Mounting fluxgate compass off Centerline??

[alanperry]

Does anyone have any experience mounting their fluxgate compass off the centerline of the boat?

I have a ComNav Nexus Fluxgate compass that I need to mount about 4 feet off the centerline. The Mfg says it's ok but try to keep it close to the centerline.

I can't think of a reason it would matter, unlike a regular compass that you sight down in the cockpit. The fluxgate doesn't know it's not in the center.

Any ideas or tales of your experience will be much appreciated.

Alan Perry
S/V Oceanus
Seattle WA

[coot]

Neither my mechanical compass nor my autopilot compass is on the center line.

I had a compass adjuster calibrate my mechanical compass. The deviation card shows +1 degree on all headings except for +0 for due north. As far as I'm concerned, 1 degree is about at the resolution of the compass and way below my steering ability. I have no trouble with the compass.

My autopilot's flux gate compass is even further off the center line in a hanging locker in the aft cabin. The only placement-related problem I've come across is that it gets an incorrect reading if you put a big chunk on iron on the shelf above it.

For example, a small wall-wart has enough iron in it to be readily apparent on the heading display. That is, you look at the heading and say "that can't be right", then you go move that object and everything is fine. Even if you don't correct the problem, the autopilot is still capable of "keep going this direction" as long as you get the boat pointed before you hit the "auto" button.

I agree that I can't think of any reason why a compass would need to be on the center line. If the sensor is fixed to the boat, it's relationship to the earth's field should be the same no matter where it is. (If the center line of your boat is not pitching/rolling/yawing at the same time as the rest of the boat, you have far worse problems than whether your compass works.. )

[Paul L]

The point of putting the fluxgate compass on the center line is to reduce motion. There is less motion in the center of the boat than off its axises.

[S/V Elusive]

Paul is correct. Also, the closer the compass is to the waterline will also cause less motion to be felt by the fluxgate. The Fluxgates are limited in how much they can compensate for angles.

[GordMay]

All good answers - but it’s also more difficult to achieve an accurate fore & aft alignment, when installing off centreline.

[alanperry]

Fore and aft alignment? Most of the fluxgate compasses I know about have mounting planes and holes and all you have to do is place them on a flat vertical surface to be mounted.( like the ComNav) According to ComNav the CRITICAL alignment is whether the compass is facing forward and there is a big arrow on top to make sure you don't screw that up. I am not sure I understand what you mean by fore and aft alignment.

I agree about the motion. I am able to mount mine below the waterline in the center of the boat, fore and aft wise, in a cabinet on the vertical aft wall, my goal is to keep it in the driest place possible. This cabinet fits that bill. I could mount it closer to the centerline in another "dry' cabinet but that is our "wet" locker for foulies. The only other place on the centerline that I can mount it is too close to a huge hunk of metal known as 8D batteries. That wont work. I was hoping someone would have mounted theirs off center and had some experience with that location. I just dont have a lot of choices in my boat to get over the centerline and stay dry.

Thanks heaps for all the replies, It always helps to hear the advice.

Alan Perry

[Talbot]

When you decide where to mount it, take your hand held compass into that area and see if you get any deflections with and without instruments/engine working. Then make sure you dont stow a heap of food (or beer) in tins anywhere near!

[GordMay]

By “Fore & Aft alignment”, I mean “facing forward (“... and there is a big arrow on top ...”). Facing forward means facing exactly in the direction of travel (parallel with the keel).
Assuming that you mount the compass on a bulkhead, you must ensure that the bulkhead is at exactly 90 degrees horizontally perpendicular to the centreline axis of the boat.
As long as you've mounted it with this "critical alignment", there will be no problem, with an off-centre location, except possibly in the very most extreme rolling conditions (decks awash).

[coot]

Originally Posted by S/V Elusive

Paul is correct. Also, the closer the compass is to the waterline will also cause less motion to be felt by the fluxgate. The Fluxgates are limited in how much they can compensate for angles.

How does motion affect the reading?

What angle is less on the center line at the waterline than it is in other places on the boat?

[Talbot]

I suspect that centreline and in the centre of the boat in order to minimise vertical acceleration.

[S/V Elusive]

Coot - if you notice with your regular compasses, they can only compensate to a certain degree of angle - either heel or pitch (or combinations). There are a couple of reasons for that - primarily, the magnetic fields are at such a high degree of angle that they will not register correctly with the fluxgate.

Imagine, if you will, a small boat, lost in the night, being violently tossed about; seemingly at the will of the gods. Two people, clinging hopelessly to each other, saying their last goodbyes and promising to always be with each other, when the boat pitches forward into a huge swell and turning the boat 180 degrees on end. In those last moments of desperation the man reaches out and
Checks his compass bearing? Turns on his autopilot? Wonders if he paid his Visa bill?

[coot]

Originally Posted by S/V Elusive

Coot - if you notice with your regular compasses, they can only compensate to a certain degree of angle - either heel or pitch (or combinations). There are a couple of reasons for that - primarily, the magnetic fields are at such a high degree of angle that they will not register correctly with the fluxgate.

How does placing the flux gate on the center line fix that problem?

[Talbot]

Originally Posted by coot

How does placing the flux gate on the center line fix that problem?

It doesnt, but it does have a big impact on vertical accelerations.

[S/V Elusive]

Talbot is correct - more maybe a different way of putting it - the delta (change) rate will confuse and over power your computers ability to compensate and display (track) a correct course.

[Rick]

The electronic output from the fluxgate compass is affected in 3 dimensions of alignment as well as any velocity AND accelertation from an "at rest" position. One or more time constants are associated with the settling time of that electonic output effectively cause a time delay in obtaining a true representation of heading. These time delays are longer with velocities and accelerations of the sensors.

In addition, the sensitivity of the sensor to the earth's magnetic field is made worse by proximity of ferrous materials and current carrying wires and magnets. The proximity decreases the signal-to-noise ratio which ALSO adds to the time delay of a true representation of the direction of the vessel. Bad, bad, bad.