Join Date: Aug 2010
Location: New Mexico, USA (Heron, Elephant Butte lakes); Arizona (Lake Pleasant)
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Re: need help my brain is toast
It seems some folks make it harder than perhaps it needs to be. As I understand things and could be very, totally, dead wrong...
V variation ... difference between true and magnetic; in the charted region for the problem magnetic is to the left/west of true as you look north
D deviation, the difference between magnetic and your particular compass
Here's sort of a backwards way of looking at it:
In the first problem, boat was on heading 300 (compass) (roughly northwest).
The angle of the boat's heading to the range via pelorus angle measure was 130 to starboard, which I assume is measured from the bow (duh), so same as 40 degrees behind your starboard beam.
If compass direction were to have been the same as magnetic, then the range would have been on the line 300 + 130 = 430 minus the 360 you don't need is 70 degrees. Sighting straight down the range should have shown you a "70" and that would also have been its magnetic direction. If the magnetic direction of the range plotted off the chart was something different from 70, you'd have that much deviation... for this particular heading. (Do this lots of times for different directions and you could construct a "deviation card" and then be done with this sort of thing.)
But, to solve the problem, I think you need to know something about that range... either an observed compass bearing, or its direction after plotting a line for the range and walking it up to the compass rose, or a published bearing for the range... something.
I'm lazy and like to stay with magnetic, but maybe the navy and wanna-be navy folks like to have more numbers to play with and think that converting everything to True is intrinsically more virtuous or gives them better feng shui or something. That's fine on big ships, but simplicity is beautiful on a small, bouncing boat. Especially if squinting over a chart down below is risky to your feeling of well being.
Oh, and if you don't like the variation where you are navigating, just wait long enough and it'll change. Only a little in the short term (chart should show annual change in variation for its area), but give it 10 000 years or so and you just might see the big magnetic pole flip.
And another option is to just sail in Lake Michigan, Kentucky or Tennessee, Georgia, or parts of the Bahamas, where navigators don't have to worry about much steekeen variation.
A wicked thought... see just how good your instructor is by having him work problems like this while skippering a small, heeling, bouncing sailboat in a lively seaway while dodging holiday weekend traffic and keeping a good lookout. No doubt he'll always get perfect, textbook results. Yeah, right.
Last edited by rgscpat; 09-29-2014 at 04:20 AM.