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Navigation Equipment—Compass

Navigation Equipment—Compass

The compass is an instrument that provides a directional datum and is used by ships to determine the course and the bearing of the observed object. There are two kinds of compasses: magnetic compass and gyrocompass, and generally sea ships are equipped with both magnetic compass and gyrocompass. The former is simple and reliable, while the latter is easy to use and accurate.

Magnetic compass

Magnetic compass: It is made by using the characteristics of the magnetic needle pointing north. The compass is the original type of magnetic compass, which is one of the four great inventions of ancient China. The compass used for navigation is also known as the compass. After the advent of iron ships, the magnetic meridian produced self-difference. After the 19th century, the method of eliminating self-difference was proposed, and by the beginning of the 20th century, the liquid compass with stable performance and less friction of the shaft needle was made and used in most ships.

CPT-130A CPT-130B

The magnetic compass has a magnetic difference, which is caused by the inconsistency of the magnetic pole with the earth’s pole. The angle that exists between the magnetic north and the true north, i.e., the magnetic declination. The charts are marked with local magnetic differences and annual rates of change, which can be used to correct readings when using a magnetic compass. The magnetic compass structure is mainly composed of a compass cabinet and a compass basin, and a compass card with a magnetic needle is installed in the basin.

According to the structure, the magnetic compass can be divided into two types: dry compass and liquid compass; According to the use, it can be divided into standard compass, steering compass, emergency compass, boat compass, etc. Modern ships often have a standard compass on the open deck above the bridge that projects compass card readings into the wheelhouse, which doubles as a steering compass. There are two kinds of compasses: reflective and projectional, and their basic structure is similar to that of ordinary compasses, with only one more optical projection system.

CGT 165

The error of a magnetic compass includes magnetic difference and self-difference. Magnetic aberration is the angle between magnetic north and true north due to the inconsistency between the magnetic pole and the earth’s pole, also known as magnetic declination. The self-difference is due to the magnetic magnetization of the ship’s steel by the geomagnetic magnetization of the magnetic compass to act on the magnetic compass, causing the magnetic needle to deviate from the magnetic north, and the difference angle formed is even tens of degrees. Correcting the self-deviation is to place a corrector that can produce a force opposite to the ship’s magnetism around the compass, and use the magnetic rod and soft iron in it to counteract the influence of permanent ship magnetism and induced ship magnetism on the magnetic needle. The corrected remaining self-deviation shall be measured to make a self-deviation table or self-deviation curve, and the absolute value should not be greater than 3°. The main feature of the self-deviation is that it changes with the course, and different courses should be corrected with the corresponding self-difference.

So when do you need to correct the tolerance?

The self-deviation of the magnetic compass needs to be corrected in the following cases:

(1) Magnetic compass on a newly built ship.

(2) The magnetic compass on the ship after the ship is repaired (including major repair, medium repair, and annual repair).

(3) The magnetic compass of the ship that has vibrated violently (such as collision, grounding, landing, lightning strike, etc.).

(4) After the compass installation position on the ship is changed.

(5) Excluding the constant self-difference, when the self-deviation of the standard magnetic compass is greater than 3° of soil (when the self-deviation of the steering compass is greater than +5°).

(6) Magnetic compass when the ship is docked in a fixed course for more than 1 month.

(7) Magnetic compass on board after unloading of a large number of ferromagnetic cargo.

Electronic compass

Gyrocompass: Also known as gyrocompass, it is an instrument that uses the axial and precession of gyroscope, combined with the earth’s rotation vector and gravity vector, and is made with control equipment and damping equipment to provide a true north datum. The gyrocompass is composed of the main compass and the compass, the power converter, the control box and the control box and other auxiliary equipment.


There are latitude errors, velocity errors, impact errors, sway errors and baseline errors in gyrocompass. The gyrocompass using the vertical axis damping method has a latitude error, which is a principle error. The speed error is not related to the compass structure parameters, but to the speed, course and latitude of the ship. The latitude error and velocity error are regular, and can be corrected by the look-up table method, the moving baseline or dial method, the moment compensation method, etc. The impact error caused by the influence of inertial force on the gyrocompass when the ship is maneuvering can be eliminated by cutting off the damper and making the equal amplitude swing period of the compass equal to 84.4 minutes or cutting off the electromagnetic pendulum. The gyrocompass has a device to reduce the sway error caused by the sway of the ship, so this error can generally not be considered. The baseline error caused by the poor installation of the baseline of the main compass or compass is a fixed error, and after measurement, the base of the main compass or compass can be rotated to make the baseline parallel to the bow and stern line to correct. On a calm sea, when the ship sails at constant speed and direction, the error of the corrected gyrocompass should not be greater than 1°.

The gyrocompass usually consists of two parts: the main compass and the auxiliary instruments. Auxiliary instruments include power converters, control boxes or control boxes, and compasses, etc., which are necessary equipment to ensure the normal operation of the main compass. In order to reduce the number of gyrocompass parts, the power converter can be assembled with the control box or with the main compass. The main compass can generally bring 8~20 sub-compasses to show the course of the main compass. Modern gyrocompasses are small in size, light in weight, long in life, easy to maintain, and easy to operate

Regulatory staffing requirements

Technical Rules for Statutory Survey of Domestic Navigation Sea Vessels:

Standard magnetic compass: ships of 300 gross tonnage and above are required to be equipped, and ships of less than 300 gross tonnage can only be equipped with steering magnetic compass (92 regulations: Class I. navigation area of 150 gross tonnage and above is required to be equipped with II., III. Category navigation area of 300 gross tonnage and above is required to be equipped)

Steering magnetic compass: Ships with reflective magnetic compass are exempt.

Standby standard magnetic compass: ships of more than 300 gross tonnage are required to be equipped, and ships that have been equipped with a steering magnetic compass or gyrocompass can be exempted.

Gyrocompass: passenger ships of 3,000 gross tonnage and above and cargo ships of 5,000 gross tonnage and above are required to be equipped (“11 regulations”: ships of 500 gross tonnage and above are required to be equipped) (“92 regulations”: I. requirements of 500 gross tonnage and above in Class II navigation areas can be exempted for passenger ships of 3000 gross tonnage and below and cargo ships of 5000 gross tonnage and below).

Gyrocompass azimuth: If the compass is installed on the decks of both wings of the deck outside the wheelhouse and the top of the deck is shaded, a separate compass should be added to the open deck above the wheelhouse (installation requirements are not mentioned in Regulation 11).

The gyrocompass should be configured in the main steering position (except if it can be clearly read from the main compass) and the emergency steering position (the “92 regulations” require: in the following places: wheelhouse, chart room, emergency wheelhouse, captain’s room).

Heading Conveyor (THD): “06 Change” requires passenger ships under 3,000 gross tonnage and cargo ships of 500 gross tonnage to 5,000 gross tonnage, if not equipped with a gyrocompass, should be equipped with a heading conveyor for inputting the head information of the magnetic compass into the AIS and radar equipped with the ship. This requirement should also be met for existing vessels with new AIS installations. (Rule 11: Ships under 500 gross tonnage, if not equipped with gyrocompass, should be equipped with a head conveyor.

Apparatus for measuring orientation in a 360° range of the water level: all ships are required to equip it.

Check the points

  1. Whether it is equipped with a standard compass, steering compass or gyroscopic compass and sub-compass as required.
  2. Whether the driver’s platform is posted with a magnetic compass self-check form, and whether the self-check table is within the validity period. Whether the self-inspection and correction have been completed.
  3. Whether the remaining self-deviation of the standard magnetic compass exceeds ±3°, and whether the remaining self-deviation of the steering magnetic compass exceeds ±5°. Whether the reading error between the main compass and the compass exceeds ±0.5°.
  4. Whether the bow sensor is installed as required. Whether the heading data is transmitted normally.
  5. Whether there are bubbles or steam in the magnetic compass box will affect the reading.
  6. Whether the directional ring can be restored to a horizontal state after rotating the compass box.
  7. Adjust whether the power supply and dimming functions of the compass are normal.
  8. Whether the magnetic compass electrical repeater works normally and whether there are two power supplies: main power supply and emergency power supply (standby power supply).
  9. Whether the ship is equipped with the following accessories: azimuth reader, compass disc reading magnifying glass, compass spare compensation magnetic rod.

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