Compass deviation problem

[Caer Urfa]

A compass mounted on top of the plastic helm control panel of one of our training boats is deviating about 10/15 degree at times,
we feel this may be that it is being magnetically affected by the helm wheel mechanism inside the panel.
Has anyone any suggestions as to what material to use if we fit some sort of barrier backing pad inside the panel under the compass so it is not being magnetically affected

 

[Buck Turgidson]

I would shift the radio first !

 

[MontyMariner]

Speakers are a common source of magnetic field. You need to turn off / move each electronic device in turn and look at any deviation change +/- . It's quite a job, as it should be done on different headings, then decide what to do about the worst offenders.
In your favour is you can put the boat on a trailer and swing it through 360.
You might end up having to fit some compensation magnets if you can't adjust the compass internal magnets.

 

[William_H]

" Deviating 10 to 15 degrees at times" Does that mean at times it is fine. Or is that the deviation is constant but only noticed on different headings? Magnetic interference from radio speaker or other instruments with magnets in would be constant but to varying degrees depending on heading. Fix as mentioned above.
I do not believe a shield or barrier under compass would be a way forward. Unless perhaps parts of the steering gear have become magnetised.
If that is the case then it needs to be degaused. This I believe was done to whole steel ships. Involves a huge coil of many turns of wire around the ship hit with alternating current that magnetises then reverses the magnetism. You reduce the AC current slowly to reduce the residual magnetism.
Anyway this is done on a small scale on small aircraft engine over haul shops where steel components of the engine can become magnetised during NDT. The component is put into a coil with AC current then slowly removed to a distance before current is cut.
Probably not practical but if you made a large coil to drop over the console of several turns of heavy wire fed by low voltage AC (<5v ) you could then degause the components. Then no magnetic interference. But beware this could disable instruments and radio that need a permanet magnet. (gause is a unit measurement of magnetism) ol'will

 

[Sandy]

Has anyone any suggestions as to what material to use if we fit some sort of barrier backing pad inside the panel under the compass so it is not being magnetically affected

Is that a handheld VHF beside the compass in the photo or a knife / vape on the other side?

Have you swung your compass?

Does this happen with every helm?

You mention other boats are they all identical, if so what is different about this one?

If you find a material that absorbs magnetism make sure you patent it.

 

[Caer Urfa]

Hi gents thanks for the comments and all noted

The compass deviates even without the hand held radio next to it, we do however keep a small knife permanently for emergency use on the same top would that cause a problem as we use the boat for RYA training and as a club safety boat

we have tried to adjust the Richie compass itself but only a slight improvement so we feel it must be the helm steering mechanism inside the panel about 300mm lower than the compass which is outside the panel on the top

 

[Bilgediver]

it needs to be degaused. This I believe was done to whole steel ships. Involves a huge coil of many turns of wire around the ship hit with alternating current that magnetises then reverses the magnetism. You reduce the AC current slowly to reduce the residual magnetism.

The degassing coils on old ships had nothing to do with the compass. They were fitted to reduce the chance of a ship triggering a magnetic mine. They had their own generator and only operated where mines might be expected.

Compass deviation is taken care of by the two steel balls you see on some binacles and on other smaller binaries there may be other adjustment . If no means of adjustment then beware of mounting the wrong items within 1 metre of the compass. The compass manual should give details of any adjustment in their product.

Could always call in a compas adjuster.

 

[Alex_Blackwood]

Hi gents thanks for the comments and all noted

The compass deviates even without the hand held radio next to it, we do however keep a small knife permanently for emergency use on the same top would that cause a problem as we use the boat for RYA training and as a club safety boat

we have tried to adjust the Richie compass itself but only a slight improvement so we feel it must be the helm steering mechanism inside the panel about 300mm lower than the compass which is outside the panel on the top

Yes the knife could be the problem, or part of, Anything magnetic within a couple of mtrs. or so could also cause the problem even a mobile phone in the Cox's pocket.

 

[srm]

Knife may be a cause, why not test it to see if it is magnetic, or just move it around the compass and watch the card. It could be the steering gear, again watch card as wheel is turned with boat stationary.
Would suggest permanently keeping all the clutter with ferrous metal or magnets a safe distance from the compass. (Including stuff on or around the driver's person). If steering gear is shown to be the problem then they must be separated to a safe compass distance. A taller, none magnetic, binnacle mount may help. I used that method on my boat with compass on the steering pedestal and a wheelpilot fitted.

Must say I am very surprised that a "marine surveyor" and "RYA Instructor" is asking such basic questions.

 

[NealB]

Must say I am very surprised that a "marine surveyor" and "RYA Instructor" is asking such basic questions.

I'm with Caer Urfa on this.

After a working life in post-graduate professional training, the best learners were those that were not afraid to ask potentially embarrassing questions. They also helped the less brave to learn.

 

[Alex_Blackwood]

The degassing coils on old ships had nothing to do with the compass. They were fitted to reduce the chance of a ship triggering a magnetic mine. They had their own generator and only operated where mines might be expected.

Compass deviation is taken care of by the two steel balls you see on some binacles and on other smaller binaries there may be other adjustment . If no means of adjustment then beware of mounting the wrong items within 1 metre of the compass. The compass manual should give details of any adjustment in their product.

Could always call in a compas adjuster.

Further to above. The Degaussing coils are DC not ac The current can be increased, decreased, forward or reversed depending on the ships position. and course. Coils on Merchant ships were simply one coil, probably of more than one turn of cable run round the upper deck of the vessel. Naval vessels have a far more complex arrangement of coils in various positions and directions, some are "Thwart ships" Degaussing systems usually incorporate a compass correction unit which is switched on when the DG is in use.
Degaussing can also be applied to cathode ray tube equipment ( old fashioned radars and computer/TV screens etc. which are affected by variations in the Earths magnetic field. For example a TV designed for UK use would not function correctly in some other areas of the globe. "Blooming" of the screen is one effect.
As for compass correction, ships have, as stated above "Kelvins Balls" which can be adjusted for correction. Also they have "Flinders Bars" usually a heavy rod of iron, normally in a brass tube just in front of the binnacle and again used correction.

 

[srm]

Degaussing coils around hulls were used to negate the ships magnetic field so that they would not trigger the magnetic detonator on mines.
Kelvins Balls and Flinders Bars are soft iron. These are temporarily magnetised by the earth's magnetic field and are positioned to counteract the ship's similarly induced magnetism. Correcting magnets are also needed to counteract the ship's permanent magnetism.

After a working life in post-graduate professional training, the best learners were those that were not afraid to ask potentially embarrassing questions. They also helped the less brave to learn.

Agree totally having spent a significant part of my career lecturing in Maritime Studies, and land survey for a Master's degree. However, I am surprised that someone adverting services as a "marine surveyor" needs to ask if a knife can effect a compass. It appears to show a general lack of understanding of the properties of various metals that can typically be found on the vessels he reports on.

 

[Caer Urfa]

Must say I am very surprised that a "marine surveyor" and "RYA Instructor" is asking such basic questions.

In my 16 years of surveying and 20 years of a voluntary instructor with the RYA I have not surveyed a rotomould hull or a 3.9m plastic dinghy and our sailing club has only recently installed this compass and you never can beat other peoples experience

 

[srm]

In my 16 years of surveying and 20 years of a voluntary instructor with the RYA I have not surveyed a rotomould hull or a 3.9m plastic dinghy and our sailing club has only recently installed this compass and you never can beat other peoples experience

I understand that and suspect that rotomoulding postdates many of Colvic's boats. My concern is stated at the end of my post #12:
"However, I am surprised that someone adverting services as a "marine surveyor" needs to ask if a knife can effect a compass. It appears to show a general lack of understanding of the properties of various metals that can typically be found on the vessels he reports on."
As a retired surveyor I hope that I am wrong and a more detailed explanation is nor required.

 

[William_H]

Is that a handheld VHF beside the compass in the photo or a knife / vape on the other side?

Have you swung your compass?

Does this happen with every helm?

You mention other boats are they all identical, if so what is different about this one?

If you find a material that absorbs magnetism make sure you patent it.

I do remember MuMetal was one metal used to shield things like cathode ray tubes from external magnetic field. Iron I think in any form will redirect magnetic field through the metal. ie tend to shield field from passing through. MuMETAL® High Permeability Magnetic Shielding Alloy ASTM A753 ol'will

 

[thinwater]

I assume you tried taking the compass off the boat. Also cutting the power. As for the knife, I assume you tried moving it. It obviously can be relocated. You can also find one, perhaps, that is less magnetic.

Yup, not unlikely there is iron right under the compass. You cannot block it, but you can place other metal to compensate. Or you could relocate the compass.

Finally, you can always carry a hand bearing compass. More useful in many ways.

 

[RunAgroundHard]

That type of compass is useless on a Rib in any waves at speed. Fit a digital compass where the sensor can be mounted well clear of any ferrous materials, remotely from the actual compass read out head.

Do you actually need a compass?