Figure 1
wind
B
D.R. 1400
ing of earlier LOPs depends on accurate esti-
mates of current and leeway.
C
Taking Visual Bearings
y
lee
w
a
D E.P 1400
Because of the difficulty of using a steer-
ing compass for taking visual bearings, it is
E
fix 1400
A
very useful to have a hand-bearing compass
fix 1300 on board. Because a hand-bearing compass
Figure 1 illustrates the different kinds of
moves around the boat with you, it is impos-
positions. If point A is the last reliable fix
sible to account for deviation in the same
(obtained at, say, 1300 hours), and line AB is
manner as your fixed compass. In general,
the course steered and the distance traveled
if you take prudent measures to ensure that
based on the speedometer for one hour; then
you’re not holding the compass near any mag-
B is the DR position for 1400 hours—i.e.,
netic material, any deviation should be minor.
the position the vessel would be in if there
If you are concerned about possible devia-
were no leeway, no current, the course had
tion, you can check it by choosing a series
been accurately steered, and the distance run
of ranges that can be plotted on a chart, and
through the water accurately indicated.
checking them against your hand-bearing
If the vessel had been making leeway (angle
compass when you are on the range. Move
BAC) due to a northerly wind, but was not
around the boat, taking the same bearing
affected by current, then C would be the esti-
from several different positions and see if it
mated position at 1400 hours. Note that the
changes appreciably. Remember that if you
distance AB and AC are the same.
take bearings to particularly close objects, the
If, however, a current setting at, say, 200° T
distance between the bow and stern of your
at 3 knots were encountered, then you’d lay
boat could be enough to change that bearing. t
ing
offf the set and drift for one hour (200° - 3
A few other factors to keep in mind when
ilo
miles) on the chart from point C — i.e., CD,
choosing targets for visual bearings:
and point D is now the EP for 1400 hours.
• LOPs based on directions to fixed objects
Should a reliable fix, E, be obtained at 1400
ashore or to beacons fixed to the sea bot-
hours, you could determine the current actu-
tom are more reliable than those taken
o
astal P
ally experienced during the last hour by mea-
from floating buoys.
C
suring the direction and distance from C to E
• Bearings to objects on the beam and
(not drawn in figure). The track made good
nearby a moving vessel change more rap-
over the ground would be the straight line AE.
idly than objects ahead, astern or more
Establishing a Fix
distant.
ion
• A single bearing to an object near your
Sailing in sight of land, you can fix your
at
beam can give you an accurate reference
position by taking a compass bearing to an
ig
point when making a turn (a turning
object noted on the chart and plotting a line
bearing) and when determining if your
a
v
back from the object to where you ought
N
hook has set or is dragging when anchor-
to be. This is a line of position (or LOP);
ing (an anchor bearing).
you know you are somewhere along that
There are other occasions when a single
line. The intersection of two or more lines
position line may prove useful. For example,
of position (LOPs) marks a fix. You should
you can use: a) an LOP parallel or nearly
use more than two LOPs whenever possible
parallel to your intended track to determine
because it’s difficult to gauge the accuracy of
whether or not your vessel is on track; b) a
the lines when only employing a pair. Try to
position line that crosses your track at or
obtain the LOPs as nearly simultaneously as
near 90° to check on speed over the ground;
possible. If there’s a significant time interval
or, c) a single line tangent to a hazardous area
between the observations you can get a so-
(a danger bearing) to keep on the safe side.
called “running fix” by advancing your plots
• Bearings to ranges (objects in line) can
of earlier LOPs up to the time of the last one.
be very accurate and require no compass
A simultaneous fix is usually more reliable
correction. You can use any two objects,
than a running fix because accurate advanc-
however dissimilar, whose locations are
837