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Lesson 5: Navigation Instruments. Lesson 5: Navigation Instruments. AGENDA: Measurement of Depth Measurement of Direction Measurement of Distance Measurement of Speed Applicable reading: Hobbs pg. 110-128. Measurement of Depth. Echo Sounder (fathometer) Hand leadline. Lead Line.

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Lesson 5: Navigation Instruments


    • Measurement of Depth

    • Measurement of Direction

    • Measurement of Distance

    • Measurement of Speed

  • Applicable reading: Hobbs pg. 110-128.

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Measurement of Depth

  • Echo Sounder (fathometer)

  • Hand leadline

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Lead Line

  • Manual method of determining depth

  • Also allows for a sampling of the bottom type

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Measurement of Direction

  • Bearing: the horizontal direction of one terrestrial point as measured from another, expressed as an angle from 0 to 360 degrees.

  • There are three references for bearings:

    • Relative

    • Magnetic

    • True

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000T / 270R

270T / 180R

090T / 000R

180T / 090R

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Measurement of Direction

Visual Bearings can be measured in:

- Degrees Relative ( # # # 0R )

- Degrees Magnetic ( # # # pstgc OR 0M )

- Degrees True ( # # # 0T)

The navigator must convert any of these types of bearings to True before they can be plotted on the chart.

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Bearing Circle/Alidade

  • Azimuth or Bearing Circle

  • Telescopic Alidade

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Bearing Circle

  • Telescopic Alidade

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Measurement of Distance

Radar-Distance is obtained by a radio wave

that is transmitted into the air and returns as

an echo. The time

interval between

transmission and

return can be

converted into

a physical range.

Used for distance (range) only!!

Not normally used for bearings

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Measurement of Distance


Used to determine precise ranges between ships in formation

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Making an Observation with the Stadimeter

  • Distance is determined by:

    • distance = height of object / tan (angle)

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Plotting Instruments

Parallel Ruler

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Plotting Instruments

Parallel Plotter

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Plotting Instruments

Parallel Motion Protractor

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Plotting Instruments

  • Compass

  • Dividers

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Plotting Instruments

Nautical Slide Rule

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Operation of Nautical slide Rule

  • Given two of the following, find the third:

    • Speed

    • Distance

    • Time

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Measurement of Speed

  • Speed over Ground (SOG) = true speed

  • Speed through the water


  • Impeller Log

  • Pitotmeter Log

  • Doppler Speed Log

  • Shaft RPM


  • Direct output of many electronic navigation systems (GPS, Loran)

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Measurement of Speed

Impeller Log

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Measurement of Speed

Pitometer (Pit) Log

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Measurement of Speed

Doppler Speed Log

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3 Minute Rule

  • The Distance in yards that a ship travels in 3 minutes (D) is equal to the Ship’s Speed (Spd) in knots multiplied by 100.

    D = Spd x 100

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D =

6 Minute Rule

  • The Distance in nautical miles that a ship travels in 6 minutes (D) is equal to the Ship’s Speed (Spd) in knots divided by 10.

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  • What does a fathometer measure?

  • What does D=Speed x 100 tell you?