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Old 09-28-2014
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need help my brain is toast

Problem solved! Thanks to the crew of Sailnet.....getting ready for ASA 105 written test and for the life of me I can't figure these two problems out? any step by step solution would be greatly appreciated. Your are on a compass course of 300 degrees when you note the monument and the lookout tower on Cuttyhunk Island are in line. Using s Pelorus you determine that the transit formed by the monument and the tower is 130 degrees to starboard. What is the deviation of your compass for this heading?....and the second question is a compass course of 317 degrees and a pelorus transit of 064 degrees to starboard again solving for deviation. I'm too old to be school material, in my youth I was a math wiz but now too many brains cell have retired on me when I need them the most. I just don't understand the process to solve these questions thanks augie

Last edited by MrA; 09-30-2014 at 02:16 PM.
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Old 09-29-2014
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Re: need help my brain is toast

Some of us don't have that chart, so we don't know the magnetic direction of the line made by the range itself. That combined with the perlorus bearing should give you the info to know what your magnetic heading should be and then you should be able to compare that to your compass bearing to get deviation -- for this heading, anyway.
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Old 09-29-2014
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Re: need help my brain is toast

both problems were solve without a chart? answers were 005 and 003 west. That's why I'm perplexed. Since in both problems you are traveling in a north direction and the transit is behind you, solving the problems takes you through 360 degrees and here is where I get lost? anyway the teacher did it without a chart? was it that he knew the bearing? If I plot it on a Rose it still makes no sense to me or am I just going about the process to solving the wrong way thanks
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Old 09-29-2014
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Re: need help my brain is toast

I would draw a line through the lookout tower and Cuttyhunk Island
Then I would put a dot anywhere on the line preferably in open water.
Then through that dot pointing somewhere west of north I would draw a line through the dot so that the angle between my northwest line and my south east line was 130 degrees.
Then I would use a parallel rule to find out the magnetic heading of this second line.
The difference between the magnetic heading of the second line and 300 is your deviation.

There are real navigators on this forum but maybe this will get you started.

This is how I thought about it.

If you see the lookout tower and the Island in line then you have to be somewhere on a line that connects those two.
The only question is where on the line and which direction you are pointing.
We don't know nor care where on the line but we know we are pointing 130 degrees west of the line because that is what the Pelorus says.

Now we can tell what our magnetic heading should be.
If it doesn't match our compass the difference must be the deviation.

It looks like the second question is the same as the first just different numbers.

I'm just guessing here but does this make sense?
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Last edited by davidpm; 09-29-2014 at 01:50 AM.
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Re: need help my brain is toast

just Saturday morning I drove an hour to see the teacher, he's not teacher material too long in the Marines, he believes every new sailor should be as smart as he, well he solved the problems right in front of me and still I can't get back to his numbers T=299, V=015W, M =314, D=003W, C=317 and the other problem T=290, V 015W, M=305, D=005E, C=300, thanks
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Re: need help my brain is toast

Quote:
Originally Posted by davidpm View Post
I would draw a line through the lookout tower and Cuttyhunk Island
Then I would put a dot anywhere on the line preferably in open water.
Then through that dot pointing somewhere west of north I would draw a line through the dot so that the angle between my northwest line and my south east line was 130 degrees.
Then I would use a parallel rule to find out the magnetic heading of this second line.
The difference between the magnetic heading of the second line and 300 is your deviation.

There are real navigators on this forum but maybe this will get you started.

This is how I thought about it.

If you see the lookout tower and the Island in line then you have to be somewhere on a line that connects those two.
The only question is where on the line and which direction you are pointing.
We don't know nor care where on the line but we know we are pointing 130 degrees west of the line because that is what the Pelorus says.

Now we can tell what our magnetic heading should be.
If it doesn't match our compass the difference must be the deviation.

It looks like the second question is the same as the first just different numbers.

I'm just guessing here but does this make sense?
the chart is a permanent fixture on the kitchen table, in the AM I will calculate your reply thanks
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Old 09-29-2014
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Re: need help my brain is toast

Quote:
Originally Posted by MrA View Post
just Saturday morning I drove an hour to see the teacher, he's not teacher material too long in the Marines, he believes every new sailor should be as smart as he, well he solved the problems right in front of me and still I can't get back to his numbers T=299, V=015W, M =314, D=003W, C=317 and the other problem T=290, V 015W, M=305, D=005E, C=300, thanks
Please expand on what the letters mean.
What is D C etc.

Oh I just remembered True, Varitation, Magnetic, devation, compass

Also please quote the problem exactly, word for word with nothing added or left out including exact punctuation.
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Last edited by davidpm; 09-29-2014 at 02:30 AM.
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Re: need help my brain is toast

Quote:
Originally Posted by MrA View Post
just Saturday morning I drove an hour to see the teacher, he's not teacher material too long in the Marines, he believes every new sailor should be as smart as he, well he solved the problems right in front of me and still I can't get back to his numbers T=299, V=015W, M =314, D=003W, C=317 and the other problem T=290, V 015W, M=305, D=005E, C=300, thanks
I suspect their is a missing piece of the puzzle.
In what way did he solve the puzzle with 299,05,314,003,317

To me the number 300 has to be in the mix someplace because according to the question the compass is 300 yes?
So one way or another if you have the calculated or plotted magnetic course and you have the compass you just subtract to get the D.
So in your solution where is my 300 compass which is the given.
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Re: need help my brain is toast

Quote:
Originally Posted by MrA View Post
both problems were solve without a chart? answers were 005 and 003 west. That's why I'm perplexed. Since in both problems you are traveling in a north direction and the transit is behind you, solving the problems takes you through 360 degrees and here is where I get lost? anyway the teacher did it without a chart? was it that he knew the bearing? If I plot it on a Rose it still makes no sense to me or am I just going about the process to solving the wrong way thanks
If you know the magnetic of the range then without a chart you can just go west 130 degrees and have a magnetic bearing.
The difference between magnetic and comp is deviation.

But unless you have the bearing of the range in either true or magnetic how do you start to calculate?

Maybe that number was part of the prior problem.
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Old 09-29-2014
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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...

Letters are
T true
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
M magnetic
C compass
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.
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Last edited by rgscpat; 09-29-2014 at 04:20 AM.
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