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Variation and deviation, from charted course to compass course
Variation plus deviation gives the total compass error; it is subtracted from the true course.
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Variation is printed on the chart and changes from year to year; deviation depends on the boat and changes with every new speaker fitted beside the compass. If you know neither of them, you steer a course that is several degrees out; over forty nautical miles, five degrees puts you more than three nautical miles off track.
Electronic navigation does not make this redundant. If the plotter fails, the magnetic compass is the only heading reference left, and then the deviation card has to be right.
For insurance, navigation is a question of due care. Running aground because the course was never checked is usually covered, as grounding falls under comprehensive hull cover; check whether consequential damage is included. Anyone who can be shown to have put to sea with no navigation at all, however, must expect the question of gross negligence to be raised.
Frequently asked questions
Why does the compass differ from the charted course?
Because of the variation of the earth's magnetic field and the deviation caused by iron and
electrics aboard.
Is grounding caused by a navigation error insured?
Usually yes, through comprehensive hull cover;
whether consequential damage to the drive train is included depends on the wording.
Worked out for your type of boat
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