Back to the Future Flux Capacitor thingy

[mikehibb]

When my boat was delivered recently to its new home the skipper informed me that the Flux gate on the autohelm was reading off by 20 deg when compared to the compass (compass had been checked to be correct)

As an engineer by trade, one is normally reluctant to read the manual, however in this case I have done so.

Apparently I have to motor around slowly through a full circle no quicker than one full circle in 3 minutes in order to reset the flux gate.

My question, is it as easy to reset as it says in the manual?

Also how often should this be done? Reguarly, or just when you notice a variation?

As always thanks in advance for the replies.

 

[pvb]

Think you're confusing two different things here. The skipper's report that the autohelm was reading off by 20 degrees presumably means that the digital readout on the autohelm was different from the compass. To cure this, you either need to rotate the body of the fluxgate compass until the readout agrees with the steering compass, or to align the heading through the calibration procedure (depending on what sort of system you have).

As a separate procedure, motoring around in a circle is called "linearising" the compass, and is a means of allowing the compass to correct minor variations automatically. It won't correct a basic misalignment.

 

[peterb]

You have to motor round in slow (very slow) circles until the system says "Enough!". But that only gets rid of deviation due to iron or steel. I suspect that your major error is in misalignment of the fluxgate and the boat's centre line. After you've finished the circles you have to align the fluxgate so that the fluxgate gives the same reading as the main steering compass. And, of course, there's not much point in doing that unless the steering compass has itself been corrected.

The best way is to get someone to show you how to correct compasses (both flux gate and magnetic) and then to spend a day or so correcting your own. For the rest of the season keep an occasional eye on the difference between the two compasses, and re-correct if the difference exceeds two or three degrees.

A word of warning; don't try correcting the fluxgate in an area which might have stray magnetic effects. We once thought that our compass was playing up, so corrected it by circling in some nice calm water near Rendsburg, in the Kiel Kanal. We ended up with errors of nearly 20 degrees!

Edit: That's the problem with being a slow typist!

But don't get the order wrong; you must 'linearise' the compass first (to get rid of magnetic errors), then deal with the misalignment.

 

[jwilson]

If it's Raymarine ST60 setup you do the big slow circles bit in still water until the readout says it's finished then you do an overall +- adjustment to match the fluxgate heading either to a corrected magnetic compass reading, or a transit bearing. I had to do it a couple of times before it seemed to 'stick', but since then it's been fine.

 

[Fr J Hackett]

If you are unsure of the boats main compass and its deviation then it can be checked out and the flux gates heading realigned at the same time by motoring down a transit line which can be confirmed from a chart.

 

[fishermantwo]

I'm with Peterb with this one. After you have aligned the fluxgate compass, put a small bit of white paint as an alignment point. Often have clumsy people[kids etc] wanting to rotate this thing.

 

[mikehibb]

Many thanks all.

I guess that you all now know what I will be doing next week.
I thoroughly understand everything else that I need to do, I was just wondering if the sailing around in a circle bit was as easy as it says in the manual. Looks like a very slow circle is needed (and maybe continue until it says enough) and yes it is an ST60

Only problem is that I will look like the guy in the marina that has just fitted a new christmas present. Or is this a common occurence after christmas?
I can just imagine harbour masters chatting to themselves in the new year.

"Here they come every new year, carrying brand new boxes of electrickery, give them a couple of hours to plug them in and they will all be sailing in circles again"

I am also looking for a new handheld compass, any recommendations would be appreciated.

 

[BlueChip]

[ QUOTE ]
....I thoroughly understand everything else that I need to do, I was just wondering if the sailing around in a circle bit was as easy as it says in the manual. Looks like a very slow circle is needed (and maybe continue until it says enough) and yes it is an ST60



[/ QUOTE ]

You'll find a nice empty bit of sea to do your turns and then like moths to a candle every small craft within two miles will head your way. Its uncanny how it happens every time!

 

[peterb]

[ QUOTE ]
You'll find a nice empty bit of sea to do your turns

[/ QUOTE ]

I wouldn't if I were you. To do the sort of smooth slow turn that seems to be needed you need calm smooth water. Somewhere tucked up a reasonably wide river works well. We usually use the Stour or the Orwell, or even better Hamford Water in the Walton Backwaters.