Q) With the help of a block diagram explain how ship‟s speed is transmitted
to displays. (Sept-17, Jan-16)
ANS:-
Explanation:-
Distance recording is achieved by using a constant speed motor (10) which drives
the distance counter (11), via friction gearing.
The constant speed motor has been used in order that a distance indication may
be produced that is independent of the non-linear characteristic of the system.
The motor is started by contact (5) as previously described.
The main shaft (7), whose angle of rotation is directly proportional to the speed of
the ship, is fitted with a screw spindle (12).
The rotation of the shaft causes a lateral displacement of the friction wheel (13).
At zero speed, the friction wheel rests against the apex of the distance cone (14),
whilst at maximum speed the wheel has been displaced along the cone to the rim.
The distance indicator (11) is driven from the constant speed motor (10) via the
cone.
The nearer to the rim of the cone the friction wheel rides, the greater will be the
distance indication.
Revolutions of the distance shaft (15) are transmitted to the remote distance
indicator via the servo transmission system (16 and 17).
The speed unit provides the following outputs to drive both speed and distance
counters:-
o An analogue voltage, the gradient of which is 0.1 V/knot, to drive the
potentiometer servo-type speed indicators.
o A pulse frequency proportional to speed.
o The frequency is 200/36 pulses/s/knot. Pulses are gated into the digital
counter by a 1.8-s gate pulse.
o A positive/negative voltage level to set the ahead/astern indication or the B
track/W track indication.
o 2000 pulses per nautical mile to drive the stepping motor in the digital
distance indicator.
Q) What is the principle of Doppler Speed Log? (March-20, July-19, May-
19,March-19, Jan-18, Nov-18, Sept-18, May-18, Sept-17, July-17, May-17, Jan-17,
Nov-16, May-16, Jan-16) OR
Q) Describe Janus Configuration. (Jan-20, July-18)
Ans:- Principle:-
Equipment to measure ship‟s speed.
The Doppler log is based on measurement of the Doppler effect.
The Doppler effect can be observed for any type of wave – water wave, sound
wave, light wave, etc. we are most familiar with the Doppler effect because of our
experiences with sound waves. For instance, a police car or emergency vehicle
was travelling towards us on the highway. As the car approached with its siren
blasting, the pitch of the siren sound (a measure of the siren‟s frequency) was
high; and then suddenly after the car passed by, the pitch off the siren sound was
low. That was the Doppler effect – an apparent shift in frequency for a sound wave
produced by a moving source.
The Doppler Effect is a frequency shift that results from relative motion between a
frequency source and a listener.
If both source and listener are not moving with respect to each other (although
both may be moving at the same speed in the same direction), no Doppler shift
will take place.
If the source and listener are moving closer to each other, the listener will
perceive a higher frequency – the faster the source or receiver is approaching the
higher the Doppler shift.
If the source and listener are getting further apart, the listener will perceive a
lower frequency – the faster the source or receiver is moving away the lower the
frequency.
So, the Doppler shift is directly proportional to speed between source and listener,
frequency of the source, and the speed the wave travels.
FORMULA:-
Doppler effect can be further explained by following equations:
o fr is the frequency received by observer.
o ft is the transmitted frequency.
o c is the speed of sound.
o vO is Velocity of observer
o vg is Velocity of source
o If the source moves towards stationary observer,
fr = c ft / (c – vg)
o If the source moves away stationary observer,
fr = c ft / (c + vg)
o If the observer moves towards stationary source,
fr = ft (c + vg) / c
o If the observer moves away from stationary observer,
fr = ft (c - vg) / c
o If the observer & source moves away from each other
fr = ft (c - vg) / (c + vs)
o If the observer & source moves toward each other
fr = ft (c + vg) / (c - vs)
o Since, in the case of the Doppler log, the source & observer are the same.
hence,
vO is equal to vS, is equal to v
fr = ft (c+ v) / (c - v)
fr = ft (c+ v cos ) / (c – v cos )
After Further simplification
v = c (fr – ft) / 2 ft cos
Given a propogation angle of 60O, cos = 0.5 (using single transducer facing
forward)
Graphs of speed error caused by variations of the vessel‟s trim:
It follows that if the angle changes, the speed calculated will be in error because
the angle of propagation has been applied to the speed calculation formula in this
way. If the vessel is not in correct trim (or pitching in heavy weather) the
longitudinal parameters will change and the speed indicated will be in error.
To counteract this effect to some extent, two acoustic beams are transmitted, one
ahead and one astern. The transducer assembly used for this type of transmission
is called a „Janus‟ configuration after the Roman god who reputedly possessed
two faces and was able to see into both the future and the past.
After installing transducer facing aft, the Doppler frequency shift formula now
becomes:-
Frt – fra – 4 vft cos / c
Hence, v = c (frt – fra) / 4 ft cos
Therefore the transmission angle can effectively be ignored.
The advantage of having a Janus configuration over a single transducer
arrangement. It can be seen that a 3O change of trim on a vessel in a forward
pointing Doppler system will produce a 5 % velocity error. With a Janus
configuration transducer system, the error is reduced to 0.2% but is not fully
eliminated.
The addition of a second transducer assembly set at right angles to the first one,
enables dual axis speed (longitudinal speed and transverse speed) to be indicated
DOCKING OPERATION:-
The placing of the Janus configuration in a fore and aft direction is known as a
single axis system and is used to calculate speed over ground in the forward and
after direction. A dual axis system places a second grouping of Janus configured
transducers in an athwart ships direction allowing for the calculation of a vessel's
speed when moving sideways through the water, as in docking. The beam width of
the athwart ship installation is about 8 degrees to account for the possibility of a
vessel's rolling.
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. Frequencies employed are between 100 kHz and 600 kHz. There
are primarily four errors to be aware of when using the Doppler system:
Transducer orientation error caused when the pitching or rolling of the vessel
becomes excessive
Vessel motion error caused by excessive vibration of the vessel as it moves
through the water
Velocity of sound errors due to changes in water temperature or density
due to
salinity and particle content
Signal loss errors caused by attenuation ofthe vibrations during transit
through
the water or upon reflection from the bottom
The Doppler system normally measures speed over ground to about 600 feet. This
depth signals may be returned by a dense, colder layer of water located
throughout the oceans called the deep scattering layer (DSL). Signals received off
the DSL are not as accurate as signals received from bottom reflections but can
still be used to provide an indication of speed through the water instead of speed
over ground when bottom tracking. Your unit may have a manual or automatic
system which will switch from bottom tracking to water tracking at increased
depth.
The Doppler system can be connected with other electronic navigation systems
providing generally accurate speed input. The navigator should be cautioned that
precise speed should be determined not only by using the Doppler but also from
careful calculations of distances between
accurate navigational fixes.
Q) Describe the errors associated with Doppler log. (Sept-19, Jan-19)
Ans:- ERRORS OF DOPPLER LOG:- The Log speed indicated is subject to various
errors, spanning installation, equipment, data processing, varying propagation
conditions and sea conditions.
Error in transducer orientation:- The transducers should make a perfect angle of
60° with respect to the keel or else the speed indicated will be inaccurate.
Error in oscillator frequency:- The frequency generated by the oscillator must be
accurate and constant. Any deviation in the frequency will result in the speed
showing in error.
Error in propagation:- The velocity of the acoustic wave at a temperature of 16°C
and salinity of 3.2% is 1505 m/sec but taken as 1500 m/sec for calculation. This
velocity changes with temperature, salinity and pressure. To compensate the error
due to temperature change, a thermister is mounted near the transducer and
change in velocity of the acoustic wave through the water from the standard value
due to the change in sea water temperature is accounted for.
Error in ships‟ motion:- During the period of transmission and reception, the ship
may have a marginal roll or pitch and thereby the angle of transmission and
reception can change and a two degree difference in the angle of transmission and
reception can have a 0.10% error in the indicated speed, which is marginal and
can be neglected.
Error due to rolling/pitching:- The effect of pitching will cause an error in the
forward speed and not the athwartship speed. Similarly, rolling will have an effect
on the athwartship speed, not the forward speed.
Actual speed = Indicated speed/Cosß
Error due to inaccuracy in measurement of frequency:- The difference in the
frequencies received by the forward and aft transducers must be measured
accurately. Any error in this will be directly reflected in the speed of the vessel.
Error due to side lobe:- When the side lobe reception dominates over the main
beam reception, there will be an error in the speed indicated. The error is more
pronounced on a sloping bottom as the side lobe is reflected at a more favourable
angle and will have path length less than the main beam. This error can be
eliminated with the help of the Janus configuration and to reduce this error, the
beam of the transmitted acoustic wave is reduced.
Q) Differentiate between water track speed and ground track speed. (March-
20)
Ans:- Water Track Speed:- In open seas, the sound pulse from the Doppler
transducer may not reach the bottom, but get totally internally reflected from a layer
of water in between. This is known as the echo from the „Water Track”.
When the sound is bounced off a water layer, called a water track, speed
indicated is the „Speed through Water”. or from a layer of water and the echo is at a
higher frequency.
The frequency of the echo from the water track will follow the same Doppler
principles as the echo from the bottom track. However, the speed measured from the
„water Track‟ will not be „Speed over the Ground‟, but it will be „Speed through
Water‟.
Ground Track Speed:- Speed over ground is the speed of the ship with respect to the
ground or any other fixed object such as fixed buoy or island.
Speed through water is the speed of the ship with respect to the water such as
anything floating on water.
A ship with her engine stopped in water with 2 knots currents will have zero
speed through water but will have 2 knots speed over ground.
Q) Explain the principle and the errors of EM log. (March-19, July-18, March-18)
Ans:- Principles of Electromagnetic Speed Log:-
The electromagnetic log is based is upon the induction law, which states that if a
conductor moves across a magnetic field, an electro motive force (e.m.f.) is set up
in the conductor.
Alternatively, the e.m.f. will also be induced if the conductor remains stationary
and the magnetic field is moved with respect to it.
The induced e.m.f. is directly proportional to the velocity.
o Velocity when integrated with time gives distance
o The induced e.m.f. „E‟ is given by the following:
E = F X L X V
o Where F = magnetic field
o L = the length of the conductor
o V = the velocity of the conductor through the magnetic field.
Constructional Details of an electromagnetic log sensor:-
This type of log consists of:
1. Sensor
2. Amplifier
3. Indicator
Working of Electromagnetic Log:
An Electromagnet consisting of a coil carrying alternative Current (A.C.) generates
a vertical magnetic field in the water around the probe.
The SW Conductor moving horizontally through this magnetic field has an
electromotive force induced into it proportional to the seed of the vessel. In the
EM log the „F‟ and „L‟ are maintained constant, therefore the induced e.m.f. is
directly proportional to the velocity „V‟, which is the velocity of the vessel through
the water.
The speed output from an EM log depends upon the water flow by way of the
sensors.
This type of log can give only speed through water and is greatly affected by the
current flowing under the ship.
The induced e.m.f. and hence the speed indication will vary with the conductivity
of the water.
This e.m.f. is picked up by 2 electrodes.
This induced e.m.f. is very small hence the amplification is required.
The amplified signal thus drives the mechanism which is connected to indicator.
Hence, the induced e.m.f. which is directly proportional to the velocity is finally
displayed on the indicator.
More information:-
The Log Extends up to about 20 cm outside the hull.
It should be retracted in case of reduced UKC & before proceeding to dry dock.
Normally retracted from the engine room.
If sensors are also fitted athwartship, the speed in athwartship direction also can
be displayed.
Errors / Limitations:
Siting of the probe is critical. This is so since if too close to the hull then due to the
non-linearity of the hull form the speed of the water flow may give a wrong
representation of the vessels speed. This is minimized by careful siting of the
sensor as well as by calibrating the instrument while installation.
Pitching and Rolling also give rise to errors however these are reduced by having
an electrical time constant that is longer than a period of vessel motion. A welladjusted
log can have an accuracy of better than 0.1 percent of the speed range.
Sign of Speed, it can show astern speed as well, but without sign if AC current is
used, if DC current is used to create the magnetic field it will show sign of speed
range. This type of log can give only speed through water and is greatly affected
by the current flowing under the ship.
While navigating in area with greater current, one must exercise precautions.
Q) Compare advantages and disadvantages of Electro-magnetic log with
Doppler log. (March-18)
Ans:- Advantages
No moving parts
Less affected by sea growth than Pit sword
Disadvantages
Salinity and temperature of water affects calibration.
Measurements affected by boundary layer, (water speed slowed down close to the
hull by friction).
Provides boat/ship speed relative to water not ground. Current affects accuracy.
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