Navigation and Seamanship

2.1.5.Compass error-II Deviation

Deviation: Local magnetic fields in a ship may cause the resultant filed at the compass position to lie in a different direction from the Earth’s field alone. A ship is built by the steel and steel alloys, and these materials may influence the magnetic needle of the compass placed nearby. The magnetism in the hard iron component will be permanent and will remain constant whatever the direction of the ship’s head. Boats themselves can also affect compasses due to their construction, steel engines, keel, Ferro-magnetism, electromagnetism, (loudspeakers, mobile phones, hand held VHF) and deck cargo on large ships. This effect is called Deviation, and will vary with the heading of the boat.

Because of the ships’s magnetic effect the compass needle deflected from magnetic north to compass north .the difference between magnetic north and compass north is termed as ‘Deviation’

The compass course is the angle between the compass needle and the direction of the ship’s head. It defines the direction of the ship’s head relative to “ compass north”. The compass course is indicated by the position of the lubber’s line relative to the compass card. The magnetic course is the angle between the magnetic meridian and the direction fo the ship’s head. It defines the direction of the ship’s head relative to “magnetic north”. The deference between any magnetic course and the corresponding compass course is the deviation. The true course is the angle between the true meridian and the direction of the ship’s head. It defines the direction of the ship’s head relative to “true north”. The difference between any true course and the corresponding magnetic course is the variation. Deviation varies with vessel to vessel also it varies with changes in the ship’s heading.