Magnetic compass

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  • The magnetic compass is fitted on the upper bridge exactly center line of the ship 

  • It is referred to as standard compass 

 

1.Dry Card  

2.Wet Card 

  • A compass point north because all magnets have two poles north pole, south pole. 

  • The north pole of the one magnet attracted towards the south pole of another magnet 

 

DRY CARD COMPASS 

  • A light aluminum ring about 254mm in diameter is attached to a brass hub by silk cord which is threaded in and out through holes in the ring. 

  • This tight silk cord is act like bicycle wheels. 

  • There is no other connection or support between the ring and the hub. 

  • Silk cord is used because it doesn’t shrink or stretch due to moisture or change in atmospheric temperature. 

 

Card support: - 

 

  • The bottom of the hub has small cap fitted with a sapphire bearing surface. 

  • This rests on a pivot tipped with iridium which is hard nonmagnetic metal. 

  • This arrangement provides practically friction-less support, for the compass card and requires no lubrication. 

  • The sapphire cap has a polishing effect on the iridium. the smoothness of rotation of the compass card thus improves over the years. 

  • The compass card is made of rice paper because it is very light and unaffected by changes in temperature. 

  • The rice paper is divided into several segments and each is stuck to aluminum ring and to the silk cord using light waterproof adhesive. 

  • The reason for the division of cards into segments is that if a drop of water was to fall at any part of the compass card only that segments gets distorted. 

 

 

 

Directive element: - 

  • This consists of 3,4 or 5pairs of needle magnets suspended, parallel to each other, a few centimeters below the compass card 

  • Symmetrically Arranged on either side of card 

  • The ends of magnets have holes on them, silk cord pass through these and through holes in the bottom of the aluminum ring to support the magnets below the compass card  

  • By this method, center of gravity of the entire compass card, inclusive of magnets is below the tip of the pivot. 

  • This ensures that the card is not only north seeking but also remains horizontal even when the pivot tilts out of vertical during rolling or pitching. 

 

The compass bowl :-  

  • The compass card is enclosed in a cylindrical brass bowl having a transparent glass top and a frosted glass bottom 

  • The glass top is retained in position by a brass ‘verge ring’ which is secured to the bowl by equidistant,brass screw along the circumference 

  • The rubber washer between verge ring and the top glass ensures water tightness

  • The top of the verge ring is grooved to accommodate the azimuth mirror 

  • The vertical distance between the top of the hub of the card and the top glass is such that even if the bowl is inverted the card wouldn’t completely disengage from the pivot so that when the bowl is returned to upright,the card would again rest properly on the pivot as before. 

 

The lubber line:- 

  • On the forward,inside part of the bowl, there is small projection with a line marked on it. 

  • This line is called the lubber line 

  • This represents the direction of the ship’s head. 

  • The compass is fixed on the center line of the with lubber line aligned toward forward. 

 

Suspension bowl:- 

 

  • On the outside of the compass bowl ,there are two athwart ship projections called gimbals,at the same level as the compass card. 

  • The gimbals are triangular in cross-section,apex downward. 

  • These gimbals rest on V shaped depression in a horizontal ring called the gimbal ring which encircles  the compass bowl. 

  • The gimbal ring itself pivoted at its forward and after sides.if the ships rolls or pitches the bowl would remain horizontal because its COG is well below the gimbals. 

 

The binnacle:- 

  • The binnacle is a cylindrical container made of teak wood and brass. 

  • Non magnetic materials are used in its construction. 

  • If the screws are of brass and the nails,copper.the compass bowl is slung inside the top portion of the binnacle. 

  • The middle portion is accessible by a door and contains an electric bulb. 

Corrector magnets:- 

  • In the centre of the lower half of the binnacle,there are a number of horizontal holes,the fore , aft and athwartships,for hard iron or permenent corrector magnets which are meant to offset  undesirable disturbing,magnetic effects caused by the ship s steel hull. 

  • These holes are accessible by means of door in the binnacle. The number of the corrector magnets and their distances from the compass card,are decided by a qualified compass adjuster during compass adjustments of ships. 

  • The lower two third of the binnalce has a vertical  brass tube at the center in which slides a bucket. This bucket has some magnets in it called heeling error magnets. 

  • This bucket is held in position by a brass chain. 

  • The number and the location of each magnet in this bucket and the distance of the bucket from the comass card  must not be altered except during compass adjustment by a qualified compass adjuster.the door giving acccess to the corrector magnets should always remain locked and opened only during compass adjustment  

 

Quadrantal correctors :- 

 

  • These are two soft iron spheres which are fitted in brackets one on either side of the binnacle.the brackets are sloted so that the distance between the spheres can be altered  as desired during the compass adjusment  

 

Flinders bar:- 

  • This is soft iron correctors inserted in 60cm long brass case ,fitted vertically on the forward or on the aft part of the binnalce.if the ship has more superstructure abaft the compass,the flinders bar is fitted on the forward part of the binnacle and vice versa  

The helmet 

 

  • The top  of he binnacle is provided with a large brass helmet.this protects the comapss bowl from direct sunlight ,rain ,spray,dew etc. during non use. 

 

 Care and maintenance :-  

  1. The doors giving access to the corrector magnets should always be kept locked and the key is safe in custody.  
  2. The wooden parts of the binnalce should be varnished,not painted as paint may cause doors to jam.  
  3. The soft iron spheres (quadrantal correctors )and their brackets should be painted to prevent rust.  
  4. The brass part of the  binnacle should be polsihed regualarly  
  5. The binnalce light should be switched off during day time   
  6. All machinery materials such as ariels,stays,elctrical machinery and wires should be kept well away from the compass .
  7. Prevent salt and dust particle settling on the azimuth mirror and the comapass bowl.  
  8. Prevent spray and precipitation from falling on the azimuth mirror and the comapss bowl.
  9.  Protect the azimuth mirror and the comapss card from the direct sunlight  

 

    

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