The principle of Magnetic compass has not been changed for a long time and it's still being followed. It essentially consists of a magnetized needle pivoted over a horizontal plane.
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 a 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 its behavior, greater precision in construction, and better means to control damping of the compass needle.
Ship’s Magnetism
Ships are made of steel, a ferromagnetic material. They are magnetized during the construction of a ship. Magnetic properties induced on steel can be permanent or temporary, and mostly they are permanent due to energy imparted during cutting and welding.
Permanent and Sub-Permanent Magnetism
Effects of permanent magnets onboard the ship can be combined into one single force called permanent magnetic force. Permanent magnetism exists through the life of a ship. The force doesn’t change with heading or at poles, but changes slowly over time.
This change is also known as sub-permanent magnetism and corrections to this are carried out together with permanent magnetism. Sub-permanent magnetism results from cruising, vibration, or ageing. It must be corrected by permanent magnet correctors.
Induced Magnetism
Induced magnetism on ships changes depending on heading, strength of earth’s magnetic field, and magnetic latitude. It isn’t constant as it depends on ferromagnetic properties.
A ship is therefore a combination of all three types of magnetism. The ship’s apparent magnetic condition is subject to change due to vibrations, welding, and shocks.
Placement of the Compass
The magnetic compass should ideally be placed far away from magnetic materials to ensure only the earth’s field affects it. This is a challenge in modern shipbuilding, as ships built of steel contain permanent, sub-permanent, and induced magnetism, which can disturb compass readings.
These errors are not fixed, so if not corrected, compass headings and bearings cannot be relied upon.
Magnetic Compass at Sea
Magnetic compass is used to steer the ship on a fixed course. Compass readings must be accurate at sea, as even a one- or two-degree error on long voyages can add nautical miles and consume more time.
The compass is also used for taking bearings of terrestrial and celestial objects. Therefore, errors need to be nil or minimal when it comes to magnetic compass systems in UAE.
Errors and Corrections in Magnetic Compass
Variation
The angle between the true and magnetic meridian is called variation.
If the magnetic needle is drawn to the right of the true meridian, it is termed Easterly.
If to the left, it is termed Westerly.
Variation differs from place to place but does not change with the ship’s course.
Deviation
A steel ship has magnetic properties which affect the compass. The needle onboard is deflected because of the ship’s magnetism.
Corrective magnets are inserted in the housing (binnacle) to counteract this, but loading steel cargo makes the error variable. This error is called deviation.
Compass Error
Compass error is the combination of variation and deviation. These are applied by the sailor to get the corrected compass reading.
Maritronics Expertise
Maritronics, one of the leading dealers of maritime products and services, sells some of the most accurate magnetic compasses and is a leading marine navigation and communication company in Dubai.
As discussed, compass readings are subject to change due to various reasons. Experts at Maritronics explain to each client how the magnetic compass needs to be read and how they are affected by ship manufacturing.
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