Compasses don't agree

[elink]

While out sailing last week I noticed that the two compasses on our dual helm boat don't agree. They are about 30 degrees different. When using a hand held and moving from one compass to the other, we get the same difference in the readings. I am assuming the boat has some magnetic interference. We recently had a new cockpit enclosure installed which did add one stainless steel tubing support from the previous design. Being stainless I am assuming that it's non ferromagnetic properties would not affect the compass, so I am not sure if that is the cause or not. There would be more SS snaps and the zippers are black, so I am not sure what they are made of, but I am guessing they are regular steel.

I also replaced the fuel sensor and fuel gauge but those are only powered up when the engine is running and they are at least 6' and 4' away respectively from the starboard compass and even further from the port side. I replaced the VHF antenna but it is a long way from the compasses and while the antennas were not identical the new one probably has less metal.

Any thoughts on what might be going on or how best to correct the issue?

 

[Don S/V ILLusion]

Stainless is an alloy composed primarily of iron, I.e., ferrous metal.

 

[shemandr]

Did the compasses agree in the past?

 

[Davidasailor26]

I’d be pretty surprised if the bimini supports were causing a 30 degree deviation. Is the deviation consistent at all headings, or does it seem to vary as you turn? Can you tell if one side is right and the other is off, or if they’re both off by different amounts?

Do you have a picture of the helm area? Common causes of deviations around the helm are speakers (either for a stereo or a VHF handset), random guest purses, phones, etc, or the magnetic latches used for some chartplotter SD card doors.

 

[elink]

I am pretty sure they did. However, I have only owned the boat a few months and I am not 100% sure.

 

[elink]

I’d be pretty surprised if the bimini supports were causing a 30 degree deviation. Is the deviation consistent at all headings, or does it seem to vary as you turn? Can you tell if one side is right and the other is off, or if they’re both off by different amounts?

Do you have a picture of the helm area? Common causes of deviations around the helm are speakers (either for a stereo or a VHF handset), random guest purses, phones, etc, or the magnetic latches used for some chartplotter SD card doors.

Unfortunately, I am not near the boat (1800km away) so I can't provide any new pics. The deviation seems consistent for various points of sail. I couldn't really tell if one was more off than the other as the handhelds did the same thing. I will have to get a bearing while standing on the dock and see how it compares. There are speakers about 1 to 1.3 meters away at the companionway. The VHF handset is probably 1 meter or less away from the starboard compass and more for the port side. I thought the phones as well but when I moved them there was no change. The starboard compass seems to agree fairly well with the chart plotter.

 

[elink]

Stainless is an alloy composed primarily of iron, I.e., ferrous metal.

I should have used the term "non-magnetic"

 

[Maine Sail]

When was the last time you swung the compass. Here In Maine with thick fog I had it done pretty routinely. You will then have a deviation card to reference

 

[Don S/V ILLusion]

I should have used the term "non-magnetic"

Regardless, SS can upset near field magnetic. If that’s the only change and the compasses previously agreed, deductive reasoning ….

 

[JoeWhite]

You did not indicate if this error is evident while at the dock or on the water. If on the dock, disconnect the shore power and test. It could be magnetic field from a shore power transformer. Move to a guest dock and test.

 

[JamesG161]

add one stainless steel tubing support from the previous design

Is the one you added near the Compass in Question?

Jim...

PS: A compass can be affected by other metals too.

 

[kappykaplan]

PS: A compass can be affected by other metals too.

And a bag of Granny Smith apples! Don't ask me how I know.

 

[jssailem]

If you have a cell phone with a compass, you can get an app and harness the power of your phone to act as a Gauss meter and detect magnetic fields on your boat.

It may be a piece of cheap 304 stainless steel ( you know like the stainless steel used on your refrigerator at home that holds the magnets). Or there may be a speaker near enough to attract your compass.

 

[rgranger]

And a bag of Granny Smith apples! Don't ask me how I know.

Wait….!
You can’t just drop that and walk off.

 

[kappykaplan]

Wait….!
You can’t just drop that and walk off.

The autopilot started doing Crazy Ivans! Took a while to sort that out.

 

[jssailem]

I know that trick.
I set a portable radio on top of the cabinet with the EV100 sensor. Immediately the AP went berserk. A radical turn to starboard. By the time I got into the cockpit and disarmed the AP, I was on a 190 reciprocal course. Moved the radio and the AP reported. “All good here , Sir. We are back on course. The rocks to Starboard are no longer a threat!”

 

[thinwater]

I was cruising along on autopilot, easy going, easy weather. I started futzing around the deck, working on a few things. Suddenly the boat was going all over the place. Then I realized that I was wielding a metal scraper just above the autopilot compass. When I scrapped the boat turned one way, when I stopped it returned to the correct course. It was about a 20-30 degree deviation.

 

[Mark Maulden]

“When using a hand held and moving from one compass to the other, we get the same difference in the readings.”
“What is your belt buckle made from?

 

[elink]

When was the last time you swung the compass. Here In Maine with thick fog I had it done pretty routinely. You will then have a deviation card to reference

I haven't done it on the boat as yet. Newly acquired in December and only noticed the issue in late April.

 

[elink]

You did not indicate if this error is evident while at the dock or on the water. If on the dock, disconnect the shore power and test. It could be magnetic field from a shore power transformer. Move to a guest dock and test.

The problem seems evident both on the water and at dock