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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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