Chapter 7 THE SHIP'S DOCTOR'S STORY Upon the arrival of ships in Bonny and New Calabar, it is customary to hand over the rigging, lower the sails on the yards and masts, and begin to build what is called a home. It's done this way. The sailors first tie the beams and yards from mast to mast so that Semikov Sergey Alexandrovich

Chapter II. Geometry of the ship's hull and the main measurements of the ship § 4. Shape of the ship's hull

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Chapter VI. Strength of a ship's hull and its design § 24. Forces acting on the hull of a floating vessel

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Deviation is the phenomenon of deviation of the magnetic compass needle from the magnetic meridian under the influence of the ship’s magnetic field

Compass meridian is the vertical plane in which the magnetic compass needle is installed on a ship

Magnetic compass deviation - the angle between the magnetic and compass meridians

Deviation is measured in a semicircular counting system. If the compass meridian is deviated to the east of the magnetic meridian, then the deviation is considered positive. If the compass meridian is deviated to the west of the magnetic meridian, then the deviation is considered negative

The deviation of the magnetic compass depends on the ship's heading. If you change course, the ship’s magnetic field and its effect on the magnetic compass needle will change.

The deviation varies depending on the latitude of the navigation area. The deviation of the magnetic compass in the same course, but at different latitudes, will be different

To ensure reliable and accurate magnetic compass readings, it is necessary to take measures to eliminate deviation. The principle of eliminating deviation is artificial creation near the compass there are magnetic fields equal but opposite in sign to the fields generated by the ship's iron.

Effect of liquid cargo on stability

Stability is the ability of a vessel that has become tilted at a certain angle under the influence of external forces to return to its original position straight position after the force ceases. Ships always carry liquid cargo (ballast water, fuel, fresh water for various purposes, etc.), and liquid cargo ships carry standard cargo. If a liquid cargo completely fills the volume allocated to it (cistern, tank), then when the ship tilts, it will behave like a solid, non-moving cargo. The effect of such a load on stability is similar to the effect that a fixed solid load has on stability.

If the tanks have a free surface (not completely filled), then when the vessel tilts, the shape of the volume of liquid in the tank will change, and this affects the landing and stability of the vessel. From the point of view of influence on stability, a liquid load with a free surface is similar to a suspended load, the suspension point of which is located at the metacenter, and the length of the suspension is equal to the metacentric radius.

The correction Δh to the metacentric height, taking into account the influence of the free surface of the liquid, will be: Δh = -(P × ρ l) / D (P = ρ l × V l - mass of liquid in the tank; V l - volume occupied by the liquid; ρ l - liquid density).

The value of the metacentric radius for this case: r l = i x / V l (i x is the moment of inertia of the free surface of the liquid relative to the longitudinal axis passing through the center of gravity of the area of ​​this surface). => Δh = -(ρ x /ρ) × (i x / V)

The main influence on Δh is exerted by the value i x, which depends on the shape and size of the free surface. At large area of the free surface, the moment of inertia i x , and, consequently, the correction Δh will be so large that the transverse metacentric height will be insufficient and may even become negative.

Recommendations for elimination or reduction negative influence the free surface of the liquid cargo on the stability of the vessel: when accepting liquid cargo, it is necessary to strive to ensure that the tanks or compartments are filled to 95% or more; voyage supplies should be consumed first from the upper containers, and then from the lower ones in turn; when ballasting, sea water cannot be taken into several ballast tanks at once; During the voyage, you should avoid taking sea water into ballast tanks and removing it from them; ballasting must be done in a port or at a base - a shelter.

Give a description running lights power-driven vessels of 50 meters or more in length

A power-driven vessel underway must display:

1. masthead light ahead;

2. a second masthead light behind and above the forward masthead light, however, a vessel less than 50 m in length is not required, but may display such a light;

3. side lights;

4. stern fire.

Magnetic compass deviation table

The deviation table shows the dependence of the residual deviation of the magnetic compass on compass courses. It is compiled after the destruction of the deviation by a deviator (a specialist in the port deviation service). The table is valid for up to 12 months. In exceptional cases, the captain of the vessel may extend the validity of this table with his signature for 3 months. The table can be made in two versions - compass courses are set in increments of 10 or 15 degrees. The argument for entering the table is the compass heading. If the compass course is not specified, then the magnetic course can be used as an argument.

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 “ship iron”, everything creates its own magnetic field electrical appliances, 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 at 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 deviation is influenced by two components - the magnetic field of soft iron and the magnetic field of hard iron.

In the magnetic compass field, 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 solid ship iron must be compensated by permanent magnets, and the forces from the inductive magnetism of soft ship iron - using elements of soft ferromagnetic material. Correct installation compensators - this 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 reach minimum values using 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 Flinders bar, in honor of English navigator and Australian researcher 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 complex system compensation 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”