Skipper to be, please help me figure out VAR & DEV!

Posted by Decent-Mountain7591@reddit | sailing | View on Reddit | 31 comments

Hey there! I’m currently studying for my skipper exam in Croatia. I bought a training chart from and tried to solve one of the navigation exercises, but I can’t figure out what I’m doing wrong.

This is the task:

We are sailing from Donja Punta (Rab) to Koromačna (Cres). What is the compass course?

The true course I measured is 278°, which is the easy part.

The next step should be applying the magnetic variation to get the magnetic course. The chart shows a variation of 3° East.

My understanding is:

“West is best, east is least.”

So with an easterly variation, I should subtract it:

278° − 3° = 275° magnetic course

Then I still have to consider the deviation table. For the closest magnetic course listed (273°), the table gives a deviation of −9°.

So my calculation is:

275° − 9° = 266° compass course

That would be my solution.

How on earth does the book arrive at a compass course of 273°?

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ez_as_31416@reddit

Here's how I learned it:

From True to Mag:

True Virgins Make Dull Companions At Weddings

(True Var Dev Compass Add West)

268-3 = 265 - (-9) = 273

And from Mag to True:

Can Dead Men Vote Twice At Elections

(Compass Dev Mag Var True add East)

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TheMightyLizard@reddit

So I'm studying for the Day Skipper theory course at the moment, and the two mnemonics I've been taught to help are as follows:

Cadburys Dairy Milk Very Tasty (Starting with Compass bearing, factor in deviation to get magnetic bearing, factor in variation to get True).
Cadet -> From Compass bearing, add east to get to True (subtract west from compass to true, vice versa if you're going from true to magnetic/compass.

Hopefully that helps!

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Decent-Mountain7591@reddit (OP)

It's german, but this is the solution the book gives me...

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therealllama-power@reddit

This says that your true bearing (rwK) should be 268, not 278 as you stated in your original post.

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Decent-Mountain7591@reddit (OP)

Oh god, you're right - that was a typo and I used the wrong numbers. 268 is right. But 268 - 3 VAR - 8 DEV is even further away 😅

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Sangatsuko@reddit

Wrote it in another reply: you have to substract a negative nummer so you end up adding. -(-8) => +8

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Decent-Mountain7591@reddit (OP)

I cannot believe this 😂 Why would the deviation table have a negative sign for this? Just to add an extra layer of complexity or what is the logical reason for this? 🤯

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Sangatsuko@reddit

The deviation can be clockwise or anti-clockwise. The sign gives the direction in which the compass is deviated.

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Decent-Mountain7591@reddit (OP)

So -9 would mean a Deviation of 9° East? I don't really get those numbers and why I would need to substract the negative number. East is Least/West is Best does also account for DEV right?

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Diipadaapa1@reddit

Think CADDET.

When you want to calculate from Compass heading to True heading, you ADD E.

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Decent-Mountain7591@reddit (OP)

Yeah, right - I know that - but this example is exactly the other way around 😉 I have to calculate COMPASS HEADING from TRUE HEADING, therefore I have to subsctract E. W+/E-. Right?

I kinda managed to imagine my deviation table as a volume knob. If it says minus, I'm turning the knob to the left (counter-clockwise). To the Left means "westerly direction", therefore a negative Deviation means Variation to the WEST.

So DEV-9 = DEV 9°W

Then I just have to do my "West is best (+), East is least (-)"-Game to find out if I have to add or substract it.

Thank you all for your help 😄 I still think my deviation values are written in a kinda complicated way (the ones I know from various aircraft look a bit different) but at least I now found a way to remember that 😄

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Diipadaapa1@reddit

Yes, the other way around you do the opposite.

This applies to both deviation and variation

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Sangatsuko@reddit

I would actually think it is 9 degrees West. But it is all a bit complicated and I don't want to confuse you any further. Better ask your instructor.

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fgorina@reddit

They interpolated the deviation

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Necrid1998@reddit

Tomascheks übungsklausuren sind auch überall gelandet;)

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1022whore@reddit

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Cheerfulfilms@reddit

Can (Compass) Dead (Deviation Men (Magnetic) Vote (Variation) Twice (True) At Elections (Add East correcting to True from Compass).

I always drew this out when correcting which helped me visualize.

So, let's say the given is 270° true, variation is 17° W, and deviation is 5°E.

270° True + 17° W Variation = 287° Magnetic

287° Magnetic - 5° E Deviation = 282° Compass

Does that help?

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Decent-Mountain7591@reddit (OP)

I thought that is exactly what I did... Would you try this with my Numbers?

True = 268 Var = 3° E DEV = -9

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Cheerfulfilms@reddit

I would wager you need to actually add the deviation as the result of a double negative -(-9) which would lead to the books answer. I don't see deviation commonly represented by negative numbers, rather as W or E so that may be adding to the confusion.

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Known-Alfalfa@reddit

Check McFly SKS lessons on YouTube, they explain it well

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flyingron@reddit

Variance is the difference between the magnetic bearing and the true north bearing at a given point. It is a correction for the difference in locations of the true and magnetic north poles and some other effects. It is unrelated to the vessel that is actually making the observation.

Deviation is the difference between what your particular compass reads and the magnetic bearing. I.e., it's correcting for errors in your compass (either internal or due to some ferric material on the boat).

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Decent-Mountain7591@reddit (OP)

Thanks, I understood the theory, but cannot figure out what is wrong in my calculation :/

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Small_Dog_8699@reddit

The variation is -9. That means at reading 273, your compass reads 9 below actual, add the 9 back to get actual. 265+9 is 274. That’s a degree off which might be your measurement error.

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Small_Dog_8699@reddit

Worth pointing out that deviation is not a constant but a dynamic correction function. It might be 0 pointing magnetic N and different at due east or west depending on distribution of metal in your boat. It is produced by swinging the boat around and comparing the reading to a reference and noting the difference at various orientations.

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