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The Ship’s Magnetic Compass
The principle of the present day magnetic compass is in no way different from that of the compass used by the ancients. It consists of a magnetised needle, or array of needles pivoted so that rotation is in a horizontal plane.
The superiority of the present day ship’s compass results from :
- better knowledge of the laws of magnetism which govern the behaviour of the compass,
- greater precision in the construction of compass & binnacle including correctors,
- better means to control damping of the compass needle.
A properly adjusted ship’s compass is the mariner’s best friend. It does not need any power nor satellites to work. As long as earth’s magnetism exists, a properly adjusted compass will show you correct directions. It takes up a very small place onboard, normally in Monkey Island (which has least real estate value). It works well in most of the navigable waters of the world, except areas closer to magnetic poles. Readings from compass can be easily corrected, variation & deviation is readily available onboard at all times.
Magnetism is a property mainly experienced with ferromagnetic materials eg Fe Ni Co and Rare earth metals as well as its various alloys (eg steel). Any piece of ferromagnetic material on becoming magnetised, that is, acquiring the property of attracting small particles of similar ferro magnetic material, will assume regions of concentrated magnetism, called poles. Any such magnet will have at least two poles, of unlike polarity. Magnetic lines of force connect one pole of such a magnet with the other pole as indicated in the figure below. The number of such lines per unit area (flux) represents the intensity of the magnetic field in that area produced by the magnetic material, more lines of forces means more flux due to higher magnetism.
Before we dive into the ship’s magnetism, there are few basic properties of magnet & magnetic lines of forces which need to be kept in mind at all times. These can be easily proved by basic lab experiments. These are strictly NON NEGOTIABLE.
General Properties of Magnet :
- A magnet attracts magnetic materials towards itself. It does not have any effect on non-ferro magnetic materials.
- A magnet always has two poles, commonly named as North / South, Red / Blue, North Seeking / South Seeking etc… If a magnet is broken into 2 pieces, each piece will have 2 poles again. A magnet cannot have only one pole.
- Unlike poles attract each other and like poles repel each other.
- Magnetic force is greatest at its poles and least at the point between poles.
- A freely suspended bar magnet always aligns itself with the prevailing magnetic lines of forces.
- When a magnet is rubbed/placed over another ferromagnetic material, it passes some of its properties to the material.
General Properties of Magnetic lines of force :
- Magnetic lines of force start from the North Pole and end at the South Pole.
- They are continuous through the body of the magnet, thus eventually forming a closed loop.
- Magnetic lines of force can pass through Ferromagnetic materials more easily than any other medium (air/water).
- Two magnetic lines of force cannot intersect each other.
There are two kinds of magnetism – Permanent and Induced.
Permanent Magnet produces a magnetic field of its own at all times, stronger the permanent magnet, stronger is its magnetic field and stronger is its flux. They will retain its magnetism when it is removed from the magnetizing field. The important point to remember is that they are a magnet because of their inherent properties and not dependent on external magnetic forces. These are also known as Hard Iron
A ferromagnetic material when placed close to a permanent magnet will temporarily behave like a magnet as it gets induced by the magnetic lines of forces. It will, however, lose its magnetism when removed from the magnetizing field, ie if the permanent magnet is now removed. It may change polarity also if the permanent magnet is now kept in the opposite end.
Induced magnets are also known as Soft Iron.
Whether or not a bar will retain its magnetism on removal from the field will depend on:
- the strength of magnetic field and duration it is in that field,
- the degree of hardness of the ferromagnetic material (retentivity), the harder the iron the more permanent will be the magnetism acquired.
- amount of physical stress applied to the bar while in the magnetizing field.
Both Permanent & Induced magnets have their lines of forces. Magnetic forces from permanent magnets are called permanent magnetism to differentiate it from that caused due to Induced magnetism from soft irons.
The accepted theory of terrestrial magnetism considers the earth as a huge permanent magnet surrounded by lines of magnetic force that connect its two magnetic poles. These magnetic poles are near, but not coincidental, with the geographic poles of the earth. Since the north-seeking end of a compass needle is conventionally called a red pole, north pole, or positive pole, it must, therefore, be attracted to a pole of opposite polarity, or to a blue pole, south pole, or negative pole. The magnetic pole near the north geographic pole is, therefore, a blue pole, south pole, or negative pole; and the magnetic pole near the south geographic pole is a red pole, north pole, or positive pole.
Below figure illustrates the earth and its surrounding magnetic field. The flux lines enter the surface of the earth at different angles to the horizontal, at different magnetic latitudes. This angle is called the angle of magnetic dip, θ, and increases from zero, at the magnetic equator, to 90° at the magnetic poles. The total magnetic field is generally considered as having two components, namely H, the horizontal component, and Z, the vertical component. These components change as the angle θ changes such that H is maximum at the magnetic equator and decreases in the direction of either pole; Z is zero at the magnetic equator and increases in the direction of either pole.
A compass needle in line with the earth’s magnetic field will not indicate true north, but magnetic north. The angular difference between the true meridian (connecting the geographic poles) and the magnetic meridian (direction of earth’s magnetic lines of forces) is called variation. This variation has different values at different locations on the earth. These values of magnetic variation may be found on the compass rose of navigational charts. The variation for most given areas undergoes an annual change, the amount of which is also noted on all charts.
Magnetic variation is the angle between magnetic north and true north and is caused by the different locations of the Geographic North Pole and the Magnetic North Pole.
Modern ships are made of numerous plates of steel. They are cut / bent into shapes and welded in place. Steel being ferro magnetic are magnetized in the process of ship construction. This could be permanent (due to energy imparted when cutting & welding) and also induced due to the earth’s magnetic lines of forces and also when in close proximity to other permanent magnets.
Effects of all permanent magnets onboard the ship can be combined in a single force called Residual Permanent magnetic forces, or simply Permanent magnetic forces. Unless the ship undergoes a major repair or steel renewal, this permanent magnetism is generally remains more or less for the life of the ship. Its force does not change with the change in heading or whether in Northern magnetic hemisphere or Southern. Its force does change very slowly over time. This change is also known as Sub-Permanent magnetism and corrections to this will be carried out together with Permanent Magnetism.
The magnetism in the various structures of a ship, which tends to change as a result of cruising, vibration, or ageing, but which does not alter immediately so as to be properly termed induced magnetism, is called subpermanent magnetism. This magnetism, at any instant, is part of the ship’s permanent magnetism, and consequently must be corrected by permanent magnet correctors.
Unfortunately the same does not apply to Induced magnetism onboard. The force of induced magnetism changes depending on ships heading, the strength of earth’s lines of forces, magnetic latitude etc. It also depends on its ferromagnetic properties (Retentivity etc…). In short, Induced magnetism is not constant.
A ship, then, has a combination of permanent, subpermanent, and induced magnetism. Therefore, the ship’s apparent permanent magnetic condition is subject to change from deperming, shocks, welding, and vibration. The ship’s induced magnetism will vary with the Earth’s magnetic field strength and with the alignment of the ship in that field.
Magnetic Compass should be ideally placed in a place far away from all ferromagnetic material so that only forces affecting it are the earth’s magnetic lines of forces, eg on a wooden boat without any metal parts. Unfortunately, this is not possible in a modern ship which is mainly built of steel, which due to its Permanent, Sub permanent & Induced magnetism disturbs the earth’s magnetic lines of forces, thus causing an error in the compass. These errors are not fixed, thus if not adjusted/corrected, the compass headings and bearings cannot be relied upon.
Resultant induced magnetism from earth’s magnetic field
The above discussion of induced magnetism and terrestrial magnetism leads to the following facts:
- A long thin rod of soft iron in a plane parallel to the earth’s horizontal magnetic field, H, will have a red (north) pole induced in the end toward the north geographic pole and a blue (south) pole induced in the end toward the south geographic pole.
- This same rod in a horizontal plane but at right angles to the horizontal earth’s field would have no magnetism induced in it because its alignment in the magnetic field is such that there will be no tendency toward linear magnetisation and the rod is of a negligible cross-section.
- Should the rod be aligned in some horizontal direction between those headings that create maximum and zero induction, it would be induced by an amount that is a function of the angle of alignment. If a similar rod is placed in a vertical position in northern latitudes so as to be aligned with the vertical earth’s field Z, it will have a blue (south) pole induced at the upper end and a red (north) pole induced at the lower end. These polarities of vertical induced magnetisation will be reversed in southern latitudes. The amount of horizontal or vertical induction in such rods, or in ships whose construction is equivalent to combinations of such rods, will vary with the intensity of H and Z, heading, and heel of the ship.
A Short Recap:
The Earth’s Magnetism
- The Magnetic Field of the Earth is identical to the field of a short bar magnet.
- The imaginary short bar magnet has its BLUE pole towards Geographic North & it’s RED pole towards Geographic South.
- The directions in which the poles point are called Magnetic North & Magnetic South.
- These are slightly away from Geographic North & South poles & are also moving slowly.
Are those points where the lines of force of Earth’s magnetic field are vertical (perpendicular to the Earth surface).
Is an imaginary line joining those points where the lines of force of Earth’s magnetic field are horizontal (parallel to the Earth surface).
The angle at which the Earth’s magnetic field acts w.r.t. horizontal at any place is the Dip at that place. The Earth’s field has a Northward & Downwards component in the N hemisphere & in S Hemisphere it has a Northward & Upward component. Conventionally Dip is designated positive in N hemisphere.
Components of the Total Force
- The total force of the Earth’s magnetic field can be resolved into two components.
- It is the Horizontal force H which provides the Directive property to the Magnetic Compass.
- H is maximum at the Equator & reduces as the latitude increases till it becomes nil at the poles.
- The directive property of a Magnetic compass is hence highest at the equator & Nil at poles.
- The vertical component is called Z.
- It acts downwards in N Hemisphere & upwards in S Hemisphere.
- Z does not contribute to the directive property but tends to dip the card. However, the card is so constructed as to restrict dipping by keeping its COG low.
- On a wooden ship, a magnetic compass would point towards Magnetic North.
- By applying Variation True Directions can be obtained.
- However, present day ships are built of steel – a magnetic material, which also affects the Compass (deviation).
- Ship’s magnetism is of two types.
- Permanent Magnetism – In Hard Iron structures like Hull, Decks, Bulkheads & Tank tops.
- Induced Magnetism – In Soft Iron structures like beams, Girders, Masts & Funnels.
- Is acquired by the Hard Iron in the ship when it is built.
- Heading in a constant direction in the yard, continuous Heating, Hammering, Bending & Welding causes the HI molecules to align themselves in the direction of earth’s magnetic field at that location thus acquiring permanent magnetism.
- Since this is permanent magnetism, the Poles generated are also permanent & do not shift.
- The vessel acquires magnetism both in Horizontal & vertical planes.
- Blue Pole is acquired where the lines enter the material & Red pole where they leave.
Permanent Magnetism – Horizontal : Positioning of poles depends on upon the Heading of the Vessel when built
Permanent Magnetism – Vertical : Positioning of poles depend on upon the Hemisphere, where Vessel is built depending on the heading & hemisphere the positioning of poles could result in any oblique direction (Three dimensional). This permanent magnetism in any oblique directions is split into 3 mutually perpendicular directions to understand & counteract it effectively.
Ship’s Induced Magnetism
Horizontal Soft Iron is induced by H component of the Total Force. The strength of Induced magnetism depends on the heading of the Ship. Vertical Soft Iron is induced by Z-Component of the Total force. The position of the poles will depend on upon the hemisphere the vessel is in presently.
Theory of Magnetic Compass adjustment
The magnetic compass, when used on a steel ship, must be so corrected for the ship’s magnetic conditions that its operation approximates that of a nonmagnetic ship. Ship’s magnetic conditions create deviations of the magnetic compass as well as sectors of sluggishness and unsteadiness.
Deviation is defined as a deflection of the card (needles) to the right or left of the magnetic meridian. Adjustment of the compass is the arranging of magnetic and soft iron correctors about the binnacle so that their effects are equal and opposite to the effects of the magnetic material in the ship, thus reducing the deviations and eliminating the sectors of sluggishness and unsteadiness.
The Sources of Compass Error
Assuming they are constructed well, compasses on ships fail to point to true (geographic) north due to two factors:
- Magnetic variation (or magnetic declination), the angle between magnetic north and true north due to the local direction of the Earth’s magnetic field, and
- Magnetic deviation, the angle between the compass needle and magnetic north due to the presence of iron within the ship itself.
The algebraic sum of the magnetic variation and the magnetic deviation is known as the compass error.
Permanent magnetism and its effects on the compass
The total permanent magnetic field effect at the compass may be broken into three components mutually 90° apart, as shown in figure 1. The effect of the vertical permanent component is the tendency to tilt the compass card and, in the event of rolling or pitching of the ship to create oscillating deflections of the card. Oscillation effects that accompany roll are maximum on north and south compass headings, and those that accompany pitch are maximum on east and west compass headings. The horizontal B and C components of permanent magnetism cause varying deviations of the compass as the ship swings in heading on an even keel. Plotting these deviations against compass heading will produce sine and cosine curves, as shown in figure 2. These deviation curves are called semicircular curves because they reverse direction in 180°.
The permanent magnetic semicircular deviations can be illustrated by a series of simple sketches, representing a ship on successive compass headings, as in figures 3 and 4.
The ships illustrated in figures 3 and 4 are pictured on cardinal compass headings rather than on cardinal magnetic headings, for two reasons:
- Deviations on compass headings are essential in order to represent sinusoidal curves that can be analyzed mathematically. This can be visualized by noting that the ship’s component magnetic fields are either in line with or perpendicular to the compass needles only on cardinal compass headings.
- Such a presentation illustrates the fact that the compass card tends to float in a fixed position, in line with the magnetic meridian. Deviations of the card to right or left (east or west) of the magnetic meridian result from the movement of the ship and its magnetic fields about the compass card.
A compass deviation is caused by the existence of a force at the compass that is superimposed upon the normal earth’s directive force, H, a vector analysis is helpful in determining deviations or the strength of deviating fields.
For example, a ship as shown in figure 5 below on an east magnetic heading will subject its compass to a combination of magnetic effects; namely, the earth’s horizontal field H, and the deviating field B, at right angles to the field H. The compass needle will align itself in the resultant field which is represented by the vector sum of H and B, as shown. A similar analysis on the ship in figure 5 will reveal that the resulting directive force at the compass would be maximum on a north heading and minimum on a south heading, the deviations being zero for both conditions. The magnitude of the deviation caused by the permanent B magnetic field will vary with different values of H; hence, deviations resulting from permanent magnetic fields will vary with the magnetic latitude of the ship.
Induced magnetism and its effects on the compass
Induced magnetism varies with the strength of the surrounding field, the mass of metal, and the alignment of the metal in the field. Since the intensity of the earth’s magnetic field varies over the earth’s surface, the induced magnetism in a ship will vary with latitude, heading, and heel of the ship.
With the ship on an even keel, the resultant vertical induced magnetism, if not directed through the compass itself, will create deviations that plot as a semicircular deviation curve. This is true because the vertical induction changes magnitude and polarity only with magnetic latitude and heel and not with the heading of the ship. Therefore, as long as the ship is in the same magnetic latitude, its vertical induced pole swinging about the compass will produce the same effect on the compass as a permanent pole swinging about the compass. Figure 6 below illustrates the vertical induced poles in the structures of a ship.
Generally, this semicircular deviation will be a B sine curve, as shown in figure 7 above, since most ships are symmetrical about the centerline and have their compasses mounted on the centerline. The magnitude of these deviations will change with magnetic latitude changes because the directive force and the ship’s vertical induction both change with magnetic latitude.
The masses of horizontal soft iron that are subject to induced magnetization create characteristic deviations, as indicated in figure 7. The D and E deviation curves are called quadrantal curves because they reverse polarity in each of the four quadrants.
Symmetrical arrangements of horizontal soft iron may exist about the compass in any one of the patterns illustrated in figure 8.
The deviation resulting from the earth’s field induction of these symmetrical arrangements of horizontal soft iron are illustrated in figure 9, showing the ship on various compass headings. The other heading effects may be similarly studied.
Such a D deviation curve is one of the curves in figure 7. It will be noted that these D deviations are maximum on the intercardinal headings and zero on the cardinal headings.
Asymmetrical arrangements of horizontal soft iron may exist about the compass in a pattern similar to one of those in figure 10.
The deviations resulting from the earth’s field induction of these asymmetrical arrangements of horizontal soft iron are illustrated in figure 11, showing the ship on different compass headings. The other heading effects may be similarly studied.
Such an E deviation curve is one of the curves in figure 7. It will be observed that these E deviations are maximum on cardinal headings and zero on the intercardinal headings.
The quadrantal deviations will not vary with latitude changes, because the horizontal induction varies proportionally with the directive force, H.
The earth’s field induction in certain other asymmetrical arrangements of horizontal soft iron creates a constant ‘A’ deviation curve. The magnetic A and E errors are of smaller magnitude than the other errors, but, when encountered, are generally found together, since they both result from asymmetrical arrangements of horizontal soft iron.
In addition to this magnetic A error, there are constant ‘A‘ deviations resulting from:
- physical misalignments of the compass, pelorus, or gyro;
- errors in calculating the sun’s azimuth, observing time, or taking bearings.
The nature, magnitude, and polarity of all these induced effects are dependent upon the disposition of metal, the
symmetry or asymmetry of the ship, the location of the binnacle, the strength of the earth’s magnetic field, and the angle of dip.
Certain heeling errors, in addition to those resulting from permanent magnetism, are created by the presence of both horizontal and vertical soft iron, which experience changing induction as the ship rolls in the earth’s magnetic field. This part of the heeling error will naturally change in magnitude with changes of magnetic latitude of the ship. Oscillation effects accompanying roll are maximum on north and south headings, just as with the permanent magnetic heeling errors.
Adjustments and Correctors
Since some magnetic effects remain constant for all magnetic latitudes and others vary with changes of magnetic latitude, each individual effect should be corrected independently. Further, it is apparent that the best method of adjustment is to use permanent magnet correctors to create equal and opposite vectors of permanent magnetic fields at the compass, and soft iron correctors to assume induced magnetism, the effect of which will be equal and opposite to the induced effects of the ship for all magnetic latitude and heading conditions.
The compass binnacle provides for the support of the compass and such correctors. Study of the binnacle in figure 13 will reveal that such correctors are present in the form of:
- Vertical permanent heeling magnet in the central vertical tube,
- Fore-and-aft B permanent magnets in their trays,
- Athwartship C permanent magnets in their trays,
- Vertical soft iron Flinders bar in its external tube,
- Soft iron spheres.
The heeling magnet is the only corrector that corrects for both permanent and induced effects, and consequently must be readjusted occasionally with radical changes in the latitude of the ship. (It must be noted, however, that any movement of the heeling magnet will require readjustment of other correctors.)
The tabular summary of “Compass Errors and Adjustments,” figure below as printed by NATIONAL GEOSPATIAL-INTELLIGENCE AGENCY BETHESDA, MD 2004, summarizes all the various magnetic conditions in a ship, the types of deviation curves they create, the correctors for each effect, and headings on which each corrector is adjusted. Correctors should be applied symmetrically under all but exceptional conditions and as far away from the compass as possible to preserve uniformity of magnetic fields about the compass needle array.
* In Writing this article, references made listed below:
- Handbook of magnetic compass adjustment – NGIA Bethesda
- National Imagery and Mapping Agency – Compass
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