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How to Determine Deviation
Information
As you learned in the last topic, the deviation tables contain information
on the deviation for headings. The deviation table must be updated
annually and posted on or near the magnetic compass. Follow the steps
in the step action table to determine the deviation and magnetic course
to steer.
Example: Refer to figure 2-8. Your ship is on course 090° T, the gyro
fails and the OOD now wishes to make good course 117° (magnetic
course) by magnetic compass. Find the proper deviation from the
magnetic compass deviation table and recommend the correct magnetic
course to steer to make 117° good. In this example we will assume that
degaussing is turned OFF.
DETERMINING YOUR DEVIATION
Step
Action
Example/Result
1.
Determine if degaussing is ON or
OFF
OFF
2.
If
Then
OFF
Use left column
ON
Use right column
In the deviation table, locate the
course nearest your desired
course.
The nearest course is 120
3.
4.
Read the deviation
Deviation equals 3.0 W
Apply the deviation correction to
RULE: Westerly deviation
the ordered course
means that the compass reads
less than it should. You must
add W deviation to the magnetic
course.
Easterly deviation is opposite,
you must subtract the deviation
from the magnetic course to get
your course to steer or “compass
course.”
Magnetic Course
117°
Deviation
+ 3° W
Compass Course
120°
Because of the compass
deviation you must steer
compass course 120° to make
magnetic course 117° good.
2-11
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