I know this is an old thread, but I though I would add this if anyone was still curious/listening.
The topic came up in a GSP thread and I went searching for some answers but haven't found the exact one I was looking for. This thread indicates some misunderstanding of flux-gate - so what I know so far.
2-axis flux-gates, which most are, are sensitive to two things
1. level
2. magnetic materials.
change the level of the sensors by more than a couple degrees and the direction changes significantly. 3-axis flux-gates do not have this issue.
MOVE the sensors into different magnetic environments and the direction indication change also makes it unusable.
THEREFORE, it does not make a difference WHERE u place the flux-gate in the
boat as long as u calibrate it before use AND do not move it or any magnetic material in or into its space - say 3' - during use.
Again a 3-axis flux-gate has been -reported- to not suffer from this error and the reason for that is what I am searching for now.
Since magnetic fields on Earth vary in different locals, the flux-gate needs to be periodically calibrated as u move its location relative to the Earth as from port to port. - but then if the above is true re the 3-axis, then it should not require re-calibration in this instance - which is hard to believe.
I will just keep searching.