chart plotting and preparation 39
steady up on 000° on the steering compass. Have the helm call out “Mark!”
the exact moment the steering compass reads 000°, and write down the
corresponding handbearing compass reading in the Boat Heading column.
Now steer 015°. Repeat the process as before. Continue doing this for each
heading. When you are fi nished, go to the next step.- Find the deviation. Compare the Boat Heading and Steering Compass
columns. Record the diff erence in the Deviation column. Name each
deviation east or west. Use this mnemonic: Compass least, error east;
Compass best, error west. Th is means that if the Steering Compass column
is less (least) than the Boat Heading column, you should label deviation
east. If it is higher (best), label deviation west.
It’s a good idea to have your compass professionally adjusted, especially before
going on an extended cruise. Deviations of less than 3 degrees can be ignored.
Because of the amount of swing of the compass card, you’ll fi nd that even the
best helmsmen can only steer a course within 2 degrees. Th is is just one more
reason you should update your position on the chart every so oft en.
Using Your Deviation Table to Find the Course to Steer
- Plot the trackline from starting point to destination. Convert the true
course to a magnetic course. - Enter the table and look under the Boat Heading column. Find the
closest boat heading to your magnetic trackline. - Read the deviation in the second column. If the deviation is west, add
the number to the magnetic trackline. If the deviation is east, subtract
the number from the magnetic trackline.
The magnetic trackline from point A to B is 057 degrees magnetic.
Choose the closest number to 57 degrees magnetic in the Boat Heading
column (i.e., 063°).
Look across from 063 degrees and read the deviation: 3 degrees east. Apply
the deviation to the magnetic trackline to fi nd the course to steer:
057 degrees magnetic − 3 degrees east = 054 degrees.
SEA-CRET TIP
X Don’t make the mistake of using a number from the Boat Heading
column as your course to steer. Use the table only to fi nd the deviation.
Then apply the deviation to your magnetic trackline to fi nd the course
to steer.