If we ignore the minus sign and apply the sayings as above then the answer is indeed 21.
But which way is correct and why ?
I don't mind eating humble pie but someone please explain the reasoning, not just we told you so.
Incidentaly I looked up this minus being west convention but teh only navigation text book I could find says the opposite, that east is minus. It was a nautical book but its the same.
if we use the saying
"True virtue makes dull company. Add whiskey."
true+variation=magnetic+deviation=compass
The plus signs are used for westerly changes ("add whiskey" = add westerly) If the variation or the deviation are EASTERLY, then use a minus sign.
We know the following
true = 172 (Nobody disputes this 180 minus 8 degrees drift)
variation = x (to be calculated)
magnetic = y ( to be calculated)
deviation = 2
compass = 195
we know variation and deviation are westerly so we add.
172 + x = y (magnetic)
y + 2 = 195 (compass)
y must equal 193 as we know that 193 + 2 = 195
therefore 172 + x must equal 193
therefore 193 - 172 = x
therefore x (variation) = 21
therefore Answer = A
If we now do the reverse calculation using the saying
"Can dead men vote twice at elections?"
compass+deviation=magnetic+variation=true
The plus sign is used for easterly changes ("at elections." = add easterly) If the variation or the deviation are WESTERLY, then use a minus sign.
Again we know the following
true = 172
variation = x (to be calculated)
magnetic = y ( to be calculated)
deviation = 2
compass = 195
we know variation and deviation are westerly so we subtract.
195 - 2 = y
therefore y (magnetic) = 193
193 - x (variation) = 172 (true)
therefore x (variation) = 21 (as 193 - 21 = 172)
Variation = 21
the answer is A
Regards
EddieHeli
by the way unowho - chill out
have you never been wrong ?
I thought the idea of these discussions was to thrash out ideas till we found the answer.
I have supplied my reasoning with all of my answers. If my thinking is wrong well fair enough, but explain it better so that I can see why - otherwise why should my thinking change?
Someone said that convention says that -(minus) is west but I read this article which says the opposite.
Here\'s a clip from a navigation article by Jim Sexton on sailnet found at
http://www.sailnet.com/collections/a...leid=sexton050
Secondary magnetic fields in the vicinity of the compass can also affect the compass readings. These secondary magnetic fields are caused by the presence of ferromagnetic objects, electronics, and electrical wires in the boat. This error can be reduced by changing the position of the small compensating magnets in the compass case. However, it is not possible to remove all of these errors on all headings. The end result will be a compass card showing the number of degrees of error in the compass when you are on various compass headings. This error is called deviation, and like variation must also be considered when determining heading relationships.
The correction for variation and deviation is usually expressed as east or west and is computed as a correction to true heading. In order to make this computation easier we usually convert the east or west values to a plus or minus value and add them algebraically. If variation or deviation is east, the sign of the correction is minus and if west, the sign is plus. A good mnemonic for remembering this is "east is least and west is best."
regards
EddieHeli