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Amount of Anchor Cable to Use

A term used here is scope. The length of cable laid out, measured from the hawse pipe to the anchor, divided by the distance measured vertically from the hawse pipe to the se-bed, is called the scope of cable .the scope used depends upon several factors:

  1. The nature of the holding ground .Stiff clay, rock, shells, and stones are considered poor holding ground. Very soft mud can be a poor material in this respect.
  2. The amount of swinging-room available for the ship as the wind or stream changes in direction.
  3. The degree of exposure to bad weather at the anchorage.
  4. The strength of the wind or stream. As this strength increases so the ship moves astern, lifting her cable off the bottom so that it assumes long stay.
  5. The duration of stay at anchor.
  6. The type of anchor and cable.

If the cable leads from the anchor shackle in a direction 5 degrees above the shank axis the holding power of the anchor is reduced by one-quarter. If the angle becomes 15 degrees the loss of holding power is one-half. (This fact is repeated in chapter VII in view of the text contained therein) For this reason, it is most important that a length of cable shall lead from the anchor shackle along the sea-bed before rising gently to the hawse pipe. Only a good scope will ensure this. Very often, when a ship dargs her anchor, more cable is veered and the anchor holds. The action is dragging because the angle between the cable and shank axis, at the shackle, was more than zero. The veering of cable removes this angle and the anchor holds once more.

A rough rule to lay out three to eight times the depth of water in cable length is haphazard. The admiralty recommend the following lengths, which should be regarded as the minimum for calm weather and a 5-knot stream:

For wrought iron cable, lay out 25√D of cable.

For forged steel cable, lay out 25√D of cable.

For special-steel cable, lay out 25√D of cable.
(Where D is the depth of water in metres.)

It should be observed that more cable is laid in the case of the stronger chain. This represents a disadvantage of the special-steel in that it is roughly 12 ½ % lighter than wrought-iron cable, and therefore lifts from the sea-bed more easily. A heavy bight of cable must be used so that the cable partly lies on the sea-bed and its catenary, or curve, yawing. The holding power of an anchor, i.e. the types sketched in this chapter can vary from between three and fourteen times its own weight. The resistance offered by cable is only about three-quarters of its weight, and there is thus no point in laying out more cable than is necessary. Further the cable imparts a drag to the anchor, quite apart from the drag of the ship. Recent research has shown that a twin- screw ship, anchored in a 4-knot stream and a 55-knot wind, with locked propellers, imparts the following drag to her anchor.

 

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