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Old 28-07-2014, 09:01   #16
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Re: Boat compass deviation?

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Originally Posted by Ex-Calif View Post
While you acronym is correct your application of it is pretty off.

True - True course is measured with a protractor using the lines of longitude on a chart - these lines are aligned with true north
Variation - Variation is "either" East or West (plus or minus) variation and it is marked on the chart. This is the difference between true north and magnetic north at the current location. It is unrelated to direction of travel. Variation is added or subtracted from true course and results in...

Magnetic Course - True course adjusted for variation is magnetic course. The compass rose on a chart is aligned with magnetic north. You can simply skip the first two steps and use a parallel ruler to scribe a magnetic course.

Deviation - Deviation is caused by internal error in the compass and by outside influence such as metal, equipment etc. Deviation is usually described on a deviation card kept with the compass - every compass installation is different. Deviation is added or subtracted from magnetic course and results in...

Compass heading - This is the heading to steer. It does not account for current or set. If you know current and set at a given speed you can adjust mag heading for these factors.
Don't know about other charts, but NOAA charts have compass roses with two concentric rings, outer true, inner magnetic. You don't have to have a protractor to read true on a NOAA chart, you can use parallel rules to read true directly from the compass rose.

Learn to Navigate Using Nautical Charts- Compass Rose
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Old 28-07-2014, 09:41   #17
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Re: Boat compass deviation?

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Originally Posted by CaptainPatty View Post
It is :
true virgins make dull companions:
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I was taught the same acronym with the addition of, "therefore add whiskey", meaning if you have a west variation add it to true and if you have an east variation subtract it from true.

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Old 28-07-2014, 10:02   #18
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Re: Boat compass deviation?

Haven't done it myself, but a GPS is deadly accurate with direction. So this suggests the following method.
Waypoint a fixed structure that has clear water around it, little or no current.
Go to the first cardinal position according to GPS, say 1/2 mile away, align the vessel straight at structure, read the compass, subtract from GPS direction.
Repeat until the compass is boxed.

To a earlier poster the Acronym TVMDC is "Timid virgins make dull company".

incert smiley here.
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Old 28-07-2014, 11:47   #19
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Re: Boat compass deviation?

I do a pretty good job of swinging my compass while underway using my gps, but it helps to have a very directionally stable full keeler and a very calm day. Not more than a few degrees off and acceptable for me because I've never been more than a few hundred miles from a landside fix.

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Old 28-07-2014, 17:58   #20
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Re: Boat compass deviation?

> Repeat until the compass is boxed.

"Boxing" the compass is naming the thirty two points of the compass in order.

"Swinging" the compass is what you are talking about.
- "Repeat until the compass is swung"
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Old 28-07-2014, 18:34   #21
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Re: Boat compass deviation?

maybe many Boxers are Deviants and Swingers too....
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Old 28-07-2014, 18:38   #22
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Re: Boat compass deviation?

My young 20-soomething brother worked at FedEx, he was telling our 20-something aussie niece about his job - donkey kicks to get containers closed for example - and explained he spent a lot of time "smashing boxes."

She blushed and explained what smashing boxes meant in Australia.

Disturbing to learn this from my young niece to say the least - LOL...
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Old 28-07-2014, 19:23   #23
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Re: Boat compass deviation?

Two big issues with deviation. Heeling error and the changing dip angle. Both can make a nicely swung compass with small deviations inaccurate. So check deviation when the boat is heeled as well as upright, and also as you change locations, obviously variation changes, but so does the vertical component of the feild, which can change the deviations. Ie big potential change going from Alaska to Mexico. On steel boats these factors can be huge. Seen 30 degrees of Heeling error.
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Old 28-07-2014, 20:09   #24
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Re: Boat compass deviation?

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Two big issues with deviation. Heeling error and the changing dip angle. Both can make a nicely swung compass with small deviations inaccurate. So check deviation when the boat is heeled as well as upright, and also as you change locations, obviously variation changes, but so does the vertical component of the feild, which can change the deviations. Ie big potential change going from Alaska to Mexico. On steel boats these factors can be huge. Seen 30 degrees of Heeling error.
Just don't confuse dipping error with turning error.

In high northerly or southerly latitudes the compass will point downward due to magnetic influence eventually making compass navigation impossible.

Turning error is the tendency of the compass to turn when accelerated.

ANDS is an acronym that describes "Accelerate North and Decelerate South." - It is the tendency of the compass to turn to the north when accelerated and turn to the south when decelerated - however this is not a permanent error. The compass will eventually settle (when acceleration stops) on the correct heading.

It can create confusion. Imaging steering a heading of 270, slowing the boat and turning north. The compass may initially turn south! It could lag the turn and when you settle on the new compass heading, say 290 and stabilize the compass may end up pointing 300 when it stops accelerating.

BTW this is northern hemisphere. It works opposite in the southern hemisphere.

Compasses are really quite tricky...
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Old 28-07-2014, 20:51   #25
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Re: Boat compass deviation?

Hi ex, intetesting about the turning error. Hadnt heard of this for a magnetic comp, I know gyros do weird things at times. Guess ships accelerations when rolling might trigger this... Interesting.

I was more refering to I think the induced vertical coefficient caused by the dip. I remember swinging a compass on the magnetic equator to try to eliminate this error. I am guessing all these effects are pretty minimal on the average grp boat, but certainly enough to disrupt a nice deviation table.
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Old 29-07-2014, 06:09   #26
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Re: Boat compass deviation?

Quote:
Originally Posted by Ex-Calif View Post
While you acronym is correct your application of it is pretty off.

True - True course is measured with a protractor using the lines of longitude on a chart - these lines are aligned with true north
Variation - Variation is "either" East or West (plus or minus) variation and it is marked on the chart. This is the difference between true north and magnetic north at the current location. It is unrelated to direction of travel. Variation is added or subtracted from true course and results in...

Magnetic Course - True course adjusted for variation is magnetic course. The compass rose on a chart is aligned with magnetic north. You can simply skip the first two steps and use a parallel ruler to scribe a magnetic course.

Deviation - Deviation is caused by internal error in the compass and by outside influence such as metal, equipment etc. Deviation is usually described on a deviation card kept with the compass - every compass installation is different. Deviation is added or subtracted from magnetic course and results in...

Compass heading - This is the heading to steer. It does not account for current or set. If you know current and set at a given speed you can adjust mag heading for these factors.
I apparently was not clear in my posting so I will attempt to clarify now:

True North is the direction towards the geographic north pole along a meridian line where magnetic variation corresponds to your geographical location. The importance of this, declination the difference between true and magnetic is always in perpetual motion and changes occur yearly by approximately 1/2 a degree. This motion or yearly change is notated on the charts and maps.
When I stated "it is normally defined in degrees per annum, whether it is plus or minus depends on whether you are sailing west to east or east to west," I should have said when determining your heading depending upon your geographical location, the mnemonic, "East is least and West is best," meaning subtract an easterly variation and add a westerly variation.
In regards to deviation- as I stated perhaps poorly, is the error caused by disturbance in local magnetic fields and yes as you stated there are deviation charts and is individual to the vessel compass. A hand held compass can also be used to determine the unique deviation of a particular vessel's compass.

Again, I misstated Compass- not course over ground but heading to steer after taking into account deviation and variation and no, it does not take into account set and drift.
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Old 29-07-2014, 06:52   #27
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Re: Boat compass deviation?

To answer the OP's question, I checked the deviation about 1 month ago, on a small steel workboat using a transit. It was about 60 degrees East on a heading of 270 deg C. The deviation table suggested it should have been no more than a couple of degrees... Steel boats!
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