LOGS and READINGS Flashcards
Indicate next to each of the following speed logs whether they give Speed Through the Water (STW) or Speed Over the Ground (SOG)
Towed log - STW
Impellor log - STW
Doppler log - STW or SOG
pressure (pitot) log - STW
Electromagnetic log - STW
acoustic correlation log STW or SOG
GPS log - SOG
Describe with the aid of a diagram how an electromagnetic log calculates vessel speed
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
- Explain the Janus configuration with respect to a Doppler log
One transducer faces in the opposite direction to the other so that any error due to change in trim or pitching can be eliminated
Describe with the aid of a diagram how a Doppler log calculates vessel speed
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 2 astern.
This ‘doppler shift’ can be measured and converted to a speed read out.
What are two advantages of the Doppler log over the electromagnetic log?
Speed over the ground reading is possible Accurate in shallow water
Explain the dangers of using a Doppler log as a speed input to a radar
) If the radar is being sea stabilized 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.
Outline the echo-ranging principle
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
Explain the factors you would take into account when setting the alarm on an echo sounder for a coastal passage
Draught in relation to available depth of water, considering whether squat is likely to be a factor due to depth under the keel and ship speed.
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.
