Handbook of Magnetic Compass (H.O. 226)

[Richard5]

Commencing on page 9, continuing to page 12:

10. Ship's Magnetism
A ship while in the process of being constructed will acquire magnetism of a permanent nature under the extensive hammering it receives in the earth's magnetic field. After launching, the ship will lose some of this original magnetism as a result of vibration, pounding, etc., in varying magnetic fields, and will eventually reach a more or less stable condition. This magnetism which remains is the so-called permanent magnetism of the ship.

11.
The fact that a ship has permanent magnetism does not, of course, mean that in cannot also acquire induced magnetism when placed in a magnetic field such as the earth's field. The amount of magnetism induced in any given piece of soft iron is dependent upon the field intensity, the alignment of the soft iron in that field, and the physical properties and dimensions of the iron. this induced magnetism may add to or subtract from the permanent magnetism already present in the ship, depending on how the ship is aligned in the magnetic field. the softer the iron the more readily it will be induced by the earth's magnetic field, and the more readily it will give up its magnetism when removed from that field.

[Richard5]

12.
The magnetism in the various structures of the ship which tends to change as a result of cruising, vibration, or aging, but does not alter immediately so as to be properly termed induced magnetism, is called subpermanent magnetism. This magnetism at any instant is recognized as part of the ship's permanent magnetism, and consequently must be corrected as such by means of permanent magnetism correctors. This subpermanent magnetism is the principal cause of deviation changes on a magnetic compass. Subsequent reference to permanent magnetism in this text will refer to the apparent permanent magnetism at any instant.

13.
A ship, then, has a combination of permanent, subpermanent, and induced magnetism, since its metal structures are of varying degrees of hardness. Thus, the apparent permanent magnetic conditions of the ship is subject to change from deperming, excessive shocks, welding, vibrations, etc,; and the induced magnetism of the earth's magnetic field at different magnetic latitudes, and with the alignment of the ship in that field.

[Richard5]

I think that even though this information may be dated or otherwise of minimal use on the modern day cruising vessel (primarily due to use of composite materials), it offers a glimpse into the history of what brought us to this point in time. This H.O. was first published in 1963.

I think I know but I will have to research the definition of "deperming".

I did not know that vibration could alter the magnetism of metals.

All italics are from the original text.

[nigel1]

I think Deperning has the same meaning as Degausing, a process used to de-magnetise a ship. Very commonly used during WW2 to protect ships from magnetic mines.
I'm old enough to remember that the first two ships I went to sea on were fitted with degausing coils around the main deck. These were cargo ships built during the 1960's.
By the 80's most of the copper coils had ended up being hacked off by the crew and sold as scrap.

[carstenb]

They just took it down, but out at the end of Copenhagen Harbour that were 4 yellow buoys that marked the Degaussing station. Ships would sail into the area between the buoys and ZAP! the station on shore would degauss them.

It was still in use about 5 years ago

[Richard5]

A reading from the aforementioned H.O. reveals that "deperming" does refer to degaussing.

To wit: (page 112, No.4);
When a vessel is newly commissioned, all compasses shall be approximately compensated. At the first opportunity after commissioning the ship shall be swung for deviation, as follows: First, with all correctors removed; second, after adjusting, with degaussing coils off; third, with degaussing coils on, and no degaussing coils operating; fourth, with no degaussing coils on and compensating coils in adjustment. A complete analysis of the deviations of all compasses installed shall be made, both before and after adjustment, and a complete report forwarded on NBS 1102 and NBS 1106 to the Bureau of Ships.

Also, No.5:
Similar observations and reports shall be made on or near the magnetic equator, and data for the installation of Flinders bars obtained.

[Richard5]

No. 6:
Similar observations and reports shall be made as soon as possible after any considerable change in the magnetic state of the ship which may be caused by any of the following: Fishing, wiping, deperming, major alteration or any repair of hull structure or machinery, firing of main-battery guns, lying in the same direction for a period of two or more weeks, or after making a passage during which the same course has been steered for a week or more. In particular, for about thirty days after flashing or wiping, the magnetic state of the vessel will change slowly. It is, therefore, necessary not only to adjust immediately after flashing or wiping, but also to obtain and record deviations at approximate 10-day intervals for thirty days, and to readjust as soon thereafter as possible.

[Richard5]

A brief take-away is how many ship's log record not only the long/lat but the compass deviation? How often are adjustments made in compass deviation and variance?

Perhaps in these days of GPS the above seems only to be regulated to the past but since the compass is there should it not be thought of, and used as, a redundant system by which location, through DR or pilotage, can be deduced?

Tell you what, as a commercial pilot, I oft think of the wet compass as a primary instrument, even though its deviation and variance are a known, and sometimes unknown, value. I take seriously the periodic adjustments of the magnetic compass. Certainly, when I fly it is of less latitudes than when I sail. In other words, sailing to the far reaches includes a far great variance than does flying.

Perhaps this sounds like hyperbole, perhaps it is. Yet for the greater good, for security, I place a high value on the accuracy of my compass.

[carstenb]

Originally Posted by Richard5

A brief take-away is how many ship's log record not only the long/lat but the compass deviation? How often are adjustments made in compass deviation and variance?

Perhaps in these days of GPS the above seems only to be regulated to the past but since the compass is there should it not be thought of, and used as, a redundant system by which location, through DR or pilotage, can be deduced?

Tell you what, as a commercial pilot, I oft think of the wet compass as a primary instrument, even though its deviation and variance are a known, and sometimes unknown, value. I take seriously the periodic adjustments of the magnetic compass. Certainly, when I fly it is of less latitudes than when I sail. In other words, sailing to the far reaches includes a far great variance than does flying.

Perhaps this sounds like hyperbole, perhaps it is. Yet for the greater good, for security, I place a high value on the accuracy of my compass.

I'm old-fashioned too. Every year at the start of the season, I check the deviation of my compass at all points of the compass (and log it!).

Why?

Because you never know.......................

[nigel1]

Originally Posted by Richard5

A brief take-away is how many ship's log record not only the long/lat but the compass deviation? How often are adjustments made in compass deviation and variance?

Perhaps in these days of GPS the above seems only to be regulated to the past but since the compass is there should it not be thought of, and used as, a redundant system by which location, through DR or pilotage, can be deduced?

Tell you what, as a commercial pilot, I oft think of the wet compass as a primary instrument, even though its deviation and variance are a known, and sometimes unknown, value. I take seriously the periodic adjustments of the magnetic compass. Certainly, when I fly it is of less latitudes than when I sail. In other words, sailing to the far reaches includes a far great variance than does flying.

Perhaps this sounds like hyperbole, perhaps it is. Yet for the greater good, for security, I place a high value on the accuracy of my compass.

On a commercial merchant ship, it is usual practise to make a compass observation once per watch (every 4 hrs). The observation is recorded in the compass error log book, along with position, heading, etc. It is usual to compare the magnetic and gyro compass at the time.
On a sail boat, taking a compass error is not so easy, not unless your on a big sail boat with a big binnacle and azimuth ring or shadow pin.

Again on a commercial ship, I swing the compass at least once a year and correct deviations, also if crossing the magnetic equator as this helps to split co-efficient B.
We are really meant to use the services of a professional compass adjuster, but personally, I have not found them to very good at what they are meant to do

[Richard5]

Originally Posted by nigel1

We are really meant to use the services of a professional compass adjuster, but personally, I have not found them to very good at what they are meant to do

Good on you to swing the compass on a regular basis. As your comments infer, it is incumbent on the ship owner to ascertain the reliability and accuracy of the instrument.

All of which begs the question; How does the recreational sailor swing the compass? There is, in fact, a procedure for this but how many would make use of it let alone care?

[Richard5]

I forgot to ask, until now: who, when commissioning or taking possession of a used vessel for recreation (non-commercial), thinks to swing the compass?

[carstenb]

Originally Posted by Richard5

Good on you to swing the compass on a regular basis. As your comments infer, it is incumbent on the ship owner to ascertain the reliability and accuracy of the instrument.

All of which begs the question; How does the recreational sailor swing the compass? There is, in fact, a procedure for this but how many would make use of it let alone care?

If you have the item and energy, you can swing the boat from a "deviation post", but let's be honest here. Assuming you have a chart plotter or GPS, you can simply sail in a set direction and mark what heading your compass says and what heading you have on the GPS (do this for say 16 points on the compass).

Now you get out your paper chart (oh - don't have one of those? Now you know why you need them - amongst other reasons) and determine the magnetic variation at your Lat/Long. Deduct the Mag. Var. and what is left is your deviation.

[Richard5]

Originally Posted by carstenb

If you have the item and energy, you can swing the boat from a "deviation post", but let's be honest here. Assuming you have a chart plotter or GPS, you can simply sail in a set direction and mark what heading your compass says and what heading you have on the GPS (do this for say 16 points on the compass).

Now you get out your paper chart (oh - don't have one of those? Now you know why you need them - amongst other reasons) and determine the magnetic variation at your Lat/Long. Deduct the Mag. Var. and what is left is your deviation.

Nyet. A GPS is not required. I suspect you know that many ports have range markers, et al which accurately define a true course. Mark your heading in reference and note deviation. 16 points indeed is doable and warranted.

EDIT: didja know typing "point" without the "O" makes me thirsty? Thank God my compulsion to proofread before I hit SEND.

[Richard5]

Alternately, one can plot a course on those pesky paper charts (range and bearing) to construct a series of headings by which to swing the compass. IF one has the charts. IF one cares for the mental gymnastics.