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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.
NavigationInstruments
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
NavigationInstruments
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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.
NavigationInstruments
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
NavigationInstruments
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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