Abstract:
A marine vessel towing assembly includes a tow pin assembly housing including therein a self-contained horizontal roller cartridge, a self-contained vertical pin cartridge, and a self-contained hook cartridge, the tow pin assembly housing defining a clear path of removal for each self-contained cartridge from the tow pin assembly housing such that removal of each self-contained cartridge is conducted with an absence of contact with remaining self-contained cartridges of the tow pin assembly housing.

Description:
TECHNICAL FIELD 
     The present invention relates to a pin, roller and hook assembly of which the purpose is to guide and trap a wire used to connect a towing vessel with its tow. More particularly the present invention uses material and design features that improve the manufacturing process, make it less susceptible to damage, create improved accessibility to components for inspection and repair and improve the reliability and performance of the invention in its intended service. 
     BACKGROUND OF THE INVENTION 
     Towing astern in a marine environment is a towing mode in which the towing vessel is connected to its tow by a rope or wire that is stowed on a winch on the deck and terminates at a connection to the tow. Prior art tow pin assemblies are difficult to repair and result in wear and tear on the towing rope or wire. 
     There is a need for a tow pin assembly that facilitates ease of repair and that reduces wear and tear on a towing rope or wire. 
     SUMMARY OF THE INVENTION 
     The present invention is particularly intended for use on vessels that tow astern, and in particular, relates to a towing mechanism in which the towing vessel is connected to its tow by a rope or wire that is stowed on a winch on the deck and terminates at a connection to the tow. The tow pin and stern roller assembly of one example embodiment (referred to as the “tow pin assembly”) consist of a horizontal roller, multiple vertical rollers (tow pins) and a hook assembly that are supported by a steel structure (tow pin box). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a marine towing vessel, including one example embodiment of a marine vessel tow pin assembly. 
         FIG. 2  is a plan view of a marine tow vessel, including one example embodiment of a tow pin assembly, towing a towed marine vessel. 
         FIG. 3  shows a towed vessel in a marine environment. 
         FIG. 4A  shows a side view of one example embodiment of a tow pin assembly. 
         FIG. 4B  shows a plan view of one example embodiment of a tow pin assembly. 
         FIG. 4C  shows a front view of one example embodiment of a tow pin assembly. 
         FIG. 4D  shows a details of a portion of a tow pin assembly. 
         FIG. 5A  shows a side cross sectional view of an example embodiment of a set of tow pins. 
         FIG. 5B  shows a detailed view of an example embodiment of a tow pin. 
         FIG. 6A  shows a side cross sectional view of an example embodiment of an outer tube. 
         FIG. 6B  shows a plan view of an outer tube. 
         FIG. 7A  shows a side cross sectional view of an example embodiment of a pop-up pin and key. 
         FIG. 7B  shows a plan view of an example embodiment of a pop-up pin. 
         FIG. 7C  shows a front view of an example embodiment of a key cutout. 
         FIG. 7D  shows a side cross sectional view of an example embodiment of a key. 
         FIG. 7E  shows a front view of an example embodiment of a key. 
         FIG. 8  shows an example embodiment of a pop-up pin template showing guide key detail. 
         FIG. 9  is a plan view of one example embodiment of a roller bearing cage. 
         FIG. 10A  shows a side cross sectional detailed view of an example embodiment of a stern roller assembly. 
         FIG. 10B  shows a side cross sectional view of an example embodiment of a stern roller assembly. 
         FIG. 11A  shows a side cross sectional view of an example embodiment of a seal plate. 
         FIG. 11B  shows a top view of an example embodiment of a seal plate. 
         FIG. 11C  shows a corner detail of an example embodiment of a seal plate. 
         FIG. 12A  shows an exploded view of an example embodiment of a tow hook assembly. 
         FIG. 12B  shows an isometric view of an example embodiment of a tow hook assembly. 
         FIG. 12C  shows a side view of an example embodiment of a tow hook assembly arm. 
         FIG. 12D  shows a side view of an example embodiment of a tow hook assembly shaft. 
         FIG. 12E  shows a plan view of an example embodiment of a tow hook assembly shaft. 
         FIG. 12F  shows an end view of an example embodiment of a tow hook assembly shaft. 
         FIG. 12G  shows an end view of an example embodiment of a tow hook assembly jacking screw. 
         FIG. 12H  shows a side view of an example embodiment of a tow hook assembly jacking screw. 
         FIG. 12I  shows a side view of an example embodiment of a tow hook assembly large bushing. 
         FIG. 12J  shows an end view of an example embodiment of a tow hook assembly large bushing. 
         FIG. 12K  shows a side view of an example embodiment of a tow hook assembly small bushing. 
         FIG. 12L  shows an end view of an example embodiment of a tow hook assembly small bushing. 
         FIG. 12M  shows a plan view of an example embodiment of a tow hook assembly shaft end plate. 
         FIG. 12N  shows a plan view of an example embodiment of a tow hook assembly shaft hub. 
         FIG. 12O  shows a side cross sectional view of an example embodiment of a tow hook assembly shaft hub. 
         FIG. 12P  shows an siometric view of an example embodiment of a tow hook assembly shaft hub. 
         FIG. 12Q  shows a side view of an example embodiment of a tow hook assembly hook. 
         FIG. 12R  shows a cross sectional plan view of an example embodiment of a tow hook assembly hook. 
         FIG. 12S  shows an isometric view of an example embodiment of a tow hook assembly hook. 
         FIG. 12T  shows a plan view of an example embodiment of a tow hook assembly bottom plate. 
         FIG. 12U  shows a side view of an example embodiment of a tow hook assembly aft side plate. 
         FIG. 12V  shows a side view of an example embodiment of a tow hook assembly forward side plate. 
         FIG. 12W  shows a rear view of an example embodiment of a tow hook assembly end plate. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention will now be described with reference to the drawings. The present invention is particularly intended for use on vessels that tow astern, and in particular, relates to a towing mechanism  10 , also referred to as a tow pin and stern roller or tow pin assembly, in which the towing vessel  12  is connected to its towed vessel  14  by a rope or wire  16  that is stowed on a winch  18  on the deck  20  and terminates at a connection to the tow ( FIG. 1  and  FIG. 2 ). The tow pin and stern roller assembly  10  of one example embodiment (referred to as the “tow pin assembly”) consist of a horizontal roller  22 , multiple vertical rollers (tow pins)  24  and a hook assembly  26  that are supported by a steel structure (tow pin box)  28  ( FIG. 4A  and  FIG. 4B ). 
     The tow pin assembly  10  of the present invention may be welded in a manner that integrates the assembly into the supporting steel structure  30  of the towing vessel  12 . The tow pins  24  and hook  26  are raised and lowered as necessary by hydraulic rams mounted inside the tow pin box  28 . The tow pin assembly  10  must be strong enough to withstand the forces transferred through the tow wire  16  that result from external forces generated by the thrust of the vessel, the action of both the towed vessel  14  and towing vessel  12  in a seaway and the horizontal pressures generated when the towed vessel is not directly behind the towing vessel&#39;s centerline. The combination of these forces can exceed the breaking strength of the tow wire  16 . 
     The tow pin assembly  10  serves multiple functions: reduces tow wire wear; extends tow wire working life; and traps the tow wire  16  on the vessels stern which: shifts the towing vessels towing point aft; creates a safer work environment for crewmembers; and reduces the probability of the towing vessel gifting. Each of these features will be addressed in turn. 
     Reduction of tow wire wear: The tow pin assembly  10  reduces abrasion and wear on the tow wire  16  as it is being paid out, hauled in or laying in a static position during the voyage. Friction and acute bending angles can produce excessive and premature wire fatigue and will weaken the critical tow wire connection between the towing vessel and the vessel being towed. The stern roller  22  and tow pins  24  must be able to rotate under load in order to enable the tow wire  16  to roll over the bearing surfaces of the pins and roller rather than rub and abrade during these operations. The tow pins and rollers should be constructed in a manner that allow the tow pin sleeve and stern roller to rotate under a wide range of loads and speeds associated with towing operations. 
     Another source of abrasion is the horizontal and vertical movement of the tow wire  16  when the tow wire has been paid out to the desired length and the towing vessel is underway with its tow. While engaged in towing astern the towing vessel&#39;s stern will move vertically and horizontally due to the vessels yaw, pitch and roll actions in a seaway. The tow wire will move independently of the towing vessel&#39;s action and will abrade on the towing vessel contact surfaces unless restrained or provided with chafing gear. The tow pin assembly design of the present invention is intended to minimize abrasion from this movement. The space between the vertical tow pins is slightly more than the tow wires diameter minimizing horizontal movement. A tow hook  26  is designed to trap the tow wire and prevent abrasion from vertical movement. 
     Tow wire fatigue and subsequent weakening can be induced if the tow wire is bent at acute angles when under load. The tow pin and stern roller assembly fairlead the wire through bearing surfaces of a diameter sufficient to reduce wire fatigue due to sharp bends. 
     Extending Working Life of the Tow Wire: The tow pin assembly  10  lengthens the working life of the tow wire  16  by reducing abrasion and wire fatigue due to excess bending. The useful life of a tow wire averages 15,000 working hours, or several years, depending on the towing application. Acute bending angles or abrasive conditions can seriously damage the tow wire in a matter of hours. Tow wires must be continuous and cannot be spliced. If the tow wire is damaged it is either trimmed back in order to remove the damaged section or discarded completely. The tow pin and stern roller assembly  10  of the present invention help prevent premature damage and failure of the tow wire  16 . 
     Trapping the tow wire: An additional purpose of the tow pin assembly  10  is to hold the tow wire  16  in a fixed position on the towing vessel&#39;s stern. Trapping the tow wire at a location on the stern of the towing vessel makes the operation of towing astern safer. The tow pin assembly, when functioning correctly, traps the tow wire and shifts the towing point to the stern. The towing point is the last physical point on the tug that fairleads the tow wire from the towing vessel to the vessel being towed. A towing point on the stern has several benefits. 
     Safety of the crew: The safety of the crew is facilitated by preventing tow wire movement while crewmen are working on the aft deck. Crewman are called to work on the aft deck during towing operations to make tow, break tow and conduct regular inspection and maintenance of the aft deck area. The tow pin assembly  10  of the present invention traps the tow wire  16 , minimizing its movement between the towing vessel&#39;s tow winch and stern which helps prevent crewman from being struck by unexpected movements of the tow wire. 
     Girting: Girting is a term used to describe the scenario in which the strain on the tow wire causes the towing vessel  12  to capsize. Factors that contribute to girting are location of the towing vessel&#39;s towing point, heeling angle, hull resistance, propulsion and steering forces, and the direction and force of the towline. In simple terms a towline strain of sufficient force can overcome the towing vessel&#39;s inherent stability and cause the towing vessel to capsize. A common cause of this event is when the towing point is located near amidships on the towing vessel (e.g., at the tow winch) and an unexpectedly high towline strain occurs off to one side or the other. 
     The tow pin assembly  10  reduces the likelihood of this catastrophic event by shifting the towing point to a low point at the vessels stern. If a girting situation were to develop the force of the towline will tend to turn the towing vessel  12  in line with the strain rather than pull it over sideways. This feature is critical to vessel and crew safety in towing astern operations. 
     The tow pin assembly  10  is a critical piece of equipment in towing astern operations. It should be constructed in a manner that withstands the high dynamic loads, constant exposure to salt water, sea spray ( FIG. 3 ) and has a high degree of reliability. It should be constructed in a manner that facilitates inspection, refurbishment and renewal of components. 
     The present invention provides an improved tow pin assembly  10  and a process for manufacturing same that overcomes the disadvantages of prior art. The present invention is constructed in a manner in which each major component is an independent cartridge assembly that can be inspected, serviced and repaired without the extensive disassembly and remanufacturing required of prior art. In addition, the present invention uses a unique design and construction materials that improve the strength, reliability and longevity of the present invention compared to prior art. 
       FIGS. 4A-D  show one example embodiment of a tow pin assembly  10 .  FIG. 4A  shows a side view of the tow pin assembly with the vertical pins  24  and the hook assembly  26  retracted downwardly into the tow box  28 .  FIG. 4B  shows a plan view of the tow box  28 .  FIG. 4 c    shows a front view of the tow box  28  with the vertical rollers  24  and the hook assembly  26  retracted downwardly into the tow box  28 .  FIG. 4D  shows a detail of the area  4  of  FIG. 4C . The detail shown in  FIG. 4D  shows roller  22  extending upwardly above a top plate  32  of tow box  28  and inwardly of a side plate  34  of tow box  28 . 
       FIGS. 5A and 5B  show one example embodiment of tow pins  24  (vertical rollers  24 ), namely 12 inch (12″) diameter tow pins, of the present invention.  FIG. 5A  shows a single tow pin assembly  24 . There are two additional, identical tow pins  24  in the tow pin assembly  10 . Each tow pin  24  consists of a pop-up pin  35  that is raised and lowered by a hydraulic cylinder  36  (shown schematically). A steel roller sleeve or roller  42  is inserted over the pop-up pin and provides the bearing surface for the tow wire. Each tow pin includes a roller top plate  38 , that is threaded onto the hydraulic lifting ram, caps the roller  42  and is secured by ½″ stainless recessed bolts, such as fastener  40 . that connects to the pop-up pin  35 . A top plate  44  secures the hydraulic cylinder  36  to the three cylinder mount rods  46  secured to the bottom plate  48 . The bottom of the roller  42  rests on ball bearings  50  in a cage  52  and a wear ring  54 . A stainless bottom plate  48  serves as the foundation of the tow pin and a receiver for the outer tube  56 . 
     In the present invention, the top plate  38  and threaded connecting rods  46  are manufactured of stainless steel for durability, strength, corrosion resistance, and ease of disassembly for refurbishment or renewal. The prior art does not have a top plate or threaded rods but uses a clevis pin arrangement on the bottom end and the top is threaded into a steel receiver. The prior art structures are more exposed to corrosive effects and become more difficult to access and disassemble for refurbishment or renewal. The roller top plate  38  is manufactured of stainless steel. The prior art uses a roller top plate fabricated from mild steel which is less resistant to corrosion. The bottom of the roller  42  rests on 5/16″ stainless ball bearings  50  contained in a bronze cage  52 . This provides an extra bearing surface, not found in prior art, that enhances the ability of the tow pin to rotate and reduces wear on the bottom end of the roller. This extends the life of the tow pin assembly. In addition, the bearing cage  52  is supported underneath by a stainless wear ring  54  that provides an expendable wear surface. When excessive wear is evident the pop pin no longer maintains a true vertical position and wobbles when rotating. The present invention allows easy removal of the pop-up pin assembly from the tow pin assembly  10  and simple replacement of the stainless wear ring rather than the extensive disassembly and repair required by prior art. The prior art does not provide a mechanical bearing surface for the bottom of the roller. The bottom of the roller is a steel on steel interface. The bottom of the prior art roller is prone to galling causing a buildup of material using up all the clearance resulting in the roller not turning freely. In prior art devices, when the bottom becomes worn the whole roller must be replaced. During use of the present, one only has to replace the stainless wear ring  54  and bearings  50 . 
     The hydraulic cylinder  58  that lifts and retracts the pop-up pin is secured in place on the bottom by a locating socket in the bottom of the cylinder, three stainless steel threaded rods  46  with stainless nuts and a stainless steel top plate  44 . This allows access to the hydraulic ram assembly from the top and facilitates ease of manufacturing and repair. If the component needs refurbishment or renewal the roller top plate is unbolted and removed, the three stainless steel nuts on the threaded rods removed and the hydraulic cylinder pulled out from the top. The prior art does not use this design for securing the hydraulic ram. The prior art uses a mild steel clevis and pin arrangement to secure the bottom of the hydraulic cylinder and does not have threaded rods to provide vertical support. The mild steel clevis and pin is subject to corrosion and seizing. In order to remove the prior art hydraulic ram, the prior art requires disassembly of the roller and pop-up pin in order to access the pin and then subsequent heating with a torch to remove rust and drive the pin out. This prior art repair process usually results in consequential damage to other components adding to the time, cost and scope of repair. 
     The present invention utilizes a stainless bottom plate  48  as the foundation for the pop pin and roller assembly and serves as the receiver for the outer tube  56 . The outer tube is welded at the top to the tow pin box structure. Access and removal of the outer tube for refurbishment or renewal is from the top. The pop-up pin assembly is removed and the weld between the outer tube and tow pin box are carbon arched allowing removal of the outer tube from the top. The prior art design does not have an outer tube. Prior art structures create a tube and foundation for the pop-up pin by using the tow pin box structure. Prior art design do not accommodate refurbishment or renewal of the pop pin tube or foundation without reconstruction of the tow pin box structure. 
     In the present invention, each pop-pin is equipped with one bronze roller bearing  60  that is full length of the roller and functions as the load bearing surface for the roller. The bearing is shrunk fit to the pop-up pin and greased via grease fittings located on the top of the roller top plate  38  and ⅛″ diameter grease channels. The prior art has two bronze bushings, an upper and lower, but none in the middle. The prior art lubricates the roller bushings by grease fittings threaded into recessed holes in the body of the roller. The prior art design grease fitting is susceptible to damage as its location on the roller is an area exposed to bearing of the tow wire and excessive wear. The prior art bushing arrangement produces an “hour glass” effect on the roller with heavy loading in the middle of the roller. Water intrusion is also common in the cavity between the upper and lower bushing. This produces corrosive effect over time and as the bushings wear, contact between the inner wall of the roller and the out wall of the pop-up pin restrict or stop the roller from turning. 
     In the present invention, the bearing surface between the pop-up pin and the outer tube is lubricated through two ¼″ stainless steel tubes that run down the inner wall of the pop-up pin 180 degrees apart. This distributes lubrication over the whole sliding surface. The prior art uses only one lubrication point and does not distribute lubrication over the whole slide surface. 
     These design and material features of the present invention utilize a “cartridge” design principle so that the pop-up pin, roller and hydraulic cylinder components can be easily manufactured and accessed for refurbishment or renewal. The prior art design does not incorporate a “cartridge” design principle. Access to the pop-up pin, roller and hydraulic cylinder components of the prior art requires extensive and time consuming disassembly and may damage or destroy surrounding unaffected components or structural members. 
       FIG. 6A  is side cross sectional view of the outer tube  56  illustrating the slotted key way  62  that guides the pop up pin  24  ( FIG. 5A ) vertically up and down.  FIG. 6B  is a top view of the outer tube  56 . 
       FIG. 7A  is cross sectional side view of the pop-up pin  24  and the key  64 , received within slot  66  of pop-up pin  24 , that rides up and down in the key way  62  of the outer tube  56  ( FIG. 6A ). FIB.  7 B is a top view of the pop-up pin  24 . The pop-up pin  24  is equipped with a guide key  64  to track the pin movement in the outer tube key way  62 . The present invention guide key pin is manufactured of stainless steel. The prior art utilizes mild steel for a guide pin. 
       FIG. 8  is a plan view of the top of the pop-up pin illustrating the detail of the keyway  62 . The present invention uses a 1¼″ wide by ¾″ deep keyway  64 . This adds to the reliability of the tow pin assembly and reduces the probability of the pop-up pin rotating. If the pop-up pin  24  rotates due to wear of the guide key there is a high probability that hydraulic lines will be severed and render the pin inoperable. The prior art utilizes a guide key depth of ¼″ and has less tolerance for wear and thus reduced reliability in comparison to the present invention. 
       FIG. 9  shows a plan view of the roller bearing cage  52 . The present invention uses stainless steel ball bearings  50  mounted in a bronze bearing cage  52  to bear the weight of the roller bottom as it rests on the pop-up pin. Sixteen stainless steel bearings  50  are mounted in a bronze steel bearing cage  52 . The bearing pattern is staggered, i.e., the bearings are positioned such that they do not all move in the same path, to increase durability and reduce the friction of the rotating surfaces. In the prior art the bottom of the roller rests on the pop up pin with no mechanical bearing surface. This generates more friction while the roller is turning. 
       FIG. 10A  shows a side cross sectional view of an end region of horizontal roller assembly  68  that is inset into the towing vessel&#39;s bulwarks.  FIG. 10B  shows the entire stern roller  22 . The horizontal or stern roller  22  of assembly  68  is subject to the downward force of the tow wire  16  ( FIG. 1 ) generated by the weight of the tow wire, and external forces such as the thrust of the vessel and the action of both the towed vessel and towing vessel in a seaway. It must be of sufficient diameter  70 , such as 20″ diameter in the example embodiment shown, to minimize bending stress on the tow wire  16  and to be able to rotate under load. 
     The stern roller  22  is constructed of a mild steel roller  22  and supported on either end by an axle shaft (roller shaft)  72  and self-aligning bearings  74 . The self aligned bearings  74  are inset in the bearing bore  76  and the seal plate assembly  78  retains it in the roller. A ⅜″ back seal plate  80  is welded to the backside of bearing insert  76 . Once the bearing  74  is inserted, outside seal plate  78  is fastened with stainless fasteners to the bearing bore insert  76 . This maximizes the self-aligning performance of the bearings  74 . The bearing insert  76  is machined so that an internal cavity  82  is created between the back seal plate  80  and the bearing  74 . When grease is applied through the external zerk fitting  84  it fills the internal cavity  82  first, passes through the bearing structure and is forced out the outside seal plate  78 , which may be referred to as a front seal plate  78 , preventing salt water intrusion into the bearings  74 . A seal  86  also acts to retain grease within internal cavity  82 . 
     Accordingly, the present invention uses self aligning bearings  74 . The stern roller assembly  68  can be subject to heavy contact with the towed vessel due to human error. The self aligning bearings can accommodate more degree of misalignment than prior art and thus are more durable. An axle shaft on each side of the stern roller is inserted through the side frame of the tow pin box and retained in the stainless  1 ″ register. This prevents shear loading on the retaining bolts. The prior art uses hat bushing pressed in place and then the roller shaft is secured by a bolted bearing cap. Heating and cooling during the manufacturing process makes prior art devices susceptible to misalignment during fabrication. Heavy contact with the towed vessel can also cause the roller to become misaligned in prior art devices. The prior art has little tolerance for misalignment and its ability to rotate freely and function properly will either be restricted or eliminated. 
     An axle shaft  72  of stainless steel is shrunk fit to a flange plate  88  and then bolted to the register retaining plate (shaft doubler)  90  with stainless fasteners. The register retaining plate (shaft doubler)  90  is welded to the side plate of the tow pin box  28 . The advantage of the present invention is that the stern roller can be easily removed by unbolting the flange plate  88 , removing the axles and lifting the roller clear of the tow pin box  28 . The prior art does not use flange plates but a half-bearing cap principle in which the axle is retained on the lower side by a built-in bearing cap receiver and on the upper side by a half-bearing cap that is secured by steel socket bolts. The disadvantage of the prior art is that the bolts are subject to shear loads and can be easily distorted by roller contact. Removal of the stern roller in the prior art is more difficult and in practice the bolts must be burned off. In addition, the bolts are exposed to damage from the tow wire riding over the top of them. 
     In the present invention, lubrication to the bearings is through a ⅛″ diameter channel  92  rifled through the center of the roller shaft  72 . Grease is applied through exterior zerk fitting  84  and fills the inner cavity  82  forcing grease out the retaining seal  86 . This prevents salt water intrusion into the bearing  74 . The prior art utilizes a bronze hat bushing and is lubricated through a zerk fitting inset into the roller. The zerk fitting of the prior art is inset in an area that the tow wire runs over and is subject to damage. The prior art bushing does not allow the same freedom of rotation as the self-aligning bearings of the present invention and cannot accommodate as much impact on the stern roller as the design of the present invention. 
     The present invention has a register retaining plate (shaft doubler)  90  that receives the bolts  94  securing the axle shaft/flange assembly to the pin box  28 . The register retaining plate  88  absorbs shear loads rather than the flange mounting bolts. Bearing cap bolts utilized by the prior art are exposed to damage or excessive wear when the tow wire or heavy chain comes over the stern roller with either the pins down or has “jumped” the pins and lays outboard of the tow pins. 
     In the present invention, the gap between the roller edge  96  and the pin box structure  28  is a distance  98  of ½″. The present invention creates a smaller gap that reduces the potential for wear on the tow wire. The gap between the stern roller and the cap rail in the prior art is 4-6″ in order to accommodate the bearing cap and bolts. This gap is of the prior art is a sufficient width to allow the tow wire to fall in and become damaged. 
     In the present invention, the bearing insert  76  is machined and heat shrunk fit. Over time the exterior wall of the roller tube  22  is subject to heavy wear in scattered locations of high use. The advantage of the present invention is that when the roller tube requires refurbishment the roller assembly  68  can be removed, the bearing insert  76  retained and re-used while the roller tube is thin walled machined and installed in a pre-machined tube. In the prior art when the exterior wall of a roller tube becomes worn the roller assembly including the axles must be removed and replaced. 
       FIG. 11A  shows a side cross sectional view of a seal plate  78 , manufactured of stainless steel, drilled and tapped ½″×13 in two places to receive jacking bolts to facilitate easy removal of the seal plate.  FIG. 11B  shows a top view of the seal plate  78 .  FIG. 11C  is a detail of the region  11  shown in  FIG. 11A . The seal plate  78  and bearing bore  76  ( FIG. 10A ) are of dissimilar metals and may be subject to bonding. The pre-drilled and tapped jacking bolt bores  100  facilitate ease of removal of the seal plate  78  from roller  22  for bearing inspection, refurbishment or renewal. The present invention includes two drill and tapped jacking bores  100  to receive jacking bolts. The prior art does not use a roller tube cartridge assembly on the roller ends. 
       FIG. 12A  shows an exploded view of a tow hook assembly  26 . The tow hook assembly  26  is the component that restricts the tow wire&#39;s  16  ( FIG. 1 ) vertical movement. It must have the structural integrity and design principles to withstand the same dynamic loading and salt water exposure that the affects the tow pins and roller. The hook assembly  26  is subject to heavy horizontal and vertical loads. The hook  102  is retracted (in this embodiment, retracted means hook  102  is lowered into tow box  28 ) when the towing operation requires an unobstructed horizontal movement of the tow wire and is raised out of tow box  28  when the towing operation requires trapping the wire  16  from movement. When the hook  102  is fully raised the backside rests against the hook box  104  adding additional strength to the assembly. The present invention utilizes the same cartridge principle for the tow hook as applied to the tow pins and roller. 
     The tow hook assembly  26  consists of a steel fabricated hook  102  mounted in a steel fabricated box  104 . The present invention uses a tapered shaft  106  that defines a shaft axis  107  about which the shaft rotates. The shaft  106  is double keyed to two keys  108  and  110  for structural strength when inserted in the hook. The shaft  106  is tapered on one end and mounted in the box by a small bushing  112  on one end and a large bushing  114  on the other and an end plate  115 . The tow hook  102  is rotated up and down by the action of the arm  116  bolted to the hub  118  which is subsequently pressed onto the tapered end  120  of the shaft. The throw of the arm can be precisely adjusted during manufacturing due to the tapered fit of the hub on the shaft. The arm is moved up and down by a hydraulic cylinder  122  (shown schematically) mounted inside the tow pin box. Hook box  104  includes side plate  124 ,  126 ,  128  and  130  ( FIGS. 12T, 12U, 12V and 12W ). 
     The prior art does not utilize a cartridge principle. The prior art box is integral to the pin box, the shaft is keyed on one side only, bushings are of equal diameter and the arm/hub assembly is welded to the shaft. Once the prior art tow hook assembly is installed, the entire assembly must be cut out of the pin box to service the tow hook components. The present invention&#39;s tow hook assembly  26  is a self-contained component of the tow pin assembly and creates an easier and more precise manufacturing process and allows ease of removal for inspection and refurbishment. During manufacturing the box  104  is welded into the tow pin box  28  and can be adjusted to accommodate different vertical wire fleeting angles (the angle created as a result of the tow winch height and distance from the tow pin assembly). 
     A steel fabricated box  104  is welded into pin box  28  ( FIG. 4B ) after tow pin box  28  is installed in vessel. The advantage is that the hook height can be adjusted by moving the hook box  104  up or down prior to final welding in place within tow pin box  28 . This facilitates the performance of the hook  102  by customizing the design to accommodate different wire vertical fleeting angles and to allow ease of the tow wire entry into the hook during use. The prior art does not use a fabricated box component. The prior art tow hook box is integral to the tow pin box and its position cannot be adjusted during installation. 
     The tow hook shaft  106  of the present invention includes two key slots  109  and  111  so as to receive two keys  108 ,  110 , in order to increase its structural strength. The hook  102  is subject to heavy horizontal loads in the raised position and the two keys prevent the hook from rotating on the shaft when in a fixed position. The prior art shaft is single keyed with half the structural connection as the present art. It is less durable and more subject to wear allowing the hook to rotate on the shaft. 
     The tow hook shaft  106  is of different diameters on either end. The larger end is fitted with a machined taper to accept a larger bushing  114  and the pressed on hub  118 . The opposite end is of a smaller diameter to accept the smaller bushing  112  and the keys  108  and  110 . The advantage of the present invention is that is that the key  108 ,  110 , can be easily removed through the larger bushing side and the shaft, hook and bushings can be removed without altering the pin box structure. Any of the hook components can be replaced without damaging the hook box  104  and the hook  102  can be removed and new bushings inserted into the hook receiver. The prior art has equal diameters on its shaft and once exposed to a salt water environment, the shaft cannot be removed due salt water corrosion. Instead the entire tow pin assembly of the prior art must be cut out of the pin box and must be scraped and replaced with a new assembly. 
     In summary, the tow pin assembly  10  is subject to heavy use in extreme environmental conditions. Components of the tow pin assembly  10  are subject to wear and require refurbishment or renewal at different times during the life of the assembly. The prior art uses materials and a design which make the tow pin assembly more subject to corrosive processes and require extensive disassembly to replace critical components. The disassembly process of the prior art regularly includes the destruction of unaffected surrounding components and structure in order to access and remove the worn component. 
     The present invention utilizes a cartridge principle in the design and manufacturing of a tow pin assembly  10  that is not found in the prior art. The major components of the tow pin assembly, including tow pins  24  in a self contained tow pin cartridge  56 , stern roller  22  in a self contained horizontal roller cartridge  76 , and tow hook  102  in a self contained tow hook cartridge  104 , are designed and manufactured as a cartridge independent of the other components and the supporting structure of the tow pin box  28 . The present invention uses a “cartridge” design principle which enables individual components to be removed for refurbishment or renewal without the extensive disassembly required of the prior art. Components requiring repair or renewal can be removed and installed without damaging or disassembling the other components. 
     In addition the present invention uses ball bearings, stainless steel rather than mild steel as used by prior art, for critical components in order to reduce corrosive processes and facilitate ease of assembly and disassembly and provide superior performance and longevity over prior art. Prior art life expectancy is 5-7 years and requires complete replacement of the tow pin assembly. The present invention allows replacement of components and has a life expectancy of 10-15 years.