True to Magnetic: When to Add, When to Subtract

Variation: 3° West.

Add it? Subtract it?

You learned this. You've been taught it at least twice. You've probably passed a practice question on it. But right now, with the chart in front of you and a course to plot, you cannot remember which way round it goes.

So you try to visualise it. True north, magnetic north, the angle between them, west means... left? And if magnetic is left of true then the number should be... bigger? Smaller? You're not even sure anymore. You pick one, do the calculation, and spend the next ten minutes wondering if you got it backwards.

This is the bit that drives people mad. Not understanding what variation is. That's easy. The part that won't stick is whether to add or subtract in this specific situation, right now, under pressure.

Here's why it won't stick: you're trying to reason it out each time. That's the wrong approach. You need a mechanical rule you can apply without thinking. One rule. No reasoning required.

Write these letters in a column:

If you need help remembering the order, there's a mnemonic: True Virgins Make Dull Companions. Nobody forgets this one. Funny, that.

This column is your conversion ladder. You're always either climbing up or going down.

Going DOWN means True to Compass. You're starting with a bearing from the chart and working toward a course to steer. Chart to helm.

Going UP means Compass to True. You're starting with what you steered and working back to plot on the chart. Helm to chart.

Now here's the rule. This is the bit you're going to memorise:

What about going up? You're now reversing what you did going down. Going down was add west. Going up is the opposite: add east.

Going down, add west. Going up, add east. If you remember the first one, the second one follows.

Some people use "Add Whisky" as a memory aid (Add W = Add West). Sailors remembering things via alcohol references? Shocking. But it works, so we'll allow it.

Course from the chart: 045° True Variation: 3° West Deviation on this heading: 2° East

We're going from True to Compass. That's going DOWN the ladder. And what's our rule? Going down, add west.

Steer 046° on the compass.

No visualising. No reasoning about angles. Just: going down, add west. (Or add whisky, if that's what gets you there.)

Before you commit to any answer, ask yourself:

Is Magnetic bigger or smaller than True?

In UK waters, variation is West (currently about 2-3°). When variation is West, Magnetic is always BIGGER than True.

So if you converted 045° True to Magnetic and got 042°, you know instantly that's wrong. Magnetic should be bigger. You subtracted when you should have added.

This one check catches most mistakes before they matter.

Different brains, different tricks. Here's an alternative.

CADET: Compass Add East for True

When converting FROM Compass TO True, you Add East (subtract West). Going the other way, reverse it.

Some people find a sentence easier than a column. Use whichever sticks.

Three reasons:

Too many mnemonics. CADET, TVMDC, "True Virgins Make Dull Companions," "Error West Compass Best." You've probably half-learned several of them and now they're fighting each other in your head.

The words are confusing. "Westerly variation" sounds like something's moving west. It actually means magnetic north is west OF true north. "Add" and "subtract" swap depending which direction you're converting. It's genuinely poorly designed.

You keep trying to understand it. Understanding is great for knowing why variation exists. It's useless for doing the calculation quickly. Stop reasoning. Start drilling.

The fix: pick ONE method, use it exclusively, and do enough practice problems that it becomes automatic.

The compass rose on your chart has two rings. Outer ring is True. Inner ring is Magnetic.

In the middle, you'll see something like:

> 3° 15' W 2020 (8' E)

This means: 3° 15' West in 2020, changing by 8 minutes East per year.

For 2026: 6 years × 8 minutes = 48 minutes of change. Updated variation: about 2.5° West.

Round to the nearest half degree. Nobody steers that accurately anyway.

Variation comes from the planet. Everyone in the same area has the same variation.

Deviation comes from your boat. Engine, electronics, wiring. It changes depending on your heading, which is why boats have deviation cards.

Apply deviation when: using the steering compass, or the exam question mentions "compass course" or bearings marked with "C".

Ignore deviation when: using a hand bearing compass (you hold it away from the boat's magnetic field), or the question only mentions True and Magnetic.

For Day Skipper, deviation is mentioned but not heavily tested. Master variation first.

Reading won't do it. Practice will.

Do twenty conversion problems. Training almanac, online quiz, make them up. Doesn't matter.

By problem five, you're still thinking. By problem ten, you're faster. By problem fifteen, it's becoming automatic. By problem twenty, you'll wonder what you were struggling with.

That's how everyone learns it. Not by understanding it better, but by doing it until the understanding becomes unconscious.

A 5° error over 60 miles puts you 5 miles off course. In the Dover Strait, that might mean Calais versus Cap Gris-Nez. Embarrassing, but survivable.

A 5° error approaching an unfamiliar coast at night, or in fog, or over a longer passage where errors compound... that's when it matters.

GPS exists. You'll probably notice before anything bad happens. But the Day Skipper standard assumes you can navigate without electronics, because electronics fail and skills don't.

1. TVMDC in a column
2. Going down, add west. (That's the rule. Learn it.)
3. Going up is the opposite: add east
4. Sanity check: West variation = Magnetic bigger than True
5. Twenty practice problems until it's boring

Going down, add west. Add whisky. Whatever works. That's the whole thing.