Echo Sounder and Speed Log

Speed Logs

 

The Impeller Log

This type of log is usually fitted in small crafts.

The standard equipment consists of the following

1.         The log tube assembly

2.         The amplifier

3.         Speed indicator and distance counter

The sensing device is at the end of a long-tube or probes, consisting of a small device called impeller (the dynamic element could be either a small propeller or a paddle or a screw) at the end of the probe, which is lowered into the water.

The tube is set with the port (opening) facing forward.

The water flow drives (or turns) the impeller and the rotation of the impeller induces an electrical signal, which is picked up at the coils. The output is fed to the amplifier and is used to operate the speed indicator. The rotating impellor's signal could also be used to provide a distance measurement.

When speed (or distance) measurement is required, the log is lowered into the water, and when not in use, is retracted inside the hull. Retraction of the log can be done manually or by a remote hoisting arrangement operated from the navigating bridge or engine room.

The log-tube may become blocked or obstructed by foreign bodies such as small fish, seaweed etc. The arrangement allows the whole tube to be withdrawn inside the vessel for inspection and cleaning. In the event of the log-tube being bent by hitting an underwater obstruction such as a sand bank or a large fish or more often caused by a wire or a rope having passed under the vessel, the log-tube must be jettison.

This type of log can give only speed through water and is greatly affected by the current flowing under the ship.

The Pressure type log (Pitot tube Log)

This type of log consist of

1.         Two openings outside the hull of the ship, static tube that provides static pressure and   impact or Pitot tube that measures dynamic pressure or the water flow of pressure

2.         Controller unit (pressure differentiator)

3.         Speed and distance transmitter

4.         Speed and distance recorder

Operations

The opening of the Pitot tube faces forward so that when the vessel moves forward, the water causes a pressure at the tube this dynamic pressure is proportional to the speed at which the vessel moves. The pressure differentiator measures the differential pressure. The Controller unit converts the pressure difference into speed and distance units.

This type of log can give only speed through water and is greatly affected by the movement of the water which would induce an extra pressure giving rise to error in readings.

ELECTRO MAGNETIC LOGS

This type of log consist of

1. Master Indicator

2. Preamplifier

3. Sensor

Operations

The sensing of speed makes use of law of electromagnetic induction

When the ship moves, the water passing through the hull acts as a conductor.

The magnetic field is produced by a solenoid, installed in such a way as to allow the field to extend into water

This produces an EMF (electromagnetic force), which is measured and converted into the speed of vessel through the water.

Principle


The electromagnetic log is based upon the Faraday-Maxwell induction law; Figure shows the principle of the log. 

The induced e.m.f. ‘E’ is given by the following:

            E = F x L x V

Where F = the magnetic field

            L = the length of the conductor

            V = the velocity of the conductor through the magnetic field.

In the EM log a direct current through the windings of a coil, generates a magnetic field. If the conductors do not move relative to the coil they do not intersect the magnetic fines of force and no voltage is induced in them.

In the EM log the ‘F’ and ‘L’ are maintained constants, therefore the induced e.m.f. is directly proportional to the velocity ‘V’, which is the velocity of the vessel through the water.

The direction of the voltage E depends on the directions of the lines of force and the direction of the velocity of the conductor water. According to the formula the induced voltage is proportional to the velocity V.

Should the velocity have the opposite direction, the direction of the voltage would change too.

The electromagnetic log is based upon the Faraday-Maxwell induction law;

A direct current through the windings of a coil, generates a magnetic field.

Four conductors (ab, bc, cd and da) are arranged in the form of a loop around the coil. 

If the conductors do not move relative to the coil they do not intersect the magnetic lines of force and no voltage is induced in them.


 

Alternating current through the coil

Instead of a direct current, suppose that we send an alternating current through the coil. Then the induced voltage that we will have would be also an, alternating voltage with amplitude that is proportional to the velocity, V.

For the electromagnetic log an alternating voltage is preferred to a direct voltage.

The speed out put from an EM log depends upon the water flow by way of the sensors. Thus 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.

Pitch and roll 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 well-adjusted log can have an accuracy of better than 0.1 percent of the speed range

This type of log can give only speed through water and is greatly affected by the current flowing under the ship. However if the water is stationary at an anchorage there will be no speed shown.

In all the above logs the flow of water past and under the hull play a major part in the accuracy of the readings.

 

DOPPLER LOG

 

Examples of the Doppler phenomenon with sound:

The Doppler principle is the effect, which makes the tone of a fire engine change as it passes the observer.

The fire engine is continuously emitting sound waves but if it is moving towards the observer the wave fronts arrive closer together, which is equivalent to a higher frequency.

As the fire engine starts to move away from the observer, the wave fronts arrive less frequently at the observer and the tone is of a lower frequency.

As the train approaches a stationary listener, the pitch (frequency) of the rumbling sound of the train is higher than when the train passes by, at which time the pitch sounds the same as if the train were stationary.

As the train recedes from the listener, the pitch decreases.

Electromagnetic waves radiated by radar, as well as sound waves, obey the Doppler principal, although electromagnetic waves travel at the speed of light and audio waves travel at the speed of sound.

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 farther apart, the listener will perceive a lower frequency - the faster the source or receiver is moving away the lower the frequency.

The Doppler shift is directly proportional to speed between source and listener, frequency of the source, and the speed the wave travels.

In above figure ‘v’ is the velocity of sound, and the propagation speed is ‘ c’, every wave is shortened due to the movement of the source by ‘d’

This shortening is equal to the source having moved a distance during the time required to generate the wave.

The Doppler log is based on measurement of the Doppler effect.


It is seen that an observer, moving with a source of sound towards a reflecting plane, receives a frequency:

Where fv is the received frequency, f the transmitted frequency, c the speed of sound and v the speed of the source of sound.

 

By measuring fv and knowing f and c, the speed of a ship with regard to the seabed can be determined.

 

Principle 

 

A transmitting transducer below the ship continuously emits a beam of sound vibrations in the water at an angle  (usually 60˚ to the keel) in the forward direction.

A second transducer aboard receives the echo caused by diffuse reflection from the seabed.

 

A Doppler log uses a higher frequency than an echo sounder. 

Advantages:

1.The resulting shorter wavelength leads to the more diffuse reflection desired; the echo from a specular reflection would not be received, in view of the oblique incidence of the beam. 

2.The shorter wavelength makes possible a smaller beam-angle and so avoids the dimensions of the radiating face of the transducer becoming too large. 

3.The emitted power of the sound vibrations spreads less and thus the echo is stronger.

 

Every point of the seabed is hit by the beam and causes a stronger or weaker echo in the direction of the receiving transducer.

All these points are situated at a different angle a to the horizontal direction;

The frequencies received aboard must differ for all these points.  However, the average frequency is approximately that from point P, at an angle a to the horizontal.

 

 

 

Hence, though the distance between the ship and the seabed does not change, the received frequency will differ (owing to the Doppler effect) from the transmitted frequency. 


From the Doppler frequency-shift, which can be measured, the speed v of the vessel can be found.

 

 

 

 

A second transmitting transducer directs a beam in a backward direction and a second receiving transducer receives its echoes.

The speed of sound waves in the water ‘c’ depends, however, on the temperature and (to a smaller degree) on the salinity and the water pressure. 

For that reason a thermistor is mounted near the transducers. (A thermistor is a resistance, the magnitude of which depends on the, temperature.)

Deviations of the sound speed ‘c’ from the normal value are passed to the system computer for correction of its calculations.

Note that the reading of a Doppler log depends solely on the speed of the sound waves;


The propagation time of the pulse and its echo plays no role.

Automatic correction for changes in speed of sound

 

In some types of Doppler log, c/cos. α is automatically kept constant.  This is done by building up each transducer from a large number (144) of electrostrictive elements. 

For simplicity only four elements are shown:

If the four elements were supplied with alternating voltages in phase, the, resulting sound waves would also be in phase, and the beam would be directed perpendicular to the radiating face of the transducer, i.e. vertically. 

However, the elements are fed with voltages that differ in phase by 120ฐ, so the sound waves have the same phase difference. 

At all points of the line AB, however, the sound vibrations are in phase.

Such a line or plane is called a wave front; propagation is always perpendicular to a wave front

Reflections

 

Both the echo sounder and the Doppler log react to reflections of sound waves from the seabed; the former measures the propagating time and the latter the difference of the two frequencies.

If the beam is propagated from one water layer into a second one of different composition or temperature, there will be reflection; there will also be a Doppler effect if the second layer moves relative to the first layer and if the beam hits this layer obliquely. 

In that case the frequency of the sound vibrations penetrating the second layer will also change, if the speed of the sound waves in the second layer is different from that in the first layer. 

For the echo, however, the reverse frequency change will occur and will cancel out the first change.

A Doppler log measures the algebraic sum of all Doppler frequency shifts experienced by the sound on its way to the bottom (or to a reflecting layer) and back again. 

To this frequency shift must be added the shift that arises at the transition of the transducer vibrations between the ship and the water, and vice versa. If the beam hits the bottom (bottom lock) the total frequency shift is, proportional to the speed of the ship with regard to the bottom. 

If there is no bottom contact, but only reflection against a water layer, the measured Doppler shift is proportional to the speed of the ship relative to that water layer (water lock).

Janus configuration

 

The placing of the two transmitting transducers, to produce forward and backward beams is called a Janus configuration.

Due to the Janus configuration a linear relationship exists between the speed of the vessel and the measured frequency shift. 

A further advantage is that vertical movements of the ship cause equal changes to the Doppler shifts in the forward and backward beams, so the difference remains the same. 

Vertical movements of the ship do not therefore influence the Doppler shift.

For measuring the athwart ship speed, a similar Janus configuration is mounted at an angle of 90 deg. with the along ships transducers;

The distance from the bridge of a large tanker to the bows may be 250 metres, so special information about the athwart ships speed both fore and aft is required when mooring. 

In that case athwart ships transmitting and receiving transducers are mounted both fore and aft.

Janus configuration. A term describing orientations of the beams of acoustic or electromagnetic energy employed with Doppler navigation systems.

The Janus configuration normally used with Doppler sonar speed logs, and docking aids employs four beams of ultrasonic energy, displaced laterally 90ฐ from each other and each directed obliquely (30ฐ from the vertical) from the ship’s bottom. This is to obtain true ground speed in the fore and aft and athwart ship directions.

These speeds are measured as Doppler frequency shifts in the reflected beams. Certain errors in data extracted from one beam tend to cancel the errors associated with the opposite directed beam.

Pitching and rolling

 

From the figure we see that the speed for the dotted position of the ship, and for the forward-directed beam increases to V1’; for the backward-directed beam V1 decreases to V1’’.

 

Results are obtained by taking the difference of the Doppler shifts for the forward beam and for the backward beam. 

In the horizontal position of the ship when this vector becomes smaller this vector becomes greater, or vice versa, so the sum of the two vectors is approximately 2v,.  Hence the Doppler measurement of the speed is not, in practice, influenced by pitching.  The same applies to the two athwartships beams during rolling.

Continuous-wave and pulse, systems

 

Hitherto it has been taken for granted that the transmitting transducers generate vibrations continuously, thus making it necessary for each beam to have a separate transmitting and receiving transducer. 

This is called a continuous-wave (c.w.) system.  Transmitting and receiving transducers are of identical construction.

 

Other types are pulse systems.  In such a system a transducer generates pulses and the same transducer receives the echo between the transmissions.  Therefore a pulse system needs only half as many transducers as a continuous-wave system.

In the continuous-wave system the reception of the echo can be disturbed by the continuously emitted vibrations of the transmitting transducer going directly from transmitting to receiving transducer (cross-noise or feedback). 

With pulse systems this cannot occur, since a pulse is transmitted only after the echo of the preceding pulse has been received, and the receiver is blocked during the transmission.

 

The majority of Doppler logs in use are pulse systems

Transducers

 

The angle of the along ships beams is about 3 deg., that of the athwart ships beam is about 8 degrees. 

The frequency used is 100 to 600 kHz – newer models have a transmission frequency of maybe 2 MHz. 

The surface area of each transducer need then be only about 10 cm2.

 

 

The high frequency and the concave shape of the surface also lead to a small beam angle. 

The higher frequency influences the reflection and the absorption but not the speed of propagation.

 

The transducers are of the electrostnctive type.

Two possibilities for a Janus configuration. 

Usually the transducers are inserted in a 'sea chest' or 'sea well’, permitting their removal for repairs or replacement without the ship requiring dry-docking. 

The diameter of the hole required in the hull plates is about 350 mm.

 

Replacement of a transducer (1) in a sea chest without the ship being dry-docked can be done in the following way. 

After the transducer (which is connected to the other apparatus by means of a cable with a plug and socket) has been disconnected, some nuts (2) are loosened and the bolts turned in the direction of the arrows.  Now the transducer (1) can be drawn upwards until it is above the flange (4) in the upper part (3) of the sea chest.  This upper part is then shut off from the lower part (6) by means of a sliding valve operated by the hand wheel (5).  In order to check that the valve is properly shut a tap (not shown in the figure), connected to the upper part (3), can be opened.  If the water in the upper part is not under pressure the bolts (7) of the flange (4) may be removed.  By using grease, the transducer can be slid easily from the top flange (8).  The sequence is reversed when a new transducer is mounted.

 

 

 

 

 

Measurement of ship's speed relative to bottom or to water

 

 

Owing to absorption by particles in the water at a depth of 200 to 400 metres, the so-called deep scattering layer (DSL), a Doppler log may only function, down to about 200 metres, unless the set is equipped to work in the layer of 10-30 metres below the surface.

When reflections are received from this layer the speed of the ship relative to that layer, and not relative to the bottom, is obtained.  Thus uncertainty and confusion may occur.

 

 

Apart from the effect of the Deep Scattering Layer, the water at 10 to 30 metres below the keel also causes an echo and Doppler effect by volume-reverberation.

This is called 'water track' (as opposed to 'bottom track').  In deep water there is a considerable difference between the time of propagation for bottom reflection and that for reflection from the mass of water at a depth of 10 to 30 metres.  Receivers can be made operative for only a short period (a certain 'window' of time) either immediately after or a short time after each pulse transmission. 

 

 

Suppose that the receiver has bottom contact, with the window occurring a short time after, transmission. 

If the Doppler log then loses bottom contact, the window is automatically shifted to occur immediately after pulse transmission. 

As a result, the receiver reacts only to reflections from the 10-30-metre water layer. 

When this happens, 'bottom track' indicator is replaced by 'water track'.

When sufficiently low frequencies are used, echoes may still arrive from a rocky bottom at a depth of 600 metres and more.

In some Doppler log, for depths less than 600 metres it is possible to switch manually to the water track mode.

 

 

Uses of the Doppler log

 

 

For, example, for a tanker of 200 000 tonnes with a residual speed when tying up of 0.2 knots (0.1 m/s), the energy to he absorbed by a pier or dolphin together with the ship's side is:

 1/2mv2 = 1000 000 joules. 

The Doppler log can measure the speed to the nearest 0.01 knot or 5 mm/s; unfortunately, however, it sometimes does not function correctly during docking if the screws of tugs cause air bubbles (which reflect sound waves) to pass through the beams (aeration).  Since the sound waves are reflected off by the water – air barrier the Doppler may give wrong readings.