Boat compass deviation?

[OldFrog75]

Who here has taken the time to determine if there is any compass deviation for your boat's compass and what did you find?

[cal40john]

http://www.cruisersforum.com/forums/...ble-22905.html

[OldFrog75]

Originally Posted by cal40john

http://www.cruisersforum.com/forums/...ble-22905.html

Oops. Thanks for the link. That poll is 5 years old and although the response %'s would probably be similar now, it doesn't indicate how meaningful the deviation was, which is what I was looking for.

[barnakiel]

There is.

As we sail by GPS mostly I tend to disregard.

Plastic boat.

b.

[Sailmonkey]

Ours ranges from 2-3 deg on east west headings to 5-6 on north south headings. Off the top of my head I don't know what directions are positive or negative though.

[leftbrainstuff]

Our Ritchie is far out of wack. It"s on my corrective maintenance list to recalibrate.

Although we use GPS and chart plotters I still primarily sail by compass dead reckoning.

We're only sailing in the bay at the moment so I can justify not completing this task immediately.

Sent from my GT-I9300 using Cruisers Sailing Forum mobile app

[Cheechako]

I swung one and it was up to 6 degrees off in spots. On one boat, it was way off when the alternator first started up and put out 80 amps. Then it would settle down in 15 mins or so. The alternator was at least 4 feet from t he compass.

[CaptainPatty]

All boats have a deviation a surveyor can do it for you



Sent from my iPhone using Cruisers Sailing Forum

[Ex-Calif]

Originally Posted by Sailmonkey

Ours ranges from 2-3 deg on east west headings to 5-6 on north south headings. Off the top of my head I don't know what directions are positive or negative though.

Similar for me.

[CaptainPatty]

I understand this conversation has gone into more of a new age technological discussion than my initial understanding of the posted question. That being said there have been several postings referring to old school navigation techniques. I do not know the backgrounds of of the people posting but have received a few messages regarding boat deviation. It is in reference to these that I again participate in this conversation.

Without use of chart plotters and relying solely upon charts, coastal pilots ( etc..) and compass. There is a simple mathematical formula to calculate heading. There is an acronym to help remember this formula.

It is :
true virgins make dull companions:
T= true ( you ascertain this by utilizing your chart and compass rose)
V = variation. You can find the annual variation on your chart. It normally is defined in degrees per annum, whether it is plus or minus depends on whether you are sailing west to east or east to west.
M= stands for magnetic, you can either find this on your compass rose of paper charts, labeled such or by using the formula above.
D= deviation- this is the magnetic interference of the electronics on your boat. This may include: antennas, ssb, radios etc that are in close proximity to your compass and sometimes structural interference. Keep in mind that new age technologies which will affect them are :!iPods, iPads I phones or any other new technology that has a location device. To test this simply place any of these questionable devices next to your compass and see what happens.
Further more an incorrect grounding wire has the potential to affect your compass reading. Back on point however: every boat has a deviation: I personally do not know how to determine this but any sound surveyor can.
C= course over ground. This takes into consideration all if the above; not reliant in gps.

Also remember and someone can correct me if I am wrong ... During war time most GPS systems are altered up to 3-4 degrees. ( and all you saltines understand that a discrepancy of only 1 degree over several days to weeks will and can effectually lead you hundreds of miles off course). You will recognize this those of you who know you were in a channel or anchorage yet your chart plotter indicates you are on land.

There is nothing wrong with electronics, I use them. However, they are known to fail: lightening strikes, salt water incursion, battery failure, engine failure :any and all if these things can and if you are out there long enough will happen.

Educate yourself because in the end a computer will not deliver you safely to shore, your knowledge and your Response to emergent situations will.




Sent from my iPhone using Cruisers Sailing Forum

[biscuitsjam]

It wouldn't be terribly difficult to measure if your compass had any deviation...

1. Find your current location on your map.
2. Find a known point that you can see (radio tower, etc)
3. Find the azimuth between the two points on your map using a protractor.
4. Find the azimuth using the compass on your boat.
5. Convert the azimuth that you got on your compass from "magnetic north" to "grid north" (a "declination diagram" on the bottom of your map will explain how to do this).
6. Compare the azimuth you measured using your compass with the one on your map.

Be aware that "magnetic north," "grid north," and "true north" are not necessarily the same thing. Grid north is "up" on your map and is usually very close to "true north" (the north pole). Magnetic north, however, is the direction towards that giant hunk of iron under the Hudson Bay in Canada. Most maps have a "declination diagram" on the bottom that explains how to convert magnetic north to grid north.

If you are terribly confused, just do a google search for "magnetic deviation".

[CaptainPatty]

Nice! Thank you.


Sent from my iPhone using Cruisers Sailing Forum

[biscuitsjam]

Originally Posted by CaptainPatty

Also remember and someone can correct me if I am wrong ... During war time most GPS systems are altered up to 3-4 degrees. ( and all you saltines understand that a discrepancy of only 1 degree over several days to weeks will and can effectually lead you hundreds of miles off course). You will recognize this those of you who know you were in a channel or anchorage yet your chart plotter indicates you are on land.

in wartime, the U.S. government can degrade GPS signals for non-military users. This would mean, for instance, that your location might be 150 meters off instead of 5 meters. This would not take you hundreds of miles off course...

The gubment could also just shut off GPS to non-military for WW3... However, even with the recent wars, the signal hasn't been messed with. I've used civilian GPS in war zones and it always worked fine.

[Ex-Calif]

Originally Posted by CaptainPatty

I understand this conversation has gone into more of a new age technological discussion than my initial understanding of the posted question. That being said there have been several postings referring to old school navigation techniques. I do not know the backgrounds of of the people posting but have received a few messages regarding boat deviation. It is in reference to these that I again participate in this conversation.

Without use of chart plotters and relying solely upon charts, coastal pilots ( etc..) and compass. There is a simple mathematical formula to calculate heading. There is an acronym to help remember this formula.

It is :
true virgins make dull companions:
T= true ( you ascertain this by utilizing your chart and compass rose)
V = variation. You can find the annual variation on your chart. It normally is defined in degrees per annum, whether it is plus or minus depends on whether you are sailing west to east or east to west.
M= stands for magnetic, you can either find this on your compass rose of paper charts, labeled such or by using the formula above.
D= deviation- this is the magnetic interference of the electronics on your boat. This may include: antennas, ssb, radios etc that are in close proximity to your compass and sometimes structural interference. Keep in mind that new age technologies which will affect them are :!iPods, iPads I phones or any other new technology that has a location device. To test this simply place any of these questionable devices next to your compass and see what happens.

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.

[StuM]

> 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...

And it's worth making it clear than Deviation is not a single figure for a compass. A compass will have different deviations for different directions. Hence the requirement to "swing the compass" (actually "swing the boat") and record a series of deviations in various directions on the card.