LOGS and READINGS

Question 1

Describe with the aid of a diagram how an electromagnetic log calculates vessel speed

Answer 1

• The transducer houses a coil through which an electrical current is passed This sets up a magnetic field around it.
• (E.M.F.) is created.
• Sensors pick up this E.M.F. which increases with increase in vessel speed.
• The log measures the voltage which it converts to a speed and distance read out

Question 2

Explain the Janus configuration with respect to a Doppler log

Answer 2

• One transducer faces in the opposite direction to the other
• so that any error due to change in trim or pitching can be eliminated

Question 3

Describe with the aid of a diagram how a Doppler log calculates vessel speed

Answer 3

• Transmits an acoustic pulse at a known frequency
• The signal which is reflected back will have an increasingly higher frequency
• the faster the vessel is moving forward and an increasingly lower frequency the faster it goes to astern.
• This ‘doppler shift’ can be measured and converted to a speed read out.

Question 4

What are two advantages of the Doppler log over the electromagnetic log?

Answer 4

• Speed over the ground reading is possible.
• Accurate in shallow water.

Question 5

Explain the dangers of using a Doppler log as a speed input to a radar

Answer 5

• If the radar is being sea stabalised by the log, it must be water tracking.
• If the seabed comes within range, the log may change to ground tracking automatically.
• The OOW may not be aware and the radar would then be ground stabilized.
• The correct aspect of targets may not be displayed for collision avoidance.

Question 6

Outline the echo-ranging principle

Answer 6

• A signal is transmitted
• The signal hits an obstruction, sending an echo back
• The time difference between sending and receiving is measured
• The Range of the obstruction can be found by D = S x T divided by 2

Question 7

Explain the factors you would take into account when setting the alarm on an echo sounder for a coastal passage

Answer 7

• Draught in relation to available depth of water
• squat
• How close the depth contours are will also be a factor
• The alarm needs to sound to give the OOW enough time to react to it and keep the ship safe.

Question 8

TOWED LOG

Answer 8

STW

Question 9

IMPELLOR LOG

Answer 9

STW

Question 10

DOPPLER LOG

Answer 10

STW OR SOG

Question 11

PRESSURE LOG

Answer 11

STW

Question 12

ELECTROMAGNETIC LOG

Answer 12

STW OR SOG

Question 13

GPS LOG

Answer 13

SOG

Question 14

Doppler Log Advantages

Answer 14

Advantages -
Highly accurate
No moving parts
Gives ground and water speed

Question 15

Doppler Log

Disadvantages

Answer 15

Expensive
Areation
Turbulence
Gives ground and speed ARPA