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Another perspective on the first example

Forum - Coastal Navigation - Another perspective on the first example
Module Content - Sea Talks2
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John Lambert
Ruckersville, VA, USA
Crew Level 0
Status: Big Time Sailor
Qualified days: 9
Another perspective on the first example
Posted 2017, Jun 04 19:58
I had trouble understanding the first example and had to re-read and ponder a lot. If I understand it correctly now, here is another way of looking at it. Start with this: T = ? V = 5W (we know this from charts that tell us the variance for the area we are in) Magnetic = ? D = 2E (we know this from predetermined deviation information about our boat) Compass = 270 (we know this from reading the ship's compass direction toward the buoy) Now we add the deviation of 2 to the compass bearing to get the magnetic bearing because it is an East deviation we are going from up the TVMDC, from Compass to Magnetic to True. Then we subtract the variance of 5 from magnetic bearing to get the true bearing because it is a West variance and we are still going up the TVMDC. This helped me a lot, so I thought I would share it in case it might help anyone else.
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Carlos Bazan
Santiago, Metropolitana de Santiago, Chile
Skipper Level I
Status: Sailor
Qualified days: 11
Reply 2018, Jul 28 23:31
For me, in particular, I was taught that the variation towards the West is always negative and towards the East it is always positive. What we do when there is a variation (east or west) and a deviation of the compass, we calculate what we call the "total correction (tC)" which is not more than the algebraic sum of the variation and the deviation, result that the we use later to calculate the true course or the compass heading. that is, if we have a variation of 14º W and a deviation of 5º E, that would be equal to: tC = (-14) + 5 = -9 (9º W). So, if we have an 80º compass heading, then the true course comes from the following formula: T = C + tC T = 80º + (-9º) = 71º (different signs are subtracted) To move from true to compas, the equation is reversed and the sign is replaced by - then it would be: C = T - tC C = 71º - (-9º) = 80º (equal signs are added) Same aplied for bearings. It's easier to remember, I think
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Eigil Olsen
Oslo, Norway
Captain Level V
Status: Really Really Big Time Sailor
Qualified days: 253
Reply 2018, Sep 14 10:49
I too thought the explanation was difficult to grasp, although I was earlier taught that when it comes to variation and deviation "plus becomes minus and minus becomes plus". Reason for my confusion: in the example, when you SUBTRACT the east deviation the degree number goes up from 270 to 272, while when you ADD the west variation, the degree number goes down, ie from 272 to 267. So to some extent adding is subtracting and vice versa here (plus is minus and minus is plus). It became Clear only once I understood that you add the west variation to the True bearing to get the Compass bearing, and vice versa. Going back to the figure at the very start of the page was suddenly much more intuitive! I finally got it!
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