Logs Flashcards
Explain how an echo sounder finds the depth, stating the basic formula and speed used
The transducer transmits an acoustic pulse. The echo sounder measures the time between transmission and receiving the echo back from the seabed.
It uses this time in the echo ranging calculation to find the depth below the transducer.
Depth = Speed x Time / 2
State two common frequencies used by an echo sounder and explain what each are used for
50 kHz –
Used for deep soundings because the more powerful signal travels further. The cone is wide and the picture of the seabed is not very detailed
200 kHz –
Used for shallower soundings. The cone is narrower giving a more detailed picture of the seabed. It is less affected by aeration.
Describe three sources of errors with an echo sounder
Any three from:
* Change in density of water can cause a change in speed of sound
* Aeration can cause change in speed of sound, affecting indicated depth.
* Mechanical and electrical interference can cause errors.
* Multiple echoes due to signal bouncing between ship/surface and seabed.
* False echoes shown due to ‘false bottom’ or ‘second trace
Explain the factors you would take into account when setting the alarm on an echo sounder for a
coastal passage
- Draught of the vessel
Squat
- The rate of rise of the depth contours
The alarm needs to sound to give the OOW enough time to react to it and keep the ship safe.
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. As the vessel moves through the water an electro-motive force (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 illuminated
Describe with the aid of a diagram how a Doppler log calculates vessel speed
The log transmits an acoustic signal from a transducer angled at 60o to the horizontal. 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 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
- More 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. Also, if the log is being used for sea stabilization of the radar, it is not certain which water layer is being used, therefore accurate aspect of targets will be affected.
State two sources of potential spurious returns that might be present on an echo sound and how they might be eliminated.
- Thermoclines: These are layers in the water where the temperature changes rapidly, which can cause sound waves to refract and return false echoes. To mitigate this, echo sounders can use multiple frequencies.
- Sea Bed Composition: Different types of sea bed (sand, rock, mud, etc.) reflect sound waves differently, which can cause inaccurate readings. To address this, modern echo sounders can adjust the gain of the return signal based on the time elapsed since the signal was transmitted.
Discuss the considerations to be made when installing a new echo sounder transducer
- Transducer Location: The transducer should be located in a place where the water is free from aeration beneath. It shouldn’t be located in the forward 1/3 of the vessel also be as far away as possible from sources of noise, such as the engine or propeller.
- Transducer Depth: Based on experience, it is wise to select a location as deep as possible for the transducers, where the water is less aerated.
- Shape of the vessel for example if it has bulbous bow: If the vessel has a pronounced bulbous bow on Inclination of the Transducer face location as well as acoustic noise will mix with the received signal and may reduce the effective range of the echo sounder
With the aid of a sketch describe and explain the Echo Soulder principale.
The echo sounder is a type of sonar that uses sound waves to determine the distance and direction of objects underwater in this case the sea floor. It is useful for navigation, fishing, surveying, and research purposes.
It send out a pulse that hits the sea bed or thermal cline and the speed in which it takes to return to the vessel is measured. Using the formular
Depth = Speed x time / 2
Two advantages and two disadvantages of a electromagnetic log over a doppler log
- An EM log is more accurate at lower speeds and less affected by the ships wake.
- Can work in both forward and reverse
