Reciprocal Courses

[GordMay]

From BoatUS “Ask the Experts” ~ Navigation: http://my.boatus.com/askexperts/

Question
Captain Larry:
If I follow a compass course from my home port to a waypoint, why is there not always a 180 degree difference between that course and the compass course that I follow to return home? It usually varies by a few degrees. Shouldn't the two courses it be exactly 180 degrees apart?
Jack

Answer
Jack:
There are two forces that cause errors in your magnetic compass. One is called Variation - the angular difference between True North and Magnetic North. The other is called Deviation - the error in your compass caused by onboard electro-mechanical equipment, and in some cases by the vessel itself.

The question involves what we call "reciprocal" courses. That is, when you travel one course from point "A" to point "B" and another from point "B" to point "A" in exactly the opposite direction as your original course. The interference that causes deviation in a magnetic compass is sometimes between your magnetic compass and the North pole and sometime it is not. So, as you change your course, you change the amount of deviation.

If your course to your first waypoint is 261 degrees magnetic and the reciprocal course back to your original departure point is 078 degrees magnetic, there is a 183 degree difference. The additional 3 degrees is due to the difference in deviation experienced on both courses.

Captain Larry Walker

[Sunspot Baby]

The answer as published seems to assume perfect conditions and that course steered is the same as course made good. Even a little wind causes leeway and the set and drift of current are big factors.

A flux gate compass can have very little deviation and still the course home may not be exactly 180 from the course out.

My experience indicates variables in the environment have greater and more frequent impact on course than compass deviation.

Sunspot Baby

[GordMay]

Sunspot Baby correctly points out that ERRORS are ADDITIVE.
Cpt Larry was merely indicating the common error of using simple numerical reciprocals, without reference to compass ‘deviation’. He was only answering a specific question.
Of course ‘set & drift’ must also be vectored into the CTS calculation.