Magnetically hiding an engine?

[Seagreen]

Is this possible? The only reasonable position for a magnetic steering compass is about 2 feet from the engine - not ideal by any means. So I thought that covering most of the engine box in a wire mesh (securely holding on the insulation anyway) would help to create a Faraday cage effect and neutralise the Deviation in some way. Is this likely? I'm going to do a Dev. table for the compasses anyway, but I'd like to try and reduce the errors. I think I get a 45 deg. error at the moment.

The only other alternative is an edge reader like a Sestrel Moore type, but again, that would be very vulnerable to being torn off or stepped on.

 

[cagey]

Re: Magnetically hiding and engine?

good post
for years have been attempting same including insts on binnacle.
surely today we could shield our compass (magnetic and fluxgate ) from the influence of cables and lumps
Advice from experts please

 

[Seagreen]

As for the fluxgate compass, it sits about 2 feet itself above a very long cast iron keel, yet the previous owner said it had negligible Dev. I doubt this but I haven't been able to test it out so far. Can this be treated in the same way? If the fluxgate compass is affected, I can always move it off the centreline and forrard away from the keel and the engine.

 

[VicS]

[ QUOTE ]
The only reasonable position for a magnetic steering compass is about 2 feet from the engine

[/ QUOTE ] That is not a reasonable position for a compass.

A Faraday cage would prevent lightning striking the engine but I don't think it will have any significant effect on the magnetic field. A soft iron enclosure might!

 

[roly_voya]

Only way out is to use a fluxgate with a remote sensor. If there is nowhere in the ship that the sensor can be sited away from magnetic problems you can mount it on the mast but will get some degredation from movement which will make it a bit less stable. Find a position to mount a magnetic compass in case you lose power even if it is vunerable/awkward to see.

 

[john_morris_uk]

I am afraid that Faraday Cages don't shield magnetic influences. They do shield electromagnetic waves like radio waves - but not magnetic fields...

 

[Talbot]

The only way this is achieved on a metal ship is by having two large globes of metal either side of the magnetic compass equidistant apart.

 

[cliffordpope]

[ QUOTE ]
The only way this is achieved on a metal ship is by having two large globes of metal either side of the magnetic compass equidistant apart.

[/ QUOTE ]

That's right - a traditional binnacle has a large red and green globe mounted either side. Are they just lumps of iron, or are they magnetic?

I remember having an ex-RAF grid steering compass years ago and it came with a small device for mounting underneath the compass. It had a key for moving the positions and orientations of one or more magnets inside the box. We never did get round to following the instructions for setting it up, but I think the idea was that by painstaking checking and adjustment it was possible to eliminate or at least reduce the errors.

 

[john_morris_uk]

[ QUOTE ]
[ QUOTE ]
The only way this is achieved on a metal ship is by having two large globes of metal either side of the magnetic compass equidistant apart.

[/ QUOTE ]

That's right - a traditional binnacle has a large red and green globe mounted either side. Are they just lumps of iron, or are they magnetic? <span style="color:blue"> They are lumps of iron (non magnetised) called 'Flinders Bars' </span>

I remember having an ex-RAF grid steering compass years ago and it came with a small device for mounting underneath the compass. It had a key for moving the positions and orientations of one or more magnets inside the box. We never did get round to following the instructions for setting it up, but I think the idea was that by painstaking checking and adjustment it was possible to eliminate or at least reduce the errors. <span style="color:blue"> Many compasses have adjustment magnets - usually two opposed magnets of equal strength in the N-S and E-W planes arranged with gearing to allow you to control the amount of magnetic influence to counteract deviation from the other magnetic influences nearby. Even cheap Plastimo compasses have such adjusters...</span>

[/ QUOTE ]

 

[thurloo]

hi i have a steel boat i solved this problem by mounting a aluminium pipe to my deck.in the cockpit, about 1halfmts long . and put a plate of same on top, then put compass on that . and its 100 percent and its been there now for 5yrs , hope this helps as its simple to do dave

 

[pampas]

The two soft Iron balls are Quadrantal correctors and flinder Bars are the vertical ones beneath the compass,in addition thete may be small permenant magnets as well. might as well use the correct terminoligy.

 

[AntarcticPilot]

This is a problem solved in the 19th century, when ships were first built of iron. It is, perhaps, interesting that the first tests were carried out when SS Great Britain was being built (one of the arguments against iron ships was that the compass would be unreliable; given about 150 years of experience, I think we can safely say this isn't so!). The bad news from your point of view is that you need a compass mount that can incorporate compensating magnets. Setting this up for a magnetic compass is a skilled job; you will need the services of a compass adjuster.

Fluxgate compasses can automagically compensate for the effects of local magnetic fields; this is usually part of the setting up and simply requires the boat to be steered in a circle a couple of times.

 

[saltwater_gypsy]

There are materials like Mu-metal, which are available in foil form, which shield magnetic fields. They work by having a very low level of magnetic saturation. Unfortunately in shielding your engine it would also shield the earths magnetic field and you would be no further on.
Otherwise compensation is required. (Not necessarily big balls)

 

[Seagreen]

No, 2 feet from the engine isn't a reasonable position for a compass, but it's the only feasible and usable place for it.

I'll explain further.

The compass sits in a wooden box in the front face of the cockpit bulkhead. Luckily, I can arrange for two iron spheres to be placed either side of this box, but accessible from inside the boat which would be ideal. There would also be room for a small series of corrector magnets to be set up underneath the compass, either in the box or underneath, assuming that 9mm ply is magnetism transparent! The total arrangement would be a sort of "binnacle in a bulkhead" with just the compass visible from the cockpit.

I've also two different compasses (belt 'n' braces), a heath type sailing compass and a brass sestrel one. Neither have integral corrector magnets like the Plastimo Contest I used to use (V good but prone to leaking) though I could source the Spheres and corrector magnets from elsewhere. I think SIRS navigation might be the obvious choice for these.

As for adjusting these, I'll have to go back and re-read my Bowditch. I may have skimped on that chapter.

Would this arrangement be successful?

 

[johnalison]

As far as I remember, the trouble with your engine is that it will cause heeling error which can't be corrected by magnets, only by the iron balls mentioned. While correction of the other errors is a skilled kob, it's not impossible - I had 14 deg on one boat from a fire-extinguisher bracket and got it down to less than a degree with the correcting magnets on a Sestrel Minor. (You might well ask why I didn't move the extinguisher; but I like a challenge).

 

[Sixpence]

[ QUOTE ]
(You might well ask why I didn't move the extinguisher; but I like a challenge).

[/ QUOTE ]

Yes, I was going to ask that /forums/images/graemlins/grin.gif
That's a pretty hefty error though, out of interest, I presume you've done a compass swing to check that it's not still interfering ? and what's going to happen if you remove the extinguisher ?.

 

[wotayottie]

Must admit I thought that the software in a modern electronic compass eliminated deviation ie sensed it and corrected for it.

mind you I doubt that it would counteract on / off 12 v powered electrics on the engine.

chicken wire would have no noticeable effect .

check the deviation in the location you are thinking of using. the magnetic field of the engine block may well be pretty small. my worry would be about the electrics so check with engine off and engine running.

my autohelm compass is something like 2 ft away from my engine. must admit that I dont use it to read off a course in degrees so much as to say "we are on course - maintain that" Which it does effectively. So if you dont want a precise reading in degrees, my guess would be that you would be OK.

 

[hightech]

Seperation is the only remedy, however if you do come up with something that works you would make a fortune.

Generally, a 1 metre seperation for an engine is sufficient and a compass adjuster could allow for that easily.

When installing utopilots I have always used a 1 metre minimum and it works for most non ferrous hulls. Ferrous hulls need to gu the mast or a pole at least 2 metres.

 

[grumpy_o_g]

Having swung many, many aircraft compasses that are stuck in a cockpit near all sorts of dodgy influences it's certainly worth a try. On the old E2B emergency compasses (bit like the cheapo car compasses you used to get) we'd often take out 45 degrees of error. Usually managed to get them well below 5 and often down to a couple of degrees of deviation, which was much straighter than most of the pilots could fly.

I'd be more worried about the effect of the engine electrics on the compass when it's running. You could watch the compass swing as you operated the (electric) seat mechanism on some a/c. You'll need to swing it twice, once with the engine running and once with it off and may need to produce two cards.

Apologies if I'm telling you what you already knew...

 

[jimboaw]

I confess I hardly ever use the steering compass these days. Have had laptop charting programs (on two laptops) for the past 10 years and back up independant autopilots with their own fluxgates. I also have a cheap radar. I steer out of the harbour or away from a mooring/anchorage but that's about it. The rest I leave to technology. Don't get me wrong, I can do it if I have to, many channel crossings with nothing more than a compass, not even a log but why bother?


Ducks below parapet