3dw.
Name;
Date
Section
Points. 10/10
Test Topic 2. Echo Sounder.
Type or write your answer with a clean paper, screen shot the answer
and send to my messenger today.
1. The echo sounder transmits a bundle of ultrasonic ____ to the
bottom, the propagation speed of these waves is known.
2. ____ means sound speed is the weighted average sound speed
down to the depth of interest.
3. The resulting ____ maps are more detailed than those obtained
using single beam mapping.
4. ____ the most common system for measuring water depth, and
preventing collisions with unseen under water rocks, reefs, etc.
5. The sound ____ are sent out regularly as the ship moves along
the surface, which produces a line showing the depth of the
ocean beneath the ship.
6. The echo sounders ___ the Under Keel Clearance (UKC).
____ echo can be caused by school of fish, air bubbles under the
ship, wake of other ships, outlets of sewer and factories.
7. The ____ consists of a line and
8. a weight of lead connected to it, example, 2 meters red cloth.
9. At the moment the ____ knot emerge this was called aft.
10. Loss of ____ is caused by refection and absorption.
Topic 3.
Speed Measurement Equipment
Deck watch keeping 2
Speed log
There are different types of speed logs. The most common types
used are:
Doppler log
Electromagnetic log
The Doppler effects (or Doppler shift) is the change in frequency of
a wave for an observer moving relative to the source of the waves.
It is commonly heard when a vehicle sounding a siren or horn
approaches, passes, and recedes from an observer.
The received frequency is higher (compared to the emitted
frequency) during the approach, it is identical at the instant of
passing by, and it is lower during the recession.
The most accurate means of ships speed measurement makes use of
the Doppler measurement, either derived acoustically by use of
Doppler sonar or radio by Doppler measurement of satellite signals
such as those from Global Positioning System (GPS).
Doppler effects
The electromagnetic log
An electromagnetic log, sometimes called an "EM Log", measures
the speed of a vessel through water.
It operates on the principle that when a conductor (such as water)
passes through an electromagnetic field, a voltage is created and the
amount of voltage created increases as the speed of the conductor
increases.
The process is:
The EM Log creates an electromagnetic field
A voltage is induced in the water; the magnitude of the voltage
varies depending upon the speed of the water flow past the sensor.
The EM Log measures the voltage created and translates this into
the vessel's speed through water.
Speed Logs
Speed measurement has always been of the utmost importance to the
navigator.
The accuracy of a dead reckoning position plotted after a long passage
without star sights being taken, is dependent upon a sound
knowledge of the vessel’s heading and speed.
States describes the difference between ground-
reference speed and water-reference speed
• the speed of any object must be measured relative to some other point.
• At sea, speed may be measured relative to either the seabed
• (ground reference speed) or to the
• water flowing past the hull (water reference speed).
• Both of these types of speed measurement are possible and both have their
place in modern navigation systems.
Describes the basic principles of the electromagnetic speed log.
The operation relies upon the principle that any conductor which is
moved across a magnetic field will have induced into it a small
electromotive force (e.m.f.).
Alternatively, the e.m.f. will also be induced if the conductor remains
stationary and the magnetic field is moved with respect to it.
• Describes the basic principles of the pressure-tube log
• When a tube, with an opening at its base, is vertically
submerged in water, a pressure, proportional to the depth to
which the tube is submerged, will be developed in the tube.
• If the tube is held stationary the pressure remains constant and
is termed ‘static’ pressure.
• If the tube is now moved through the water, whilst keeping the
depth to which it is submerged constant, a second pressure
called ‘dynamic’ pressure is developed.
• The total pressure in the tube, called a Pitot tube, is therefore
the sum of both the static and dynamic pressures.
• Describes the basic principles of the acoustic-
correlation log
• Speed measurement using acoustic correlation techniques
• Unlike the previously described speed log, which measure the
vessel’s speed with respect to water
• only, the SAL-ICCOR log measures the speed with respect to
the seabed or to a suspended water mass.
• The log derives the vessel’s speed by the use of signal acoustic
correlation. Simply, this is a way of
• combining the properties of sonic waves in seawater with a
correlation technique. Speed measurement is achieved by
bottom-tracking to a maximum depth of 200 m. If the bottom
echo becomes weak.
• Describes the basic principles of the Doppler speed log
• The Doppler system calculates speed to within an accuracy of
about 0.5 percent of the distance traveled. It functions well for
all speeds that modern vessels can attain and works from a
minimum depth of about 1.5 feet to a maximum depth of about
600 feet.
There are primarily four errors to be aware of when using the
Doppler system:
1. Transducer orientation error caused when the pitching or
rolling of the vessel becomes excessive
2. Vessel motion error caused by excessive vibration of the vessel
as it moves through the water
3. Velocity of sound errors due to changes in water temperature
or density due to salinity and particle content
4. Signal loss errors caused by attenuation of the vibrations
during transit through the water or upon reflection from the
bottom.
Bottom track: speed is measured relative to seabed up to a depth of
about 200 metre.
When the bottom is hard, deeper is possible
Water track: speed measured relative to a water layer.
