Speed logs

Question 1

Function principle of the electro magnetic log

Answer 1

An electro magnetic coil has current flowing through it to produce a magnetic field.

Sea water being an electrolyte flows through this field and changes it, thereby changing the current in the coil.

Sensors feed this info to displays

Question 2

Advantages of an electromagnetic log

Answer 2

Easy operation
High sensitivity
High level of calibration sensitivity
Relative freedom from fouling as they are mostly flush on the hull
Low maintenance

Question 3

Electromagnetic log diagram

Answer 3

Question 4

Function of the Doppler Log

Answer 4

Speed is computed from the Doppler Shift of the signal’s
frequency from transmission to reception

Measures water speed by measuring distance off sea particles (thermoclines) at around 10-30m

Measures ground speed at shallow depths

Question 5

Describe the doppler effect

Answer 5

Doppler Effect is the change of frequency of
transmitted wave energy as a result of relative motion
between a transmitter and receiver (or reflector). The received frequency will therefore have a
higher value than the transmitted frequency due to the
extra cycles received as a result of the movement of (R)
and (I). The beat frequency is then measured.

Question 6

Doppler effect (diagram)

Answer 6

Question 7

Advantages of the Doppler Log

Answer 7

High degree of accuracy both fore and aft

Independent of weather conditions, current and depth

Independent of pitch and rolling (8 degrees) and trim (3 degrees) due to Janus configuration

Independent of ships velocity and water temperature

Question 8

Speed log errors

Answer 8

Calibration errors (as per manufactures instructions)

Pitch and roll/trim errors

Errors due to siting of Transducer

In addition for Doppler logs
Aeration of the water
Nature of the bottom, uneven, Material, Shelving (only in ground track mode)