What is compass deviation and how do you apply it?

01 August 2016

We have already discussed magnetic variation. In addition to variation, we must also allow for compass deviation when transferring a bearing from the chart (where we work in relation to True North) to the helm, who is using the boat’s compass to steer a course.

Deviation is caused by the effect of magnetic fields generated on board the vessel (such as the engine block), whereas variation is generated regionally by local effects (for example large areas of granite in the earth’s surface).

Deviation is the amount by which the ship’s compass is affected by the characteristics of the vessel (such as the engine block and wiring / other ferrous metal objects, etc). On modern fibreglass vessels deviation is rarely more than 2 degrees although deviation may change from +2 degrees to -2 degrees dependent on the heading of the vessel. To establish the vessel’s deviation for a given heading, simply refer to the ship’s deviation card. 

A simple mnemonic used by many to remember how we should apply variation and deviation and in what order is the following somewhat politically incorrect statement;

“True Virgins Make Dull Company”. We can’t comment on the wisdom of the phrase, but it’s certainly memorable and useful to the salty sea dog in us all. We use it as follows:

T    -     True Course in degrees

V    -    Apply Variation

M    -    to give us Magnetic course

D    -    Apply Deviation

C    -    to give us a Compass Course.

It is important that we make the amendments in this order. We also use the mnemonic CADET to remember whether we are adding or subtracting the Easterly and Westerly corrections. Check out our blog on variation for more details.

When talking courses to steer and bearings, always make it clear what you are saying. Is the bearing a True bearing, Magnetic bearing or Compass bearing. All three may vary dependent on the vessel’s heading and position.

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