Use of deviation on running fixes

[davefromoregon]

I am preparing for my asa 105 exam so this is a bit academic....

I look up deviation relative to the track per ships compass course.

Then I am given fixes with other bearings again per ships compass.

So, do I use the deviations that correspond to the fixes or the track?

[Hydra]

The deviation measures the influence of the ship's magnetic masses on the magnetic field, which acts on the compass. This depends on the ship's heading, not the direction one is looking.

I prefer to avoid the word "course" because it can have 2 meanings: heading or track.

Alain

[bletso]

If ship ran down the bearing then use deviation. If you used a hand bearing compass then not.

[Don C L]

Originally Posted by davefromoregon

I am preparing for my asa 105 exam so this is a bit academic....

I look up deviation relative to the track per ships compass course.

Then I am given fixes with other bearings again per ships compass.

So, do I use the deviations that correspond to the fixes or the track?

Ordinarily running fixes are made with a hand-bearing compass (at least I do) and thus deviation should be close to nothing. But if you are being given fixes with other bearings using the ship's compass, and the ship is not changing course to get those fixes, then the deviation remains the same as for the given compass course. The deviation is determined by the ship's heading.

[Mirage35]

The real question is what they are expecting you to do on the exam. If course notes or other material provide an indication, follow it.

I wrote [the Sail Canada equivalent of] that exam not long ago, and my notes and exercises all show variation calculated but not deviation. So, apparently they were assuming a hand bearing compass or equivalent. (I got 98% on the exam, by the way. )

[davefromoregon]

Thanks. If the problems had mentioned a pelorus or relative bearing, it would have been more obvious. Since it did say "per ships compass", I found it misleading.

All I have to work with is the the ASA guide which isn't explicit on this point as far as I can tell.

http://www.americansailacademy.com/sail_4.phtml

[Hoofsmit]

Both
If it says the ships compass is used for the track and fixes, the deviation is applied on all points as stated on the ship deviation card
Better still - tell them to get it swung properly


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[Don C L]

Originally Posted by davefromoregon

Thanks. If the problems had mentioned a pelorus or relative bearing, it would have been more obvious. Since it did say "per ships compass", I found it misleading.

All I have to work with is the the ASA guide which isn't explicit on this point as far as I can tell.

Coastal Navigation

That is strange, but since most (or do any?) ships don't change course to get a fix, I'd guess they are talking about a pelorus or relative bearing as you said. Well you could always calculate it all three ways and really impress them!

[davefromoregon]

Originally Posted by Don C L

That is strange, but since most (or do any?) ships don't change course to get a fix, I'd guess they are talking about a pelorus or relative bearing as you said. Well you could always calculate it all three ways and really impress them!

Unfortunately, the questions typically are asking you tell determine your location. So, it isn't going to help much to give three locations.

That said, I'll go with your thought, and that of others, and assume the deviation is based on the boats track for all of the fixes.

[Lodesman]

Originally Posted by davefromoregon

I am preparing for my asa 105 exam so this is a bit academic....

I look up deviation relative to the track per ships compass course.

Then I am given fixes with other bearings again per ships compass.

So, do I use the deviations that correspond to the fixes or the track?

It does say "ships compass", implying a compass affixed to the ship. A hand bearing compass will not have a deviation card after all. Deviation is read from the card under the column that best corresponds to the ship's compass heading - that same figure is applied to all bearings of the fix.

[StuM]

Originally Posted by davefromoregon

I am preparing for my asa 105 exam so this is a bit academic....

I look up deviation relative to the track per ships compass course.

Then I am given fixes with other bearings again per ships compass.

So, do I use the deviations that correspond to the fixes or the track?

In this case, I'd go with "deviation per ships heading" since the bearings are per ships compass. If it had been a separate hand bearing compass, I would not apply a deviation.

You should be careful in your terminology and be aware of the difference between route, course, track and heading.

[Hoofsmit]

Dave
are they giving a 3 point fix or a running fix ?
You can't use the same deviation as the track at each point will have a different deviation
See the different deviation on each bearing on this example of a card
Remember also..... compass to true...... you subtract west and add east
Hope that helps !




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[Lodesman]

Originally Posted by Hoofsmit

You can't use the same deviation as the track at each point will have a different deviation
Attachment 141034

Sorry that's wrong. See your deviation card - you read deviation to the ship's head. Unless you point the bow at each point of your fix, you apply the same deviation figure to all the points of the fix.

[Hoofsmit]

The two bearings will have different deviations in the example below
In truth the track is not your heading / course steered as leeway and drift need to be taken into account ( which is what happens in real Life) as Stu points out, terminology is important



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[Hoofsmit]

And yes I am proved wrong
Thankyou lodesman you are correct it is the ships heading that applies and if the fix is taken on that constant heading the same deviation is applied
Sorry if I confused you


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