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Old 11-05-2008, 05:30   #1
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Deviation card

Yay! Boat's in the water. Now need to get the mast up and then start on a list of other chores, including a deviation card for the compass.

I had planned to put the Admiral in the bow with a bearing compass and record the differences on various compass courses (or maybe the other way around, since she's not great with the bearing compass but she can drive).

But it occurs to me ... If I'm moving, the GPS should give me a heading, which I could compare with the compass from the cockpit. And the GPS heading should be immune from local ferric influences, perhaps even more than a bearing compass in the bow.


Yes? No? Or would the GPS be giving me true, rather than magnetic?

Connemara
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Old 11-05-2008, 05:43   #2
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Connemara,

You can "swing ship" with a GPS. The GPS will give you the boat's Course Over Ground. The actual heading of the boat indicated on the compass will be affected by any wind and/or current. You would need to run identical, but reciprocal courses to account for this effect.

The GPS can be set to True or Magnetic heading. Look in the Setup menu.
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Old 11-05-2008, 05:47   #3
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Quote:
Originally Posted by Connemara View Post
... If I'm moving, the GPS should give me a heading, which I could compare with the compass from the cockpit. And the GPS heading should be immune from local ferric influences, perhaps even more than a bearing compass in the bow.
Yes? No? Or would the GPS be giving me true, rather than magnetic?
Connemara
The GPS won't give you a heading, but a Course Made Good/Over Ground.
The CMG/COG will only approximate a Heading, absent any Set & Drift.

Heading:
The direction in which a vehicle is moving. For air and sea operations, this may differ from actual Course Over Ground (COG) due to winds, currents, etc.
Course Made Good (CMG):
The bearing from the 'active from' position (your starting point) to your present position.
Course Over Ground (COG):
Your direction of movement relative to a ground position.
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Old 11-05-2008, 05:52   #4
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But if I was in sheltered waters -- without set and drift -- it would work, then.


Thanks for the swift answers ... on my way to the club shortly to start preparing my mast.


Connemara.
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Old 11-05-2008, 06:15   #5
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Things to remember. A compass, relate to MAGNETIC only . This includes local magnetic anomaly's. A GPS is very accurate but the numbers you see are related to what you have asked it to provide you in the first place. The gps can provide TRUE north, as long as you keep moving...and I mean keep moving in the same direction........ Hint if you have an auto pilot use that to steer the boat. Do your four classic runs at least twice. Average those headings and then computate your deviations. While your at it check your hand held compass. And maybe make sure that all your GPS,s are on the same datum. ( and the same as your charts.) When you have traveled for a few miles from your port do a bearing (straight ahead) versus gps and check your deviation chart to see how close your main compass is. Dont be at all surprised if its 2 to 4 deg..out.



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Old 11-05-2008, 22:08   #6
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With a 27 foot boat I am guessing that the compass is rather small. I would shoot the bearings using fixed marks, but then that's me..
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