Magnetic compass: The coefficients are the mathematical ... - Rajnav

Magnetic compass:
The coefficients are the mathematical representation of the deviation caused in the compass due to permanent
and induced magnetism.
Coefficient A:
Real A and Apparent:
Real A is the deviation caused by the Magnetism induced in the unsymmetrical horizontal soft iron.
Correction: Not corrected and allowed for like gyro error.
Apparent A
The deviation caused due to various physical factors like:
• The magnetic axis of the needle not aligned on the north south line of the card,
• The lubber line not on the centre line of the vessel
• The compass not set in the F/A line and on the F/A line
• Variation application error
• Calculation error when calculating the LHA of the sun.
• Prismatic errors by the observer
Correction: Not corrected and generally allowed for. In case of lubber line misalignment shift the lubber
line.
Coefficient B: the algebraic sum of Bi and Bp.
Induced B (Bi)
Caused by magnetism induced in the vertical soft iron components around the compass
Varies with the latitude as the vertical component of the earths field
Maximum at the poles and minimum at the equator.
Changes signs with the hemisphere
Deviation is Maximum on the East west courses.
Deviation is nil on N/S course
Correction:
Corrected by using the Flinders bar either forward or aft of the compass (depends on the ship construction)
The height is adjusted using wooden supports to place the pole in line with pole of magnetic needle.

Permanent B (Bp)
Caused by the permanent magnetism displayed by the vessels permanent structure in the Fore and Aft direction
Polarity depends on the direction where the vessel was headed when being built.
Remains permanent and does not change signs with change of hemisphere
The value remains same in all latitude
Maximum deviation is caused at the E/W headings.
Nil deviation on N/S headings
Correction:
Corrected using the permanent magnets in Fore and Aft direction (Like for like principle use and equal and
opposite pole)
An easterly deviation indicates that the ship has a blue pole attracting the red needle (+Bp) hence this is
corrected by using a Fore and Aft magnets with red pole facing forward.
Correction can be effected by using as many magnets required to nullify the deviation.
Use of single magnets is not advisable and better correction by uniform field can be achieved by the use of a
number of magnets placed at varying distance from the needle.
The F/A correctors are placed in two lines on port and starboard side due to the presence of the Heeling error
magnet bucket in the centre ( in order to maintain symmetry on both sides
The corrector magnets are not to be placed nearer than twice their length to enable uniform magnetic field.
Correction procedure for B
Split the components in to Bp and Bi.
Following methods can be used:
Magnetic equator method:
This is the easiest and practical method and can be carried out every time the vessel is in magnetic equator.
At the equator the Bi is nil since the vertical component of the earth’s field does not exist and all deviation of B
is due to Bp only.
Take a magnetic bearing by observing a distant object (15-16 miles) and obtain bearings on 8 cardinal points
and average the values to obtain the magnetic bearing. Take a bearing on East and west and determine the value
of Bp. Head the ship on the east or west direction and adjust the position /number of the Fore aft and magnets to
nullify the deviation.
Once the vessel is back in higher latitudes, Repeat the above procedure to eliminate and calculate the deviation
due to Bi but eliminate the deviation by adjusting the Flinder bar’s position and numbers (Having corrected the
Bp at the equator, all the deviation being caused at the East/West headings is due to Bi only)

Compass adjuster method:
With his experience on similar vessel the adjuster allots a value of deviation for Bi and corrects eliminates that
amount if deviation caused by Bi by adjusting the position of Flinder bars. Then the Bp is eliminated by
adjusting the fore aft permanent magnet position and numbers.
Home coaster method: Since the vessel is going to be on the coast and there will be no appreciable change of
latitude and all the deviation caused on the E/W courses is corrected by the use of F/A method. Since the
Latitude is not changing the Bi will also not change and has been eliminated by the F/A method.
Two different latitude method:
The coefficient of B is noted at 2 different latitude that are widely separated. No correction or adjustment is
made to the corrector magnets. A calculation is made and then Bp and Bi values found are eliminated separately
using Flinder bars and F/A magnets
Co-efficient C:
This is the algebraic sum of Ci (Induced) and C (p).
Ci is the deviation caused due to magnetism induced in the vertical symmetrical soft iron on the port and
starboard side of the compass.
Usually Ci gets cancelled out if the compass is situated in the centre. In case where the symmetry is disturbed or
if the compass is not lying at the center the Ci can cause deviation.
Correction:
This is eliminated by the use of Flinder bar which is slewed out to cancel the effects of asymmetry.
Co-efficient Cp:
This is the deviation caused by the permanent magnetism of the ships athwart ship component. This is formed
during the ships construction.
This is maximum on north south courses
Does not change value with latitude or Hemisphere
Correction:
This deviation is corrected by the use of Athwartship permanent magnets.
An equal and opposite pole to that of the ships pole is used to nullify the deviation.
Coefficient D:
This is has 2 components the Fore / Aft and Athwartship component but the deviation is mainly caused by the
Athwart ship soft iron only. The proximity of the compass in the usual construction of vessels results in the
atwartship component causing the deviation. (ICHAS – Induced continuous horizontal athwartship soft iron).
This deviation is maximum on quadrantal courses and nil on cardinal courses. Thus it is called a quadrantal
error. Change in latitude/hemisphere does not change the net deviation since both the horizontal and athwartship
components change proportionately.
Correction:
Quadrantal error corrector sphere are used (Soft iron spheres also called the Kelvin’s sphere)
The vessels construction results in +D (Easterly deviation in NE course)

Coefficient E
This is caused due to induced magnetism in the horizontal soft iron located symmetrically about 45 deg from
the compass position.
This is corrected by the slewing of Kelvin’s spheres at their athwartship position to cancel this deviation.
Coefficient J (Heeling error)
This is the mathematical representation of the deviation caused when the vessel is heeled to port or stbd by 1
deg.
This has 4 components:
1. Deviation due to Vertical component of the ships permanent magnetic field. Maximum deviation in the north
and south courses and does not change with latitude or hemisphere. (Corrected by Heeling error magnets)
2. Deviation due to Induction of vertical soft iron component below the compass. Maximum on North and
South courses but changes in strength and polarity with latitude and Hemisphere (Corrected by Heeling error
magnets)
3. Vertical induction of Horizontal athwartship soft iron as the vessel rolls. Changes in polarity with the
hemisphere. In southern hemisphere it will be low sided deviation and in the northern hemisphere it will be high
sided deviation. This is corrected by the presence of Kelvin’s spheres. Maximum deviation on North and South
courses
4. Horizontal induction of Fore and Aft soft iron, terminating below the compass position. Maximum on North
south courses and nil on East/West courses.
Heeling error is corrected by the use of Vertical permanent magnets Heeling error magnets in a bucket placed
below the compass.
The bucket is attached to a chain and its position can be adjusted by adjusting the chain. The bucket has holes to
vary the number of magnets.
At the equator the bucket is placed at its lowest position and farthest from the compass. As the latitude increase
the bucket is placed closer to the compass by adjusting the bucket.

3.
Magnetic bearing of an object is found out by taking bearings of a chosen object from all cardinals adding all
the values and then dividing by the number of cardinals.
4.
The object chosen for swinging should be at least 15 miles (to avoid parallalax error)
5.
The heeling error bucket is in the way and hence in order to maintain symmetry the F/A correctors are placed on
both sides for Bp correction.
6. Two correctors at bottom are preferable to one single corrector at the top in order to avoid parallax error and
also to produce uniform field.
7.
This is a combination of alcohol and distilled or Bayol. The alcohol is used to prevent the water from freezing
and the water is added to prevent the alcohol from evaporating. In case of Bayol it is a special liquid and has the
R.D to give horizontal stability to the card movement and also the buoyancy for the card itself to reduce friction
at the pivot point.
8.
One magnetic compass is required and one more if there is no Gyro at the steering position
9.
It should be kept safely stowed where it is away from magnetic influence and any shock.
10.
The following information is available from the deviation card:
Size, position and number of all correctors
Deviation curve
Name of the vessel
Name of the adjuster
Date and place of adjustment of compass
11.
The efficiency of the compass can be ascertained by the following:
The value of the deviation after correction should be low and symmetrical on all headings
The directional force especially at higher latitudes
Steady movement on all headings
12.
Terminating material is those ships material that can produce magnetic poles above or below the compass in
any direction
13.

When the new vessel has just been delivered from the yard
After one voyage of the new vessel where the magnetic characteristics have settled down
When in an old vessel the deviations begin to be large and unsymmetrical etc
At least once every two a year to check the total deviation on all headings
When the vessel has been laid upon one heading for a long time
When considerable alteration has been done to the ships steel work
When the vessel has suffered severe force by way of collision or contact with structures or the vessel was on
fire or struck by lightening
When the ship has loaded magnetic cargo
14.
The Coefficient B has to be adjusted since it has not been split correctly.
15.
Large yaw appearing on North/south course is due to Heeling error. Correct it with the heeling error bucket.
(Latitude correction)
16.
The larger of the two should be corrected first
17.
At the lowest position farthest from the Compass needle
18.
The Flinder bars are offset to eliminate the Ci deviation due to unsymmetrical VSI on one side.
19.
Head the vessel in East or west heading. Check the deviation. Invert the compass and wait for a while. The
deviation should come back to the same value if it does not then the Flinder bars is exhibiting hard iron
properties and needs to be annealed to bring back its soft iron properties.
20.
Head on quadrantal course and check the deviation. Then Slew the spheres through 90 degrees and 180 degrees
and wait for some time after which recheck the deviation if not same then the spheres have lost their soft iron
properties and needs to be annealed.
21.
The process of annealing removes the permanent magnetism and returns them to their original state of soft iron
(ensure to cool them slowly in air)

22.
The vessel should be in sea going condition with all her equipments in place and secured (Cranes etc)
The vessel should be upright and possible on even keel
Any unnecessary metal/magnetic materials to be removed from the vicinity of the compass
Check the initial position of all correctors as per the deviation card
Check the condition of the compass (binnacles, lights, movement of card, Bubbles etc)
Ensure that a good swinging area free from traffic, magnetic anomaly, having good under keel clearance,
devoid of obstructions (magnetic materials), away from off shore structures is used
Ensure all spare correctors, binnacles etc is stowed away.
Check for apparent A error causes (Lubber line, Magnetic compass positioning, Prism errors etc)
23.
Yes as long s the new compass is in good condition.
24.
They correct the Quadrantal error (Coefficient D)
They improve the direction force of the compass and hence the readings are better
They also correct the 3 rd cause of Heeling error
25.
Measure the fore and aft line accurately using yard sticks and realign the bolts on the compass bottom to bring it
in the line and on the line
26.
The ships multiplier also referred to as λ2 and is the ratio of the ships directional force and the earth’s
directional force and is a figure little less than 1. Used for calibrating the VFI prior correcting the Heeling error.
27.
28.
Check deviation card details
Ascertain if all the details in the card and the correctors are in place.
Check the condition of the compass bowl and the glass
Check for any bubbles
Check for the free movement of the gimbals
Check for the free movement of the card by using a magnet to deflect the card
Check the Flinder bars and the spheres for any signs of Permanent magnetism
Check if the binnacle is in the F/A line and on the fore aft line
Check the accuracy of the mirro by taking one celestial bearing and one terrestrial bearing.

29.
15 miles from a conspicuous object
Free from traffic / focal point
Away from metal structures, outfalls etc
Deep water (UKC / Turning circle)
No Magnetic anomalies
30.
They are correcting for Coefficient E (Induce asymmetrical horizontal soft iron in F/Aft plane).
Procedure for compass correction
Tentative swing:
Decide on the direction of swing
Commence the swing (Tight turning circle)
Obtain magnetic bearing of a distant object at all cardinals and get the average of this value which is the
magnetic bearing.
Head East take bearing and remove all deviation using F/A correctors (Bp)
Head South take bearing and remove all deviation using Athwartship correctors (cp)
Head West take bearing and remove Half of Deviation using F/A correctors
Head North take bearing and remove Half of Deviation using athwartship correctors (cp)
Head in NE (inter cardinal) heading take a bearing and remove half of the deviation using Spheres
Head in SE take bearing and remove half of the deviation using F/A correctors (Bp)
Make a full swing and get a deviation curve and see if the deviation observed is symmetrical and of low value.
Analysis method:
Analysis of the various deviations are carried out and correctors placed. A magnetic bearing is obtained and
used as a reference to correct the deviation. If vessel’s position is exactly known the true bearing can be
obtained and taken out from the variation gives the Magnetic bearing. A true bearing of the astronomical object
should be continuously taken to ascertain the True bearing and subsequently the magnetic bearing can be
obtained by taking off the variation.
Co-efficient Bi – Flinder Bars
Flinder bar is first placed to adjust the Bi. An approximated value is corrected for 3-4 degrees when the
compass is heading east or west.
On ships with +ve Bi (usually) the Flinder bar is placed Forward of the binnacle.
In cases where there is –ve Bi (Funnel /Mast forward of the binnacle) the Flinder bar is placed aft of the
Binnacle.

Coefficient D – Kelvin’s spheres
Take a bearing on the cardinal heading (all four) and average out the magnetic bearing.
Now obtain Bearing on Inter cardinal headings and average out the magnetic bearing.
The difference in bearing is the deviation due to D. Correct it by adjusting the Spheres. Move the spheres closer
till the deviation is removed.
Heeling error correction: Co-efficient J
Obtain the value of ships multiplier and adjust the shore to ship Hf ration using the VFI (Vertical force
instrument). Use the VFI instrument and adjust the heeling error bucket position to cancel out the heeling error.
The VFI is calibrated ashore. Vessel is steadied on the east west course and the bowl removed and replaced with
the VFI in the same position as the compass needle would be. The Heeling error magnets are adjusted in
number and position of buckets adjusted so as to make the VFI needle horizontal. Thus the heeling error is
removed for that latitude.
The Larger of the B or C should be corrected first.
Permanent Bp –
Head the vessel in East west direction obtain the Bearing and compare with the magnetic bearing obtain the
deviation and using F/A magnets correct it.
Permanent Cp -
Head the vessel on North / South heading and obtain Bearing compare with magnetic bearing and obtain
deviation. Using athwartship magnets correct it.
Now corrections should be made for Ci and E and A if any. These are best left uncorrected and applied for.
Carry out a complete swing and obtain deviation card. Confirm the low value of deviation and also the
symmetry.
Record all the position of the correctors. (Number and color)
Position of carrying out the swing.
Order of placing magnetic correctors:
1. Flinder bars: These should be placed first since they will correct Bi but create a D error which has to be
corrected by the spheres.
2. The Kelvin’s spheres are to be used to correct D and they will also be correcting the 3 rd cause of Heeling
Error (Athwartship horizontal). Their presence increase the directive force hence the bearings are better
in quality and accuracy.
3. The heeling error corrector bucket for correcting (J)
4. Bp and Cp which ever of them is the largest.

Additional questions:
How to correct heeling error if noVFI available
Head the vessel on the North/South course and Heel the vessel and adjust the heeling error magnets and bucket
position to correct deviation due to heeling error.
What is the distance of the corrector from the compass needle
Not less than twice the length of the correctors in order tohave a uniform correcting magnetic filed.
Vessel heading east – Deviation is 9 degrees, you found the Blue end of the corrector in forward position,how
will you correct
This ship has a Red forward hence has been corrected by using the blue end forward. But too much of Blue
corrector so either reduce the Magnet or increase the distance to reduce the deviation by 8 degrees
Vessel heading SE- westerly deviation how to correct
Bring the quadrantal error correctors (Kelvin’s spheres) more close as they are supposed to produce +E
correction
Vessel laid up in Fleetwood for 2 months and now proceeding of westerly heading found large deviation what is
the deviation likely to be easterly or westerly?
Easterly
Why is the ship swung slowly
To avoid gaussing error