TOPIC SIX COMPASSES, REGULATIONS AND SAFETY CONVENTIONS 6.1. Compasses Flashcards
What actions do you take Prior to Adjusting the Compass:
♣ Add compass fluid through the filling plug on the compass bowl. If an appreciable amount of compass liquid has leaked out, check the sealing gasket and filling plug for leaks.
♣ Take the compass to a place free from all magnetic influences except the earth’s magnetic field for tests of moment and sensibility. These tests involve measurements of the time of vibration and the ability of the compass card to return to a consistent reading after deflection. These tests will indicate the condition of the pivot, jewel, and magnetic strength of the compass needles.
♣ Move the spheres as close to the compass as possible and slowly rotate each sphere separately. Any appreciable deflection (2° or more) of the compass needles resulting from this rotation indicates residual magnetism in the spheres.
♣ The Flinders bar magnetization check is preferably made with the ship on an east or west compass heading. To make this check:
(a) note the compass reading with the Flinders bar in the holder;
(b) invert the Flinders bar in the holder and again note the compass reading. Any appreciable difference (2° or more) between these observed readings indicates residual magnetism in the Flinders bar.
♣ Spheres or Flinders bars which show signs of such residual magnetism should be annealed, i.e., heated to a dull red and allowed to cool slowly.
♣ Correct alignment of the lubber’s line of the compass, gyro repeater, and pelorus with the fore-and-aft line of the ship is important.
♣ Ensure the Code Flags “Oscar Quebec” are flown during compass swings. (Int’l Code of Signals)
What are the meaning of the following terms:
- Deviation
- Permanent magnetism
- Induced magnetism
- Dip
- Variation
Deviation: Deflection to the right or left of the magnetic meridian.
Permanent magnetism: A metal retains its magnetism when removed from a magnetic field.
Induced magnetism: A metal slowly loses its magnetism when removed from a magnetic field.
Dip: Angle at which the flux lines enter the earth to the horizontal.
Variation: The angular difference between the true meridian (great circle connecting the geographic poles) and the magnetic meridian (direction of the lines of magnetic flux).
(a) Describe the analysis method of correcting a compass
Calculate the co-efficients and correct by placing correcting magnets, spheres and Flinder’s Bar.
1) Head ship in a E/W direction and place Flinder’s Bar
to correct for an induced co-efficient B. (Fwd/Aft)
2) Place spheres at mid-position in bracket arms.
3) Swing the vessel through 360° and read deviations
on Cardinal and Inter-cardinal headings.
4) Analyze the co-efficients: Residual B (fwd/aft),
Permanent C (Athwartships), Residual D (Quadrant)
5) Head vessel on each Inter-cardinal heading and
adjust spheres to correct for residual D.
6) Head ship N/ S magnetic and correct heeling error
by placing permanent magnets in the heeling error
bucket.
7) Head 090° and 270° and correct for residual B by
placing fore-and-aft magnets in the binnacle.
8) Head 000° and 180° and correct for permanent C by
placing athwartships magnets in the binnacle.
9) If the co-efficient C is greater than B then correct co-
efficient C first.
10) Swing vessel through 360° and read deviations on
all cardinal and inter-cardinal headings.
11) Draw up a compass card.
(b) Describe the tentative method of correcting a compass
1) Check the Soft Iron Correctors for permanent
magnetism. (Spheres and Flinders Bar).
2) When there is a slight swell running, head the vessel
N or S by compass. As the vessel rolls, steady the
compass card as it oscillates by adjusting the
position of the heeling magnet until the oscillations
have reached the average minimum.
3) Steady the ship on an easterly or westerly heading
and adjust the fore and aft magnets to remove any
deviation.
4) Steady the ship on a northerly or southerly heading
an adjust the athwartships magnets to remove any
deviation.
5) Steady the ship on any inter-cardinal heading and
adjust the spheres to remove any deviation.
6) Adjusting the spheres may affect the deviation in (3)
or (4), so the vessel must be steadied on a heading
opposite to the one used in (3) and (4), and half of
the deviation removed by adjusting the fore and aft
and athwartships magnets for the corresponding
heading.
7) The adjustments made in (6) may have an effect on
the adjustments made on the spheres, as a result
the ship should be steadied on the opposite inter-
cardinal heading made before, and half of the
resultant deviation removed by once again moving
the spheres.
8) Swing the ship slowly through 360° and observe the
deviations for cardinal and inter-cardinal headings.
9) Draw up a deviation card.
(c) Describe the behaviour of the compass when passing from one hemisphere to another
♣ Vertical Soft Iron (VSI), such as the ship’s masts, cranes or funnels is induced by the vertical component of the earth’s force (Z). As a result the induction is maximum at the magnetic poles and zero at the equator since the vertical component Z is maximum at the magnetic poles and zero at the equator.
♣ When changing from one Hemisphere to another the polarity is reversed. A C-Rod will have a red end at the top in the Northern Hemisphere, and will be opposite in the Southern Hemisphere.
♣ From the direction that the Flux lines pass through the earth from South to North, it can be seen that a rod held vertically in the Southern Hemisphere would have lines passing from the bottom to the top in the direction of Z. Therefore, as Flux Lines always travel red to blue it stands to reason that red is on the bottom and blue is on the top.
(d) Identify possible causes of unreliable or fluctuating compasses or large deviation for certain headings and practical steps to correct these
Causes:
a) The vessel has been standing for an extended time
in dock or alongside in port.
b) The vessel may have suffered a grounding.
c) The vessel may have had structural changes or
suffered some damage to the structure/ hull.
d) The vessel may have been struck by lightning.
e) Soft Iron magnets may have become permanently
magnetized.
f) An object with residual magnetism may have been
stowed near the compass. (Tools near Binnacle,
reels of cable, small loose metal objects such as
pens or glasses.)
g) An electrical item may have been retro-fitted too
near the Binnacle. (Electric motors, loudspeakers,
searchlights, radar equipment.)
h) Heeling error has either been done incorrectly, or if
done correctly the remaining magnets have not
been placed correctly after the correction for
heeling has not been done.
i) Age of the magnetic compass, which will have an
effect on moment and sensibility.
Occasionally, the permanent magnetic effects at the location of the compass are so large that they overcome the earth’s directive force, H. This condition will not only create sluggish and unsteady sectors, but may even freeze the compass to one reading or to one quadrant, regardless of the heading of the ship.
Should the compass become so frozen, the polarity of the magnetism which must be attracting the compass needles is indicated; hence, correction may be effected simply by the application of permanent magnet correctors, in suitable quantity to neutralize this magnetism.
Whenever such adjustments are made, it would be well to have the ship placed on a heading such that the unfreezing of the compass needles will be immediately evident. For example,
a ship whose compass is frozen to a north reading would require fore-and-aft B corrector magnets with the positive ends forward in order to neutralize the existing negative pole
which attracted the compass. If made on an east heading, such an adjustment would be practically complete when the compass card was freed to indicate an east heading.
(e) Describe how and why to keep a compass in good order
a) Maintain the binnacle especially if made of wood so
that it does not rot and allow moisture to damage
the compass.
b) Cover the binnacle with canvas to help reduce
exposure to moisture, salt spray and sunlight.
c) Ensure the base is stable and maintained in a good
condition.
d) Keep mirror clean and free of moisture.
e) Ensure that spare light bulbs are on hand, and
regularly check that the 12 and 220 Volt power
supplies are operational.
f) Ensure the Spheres and Flinders Bars are kept free
from rust. However, avoid painting the Spheres as
paint (particularly black paint) contains traces of
metal which will have an effect on the magnetism.
g) Ensure the compass is swung regularly by qualified
personnel.
When Should a Compass be Swung?
a) There have been major structural changes have
been made to the vessel.
b) The ship has been I dry-dock or on an extended
voyage on the same heading, causing the
permanent magnetism to change.
c) On reaching the magnetic equator if you require
Flinders Bar data.
d) When changing the Heeling magnet position, if a
Flinders Bar is present.
e) If any correctors have been adjusted.
f) On a newly built ship during commissioning.
g) If magnetic equipment has been altered near the
binnacle.
h) At least once annually.
(f) Discuss the effects of external influences on the compass
a) An object with residual magnetism may have been
stowed near the compass. (Tools near Binnacle,
reels of cable, small loose metal objects such as
pens or glasses.)
b) An electrical item may have been retro-fitted too
near the Binnacle. (Electric motors, loudspeakers,
searchlights, radar equipment.)
The above would cause the compass to fluctuate or
be induced, creating varying deviations.
(g) Identify specific regulations as to the siting, construction and number of compasses
South Africa - Safety of Navigation Regulations
Singapore - Merchant Shipping Act Chapter 179
Merchant Shipping (Safety Convention)
Regulations Chapter 5, Regulation 12.
(h) Understand the content and method of drawing up a deviation card and a compass deviation book
Content:
♣ Name of Vessel;
♣ Name of Master;
♣ Date of Swing;
♣ Position of vessel in Latitude and Longitude;
♣ Amount, height and orientation (red or blue forward)
of permanent magnets;
♣ Distance of Spheres from centre and their
orientation;
♣ Length of Flinders Bar and position on binnacle
(forward or aft).
♣ Position of heeling magnets and orientation (red or
blue up).
♣ Residual deviation for headings plotted on a graph.
Method:
After correcting the compass, all correctors are secured in place and the vessel swung for residual deviations. The vessel is swung and steadied every 15 Degrees, and the gyro and magnetic compass headings are recorded. After applying the variation for the particular latitude, a list of deviations for every 15 degrees of heading change will remain. These deviations may either be entered on a table or a graph on the Deviation Card, along with the above-mentioned information.
