You’re off sailing. You’ve plotted your course on the chart, so isn’t it now just a case of reading it off and asking the helm to steer it? Unfortunately not, and here’s why.
The course on your chart is plotted relative to “true north” which is the direction along the earth’s surface towards the geographic North Pole.
Your compass however points to a “magnetic north” which is different. Why? Fluid iron in the earth’s core acts like a huge magnet, creating a relatively weak magnetic field. The needle in a compass is magnetised and freely suspended, and it aligns itself with the earth’s magnetic field.
This difference between true north and magnetic north is known as variation. It’s measured in degrees and minutes (either east or west) and is shown on your chart in the compass rose.
The size of the variation depends on two factors, where you are on earth and when you’re there. This site will tell you the variation for your location (it’s not great from a mobile). In some places variation is minimal (2 or 3 degrees) and given most people on the helm can’t steer to that level of precision, it’s not worth accounting for. However in other places it can be up to 30 or 40 degrees so it most certainly needs factoring in. Variation changes over time (this animation shows how) and it can be predicted, so your chart will show the yearly change relative to the date it was printed.
With me so far?
We just said your compass points to magnetic north but annoyingly, that’s not entirely true. Objects on your boat (engine block, radio, gas bottles, mobile phones) create their own magnetic fields which mess with the compass. This is called “deviation”.
Unlike variation, deviation changes depending on the boat (with their different objects and layouts) and also on your heading. Why on the heading? Imagine you’ve a radio installed in front of the steering compass. When heading north your compass needle will be attracted towards it, which is fine because that’s the direction you’re heading anyway. Head east and the needle is pulled in two directions; towards magnetic north and towards the radio. To help you know how much deviation to account for, you should have a deviation card made for your boat. It’ll tell you the degrees east or west given a specific course. Here’s what they look like (bottom of the page).
OK, now what?!
We need to convert the course from your chart into a compass course to steer. Of course you’ll also need to convert your compass course back into a true course if you want to transfer it onto your chart.The main thing to remember is whether to add or subtract variation and deviation. From true to compass add the degrees if they’re west (and subtract east), and from compass to true do the opposite; subtract west (and add east). There are a few different mnemonics to help you remember, this is my favourite. 😊
In case this one doesn’t stick for you, try these:
- “Error west compass best” and “Error east compass least” – best in this case meaning the biggest, so add it!
- CadET: To get from compass © to true (T) you ‘ad’ (add) easterly errors.