Measuring altitudes of celestial bodies and horizontal angles between terrestrial objects vertical angles of terrestrial objects.
Principle of Sextant:
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.
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:-
Sextant Formulae
To
prove angle S = twice angle Q
a = Q + Q
Q
= a – Q
Multiplying
by 2, 2Q = 2 a – 2 Q ….. (i)
Again 2 a = 2 Q + S
(Ext.
angle = sum of interior opposite angles)
Substituting in (i), 2Q = 2Q + S – 2Q = S
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:-
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:
Earth’s Magnetic Poles
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.
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.
‘Variation’ and ‘Deviation’ of Magnetic Compass:
VARIATION:
Variation of Magnetic Compass
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:
Deviation of Magnetic Compass
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.
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.
Sketch & label a wet card Magnetic Compass:
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.
Correctors on a compass binnacle and why are they required, various corrections to be applied to magnetic compass
The Binnacle – 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.
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:
Tilt the bowl until the ‘filter hole’ comes uppermost. This hole is provided on the side of the bowl. Unscrew the stud/ screw provided.
Top up with ethyl alcohol. If this is not available, distilled water would do.
Screw the stud/ screw back into place.
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.
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.
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.
What is Dip? How compass is kept horizontal in varying latitudes?
Magnetic dip Error:-
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.
With reference to oil monitoring of bilge and tanker ballast discharges: Describe with aid of a sketch, the general arrangements of an oil monitoring system.
Oil content monitoring – Working principles & procedure for ship service systems:-
In the past, an inspection glass, fitted in the overboard discharge pipe of the oil/water separator permitted sighting of the flow. The discharge was illuminated by a light bulb fitted on the outside of the glass port opposite the viewer. The separator was shut down if there was any evidence of oil carry over, but problems with observation occurred due to poor light and accumulation of oily deposits on the inside of the glasses.
Present-day monitors are based on the same principle. However, whilst the eye can register anything from an emulsion to globules of oil a light-sensitive photo-cell detector cannot. Makers may therefore use a sampling and mixing pump to draw a representative sample with a general opaqueness more easily registered by the simple photo-cell monitor. Flow through the sampling chamber is made rapid to reduce deposit on glass lenses. They are easily removed for cleaning.
Bilge or ballast water passing through a sample chamber can be monitored by a strong light shining directly through it and on to a photo-cell (Figure 1). Light reaching the cell decreases with increasing oil content of the water. The effect of this light on the photo-cell compared with that of direct light on the reference cell to the left of the bulb, can be registered on a meter calibrated to show oil content.
Another approach is to register light scattered by oil particles dispersed in the water by the sampling pumps (Figure 2), Light reflected or scattered by any oil particles in the flow, illuminates the scattered light window. This light when compared with the source light increases to a maximum and then decreases with increasing oil content of the flow.
Fibre optic tubes are used in the device shown to convey light from the source and from the scattered light window to the photo-cell. The motor-driven rotating disc with its slot, lets each light shine alternately on the photo-cell and also, by means of switches at the periphery, causes the signals to be passed independently to a comparator device, These two methods briefly described, could be used together to improve accuracy, but they will not distinguish between oil and other particles in the flow.
Methods of checking for oil by chemical test would give better results but take too long in a situation where excess amounts require immediate shut down of the oily water separator.
Monitor for oily water using direct light (Figure-1)
Monitor based on scattered light (courtesy Sofrance) (Figure-2)
Tanker ballast:-
Sampling and monitoring equipment fitted in the pump room of a tanker can be made safe by using fibre optics to transmit light to and from the sampling chamber (Figure 3). The light source and photo-cell can be situated in the cargo control room together with the control, recording and alarm console. The sampling pump can be fitted in the pump room to keep the sampling pipe short and so minimize time delay.
For safety the drive motor is fitted in the machinery space, with the shaft passing through a gas-tight seal in the bulkhead. Oil content reading of the discharge is fed into the control computer together with discharge rate and ship’s speed to give a permanent record.
Seris Monitoring System for Tanker Ballast (Figure-3)
With reference to oil monitoring of bilge and tanker ballast discharges: State the Inputs that are recorded and output operation.
List of items to be recorded:- (Marpol Annex I)
(A) Ballasting or cleaning of oil fuel tanks
Identity of tank(s) ballasted.
Whether cleaned since they last contained oil and, if not, type of oil previously carried.
Cleaning process:
position of ship and time at the start and completion of cleaning;
identify tank(s) in which one or another method has been employed (rinsing through, steaming, cleaning with chemicals; type and quantity of chemicals used, in cubic metres);
identity of tank(s) into which cleaning water was transferred.
Ballasting:
position of ship and time at start and end of ballasting;
quantity of ballast if tanks are not cleaned, in cubic metres.
(B) Discharge of dirty ballast or cleaning water from oil fuel tanks referred to under section (A)
Identity of tank(s).
Position of ship at start of discharge.
Position of ship on completion of discharge.
Ship’s speed(s) during discharge.
Method of discharge:
through 15 ppm equipment;
to reception facilities.
Quantity discharged, in cubic metres.
(C) Collection and disposal of oil residues (sludge and other residues)
Collection of oil residues: Quantities of oil residues (sludge and other oil residues) retained on board. The quantity should be recorded weekly:* (This means that the quantity must be recorded once a week even if the voyage lasts more than one week)
Gyroscope having three degrees of freedom is called “FREE GYROSCOPE”
Free Gyroscope
Properties
of Free Gyroscope:-
Gyroscopic inertia or rigidity in space
Precession
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.
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.
Gyro Precession
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.
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 = 15O sine Azimuth * cosine Latitude
Rate of Drift in degrees per hour = 15O 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 360O and the star would again be rising.
This rate of tilting of 360O in a day is a rate of 15O 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 0O to zero when the latitude is 90O. That is the rate of tilting varies as the Cosine of the Latitude.
Rate of tilting in degrees per hour = 15O 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 360O in one sidereal day, that is at a rate of 15O 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 = 15O 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.
Tilting & Drifting of a Gyroscope
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.
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 1/3rd 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.
Latitude course & speed error with respect to the Gyro CompassExplanation
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.
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.
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.
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.
Power Card is the measurement of the variation of pressures in a cycle.
Irregularities in the shape of the diagram will show operational faults. Maximum or peak pressure may be measured to scale between the atmospheric line and the highest point on the diagram.
Compression diagram is taken in a similar manner to the power card but with the fuel shut off from the cylinder.
The height of this curve shows maximum compression pressure. If compression and expansion lines coincide, it shows that the indicator is correctly synchronized with the engine.
Reduction in height of this diagram show slow compression, which may be due to worn cylinder liner, faulty piston rings, insufficient scavenge air or leaky exhaust valve, any of the which will cause poor combustion.
Draw card or out of phase diagram is taken in a similar manner to the power card, with fuel pump engaged but with the indicator drum 90″C out of phase piston stroke. This illustrates more clearly the pressure changes during fuel combustion. Fuel timing or injector faults may be detected from its shape.
Light or Weak spring diagram is again similar to the power card and in phase with the engine, but taken with a light compression spring fitted to the indicator showing pressure changes during exhaust and scavenge to an enlarged scale.
It can be used to detect faults and scavenge to an enlarged scale. It can be used to detect operations.
Arrangements required for ensuring safe engineering watch when carrying dangerous cargo.
Routine Work during watchkeeping:
Soot Blowing – Soot deposits are un-burnt carbon particulates which accumulate on the gas side of the boiler. Unnecessary buildup of soot deposits impedes heat transfer and causes exhaust pipe uptake fires. Soot blowing a boiler is done once every 12 hours using pressured steam or air. Soot blowers are operated manually or driven by an electric or pneumatic motor.
Pumping Out bilges through OWS – Water and oil leaking from the pump glands and other machinery are collected in the engine room bilge. The content of the bilges are passed to the OWS to separate the oil and to ensure that the water being pumped out does not content more than 15 ppm of oil. Each operation must be recorded and completed operation must be signed by the Engineer in charge and Chief Engineer.
Turbocharger Blower washing – Depending on the maker’s instruction, the blower side of the turbocharger may be water washed daily to remove the deposits accumulated during operation.
Transfer of Oil from Bunker Tank to Settling Tank – Fuel is transferred from the Bunker tanks to the settling tank and water and impurities are allowed to settle. This oil is then purified using purifier before transferring it to the Service tank. All line valves must be correctly opened /closed for a smooth operation.
Draining of Air bottles – This is a very important routine operation. Water in the compressed air causes corrosion in the air bottles and the pneumatic control systems.
De-sludging of Purifier – Usually the purifier will run in auto-mode. If they are run in manual mode, then de-sludging operation must be carried out manually once in a watch to remove the sludge deposits accumulated in the bowl.
Boiler Blow down – Boiler is blown down atleast once in a day to get rid of the suspended and dissolved solids. This is done by using the upper scum valve and bottom blow-down valve.
Boiler water tests – Boiler water is tested for alkalinity, chlorides and dissolved solids once in a day by following correct procedures. Treatment chemicals are added according to the test results.
Other routine jobs include – draining of lube oil sumps off water, LO analysis, incineration, cleaning or changing of filters, etc.
Q. What is Broaching? Explain the effects with suitable sketch.
Broaching
Broaching:- when a steep following sea causes the vessel to ‘surf’ forwards controllably, the bow tends to ‘dig’ into the wave ahead, decelerating the vessel rapidly.
The forces on the stern will cause the stern to swing violently to the left or right and the vessel will come to rest broadside to the waves. A rapid “broaching” may cause a capsize.
Q) Explain why regular testing of water in Auxiliary Boilers is desirable.
Ans:- Boiler water tests are
tests to find a particular chemical compound or excess chemical compound in the
boiler water. If you do the boiler water test correctly and regularly you can
find excess compound or impurities in the boiler water and prevent following
factors:-
Prevent corrosion in the boiler feed system by maintaining the boiler water’s alkaline condition.
Prevent scale formation in the boiler tubes.
Remove dissolved gases such as oxygen from water.
Control sludge formations which prevent carry over with the steam.
Determine the amount of impurities by which we can select appropriate treatment.
Exercise careful control over boiler treatment chemicals.
Maintain and provide residual reserve of chemicals.
Improve efficiency, thus increasing boiler life and service period of boilers.
For economical operation of boiler.
Q) For each test normally carried out, state – Reasons for making the test.
Ans: – Different Boiler Water
Tests and its Purpose:
Hydrate Alkalinity – For testing the presence of hydrates.
Condensate PH – Measuring the acidity or alkalinity of the water.
Chloride ppm CI Test- measure the chlorine content or sea water presents.
Phosphate – For testing the presence of phosphate compounds. It is important to maintain minimum of 10 ppm of phosphate.
Deha/Oxygen test – For testing oxygen and Chemical content.
Neutralized Conductivity – For testing the presence of ferrous ions and metal contents.
Feed Water Hardness – For testing the presence of carbonate compounds.
Q) For each test normally carried out, state – Acceptable values for any particular type of auxiliary boiler.
Acceptable values for any particular type of auxiliary boiler
Q) Action required when measured values differ appreciably from desired values.
Ans:- When measured values differ from acceptable standards, a corrective action chart is to be referred and suitable action taken accordingly. This chart is reproduced:-
Action required when measured values differ appreciably from desired values
Q) Sketch & describe the Reverse Osmosis system for the production of drinking water on board the ships.
Principle of Reverse Osmosis – (Figure-1)
Reverse Osmosis:-
One of the methods of producing fresh water is by using the principle of Osmosis. Osmosis term is used to describe passage of pure water from one side off a semi-permeable membrane into a salt or other solution on the other, with the result the salt solution is diluted but the water remains pure.
Reverse Osmosis is a water filtration process, which makes use of semi-membrane materials. Salt water on one side of the membrane is pressurized by a pump and forced against the material. Pure water passes through but not thee salts see figure 1.
Pressure required to force the pure water through, is called osmotic pressure & requirement of osmotic pressure is higher if the salinity of salt water is higher. For production of large amounts of pure water, the membrane area must be large and it must be tough enough to withstand the pump pressure.
Also, as continuous supply of sea water is used as feed, the salinity of feed is steady, than the osmotic pressure required to force fresh water through is also steady, and steady pressure is taken from a continuously running pump, see figure 2. Semi permeable membrane may be provided by a bunch off cartridge as shown in figure 3.
Reverse Osmosis System for continuous supply of Fresh Water (Figure -2)
Cartridge used for Reverse Osmosis (Figure -3)
Q) State –
Pre-treatment used with RO equipment.
Ans:- Due to the low temperature of operation in low pressure evaporators, the fresh water produced by such a FWG may be harmful to drink as it may contain bacteria. It may not even contain the minerals that are needed. Therefore, it needs to be treated, in order for it to be potable. The treatment of fresh water can be done using the following methods:
Chlorine Sterilization: The sterilization by chlorine is recommended and it may be done by an excess dose of chlorine provides by sodium hypochlorite in a liquid form or by adding calcium chloride granules. The chlorine content may be upto 0.2 ppm, for it to be effective. The water is then de-chlorinated in a bed of activated carbon to remove the excess chlorine. Any colour, taste and odour, which were present in the water, will also be removed by the carbon.
Silver Ion Sterilization: In this method, silver ions are injected into the distillate, by means of a silver anode. This method of sterilization is effective since silver is toxic to the bacteria present in the water. Unlike chlorine, the silver ions do not evaporate. The amount of silver ion released into the water is controlled by the current and the silver ion content may be upto 0.08 ppm, for it to be effective.
Ultraviolet Light Sterilization: A temporary but immediately effective sterilization is by means of UV light. Chlorine and silver ion methods although long lasting, change the taste of the water and require efficient carbon filtration to remove the odour. UV light on the other hand does not cause any physical or chemical change in the water. This method uses UV lamps to produce short wave radiations that destroy the bacteria, viruses and other organisms in the water. This method is usually used on the discharge side of the water storage tank so that the water is sterilized immediately before use.
Sterilization by Ozone: Use of ozone for sterilization is very effective. It is an effective oxidant. However, the equipment is costly and has high running cost.
An example of treatment of fresh water produced by the FWG is shown in the flowchart below. The system may be different on different ships. The end result should however be that the produced water made available for drinking is slightly alkaline, sterilized, clear and good in taste.
Flow chart of pre-treatment of Drinking Water
Q) State – The Post-treatment necessary.
The Permeate (product water) from the above process may contain small quantities of dissolved salts and is suitable as drinking water if the salt content is below 250 ppm, well within the WHO limit.
However, if the water is to be used for boiler feed, it has to be completely free from salts and the product water may require further treatment to get rid of small quantities of salts by Ion Exchange Treatment.
