UNITED24 - Make a charitable donation in support of Ukraine!


Appendix D

Wire And Nylon Towlines

A towline carries most of the load in a tug-and-tow operation. This appendix describes the use, maintenance, precautions, and life expectancy of wire and nylon towlines.



D-1. Wire rope towline is very expensive. It is important to keep it in excellent condition, protect it against excessive wear, and inspect it regularly.



D-2. Wire rope, like a machine, is made up of many moving parts. The steel fibers slide independently and must be protected by adequate lubrication against the effects of stress. Corrosion damage is also a danger. There is no way of estimating the exact loss of strength resulting from corrosion.


D-3. During manufacture, wire rope is thoroughly lubricated. However, this lubrication is lost during operation and must be renewed periodically and after each use. Use relatively fluid oil of light viscosity (such as linseed oil). If the oil is stiff, it must be heated. Apply the oil so that it penetrates through all the strands to the core. Apply the oil by swabbing, using a brush, or by pouring the oil on the rope.


D-4. If wire rope is to be stored for some time, lubricate it with a heavy oil (such as crude petroleum) to which a small amount of graphite has been added.



D-5. Ideally, only one layer of wire rope should be wound on a drum. Where this is impractical, the rope must be carefully wound so that each layer spools evenly over the preceding layer. The drum must be the correct size for the rope size.



D-6. A towline can be reversed occasionally to redistribute the wear, or one end can be cut. These are complicated operations because of the construction of wire rope. They must be done in a shipyard.



D-7. Suddenly applying a load to wire rope by rapid acceleration causes stress much greater than the weight or resistance of the tow. Avoid such strain on the rope by gradual acceleration.



D-8. When not in use, stow wire rope in a dry place where there is no acid. Keep the outer layer lubricated.



D-9. New wire rope is broken in by using it first with a light load or no load. A towline may be streamed aft before connecting to the tow. This procedure ensures a straight fairlead.



D-10. Inspect the rope thoroughly as it is being wound after each use. Look for broken strands in the outer layer. When one-half the diameter of the fibers of the outer layer is worn away, the rope should be replaced. Worn wire rope is an indication of chafing. The cause of the chafing should be investigated. Ensure that the rope is properly lubricated and stored after use and inspection.


Note: Wire rope must not be thrown away; it is extremely expensive and in port can be cut into usable lengths for other purposes.



D-11. After using a wire rope towline in seawater, wash it with freshwater and lubricate it with linseed oil. If the rope is to be submerged for a long time, saturate it with a heated preservative made of one part Stockholm tar to one part fresh slaked line, boiled.



D-12. It is extremely important that wire rope be used and maintained properly because of the great expense involved. It should not be subjected to any of the following common abuses:

  • Chafing.
  • Rope of incorrect size.
  • Drum of inadequate size.
  • Improper winding on drum.
  • Improper or insufficient lubrication.
  • Exposure to acid fumes.
  • Lack of protection against moisture and saltwater.
  • Kinks.
  • Sudden acceleration.
  • Touching the seabed.

Note: Simple shipboard action can prevent damage to wire rope. Adequate use of chafing gear, puddings, and fairleads; and timely change of nip will greatly increase the life of wire rope.



D-13. The tensile strength of nylon line is approximately twice that of manila line of equal size. It will stretch under strain and return to its normal length. Nylon line resists rot and mildew. If properly maintained, it should last five times longer as manila line.



D-14. Chafing gear should always be used at points of abrasion where there are sharp metal edges. If possible, ensure that all holding devices have smooth surfaces. If the yarn becomes abraded, cut away the chafed section and splice the ends.


D-15. Glazed areas may appear where the line has worked against bitts or chocks. These areas do not affect the strength of the line. When reeling in, ensure that thimble and connecting links do not chafe or cut hawers.



D-16. If nylon becomes oily or greasy, scrub it with freshwater and a paste-like mixture of granulated soap. For heavy accumulation of oil or grease, soak the line overnight in a 5 percent solution of a high alkaline detergent and freshwater (about 1 cup per gallon of water). Then scrub as directed above.



D-17. Nylon line can stand repeated stretching. It thins out when under load and returns to normal if the stretch does not exceed 40 percent. Beyond that, the line may part. When the stretch becomes excessive, double the lines by passing the bight. This reduces the danger of snapback.



D-18. New cable-laid nylon hawers may be stiff. Tension the cables at 30 percent extension for 20 minutes.


D-19. When new lines are strained, they produce a sharp crackling sound. This is the result of readjustment of the line strands to stretching and should not be alarming.


D-20. To uncoil new line, unreel it as you would wire rope. Do not pull the end through the eye of the coil.



D-21. When not using a powered reel, coil the line in a different direction each time. This prevents unbalancing the lay.



D-22. Stow nylon rope away from strong sunlight, heat, and strong chemicals. Cover the rope with tarpaulins. If the line is iced over, thaw it carefully and drain it before stowing.



D-23. When using sets of lines in parallel (for example, purchases), use nylon line with nylon line only, not with wire or manila. Other kinds of line must not be used on the same chock or bitt with nylon. Always use a nylon stopper with nylon towlines under load.



D-24. Plain-laid nylon hawsers may elongate around the bitts under load. Minimize by taking a turn under the horn and crossing the line on itself before taking more turns.


D-25. Sudden surges may occur when nylon hawsers are used on capstans for heavy towing or impact loading. Take six turns on the capstan. The standing part of the line is led to a set of bitts and figure eighted. This provides a back up and is a precaution against surges.



D-26. Make up forward and backing towlines as closely as possible. Take up slack in the relaxed line while the other line is under load. It may be necessary to reverse the tug's engines slightly when easing pull. This counteracts the elasticity of the nylon and prevents snapback.



D-27. Snapback may occur after a 50 percent stretching. Ensure that personnel are away from the direct line of pull when applying heavy loads.


D-28. Nylon line around bitts may slip when eased out under heavy load because its coefficient of friction is below that of manila. This may cause injury to personnel who have not been thoroughly instructed in the peculiarities of nylon line. Take two or three turns on the bitt before figure eighting the line; this provides closer control. Stand well clear of the bitts.


Join the GlobalSecurity.org mailing list