Category: Bridge Equipment

Working of a Ship’s Auto Pilot with Sketch:

• An auto pilot is the ship’s steering controller which automatically manipulates the rudder to decrease the error between the reference heading and actual heading.
• Autopilot relieves the helmsman to great extent but definitely autopilot is not a substitute for helmsman.
• Autopilot also reduces fuel consumption as the zig-zag course is avoided.

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Working of Auto Pilot:-

• Course is selected by the course selector.
• Present heading is indicated by the compass.
• The output from the compass is fed to the comparator in the control unit. The signal from the course selector is also fed to the comparator.
• Difference between the two signals is causing the output error signal detected by the comparator.
• Integrator and differentiator also analyze the signal.
• The signals from the comparator, integrator and differentiator are fed to summing amplifier (control unit).
• The summing amplifier in turn, passes the signals to error amplifier which also receives feedback from the steering gear.
• The output of error amplifier is transmitted to steering gear via telemotor transmitter and telemotor receiver.
• A torque motor may be fitted instead of a telemotor.

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Controls available in Auto Pilot console:

The Autopilot Control Unit – The PID Control Unit:- In order to maintain the ship’s course accurately, the deviation signal has to be generated under the following conditions:

1. When the set course is changed (by the navigator).
2. When the ship deviates from the set course (due to external factors).

For this purpose, the helm must be provided with data regarding the ship’s movement relative to the course to steer line.

This is achieved by electronic circuits with the help of the following:

• Proportional control
• Derivative control
• Integral control

Proportional Control:-

• The effect on steering, when only the proportional control is applied, causes the rudder to move by an amount proportional to the off-course error from the course to steer.
• When the ship has gone off-course to port, an error occurs and helm, proportional to the deviation and hence error signal, is used to bring her back to the set course.
• As the ship starts to return to the set course, the helm is gradually eased and finally removed when the ship is back on the set course.
• The rudder will be amidships when the ship reaches its set course and then the heading overshoots resulting in the vessel to go more to starboard. Correcting helm is now applied causing the ship to return to port and back to the original course.
• The vessel thus keeps on oscillating to port and starboard of the course line.

Derivative Control:-

• In derivative control, the rudder is shifted by an amount proportional to the rate of change of the ship’s deviation from the course. Any deviation of course to port will cause correcting rudder to be applied to starboard.
• As the rate of change of course decreases, the automatic rudder control decreases and at a point X, the rudder will return to midships before the vessel reaches its set course.
• The ship will now make good a course parallel to the required course.

Integral Control:-

• Certain errors due to the design of the ship (bow going to port due to transverse thrust, shape of the hull, current draft, etc.) have an impact on the steering capabilities of the ship and have to be corrected for effective overall steering performance.
• In order to achieve this, signals are produced by sensing the heading error over a period of time and applying an appropriate degree of permanent helm. The rudder used to correct the course will now be about this permanent helm. That is, the permanent helm will now act as midships.
• Additionally, there are various controls provided on the autopilot system along with a filter system for the action of the winds and waves which supply more data to the autopilot which optimizes the performance of integral control.
• The output of these three controls is combined and the net resultant thus obtained drives the rudder maintaining the ship on the set course. This type of auto pilot is referred to as PID auto pilot.

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Working of “Weather Control” in Auto Pilot System:

Rough weather and hostile sea conditions have adverse effects on the performance of the auto-pilot. Uncontrolled yawing of the ship can result in excessive rudder movement. Modern auto-pilot system has Weather control option in which the system automatically adjusts the setting to adapt to the changing weather and sea conditions. It also provides an option for the user to manual set a specific value.

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Working of “Yaw Control” in Auto Pilot System:

The setting of the Yaw Control depends upon the wind and weather condition and their effect on the course keeping ability of the ship, in bad weather this setting should be set high and calm weather this should be set low. If Yaw Control is not set properly, the steering gear will over work & there will be excessive load on the system.

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Working of “Off Course Alarm” in Auto Pilot System:

Off Course Alarm:- Usually an Off Course Alarm is fitted on the Autopilot. This can be set for the required amount of degrees. So that if at anytime the difference between the actual course and the Autopilot set course is more than the preset degrees, an alarm will warn the officer.

There is however, one limitation which should be noted. In case, the gyro compass itself begins to wander the Autopilot well steer so as to follow the wandering compass and the Off Course Alarm will not sound. It does not ring unless the difference between the course setting and gyro heading is more than the preset limit.

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Working of “Rudder Limit” in Auto Pilot System:

Rudder Limit:- This setting specifies the maximum amount of rudder to be used when correcting the ship’s head or when altering course on autopilot. That is, if a setting of 10^O is applied for rudder limit, when altering course the rudder will move to a maximum of 10^O. This limit can be varied according to the requirements of the navigator.

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Purpose of following settings in Autopilot: Rudder

• This control determines the amount of rudder to be used to correct the slightest amount deviation from the set course.
• The higher is setting the larger the rudder angle is used to correct a course deviation and this may result in over correcting.
• But if setting is less, the rudder angle is used to correct deviation may not be sufficient and will take longer time to return to set course.
• This is proportional controller which transmits a signal which is proportional to course error
  • Controller output = constant (Kp) x Deviation
• The ratio can be changed by settings (i.e. the ratio between instantaneous heading error and rudder command) also called rudder multiplier.
• Control Knob alters the ratio of output.
• Higher setting – Larger rudder angle (results in overcorrecting – overshooting)
• Lower setting – Less rudder angle (Long time to return to set Co-Sluggish).
• Therefore, optimum setting required.

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Purpose of following settings in Autopilot: Counter Rudder

• This control determines the amount of counter action by the rudder to be used to steady the ship on the set course keeping the overshoot to minimum.
• Too low setting will allow the ship to overshoot and too high setting will bring the ship back in long time. 
• This is Derivative control.
• Purpose is to apply a relatively greater amount of helm at the beginning of a course alteration to get the ship turning. Once the ship is turning, just enough helm is applied in order to keep her coming around. When new heading is approached, opposite helm is applied to stop the swing. As the ship settles on new heading and the yaw rate disappears, the helm is removed.
• Produces an output when course of vessel is changing.
• Depends on rate of change of course:
  • Controller output = constant ( KD ) x change of error / time
• Determines amount of counter rudder to steady the ship on set course.
• Keeps over shoot to minimum.
• Greater the ship’s inertia, greater the setting required. If ship has good dynamic stability, relatively small settings of counter rudder will be sufficient. If the ship is unstable, higher settings will be required.
• Depends on ship’s characteristics, loaded/ballast conditions and rate of turn.
• Too high setting will bring the ship to set Co slowly.
• Too low setting allows overshoot.
• As counter rudder settings increase, counter rudder increases.
• KD – Counter rudder time constant (Calibration done at sea trial to set KD).

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Purpose of following settings in Autopilot: Constant Helm

Constant / Permanent Helm:

• This is integral controller. (In NFU this control is out of action).
• When ship has known imbalance to one side, requiring a certain amount of bias helm (e.g. TT of propeller) manual setting of the approximate bias speed up the effect of the AUTOMATIC PERMANENT HELM calculator, because it started off nearer to its target.
• Whether the control setting is estimated correctly or left at zero has no effect on the final steering accuracy but only in the time it takes to reach this heading accuracy.
• If not used as described above , the permanent helm should be left at ZERO and the automatic permanent helm will function normally.
• Produces output as long a course error persists.
• Used when beam winds; couple formed causing ship to turn into wind.
• Rudder position required to counteract is permanent helm.
• Continuous control calibrated from 20 (P) to 20 (S).

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Purpose of following settings in Autopilot: Weather

The setting of the yaw control depends upon the wind and weather condition and their effect on course keeping ability of the ship in bad weather this setting should be set high and calm weather this should be low.

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Purpose of following settings in Autopilot: Rudder Limit

Rudder limit: This control specifies the maximum amount of rudder to be used, when correcting the ship’s head or altering the ship’s course.

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Auto Pilot should not be used in the following conditions:

• In narrow channels.
• At slow speeds.
• During manoeuvring.
• During pilotage.
• During heavy weather conditions.
• During large alteration of course.
• Near or in area of restricted visibility.
• When passing close to vessels etc.

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Principle & Working of Doppler Log: 

• 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.

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Explanation of how ship’s speed is transmitted to remote displays:

• 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:-
  • An analogue voltage, the gradient of which is 0.1 V/knot, to drive the potentiometer servo-type speed indicators.
  • A pulse frequency proportional to speed.
  • The frequency is 200/36 pulses/s/knot. Pulses are gated into the digital counter by a 1.8-s gate pulse.
  • A positive/negative voltage level to set the ahead/astern indication or the B track/W track indication.
  • 2000 pulses per nautical mile to drive the stepping motor in the digital distance indicator.

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FORMULA of Doppler Log:-

• Doppler effect can be further explained by following equations:
  • fr is the frequency received by observer.
  • ft is the transmitted frequency.
  • c is the speed of sound.
  • v_O is Velocity of observer
  • v_g is Velocity of source
• If the source moves towards stationary observer, fr = c ft / (c – v_g)
• If the source moves away stationary observer, fr = c ft / (c + v_g)
• If the observer moves towards stationary source, fr = ft (c + v_g) / c
• If the observer moves away from stationary observer, fr = ft (c – v_g) / c
• If the observer & source moves away from each other, fr = ft (c – v_g) / (c + v_s)
• If the observer & source moves toward each other, fr = ft (c + v_g) / (c – v_s)
• Since,
  in the case of the Doppler log, the source & observer are the same.

Hence,

v_O is equal to v_S, is equal to v

fr = ft (c+ v) / (c – v)

fr = ft (c+ v cos a) / (c – v cos a)

After Further simplification

v = c (fr – ft) / 2 ft cos a

• Given
  a propogation angle of 60^O, cos a = 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:-

        Fr_t – fr_a – 4 vf_t cos a / c

Hence, v = c (fr_t – fr_a) / 4 ft cos a

• 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 3^O 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.

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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 of the 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.

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Errors in a Doppler log & how are some of these errors overcome by the Janus Configuration:

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.

THE ‘SPEED’ FORMULA WITH SHIP MOVEMENTS CORRECTION – JANUS CONFIGURATION:-

As the ship moves forward, she also has an up and down motion in the vertical direction, called ‘heaving’. The vertical motion component is v sin α.

As this movement of the ship has an effect on the frequency shift, it should be accounted for. This is done by installing a second set of transducers (for transmitting and receiving) in the aft direction at the same angle of 60º. (Refer figure). This type of installation setup is called Janus Configuration.

The effect of frequency shift due to vertical motion (the component v sin α ) of the ship gets cancelled out in Janus Configuration and the resultant ship speed is calculated by the formulae:

v = c (Frf – Fra) / 4 Ft cos α

Where,

• v= ship’s speed
• c= speed of acoustic wave in water
• Frf = Freq. of the received wave, from fwd direction
• Fra = Freq. of the received wave, from aft direction
• Ft = Freq. of the transmitted wave
• α = angle of acoustic wave transmission

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Basic Principles of Echo Sounder:

Short pulses of sound vibrations are transmitted from the bottom of the ship to the seabed. These sound waves are reflected back by the seabed and the time taken from transmission to reception of the reflected sound waves is measured. Since the speed of sound in water is about 1500 m/sec, the depth of the sea bed is calculated which will be half the distance travelled by the sound waves.

The received echoes are converted into electrical signal by the receiving transducer and after passing through to stylus which burns out the coating of the thin layer of aluminum powder and produces the black mark on the paper indicating the depth of seabed.

Components of Echo Sounder:

• Basically an echo sounder has following components:
• Transducer – to generate the sound vibrations and also receive the reflected sound vibration.
• Pulse generator – to produce electrical oscillations for the transmitting transducer.
• Amplifier – to amplify the weak electrical oscillations that has been generated by the receiving transducer on reception of the reflected sound vibration.
• Recorder – for measuring and indicating depth.

CONTROLS:-

• An echo sounder will normally have the following controls:
• Range Switch – to select the range between which the depth is be checked e.g.  0- 50 m, 1 – 100 m, 100 – 200 m etc.  Always check the lowest range first before shifting to a higher range.
• Unit selector switch – to select the unit feet, fathoms or meter as required.
• Gain switch – to be adjusted such that the clearest echo line is recorded on the paper.
• Paper speed control – to select the speed of the paper – usually two speeds available.
• Zero Adjustment or Draught setting control – the echo sounder will normally display the depth below the keel.  This switch can be used to feed the ship’s draught such that the echo sounder will display the total sea depth.  This switch is also used to adjust the start of the transmission of the sound pulse to be in line with the zero of the scale in use.
• Fix or event marker – this button is used to draw a line on the paper as a mark to indicate certain time e.g. passing a navigational mark, when a position is plotted on the chart etc.
• Transducer changeover switch – in case vessel has more than one switch e.g. forward and aft transducer.
• Dimmer – to illuminate the display as required.

Working:

• The acoustic pulses of very short duration are transmitted vertically at the rate of 5 to 600 pulses per minute having a beam width of 12 to 25°.
• These pulses strike the seabed and get reflected back towards the receiving transducer as echoes.
• These received echoes are converted into electrical signals by the receiving transducer and after passing through the different stages of the receiver, the current is supplied to the stylus which bums out the coating of the thin layer of aluminium powder and produces a black mark on the paper indicating the depth of the seabed.

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Principle used in the working of an Echo Sounder:-

There are two techniques:-

• Ranging
  
• Phasing

Ranging:-

• In echo sounder the stylus is mounted on circular belt driven by means of a stylus motor which moves at certain speed and transmission takes place when the stylus passes the zero marks.
• A magnet fixed on the stylus belt triggers the transmitter to transmit a pulse every rotation of belt when stylus is at zero mark on the paper scale, the transmission of the acoustic waves from the transducer is synchronized with the stylus at the zero mark.
• The acoustic waves are reflected from the seabed and echoes are received by the transducer and after passing through various stages eventually the current is supplied to stylus which burns out the coating of the thin layer of aluminum powder and produces the black mark on the paper indicating the depth of seabed.
  
• This cycle is repeated for every rotation so as the paper is pulled across the display, the profile of seabed is obtained.
• Suppose the lowest range scale is 0 to 50 M, the transmission will take place when stylus reaches at the zero mark.
• When the higher range is selected say 0 to 100 M, in order to cater for this range scale, the speed of the stylus motor is reduced, in this process the scale magnification is lost and as we switch over to higher ranges the scale becomes more & more congested.
• To overcome this problem some of echo sounding machines work on phasing technique.

Phasing:-

• In phasing the speed of the stylus motor remains constant.
• Instead of changing the speed of the stylus, the transmission point is advanced.
• If the first range is 0 to 50 M the second range will be 50 to 100 M (instead of 0 to 100 M).
• Various sensors are positioned around the stylus belt, the magnet generates the pulse when it passes the sensors which in turns activates the transmitter.
• In the below diagram, when we select the lowest range i.e. 0 to 50 M, the magnet mounted on the stylus belt will activate sensor no. 1, transmission takes place when the stylus exactly passes over the zero mark, when we switch over to higher range, say 50 to 100 M, the magnet mounted on the stylus belt will activate sensor no.2 and transmission will take place early, at the time of the transmission, the stylus will not be passing over 50 M mark on display unit, in other words there will be delay introduced by delay unit no.2 & the stylus will reach the 50 M on display unit after delay of 0.067 seconds. (50 x 2 / 1500, where 50 correspond to the range, multiplied by 2 because double of distance is covered by acoustic waves & the echoes and 1500 is the speed of acoustic waves).
• Likewise, when we switch over to higher range say, 100 to 150 M, magnet mounted on the stylus belt will activate sensor no. 3 & more delay will be introduced for the stylus to pass over the 100 M.

Caution when using phasing technique: – We must always start sounding at lowest range and check for echoes, adjust the gain control if required and then only switch over to higher range.

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Errors of Echo Sounder:

• Velocity of propagation in water:- The velocity changes with temperature salinity & pressure. The velocity of the acoustic wave assumed at the temperature of 16 degree C & Salinity of 3.4% is 1505 m/sec, but generally it is taken as 1500 m/sec for calculations. As velocity is varying hence depth recorded will be erroneous. Depth indicated in Fresh water can be about 3% higher than the actual depth. NP 139 can be referred in order to obtain the corrections. To compensate the error due to temperature variation, a component called “thermistor” may be mounted near the transducer & change in velocity of the acoustic wave through water from the standard value due to the change in sea water temperature is accounted for. Error due to pressure is not so significant.
• Stylus speed error:- The speed of the stylus is such that the time taken by the stylus to travel from top to bottom on chart is same as the time taken by sound waves to travel twice the range selected, but due to fluctuation in voltage supplied to stylus motor, will cause error in the recorded depth.
• Pythagoras error:- This error is found when two transducers are used, one for transmission and the other one for reception. This error is calculated using the Pythagoras principle. This error becomes prominent whenever distance between two transducer is more than 2 mtrs, manual should be referred in order to use the table for corrections.
• Multiple echoes:- The echo may be reflected no. of times from the bottom of the sea bed, hence providing the multiple depth marks on paper.
• The thermal and density layers:- The density of the water varies with temperature and salinity, which all tends to form different layers. The sound wave may be reflected from these layers.
• Zero line adjustment error:- If the zero is not adjusted properly, it will give error in reading.
• Cross noise:- If sensitivity of the amplifier is high, just after zero marking a narrow line along with the several irregular dots and dashes appear and this is called cross noise. The main reasons for the cross noise are aeration and picking up the transmitted pulse. If intensity of cross noise is high, it will completely mask the shallow water depths. This is controlled by swept gain control circuit.
• Aeration:- When the sound wave is reflected from the reflected from the air bubbles, it will appear as dots, this is known as aeration.
  • Aeration can be due to pockets of bubble due to heavy weather.
  • Rudder hard over causing drastic alteration of course.
  • Pitching in light condition.
  • Whilst astern propulsion. (Switch over to forward transducer if available.)

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Electrostrictive Transducer with respect to Echo Sounder:

• This type of transducer works on the basic principle of piezo-electric effect, i.e., certain crystals such as quartz, have a property that when pressure is applied to the two opposite faces, a difference of potential is created which is proportional to the applied pressure or when an alternating voltage is applied,
  the crystals start vibrating or oscillating. This type of transducer is also known as Piezostrictive transducer.
• The electrostrictive transducer uses the property of a crystal for transmission and reception of acoustic waves in water. The crystal is firmly fixed between two steel plates so that they act as a single unit.
• The purpose of the steel plates is to provide solid and robust housing for the crystal as well as a suitable contact surface for seawater.
• When an alternating voltage is applied between the steel plates, the quartz and the steel plates start vibrating together. The vibration will be of very high amplitude, if the frequency of the alternating voltage is equal to the resonance frequency of the crystal. The lower of the two steel plates is in direct contact with the water, which will cause the vibration in the seawater.
  The vibration is always perpendicular to the plate and hence always kept horizontally.
• Generally, only one transducer is used for transmission and reception of the signals and this transducer is always mounted as pierced hull.

Purpose of BNWAS:-

• The purpose of a bridge navigational watch alarm system is to monitor bridge activity and detect operator disability which could lead to marine accidents.
• The system monitors awareness of the Officer of the Watch (OOW) and automatically alerts the Master or another qualified person if, for any reason, the OOW becomes incapable of performing OOW duties.
• This purpose is achieved by series of indications and alarm to alert first the OOW and, if he is not responding, then to alert Master or another qualified person.
• Additionally, the BNWAS provides the OOW with means of calling for immediate assistance if required.

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Operational Modes of BNWAS:-

BNWAS should incorporate the following 3 operational modes:

1. Automatic
2. Manual ‘ON’
3. Manual ‘OFF’

1. Automatic: The BNWAS is automatically activated when the vessel is navigating by means of heading or track control system (autopilot / trackpilot) and inhibited when this system is deactivated.
2. Manual ON: The BNWAS is always in operation.
   • Authorized person (Master) switches on system by single turning the key-switch to position “ON/OFF”.
   • Once dormant period (3 min, 9 min, 12 min) is set, the authorized person pulls out the key.
   • Dormant period starts from the moment the system has been switched on.
3. Manual OFF: The BNWAS is turned off completely.

ALERT SEQUENCE: – If dormant period is over without the OOW resetting, the system activates all reset units by flash light.

ALERT STAGE 1:- If dormant period and flash light period (15 sec) are over and the OOW has not yet resetted the button, the system activates electronic buzzer on terminal board as well as additional buzzers located on the bridge and wing area.

ALERT STAGE 2:- If dormant period, flash light period (15 sec) and alert stage 1 (15 sec) are over without OOW’s reset, system activates further alert via optic / acoustic alarm devices in officer’s area (cabins or staircase) as well as for VDR link.

ALERT STAGE 3:- If dormant period, flash light period (15 sec), alert stage 1 (15 sec) and alert stage 2 (90 sec) are over without OOW’s reset, the system activates the General Alarm as well as for VDR link.

EMERGENCY CALL:-

• The OOW generates an emergency call by pushing any reset unit longer than 5 sec.
• That immediately activates alert stage 2 and subsequently alarm stage 3.
• After emergency call has been released, reset is possible by pushing a reset unit.

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Requirements of BNWAS:-

• The bridge navigational watch alarm system shall be in operation whenever the ship is underway at sea.
• System is powered by ships main power and MUST have a battery back up giving a minimum of 6 hours usage.

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Carriage Requirements of Bridge Navigational Watch Alarm System (BNWAS):-

1. Cargo ships of 150 grt and upwards and passenger ships irrespective of size constructed on or after 1 July 2011;
2. Passenger ships irrespective of  size constructed before 1 July 2011, not later than the first survey after 1 July 2012;
3. Cargo ships of 3000 grt and upwards constructed before 1 July 2011, not later than the first survey after 1 July 2012;
4. Cargo ships of 500 grt and upwards but less than 3000 grt constructed before 1 July 2011, not later than the first survey after 1 July 2013; and
5. Cargo ships of 150 grt and upwards but less than 500 grt constructed before 1 July 2011, not later than the first survey after 1 July 2014.

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Marine Sextant Explanation

Sextant is a precision instrument used for –

• Measuring altitudes of celestial bodies and horizontal angles between terrestrial objects vertical angles of terrestrial objects.

Principle of Sextant:

1. When a ray of light is reflected by a plane mirror, the angle of incidence is equal to the angle of reflection, while the incident ray, reflected ray and the normal lying in the same plane.
2. When a ray of light, suffers two successive reflections in the same plane, by two plane mirrors, the angle between the incident ray and the final ray is twice the angle between the mirrors.

Formulae for Sextant:-

Reading of Sextant:

• When the sextant reads zero, Index mirror and horizon glasses are parallel to each other.
• When the index bar is rotated through an angle, the angle between the incident ray and the final reflected ray.
• The arc of the sextant is only 60° in extent, Micrometer is provided to measure accurate reading upto 0.1°.

Errors of Sextant:

Two types:

• Adjustable errors:
  • Error of perpendicularity:- Caused when the index mirror is not perpendicular to the plane of the sextant.
  • Side error:- Caused when the horizon glass is not perpendicular to the plane of the sextant.
  • Index error:- When the index bar is set at zero, the plane of the index mirror and horizon glass are NOT parallel to each other.
  • Error of collimation:- When the axis of the telescope is not parallel to the plane of the sextant.
• Non-Adjustable errors:
  • Graduation error:- due to inaccurate graduation of the scale on the arc or of the micrometer/vernier.
  • Shade error:- due to the 2 surfaces of the coloured shades not being exactly parallel to each other.
  • Centering error:- pivot of the index bar not coincident with the centre of the circle of which the arc is a part.
  • Optical Error:- may be caused by the prismatic errors of the mirror or aberrations in the telescope lenses.
  • Back-lash:- Wear on the rack and worm, which forms the micrometer movement would cause a back-lash, leading to inconsistent errors.
• Index error, how to determine:
  • During day time, clamp the index bar at zero and holding the sextant vertically, view the horizon through the telescope.
  • If the true horizon and its reflection appear in the same line, Index error is not present.
  • If they appear displaced vertically, turn the micrometer drum till they are in the same line.
  • The micrometer reading then is the index error, which is on the arc if the micrometer reading is more than zero, off the arc if it is less than zero.

Corrections of Sextant Altitude:-

• Visible horizon: Is the small circle on the earth’s surface, bounding the observer’s field of vision at sea.
• Sensible horizon: Is a small circle on the celestial sphere, the plane of which passes through the observer’s eye, and is parallel to the observer’s rational horizon.
• Rational horizon: The observer’s rational horizon is a great circle on the celestial sphere every point on which is 90° away from his zenith.
• Observed altitude: Of a celestial body is the angle at the observer between the body and the direction to the observer’s visible or sea horizon. The observed altitude is therefore, the sextant altitude corrected
  for any index error.
• Dip: Is the angle at the observer between the plane of observer’s sensible horizon and the direction to his visible horizon. Dip occurs because the observer is not situated at the sea level. The value of dip increases
  as the observer’s height.
• Apparent altitude: Is the sextant altitude corrected for Index error and dip.

A freely suspended Magnet in a Magnetic Compass points towards the North:

• A magnetic compass works because the Earth is like a giant magnet, surrounded by a huge magnetic field. The Earth has two magnetic poles which lie near the North and South poles. The magnetic field of the Earth causes a magnetized ‘needle’ of iron or steel to swing into a north-south position if it is hung from a thread, or if it is stuck through a straw or piece of wood floating in a bowl of water.
• A compass works by utilizing the Earth’s magnetism in order to find directions. Its invention enabled people to perform navigation over long distances, opening up the sea for exploration
• A compass points north because all magnets have two poles, a north pole and a south pole, and the north pole of one magnet is attracted to the south pole of another magnet.
• The Earth is a magnet that can interact with other magnets in this way, so the north end of a compass magnet is drawn to align with the Earth’s magnetic field. Because the Earth’s magnetic North Pole attracts the “north” ends of other magnets, it is technically the “South Pole” of our planet’s magnetic field.

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Notes on Earth’s Magnetism:

• Earth’s magnetic field, also known as the geomagnetic field, is the magnetic field that extends from the Earth’s interior to where it meets the solar wind, a stream of charged particles emanating from the Sun.
• Its magnitude at the Earth’s surface ranges from 25 to 65 microtesla (0.25 to 0.65 gauss). Roughly speaking it is the field of a magnetic dipole currently tilted at an angle of about 10 degrees with respect to Earth’s rotational axis, as if there were a bar magnet placed at that angle at the center of the Earth.
• Unlike a bar magnet, however, Earth’s magnetic field changes over time because it is generated by a geodynamo (in Earth’s case, the motion of molten iron alloys in its outer core).
• The North and South magnetic poles wander widely, but sufficiently slowly for ordinary compasses to remain useful for navigation.
• However, at irregular intervals averaging several hundred thousand years, the Earth’s field reverses and the North and South Magnetic Poles relatively abruptly switch places.
• These reversals of the geomagnetic poles leave a record in rocks that are of value to paleomagnetists in calculating geomagnetic fields in the past. Such information in turn is helpful in studying the motions of continents and ocean floors in the process of plate tectonics.

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‘Variation’ and ‘Deviation’ of Magnetic Compass:

VARIATION:

• The true North Pole and the magnetic north pole are not located at the same spot. This variation causes a magnetic compass needle to point more or less away from true north.
  The amount the needle is offset is called variation because the amount varies at different points on Earth’s surface. Even in the same locality variation usually does not remain constant, but increases or decreases at a certain known rate annually.

The variation for any given locality, together with the amount of annual increase or decrease, is shown on the compass rose of the chart for that particular locality.

DEVIATION:

• The amount a magnetic compass needle is deflected by magnetic  material  in  the  ship  is  called  deviation.
• Although deviation remains a constant for any given compass heading, it is not the same on all headings.  Deviation  gradually  increases,  decreases, increases,  and  decreases  again  as  the  ship  goes  through an entire 360° of swing.
• The magnetic steering compass is located in the pilothouse, where it is affected considerably by deviation.  Usually the standard compass is topside, where the magnetic forces producing deviation are not as strong.
• Courses and bearings by these compasses must be carefully differentiated by the abbreviations PSC (per standard compass), PSTGC (per steering compass), and PGC (per gyrocompass).
• The standard compass provides a means for checking the steering compass and the gyrocompass.

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Notes on Ship’s Magnetism:

• On a ship built up of wood, a magnetic compass would point to magnetic north knowing the variation at that place, the magnetic direction corrected and the true direction obtained, the ships built up of steel structures is of two types, soft iron magnetism and the hard iron magnetism.
• Soft iron magnetism is the induced magnetism and hard iron magnetism is the permanent magnetism.

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Sketch & label a wet card Magnetic Compass:

• Necessity:- The dry card compass is too sensitive for steering purposes, especially in bad weather. Even small disturbances cause the dry card to oscillate. In the wet card compass oscillations are damped, without loss of accuracy, by immersing the card in a liquid. The card therefore has a ‘dead beat’ movement.
• The card:- The wet card is made of mica and is only about 15 cm in diameter. The card is attached to nickel- silver float chamber that has a sapphire cap. The cap rests on iridium tipped pivot. The sapphire has a polishing effect on the iridium tip. This arrangement is practically frictionless.
• The directive element:- In modern wet card compasses the directive element is a ring magnet fitted around the base of the float. The ring magnet offers less resistance to movement and causes less turbulence.
• The bowl:- The diameter of the bowl is about 23 cm in order to reduce disturbances caused by turbulence in the liquid during rotation of the card. The top of the bowl is of transparent glass. The bottom is of frosted glass.
• Allowance for expansion:- One method is to have a small accordion – like expansion chamber attached to the bowl. The chamber increases or decreases in volume, as necessary, as the liquid in the bowl expands or contracts due to changes in atmospheric temperature.
• Suspension:- The bowl of the wet card compass is suspended by gimbals. This bowl, being considerably heavier than that of the dry card compass, does not have a glass hemisphere of alcohol and water attached to its underside. Instead, a ballast weight consisting of a ring of lead, enclosed in brass, is attached along with circumference of the underside of the bowl to bring its centre of gravity below the gimbals.
• Care and Maintenance:- The care and maintenance required for a wet card compass and its binnacle is the same as that for a dry card compass. The only changes / differences are:-
  • The wet compass card, if found defective owing to stickiness of movement, has to be renewed by the manufacturer or his authorized agent. Hence, no spare wet card is carried. Instead, an entire bowl is carried as a spare.
  • In rare cases, a bubble may develop in the wet compass bowl. This has to be removed at the earliest opportunity.

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Correctors on a compass binnacle and why are they required, various corrections to be applied to magnetic compass

The Binnacle:- The binnacle is a cylindrical container made of teak wood and brass. No magnetic materials are used in its construction. Even the screws are of brass and the nails, copper. The compass bowl is slung inside the top portion of the binnacle. The middle potion is accessible by a door and contains an electric bulb. Light from this bulk passes upwards through a slot, through an orange coloured glass fitted over the slot, through the bottom of the compass bowl, to illuminate the compass card from below. The orange colour ensures that the night vision of the observer is not adversely affected.

• Corrector Magnets:- (See Figure) In the centre of the lower half of the binnacle, there are a number of horizontal holes, both fore & aft and athwartships, for ‘hard iron’ or ‘permanent’ corrector magnets which are meant to offset undesirable, disturbing, magnetic effects caused by the ship’s steel hull. The lower two-thirds of the binnacle has a vertical brass tube, at the centre, in which slides a ‘bucket’. This bucket has some magnets in it called ‘heeling error correctors’. The bucket is held in position by a brass chain.

• Quadrantal Correctors:-
  (See Figure) These are two ‘soft iron’ spheres which are fitted in brackets, one on either side of the binnacle. The brackets are slotted so that the distance between the spheres can be altered as desired during compass adjustment.
• Flinders Bar:- (See Figure)
  This is a soft iron corrector, (diameter about 7.5 to 10 cm) inserted in a 60 cm long brass case, fitted vertically on the forward or on the after part of the binnacle.  If the ship has more superstructure abaft the compass, the Flinders bar is fitted on the forward part of the binnacle and vice versa.

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Remove an air bubble from the compass bowl

Removal of bubble: A bubble may form in the bowl owing to the fact that some of the liquid has somehow escaped from the bowl. This is a rare occurrence and must be remedied by following the manufacturer’s instructions. In most compasses:

1. Tilt the bowl until the ‘filter hole’ comes uppermost. This hole is provided on the side of the bowl. Unscrew the stud/ screw provided.
2. Top up with ethyl alcohol. If this is not available, distilled water would do.
3. Screw the stud/ screw back into place.
4. Gently let the bowl return to upright.

In some modern compasses, small bubbles may be removed as follows:-

• Invert the bowl gently. This would cause the bubble to enter a bubble trap provided for this purposes. Gently return the bowl to upright. The bubble should have disappeared.

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Advantages of wet compass over Dry Compass Card

The dry card compass is generally used as a standard compass & the wet card compass as a steering compass. The dry card compass is very sensitive. Even a slight disturbance makes the dry card oscillate. In the wet card compass, the oscillation is damped in the liquid and hence more useful as a steering compass. In some ships, the wet compass is now used as a standard compass, mainly because of the availability of the gyro compass as the main direction indicating instrument.

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Why the vessel is required to be swung once in a year to verify Magnetic Compass Deviation Card?

Explanation:-

• Swinging the compass, or swinging the ship (as the operation is sometimes more accurately called as the ship swings around the compass card which, ideally, remains pointing north), involves taking the vessel to a suitable location in open water with plenty of room for maneuvering. With the vessel steady on each of the eight primary compass points, existing compass headings or bearings are compared with what we know the actual magnetic headings or bearings should be, the difference being the deviation.
• During the process, any magnetic fields, created by the ship’s structure, equipment, etc, which cause the compass to deviate are reduced or, if possible, eliminated, by creating equal but opposite magnetic fields using compensating correctors. These are placed inside the compass binnacle or adjacent to the compass:
  • Magnets are aligned fore and aft and athwartships to create horizontal magnetic fields to compensate for the permanent horizontal components of the ship’s magnetism.
  • Soft iron correcting spheres or plates and the Flinders bar compensate for the induced magnetism caused by the effect the earth’s magnetic field has on the ship’s magnetism.
  • Heeling error magnets compensate for the vertical component of the ship’s magnetism.
• The timing and logistics of this operation are often governed by the tide, the weather and other vessels in the vicinity. The time it takes to swing and adjust the compass is also influenced by the condition and accessibility of the compass and correctors, the manoeuvrability of the vessel, the skill of the helmsman and the complexity of, and reasons for, the deviating magnetic fields involved.
• On successful completion of compass swing, a table recording any remaining residual deviation and a statement as to the good working order of the compass will be issued. A current deviation card / certificate of adjustment is a legal requirement on all sea going commercial vessels.
• Deviation can be determined by a number of methods: the sun’s azimuth or known bearings of distant objects, such as a mountain peak or lighthouse are considered most accurate. In certain circumstances, such as poor visibilty, calibration is carried out by making comparisons with other navigation instruments, such as a gyro or GPS compass.
• Using other navigation instruments to find deviation is only satisfactory if the absolute accuracy of these instruments has first been verified, or any known error is factored into the calculations. Most professionals prefer something tangible, such as a fixed landmark, with a known position and bearing to work with.

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What is Dip? How compass is kept horizontal in varying latitudes?

Magnetic dip Error:-

• Magnetic dip is the tendency of the compass needles to point down as well as to the magnetic pole.
• Dip is greatest near the poles and least near the Magnetic Equator.
• The compass card is designed to operate in the horizontal, therefore, any movement from the horizontal plane introduces dip error.
• The needle of your magnetic compass will be parallel with Earth’s surface at the Magnetic Equator, but will point increasing downward as it is moved closer to the Magnetic Pole.
• Northerly turning error is due to the mounting of the compass. Since the card is balanced in fluid, when the vessel turns, the card is also banked as a result of centrifugal force.
• While the card is banked, the vertical component of the Earth’s magnetic field causes the north-seeking ends of the compass to dip to the low side of the turn. When making a turn from a northerly heading, the compass briefly gives an indication of a turn in the opposite direction.
• When making a turn from the south, it gives an indication of a turn in the correct direction but at a faster rate.

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Explanation of a Free Gyroscope:

Gyroscope having three degrees of freedom is called “FREE GYROSCOPE”

Properties of Free Gyroscope:-

1. Gyroscopic inertia or rigidity in space
2. Precession

1. Gyroscopic Inertia:- A freely spinning gyroscope will maintain its axis of spin in the same direction with respect to space irrespective of how its supporting base is turned. It resists any attempt to change its direction of spin. Thus a free gyroscope has high directional stability. This property is called GYROSCOPIC INERTIA or RIGIDITY IN SPACE.
2. Gyro Precession:- Precession is the angular displacement of the spin axis of the gyroscope when a torque is applied to gyroscope. Hence, when a torque is applied to the spin axis the resulting movement will be in the direction at right angle to the applied torque. Earth is also a free gyroscope pointing north axis toward Polaris. (rigidity in space). We all are also aware that earth also possesses force of gravity.

First property of free gyro scope is useful. However, due to the placing of this gyroscope on the surface of the earth it will be moved along the direction of rotation of the earth. As such the gyroscope will have an apparent motion. For example, at night if the gyroscope is made to point in the direction of a star, then the gyroscope will follow the star as the earth rotates and the star apparently moves in the sky.

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Tilt and Drift in a Gyro compass

• Tilt is elevation or depression of the spin axis above or below the horizon.
• Drift is the movement of the spin axis in the direction of azimuth.
• Rate of tilting in degrees per hour = 15^O sine Azimuth * cosine Latitude
• Rate of Drift in degrees per hour = 15^O sine Latitude

Tilt:-

• If a free gyroscope is situated on the equator and lies with its axis East West and horizontal, it can be assumed of as pointing to a star with zero declination and is about to rise.
• The East End of the gyroscope axis will follow the movement of this star and will tilt upwards as the star rises.
• After nearly six hours the axis will be vertical and after nearly twelve hours the gyroscope will have turned completely over with the axis again horizontal but now the original East end of the axis would be pointing to the star setting due West.
• After one sidereal day, the gyroscope would have tilted through 360^O and the star would again be rising.
• This rate of tilting of 360^O in a day is a rate of 15^O per hour.
• If the gyroscope had been situated on the equator with its axis lying in the North – South direction, then the North end would be pointing towards the Pole star and would then have no apparent movement relative to the Earth.
• The rate of tilting thus varies from zero when the axis is lying North – South to a maximum when it is lying East – West. That is the rate of tilting varies as the Sine of the Azimuth.
• A free gyroscope situated at a pole with its axis horizontal would have an apparent turntable motion due to the Earth’s rotation.
• That is it would follow a fixed star around the horizon but it would not rise or set.
• The rate of tilting thus varies from a maximum when the latitude is 0^O to zero when the latitude is 90^O. That is the rate of tilting varies as the Cosine of the Latitude.
• Rate of tilting in degrees per hour = 15^O sine Azimuth * cosine Latitude.
• The direction of tilting is such that the end of the gyroscope axis, which lies to the East of the meridian, tilts upwards and the end of the axis, which lies to the West of the meridian tilts downward.

Drift:-

• Drift is the apparent movement of a gyroscope in azimuth.
• A free gyroscope situated at the North Pole with its axis horizontal will have an apparent movement, which is entirely in the horizontal plane.
• Its axis will appear to move in a clockwise direction when viewed from above. This would be due to the real counter clockwise rotation of the earth beneath, this circular motion causes the gyroscope to drift through 360^O in one sidereal day, that is at a rate of 15^O per hour.
• A free gyroscope situated at the equator with its axis horizontal will not drift at all, irrespective of whether its axis is set in the North – South or East – West line.
• The rate of drift for a gyroscope with its axis horizontal thus varies from a maximum at the poles to zero at the equator.
• That is the rate of drift varies as the sine of the latitude. For a free gyroscope with its axis horizontal: Rate of Drift in degrees per hour = 15^O sin Latitude. The direction of drift depends upon hemisphere so that the North end of a horizontal gyroscopic axis drifts to the eastwards in the Northern hemisphere but to the Westwards in the southern hemisphere.

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Course, Latitude and Speed Error in a Gyro Compass:

• The gyro compass settles in the N/S direction by sensing Earth’s spinning motion. Same gyro compass when placed on a ship also senses the ship’s motion. And therefore, the axis of gyro compass settles in a direction which is perpendicular to the resultant of the Earth’s surface speed and the ship’s velocity.
• The direction, in which the compass settles, is therefore, different to the direction of the True North and depends on ship’s course, speed and latitude of the observer.
• This error also increases as the observer’s latitude increases. The error is westward on all Northerly courses and vice-versa. In exactly E-W courses, the error is nil. In exactly N-S courses, the error is maximum.
• To compensate for steaming error, a speed rider is provided, which in association with the latitude rider, shifts the lubber line equal to steaming error in the appropriate direction.

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How is the Gyro Compass System made North Seeking?

North Seeking Gyro:-

• In order to damp unwanted oscillation, we need to achieve damping in tilt.
• This is done by means of offset slightly to the east of vertical, resulting in component of the same force producing the required torque.
• The magnitude and direction of this force is pre-calculated to achieve the required damping oscillation.
• The amplitude of each oscillation is reduced to 1/3^rd of previous oscillation.
• The spin axis reaches equilibrium and settles in a position at which drifting is counteracted by control precession & the damping precession counteracts tilting.
• Finally, the gyro settles in the meridian & becomes north seeking.

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Latitude course & speed error with respect to the Gyro Compass Explanation

Course, Speed and Latitude Error (Speed Error):-

• The gyro compass settles in the N/S direction by sensing Earth’s spinning motion.
  Same gyro compass when placed on a ship also senses the ship’s motion and therefore, the axis of gyro compass settles in a direction which is perpendicular to the resultant of the Earth’s surface speed and the ship’s velocity.
• The direction, in which the compass settles, is therefore, different to the direction of the True North and depends on ship’s course, speed and latitude of the observer.
• This error also increases as the observer’s latitude increases. The error is westward on all Northerly courses and vice-versa.
• In exactly E-W courses, the error is nil. In exactly N-S courses, the error is maximum.
• To compensate for speed error, a speed rider is provided, which in association with the latitude rider, shifts the lubber line equal to speed error in the appropriate direction.
• This error can be corrected automatically by a mechanism which moves the lubber line by an amount equal to the error, or it can be found from correction tables or from a portable correction calculator and then applied as necessary.

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Three Degrees of Freedom:

As a mechanical device a gyroscope may be defined as a system containing a heavy metal wheel or rotor, universally mounted so that it has three degrees of freedom: spinning freedom, about an axis perpendicular through its center; tilting freedom, about a horizontal axis at right angles to the spin axis; and veering freedom, about a vertical axis perpendicular to both the other axes. The three degrees of freedom are obtained by mounting the rotor in two concentrically pivoted rings, called inner and outer rings. The whole assembly is known as the gimbal system of a free or space gyroscope. The gimbal system is mounted in a frame, so that in its normal operating position, all the axes are mutually at right angles to one another and intersect at the centre of gravity of the rotor.

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Gyro Compass ‘Tangent Error’ Explanation

• On a non-pendulous gyrocompass where damping is accomplished by offsetting the point of application of the force of mercury ballistic, the angle between the local meridian and the settling position or spin axis.
• Where the offset of the point of application of mercury ballistic is to the east of the vertical axis of the gyrocompass, the settling position is to the east of the meridian in north latitudes and to the west of the meridian in south latitudes.
• The error is so named because it is approximately proportional to the tangent of the latitude in which the gyrocompass is operating.
• The tangent latitude error varies from zero at the equator to a maximum at high northern and southern latitudes.

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Starting a Gyro Compass:

• A gyro needs time to settle on the meridian, the time taken will depend on the make, model & geographical location of the gyro.
• The settling time may be between one & several hours, manual provided by the manufacturer has to be consulted before switching on the gyro.
• If compass has been switched off, it will take longer time to bring compass into use.
• Following is the procedures for Sperry MK 37 digital.
• At power-up and prior entering the settling mode, system performs automatic procedure to determine if the equipment is operating within specified parameters.
• If gyro is stationary the system opts for cold start, if rotating a hot start if programmed.
• During a cold start, if no heading data is input to system when requested the gyro selects automatic. Once the power is switched on, two bleeps prompts for heading input, if the heading data is not entered within 5 minutes, the gyro switches to an auto level process. (In some older make, the slewing is done manually, a
  special key is provided for the same which is inserted into a slot).
• If heading data is fed the rotor is automatically slewed.
• The rotor is brought up to required speed within 14 minutes and the gyro will subsequently settle within an hour.
• If heading data is not fed, the gyro will settle within 5 hrs.

Some more points:-

• If entered heading is in error by more than 20 deg, gyro may take about 5 hours to settle.
• Once gyro is settled, synchronize the repeaters (radar & ECDIS also need synchronization.)
• If speed & latitude is fed manually, it should be done prior to starting the gyro.
• Once settled, compass error should be checked & compasses should be checked more frequently.

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Emergency Position Indicating Radio Beacon (EPIRB):

• EPIRB stands for Emergency Position Indicating Radio Beacon.
• An EPIRB is meant to help rescuers locate you in an emergency situation, and these radios have saved many lives since their creation in the 1970s.
• Boaters are the main users of EPIRBs.
• A modern EPIRB is a sophisticated device that contains:
  • A 5-watt radio transmitter operating at 406 MHz (see How the Radio Spectrum Works for details on frequencies).
  • A 0.25-watt radio transmitter operating at 121.5 MHz.
• A GPS receiver once activated, both of the radios start transmitting. Approximately 24,000 miles (39,000 km) up in space, a GOES weather satellite in a geosynchronous orbit can detect the 406-MHz signal. Embedded in the signal is a unique serial number, and, if the unit is equipped with a GPS receiver, the exact location of the radio is conveyed in the signal as well. If the EPIRB is properly registered, the serial number lets the Coast Guard know who owns the EPIRB. Rescuers in planes or boats can home in on the EPIRB using either the 406-MHz or 121.5-MHz signal.

How do VTS contribute to safety of life at sea?

VESSEL TRAFFIC SERVICE (VTS):- A vessel traffic service (VTS) is a marine traffic monitoring system established by harbour or port authorities, similar to air traffic control for aircraft. Typical VTS systems use radar, closed-circuit television (CCTV), VHF radiotelephony and automatic identification system to keep track of vessel movements and provide navigational safety in a limited geographical area

SOLAS CHAPTER V – REGULATION 12 – Vessel traffic services:-

1. Vessel traffic services (VTS) contribute to safety of life at sea, safety and efficiency of navigation and protection of the marine environment, adjacent shore areas, work sites and offshore installations from possible adverse effects of maritime traffic.
2. Contracting Governments undertake to arrange for the establishment of VTS where, in their opinion, the volume of traffic or the degree of risk justifies such services.
3. Contracting Governments planning and implementing VTS shall, wherever possible, follow the guidelines developed by the Organization*. The use of VTS may only be made mandatory in sea areas within the territorial seas of a coastal State.
4. Contracting Governments shall endeavor to secure the participation in, and compliance with, the provisions of vessel traffic services by ships entitled to fly their flag.
5. Nothing in this regulation or the guidelines adopted by the Organization shall prejudice the rights and duties of Governments under international law or the legal regimes of straits used for international navigation and archipelagic sea lanes.

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Benefits of implementing a VTS

The purpose of VTS is to improve the maritime safety and efficiency of navigation, safety of life at sea and the protection of the marine environment and/or the adjacent shore area, work sites and offshore installations from possible adverse effects of maritime traffic in a given area. VTS may also have a role to play in security.

The benefits of implementing a VTS:-

• It allows identification and monitoring of vessels, strategic planning of vessel movements and provision of navigational information and navigational assistance.
• It can assist in reducing the risk of pollution and, should it occur, coordinating the pollution response. Many authorities express difficulty in establishing justifiable criteria for identifying whether VTS is the most appropriate tool to improve the safety and efficiency of navigation, safety of life and the protection of the environment.
• A VTS is generally appropriate in areas that may include any, or a combination, of the following:
  • high traffic density;
  • traffic carrying hazardous cargoes;
  • conflicting and complex navigation patterns;
  • difficult hydrographical, hydrological and meteorological elements;
  • shifting shoals and other local hazards and environmental considerations;
  • interference by vessel traffic with other waterborne activities;
  • number of casualties in an area during a specified period;
  • existing or planned vessel traffic services on adjacent waterways and the need for cooperation between neighbouring states, if appropriate;
  • narrow channels, port configuration, bridges, locks, bends and similar areas where the progress of vessels may be restricted; and
  • existing or foreseeable changes in the traffic pattern in the area.

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Objectives of VTS:

1. The purpose of vessel traffic services is to improve the safety and efficiency of navigation, safety of life at sea and the protection of the marine environment and/or the adjacent shore area, worksites and offshore installations from possible adverse effects of maritime traffic.
2. A clear distinction may need to be made between a Port or Harbour VTS and a Coastal VTS. A Port VTS is mainly concerned with vessel traffic to and from a port or harbour or harbours, while a Coastal VTS is mainly concerned with vessel traffic passing through the area. A VTS could also be a combination of both types. The type and level of service or services rendered could differ between both types of VTS; in a Port or Harbour VTS a navigational assistance service and/or a traffic organization service is usually provided for, while in a Coastal VTS usually only an information service is rendered.
3. The benefits of implementing a VTS are that it allows identification and monitoring of vessels, strategic planning of vessel movements and provision of navigational information and assistance. It can also assist in prevention of pollution and co-ordination of pollution response. The efficiency of a VTS will depend on the reliability and continuity of communications and on the ability to provide good and unambiguous information. The quality of accident prevention measures will depend on the system’s capability of detecting a developing dangerous situation and on the ability to give timely warning of such dangers.
4. The precise objective of any vessel traffic service will depend upon the particular circumstances in the VTS area and the volume and character of maritime traffic as set forth in 3.2 of these Guidelines and Criteria.

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Use of VTS in navigation:

• Automatic Identification System (AIS) is a system that makes it possible to monitor and track ships from suitably equipped ships, and shore stations. AIS transmissions consist of bursts of digital data ‘packets’ from individual stations, according to a pre-determined time sequence.
• AIS makes navigation safer by enhancing situational awareness and increases the possibility of detecting other ships, even if they are behind a bend in a channel or river or behind an island in an archipelago.
• AIS can also solve the problem inherent with radars, by detecting smaller craft, fitted with AIS, in sea and rain clutter.

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Reporting procedures of VTS and SRS:

Reporting procedures of VTS and SRS:- Standard Reporting Procedures, IMO Resolution A.851 (20) – ‘General Principles for Ship Reporting Systems and Ship Reporting Requirements’.

Types of Communication Messages and Message Markers:-

• To facilitate shore-to-ship and ship-to-shore communication in a VTS environment, one of the following eight message markers should be used to increase the probability of the purpose of the message being properly understood.
• It is at the discretion of the shore personnel or the ship’s officer whether to use one of the message markers and, if so, which marker is applicable to the situation.
• If used, the message marker is to be spoken preceding the message or the corresponding part of the message.
• The contents of all messages directed to a vessel should be clear; IMO Standard Marine Communication Phrases should be used where practicable.

Elements of the Ship’s Routeing System:

The objective of ships’ routeing is to “improve the safety of navigation in converging areas and in areas where the density of traffic is great or where freedom of movement of shipping is inhibited by restricted sea room, the existence of obstructions to navigation, limited depths or unfavourable meteorological conditions”. Ships routeing systems can be established to improve safety of life at sea, safety and efficiency of navigation, and/or increase the protection of the marine environment.

Elements used in traffic routeing systems include:

• Traffic separation scheme: a routeing measure aimed at the separation of opposing streams of traffic by appropriate means and by the establishment of traffic lanes.
• Traffic lane: an areas within defined limits in which one-way traffic is established, natural obstacles, including those forming separation zones, may constitute a boundary.
• Separation zone or line: a zone or line separating traffic lanes in which ships are proceeding in opposite or nearly opposite directions; or separating a traffic lane from the adjacent sea area; or separating traffic lanes designated for particular classes of ship proceeding in the same direction.
• Roundabout: a separation point or circular separation zone and a circular traffic lane within defined limits.
• Inshore traffic zone: a designated area between the landward boundary of a traffic separation scheme and the adjacent coast.
• Recommended route: a route of undefined width, for the convenience of ships in transit, which is often marked by centreline buoys.
• Deep-water route: a route within defined limits which has been accurately surveyed for clearance of sea bottom and submerged articles.
• Precautionary area: an area within defined limits where ships must navigate with particular caution and within which the direction of flow of traffic may be recommended.
• Area to be avoided: an area within defined limits in which either navigation is particularly hazardous or it is exceptionally important to avoid casualties and which should be avoided by all ships, or by certain classes of ships.

Before Implementing or starting a TSS or Vessel routeing system the below mentioned information should be collected:

1. Data about the area and problem or threat thereof:
   • Resources within are
   • Potential navigation hazard.
   • Environmental factors
2. Data about the ship traffic (e.g., vol., traffic patterns)
3. Information regarding existing measures
4. Foreseeable changes in traffic patterns
5. Information regarding incident history
6. Existing aids to navigation
7. Charts (are they up to date?)
8. IMO documents (models)

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VTS Category – Information Service:

Defined by IMO as ‘a service to ensure that essential information becomes available in time for on-board navigational decision-making’. The information service comprises broadcasts of information at fixed times or when deemed necessary by the VTS Authority or at the request of a vessel, and may include for example :

1. Reports on the position, identity and intentions of other traffic;
2. Waterway conditions;
3. Weather;
4. Navigational hazards;
5. Any other factors that may influence the vessel’s transit.

Navigational Assistance Service: Defined by IMO as ‘a service to assist on-board navigational decision-making and to monitor its effects, especially in difficult navigational or meteorological circumstance or in case of defect or deficiencies.’ There may be occasions when an increased or new risk makes it appropriate to enhance the service through the additional provision of a Navigational Assistance Service. The IMO Resolution explains the key tenets of this service as:

1. A service that is intended to assist in the navigational decision making process on board and to monitor its effects.
2. Particularly relevant to:
   • Difficult navigational circumstances;
   • Difficult meteorological conditions;
   • Vessel defects or deficiencies.
3. A service that is rendered at the specific request of a vessel or by a VTS Authority when deemed necessary.
4. A service that is provided only on specified occasions and under clearly defined circumstances.
5. The beginning and end of navigational assistance should be clearly stated by the vessel or the VTS and acknowledged by the other party.

The IALA VTS Manual indicates that Navigational Assistance Service can fall into one of two categories, depending on whether navigational information or advice is given. Navigational Assistance Service consisting only of the giving of navigational information is referred to in this guidance as Contributory. Navigational Assistance Service consisting of the giving of navigational advice as well as navigational information is referred to as Participatory. The definitions, particularly of the Participatory service, are open to interpretation and for the avoidance of doubt their meaning is refined and expanded as follows.

1. Contributory Navigational Assistance Services:

A Contributory Navigational Assistance Service is solely the provision of factual navigational information to assist the on-board decision making process.

The information is provided either in response to a specific request from a vessel or when the VTS Authority perceives that the information would be of use to the vessel.

A Contributory Navigational Assistance Service may include information on :

1. Courses and speeds made good;
2. Positions relative to fairway axis and waypoints;
3. Positions, identities and intentions of surrounding traffic;
4. Warnings of dangers.

1. Participatory Navigational Assistance Service:

In a Participatory Navigational Assistance Service, the VTS can become involved in the on-board decision making process by providing navigational advice. Through the exchange of information between vessel and VTS, an agreed course of action may emerge. However, any recommendations from the VTS must be result orientated and must not include specific instructions on courses to steer and speed through the water. As with the Contributory service, it is provided on specific request or when perceived necessary by the VTS Authority, in the interests of safety.

Dependent on the complexity of the situation and the level of risk mitigation required, consideration should be given to the following :

(1) Authorisations of operators providing the service and recording of such authorisations;

(2) The need to reflect this category of service in the On the Job Training of VTS Operators;

(3) Operator work load during Participatory Navigational Assistance Service, including other responsibilities and activities, and the number of vessels being monitored or advised;

(4) Use of a discrete frequency;

(5) Increased traffic restrictions;

(6) The requirements of the Pilotage Act 1987.

Traffic Organisation Service: Defined by IMO as ‘a service to prevent the development of dangerous maritime traffic situations and to provide for the safe and efficient movement of vessel traffic within the VTS Area.’

The provision of a Traffic Organisation Service includes a comprehensive and dedicated service, throughout the declared service period, without which the long term planning of traffic movement and developing situation would not be possible. This service is, by its nature, more comprehensive than an Information Service, the capability of which it necessarily includes.

Where the risks identified through the formal risk assessment are such that the only appropriate mitigating measure is the provision of service that monitors vessel traffic movement and enforces adherence to governing rule and regulation, a Traffic Organisation Service should be considered appropriate.

A Traffic Organisation Service is concerned with, for example :

1. Forward planning of vessel movements;
2. Congestion and dangerous situations;
3. The movement of special transports;
4. Traffic clearance systems;
5. VTS sailing plans;
6. Routes to be followed;
7. Adherence to governing rules and regulations.

Instructions given as part of a Traffic Organisation Service shall be result orientated, leaving the details of the execution to the vessel.

Rate of Turn Indicator

Introduction:

• IMO Recommendations on passage planning lay stress on controlled navigation. The passages in narrow channels or harbors are either along straight courses or along arcs of circles.
• As per SOLAS 2000 Amendment Chapter V Regulation 19.2.9, it is mandatory for ships over 50,000 GRT to have a rate of turn indicator. IMO recommends that large alteration of courses have to be planned along circular tracks with wheel over point marked.
• The Rate of Turn Indicator (ROTI) is a device which indicates the instantaneous rate at which the ship is turning. It is fitted on ship as an independent fitment integrated with the steering gear/auto pilot.

• When the wheel is turned over, the ship actually traverses along a curved track rather than performing a sharp turn about a point. It is very useful knowing the nature of this traversed path the ship takes which can help in planning:
  • The desired turn with given radius
  • Desired speed of the vessel to execute the planned turn.
  • When to apply the turn (wheel over point).

RATE OF TURN FORMULAE:

ROT = v/R

Where,

v – Speed of the vessel .

R – Radius from a fixed point around which to turn the ship.

Note: ROT is directly proportional to the speed.

ROT is inversely proportional to radius.

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Use of ROTI (Rate Of Turn Indicator):-

1. The rate of turn indicator is equipment which indicates the instantaneous rate at which the ship is turning.
2. This indicator is fed 60 to 200 pulses per minute from the steering repeater and from this input it works out the instantaneous rate of turn.
3. The dial is marked usually 0^O to 60^O on either side. As per IMO performance standard the dial should be marked not less than 0^O to 30^O per minute on either side and graduated in intervals of 1^O per minute.
4. As we know that when ship turn she actually traverses some distance round the arc of a circle and cannot execute a sharp turns about a point.
5. When ship is making a turn it precise the ship track uncertain due to her characteristic, condition, weight and UKC.
6. Therefore navigator uses the touch of ship track during the turn that is uncertain of position until the ship is steadied on the new course.
7. IMO recommends for passage planning is not only monitor the position on straight course but also on curve section of passage. This can be achieved by the technique called radius turn by the help of roti and ship’s log.

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