Marine speed log consist em log, Doppler log

1. MARINE SPEED LOGS

2. TYPES OF SPEED LOGS
• The speed log is a marine electronic device
used to measure the speed of a moving
vessel. There are three types of speed logs
used on commercial vessels, using different
principles of speed measuring technologies
such as “electro magnetic induction”,
“Doppler Effect” and water pressure”

4. PRESSURE TUBE LOG
• operates on “Pitotmeter Principle” based on the
pressure developed when an open-ended tube is
exposed to water movement due to a vessel’s speed.
• If the tube is held stationary the pressure remains
constant and is called static pressure.
• If the tube now is moved through water while
keeping depth constant , a second pressure called
dynamic pressure is devloped.
• The total pressure in the tube called the pitot tube ,
is the sum of both static and dynamic pressures.
•

5. PRESSURE TUBE LOG
• To get the dynamic pressure reading , and thus speed of ship
ship the static pressure need to be subtracted.
Ths is is done by installing a second tube to the first so that
static pressure is produced.
To get difference of head pressure of a Static tube and a
Pitot tube is compared in a pressure box, being applied to
opposite sides of a flexible diaphragm.
A mechanical arrangement employing the servo principle
converts the movements of the diaphragm and a synchro
transmitter coupled to a drive motor shaft transmits the
vessel’s speed electrically to remote synchro receivers to
drive display units of speed and distance.

8. OPERATION
The pitot tube which is penetrating into sea will give
dynamic pressure.
The static tube is only opening in ship hull.
Both tubes are controlled by valves.
Both static and dynamic pressures are transferred via
air controller and strainer valves to the pressure
chember.
The pressure chamber will transfer the resultant
pressure to speed indicator on bridge.

9. ELECTRO MAGNETIC LOGS

10. EM LOGS – PRINCIPLE
• It operates on the principle that:
• when a conductor (such was 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
1. the EM Log creates an electromagnetic field
2. a voltage is induced in the water; the magnitude of the
voltage varies depending upon the speed of the water flow
past the sensor.
3. the EM Log measures the voltage created and translates this
into the vessel's speed through water.

12. EM LOG - LIMITATIONS
• AC supply to solenoid inductive pick up between
coil and wires that carry the signal. It produces a
zero error which need to be corrected.
• Induced e.m.f. is small about 100uV per knot.
• Induced e.m.f. and hence speed will vary the
conductivity of the water.
• EM log measures speed to water flowing pass hull.
Any obstruction in hull surface will affect speed
reading
• Ocean currents may introduce error.
• Rolling pitching will affect.

14. EM LOG
SENSOR
PRODUDING
FIXED PROBE

15. EM LOG SENSOR
RETRACTABLE
PROBE

16. SPEED LOG - DOPLER

17. DOPPLER LOG - PRINCIPLE
• Works on the principle of the Doppler effect, which is
a shift in frequency between a transmitted signal and a
received signal caused by the motion of a vessel over
the sea bottom.
• A transducer broadcasts a continuous beam of sound
vibrations at about a 60-degree angle from the keel.
• A second transducer receives the diffusely reflected
signal returning from the seabed.
• Doppler speed log registers the change in frequency
between the transmitted signal and the received signal
and then calculates the velocity of the vessel based on
the amount of the frequency shift.

18. DOPPLER LOG - PRINCIPLE
• Doppler beams are continuous, narrower (about 3
degrees in width), and higher in frequency. In addition
to the transducer set facing forward, there is a second
transducer set facing aft. This is called a Janus
configuration (named for the two-faced Greek god)
and allows the system to calculate frequency shift in
two directions thus insuring a more accurate speed
measurement. second grouping of Janus configured
transducers in an athwartships direction allowing for
the calculation of a vessel's speed when moving
sideways through the water, as in docking. The beam
width of the athwartship installation is about 8 degrees
to account for the possibility of a vessel's rolling.

19. DOPPLER LOG - ERRORS
• 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

22. GLOSSARY
• Aeration - formation of bubbles on transducer face causing
error in system.
• Alpha – atlas low frequency phased array – transducer beam
width
• BITE – built in test circuit – self test or diagnostic test.
• CW mode – Cont. wave transmission. Both transmitter and
receiver are active the whole time.
• G/T – ground tracking or ground referenced speed – with
reference to ocean floor
• Pulse mode – acoustic energy is transmitted in form of
pulses similar to echo sounder or radar.
• Translating system – electronic section of logging system
that produces the speed indication from a variety of data
• W/T – water tracking or water reference speed.- water