What's your compass deviation...

I don't have a deviation card, but I'm thinking about making one this year. I know that my lower helm compass probably needs some compensation work, as it seems to be off by more than 5 degrees depending on which direction I'm traveling.

That said, on most small boats, I think I'd worry more about my ability to steer an accurate, straight course!
 
I beleive the deviation in the Great lakes is about 9 degrees. I believe it tells you on paper maps. Now that I think about it does a GPS take that into consideration when using the compass page???
 
Brian, you're talking about magnetic variation, which is different. Deviation is a local effect caused by iron and steel in the vicinity of the compass.

Most GPS units will allow you to choose whether you want to show true readings or magnetic.
 
I think the bay area is 12 degrees. Doesn't this change over time? So if it was 12 degrees twenty years ago when I took my safe boating course, it might be 11 or 13 now?
 
Apparently not many know the difference between Variation and Deviation....
 
Willie

Give us a refresher course! It's been 20 yrs and I never navigate offshore.......
 
For those who don't know:
Variation is the difference between True North and Magnetic North which varies by area and is indicated on each compass rose on a chart. Deviation is the error on a compass caused by metal, motors, electronics, etc on a boat. It can be adjusted on the compass. A compass with no deviation will show magnetic north.

When Variation is given as 15 deg West, for example, you subtract 15 degrees from your compass reading to get true direction.
 
Thats one nice thing here, Variance is less then 1/2 degree.
 
Deviation is the effect of equipment, mostly electrical on your magnetic compass. Remember it this way, deviation is caused by all those "deviates" in your cockpit.

As to variation, it is far better to remember the converstion from True to Mag, "East is least, West is Best". If you need to go the other way, reverse it.

For using the magnetic compass remember ANDS "Acceleration North, Deceleration South" as well as "OSUN" Overshoot North, Undershoot South. How far do you over or under shoot, "why my latitude sir!"

Now go practice.
 
Old guys like me may remember it this way:

Can Dead Men Vote Twice at Elections
Compass Deviation Magnetic Variation True Add East

True Virgins Make Dull Company at Weddings
True Variation Magnetic Deviation Compass Add West

BTW, when you read the Variation at the Compass Rose (nearest your AP)on your chart, don't forget to calculate the annual increase or decrease based on the current date and the date of your chart. Of course with GPS all of this may be moot as long as your GPS is set to True!
 
True course +/- variation = magnetic course, +/- deviation = course or heading. Did I remember that right from a pneumonic from my flying days: True Virgins Make Dull Company?
Greg
 
Alex:

To respond to your curiosity, my bridge compass, a binnacle mounted Ritchie with a 4.5" card, has a westerly deviation of 5 degrees on a SW heading. Otherwise it is within 2 degrees on all other headings. In trying to reduce the SW deviation, we wound up increasing others. Made up a card with the stated deviation. Since it is next to impossible to keep my 36 Uniflite within 3 degrees, the other headings are relatively immaterial.

The lower helm compass is a smaller Danforth and quite frankly, never having run from that station, I never bothered to box it.

And to those who call it a relic, that is how I learned to navigate. If the Chinese and the Vikings were able to navigate the world with it, I see no reason to cast aside 2,000 years or so of reliability. Yes, I do have an operable RDF and LORAN C (other relics?) as well as a WAAS enabled GPS chart-plotter interfaced with my radar. When all else fails, the compass and paper charts can get me home.

Gene
 
Guys, thanks for the lessons! It all sounds vaguely familiar........when I'm in my slip, my compass and my GPS say I am pointed SW +/- 2 degrees. That's close enough for me in the Delta!
 
"And to those who call it a relic, that is how I learned to navigate. If the Chinese and the Vikings were able to navigate the world with it, I see no reason to cast aside 2,000 years or so of reliability"

Yep, I still look at the old "whiskey compass" on taxi out and compare it to the stabilized unit driven by one of three Honeywell Inertial Reference Units. You can navigate safely and accurately thousands of miles with a good mag compass.
 
I was taught "True Virgins Make Dull Companions until you Add Wiskey". I made many a Gulf crossing using only a magnetic compass and flasher depth finder. Loran "A" (first one I had was surplus from a B-17) just confirmed the D.R.
 
One thing to keep in mind is that a GPS cannot tell you which direction you are heading. It can only tell you which direction you are MOVING and can only do so when you are moving. Also, it can only directly measure TRUE track; in order to display MAGNETIC track it uses a look-up table based on your location to find and apply the magnetic variation.

So, unless your GPS has a magnetic heading sensor its heading display (more properly a track or course display) is of absolutely no value when you are at anchor or tied up.

Mike
 
Yep, everyone has it right. Many (many) years ago I became a master at swinging compasses on airplanes. I wont go into the details but will say that Ritchie describes the procedure in all of their manuals and on their website well. That said, there is almost no practical way to swing, adjust or record deviation on a ships compass unless it is off by at least 10 degrees in a major direction. The two exceptions to this would be:

.1 to have it on dry land where you can rotate/turn the ship (it would be straightforward on a trailer boat),
2. have a slaved compass integrated with GPS so magnetic course can be compared to the ships heading - even this would require near perfect conditions so the ship is pointed exactly where it is going.

Bottom line, unless you have a trailer boat, determining and applying deviation in any reliable way on a boat is impractical.

If your compass is properly installed and you guard its location, deviation will be less than normal steering errors and wiggles.
 
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