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Page Title: Precision Anchoring, Continued
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Precision  Anchoring,  Continued Terms After the anchorage position has been determined, the navigator is ready to Associated  With begin plotting the anchorage. In so doing, reference is often made to the Anchoring following  terms: Term Definition Approach  track This is the track along which the ship must proceed in order to arrive at the center of the anchorage. Its length will vary from 2,000 yards or more for a large ship to 1,000 yards for a ship the size of a Navy destroyer or smaller.  Under  most  circumstances,  it  should  never  be shorter than 1,000 yards. Head bearing If at all possible, the navigator selects an approach track such that a charted NAVAID will lie directly on the approach track if it were extended up to the aid selected. The bearing to the aid thus described is termed the head bearing; it should remain constant if the ship is on track during the approach. Letting-go circle This is a circle drawn around the intended position of the anchor at the center of the berth, with a radius equal to the horizontal distance from the hawsepipe to the pelorus. Letting-go bearing Sometimes referred to as the drop bearing, this is a predetermined bearing drawn from the intersection of the letting-go circle with the approach track to a convenient landmark or NAVAID, generally selected near the beam. Range circles These are preplotted semicircles of varying radii centered on the center of the anchorage, drawn so that the areas are centered on the approach track. Each is labeled with the distance from that arc to the letting-go circle. Swing circle This is a circle centered at the position of the anchor, with a radius equal to the sum of the ship’s length plus the length of chain let out. Drag circle This is a circle centered at the final calculated position of the anchor, with a radius equal to the sum of the hawsepipe to pelorus distance and the final length of chain let  out.  All  subsequent  fixes  should  fall  within  the  limits of the drag circle. Note:  The actual radii of both the swing and drag circles will in reality be less than the values used by the navigator in plotting them on the chart, because the catenary of the chain from the hawsepipe to the bottom is disregarded. Thus, a built-in safety factor is always included in the navigator’s  plot. 12-15

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