Everything you need to know about Magnetic compass

Introduction

Magnetic Compass is a means of finding direction onboard the ship. It is fitted on monkey island above ship’s bridge. It is fitted along the center line of ship. It does not need any power nor satellites to work.

Ideally, the compass should be installed on the vessel’s centre line so that deviating magnetic forces are mostly symmetrical around the compass. On certain vessels, such as aircraft carriers, some fishing vessels and some modern container ships with a narrow superstructure section, the compass is offset, and this can create interesting challenges for compass adjusters.

SOLAS Requirements for Magnetic Compass

Solas Chapter V Reg 19 states,

“All ships irrespective of size shall have:
2.1 a properly adjusted standard magnetic compass, or other means, independent of any power supply to determine the ship’s heading and display the reading at the main steering position;”

“All ships of 150 gross tonnage and upwards and passenger ships irrespective of size shall, in addition to the requirements of paragraph 2.1, be fitted with:
a spare magnetic compass interchangeable with the magnetic compass, as referred to in paragraph 2.1.1, or other means to perform the function referred to in paragraph 2.1.1 by means of replacement or duplicate equipment.”

Why do we require a spare magnetic compass?

The wet compass card, if found defective owing to stickiness of movement, has to be renewed by the manufacturer or his authorized agent. Hence no spare wet card is carried. Instead, an entire bowl is carried as a spare.

Keep reading to know more about wet compass card.

Property of magnetic compass

Two most important properties of magnets

1. A freely suspended magnet always points in the north-south direction.
2. Like magnetic poles repel each other; unlike magnetic poles attract each other.

Finding Directions

The property of a freely suspended magnet to align itself always along the North-South of the earth is used to find directions at unknown places.

(1) For centuries, travellers on land and at sea have been using this property of magnets to find directions.

(2) In olden days, travellers to far off places used to carry natural magnets (which were strips of magnetite) and thread with them. They used to find directions along their way by suspending the natural magnet with a thread.

When artificial magnets were available, an instrument (or device) called compass was developed to find the directions more conveniently. In a compass, the magnet is not suspended from a thread. The tiny magnet in a compass is free to turn (or rotate) on a pivot or pin. It is called freely pivoted magnet. Just like a freely suspended magnet, a freely pivoted magnet also always comes to rest along the north-south direction.

Earth as a magnet

Magnetic compass is one of the major maritime navigational equipment and its usage is critical. Simply put, a magnetised needle, suspended freely, points North because of the forces caused by the Earth’s magnetic field. Once North is known, the other directions are easily found. A correctly adjusted compass of a ship is always sailor’s best companion. The main advantage of a magnetic needle is that it doesn’t need any external power. As long as earth’s magnetism exists, a properly adjusted compass will show you correct directions. 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.

The present day Magnetic compass has been developed from understanding the laws of magnetism which govern the behavior of the compass, greater precision in the construction of compass and better means to control damping of the compass needle.

• 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.

What is the Geographic (True) North Pole?

The Earth rotates on the geographic north and south poles. The geographic north and south poles are where lines of longitude (meridians) converge in the north. The south and north pole are directly opposite to one another.

The North Pole is located in the middle of the Arctic Ocean. Scientists have tried marking the North pole. Because the water here is permanently covered with moving sea ice, it’s practically impossible to construct any type of permanent station at the true North Pole.

What is the Magnetic North Pole?

The Earth is one big magnet. The Magnetic North Pole (also known as the North Dip Pole) is a point on Ellesmere Island in Northern Canada where the northern lines of attraction enter the Earth.

A compass needle rests freely in its casing so it can maneuver itself. When you pull out a compass, it aligns itself with the Earth’s magnetic field. The small magnetic pin is how a compass responds to Earth’s magnetism.

This means that a compass needle will point to the Magnetic North Pole – which is different from the geographic north.

Why is magnetic north not the same as geographic north?

Earth’s geographic and magnetic poles are not exactly aligned because they arise from different mechanisms. Earth’s magnetic field is caused by circulating currents of liquid iron in the outer core.

Furthermore, earth’s magnetic poles are constantly changing location relative to earth’s geographic poles. Currently, the magnetic south pole lies about ten degrees distant from the geographic north pole, and sits in the Arctic Ocean north of Alaska. The north end on a compass therefore currently points roughly towards Alaska and not exactly towards geographic north.

The Geographic North Pole differs from the Magnetic North Pole by about 500 kilometers.

Boxing the compass

Boxing the compass is the action of naming all thirty-two principal points of the compass in clockwise order. Such names, formed by the initials of the cardinal directions and their intermediate ordinal directions, are accepted internationally, even though they have their origin in the English language, and are very handy to refer to a heading (or course or azimuth) in a general or colloquial fashion, without having to resort to computing or recalling degrees.

Construction Of Magnetic Compass

There are two types of Magnetic Compass

• Dry Card
• Wet Card

The Dry card compass which was used in olden days was too sensitive for steering. to compensate for it a wet card compass was provided. Nowadays wet card compass is fitted for navigation.

An optical projector system allows helmsman in he wheelhouse to use the compass on the upper bridge for steering.

The wet card compass

The wet card is made of mica and is only about 15 cm in diameter. The graduations are photographically printed on it. The card is attached to a nickel silver float chamber that has a sapphire cap. The cap rests on an iridium-tipped pivot.

The sapphire has a polishing effect on the iridium tip. The smoothness of rotation of the compass card thus improves over the years! Though the weight of the wet card is considerable, the buoyancy of the float chamber suitably reduces the load on the pivot. This arrangement is practically frictionless.

The directive element

This is fitted below the card, enclosed in nickel-silver to avoid corrosion. In older types it consisted of two cylindrical bar magnets, one on each side of the float. In modern wet card compasses the directive element is a ring magnet fitted around the base of the float.

The ring magnet offers less resistance to movement and causes less turbulence. The ring magnet wet card compass is the most efficient type of marine magnetic compass.

The liquid

The bowl is filled with a mixture of distilled water and pure ethyl alcohol so that the mixture has
the following properties:
(a) Low freezing point about – 30″C.
(b)Small coefficient of expansion.
(c) Does not discolour the card.
(d) Low relative density-about 0.93

By immersing the card in a liquid, oscillations caused by vibration, rolling & pitching are damped, without loss of accuracy.

The bowl

Though the wet card is only about 15 cm in diameter, the diameter of the bowl is about 23 cm in order to reduce disturbances caused by turbulence in the liquid during rotation of the card. The top of the bowl is of transparent glass. The bottom is of frosted glass to diffuse the light coming from the bulb below.

The lubber line

In the forward, inside part of the bowl, there is a small projection with a line marked on it. This line is called the ‘lubber line’ and it represents
the direction of the ship’s head.

The compass is fixed on the centre line of the ship with the lubber line aligned towards forward. The reading of the compass card, which coincides with the lubber line, is the compass course of the ship at that time. In some compasses, there are three more lubber lines indicating starboard beam, port beam and right astern.

Allowance for expansion

Different methods are adopted for coping with the expansion and contraction, of the liquid in the bowl, resulting from increase and decrease of atmospheric temperature.

One method is to have a small accordion-like expansion chamber attached to the bowl, similar to that of an aneroid barometer. The chamber increases
or decreases in volume, as necessary, as the liquid in the bowl expands or contracts due to changes in atmospheric temperature.

Suspension of bowl

On the outside of the compass bowl, there are two athwartship projections, called gimbals, at the same level as the compass card. The gimbals are triangular in cross-section, apex downwards.

These gimbals rest on ‘V shaped depressions in a horizontal ring called the ‘gimbal ring’ which encircles the compass bowl . The gimbal ring itself is pivoted at its forward and after sides. If the ship rolls or pitches, the bowl would remain horizontal because its centre of gravity is well below the gimbals. A ballast weight consisting of a ring of lead, enclosed in brass, is attached along the circumference of the underside of the bowl to bring its centre of gravity below the gimbal.

The binnacle

The binnacle is a cylindrical container made of non- ferrous metal. In olden days, teak wood was used. No magnetic materials are used in its construction. The compass bowl is slung inside the top portion of the binnacle. The middle portion is accessible by a door and contains corrector magnets in the centre and the compass projector at the forward part.

Corrector magnets

In the centre of the lower half of the binnacle, there are a number of horizontal holes, both fore & aft and athwartships, for ‘hard iron’ or ‘permanent’ corrector magnets which are meant to offset undesirable, disturbing, magnetic effects caused by the ship’s steel hull. The number of corrector magnets, and their distances from the compass card, are decided by a qualified ‘compass adjuster‘ during the compass adjustment of the ship.

In the lower two-thirds of the binnacle there is a vertical brass tube, at the centre, in which slides a ‘bucket’. This bucket has some magnets in it called ‘heeling error correctors’. The bucket is held in position by a brass chain. The number and location of each magnet in this bucket, and the distance of the bucket from the compass card, must not be altered except during compass adjustment by a qualified compass adjuster.

The door giving access to the corrector magnets should always remain locked and opened only during compass adjustment.

Quadrantal corrector

These are two ‘soft iron’ spheres which are fitted in brackets, one on either side of the binnacle. The brackets are slotted so that the distance between the spheres can be altered as desired during compass adjustment.

Flinders bar

This is a soft iron corrector, (diameter about 7.5 to 10 cm)inserted in a 60 cm long brass case, fitted vertically on the forward or on the after part of the binnacle.

If the ship has more superstructure abaft the compass, the Flinders bar is fitted on the forward part of the binnacle and vice versa. The length of the Flinders bar may be altered during compass adjustment. Since the upper end of the Flinders bar must be in line with the compass card, cylindrical wooden chocks are inserted, as necessary, at the bottom of the brass case.

The helmet

The top of the binnacle is provided with a large non-ferrous metal helmet. This protects the compass bowl from direct sunlight, rain, spray, dew, frost, etc during non-use.

Compass projector

A compass projector allows the relevant part of the standard compass card and the lubber line on the monkey island to be seen by the
helmsman in the wheel house, one deck below, clear enough to steer the ship.

On the forward part of the inside of the binnacle, a brass tube of about 15 cm diameter is fitted. This tube extends into the deck head (roof) of the wheel house and houses the optical system.
An electric bulb, above the compass card, provides the necessary illumination. In case of power failure electric supply to this bulb would be available from the ship’s emergency lighting arrangements.

The bowl has a transparent glass bottom. The compass card is of the ring magnet mica type and has the degree markings photographically imprinted on it
on a black background.

Light from the electric bulb passes through an optical condenser. The bulb is situated at the focal point of the upper lens of the
condenser. This sends down the light as a parallel beam through the lower lens of the condenser. The concentrated beam then passes through the transparent compass card and down the tube.

An objective lens and a field lens magnify the image of the relevant part of the compass card and the lubberline. The image available after field lens is inverted — lubber line aft! Hence the beam is passed through an erector lens and on to a grey glass screen. The picture would now be correct as seen from BELOW the compass card – lubber line would be forward but the numbers on the card would be laterally inverted.

To correct this, the image is viewed through a mirror. The picture seen on the mirror would extend about 25° on either side of the lubber line, be suitably
magnified and appear as if seen from ABOVE the compass card.

How to remove bubble from magnetic compass?

A bubble may form in the bowl owing to the fact that some of the liquid has somehow
escaped from the bowl. This is a rare occurrence and must be remedied by following the manufacturer’s instructions.

In most compasses:
1) Tilt the bowl until the ‘filler hole’ comes uppermost. This hole is provided on the side of the bowl. 2) Unscrew the stud/screw provided.
3) Top up with ethyl alcohol. If this is not available, distilled water would do.
4) Screw the stud/screw back into place.
5) Gently let the bowl return to upright.

In some modern compasses, small bubbles may be removed as follows:
a) Invert the bowl gently. This would cause the bubble to enter a ‘bubble trap‘ provided for this purpose.
b) Gently return the bowl to upright. The bubble should have disappeared. If the bubble is large, it would have to be removed as described in above.

Care and maintenance of Magnetic Compass

• 1) The doors giving access to the corrector magnets should always be kept locked and the keys kept in safe custody.
• 2) If the binnacle is of wood, it should be varnished, not painted, as paint may cause the doors to jam.
• 3) The soft iron spheres (quadrantal correctors) and their brackets should be painted. This prevents rust.
• 4) The brass parts of the binnacle should be polished regularly.
• 5) All magnetic materials such as aerials, stays,electrical machinery, electric wires, etc, should be kept well away from the compass.
• 6) The helmet of the binnacle should be in position always except during the short periods when bearing are actually being taken.
• 7) In rare cases, a bubble may develop in the wet compass bowl. This has to be removed at the earliest opportunity.

Errors and corrections in ship magnetic compass

Variation: Is the angle between the true and magnetic meridian, that is to say, the angle that the freely suspended magnetic needle makes with the True Meridian. If the magnetic needle is drawn to the right of the true meridian, the variation is termed Easterly and if the needle is drawn to the left of the True Meridian, the variation is termed Westerly.

Variation differs from place to place but does not change with the direction of the ship’s head (course of the ship)

Deviation: A ship is made of steel, acquires a certain magnetism and so inherently has an effect on the magnetic compass. In other words, the compass needle on board does not lie on the magnetic meridian but is deflected on one side or the other because of the ship’s magnetism. Although corrective magnets are inserted in the housing (also called binnacle) of the compass to counteract this, the system is not perfect because the ship also loads steel cargo which makes the error variable. Also, the error thus caused is found to vary as the ship points in different directions (different headings). This error is called ‘deviation’ and is named East if the compass needle is drawn to the right and West if the needle is drawn to the left.

Factors affecting deviation

• Carriage of cargoes like Iron ore, concentrates, containers, or any steel cargo in large quantities.
• Any major steel repairs made on board ship (adding or removing steel).
• Any new antenna’s or aerials rigged.
• If the ship is alongside on the same heading for a long time in port.This causes a RETENTION error due to the induced magnetism on the compass needle

Compass error is, simply, the combination of the errors of Variation and Deviation, and is applied by the navigator to the compass reading to get the corrected compass reading.

Deviation curve

In practice, the deviation in a ship’s magnetic compass is reduced to minimum by use of permanent magnets and soft-iron correctors by a compass adjuster. The residual deviation is found by swinging the ship through 360 degrees and tabulating that residual deviation for the various compass headings. It may also be shown in the form of a curve.

It is preferable to tabulate deviation of the compass in a place which has the least variation or a zero variation for better accuracy. It is recommended that the deviation curve be re-drawn by swinging the vessel, every year. In case deviation exceeds 5 degrees at more than two headings, then the compass needs to be adjusted.

Compass adjustment is done by a qualified compass adjuster. In case a vessel is fitted with a transmitting magnetic heading device
(TMHD), a separate deviation curve for TMHD needs to be drawn up and posted in the wheel house. This is due to difference in distance between the magnetic correctors from the standard compass card and the transmitting element of TMC resulting in some error (up to 2.5 degrees) in TMHD readings.

Swinging of compass for preparing Deviation Card

SOLAS CHAPTER V, REGULATION 19.2.1:
All ships irrespective of size shall have:

1. a properly adjusted standard magnetic compass, or other means, independent of any power supply, to determine the ship’s heading and display the reading at the main steering position.
2. a pelorus or compass bearing device, or other means, independent of any power supply, to take bearings over an arc of the horizon of 360°”.

ISO 25862 : 2009 (E):  States that all SOLAS vessels should have their compass swung/adjusted and a new deviation card issued at maximum two yearly intervals. When a new vessel is commisioned, compass deviation on any heading should be no more than 3°. Thereafter, deviation on any heading should be 5° or less.

Vessels transiting the Panama Canal are required by the canal authorities to have had a valid compass deviation card issued within the previous 12 months.

There are several methods for doing this but in general terms the procedure is as follows:

1. The vessel is anchored securely in midstream
2. A transit line of known bearing (magnetic bearing from the chart) is established
3. The vessel moves through each of eight compass bearings (cardinal and inter-cardinal points)
4. For each of the eight points the bearing of the transit line is taken and any discrepancy between this compass bearing and the known chart bearing is noted. If the compass bearing is greater the deviation is west, if less the deviation is east. 

Transmitting magnetic heading device (TMHD)

A transmitting magnetic heading device (TMHD) is an electronic device which uses the geomagnetic field to obtain and transmit information about the ship’s heading. The TMHD is designed to transmit heading information to other equipment.

It comprises of a sensing part which provides for a sensing function of detecting any heading information connected to the transmitting device. Transmitting part receives heading information from the sensing part and converts to the required accurate signal. TMHD are usually mounted under the magnetic compass.

Transmitting magnetic heading devices (TMHD) may comprise of:
1) a standard magnetic compass equipped with a magnetic sensor and electronics for generating a suitable output signal for other devices. The compass used should be the standard magnetic compass or

2) an electromagnetic compass consisting of the sensor part and electronics for
generating a suitable output signal for other devices or

3) any type as defined above and additionally equipped with a rate gyro to improve
dynamic performance.

A fore-and-aft mark is inscribed on the magnetic sensor housing, which is installed in parallel to the ship’s fore-and-aft line. The accuracy of the fore-and-aft mark is within + 0.5° to the fore-and-aft direction of the housing. If a rate gyro is installed it is marked in the same way and additionally be marked with top or bottom.

All displays and outputs of TMHD need to be corrected for true heading by applying variation and deviation. The system allows for automatic compensation if variation for the area and the complete deviation card details are fed to the device beforehand.

An alarm is provided to indicate malfunctions of the THD or a failure of the power supply.

The mounting arrangements of the magnetic sensor are so fitted to allow for correction of
any misalignment, up to + 5°, with respect to the fore-and-aft line.

Steering Using Magnetic Compass

Some of the autopilots are provided with an additional option of steering with reference to magnetic compass in case of gyro failure. A transmitting magnetic heading device may be installed on such ships.

Procedure for change over from gyro to magnetic
1) Equipment manufacturer’s instructions shall be followed as mentioned in user’s manual for changing over auto pilot sensing to TMHD from gyro.
2) Prior change over the course to steer shall be checked along with the corresponding magnetic heading.
3) Steering shall be changed to hand steering and vessel steadied on the desired magnetic course.
4) Magnetic course to be steered shall be set in the autopilot panel by course setting knob.
5) Auto pilot settings shall be checked and adjusted if required. Off-course alarm shall be set for the corresponding magnetic heading.
6) The steering mode shall be changed over to magnetic by the changeover knob.
7) The course steered shall be observed for some time and regularly during the watch.
8) Careful check shall be kept on the changes in variation and ship’s deviation in order to set correct course to steer.