Magnetic field of the ship. Basic characteristics of earth and ship magnetic fields. Poisson and A. Smith equations. Ship magnetic forces (SMF). Application. Reprint reproduction of an extract from the ship's log of the schooner "St. Anna"

Let me remind readers that the question being analyzed is as follows: is it possible to continue sailing with a compass whose deviation has increased to 60° as a result of a lightning strike, if one knows its correction?

In the first two parts, we looked at the magnetic properties of ferromagnetic materials, studied the basic definitions, and also remembered what the Earth’s magnetic field is.

The third participant in the process of developing a course using a magnetic compass, in addition to the compass itself and the Earth’s magnetic field, is the magnetic field of the yacht. This is what we’ll talk about in the next part of the series “Magnetic compass business. Brief summary."

Deviation

Today, the vast majority of yachts have on board devices and mechanisms made from certain ferromagnets. In addition to the “ship’s iron”, all electrical devices create their own magnetic field, of which there are more and more on board every year. Obviously, all these sources of magnetic field distort the Earth’s magnetic field, so the compass card installed on the yacht shows not the magnetic meridian, but its own compass meridian. I think it would be appropriate to recall that the angle between the magnetic and compass meridians is called deviation.

The deviation of a magnetic compass installed on a ship is not a constant value, but changes during navigation for a number of reasons, in particular, when the ship’s course and the magnetic latitude of navigation change. All ship iron can be magnetically divided into soft and hard. Solid iron, having become magnetized during the construction of the ship, acquires a certain residual magnetism and acts on the compass card with a certain constant force. When the ship changes course, this force, together with the ship, changes its direction relative to the magnetic meridian and therefore, at different courses, causes a deviation of unequal magnitude and sign.

When the course changes, the ship's iron, which is soft in magnetic terms, is remagnetized and acts on the card with a force of variable magnitude and direction, also causing unequal deviation. When the magnetic latitude of navigation changes, the strength of the Earth's magnetic field and the magnetization of soft ship iron change, which also causes changes in deviation.

Thus, three forces act on the card of a magnetic compass installed on board a ship: the constant magnetic field of the Earth, the constant magnetic field of hard ship iron, and the alternating magnetic field of soft ship iron. The interaction of these fields creates a certain total magnetic field strength. The needle of a magnetic compass occupies a position along the tension vector, and the compass meridian can differ greatly from the magnetic one. And here we finally come to the answer to the question posed at the beginning of our summary: what to do if the deviation of the magnetic compass suddenly, “as a result of a lightning strike,” became very large, for example, more than 60°. Does it need to be destroyed or can the movement continue by determining an amendment?

With a large deviation, i.e. with a significant strength of the ship's magnetic field, the Earth's magnetic field may, on some courses, be almost completely compensated by the ship's magnetic field. In this case, the compass card will be in a state of indifferent equilibrium, and the compass will stop working: on some courses, the card will rotate with the ship due to the same increment in the course and deviation angles; on other directions, the sensitive element will be carried away by friction in the support due to an excessive decrease in the guiding force .

In addition, looking ahead, we note that with large deviation values ​​its determination itself becomes difficult and inaccurate, since the procedure for determining deviation assumes that the ship is on one or another known magnetic course. With large deviation values, when the course changes, it quickly changes its value, and even small errors in the course, which are inevitable, begin to significantly affect the accuracy of the determinations.

Thus, the clear answer to the question posed is that it is dangerous to continue moving with a compass that has a large deviation. It is imperative to destroy it, then determine the residual values, and only then can you safely continue moving.

The total magnetic field strength of ship's iron in the theory of magnetic compass business is described by Poisson's equations. Of its three components, the magnitude of the deviation is influenced by two components - the magnetic field of soft iron and the magnetic field of hard iron.

In the magnetic compass business, the forces that form the ship’s magnetic field and, accordingly, the deviation they cause are conventionally divided into constant, semicircular and quarter. The magnitude of the constant deviation does not depend on the course and does not change when the magnetic latitude changes, which is why it is called constant. The constant deviation is caused by the influence of longitudinal and transverse soft ship iron.

Semicircular deviation is a deviation that, when the ship’s course changes by 360⁰, changes sign twice, taking two times zero values. Semicircular deviation is caused by the magnetic field from vertical soft and any magnetically hard ship iron.

Semicircular deviation graph

Quarter deviation is a deviation that, when the ship's course changes, changes in direction twice as fast as the course. When the course changes from 0⁰ to 360⁰, the deviation changes its sign four times and passes through zero the same number of times. The quarter deviation is caused by the magnetic field from the longitudinal and transverse ship's soft iron.

Quarter deviation chart

Since the source of deviation is the longitudinal and transverse ship iron, the destruction of deviation is also carried out using longitudinal and transverse destroyer magnets.

Of all the forces that cause deviation of the magnetic compass, the weakest are the forces that cause constant deviation. Its value, as a rule, does not exceed 1⁰. Therefore, this force is not compensated, but taken into account in the form of a compass correction.

Semicircular deviation occurs under the influence of all hard and vertical soft ship iron. These forces are compensated by longitudinal and transverse magnets - destroyers installed inside the binnacle. In order to compensate for one or another magnetic force, it is necessary to apply an opposite directional force to the compass card. This is achieved by using appropriate compensators. When destroying deviations, they are guided by the following rule: forces originating from hard ship iron must be compensated using permanent magnets, and forces from the inductive magnetism of soft ship iron must be compensated using elements made of soft ferromagnetic material. Correct installation of compensators is the task that needs to be solved to eliminate deviation.

Binnacle of a modern magnetic compass with compensators and correctors

Quarter deviation occurs under the influence of only soft horizontal ship iron. The forces causing quarter deviation are brought to minimum values ​​with the help of quarter deviation compensators - bars, plates or balls made of soft ferromagnetic material, installed outside the binnacle, in its upper part.

It should be noted that quarter deviation is more stable than semicircular deviation. Therefore, the destruction of the quarter deviation is carried out, as a rule, once - immediately after the construction of the vessel. Subsequently, the residual quarter deviation practically does not undergo noticeable changes for many years, which cannot be said about the semicircular deviation.

In addition to quarter and semicircular deviation, when the ship’s hull is tilted, i.e. when heeling, trimming or during pitching, an additional error in the magnetic compass occurs - heel deviation. With roll or lateral roll, the roll deviation is maximum on courses N and S. With longitudinal roll and pitching, on courses E and W, respectively. Roll deviation can reach values ​​of 3⁰ for each degree of roll. To destroy it, a special compensator is provided inside the binnacle - an inclination magnet. It is installed vertically, under the compass bowl.

To prevent instability of semicircular deviation due to changes in magnetic latitude when the ship is sailing, the compass is equipped with another device - a latitude compensator. This is a vertical rod made of soft ferromagnetic material, mounted on the outside of the binnacle. It eliminates the variable (latitudinal) part of the semicircular deviation.

It is curious that this latitudinal compensator is called a Flinders bar, in honor of the English navigator and Australian explorer Matthew Flinders. By the way, it was he who named Australia Australia. During an expedition in 1801, he, making systematic determinations of declination using two compasses, discovered that in the Northern Hemisphere the northern end of the compass needle was attracted by an unknown force to the bow of the ship, and in the southern hemisphere - to the stern.

Matthew Flinders

Analyzing the results obtained, Flinders came to the conclusion that the cause of the deviation was the ship's iron, which, with changes in latitude, changed the magnitude and polarity of its magnetism under the influence of the Earth's magnetic field. Since most of the ship's iron was in pillars, i.e., vertical posts supporting the deck of a wooden ship, the famous navigator came up with the idea of ​​eliminating the deviation by placing a vertical bar of iron near the compass, which is still used today under the name Flindersbar.

Flinders bar - vertical pipe on the left of the binnacle

So, we have received a scientifically based answer to the question posed by Fyodor Druzhinin. At large deviation values ​​- several tens of degrees - it is difficult and sometimes dangerous to use a magnetic compass without destroying it, since the uncompensated forces causing the deviation will balance the Earth’s magnetic field so that the magnetic compass will no longer act as a heading indicator.

Modern yacht magnetic compasses are structurally somewhat different from classic instruments with a high binnacle and a complex system of compensating magnets. Nevertheless, the task of eliminating deviation is relevant for them as well.

What methods exist for eliminating deviation, how to eliminate deviation on a yacht magnetic compass, and much more, I will tell you next time.

To be continued…

Used literature: P.A. Nechaev, V.V. Grigoriev “Magnetic-compass business” V.V. Voronov, N.N. Grigoriev, A.V. Yalovenko “Magnetic compasses” NATIONAL GEOSPATIAL-INTELLIGENCE AGENCY “HANDBOOK OF MAGNETIC COMPASS ADJUSTMENT”

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All sea vessels are equipped with magnetic compasses. The main advantage is their high degree of autonomy and reliability with the simplicity of the device. The main disadvantage is the low accuracy of determining directions. Sources of errors are: inaccurate knowledge of magnetic declination, deviation, inertia and insufficient sensitivity of the magnetic needle system to the Earth's magnetic field. Errors especially increase when pitching.

Typically, two magnetic compasses are installed on a ship - main(GMC) to determine the position of the vessel and travel(PMK) - for controlling the vessel. The MMC is installed in the DP, usually on the upper bridge in a place of best protection from the influence of the ship's magnetic field; the PMC is installed in the wheelhouse. Often, instead of two magnetic compasses, a ship is equipped with one compass on the upper bridge, but with optical transmission of readings to the wheelhouse.

The reliability of determining directions using a magnetic compass largely depends on the accuracy of knowledge of its deviation.

A large deviation leads to the fact that the magnetic compass stops responding to the Earth’s magnetic field and, in fact, is no longer a heading indicator. Therefore, the deviation of the magnetic compass must be compensated by creating an artificial magnetic field. This process is called destruction of deviation. Under normal sailing conditions, the destruction of magnetic compass deviation is carried out at least once a year using special methods studied in the deviation course. The deviation remaining after destruction is called residual deviation; it must be determined by navigators and should not be more than 3° at the main compass and 5° at the directional compass. The residual deviation must be determined:

1) after each destruction of deviation,

2) after repair, drydocking, demagnetization of the vessel;

3) after loading and unloading cargo that changes the magnetic field of the ship;

4) with a significant change in magnetic latitude;

5) when the actual deviation differs from the table deviation by more than 2°.

The essence of determining residual deviation is to compare the measured compass bearing with the known magnetic bearing of the same landmark:

Since deviation depends on the ship’s heading, it is determined on 8 equally spaced main and quarter compass courses. After that, for each magnetic compass its own deviation table is calculated after 10° of the compass course. An example of a residual deviation table is shown in table. 1.2.

Table 1.2.

The residual deviation is determined by two observers. It must be borne in mind that after each turn the magnetic compass card comes to the meridian in 3-5 minutes and therefore the compass cannot be used at this time.

Let's consider the main methods for determining residual deviation.

1. On target(Fig. 1.26).

This is the most accurate method. Some ports even have special deviation points. The vessel crosses the target using each of the 8 main and quarter compass courses and at the moment of crossing the target, the navigator measures the compass bearing of this target. Magnetic bearing is calculated using the formula (1.17) MP=IP-d. IP is taken from the map, d is also determined from the map and reduced to the year of voyage.

The Earth's magnetic field can be detected using a magnetic needle. If the arrow is hung so that it can rotate freely in the horizontal and vertical planes, then at each point on the earth’s surface, under the influence of magnetic forces, it tends to take a very specific position in space. The Earth's magnetic field exists on the surface, underground and in space. The earth's magnetic field is caused by processes within its crust and in outer space and is closely related to the activity of the Sun.

The Earth's magnetic field strength is on average 40 A/m.

In general, the Earth's magnetic field is non-uniform, but in the limited space of a ship it can be considered uniform.

Let us decompose the tension, as a vector, into individual components, called the elements of earthly magnetism. These include (see figure) the horizontal component of the Earth's magnetic field strength H, vertical component Z and magnetic declination d– horizontal angle formed by the direction of the true meridian ON and component H, which lies in the plane of the magnetic meridian. In addition to these elements, the magnetic field strength vector includes magnetic inclination I– vertical angle between the horizontal plane and the direction of the earth’s magnetism vector.

From the figure we can establish the following connection between the elements of terrestrial magnetism:

If you need to determine the projection of the vector of terrestrial magnetism onto the direction of the true meridian or the first vertical, then you can use the following equalities

Lines connecting equal values ​​of H and Z are called isolines (lines of equal strength). Magnetic declination isolines are isogons, magnetic declination isolines are isoclines. Such lines are drawn on a special map of terrestrial magnetism. Isoclins of zero inclination form the magnetic equator.

Let us decompose the vector of terrestrial magnetism into ship coordinate axes:

Projections of the earth's magnetic field strength onto ship axes:

The horizontal component, which determines the operation of the magnetic compass, varies in different places on the globe from zero (at the magnetic poles) to 32 A/m at the southern tip of Asia. The decrease in this component occurs in the direction from the equator to the poles.

The vertical component of the Earth's magnetic field strength varies from zero (at the magnetic equator) to 56 A/m in the polar regions.

Topic 3 (2 hours) magnetic field of a ship. Poisson equations and their analysis.

The ship's hull, its engine, and ship's mechanisms are made of materials that have some residual magnetization. In addition to the residual permanent magnetization acquired during construction, the ship's hull and its mechanisms have not lost the ability to be magnetized in the Earth's magnetic field, which constantly affects the ship. Thus, two components can be distinguished in ship iron: the hard component is magnetized during construction and remains constant, the soft component is magnetized in the Earth’s magnetic field. Permanent ship magnetism and the magnetization of soft ship iron affect any magnetic device on the ship. In this case, it is customary to say that the ship’s magnetic field operates in the space surrounding the ship.

The ship with all its equipment is a body of very complex shape, so it is difficult to count on it being magnetized uniformly. However, the magnetization of a ship during construction and during subsequent periods of its voyage occurs in the weak magnetic field of the Earth, and moreover, the magnetic susceptibility of the ship as a whole is low. Therefore, the inhomogeneity of its magnetization turns out to be insignificant; it can be neglected and proceed from the average value of magnetization for the entire vessel as a whole.

Therefore, we can use Poisson’s theorem on the uniform magnetization of bodies.

Poisson's theorem is formulated as follows: magnetic potential U of a uniformly magnetized body is equal to the scalar product of the body’s magnetization vector taken with a minus sign on the gradient of the attractive force potential created by the mass of a given body:

Where: -
- components of the ship’s magnetization along the ship’s axes

- derived quantities V along these axes, proportional to the potential of attraction caused by the mass of the vessel.

To move from the potential to the projections of the magnetic field strength onto the ship’s axes, we differentiate (16) with respect to the variables x, y, z , Where J– constant value:

The magnetization vector of the body is expressed by formula (16). Let's break it down into components along the ship's axes:

Where: X, Y, Z - projections onto these axes of the magnetizing field - the Earth’s magnetic mole.

Let's substitute these values ​​into the previous three equations:

Let us open the brackets in each of these equations and introduce the notation

Using these notations, we can write it like this:

These equations express the projection of the magnetic field strength of the ship at point O (see figure). If there is a compass at point O, then it will show not only the ship’s magnetism, but also the influence of the Earth’s magnetic field. Let us add algebraically the projections of the field strengths of the ship and the Earth to express their joint action:

where with a prime are projections on the ship’s axes of the total magnetic field, without a prime are projections on the same axes of the Earth’s magnetic field, and with a zero are projections of the ship’s magnetic field strength. From here:

These equations are called Poisson's equations, since they were derived on the basis of Poisson's theorem on the uniform magnetization of bodies.

a, b, c,… k– Poisson parameters. They characterize soft iron: its magnetic qualities, shape and size, location relative to the center of the compass.

Components P, Q, R express the magnetic field of permanent ship magnetism caused by the action of hard iron.

All these quantities practically do not change for a given compass and for a given magnetic state of the ship. If large masses of iron are moved on a ship relative to the compass or the compass itself is moved, then these values ​​will change.

The ship's heading does not affect these values; magnetic latitude has a very weak effect only on the Poisson parameters. Shaking the ship and loading the ship affect its magnetic state.

Magnetic compass deviation. Correction and translation of rhumbs

The metal hull of the ship, various metal products, and engines cause the magnetic needle of the compass to deviate from the magnetic meridian, i.e., from the direction in which the magnetic needle should be located on land. The magnetic field lines of the earth, crossing ship iron, turn it into magnets. The latter create their own magnetic field, under the influence of which the magnetic needle on the ship receives an additional deviation from the direction of the magnetic meridian.

The deviation of the needle under the influence of the magnetic forces of the ship's iron is called compass deviation. The angle between the north part of the magnetic meridian Nm and the north part of the compass meridian Nk is called the deviation of the magnetic compass (betta) (Fig. 44).

Deviation can be either positive - eastern, or core, or negative - western, or leading. Deviation is a variable quantity and varies depending on the latitude and course of the ship, since the magnetization of the ship's iron depends on its location relative to the earth's magnetic field lines.

To calculate the magnetic course of the MK, it is necessary to algebraically add the value of deviation 6 on this course to the value of the compass course of the KK:

Kk+(+-(betta)) = MK

Or MK-(+ - (betta)) = KK.

For example, the compass heading of the KK is 80°, while the deviation of the magnetic compass (betta) = 20° with a plus sign. Then using the formula we find:

MK = KK + (+-(betta)) = 80°+ (+ 20°) = 100°.

If the ship's own magnetic field is large, then it is difficult to use the compass, and sometimes it stops working altogether. Therefore, the deviation must first be destroyed with the help of compensation magnets located in the compass box and soft iron bars installed in the immediate vicinity of the compass.

After eliminating the deviation, they begin to determine the residual deviation at various courses of the ship. The destruction and determination of residual deviation and the compilation of a deviation table for a given compass is carried out by a deviator specialist at a deviation range specially equipped with leading signs. The deviation is considered to be destroyed quite satisfactorily if its value on all courses does not exceed +4°.

Figure 44. Correction and translation of rhumbs

As already mentioned, true courses and bearings must be plotted on maps. To obtain true courses and bearings, it is necessary to make a certain correction to the readings of the compass installed on the ship, since it shows the compass course and compass bearings. The compass correction (delta) k is the angle between the north part of the true meridian N and and the north part of the compass meridian Nk. The compass correction (delta)k is equal to the algebraic sum of deviation (betta) and declination d, i.e.:

(dela) k = (+-betta) + (+-d)

It follows that to obtain the true values ​​it is necessary to add the compass correction with its sign to the compass values:

IR = KK + (+ -(delta) k)

Or CC = IR-(+ (delta)k).

In Fig. 43 shows the transition from MK to KK through declination.

In Fig. Figure 44 shows the relationship between all quantities on which the correct determination of true directions at sea depends. The angles formed by the lines NK, Nu, Nn and the heading and bearing lines have the following names:

Compass course K K - the angle between the compass meridian line NK and the course line.

Compass bearing KP - the angle between the compass meridian line NK and the bearing line.

Magnetic course MK - the angle between the magnetic meridian NM and the course line.

Magnetic bearing MF - the angle between the magnetic meridian line NM and the bearing line.

True course IK - the angle between the true meridian line Na and the course line.

The true bearing of the IP is the angle between the true meridian line and the bearing line.

Deviation (betta) is the angle between the compass meridian line NK and the magnetic meridian line NM.

Declination d is the angle between the magnetic meridian line NM and the true meridian line Nu.

Compass correction (delta) k - the angle between the true meridian line N" and the compass meridian line N K.

There is a mnemonic rule that helps the navigator correctly operate with the values ​​of true magnetic and compass directions. To fulfill this rule, you must remember the sequence: IR-d-MK-(betta)-KK. If we algebraically subtract the declination d from the IR, we obtain the value MK, which is located next to the right of the IR; If we algebraically subtract the deviation (beta) from the MC, we obtain the value KK, which is located next to the right of the MC. If we algebraically subtract from the IR both quantities d - declination (beta) -deviation to the right of the IR, we obtain KK. Provided that we have a compass course and need to obtain the MK, we perform the opposite actions: to the compass course KK we add the algebraic deviation 6 to the left of it and we obtain the magnetic course of the MK. If we algebraically add the declination d, which is to the left of the magnetic course, to the magnetic course, we obtain the true IR course. and, finally, if we algebraically add deviation (betta) and declination d to the compass heading, which are nothing more than the compass correction DK, then we get the true heading - IR.

An amateur navigator, when making calculations and working on a map, uses only the true values ​​of courses, bearings and heading angles, and magnetic compasses give only their compass value, so he has to make calculations using the above formulas. The transition from known compass and magnetic values ​​to unknown true ones is called the correction of bearings. The transition from known true values ​​to unknown compass and magnetic values ​​is called the translation of rhumbs.