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Old 11-06-2018, 00:49   #91
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Re: true vs magnetic bearing

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Originally Posted by Lodesman View Post
Deperming isn't done to alter natural magnetic anomalies; it's done to remove induced magnetism from warships, so they will (hopefully) not set off magnetic-influence mines. It involves surrounding the ship with huge electric coils, and turning the ship into a giant electro-magnet. As you can imagine, a destroyer-sized magnet will have a significant local effect on magnetic compasses.
Lodesman, I do agree with your first statement but not your last. Depressing / degaussing stations should have no magnetic effect within the local area especially when it is not active (and very little even when active). TreblePlink gives the reasons below and to recap, the degaussing process is to create a rapidly changing large magnetic field (NS to SN and back again etc). The frequency is often just AC mains frequency (60 / 50 Hz) and slowly reducing the field to zero. Any nearby compass will never respond to an equal and alternating magnetic field at those frequencies. When the degaussing station is inactive, there is no magnetic field being produced.
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Originally Posted by TreblePlink View Post
Degaussing normally refers to reducing residual magnetism in ferrous metal. For reasons already stated, this is done to ships to lessen vulnerability to certain mines. A great antenna engineer and author - Dr. John Kraus worked on perfecting the process in WWII.

Generally, an AC current is applied to coils on or adjacent to the metal, and then the current is gradually reduced to zero.

https://en.wikipedia.org/wiki/Degaussing
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Originally Posted by El Pinguino View Post
I do so wish that people would stick to traditional maritime terminology..ie ...variation, deviation, and dip... and leave that other stuff to the dirt dwellers..

What next... left and right instead of port and starboard?

COMPASS ERRORS | VARIATION | DEVIATION

What are errors of magnetic compass? |
I join you with that wish but I'm afraid it is just going to be .
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Old 11-06-2018, 15:31   #92
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Re: true vs magnetic bearing

It's going to be difficult to stay current with magnetic North on paper charts. It might be because the magnetic North Pole happens to be racing 89 miles annually towards Kamchatka Russia. Read it and weep !
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Old 11-06-2018, 16:35   #93
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Talking Re: true vs magnetic bearing

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Originally Posted by Wotname View Post
Lodesman, I do agree with your first statement but not your last. Depressing / degaussing stations should have no magnetic effect within the local area especially when it is not active (and very little even when active). TreblePlink gives the reasons below and to recap, the degaussing process is to create a rapidly changing large magnetic field (NS to SN and back again etc). The frequency is often just AC mains frequency (60 / 50 Hz) and slowly reducing the field to zero. Any nearby compass will never respond to an equal and alternating magnetic field at those frequencies. When the degaussing station is inactive, there is no magnetic field being produced.
I believe you are still confusing degaussing with deperming, and you may be confusing that with TV screen degaussing. Degaussing stations have no effect; as I said before they just measure the effectiveness of a ship's degaussing system. Warships use built-in degaussing coils; which I believe (though am not certain) are DC - and when energized create small magnetic fields that are meant to be equal and opposite to the permanent and induced magnetic fields of the ship - thus nullifying the ship's effect on the local magnetic field, and making it invisible to mines which are triggered by a shift in the local magnetic field. Deperming is done to ships to remove the magnetism that accrues over time from flexing and pounding and just being in the Earth's magnetic field. It "resets" the ship's magnetism to a known and manageable quantity, so that the DG coils are more effective. In this process the ship is surrounded by a large copper coil and it is energized with high-amperage Direct Current. Depending on the size of the ship and its magnetic make-up, this charge could be anywhere from a couple seconds to a minute or more. The coil is then de-energized, and allowed to cool, before it is energized in the opposite polarity for an equivalent amplitude and duration. This process is repeated while stepping down the amplitude, until the desired magnetic bias is achieved. When I did this, we had to remove the magnetic compasses from the ship, and because it was back in the day - all floppy discs and VHS tapes.
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Old 11-06-2018, 17:25   #94
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Re: true vs magnetic bearing

This thread reminds me of my early foray into navigation taken at VVI(Vancouver Vocational Institute). It was a required course of study for my Mates Ticket back in the ‘60’s. Learned a lot and worked commercially for over 20 years and never put a boat up on the putty!
My first experience with Celestial Navigation(sorry for the thread drift) was computing our position on the shore of English Bay. Unfortunately, I placed us just north of Canmore, Alberta as I recall.
Went on to pass the course, however and never looked back(except at the black oil barge bearing down on our tow boat after the decky ran us onto Sheringham Rock in the Straits of Juan de Fuca one cold February night). The tow boat sank but was refloated a week or so later... barge was fine.
I miss those days but not enough to try and relive them!
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Old 11-06-2018, 18:54   #95
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Re: true vs magnetic bearing

Quote:
Originally Posted by Lodesman View Post
I believe you are still confusing degaussing with deperming, and you may be confusing that with TV screen degaussing. Degaussing stations have no effect; as I said before they just measure the effectiveness of a ship's degaussing system. Warships use built-in degaussing coils; which I believe (though am not certain) are DC - and when energized create small magnetic fields that are meant to be equal and opposite to the permanent and induced magnetic fields of the ship - thus nullifying the ship's effect on the local magnetic field, and making it invisible to mines which are triggered by a shift in the local magnetic field. Deperming is done to ships to remove the magnetism that accrues over time from flexing and pounding and just being in the Earth's magnetic field. It "resets" the ship's magnetism to a known and manageable quantity, so that the DG coils are more effective. In this process the ship is surrounded by a large copper coil and it is energized with high-amperage Direct Current. Depending on the size of the ship and its magnetic make-up, this charge could be anywhere from a couple seconds to a minute or more. The coil is then de-energized, and allowed to cool, before it is energized in the opposite polarity for an equivalent amplitude and duration. This process is repeated while stepping down the amplitude, until the desired magnetic bias is achieved. When I did this, we had to remove the magnetic compasses from the ship, and because it was back in the day - all floppy discs and VHS tapes.
You are probably right, I may well be confusing degaussing / deperming; I know you are using the terms in a different manner than me . I have always taken the view the two terms are almost interchangeable and a quick google search confirms this view as far as the layman in concerned but a CF thread is no place for the layman ().

I accept your explanation is one way the terms are used (and used correctly).

Here is another way that I located.

"A Naval instructional film which explains various methods of Ranging to determine a ship's vertical magnetic field prior to degaussing (permanently removing or neutralising the magnetic field). The processes of Deperming, the removal or neutralising of permanent longitudinal magnetism from a ship, and Wiping, used to degausse smaller ships, are outlined." From https://www.iwm.org.uk/collections/i...ect/1060029729

Who would have thought the terms needed to be resolved for a different axis of a magnetic field?
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Old 11-06-2018, 20:15   #96
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Re: true vs magnetic bearing

You guys are all too smart for me!
As I recall from my old minesweeper days, a degaussing Range would need to be visited periodically by any steel or iron ship usually within 5-8 years of the original visit, probably due to the flexing and movement of the metal hull.
There were heavy magnets set up on either side of the bridge binacle to compensate for the magnetic influence of the vessel. These could be changed out depending on the magnetic influence that changed over time because their magnetism was variable. I recall that the compass was usually set up for true rather than magnetic.
You can still find these on older steel boats but are probably far out of date being replaced by the fancy new electronic gizmos! I know the old iron towboat that I drove for years up and down the BC coast had them.
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Old 11-06-2018, 20:47   #97
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Re: true vs magnetic bearing

After 7 pages of mainly on topic posts, I guess it's Ok to digress some more and share some bar tales.

Some years back, I had to deal with a perplexing magnetic field problem in a helicopter. It took some months and much head scratching to resolve although I sorta knew what the problem was from the start.

The helicopter had two gyro compass systems and after some months, they would drift apart by a degree or so but in different directions; when the difference reached about 2 or 3 degrees, the autopilot would fail as it required both gyro compass to agree with each other - normal failsafe approach.

So we would re-swing the gyro's magnetic field sensors and all would be good for a few months but then the same issue would re-occur. Clearly the owner wasn't happy.

I worked out the problem was probably due to some hard steel fasteners that were in close proximity to the magnetic sensors (within a few inches) and these should be changed out for non-magnetic fasteners. This wasn't an easy option, there would be a lot of paperwork and redesign of the structural aspects in changing the fasteners. The owner didn't want to go that route and he argued that it wasn't the problem as other helicopters of the same design not not exhibit the same fault. This was a reasonable argument and had some merit but nothing else seemed to be causing the problem.

More head scratchings and then I realised how the helicopter was hangared. It was always hangared in the same spot and facing the same way. It would sit like that for sometimes 2 or more months without being moved. My theory suggested the earths magnetic field was magnetising the hard steel fasteners and as they weren't concentric to the gyros magnetic sensors, the effect caused one to move one way and the other, the other way.

First step was to degauss the fasteners and re-swing the gyros. Then we degaussed the fasteners every week for three or four months. The problem went away.

The next step was to convince the owner to change the positioning of the helicopter in the hangar every week (without any degaussing). Although disbelieving the theory, he did so (much cheaper / easier than changing the fasteners); the helicopter was rotated 90 degrees every week. The problem never occurred again. The reason other similar helicopters never had the problem was they were never left in the same spot oriented the same way for months and months.

The earth's magnetic field may seem weak but it has strong effects!
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Old 11-06-2018, 22:01   #98
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Re: true vs magnetic bearing

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Originally Posted by Don C L View Post
Oh yeah, I'll have to dig out the charts but pretty sure you can see different variations on different compass roses on the same chart in my neighborhood. But still I remember as a kid sailing down the coast at night, my buddy's boat had no light in the compass so I'd shine a flashlight on it once in a while. I got tired of that and used the stars... which of course MOVE! By morning we were far out of sight of land with no idea of position... now that was adventure!
That is a great memory. Now imagine doing that on an island in the middle of the big salt. God hath provided the stars at night and the sun by day.
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Old 11-06-2018, 22:11   #99
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Re: true vs magnetic bearing

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Am I misunderstanding you here? To be 60 miles off on a 1 degree steering error you would have to run a course of 3437 miles ... not many people DR for that long.

A 5 degree error on a 100 mile course will put you almost 9 miles off target ... but still within sight of an island with any topography.
That looks good on a chalk board. Add current and maybe some haze or mist and you have gone missed. I suppose that's for a different thread.

"Hey, what's that land over there! It's supposed to be right in front of us. That can't be our island. It must be the next island."

Or, like the famous cartoon where the copilot looks to the pilot, "what's that goat doing in that cloud?' The other navigators-aviators-get it.
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