Preparation
Before reading this page you will need to have a copy of the training almanac and chart (and a real ones if possible).
You will need to be happy converting between magnetic and true north.
Why is this important?
The magnetic effect of our boat can deflect our compass. It is possible to allow for this error in the compass bearing.
Deviation & compass bearings
All the concepts that were covered in the Day Skipper course are still the same on the Coastal Skipper course with one additional problem thrown in.
A magnetic compass will generally point in the direction of magnetic north. However, if other magnetic, metal or electrical objects are brought close to the compass they will deflect the compass.
We can avoid this in many cases be removing the problem and being careful when positioning equipment. For example, not positioning the VHF (with a large magnetic in the speaker) near the radio, this is a common problem in RIBS when all instrutments are contained in a small area.
Other magnetic objects cannot be removed, for examples the engine, cooker or event the hull on a steel boat. These have a constant effect on the compass which is known as deviation.
The one complication with deviation is that it varies depending on which way you point the boat.
Allowing for deviation
To find out the effect of deviation we must “swing the compass” of our boat. For details of how this is done see this Sail Train article [swinging a compass…]
Once your compass has been swung you will have a compass deviation card (like the one on the right), these are different from boat to boat but will generally follow a sine wave.
For example, on the compass card to the right you can see that if your boat in on a course of 090^{o}(vertical scale) the deviation which needs to be applied is 4^{o}E.
You can download a blank Deviation Card from the Cockpit Cards website [Cockpit Cards]
Method 1 – Calculation
First recall the following saying:
“Variation West, Compass Best – Variation East, Compass Least”
Translated this means, when the variation (or deviation) is west the magnetic (or compass) bearing is biggest (best); when the variation (or deviation) is east the magnetic (or compass) bearing is smallest (least).
You now just need to know that the order of adjustments is as follows (move from left to right or right to left):
True |
Variation (from the chart) |
Magnetic Bearing | Deviation (from the compass card) |
Compass Bearing |
This means:
True to Compass: From the true bearing you must first make the variation adjustment (as before) to find the magnetic bearing and apply the deviation adjustment (finding the deviation on the compass card) to find the compass bearing.
So for example: If you read a true bearing is 340^{o} from the chart and the variation is 4^{o}W then, in this case, the magnetic bearing is the largest (best) so we add 4^{o} so we get a magnetic bearing of 344^{o}. From the compass card the deviation is 5^{o}W to the nearest whole number. In this case the compass bearing is best so we add 5^{o} to get a compass bearing of 349^{o}
True |
Variation (from the chart) |
Magnetic Bearing | Deviation (from the compass card) |
Compass Bearing |
340^{o} | 4^{o}W | 344^{o} | 5^{o}W | 349^{o} |
Compass to True: From the compass bearing you must first make the deviation adjustment to find the magnetic bearing and apply the variation adjustment to find the true bearing.
So for example: If the compass bearing is 056^{o} we can read off the compass card a deviation of 1^{o}E so the compass bearing will be “least” so we add 1^{o }to make a magnetic bearing of 057^{o} the variation is 7^{o}W then in this case the magnetic bearing is the “best” so we subtract 7^{o} so we get a true bearing of 050^{o.}
True |
Variation (from the chart) |
Magnetic Bearing | Deviation (from the compass card) |
Compass Bearing |
050^{o} | 7^{o}W | 057^{o} | 1^{o}E | 056^{o} |
Method 2 – CADET
First recall the following saying:
“Compass ADd East True or CADET”
Translated this means, if you have the compass bearing you should add east deviation to get the true bearing.
Therefore you can reverse the above statement so that for an east deviation you subtract the variation to translate from true to compass.
You now just need to know that the order of adjustments is as follows (move from left to right or right to left):
True |
Variation (from the chart) |
Magnetic Bearing | Deviation (from the compass card) |
Compass Bearing |
This means:
True to Compass: From the true bearing you must first make the variation adjustment (as before) to find the magnetic bearing and apply the deviation adjustment (finding the deviation on the compass card) to find the compass bearing.
So for example: If you read a true bearing is 340^{o} from the chart and the variation is 4^{o}W then in this case we add 4^{o} so we get a magnetic bearing of 344^{o}. From the compass card the deviation is 5^{o}W to the nearest whole number. Therefore, we also add 5^{o} to get a compass bearing of 349^{o}
True |
Variation (from the chart) |
Magnetic Bearing | Deviation (from the compass card) |
Compass Bearing |
340^{o} | 4^{o}W | 344^{o} | 5^{o}W | 349^{o} |
Compass to True: From the compass bearing you must first make the deviation adjustment to find the magnetic bearing and apply the variation adjustment to find the true bearing.
So for example: If the compass bearing is 056^{o} we can read off the compass card a deviation of 1^{o}E so we add 1^{o }to make a magnetic bearing of 057^{o}. The variation is 7^{o}W then therefore we subtract 7^{o} so we get a true bearing of 050^{o.}
True |
Variation (from the chart) |
Magnetic Bearing | Deviation (from the compass card) |
Compass Bearing |
050^{o} | 7^{o}W | 057^{o} | 1^{o}E | 056^{o} |
Links
Sail Training: Variation & Deviation
An explanation of Variation and Deviation with on questions to practice (including swinging a compass, an understanding of this is required for Yachtmaster).
Progression
Now that you understand these topics you are ready to navigate, whether calculating a Fix, a Dead Reckoning position or a Course to Steer.
Feedback
The content of these pages is put together in good faith and is constantly evolving. It is possible that errors exist within this content. If you spot an error or would like to add anything to these pages please contact use via email.
Reading the content of these pages is not a substitute for completing a RYA Shorebased course or similar.