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    Lesson 9: ShipboardCompasses

    Learning Objectives:

    Comprehend the basic principles ofoperation of the gyrocompass and itsadvantages and disadvantages.

    Apply correct procedures indetermining and correcting forgyrocompass and magnetic error.

    Comprehend the differences betweentrue, magnetic, gyrocompass, andrelative direction reference systems,and apply proper procedures to makedirection conversions from any onesystem to any other.

    Comprehend the basic principles ofoperation of the magnetic compassand its advantages and disadvantages.

    Comprehend the reasons for variationand deviation and how these mightaffect the magnetic compass.

    Apply proper procedures inconverting from true direction tocompass direction and vice versa.

    Apply correct procedures to determinevariation using navigation charts.

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    Onboard ship, there are three principal references fordirection: the ships longitudinal axis, the magnetic

    meridian, and the true or geographic meridian. Bearing: The horizontal direction of one terrestrial

    point from another, expressed as an angle from0000clockwise to 3600.

    Relative bearings (abbreviated with an Rfollowing the bearing):Bearings measured

    with reference to the ships longitudinal axis. Magnetic bearings(abbreviated with an M

    following the bearing): Bearings measuredwith respect to magnetic north. They aremeasured with a magnetic compass.

    True bearings(abbreviated with a T following

    the bearing): Bearings that are measured withrespect to true or geographic north. They aremeasured with a gyrocompass of known error.

    Ships head, orheading: A special bearingdenoting the direction in which the ship ispointing. It can be be expressed with reference tomagnetic or true north.

    True bearings are only plotted on chart. Magneticor Relative bearings must be converted to True inorder to plot on chart.

    Shipboard Compasses

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    Used to obtain precise information onheadings and directions

    Two types:


    Used the most onboard ship Provides you with true bearings

    Magnetic compass

    Used as a backup because it requires

    no electricity to operate Primary means of checking


    Found on every oceangoing vessel

    used by smaller vessels as primaryreference for direction

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    Magnetism - Physical force between two

    objects of metalMagnet metallic element that has theproperty for attracting iron and producinga magnetic field (lines of force) arounditself

    Lines of force are magnetic meridians

    Earth has magnetic properties and can bethought of as having a powerful magnetnear its center

    Internal magnet is not aligned withEarths axis

    Earths magnetic poles are at differentlocations as Earths geographic poles

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    Magnetic North

    PoleTrue North


    Notice that

    the two poles


    together. The



    point to themagnetic

    pole, and this

    differencefrom true

    North gives

    us VARIAT


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    Law of Magnetism

    Every magnet has two poles

    Each pole has opposite characteristics


    North pole attracts a south pole but itrepels another north pole

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    Variation for any area on earth is always

    equal to the angular difference betweenthe value of true north and magneticnorth

    Expressed in either degrees East or Westto indicate on which side of the

    geographic meridian the magneticmeridian lies

    True NorthMagnetic


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    Caused primarily by the fact that the earths magnetic

    and geographic poles do not coincide Compass align with magnetic lines of force flowin

    from north and south magnetic pole

    Also caused by the magnetic abnormalities in earthscrust

    Some locations have similar values of variation as atother locations

    Isogonic lines line along which measuredvariation is the same

    Example: isogonic line chart

    Magnetic field of earth does not remain constant =

    continually changing in both direction and intensity Magnetic poles wander slightly over the earths

    surface from year to year

    Variation changes slightly from year to year

    Determine the value of variation at any given position

    by referring to compass rose in area on chart whereyou are located

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    How to determine LocalVariation

    Locate the compass rose nearest to thearea in which the ship is operating

    Locate the variation and annualincrease/decrease from the center

    Locate the year from the center of thecompass rose

    Subtract the year indicated fromcurrent year

    Multiply the number of years times the

    annual change (sum) Add the sum(or subtract if decreasing)

    to the variation in the center of thecompass rose

    Round the total off to the closest degree


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    Magnetic Compass

    CNO requires that each self-propelled shipand service craft of the USN be equippedwith one or more magnetic compassessuitable for navigation

    Exception of nuclear-powered

    submarines, all ships and craft musthave a magnetic compass at the primarysteering station

    Many ships have more than onemagnetic compass

    Primary magnetic compass is called thesteering compass

    Normally located on the centerline inthe ships pilothouse where it can bebest seen by the helmsman

    Readings from the steering compassare labeled per steering compass(PSTGC)

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    Standard and SteeringCompasses

    Secondary magnetic compass iscalled the standard compass

    Normally located on the centerlineat the secondary conning station

    Readings from the standardcompass are labeled per standardcompass (PSC)

    Newer U.S. Navy ships will typicallyhave one steering compass due tofact that ships are being outfittedwith two redundant gyrocompasssystems

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    Magnetic CompassCautions

    Magnetic compass cannot be expected togive reliable service unless it is properlyinstalled and protected from disturbingmagnetic influences

    Precautions to observe in vicinity ofmagnetic compass

    Compass should not be placed near ironor steel equipment that will be movedfrequently

    Immediate vicinity should be kept freeof sources of magnetism, particularlythose of changing nature

    No source of magnetism should be

    permitted within a radius of several feetof magnetic compass

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    Magnetic CompassOperation

    Small bar magnet freely suspended in themagnetic field of earth will always alignitself parallel to the lines of force of thatfield, establishing direction

    U.S. Navy standard No. 1, 7-inch magneticcompass


    Circular card graduated in degrees from0 to 359

    Bowl of compass fluid that supports thefloating card

    Bar magnets correct and align compasscard

    Gimbals act as pivots that rest in metal

    ring, allowing compass to remain leveldespite motion of ship

    Binnacle is housing/stand for compass

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    Magnetic CompassAdvantages and


    Backup in case of gyro failure

    Simple, self-contained mechanism Operates independent of electrical

    power supply

    Requires little or no maintenance

    Not easily damagedDisadvantages:

    Seeks magnetic meridian instead of truemeridian

    Cannot be used near earths magneticpoles

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    Magnetic CompassError

    Before using magnetic compassonboard ship, must first correct for themagnetic influences that make the

    compass deviate from geographic north Variation


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    Deviation is defined as the amount thatthe compass is deflected from themagnetic meridian because of the effectsof the ships iron

    Expressed in degrees East or West

    Caused by the interaction of the shipsmetal structure and electrical currentswith the earths magnetic lines for forceand compass magnets

    Permanent magnetism created inthe ships structure during the

    building process Gains its own unique magnetic

    field based on the angle that thekeel is laid

    Induced magnetism varies

    according to the intensity of thecomponent of Earths field

    Amount of deviation varies as the shipchanges course and with equipmentalterations

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    Shipboard DegaussingSystem

    Also has an effect on deviationDegaussing system - electrical installationdesigned to protect ships against magneticmines and torpedoes

    When a ship is close to a magnetic mine

    or torpedo, the magnetic field of theship actuates the firing mechanism

    Purpose counteract the ships magneticfield and establish a condition such thatthe magnetic field near the ship is, asnearly as possible, just the same as if theship were not there

    Degaussing installation consists ofpermanently installed degaussing coilswrapped around ship on underside of hull,control unit to control the coil current, and

    compass compensating equipment toprevent disturbances to mag compasses

    Coil is a large diameter electrical wire

    A, F, L, M, Q Coils

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    Ships magnetic effects may becorrected by the proper placement ofvarious correctors Process for correcting deviation error is

    called swinging ship

    Swing the ship through 360 degrees,stopping each 15 degrees andcomparing the compass heading tothe properly functioninggyrocompass

    Results are recorded on magneticcompass deviation table

    Deviation Tables provide a meansfor knowing the deviation of themagnetic compass for any heading

    Information is crucial if thegyrocompass fails

    Updated annually and postedon/near magnetic compass

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    Sample DeviationTable

    Top portion: name of ship, location ofcompass, binnacle type, and compasstype

    Middle section: ships heading every 15degrees and deviation data

    DG OFF degaussing off

    DG ON degaussing on

    Bottom portion: information on magnetand bar placement that corrects for

    excessive deviations

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    Deviation Tables

    Example: Your ship is on course 090degrees true and the OOD nowwishes to make good course 117degrees (magnetic course) bymagnetic compass

    Determine if DG ON or OFF

    Locate the course nearest to yourdesired course on the deviationtable

    Nearest course is 120

    Read the deviation

    2.0 W Apply the deviation correction to

    the ordered course

    Westerly deviation meanscompass reads less than itshould = add WEST orsubtract EAST

    117 degrees

    2 degrees W

    119 degrees

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    Compass ErrorCalculations

    Three lines of reference have beenestablished:

    True heading - direction of true north

    Magnetic heading - direction of

    magnetic north Compass heading - direction of north

    point of compass

    Ships head







    Compass Error

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    Converting fromCompass to True

    When converting from steering compass heading totrue heading, navigator must take into accountvariation and deviation

    Sequence of conversion:

    Apply deviation to steering compass heading toobtain magnetic heading

    Apply variation to the magnetic heading toproduce the desired true heading

    Westerly errors subtracted and easterly errorsadded

    The following memory aid is used to helpremember the steps in converting steeringcompass heading to true heading:

    an Dead Men Vote Twice At Electionsompass Deviation Magnetic Variation True +East

    head head head

    The most challenging calculation is determiningthe correct deviation to apply.

    Standard deviation is based on ships headmagnetic. Due to this fact, when convertingfrom compass heading to true heading, it isnecessary to interpolate twice if the shipshead lies between two magnetic headingslisted on the deviation tables.

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    Converting fromCompass to True

    an Dead Men Vote Twice At Elections

    ompass Deviation Magnetic Variation True +Easthead head head

    First interpolation steering compassheading can be considered anapproximation of the magnetic head

    Second interpolation magnetic headcomputed again as better approximation

    than steering compass headingExample: A ships heading is 305 p stg c.What is the ships magnetic heading ifDEG OFF?

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    Example: Ships compass head is 3050 withdegaussing OFF.

    3000 1.00W


    3150 2.50W

    The desired deviation is 5/15 or 1/3 of the

    difference between 1.00

    W and 2.50

    W:5/15 * (2.5 - 1.0)=.5; D= 1.5W

    This value is subtracted from 3050to get a 303.50M.The first interpolation gives a good estimation ofships head, so a second interpolation can beperformed in order to more accurately account for

    deviation: 3000 1.00W


    3150 2.50W

    3.5/15 * (2.5-1.0)=.4; D=1.40W

    The required deviation , rounded to the nearest .50is 1.50W.

    Results in ships magnetic head 305 - 1.5W =303.5M

    Shipboard Compasses

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    Converting from True to Compass: It

    may be necessary to convert a trueheading to a compass heading in theevent a gyrorepeater fails and a certaindesired course is to be steered.

    In order to do this, corrections areapplied in a reverse order accordingto sequence:

    T V M D C A W

    True Variation Magnetic Deviation Compass + West

    head head head

    *Only one interpolation is required whenconverting from true to compass

    Shipboard Compasses

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    True to CompassConversion

    Example: While steaming on a heading of 1490T, the ships gyro tumbled. What steeringcompass course should be steered to keep theship on the same true course?

    Assume a variation of 9.00E, with degaussingOFF.

    T V M D C1490T 9.00E 1400M

    Calculation for deviation:

    1350 1.50W






    W5/15 * -1.0 = -.33, rounded to -.5, applied to 1.5 ;D=1.00W

    Note: Only one interpolation is required whenconverting from true to compass.

    T V M D C1490T 9.00E 1400M (+) 1.00W1410W

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    GyrocompassMain source for determining direction thusindicating true north

    Gyroscope rapidly spinning body having threeaxes of angular freedom

    Must be lit off a minimum of 4 hours prior to use

    Checked for error at least once daily while ship isunderway

    Proper function if error is 2 degrees or less

    Powered by electricity and consists to two maincomponents

    Master gyrocompass consists of a controlcabinet, power supply, speed unit, alarm unit,

    and transmission unit that is located within theships hull where it is least affected by pitch androll

    Repeaters receive signal transmitted frommaster gyro for real-time data

    Relative bearings on outside circle

    True bearings on inside circle

    Normally found at all ships control stations:pilothouse, bridgewings, aftersteering

    Additional spaces: COs cabin, CIC

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    GyrocompassAdvantages and

    DisadvantagesAdvantages: Seeks true meridian instead of magnetic


    Can be used near the earths magnetic poles

    Not affected by surrounding material

    Signal can be fed into integrated navigationsystems and automatic steering systems

    Extremely accurate, highly reliable, and easy touse

    Disadvantages: Highly complex instrument requiring periodic

    maintenance by qualified technicians

    Dependent on electrical power supply

    Subject to electronic and mechanical failures of

    its component parts

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    Gyrocompass Error

    Several sources of error caused by the

    transmission network but error is small Most normally functioning gyrocompasses

    will not have an error of more than 2.0degrees East or West

    Must take error into account during plot

    At sea, QMs must determine gyrocompass

    error at least one a day via the followingmethods

    Observe a natural or artificial range. Abearing is shot to the range when linedup, then compared to the chartedbearing. The difference is equal to thegyro error.

    If the ship is at a known location, such asa pier or an anchorage, a gyro error canbe obtained by comparing a knownbearing to an object ashore, as measuredon a chart.

    Comparing the ships heading while

    pierside to the known heading of thepier will give gyro error

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    If the ship is not underway, a trial anderror adjustment of three or moresimultaneous lines of position until a pointfix results. If the lines initially meet at apoint, there is no gyro error. If they form atriangle, they are adjusted by successive

    additions or subtractions of 10, then ifnecessary, .50to the bearings until theymeet at a point fix. The total correctionapplied to any one LOP is the gyro error.

    Compare the gyrocompass to another

    gyrocompass of the same error. At sea using sun as reference

    Shipboard Compasses

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    Compass Error

    Compass Best, Error West If the gyrocompass bearing is higher

    than the actual bearing, the error is west

    Compass Least, Error East

    If the gyrocompass bearing is lowerthan the actual bearing, the the error iseast

    G.E.T. - Gyro + East = True

    Gyro Gyro Error True

    80 degrees ? 182 degrees

    62 degrees ? 060 degrees

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    ReadMarine Navigation Chapter 11on Tides

    BringMarine Navigation to class

    Workbook problems Chapter 9

    Section 3: 1, 2A, 2B, 7