desired depth when the cable ferry passes.

[0060] Bottom attachments can be effected by connecting elastic tethers 38 between the cable 18 and anchors 32 placed accordingly and set in the bottom as shown in FIGS. 8 and 9. In this case the cable 18 itself will be made of floating material. The cable will resemble the inverse of a suspension bridge. The elasticity of the tethers is illustrated in FIGS. 9 and 22 by the differing lengths of the solid sections of the broken lines representing tethers 38. The traction winch may be modified so that the main cable 18 is drawn by the sheaves but the tether 38 is by some means diverted away from the sheaves with a guide, idlers or attachment such as a fairlead as used in commercial longline gear hauling. One cable may be used with the proper configuration of winches but two outboard cables may provide redundancy and more directional control depending on the application. Indeed, on a ferry with a winch on either side, by varying the winch speed a certain amount of steerage can be accomplished with a two cable system. Thus by reducing cable tension, the anchors 32 and tethers 38 may allow changes in route direction to go around obstacles such as islands and reefs that has not be possible before for a cable ferry. This would allow cable ferry routes to be designed to allow the terminus of the route to be in sheltered waters to afford safe harbor for the ferry. The ferry 10 may be designed as a catamaran or other multiple hull vessel to permit use of one cable system with winches mounted mid-ships between the catamaran's hulls.

[0061] With control in the xyz planes to orient the cable, long cable routes, even across intercontinental shipping routes can be created to support barge or other towing operations, freighters, and any other vessels traveling routinely on a particular marine traffic lane to provide the advantages of efficiency and other advantages of cable ferries. Examples of routine traffic lanes using barge traffic are where barges are delivering chips to pulp mills or freight to island communities, as well as raw log booms, and providing supplies along tourist routes.

[0062] With the use of the foregoing VSGD cables, cable profiles can be designed to fit the requirements of each individual ferry route. This can be applied to cable ferry routes where it is desirable to have the cable fully suspended from shore to shore. VSGD cables can be used on longer cable ferry routes that will not necessarily require bottom contact to limit tension on the cable, though bottom contact may be allowed, and in some cases, be desirable. The specific gravity of the cable, typically near the section at the center of the route, can to be reduced and thereby decrease the overall tension being exerted on the cable which will allow longer cable spans and therefore longer cable ferry routes. A significant advantage to having a cable suspended clear of the bottom includes much longer cable life and less wear on mechanical parts. It will allow for cable ferries to be used on longer routes than has been possible before using uniform, typically plastic wrapped steel wire rope cables.

[0063] By varying both specific gravity and the diameter of a cable, and by using tethers to floats and anchors in some cases, the specific challenges of cable ferry routes can be overcome, including, but not limited to, route length, current, environmental sensitivity, vessel traffic, and ferry propulsion efficiency. Additional applications of the VSGD cables are described as follows.

[0064] As shown in FIGS. 16 and 17 in side view, VSGD may be used to prevent cable scouring of sensitive ocean bottom near terminals, such as habitat for species at risk or other productive sub-tidal areas. A buoyant section of cable 50 may be used in the area 52 where the sensitive habitat exists. Sinking cable 54 may be used where there is more water depth. The added weight allows proper cable tensioning and allows other traffic to cross the cable. The results are an asymmetric side profile of the cable. Navigation markers 56 may be used to warn traffic where the cable will always be too shallow to cross, as illustrated in FIG. 17. VSGD cables may also be used to avoid contacting transmission lines, such as pipelines, which are typically found at the narrowest crossing distance where a cable ferry may also be sited, as illustrated in FIG. 18 in side view.

[0065] Steep or irregular bottom structure 60 (FIG. 19) may obstruct the cable and cause it to wear in one place. Boulders and ledges 60 may cause snagging of the cable. As illustrated in FIG. 19 in a side view, VSGD cables 54/50/54 having end sections 54 of denser specific gravity and a central section 50 of neutral, negative or positive buoyancy may address this issue by being suspended above the hazards 60. A side profile in FIG. 23 of a standard cable 18 illustrates several chafing points 61 that can cause damage to the cable.

[0066] Other vessel traffic crossing the route may be a design consideration for some potential cable ferry routes. Large ships 62 can pass over the cable through a marked navigation channel as shown in FIG. 20 in a side view. Increased cable density and/or diameter allows a section 54 of the cable to be deeper. Lighter material 55 is used near the terminals 14, 16 which allows overall tension to be reduced. Ships 62 may cross the cable even when the cable ferry is in transit. This allows using cable ferries on navigational routes where preventing the passage of all shipping while the cable ferry is in transit is not feasible. VSGD cables can thus be used for permitting passage of shipping in conjunction with navigation markers.

[0067] Varying cable diameter may also be used to minimize the side forces caused by current acting on the cable. Cable diameter as well as density may be varied with modern materials. Varying cable diameter and density would be useful where there is a cross-current caused by tidal flow, rivers or other circumstances, shown in a top view in FIG. 21. Cable diameter is reduced in neutrally buoyant central section 57 while density and diameter is increased in end sections 54. As compared to a standard homogeneous cable 18, less lateral force caused by the current acts on the cable 54/57/54. The benefits include: i) keeping the cable ferry more precisely on its heading and within its route, ii) keeping the cable deeper to avoid other traffic across the route, and iii) minimizing the work required by the cable ferry to transit its route.

[0068] Floats and anchors may be used for a cable ferry featuring traction winches mounted on either side of the ferry. In top view, FIG. 22 shows the cable ferry 10 in transit stretching the cables up and inward toward the cable ferry. Here two floating cables 50 are attached by tethers 38 to anchors 32 as illustrated above in relation to FIG. 9. The tensile force acts in the vertical and horizontal planes. More stretching of the tethers 38 that is caused by the passing of the ferry is permitted without lifting the anchors 32. This is useful in shallow water. Displacing the cable outside of the route keeps the course of the ferry 10 more central.

[0069] While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. For illustrative purposes one cable only is shown for the cable ferries. The same concepts apply to cable ferries using multiple drive and/or guide cables. There are two ships illustrated though normally only one ship would serve the system. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are consistent with the broadest interpretation of the specification as a whole.