Page 7
Copyright Wet Paper Publications 2014
Swinging the compass
To find whether or not deviation exists in a vessel’s compass,
the compass has to be swung (see figure 7.2).
If the vessel is a large commercial vessel intended for coastal
and offshore work, the compass is swung by a compass adjuster
who will make up a deviation table for that ship (see previous
page).
The deviationof a compass in a smaller vessel canbe determined
by lining up a fixed mark and/or a charted mark with a distant
conspicuous mark as shown in Figure 7.2.
For example, compare the magnetic bearing with the compass
bearing on the line as shown.
The deviation here at point
A
is 2
o
East.
The vessel is then moved through each of the four compass
bearings along the transit line and a note is taken between the
compass bearing and the known chart bearing and a table
constructed.
Use in navigation
Deviation is used in calculations that enable a ship's
navigator to plot a course and locate positions on
a voyage. This is discussed later in Section 3.
Pelorus
A pelorus, as shown in Figure 7.3, is
another navigation device and may be
described as a dumb compass card
engraved on metal or plastic.
• The dumb card ismountedon a central
axis so that it can be rotated freely or
clamped in any position. Sight vanes
that may be rotated are fitted to read off a
bearing from the card.
• The pelorus is used to take bearings that are not
visible from the steering compass.
• It is usually mounted on either sides of the bridge to allow
the master easy access for taking bearings.
Figure 7.3 A ship's pelorus
0
0
010
020
350
340
030
330
320
310
040
050
060
070
080
300
290
280
270
260
250
240
230
220
210
200
190
180
170
160
150
140
130
120
110
100.
Increasing about2 ' annually
Mag.Varn 8 00'E. (1990)
090
0
0
010
020
350
340
030
330
320
310
040
050
060
070
080
090
300
290
280
270
260
250
240
230
220
210
200
190
170
160
150
140
130
120
110
100.
Increasing about2 ' annually
Mag.Varn 8 00'E. (1990)
180
0
0
010
020
350
340
030
330
320
310
040
050
060
070
080
090
300
290
280
260
250
240
230
220
210
200
190
180
170
160
150
140
130
120
110
100.
Increasing about2 ' annually
Mag.Varn 8 00'E. (1990)
270
South
heading
N
E
S
W
East
heading
N
E
S
W
West
heading
N
E
S
W
273
0
C
087
0
C
182
0
C
Leading
markers
Chart reads 0
o
magnetic
Compass reads 358
o
}
Deviation is 2
0
E
North
heading
0
0
010
020
350
340
030
330
320
310
040
050
060
070
080
090
300
290
280
270
260
250
240
230
220
210
200
190
180
170
160
150
140
130
120
110
100.
Increasing about2 ' annually
Mag.Varn 8 00'E. (1990)
A
Transit markers
always in line
Earth's
magnetic
field
Deviation in
compass at helm
Figure 7.1 Deviation
Figure 7.2 Swinging the compass
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