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NAV & RADAR: 2 > Speed Logs > Flashcards

Speed Logs Flashcards

1
Q

Advantages of an Electromagnetic Log

A

Advantages:

  1. Simple operation
  2. Requires minimal maintenance
  3. Liniar speed vs voltage
  4. Works ahead & astern
  5. Accurate at low speeds
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2
Q

Disadvantages of an Electromagnetic log

A

Disadvantages:

  1. Externally mounted, making maintenance challenging.
  2. Cheaper units are not as accurate
  3. STW only
  4. Calibration can be difficult in current or if the temperature/ density of the water changes
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3
Q

Electromagnetic log

A

EMF: Electric Magnetic Field

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4
Q

Advantages of a Doppler log

A

Advantages:

  1. Highly accurate (0.1% error)
  2. Can give speed in multiple axis
  3. Gives ground and water speed
  4. No moving parts
  5. Easily fitted. (The transducer may be fittted flush with the hull, so there is no drag or fouling encountered
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5
Q

Disadvantages of a Doppler Log

A

Disadvantages:

  1. Expensive
  2. Susceptible to aeration, turbulance & cavitation
  3. Reflectivity of the seabed (In BT)
  4. Changes in thermoclines will change the propagation of the speed of sound in water.
    4a. Also, on ‘Propagation’ salinity will also change the reading given.
  5. Gives ground and water speed (ARPA)
  6. Gives errors if poorly trimmed
  7. Can be inaccurate in shallow water
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6
Q

The Doppler Shift Principle

A

Doppler effect is the change of frequency of transmitted wave energy as a result of relative motion between a transmitter & receiver (or reflector).

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7
Q

Doppler log is also useful for:

A
  1. Berthing, including mooring to jetties & single point mooring
  2. Low speed cruising
  3. Dropping or weighing anchor
  4. Approaching fixed docks

Most models display both athwart ships & fore & aft velocities.

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8
Q

Doppler log: At what range does the SONAR energy reflect before coming back to the transducer?

A

Reflects pulses of SONAR energy off either the bottom of the sea or from particles in the water 10-30m below the keel, measuring the beat frequency generated by mixing the transmitted & recieved frequency received which differ due to the doppler effect.

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9
Q

Dopper effect equation

A
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10
Q

Faraday’s Law

A

“Any conductor moved across a magnetic field will have an EMF (electric magnetic fields) induced in it.”

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11
Q

Electromagnetic log system

A
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12
Q

Doppler log. Advantages over electromagnetic

A
  • Can read speed over the ground
  • If twin axis can also read athwartships speed
  • Accurate in shallow water
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13
Q

Explain the principle of the electro magnetic log

A

Electromagnet causing magnetic field.
When field cut by conductor (seawater) an emf produced.
Greater the speed , greater the emf.
Emf detected by electrodes and converted to speed and distance read out.
(7)

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14
Q

Describe the difference between water speed and ground speed

A

Water speed = spd thru water & disregard s tide.
Ground speed = Speed of ship over the sea
bed and includes the effect of tide. (2)

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15
Q

State the type of speed provided by an electromagnetic log.

A

E.M log provides speed thru the water. (2)

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16
Q

It is recommended that the log input to a true motion radar display
should be water referenced. Clearly explain why.

A

Ships heading and water ref log produce true motion info which indicates
targets aspect. Collision regs based on aspect. (4)

17
Q

Describe three of the principle errors associated with the Doppler log. (10)

A

a) Speed error induced by vessel pitching
b) Cavitation/aeration
c) Refraction

18
Q

Describe the principle employed by a Doppler log to determine the
speed of the ship

A

Doppler shift, diagram showing wave compressed as target gets closer,
increase in frequency.
Show beam ahead of ship.
Use of bottom to reflect beam.
Use of cold water layer in deep water.

19
Q

Explain the critical factors to take into account when siting the Doppler
log transducer. (8 mark)

A

Aeration.
Cavitation
Pitching of ship.
Refraction.

NAV & RADAR: 2 (16 decks)

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