Ans:- Swinging the compass or swinging the ship:-
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:
o 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.
o 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.
o 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 maneuverability 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.
Q) Draw a neat sketch of Magnetic Compass binnacle with all correctors. (Jan-20, Sept-19)
Ans:- 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 superstructures abaft the compass, the Flinders bar is fitted on the forward part of the binnacle and vice versa.
Q) With a suitable sketch explain the construction of a Wet Card compass. (March-19, Nov-18, Aug-16)
Q) Draw a neat sketch of magnetic compass bowl and label different parts of it. (May-19)
Ans:-
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:-
o 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.
o In rare cases, a bubble may develop in the wet compass bowl. This has to be removed at the earliest opportunity.
Q) What is Dip? How compass is kept horizontal in varying latitudes? (Nov16)
Ans:- 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.
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.
Q) What is Magnetic variation, why it varies and where you fill find its value? (July-18)
Ans:-
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.
Q) What are the advantages of wet compass card over dry compass card? How will you remove an Air Bubble from the compass bowl? (Nov-17, Nov-16)
Ans:- Steps to remove bubbles from magnetic compass: 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 bare occurrence and must be remedied by following the manufacturer‟s instructions. In most compasses:
Tilt the bowl until the “filter hole” comes upper most. This hole is provided on the side of the bowl.
Unscrew the stud/screw provide.
Top up with ethyl alcohol. If this is not available distilled water would be added.
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.
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