Compass lrg


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  • When north becomes south At remote periods in the past, north has been south and south north. The Earth's magnetic field has proved a fickle lodestone, if you take a long enough view.
    The magnetic field reversals, and even the origin of the magnetic field itself, are among the most perplexing problems in geology. Einstein is said to have put the origin of the field as one of his all-time five unsolved problems in physics.
    Today geophysicists are pretty sure they know where the field comes from. Although the Earth's core contains plentiful amounts of iron and nickel, both magnetic materials, it cannot be a permanent magnet because temperatures at the centre are far above those at which these metals lose their magnetism. The magnetic field must be caused by the combination of rotation of the core, and the fact that it can conduct electricity. Movement plus current creates a magnetic field.
    The Earth's field is known to reverse every 220,000 years. The actual reversal would take place over a period of about 1,000 years and then remain in its reversed orientation for about 4,000 years.
    What would happen if the real magnetic field flipped? Sticky times. The magnetic field helps protect the Earth from damaging radiation from the Sun, so being without it for 1000 years would not only make navigation tricky. It might also throw human survival into question. Fortunately there's no evidence that this is going to happen any time soon.
  • Angle of Dip
    When the direction of the earth's magnetic field is compared with the local horizontal, the angle between this field and the horizontal component is known as the angle of dip.
  • Compass lrg

    1. 1. Grunt Productions 2007 CompassesCompasses A brief by Lance GrindleyA brief by Lance Grindley
    2. 2. Grunt Productions 2007 Gyro CompassesGyro Compasses Seamanship BooksSeamanship Books ManualsManuals Makers GuidelinesMakers Guidelines Course NotesCourse Notes Navigational TextsNavigational Texts
    3. 3. Grunt Productions 2007 Gyro CompassesGyro Compasses Using a Gyroscope or Ring Laser Gyro to detect rotation about a defined axis, a gyro compass uses a gravity reference to detect the Earth’s rotation about its North/South axis and hence define the direction of True North.
    4. 4. Grunt Productions 2007 Gyro InputsGyro Inputs ►All gyro compasses require:All gyro compasses require: ►Speed inputSpeed input ►LatitudeLatitude ►Speed input corrects for errors causedSpeed input corrects for errors caused by ship’s movementby ship’s movement ►Latitude input ensures that the gyro isLatitude input ensures that the gyro is processed at the correct rate to counterprocessed at the correct rate to counter the earth’s rotationthe earth’s rotation
    5. 5. Grunt Productions 2007 Gyro AlarmsGyro Alarms ►Silence audible alarmSilence audible alarm ►Identify which gyro compass isIdentify which gyro compass is affectedaffected ►Use alternative heading referenceUse alternative heading reference ►Alert Master, engine room andAlert Master, engine room and maintainer.maintainer.
    6. 6. Grunt Productions 2007 Compass ChecksCompass Checks ► Transit which should be sufficientlyTransit which should be sufficiently sensitive (pre-planned duringsensitive (pre-planned during pilotage).pilotage). ► Azimuth of heavenly body either atAzimuth of heavenly body either at sunrise/set .sunrise/set . ► Bearing of distant object usually takenBearing of distant object usually taken alongside from accurately fixed berth.alongside from accurately fixed berth. ► Reduction of a “cocked hat” - notReduction of a “cocked hat” - not recommended.recommended.
    7. 7. Grunt Productions 2007 Alignment of RepeatsAlignment of Repeats CorrectCorrect BearingBearing BearingBearing LowLow
    8. 8. Grunt Productions 2007 The Magnetic CompassThe Magnetic Compass ► Emergency heading reference in mostEmergency heading reference in most vessels.vessels. ► Fitted in all vessels to conform asFitted in all vessels to conform as closely as possible to SOLASclosely as possible to SOLAS regulations.regulations. ► Compass must be rotating card type,Compass must be rotating card type, not reliant on any power supply.not reliant on any power supply. ► Fitted remotely from the bridge,Fitted remotely from the bridge, normally above bridge deck.normally above bridge deck.
    9. 9. Grunt Productions 2007 Magnetic CompassMagnetic Compass
    10. 10. Grunt Productions 2007 The Earth’s Magnetic FieldThe Earth’s Magnetic Field North Magnetic Pole Magnetic Equator
    11. 11. Grunt Productions 2007 Magnetic DipMagnetic Dip H Z Horizontal Component Vertical Component LineofTotalForce Angle of Dip
    12. 12. Grunt Productions 2007 Factors AffectingFactors Affecting MagnetismMagnetism ►Vibration (when lying in magneticVibration (when lying in magnetic field)field) ►Temperature (acts like vibration)Temperature (acts like vibration) ►Rust (oxidisation reducesRust (oxidisation reduces magnetism)magnetism) ►Presence of electrical currentPresence of electrical current ►Induction from magnetic fieldInduction from magnetic field
    13. 13. Grunt Productions 2007 Ship’s MagnetismShip’s Magnetism ►PermanentPermanent -- caused by vibration and heat duringcaused by vibration and heat during building (varies with heading and latitude on building)building (varies with heading and latitude on building) ►Sub-permanentSub-permanent -- slowly changing. Acquired byslowly changing. Acquired by intermediate iron when ship has been on steadyintermediate iron when ship has been on steady headingheading ►InducedInduced -- constantly changingconstantly changing ►Forces acting on the compassForces acting on the compass  Earth’s magnetism (H) north seeking elementEarth’s magnetism (H) north seeking element  Ship’s permanent, sub-permanent and inducedShip’s permanent, sub-permanent and induced magnetism causing DEVIATIONmagnetism causing DEVIATION
    14. 14. Grunt Productions 2007 Effect of Earth’s MagnetismEffect of Earth’s Magnetism Earth’s Total Force in relation to vessel Vertical Component Fore and Aft Component Athwartships Component
    15. 15. Grunt Productions 2007 Occasions forOccasions for AdjustmentAdjustment ► After a considerable change in magnetic latitudeAfter a considerable change in magnetic latitude ► After dry-dockingAfter dry-docking ► If any of the correctors have been disturbedIf any of the correctors have been disturbed ► After modifications/repairs to electronic equipmentAfter modifications/repairs to electronic equipment ► Before sea trials of a new shipBefore sea trials of a new ship ► Annually.Annually. ► After extensive structural alterationsAfter extensive structural alterations ► After collision or grounding.After collision or grounding. ► After carriage of strong magnetic cargo (Steel girders,After carriage of strong magnetic cargo (Steel girders, iron ore, steel pipe.)iron ore, steel pipe.)
    16. 16. Grunt Productions 2007 Changing Ship’s HeadChanging Ship’s Head RED RED RED BLUE BLUE BLUE Induced Magnetism Fore/Aft Induced Magnetism Athwartships Lines of Force - Earth’s Field
    17. 17. Grunt Productions 2007 Any Questions?Any Questions?