Handbook of Magnetic Compass (H.O. 226)

[S/V Alchemy]

Anyone here ever sailed over a nice big iron deposit and watched what it did to the compass? Good times!

[Capt Phil]

There are quite a few oddball compass inflicted deposits in the PNW but if your moving it doesn't seem to bother your course too much although those were the days when the autopilots were mostly Wood Freemans or a short lashing in the wheelhouse.
There is an island off the west coast of Australia called Magnetic Island that gave mariners fits until they sorted it but there are lots that give mariners fits 'down under', mostly the cost of marine and aviation equipment, I understand! Phil

[noelex 77]

In most non steel boats the errors in a reasonably mounted compass are small.
By far the most common problems are siting something magnetic close by. Headphones, even some glasses cases have magnets installed.

Two steering compasses in separate locations are good insurance. One of these can be an electronic flux gate compass. Comparing the results will indicate if a problem has developed as a local magnetic anomaly will not effect two disparate compasses equally. An error between the readings indicates a problem.

The GPS and COG provides a further check, but COG will not match heading unless tide and leeway are insignificant, so take these factors into account before condemning the compass.

[David M]

There also used to be a degausing range on the east side of Angel Island in the SF Bay.

[S/V Alchemy]

Originally Posted by noelex 77

In most non steel boats the errors in a reasonably mounted compass are small.
By far the most common problems are siting something magnetic close by. Headphones, even some glasses cases have magnets installed.

Two steering compasses in separate locations are good insurance. One of these can be an electronic flux gate compass. Comparing the results will indicate if a problem has developed as a local magnetic anomaly will not effect two disparate compasses equally. An error between the readings indicates a problem.

The GPS and COG provides a further check, but COG will not match heading unless tide and leeway are insignificant, so take these factors into account before condemning the compass.

Indeed. I have an old but reliable KVH AC103 fluxgate at the helm, with the sensor mounted far from the steel in the center of the aluminum pilothouse roof. I compare a known bearing (derived from visual daymarks lining up) with the fluxgate, the GPS and the Ritchie to confirm they are in the same ballpark. The Ritchie has a little delay in finding its course, but it is usually within two degrees of the fluxgate and the GPS and seemingly on all points, accounting, of course, for my local variation.

[Lodesman]

Originally Posted by Richard5

Thank you for your comments. But while you state that deperming and degausing are not the same and you state how deperming is accomplished you do not state why deperming is done. Please elucidate.

I thought I had made that clear; I guess not

When a warship is brought into service the DG equipment is set-up to counter the permanent magnetism, and it is "tuned" by the adjustable settings to counter the induced magnetism - these are explained in your original posts. The subpermanent magnetism happens over time - caused by thermal cycling, flexing, pounding, added/removed equipment, etc., etc. This causes a change in the ship's magnetic signature that can't be easily measured or accounted for, and is generally dismissed as insignificant for the purpose of degaussing. This is the reason ships run the DG ranges in various ports - solely to measure the effectiveness of the DG equipment and give an indication that the subpermanent field is below limits. When there is so much subpermanent magnetism created that the DG equipment no longer satisfactorily negates the ship's magnetic field, then it has to be depermed. Deperming erases the subpermanent magnetism, effectively resetting the ship back to its original permanent signature, for which the DG equipment has been optimised. I suppose a ship could be depermed also to aid compass adjustment, but I don't imagine anyone would go to the expense of it.

[nigel1]

Originally Posted by S/V Alchemy

I would have thought there was absolute zero chance of that being true...

Might be a grain of truth to it, here's on example
http://www.texasnavy.com/Newsletter/...Correction.pdf

Google Kelvins Balls Magnetic Compass, lots of stuff out there

[Wotname]

Originally Posted by Capt Phil

There are quite a few oddball compass inflicted deposits in the PNW but if your moving it doesn't seem to bother your course too much although those were the days when the autopilots were mostly Wood Freemans or a short lashing in the wheelhouse.
There is an island off the west coast of Australia called Magnetic Island that gave mariners fits until they sorted it but there are lots that give mariners fits 'down under', mostly the cost of marine and aviation equipment, I understand! Phil

Maybe your compass needs adjusting or maybe you not used to how we do things downunder but we call this part of the continent the east coast