Magnetic compass: The coefficients are the mathematical ... - Rajnav

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Permanent B (Bp) Caused by the permanent magnetism displayed by the vessels permanent structure in the Fore and Aft direction Polarity depends on the direction where the vessel was headed when being built. Remains permanent and does not change signs with change of hemisphere The value remains same in all latitude Maximum deviation is caused at the E/W headings. Nil deviation on N/S headings Correction: Corrected using the permanent magnets in Fore and Aft direction (Like for like principle use and equal and opposite pole) An easterly deviation indicates that the ship has a blue pole attracting the red needle (+Bp) hence this is corrected by using a Fore and Aft magnets with red pole facing forward. Correction can be effected by using as many magnets required to nullify the deviation. Use of single magnets is not advisable and better correction by uniform field can be achieved by the use of a number of magnets placed at varying distance from the needle. The F/A correctors are placed in two lines on port and starboard side due to the presence of the Heeling error magnet bucket in the centre ( in order to maintain symmetry on both sides The corrector magnets are not to be placed nearer than twice their length to enable uniform magnetic field. Correction procedure for B Split the components in to Bp and Bi. Following methods can be used: Magnetic equator method: This is the easiest and practical method and can be carried out every time the vessel is in magnetic equator. At the equator the Bi is nil since the vertical component of the earth’s field does not exist and all deviation of B is due to Bp only. Take a magnetic bearing by observing a distant object (15-16 miles) and obtain bearings on 8 cardinal points and average the values to obtain the magnetic bearing. Take a bearing on East and west and determine the value of Bp. Head the ship on the east or west direction and adjust the position /number of the Fore aft and magnets to nullify the deviation. Once the vessel is back in higher latitudes, Repeat the above procedure to eliminate and calculate the deviation due to Bi but eliminate the deviation by adjusting the Flinder bar’s position and numbers (Having corrected the Bp at the equator, all the deviation being caused at the East/West headings is due to Bi only)
Compass adjuster method: With his experience on similar vessel the adjuster allots a value of deviation for Bi and corrects eliminates that amount if deviation caused by Bi by adjusting the position of Flinder bars. Then the Bp is eliminated by adjusting the fore aft permanent magnet position and numbers. Home coaster method: Since the vessel is going to be on the coast and there will be no appreciable change of latitude and all the deviation caused on the E/W courses is corrected by the use of F/A method. Since the Latitude is not changing the Bi will also not change and has been eliminated by the F/A method. Two different latitude method: The coefficient of B is noted at 2 different latitude that are widely separated. No correction or adjustment is made to the corrector magnets. A calculation is made and then Bp and Bi values found are eliminated separately using Flinder bars and F/A magnets Co-efficient C: This is the algebraic sum of Ci (Induced) and C (p). Ci is the deviation caused due to magnetism induced in the vertical symmetrical soft iron on the port and starboard side of the compass. Usually Ci gets cancelled out if the compass is situated in the centre. In case where the symmetry is disturbed or if the compass is not lying at the center the Ci can cause deviation. Correction: This is eliminated by the use of Flinder bar which is slewed out to cancel the effects of asymmetry. Co-efficient Cp: This is the deviation caused by the permanent magnetism of the ships athwart ship component. This is formed during the ships construction. This is maximum on north south courses Does not change value with latitude or Hemisphere Correction: This deviation is corrected by the use of Athwartship permanent magnets. An equal and opposite pole to that of the ships pole is used to nullify the deviation. Coefficient D: This is has 2 components the Fore / Aft and Athwartship component but the deviation is mainly caused by the Athwart ship soft iron only. The proximity of the compass in the usual construction of vessels results in the atwartship component causing the deviation. (ICHAS – Induced continuous horizontal athwartship soft iron). This deviation is maximum on quadrantal courses and nil on cardinal courses. Thus it is called a quadrantal error. Change in latitude/hemisphere does not change the net deviation since both the horizontal and athwartship components change proportionately. Correction: Quadrantal error corrector sphere are used (Soft iron spheres also called the Kelvin’s sphere) The vessels construction results in +D (Easterly deviation in NE course)
Page 1: Magnetic compass: The coefficients
Page 5 and 6: 3. Magnetic bearing of an object is
Page 7 and 8: 22. The vessel should be in sea goi
Page 9 and 10: Coefficient D - Kelvin’s spheres

Magnetic compass: The coefficients are the mathematical ... - Rajnav

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