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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:

Gyrocompass

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

gyrocompass

Found on every oceangoing vessel

used by smaller vessels as primaryreference for direction

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Magnetism

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

Pole

Notice that

the two poles

arent

together. The

magnetic

compass

point to themagnetic

pole, and this

differencefrom true

North gives

us VARIAT

Variation

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Law of Magnetism

Every magnet has two poles

Each pole has opposite characteristics

OPPOSITES ATTRACT; LIKES REPEL

North pole attracts a south pole but itrepels another north pole

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Variation

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

North

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Variation

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

Example

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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

Components:

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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Backup in case of gyro failure

Simple, self-contained mechanism Operates independent of electrical

power supply

Requires little or no maintenance

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

Deviation

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Deviation

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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Deviation

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

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 north Compass heading - direction of north

point of compass

TrueNorth

Mag

NorthCompass

North

Variation

Deviation

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:

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

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

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

3050

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

303.50

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

*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

1400

150

0

0.5

0

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

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

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meridian

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.

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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Homework