Magnetic Compass Alignment

[wgkenney]

Hi all,

We recently purchased a 2006 Beneteau 473. Boat is in great shape. But I have an issue with my heading indicators. My wife and I have been chartering for about 20 years and, as a result, usually sail the generic singly helm versions so boats. Never really had autopilot etc.

Our Beneteau has the Raymarine electronics package, ST60, Rate Gyro, Pathfinder, Autohelm, etc. This boat also has 2 magnetic compasses. One over each helm. As I was using the autohelm the other day I noticed the compass over the port helm was about 5 to 6 degrees off from the reading on the autohelm and the starboard station magnetic compass was about 10 degrees off from the port station.

I'm just learning about the electronics on board so I assume that the chartplotter is getting its input from a source other that the magnetic compasses. As I started to watch them I notice that the difference tends to vary a lot as well.

I'm use to just having one compass to look at (now wondering how accurate those were) so I'm trying to decifer which one to believe. We are in the Chesapeake Bay so we visually navigate most of the time. But I'm curious if it is normal to have such a variance in the magnetic heading sources if it is normal for their delta to shift so much.

We do have stereo speakers in the cockpit, a generator in the aft starboard locker etc. So there are fields and hunks of iron there along with the diesel. But I would have assumed that the compasses over the helms were calibrated with that in mind.

 

[Rick D]

Well, of course that assumes they were in fact calibrated. I wouldn't count on it. You'll want to calibrate them yourself or have someone swing them. As for the autopilot compass, it has an adjustment you can make. However, it isn't unusual for it and the mag compass to be off 5 degrees or so. RD

 

[Witzend]

I have the same issue with my auto pilot's flux gate and mag. compass. Depending on the heading it can be right on, or 5-7 deg. off. The Northern Chessy where I sail most can be fickel as well, adding to the error. The annoying thing for me is that my Raymarine C90W heading vectors are off because the flux gate provides the signal. I have calibrated it a number of times, but still there is a discrepancy. I probably need to swing my compass as well.

 

[Joe]

you can google "how to swing a compass" or get a copy of John Rousmaniere's "Annapolis Book of Seamanship" where you'll find specific instructions on how to calibrate/swing a boat's compass.

 

[wgkenney]

Thanks all,

Looks like this is not that uncommon. I need to read up more on how to swing my compass. Maybe have one of the techs at my marina do this with me the first time.

 

[Ross]

I have the same issue with my auto pilot's flux gate and mag. compass. Depending on the heading it can be right on, or 5-7 deg. off. The Northern Chessy where I sail most can be fickel as well, adding to the error. The annoying thing for me is that my Raymarine C90W heading vectors are off because the flux gate provides the signal. I have calibrated it a number of times, but still there is a discrepancy. I probably need to swing my compass as well.

Witz, Up on the elk River there are local magnetic disturbances besides the 11.5 degree variation.

 

[Witzend]

Yea, I think the proximity of Iron Hill may be part of the problem. The other issue I have is regardless of the geographical anomaly's my ship's compass and flux gate should read the same, and they are not, at all points of sail. so something is not right. I really need to swing my compass as well, but really need to do it in a different area.

 

[CompassAdjuster]

Magnetic Compass

It is quite simple to check your magnetic compass. http://www.compassadjuster.com.au/diy.htm
outlines a simple method for checking all the headings. If the error (deviation) of each compass is observed on the cardinals (N,E,S &W) the question of alignment (the compass's lubberline being parallel to the boat's longitudinal axis) and deviation due the the vessel's magnetism at the compass site should be able to be identified.

 

[RichH]

Witz - the Upper Ches. indeed is variable with respect to magnetic compass headings - I get upwards of 5 deg. 'oscilations' from month to month. Worton to Elk R.

Lots of web directions on how to swing a compass; but, ultimately you'll probably not ever get any compass to 'equate' to a flux gate, and mag. compass to mag compass. For that you ultimately will need a correction card so you can know how much to add or subtract to the 'read' reading.

And just to make things worse for you, the earth's entire magnetic field is starting to get 'very funky' with regard to geomagnetic flux readings. www.ngdc.noaa.gov/stp/geomag/kp_ap.html

 

[Maddog]

Thank you for those links. You can also use the "bearing to" function of a gps to help you align and check against a known location. I took my handheld out to Angels Gate in San Pedro and marked the way point physically. I am also lucky that I have about 2 to 3 miles of breakwater that runs damn near E-W true that is a big help.

 

[Ross]

When you try to use GPS for headings to check your magnetic compass beware the cross currents. You may be pointed east but going ESE over the ground. The GPS will show your true course.

 

[Witzend]

Hey Rich,
This is some pretty heavy stuff being tracked by NGDC!! I guess the only thing you really know is which way up is. As you are probably already aware the earth has had a history of reverse poling. One theory has it that the dynamo (molten iron at the center) periodically gets in a funk and causes a reversal. The last one was ~780K ago. They seem to be pretty random events. Boy, would'nt that send the art of navigation in a spin!

 

[RichH]

Witzend ...... stay tuned as there are some areas in the S. Hemishpere that are beginning to be 'already' showing up with NORTH polarity.

I wonder when the 'reversal' happens, will clocks, electric meters, computers, DC motors, etc. run backwards? ;-D

 

[Witzend]

No worries mate, a magnet used in a motor app will not change it's N/S poll ends if the earth's polls swap, but any magnetic compass will be waked out.

 

[wonkodsane]

It's OK. When the poles reverse, I'll just use a mirror to look at my compass.

 

[Seattle Scott]

Remember that the compass heading and the chartplotter / GPS heading will almost never match up. The chartplotter tell you where your boat is headed and the compass tell you where your boat is pointed.

We were sailing south according to the compass and where the bow was pointed. The knot meter said were were going 2 knots. The chartplotter said we were going 1 knot north ( which was true) because there was a 3 knot current out of the south.

 

[Ron McCoy]

Some good advice by all. From a practical point of view try a technique that's worked for me. On a calm day, with calm seas and no/or little tide affect point the boat set up a 3-4 kt course. Note the course over ground on the chartplotter (the fluxgate is feeding this data to the chartplotter). Note the COG heading. Then with a copper compass adjustment tool, adjust the magnetic helm compass to match this COG heading (from the chartplotter). This is the first adjustment you'll make. Now, turn the boat and head 180 degrees from the original course. Note the COG and adjust the magnetic compass again to match. Now, turn 45 degrees and repeat the process. Lastly, turn the boat 180 and make the fourth adjustment to the magnetic compass. If you have time, once one of the helm magnetic compasses at the heom are adjusted it's just a matter of swinging one compass to match the other. The idea is to obtain a "good" gps heading (COG) and use those reading to adjust your magnetic compass(s). Remember to use all four headings to ensure the N/S and E/W are properly calibrated.

 

[Stu Jackson]

We were sailing south according to the compass and where the bow was pointed. The knot meter said were were going 2 knots. The chartplotter said we were going 1 knot north ( which was true) because there was a 3 knot current out of the south.

Scott, that's a bit confusing as an example, because the bow is still pointing south, and may confuse some people. Your example is SOG and SOW.

Here's another example that points the difference between compass heading (bow point direction) and GPS heading but more drastically:

You're going down a river or through Deception Pass. Boat's heading west, so's the current. You hit an eddy and the bow of the boat swings north, and you get kicked back into the main flow with the bow still pointing north. But you're still heading west based on your GPS heading.

GPS headings AND boat compasses almost always SHOULDN'T agree, except if you're sailing on a lake and even then leeway from sailing will make a difference.

The time people spend trying to think they should agree means many should go back to their Chapman's and Dutton's and Annapolis BooK of Sailing and do some more reading.

What we all should be doing is checking the different sources we have on board and seeing if there are major discrepancies. Most of us have two: a GPS and a compass. Some have an autopilot, too.

 

[Witzend]

Stu you're still getting confused between GPS course or (COG) and GPS heading. We had this discussion before. Heading is where the bow is pointed....


http://forums.sbo.sailboatowners.com/showthread.php?p=660562&highlight=GPS


The Flux gate compass provides to my GPS/chart plotter the heading information. Therefore the boat's compass and GPS heading should be the same if both the Flux Gate and Compass agree, (which in my case they don't ) even though the direction you’re moving over land may be completing different.

 

[Seattle Scott]

I agree with Stu and I dont think he is confused as claimed.

" Stu you're still getting confused between GPS course or (COG) and GPS heading"

As far as I know my GPS don't know my heading or where the boat is pointed and appears to be going. The GSP can only determine COG. My fluxgate hasn't working since I got the boat, but I thought it was just an electric compass which could tell we where the bow was pointed, the heading and if displayed on a chartplotter woudl give a heading while the GPS will show COG on the plotter. the bigger the differnece between heading and COG the more forces are at work pushing your boat around, leeway, current and the like.