Fluxgate compass mounting ideas?

[prv]

As I've mentioned here before, Ariam's autopilot compass is currently located in the bulk drinks locker. This is ok as long as it contains only plastic bottles of pop (or decent beer in glass bottles, I suppose, though there are the caps). But load up a few cans of lager and the course-keeping goes completely to pot - even if the lager remains in the cans

Any thoughts on good places for the compass? I know under the saloon table is common as it's surrounded by free space, but I think my "wooden" compression post is actually steel with timber cladding, and the table is built around it.

How far from big lumps of metal is it reasonable to be?

When moving it, can the wires be extended without affecting accuracy?

I'm not rushing to do the job - more important things to do first, and now we know about the problem we can just stow tins further away - but just starting to muse on what to do. I thought a thread to kick the idea around might help.

(This is the typical Raymarine compass as used for their autopilots since the year dot.)

Pete

 

[Graham_Wright]

Mine is at the foot of the compression post but that is 316 stainless.

I extended the cable to over thirty feet apparently without ill effects.

 

[charles_reed]

Mine is under the saloon table on its aft support. On one occasion the steering went totally haywire, the reason was an upside-down beer can - the only magnetic part, the ring pull, was about 10mm away.
The usual distance recommended, from a magnet (such as a speaker), is 900mm.
316 stainless is, of course, non-magnetic but >100mm from mild steel appears to be OK.

 

[AntarcticPilot]

Mine is under the forward end of the saloon berth; that keeps it a reasonable distance from anything magnetic. Providing the compass doesn't move with respect to any lumps of metal, the calibration procedure should take out any fixed offsets such as those due to the keel. Mine seems pretty much OK when checked against other compasses more conventionally mounted.

 

[Cardo]

Slight thread hijack - Mine's located in the aft cabin around 50-60cm or so from the prop shaft. I'm guessing the prop shaft should be 316 stainless, so not an issue, but does anyone think this is a bad idea?

The compass is reasonably accurate, though can be a few (usually <5) degrees out at particular headings.

 

[prv]

Mine is at the foot of the compression post but that is 316 stainless.

My comment about the steel post was based on wafting a hand compass around near it. I'm fairly sure the needle was deflecting by 20 or 30 degrees as I moved it closer and further, although it's just possible that I was actually turning the body of the compass in the process. Perhaps I should strap it to a 3-foot batten (to make turning the case more obvious) and take more careful readings.

Pete

 

[AntarcticPilot]

My comment about the steel post was based on wafting a hand compass around near it. I'm fairly sure the needle was deflecting by 20 or 30 degrees as I moved it closer and further, although it's just possible that I was actually turning the body of the compass in the process. Perhaps I should strap it to a 3-foot batten (to make turning the case more obvious) and take more careful readings.

Pete

As long as it's fixed with respect to the compass sensor, the calibration procedure should take out any effect anyway. It's varying magnetic fields you have to worry about. The problem with your former location wasn't that there was magnetic material nearby, it was that you drank the locker's contents and changed the magnetic field!

 

[prv]

316 stainless is, of course, non-magnetic but >100mm from mild steel appears to be OK.

Hmm, that's a much smaller distance than I was imagining. The aft end of the table (furthest from the compression post) might do the job after all.

Pete

 

[AngusMcDoon]

When moving it, can the wires be extended without affecting accuracy?

It's almost certainly a digital signal, which means it either works, becomes intermittent, or doesn't work at all. The value won't degrade with length, just whether it gets through or not, but I would not expect that to be a problem on the cable lengths on typical boats owned by people here.

 

[sailorman]

Mine is under the forward end of the saloon berth; that keeps it a reasonable distance from anything magnetic. Providing the compass doesn't move with respect to any lumps of metal, the calibration procedure should take out any fixed offsets such as those due to the keel. Mine seems pretty much OK when checked against other compasses more conventionally mounted.

much the same, hanging locker forrard of main bulkhd under the locker sole. lead keel so no issues there

 

[duncan99210]

Ours is in one of the aft cabin lockers. Fine unless through a momentary lapse of reason something silly gets stowed there like the travel backgammon......

 

[sailorman]

Ours is in one of the aft cabin lockers. Fine unless through a momentary lapse of reason something silly gets stowed there like the travel backgammon......

they are best placed near the centre line mid ships & low down

 

[lw395]

they are best placed near the centre line mid ships & low down

Except in a boat with a lot of steel in it, when up the mast often works very well...

Fluxgates with no moving parts should be a lot less susceptible to the accelerations of rolling and pitching, so away from the centre is probably OK.
It's all very well to rely on the calibration or swinging the compass, but I don't know anyone who has a separate dev card for different angles of heel?
The lines of flux are not horizontal, it's a 3d problem.

But the way many of us use the compass, small errors don't matter much, we are mostly looking for changes not an absolute heading to 1 degree.

 

[prv]

It's almost certainly a digital signal, which means it either works, becomes intermittent, or doesn't work at all.

Yep, if it's digital then I'm quite happy, but I didn't know if it was. The rudder angle sensor certainly isn't, I don't believe the depth and speed transducers are, and nor is the connection to the optional rate gyro for that matter. The connection to the compass uses five cores, which seems a slightly odd number for a digital link.

Pete

 

[prv]

Fluxgates with no moving parts should be a lot less susceptible to the accelerations of rolling and pitching

There's no revolving compass card, but there are moving parts to some extent because the workings are gimballed inside the black dome.

Pete

 

[AngusMcDoon]

Yep, if it's digital then I'm quite happy, but I didn't know if it was. The rudder angle sensor certainly isn't, I don't believe the depth and speed transducers are, and nor is the connection to the optional rate gyro for that matter. The connection to the compass uses five cores, which seems a slightly odd number for a digital link.

I should have qualified what I wrote - if it's recent, it likely to be digital (even el cheapo Nasa compass offering is digital). 5 cores does sound like analogue. Is it old?

 

[Poignard]

I once crewed on a steel boat in the Med. which had a fluxgate compass.

We left Alicante heading for Majorca and all seemed well for the first few hours. Then the wind got up sharply during my watch and I went forward to put a reef in. During this operation the boat lurched and I put my hand on the mast to steady myself. Something moved under my hand. It was the sensor for the fluxgate compass, secured to the mast with brown parcel tape. Fortunately it was a clear night so I was able to steer by a star and our landfall was pretty accurate.

 

[prv]

5 cores does sound like analogue. Is it old?

It's, as I said in the first post,

the typical Raymarine compass as used for their autopilots since the year dot.

I have circumstantial evidence that the autopilot was installed new in 2006 or 2007, so it's not ancient, but "classic" Raymarine tended to avoid needlessly tweaking the parts for their pilots. "Ain't broke, don't fix it" seemed to be the attitude. So the compass is probably a much older design.

Pete

 

[Laundryman]

Despite fitting a new flux gate compass, mine is still loosing its bearing. I have calibrated it according to the manual achieving a 6% deviation! well within tolerance. On my berth in Port Solent it agrees with my main compass. By the time I'm in the main harbour it's reading 100 degrees out. When I return to Berth it's correct again. I am hoping that there is some interference and will try various locations on board but it's frustrating to say the least. Is the flux gate really 'that' sensitive to interference from metal objects, its hard to find anywhere on the boat that doesn't have close proximity to metal! Thanks. Alan

 

[duncan99210]

they are best placed near the centre line mid ships & low down

But then it's difficult to get it out when you want to play a game.