7 Navigation Instruments

5/24/2018 7 Navigation Instruments

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Lesson 5 -

NavigationInstruments

Learning Objectives:

Comprehend the use of the azimuth circle,the bearing circle, and the alidade in the

measurement of direction. Comprehend the use of the stadimeter in

the measurement of distance.

Comprehend the methods by which speedthrough the water is determined on a ship

or boat. Comprehend the use of hand lead lines

and echo sounders to determine depth.

Recognize basic plotting instruments andapply correct procedures in their use.

Apply correct procedures in the use of thethree-minute rule, log scales and thenautical slide rule to determine speed,time, and distance given any twoquantities.


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NavigationalInstrumentsNavigational Instruments

are grouped in accordance

with instruments used tomeasure direction

measure distance

measure speedmeasure depth

plotting

miscellaneous use


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Navigation

Instruments Measurement of Direction - horizontal

direction of one terrestrial point toanother , expressed as an angle from

000

o

to 360

o

, is termed a bearing Surface navigators are concerned with

three different types of bearing :

Relative bearing-a bearingmeasured with reference to the ships

longitudinal axis. Magnetic bearing- a bearing

measured with respect to a magneticcompass needle aligned withmagnetic north

True bearing-a bearing measuredwith respect to a gyrocompassrepeater, or a magnetic compasscorrected to true north.

090

270


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N

E

S

W

TRUE

BEARINGS000T

090T

180T

270T


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Ships course

= 045T

135T

N

EW

S

TRUE BEARINGS

000T

090T

?


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N

E

S

W

090R

180R

RELATIVE

BEARING

270R

000R

090T


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Ships course

= 045T

135T

?

?

N

EW

S

RELATIVE BEARINGS

?


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Ships course

= 045T

135T

180R

345R

N

EW

S

RELATIVE BEARINGS

000R


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Navigation

Instruments Azimuth Circle

Azimuth is often used interchangeably with theword bearing, although technically the formerterm refers to the bearing of a celestial body,while the latter pertains to a terrestrial body.

most common device for obtaining a visualbearing

designed to fit on standard gyrocompass repeater

contains nonmagnetic brass ring that fits overrepeaters face.

Turned to any direction by two finger lugson ring

pair of sighting vanes mounted on oppositeends of ring

Far vane

Near vane

Mirror

Prism

Mirror

ooo

090

180

270

Finger lug

Finger lug


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pair of sighting vanes mounted on diameter ofring

peep vane at one end

vertical wire at the other

reflector of glass is attached to thebottom of vertical vane designed forobserving celestial bodies

How to observe a bearing:

the observer looks through the peep vanetoward the object to be be observed

shooting the object

observer rotates ring until the object appearsbeyond the vertical wire of the oppositefar vane.

Reflecting mirror will bring portion of

compass below vertical vane into field ofvision

observer will read the bearings - truebearings appear on the compass above therelative bearings.

Azimuth Circle

Azimuth Circle


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Telescopic alidade - similar in construction to theazimuth circle except is fitted with a telescopicobservation device rather than set of sight vanes.

The object to which a bearing is to be obtained

is sighted in the telescope and its bearing isread off of the compass card

accurate observations at night

Measurement of Distance - The navigator uses twoinstruments to measure distance - radar, (whichwill be discussed later) and thestadimeter.

Stadimeter- used to determine precise rangesbetween ships in a formation. For shorter rangesup to 2,000 yards, it is considered more accuratethan a surface search radar. The stadimeter can alsobe used to determine accurate ranges to navigationaids of known height above the water.


mirrordirect

view

telescopefitting

drumBrandon Stadimeter

mirror

directview

drum

telescopefitting

Fisk Stadimeter


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The Fisk stadimeter is the most common type ofstadimeter used in the fleet, although the Brandonstadimeter is also used by some units.

Both stadimeters incorporate two scales. One, located onthe index arm of the frame, is the object height scale,

graduated in logarithmic form for object heights between50 and 200 feet. The other, inscribed around the indexdrum beneath the frame, is the distance scale; it isgraduated in a spiral logarithmic scale for distancesbetween 50 and 10,000 yards.

Both instruments are equipped with a removabletelescope fitting in the rear view finder, a reflecting mirrorin the right side of the forward view finder, and an indexmirror under the rear view finder.

Navigation

Instruments

(Overhead 5-1)Actual

image

Correct alignment for measuringdistance

Reflected

image


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First the height of the object tobe observed is first set into theindex arm scale.

Recorded heights of ships

and navaids found inpublications

Then the object is sighted in thetelescope

Turn index drum causes

reflective image in mirror onthe right side of the forwardview finder to move up or downrelative to the direct imageobserved on the left side

When the top of the reflectedimage is superimposedalongside the bottom of thedirect image, distance of theobject is read directly from theindex drum scale

HOW TO USE THE

STADIMETER


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Surface navigators are concerned withtwo kinds of speed

true speed or speed relative to the

earth (often called speed over ground(SOG))

speed is normally calculated bymeasuring the time required forthe ship to travel a known distance

ships speed through water (actualspeed)

Speed through the water is measuredboth mechanically and empirically bymethods discussed below

logs - all marine instruments designedfor direct measurement of speedthrough the water

Measuring Speed


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Impeller log-consists of a sensingdevice incorporating a small propelleror paddle wheel located beneath thewater line just outside the hull

speed of rotation of the impellercaused by the water flow past it ismechanically or electricallytranslates into vessel speedthrough the water

Pitometer - incorporates a pitot tube,

which is a three-foot long tubegenerally located near the keel, whichcan be extended through the shipshull

contains two orifices, one of whichmeasures dynamic pressure, and

the other static pressure difference between the dynamic

and static pressure is continuallymonitored. This difference isproportional to ships speed.

When using extend the shipsdraft by 3ft.no shallow water

LOG TYPES:


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Doppler speed log- depends on oneor more sonar beams projected intothe water by a transducer mountedon the bottom of the hull of the vessel

electronically analyzing the returnof the sonar beam patternreflected back either from the seabottom or from the water itself indeeper areas provides a very

accurate determination of speed Electronic navigation systems - The

Global Positioning system (GPS), andthe Omega Navigation System canprovide speed information.

Shaft RPM-

For all larger constant-draft vessels having non-variablepitch propellers, there is a fairlyconsistent relationship betweenbetween shaft RPM and speedthrough the water (tables)


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accomplished on mostmodern ships primarily bymeans of an electronic depthfinder called the echosounder or fathometer

Fixed transducer mountedon underside of vesselshull and operated remotely

transmits a sound pulsevertically into the waterand computes the depth bymeasuring the time intervalfrom transmission of the

sound signal until thereturn of its echo from thebottom

used for > 1,000 ft

MEASUREMENT OF

DEPTH


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Hand leadline- alternate methodfor measuring depth in pilotingwaters

consists of a lead weightattached to a 25-fathom line

leadsman stationed two-thirds

aft from bow of ship to bridgeand relays readings as marks

means of identificationattached at random intervalsto identify number of

fathoms Remember 1 fathom = 6 feet


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Plotting Instruments -

Parallel rulers - two parallel barsw/ cross pivot braces of equal

length so bars are always parallelwhen open/closed

used to measure desireddirection w/ compass rose

Weems parallel plotter - semicircleprotractor for measuring coursesand bearings against chartedmeridian

Parallel Motion Protractor -rotatable protractor graduated indegrees from 0-360 with draftingarm affixed

can be locked into position byscrew or held in position

Dividers vs. compass



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Weather Instruments -

onboard naval vessels acomplete round of weather

observations is made hourlyand a synoptic report made to aNaval Oceanography CommandCenter four times daily

The basic instruments installed

on on Navy ships for thepurpose of weather observationare:

The barometer whichmeasures atmosphericpressure.

The thermometer whichmeasures temperature.

MISC.

INSTRUMENTS


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The psychrometer whichmeasures wet -and dry- bulbtemperatures from which therelative humidity , and if

desired dew point. Theanemometerwhich

measures relative wind speedand direction

True wind can be calculatedfrom the relative windmeasurements


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Speed-Time-DistanceCalculations -

Three-minute rule -

Distance traveled in yardsin three minutes =

Ships speed in knots x 100

Example: How far will yourship travel in 3 minutes at15 knots?

?Yds = 15kts x 100

= 1,500 yds


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Speed-Time-DistanceCalculations -

Six-minute rule-

Distance traveled in nmin six minutes =

Ships speed in knots x1/10

Example: How far will yourship travel in 6 minutes at 17knots?

?nm = 17 kts x 1/10

= 17/10 or 1.7 nm


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Speed-Time-Distance Calculations -

The Logarithmic speed-time-distance scale is very useful in

determining any one of the threevariables if the other two areknown. Speed is expressed in howfar a ship will go in sixty minutes.The distance can be expressed in

yards or nautical miles.


603015.1 105

Example: A ship is going 10 kts. How far will it go in sixty minutes?Step One: Place the right point of dividers on 60 and the left point on ships speed.

201

Step 1

603015105

201.1

Step 2

Without changing the spread of the dividers, place the right point on minutesrun; the left point will then indicate distance. Or, place the left point ondistance; the right point will indicate time. To find speed reverse the process.

(Overhead 5-2)


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Similar logarithmic scales can be found onmaneuvering board speed-time-distancenomogramswhere there are separate time, distance

and speed lines. To use the nomogram, pencil marksare placed on the two given quantities, and the thirdquantity is read by placing a straight edge over thetwo marks and observing the point of intersection onthe third scale.

The Nautical slide rule is similar to the maneuvering

board time-speed-distance nomogram except the threescales have been bent into circular form on a plastic baseand covered by a plastic faceplate. To use the instrument,the known values are set by rotating the slide rule to theappropriate positions, and the third factor appears by theappropriate arrow.

.1 200

1 60

25 200,000

8

16,000

60

100

201

200 4,000

Time in minutes

Distance in yards/miles

82

Speed in knots



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Shipboard Timepieces - Ships speed, distance madegood, position, and most aspects of daily routine are

dependent on time, therefore it is very important tobe able to determine time accurately.

The chronometeris considered one of the mostaccurate timepieces. It is the principalnavigational timepiece normally found on mostoceangoing vessels. No timepiece can keepperfect time, but the distinguishing feature of thechronometer is that its rate of gain or loss of timeis constant over long periods of time. Thisattribute allows the navigator to determinechronometer error with precision and calculatecorrect time when needed.

Stopwatches are used to time navigational lights

and to record observations of celestial bodies.

Distance

SpeedTime

Min Hrs

10 12

7

5

9

100

150.45

2.5

35

7

200

70000

900 30000

14000

5000

1.02000

.020

400

Navigation

Instruments


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Due Tuesday

Read Chapter 8, pgs.129-138 ofMarineNavigation

Bring Workbook pages34 and 35 to class forTuesday

Complete computed

visibility and speed-time-distance practical

HOMEWORK

7 Navigation Instruments

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