SPRING 2006 • PAGE 11
NAUTICAL CHALLENGE
Distance off - Knowing where you’re going by Tom Tursi
A
s we sail in coastal areas, it is often
necessary to determine how close
our present course will carry us to a
prominent headland or other dangerous
feature and whether we should alter course
for an added margin of safety. This
determination can be made by combining
Running Fix techniques with Relative
Bearings, and is called Distance-Off. This
method requires that you hold a steady
boat course for a period of time while the
change in relative bearing to the headland
is measured, and the following rule
applies:
Doubling the Relative Bearing to an object
while holding a steady course will produce a
Distance Off equal to the Distance Run
between the times of the two bearings.
Example:
As shown in the figure, you’re sailing on a
course of 81 degrees per ship’s compass and
note a headland off your port bow to
leeward.
You sight the headland across your pelorus
and read the relative bearing to the headland
as 36 degrees off your port bow; at that
Distance off a headland can be computed based on doubling the bow angle.
moment your distance log reads 167.3 NM.
When the relative bearing to the headland doubles to 72
An opposing current will increase the distance measured by
degrees you read the log distance as 168.7 NM. The
your log between the two relative bearings and therefore
calculated distance off the headland at that moment is
increase your estimate of the distance off. Cross currents or
the difference in the two log distances, or 168.7 - 167.3
cross winds will push you either toward or away from the point
= 1.4 NM.
of land.
Factors affecting the steering accuracy or distances such
Question: You are sailing on a course of 327ºM and observe
as current or wind leeway will affect the accuracy of this
a point of land at 40 degrees off your starboard bow; your
calculation, as follows:
log reads 47.2 NM. A half hour later you note that this point
is 80 degrees off your starboard bow and your log reads
A following current will decrease the distance measured
50.4 NM. How far away is the point of land?
by your log between the two relative bearings and
therefore reduce your estimate of the distance off.
Answer: 3.2 NM.
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