Where does the requirement to carry a magnetic compass come from?
S.I. No 312 / 2024 MS (SOLAS V Nav Eq) Regs 2024
What are the requirements of the magnetic compass?
Must be properly adjusted
Determine ships hdg at main steering position
Spare mag compass
Comp brg device / pelorus
When should you adjust the magnetic compass or carry out a full adjustment?
- On first installation
- Unreliable (high deviations)
- Ship undergoes alterations affecting it’s magnetism
- Electrical / magnetic equipment added close by
- A period of 2 years since last adjustment (may be increased if regular compass errors are kept)
Long lay up
When should you carry out a tentative adjustment?
After dry docking
If the vessel is within 50 miles of the magnetic equator
When changing service area
After adjustments for heeling error during the voyage
What would be considered high deviations
There is no number defined in the Irish M Notice but I would be taking into account the number of suspected high deviations and the amount they are out by
Who can adjust a compass
What if you can’t get a compass adjusted to adjust your compass?
A qualified individual
I.e. someone with the necessary training and experience
Contact Flag state and apply for an exemption
What is on the back of the deviation card
Ship name
Time and date of adjustment
Name of adjuster
Magnetic correctors, kelvins spheres and Flinders bar placement
Can the Master adjust the compass?
Only if he has the necessary training and there is no compass adjusted available however the compass must be adjusted again by a compass adjuster as soon as possible afterwards
The deviation card may be invalid if he is found to have no experience
If he does have enough experience he is not classed as a qualified person but can adjust a compass if no qualified person is available
What happens to the Mag Compass in higher latitudes?
It becomes less reliable as it wants to point down
It will eventually settle but can take time
What to do if there is no magnetic safe distance card on the compass
Put it as far away as possible
What to do if you find a bubble
Turn the compass upside down and top it up with water and alcohol
What is Coefficient A
How is it corrected?
Deviation caused by an unsymmetrical combination of horizontal soft iron
Constant on all headings
Not normally found on a well placed compass
To remove relocate compass
What is Apparent A and what are the causes
Mechanical issues not magnetic buf coukd look like the effects of Coefficient A
Examples include:
- compass card not centered
- N & S needles not parallel
- Lubber line not on the fore and aft line
- azimuth mirror out
What is Coefficient B
A combination of horizontal hard iron in the fore and aft line and vertical soft iron in the fore and aft line which create a fire and aft magnetic field passing through the compass
What is permanent B
How is it corrected?
Horizontal hard iron in the fore and aft line of the compass
It is corrected by placing the ship on an E or W heading and placing permanent magnets in the fore and aft line
Positive B is corrected by placing the red end forward and visa versa
What is induced B
How is it corrected?
Vertical soft iron in the fore and aft line of the compass
It is corrected by placing the ship on an E/W hdg and using the Flinders bar. For -B the Flinders bar is placed forward of the compass (opposite side to the vertical soft iron)
What is Split B
The calculation used to separate the components for Permanent B and Induced B to in the case that when going to a different latitude it was found that the Flinders bar length and fore and aft magnet placement was done incorrectly
What is Coefficient C
The combination of athwartships component of the ships magnetism on the compass. It is made up of horizontal permanent iron on the athwartships line and vertical soft iron on the athwartships line
What is Permanent C
How is it corrected?
Horizontal hard iron on the athwartships line of the compass
It is corrected by placing the ship on a North South heading and using horizontal hard magnet correctors
Red ends to stbd for a + C and red ends to port for a - C
What is Induced C?
How is it corrected?
Vertical soft iron placed on the athwartships line of the compass
It is corrected using 2 methods
1. A second Flinders bar placed on the opposite side of the vertical soft iron (port side for +C)
2. If the values of c and b are small, using the Flinders bar for induced B and slewing it slightly to counteract the combined affect of induced B and C)
What is Coefficient D
How is it corrected?
Coefficient D is a combination of horizontal soft iron in the fore and aft line of the compass or the athwartships line. It affects the headings on the quadrants.
It is corrected using the kelvins spheres generally athwartships of the compass as the compass is more affected by the transverse beams (more of them). This is + D
The ship is assumed to have positive D and the spheres are placed in the athwartships line at the mid position. The ship is then swung on the intercardinals and if the D is negative the spheres are moved out and if positive the spheres are moved in.
What is single sphere correction
If it is found that there is only a small D correction needed and both spheres are overcorrecting then the adjuster must either:
1. Replace with smaller spheres 2. Remove one sphere and use the remaining sphere to affect correction. This however
What is Coefficient E?
How is it corrected?
A combination of symmetrical horizontal soft iron lying on a 45° line passing from port bow to stbd quarter and vice versa
It generally doesn’t exist on a well placed compass but you could correct by placing the spheres on a 45° line to the compass or combining D and E and slewing the spheres appropriately
What is heeling error
How is it corrected
This is Coefficient J
It is the difference between the deviation when the ship is listed or heeled vs when it is upright and is caused by the R component of the permanent hard iron and the e and k rods of the ships soft iron being induced as the ship heels
It is corrected with hard magnets placed in a heeling bucket which needs to be raised or lowered with changes in latitude. The lactation of the heeling bucket on the chain is calculated using a Vertical force instrument for R and k rods. For e rods the ratio lamda is used by comparing the directive force next to the compass vs the directive force ashore
