compass variation

kid's inheritance

kid's inheritance

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on my day skipper homework, I have to find the difference in variation between two different years.I've poured over the training map ,especially the rose but can't find how to calculate it.Any help appreciated,I don't want the answer but a clue of how to start!
Thanks
 
I don'y have a chart handy, but I think you will find in the bottom R/H corner (in the margtin) there will be a statement of the variation when the chart was drawn (dated) and how much it was changing year on year. I could be wrong, it might be on the main compass rose near the top. But it is there somewhere and it is usually a tiny amount - like 10' per year, but with 20 year old charts the rate of variation may well have changed!
 
Yes, it's typically on the compass rose on the chart. It will say something like: VAR 11.45'W (1985), ANNUAL VARIATION 5'

You'll have gathered from the above that my charts are old, but the principle applies!
 
I think I finally have done it but,Tim, enigmatic is too much at this time of night.I have only 2 weeks in front of the mast so unless it is as clear as glass, east is least isn't helping ;)! I am sure you have all been there but I am getting a little fed up of "you want a BBQ in Port Frasier at 11.46 ST feb 12th with a lee of 3.6 and a tidal stream of 2.66666666666666 neaps 121Compass with high barometric,only 10 m chain and a fed up crew-what temperature should the barbie be?"
 
TG - that excellent little mnemonic won't help, as the OP is trying to work out annual change in Variation, not trying to find True from Compass.
 
kid's inheritance said:
on my day skipper homework, I have to find the difference in variation between two different years.I've poured over the training map ,especially the rose but can't find how to calculate it.Any help appreciated,I don't want the answer but a clue of how to start!
Thanks
Click to expand...

If it says 7˚20'W 2005 (8'E) it means that in 2005 the Variation was 7˚20'W, and that it was changing eastwards by 8 per year.

In 2010 it will have been changing for 5 years
So it should have changed by 5 x 8'= 40' E

The change is in the opposite direction to the original variation so it is reducing it:-
7˚20' - 40' = 6˚40'W

If it had said 7˚20'E 2005 (8'E) then the easterly change would have been in the same direction as the original variation, so it would have increased it:-
7˚20' + 40' = 8˚00'W

Hope this helps.

But bear in mind that in real life, these numbers are approximate (the change isn't constant) and that you can't take bearings or steer a boat that accurately anyway.
 
timbartlett said:
(snip)
But bear in mind that in real life, these numbers are approximate (the change isn't constant) and that you can't take bearings or steer a boat that accurately anyway.
Click to expand...

That was always the bit that bothered me about classroom exercises.

Then I was shown eyeball "coastal pilotage" by the guy I bought me first boat from. He did it all by eye, aligning the chart to the coast, pointing at a landmark & using judgement to transfer the bearing on the chart. Then repeating with another landmark to get a position. No compass, no rulers, pencils or plotters just common sense. Close enough for jazz, dead quick & simple.
 
kid's inheritance said:
nope but just noticed after the variation figure there is a figure in brackets((8 minutes E).Is that the amount it alters per annum?
Click to expand...

For the record putting the annual change simply in brackets is the current way of doing it.

when Searush last bought a chart it was spelt out in words! :D
 
kid's inheritance said:
I think I finally have done it but,Tim, enigmatic is too much at this time of night.I have only 2 weeks in front of the mast so unless it is as clear as glass, east is least isn't helping ;)! I am sure you have all been there but I am getting a little fed up of "you want a BBQ in Port Frasier at 11.46 ST feb 12th with a lee of 3.6 and a tidal stream of 2.66666666666666 neaps 121Compass with high barometric,only 10 m chain and a fed up crew-what temperature should the barbie be?"
Click to expand...

Congratulations. Sounds like you're doing really well - but sounds like your instructor has completely lost the plot !!!
 
sarabande said:
TG - that excellent little mnemonic won't help, as the OP is trying to work out annual change in Variation, not trying to find True from Compass.
Click to expand...



I was trying to give the OP a clue in my East Least and West best as he is doing section 4 Chartwork and Position fixing of his Dayskipper
The question asked if the variation stayed the same in 2007 and 2012 if the Variation was 7 degrees W 20 minutes (8E) in 2005 East is Least two years = 2 x 8 =16 East less from 7 degrees 20 = 7 degrees 04 W
Variation 7 degrees 20 minutes (8E) is 7 x 8(E) =56 less from 7 degrees 20 = 6 degrees 24 in 20012. If we round up or down to the nearest degree then 2007 is 7 degrees West and 2012 is 6 West degrees
To convert True to Magnetic we apply any West (Best)Variation and Subtract any East (Least) this applies to whole degrees and minutes of a degree when we look at the variation from one year to the next , we can see on the chart used for the course that Variation is decreasing by 8E minutes least(East) annualy.

Ta Tim
 
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VicS said:
For the record putting the annual change simply in brackets is the current way of doing it.

when Searush last bought a chart it was spelt out in words! :D
Click to expand...

Bought a chart? BOUGHT one! I don't waste me money like that - the last ones I got were given to me. But yes, I do have a black & white chart amongst my collection - and a real gorgeous one it is too.
 
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kid's inheritance said:
I think I finally have done it but,Tim, enigmatic is too much at this time of night.I have only 2 weeks in front of the mast so unless it is as clear as glass, east is least isn't helping ;)! I am sure you have all been there but I am getting a little fed up of "you want a BBQ in Port Frasier at 11.46 ST feb 12th with a lee of 3.6 and a tidal stream of 2.66666666666666 neaps 121Compass with high barometric,only 10 m chain and a fed up crew-what temperature should the barbie be?"
Click to expand...

You've been sat in my day skipper class.... :D

Yes - subtract or add the difference to the value that shows.... so for east values subtract (ie. "least") and for west values add... on RYA chart 3 bottom left corner the rose has

7 deg 30 minutes west in 2005 with an 8 minutes east change per year in brackets afterwards... the way I remember whether to add or delete (and I have as much experience as you!) is to look at my plotter - the east variations are the left of 0 so subtract and the west are to the right (so add)... so 7 deg 22 minutes in '06, etcetc

But watch me get it wrong when someone cleverer comes along! :rolleyes:
 
Sorry, guys, but I really do think you are making this far more complicated than it really is.

If the Variation was Westerly, but it is getting more Easterly year by year, then it must be reducing.

If the Variation was Westerly, and was getting more Westerly year by year, then it would be increasing.

Isn't that easier than trying to remember mnemonics that apply to something completely different and trying to force them to fit?
 
timbartlett said:
Sorry, guys, but I really do think you are making this far more complicated than it really is.

If the Variation was Westerly, but it is getting more Easterly year by year, then it must be reducing.

If the Variation was Westerly, and was getting more Westerly year by year, then it would be increasing.

Isn't that easier than trying to remember mnemonics that apply to something completely different and trying to force them to fit?
Click to expand...

Tim, I think I'm right in saying that your learned your nav in the world's finest school. Did the RN teach its young gentlemen by methods involving remembering whether virgins were partial to particular brands of confectionary or was it just the spectre of the least appealing of rum, sodomy and the lash being applied whihc assited your mental processes?
 
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