Masters Compass - Today’s Lesson Basic Magnetism & the Earths field Hf & Zf Induction Effect of the Earths’ field on new build vessels Compass correcting – what are we correcting? Card System Coefficients Coef. ‘B’ – Induced Coef. ‘B’ – Permanent Splitting ‘B’ Coef. ‘C’

Magnetism

LINES OF FORCE EXIT RED POLE ENTER BLUE POLE

FORCE FIELD RUNS FROM RED to BLUE RED END ‘EMITS’ BLUE END ‘COLLECTS’

N (MAG) S (MAG)

Opposite poles Attract Like poles Repel Opposite poles Attract

Hf & Zf F’wood - 68

RATIO OF Hf & Zf ASHORE Hf Hf Zf Zf HIGH & LOW NORTH LAT.

Hf & Zf @ Freetown Hf = Zf = @ NW Canada Hf =

Hf & Zf RATIO’s SHIP SHORE

Induction

WHERE FIELD ENTERS ‘BLUE’ POLE INDUCED WHERE FIELD EXITS ‘RED’ POLE INDUCED

Hard & Soft Iron Ferro-magnetic materials - iron, steel, nickel Non-magnetic - brass Hard Iron retains magnetism becoming Permanent magnets Soft Iron quickly loses it’s magnetism and is then re-induced by another field

FLINDERS BAR IS INDUCED SOFT IRON HAS HORIZONTAL POLES & VERTICAL POLES

Effect of Earths field on a New build vessel

BUILT HEADING NORTH - NORTH LAT.

BUILT HEADING SOUTH - NORTH LAT.

Compass correction What are we correcting?

‘like cures like’ Forces acting on the compass position are practically equal in all directions of the ship’s head Lost directive force on certain headings have been restored and sluggish action has been eliminated Compass card has been steadied Residual deviations are small and symmetrical

Card System Driving force? Where will it work best? Magnet system – needle or ring Magnet casing – brass Float - High centre of gravity. Jewel - sapphire Pivot - iridium Liquid - Bayol

Coefficients A coefficient is a mathematical representation of deviations on two opposite headings. Named by the deviation caused in the 1st sector. East is ‘+ve’. West is ‘-ve’. Sectors : Semicircular or Quadrantal

Semicircular

Quadrantal

Coefficient ‘B’ BI – ‘B’ induced & BP – ‘B’ permanent It is the mathematical representation of deviations on EAST & WEST headings. There are 2 causes: BI – ‘B’ induced & BP – ‘B’ permanent

Cause: induction of fore & aft, vertical soft iron near the binnacle Coef. ‘B’ Induced Cause: induction of fore & aft, vertical soft iron near the binnacle

Vertical iron induced Northern hemisphere

Vertical iron induced Southern hemisphere

( NORTHERN HEMISPHERE ) INDUCED B ( NORTHERN HEMISPHERE ) What will be the deviation?

( NORTHERN HEMISPHERE ) INDUCED B ( NORTHERN HEMISPHERE ) What will be the deviation?

( NORTHERN HEMISPHERE ) What will be the deviation? INDUCED B ( NORTHERN HEMISPHERE ) What will be the deviation?

( NORTHERN HEMISPHERE ) COMPENSATION WITH FLINDERS BAR INDUCED B ( NORTHERN HEMISPHERE ) COMPENSATION WITH FLINDERS BAR ( BLUE POLE )

( SOUTHERN HEMISPHERE ) COMPENSATION WITH FLINDERS BAR INDUCED B ( SOUTHERN HEMISPHERE ) COMPENSATION WITH FLINDERS BAR ( RED POLE )

What about at the Equator?

Cause: ships’ fore & aft magnetic field Coef. ‘B’ Permanent Cause: ships’ fore & aft magnetic field

COEFFICIENT +B E EASTERLY DEVIATION ( BLUE BOW )

COEFFICIENT +B W WESTERLY DEVIATION ( BLUE BOW )

Calculate the value of Coef ‘B’ When heading East the deviation was 10W When heading West the deviation was 8E E is +ve W is -ve Change the sign on West, find the sum of the values and divide by 2

Calculate the value of Coef ‘B’ When heading East the deviation was 10W When heading West the deviation was 8E -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- -10 -8 ----------------------------------- -18 divided by 2 = -9 Coef. ‘B’ combined

Bp - Correction

Splitting ‘B’ Magnetic Equator method 2 different latitudes method Adjusters method Home coast method

Magnetic Equator method - example voyage Compass corrected in Liverpool. Adjuster estimated the amount of Flinders Bar and corrected the remainder of Coef. B with permanent correctors Ship sailed for Argentina How is ‘B’ split?

Which ships don’t need a Flinders Bar?

Coefficient ‘C’ It is the mathematical representation of deviations on NORTH & SOUTH headings. Cause: ships’ athwartship permanent magnetic field

COEFFICIENT +C N EASTERLY DEVIATION ( BLUE STARB’D )

COEFFICIENT +C WESTERLY DEVIATION ( BLUE STARB’D ) S

C - Correction

Obtaining a Magnetic bearing & calculating the value of Coef’s B & C

A B C D E N 052° NE 049 ° SE S 058 ° SW W 062 ° NW HDG Comp. brg. Mag. Brg DEV. A B C D E N 052° NE 049 ° SE S 058 ° SW W 062 ° NW

A B C D E N 051° 055° 4°E +4 NE 049 ° 6°E +6 SE S 058 ° 3°W -3 SW W HDG Comp. brg. Mag. Brg DEV. A B C D E N 051° 055° 4°E +4 NE 049 ° 6°E +6 SE S 058 ° 3°W -3 SW W 062 ° 7°W -7 NW 13/2 =+6½ 7/2 =+3½