Why do we need to correct the compasses on board ship?

Asked by: Anne Alvarez

The magnetic compasses are a very important tool for the safety of navigation. Vessel owners and masters must ensure that magnetic compasses are maintained in good working order, adjusted and a table or curve of residual deviation is available.

Why do we need to correct the compass?

The compass error should be checked each watch and on every new course to measure the error of the gyrocompass, which is the angle the gyro north makes with the True north to establish trends in deviation on different courses at varying latitudes.

Why does the ship’s magnetic compass error need to be corrected?

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.

Why do we need to correct the compass especially on long sea passage?

The compass readings needs to be accurate when at sea, because an error of one or two degrees in long sea passages can make a difference of nautical miles in reaching the destination and might consume more time than expected.

Why compass is very important in ship’s navigation?

A compass is a navigational instrument for determining direction relative to the Earth’s magnetic poles. It consists of a magnetized pointer (usually marked on the North end) free to align itself with Earth’s magnetic field. The compass greatly improved the safety and efficiency of travel, especially ocean travel.

How do you take a compass error on a ship?

We work out the ship's true heading. So we take the ship's gyro heading and how to subtract the gyro error. In this case negative 0.1 which gives us a true heading out of 131 decimal nine degrees.

What is compass error in navigation?

Definition of compass error



: the difference between compass heading and true heading expressed as the algebraic sum of variation and deviation.

What is the process in correcting a compass?

“Correcting” is converting a magnetic (M) direction to true (T). “Uncorrecting” is converting from true to a magnetic direction. “True” direction is relative to true north (north pole). Variation is the failure of the compass to point to true north.

How do you correct a compass?

Turn the compass over. Insert the metal key (provided with your compass) into the adjustment screw. Turn the key until the declination indicator is the correct number of degrees east or west of 0° (15.6 degrees West in this example).

What is the ship’s magnetic compass magnetism?

The Principles of the compass



A Magnetic compass is a critical piece of marine navigational equipment. 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.

How do compasses work on ships?

A spinning gyroscope tends to maintain the direction of its axis and in a gyrocompass this constant direction of the axis is towards true north. Thus the compass points to true north and is not affected by magnetic fields, electrical equipment or other metal objects nearby.

How did the compass change sea navigation?

The invention of the compass made it possible to determine a heading when the sky was overcast or foggy, and when landmarks were not in sight. This enabled mariners to navigate safely far from land, increasing sea trade, and contributing to the Age of Discovery.

What is a ship’s compass called?

A binnacle is a waist-high case or stand on the deck of a ship, generally mounted in front of the helmsman, in which navigational instruments are placed for easy and quick reference as well as to protect the delicate instruments.

How many compasses are there in a ship?

There are two types of compasses in use. The dry card compass is generally used as a standard compass & the wet card compass as a steering compass. The dry card compass is very sensitive.

What keeps a compass level at sea?

Its traditional purpose was to hold the ship’s magnetic compass, mounted in gimbals to keep it level while the ship pitched and rolled. A binnacle may be subdivided into sections and its contents typically include one or more compasses and an oil lamp or other light source.

What is compass eye?

Compass Eye is designed to help navigate at sea and can be used much like a pair of Compass Binoculars. Designed by a skipper for skippers, it can also be used on land as a bearing/marching compass and as it’s fully Gimballed; in the air too.

What is the significance of binnacle to the magnetic compass?

A binnacle is the housing for a ship’s compass. The idea behind a binnacle is to counter the magnetic deviation caused by the ship being made of iron so that the compass can point to magnetic north. Metals that were used to construct binnacles were required to be non-ferrous (containing no iron) such as brass.

What is compass Bowl?

[′käm·pəs ‚bōl] (engineering) That part of a compass in which the compass card is mounted.

Why do we need to remove bubbles from magnetic compass?

If a small bubble forms in the liquid-filled capsule, it has no influence on the accuracy of the compass. Its appearance and disappearance are due to changes in temperature and atmospheric pressure.

What liquid is in a ship’s compass?

Until 2001, all Ritchie compasses (see exception) were filled with Odorless Mineral Spirits (100% Paraffin based). Ritchie now uses a fluid called Isopar L that has similar properties but some added benefits over Odorless Mineral Spirits.

How many times you must check the magnetic compass onboard?

The performance of the magnetic compass should be monitored and deviations to be recorded in a compass deviation book at regular intervals (Ref: IMO Resolution A. 382(X)), ideally at least once every watch and also shortly after a large alteration of course.

What is the principal purpose of magnetic compass adjustment?

Compass Adjustment is the method of analyzing and compensating for deviating forces of a vessel’s magnetic compass. A magnetic compass should always align steadily to the earth’s magnetic field, pointing to magnetic north.

What are the errors associated with a magnetic compass?

Oscillation. Northerly Turning Error (part of magnetic dip) Acceleration/Deceleration Errors (part of magnetic dip)

How do you adjust a marine compass?

Try to keep metals away from it most compasses will have some means of adjusting it. This one it's right here on the bottom. The east-west the north/south.

How is a compass calibrated?

Compass calibration typically involves two steps. First, compass compensators are adjusted to minimize the influence of aircraft-induced fields that generate errors; and second, a correction card is prepared to indicate remaining error.

How do you know if a compass is accurate?

How quickly does the needle settle and accurately point north? Ensure it does not ‘stick’ when settling down to the last couple of degrees. A good compass will have a jewelled pivot bearing. It should settle in less than 3 seconds with no ‘assistance’.