Fluxgate compass locaiton...

Rich Stidger · May 1, 2012

I don't have your boat, so I can't answer your location question, but the common reason that the binnacle compass and the AP don't agree is due to calibration. The fluxgate compass, unlike the magnetic binnacle compass does not inherently know where north is located. The fluxgate merely reads the flux of the earth's field. You have to tell it - by calibration- where north is located, or in which direction it is pointing.

In your owner's manual (download one if you don't have one) it will give you a procedure for linearizing the compass and for adjusting the compass to read correctly.

Linearizing the compass is where you swing the boat slowly through several circles so that the fluxgate compass system can store an electronic deviation table for your vessel. First find the fluxgate compass and be sure that there are no metal objects within a couple feet in any direction of it. After that is complete, you need to set your boat on a known heading and adjust the display reading to that direction. The direction does not need to be north or any particular major compass direction- you just need to know the precise direction, eg. 142 degrees Magnetic.

You accomplish this action by plotting your position on a paper chart and locating a mark that is 1/4-1/2 mile away from you. Determine the magnetic direction to that mark - if you measure the direction in TRUE off the chart, be sure to correct for the local variation and convert to MAGNETIC. Set your boat (calm water is best) to point to that mark, and while holding that direction, adjust the fluxgate display to read the magnetic bearing that you have obtained from the chart.

DO NOT USE THE GPS HEADING TO SET THE FLUXGATE HEADING. Use a paper chart and determine the bearing to a distant mark as described above.

If your compass and fluxgate differ by 10-30 degrees, I suspect that the reason is one of the following:

1. This calibration was never done
2. Metal or magnetic material has been placed near the fluxgate or the binnacle compass (maybe in an adjacent compartment)
3. The fluxgate compass has become detached from it's mount and is not orientated on the center-line of your vessel.
4. Defective fluxgate compass - not common.

Be aware that after you adjust the fluxgate display, the binnacle compass and the fluxgate probably will not track perfectly at all points of the compass. This is because the binnacle compass my not be calibrated itself for deviation. Also the heading that you see on the GPS will typically not agree with either the binnacle or the fluxgate due to the effect of current and wind set.

Bill Roosa · May 1, 2012

Could also be the binical compass needs to be calibrated or has some metal object near it. You need both correctly aligned to get agreement.

You can find the fluxgate compass by following the wires from the AP master control box. the manual will tell you which set of wires goes to the fluxgate. Just follow them to the source. you might want to note along the way any other wires that run along the fluxgate wires for any distance. they may be causing interferance.
Also check the terminals on everything. The AP uses 5 volts for the signals so it does not take much to drop the voltage to a level that is unusable by the various components.

chuckwayne · May 1, 2012

Our fluxgate is mounted under the nav station and works ok-But-as Rich said, you have to set it up correctly and be careful about not putting anything magnetic near it.

Mike H. · May 4, 2012

Here's my horror story.

Bought our 94 Hunter vision in 2009. Auto pilot never worked. Swung that damn fluxgate compass 20 times. Around and around and around. It never locked on.

So I traced the wires thinking I would relocate the compass to get it away from the engine or keel.

The original installer had placed the wire under the engine mount and crimped and striped the cable under the engine. He had neatly placed a small piece of black tape to cover his f*$^up.

I cut the cable back, re-spliced it, relocated the fluxgate compass under the aft stateroom berth, swung the compass and BAM! locked on!

It works great now. Need to re swing it. They deviate a little after awhile.

Roger Long · May 5, 2012

Rich Stidger said:
DO NOT USE THE GPS HEADING TO SET THE FLUXGATE HEADING.

But, you can set a distant waypoint at a mark you can see and use the bearing obtained from the GPS. This will be more accurate than a lot of the fixes you can get on a paper chart.

Rich Stidger · May 5, 2012

Roger Long said:
But, you can set a distant waypoint at a mark you can see and use the bearing obtained from the GPS. This will be more accurate than a lot of the fixes you can get on a paper chart.

Agreed. My caution was to avoid an incorrect bearing due to the set of wind or current. If the GPS is used to calculate a bearing, double check to be sure the GPS is using the correct variation for your location and that it is reading in magnetic not true.

Roger Long · May 6, 2012

Rich Stidger said:
My caution was to avoid an incorrect bearing due to the set of wind or current. If the GPS is used to calculate a bearing, double check to be sure the GPS is using the correct variation for your location and that it is reading in magnetic not true.

If someone is vulnerable to these GPS errors, they probably shouldn't be letting an autopilot steer their boat, or their hands for that matter.