How would you remove an air bubble from the liquid magnetic compass?
Answer: Invert the compass bowl in the gimbal arrangement to bring the expansion chamber uppermost. Undo the screw cap of the chamber and top up the fluid in the bowl with distilled water. This
action will fill the bowl, forcing out the air bubble.
What is the compass card made of, in a liquid magnetic compass?
Answer: Mica or melamine.
What liquids constitute the fluid inside the liquid magnetic compass?
Answer: The liquid is made up of two parts distilled water to one part alcohol (alternatively, a modern liquid compass would use Bayol a light clear oil which lubricates the pivot and reduces bearing friction).
What is the name of the cover that protects the bowl and the azimuth circle found on top of the compass bowl?
Answer: The cover, usually made of brass or fibre glass, is called the ‘helmet’. It is fitted with two hinged windows for viewing and providing access to the bowl and azimuth ring.
When correcting a magnetic compass for the average, conventional ship, what order of placing the correctors would you take?
Answer: The usual procedure would be to position:
(a) The ‘Flinders bar’ of approximately 12 in. (30.48 cm) length, into the forward, brass holder. This length would be supported by wood blocks to bring the ‘pole’ of the bar, level with the compass
needle arrangement.
(b) The soft iron spheres, (Kelvin’s balls) would be positioned midway on the side brackets and only lightly secured by the
underside bolts. The size of the spheres can vary, but their centres should be level with the magnets of the card and not
closer than 11⁄4 times the length of the longest needle.
(c) Heeling error magnets, each of a length of 9 in. (22.86 cm) are placed into the ‘bucket’ (they compensate for field ‘R’ and vertical soft iron). They also induce magnetism into the ‘Flinders bar’
and ‘spheres’ to help correct heeling error.
(d) Horizontal magnets each of 8 in. (19.32 cm) length compensate for the fore and aft and athwartships components of semi-permanent magnetism.
When positioning the ‘Flinders bar’, what is the position of the top of the bar, in relation to the magnetic needle arrangement?
Answer: The top of the bar should be positioned approximately 1/12th above the needle arrangement. This position effectively brings the pole of the Flinders bar level with the card magnets.
What do you understand by the term ‘dead beat’?
Answer: The expression indicates that the compass card is steady and not being allowed to oscillate because of the fluid around the card. A ‘dead beat’ compass is good to use as a steering compass because it is not too sensitive (unlike a dry card compass, which is difficult to steer by).
Electrical current is passed into the binnacle in order to provide power to the lamp, under the frosted, glass base of the compass bowl. Why does the electrical current not magnetically affect the
magnetism of the compass?
Answer: Power cables are always doubled, carrying direct current (DC). When positioned, they are set in opposing directions so that one electric field opposes the other, so cancelling each other out and having zero effect.
What liquid is employed in a modern liquid magnetic compass?
Answer: Bayol, which is a light clear oil. Not only will this fluid provide lubrication to the pivot but also the oily fluid makes the compass more ‘dead beat’ and reduces the risk of corrosion.
Where would you find the ‘float chamber’ on the liquid magnetic compass?
Answer: The float chamber is found in the centre of the compass card, under the centre ‘dome’. It supports the weight of the card on the pivot and reduces friction at the pivot point. Its function is to raise
the centre of gravity of the card above itself so that the card always returns to the horizontal if inclined by an external force.
You are instructed to take over as Master of a ship which has been ‘laid up’ for 3 years. What checks would you make on the compass?
Answer: I would inspect the compass and binnacle housing for overall
condition and have the ship ‘swung’ and corrected by a compass adjuster. It would be necessary to have the ship in an upright position for the swing with all derricks, cranes, lifeboats, etc., in their correct seagoing position.
The azimuth mirror would be checked for accuracy and correct alignment of the ‘lubber line’ would be ascertained. Any friction on the bearing could be noted by using a magnet at the side of the bowl,
to cause a deflection, to see if it returns to its original position without sticking. The soft iron correctors should be tested for retained magnetism and any residual magnetism detected could be removed by
annealing.
All moveable equipment near the compass position would be removed and the swing must not take place with other vessels within a threecable proximity.
When else would you consider it necessary to ‘swing’ the ship to correct the compass?
Answer: The ship would normally be swung after any of the following
circumstances:
(a) Following a collision or a grounding incident where major repairs are required to be made to the ship, which could affect
the vessels permanent and induced magnetism.
(b) In the event of a major fire on board the ship.
(c) If engaged on a long charter trading in high latitudes, i.e. ‘Hudson Bay’.
(d) After leaving the builders yard as a new ship.
(e) If loading a high capacity metallic cargo, e.g. railway lines on deck.
(f) In the event that compass errors became excessively large for no apparent reason or the compass becomes unreliable.
(g) If electrical or magnetic equipment is added or removed to/from the proximity of the magnetic compass.
(h) In the event that a period of 2 years has elapsed and the ship is without a record of deviations, or when the compass shows a
physical defect when first installed.
Where would you place in the compass ‘bucket’?
Answer: The bucket is found in the centre of the binnacle and hangs
on a chain vertically beneath the centre of the compass bowl in a tube.
It is used to hold the ‘heeling error magnets’.
The fabric casing of the binnacle is usually manufactured in either wood or fibre glass resin. What is this casing called?
Answer: The whole structural fabric of the binnacle housing is referred
to as the ‘furniture’.
A liquid magnetic compass contains a mix of alcohol and distilled water. What is the purpose of this mixture?
Answer: The inclusion of alcohol in the mix is to prevent the liquid freezing when navigating in high cold latitudes, while the purpose of the distilled water is to prevent the alcohol evaporating in the warm
middle latitudes.
The binnacle is bolted to the ship’s deck through a slewing arrangement. What coefficient correction is adjusted, by slewing the directional aspect of the binnacle in its slewing feet?
Answer: By slewing the compass in its footings the correction for an ‘Apparent A’ can be eliminated.
When carrying out maintenance on or near the magnetic compass, what paints would you avoid using and why?
Answer: Black paints should not be used on or near the compass because they have a high metallic (lead) base, which could influence magnetic effect.
What is the normal position of the ‘soft iron spheres’ (Kelvin’s balls)?
Answer: The spheres are normally placed in the centre of the track either side and equidistant from the compass bowl.
Why it is necessary to correct the magnetic compass?
Answer: The vessel has a need for an effective steering compass and correction would provide a reliable instrument for this purpose, by being provided with good, directional force.
Having joined a ship for the first time, where would you obtain information about the compass?
Answer: Information pertinent to the compass may be obtained from:
(a) the Deviation Card;
(b) the Azimuth/Deviation Record Book on the bridge;
(c) the Deck Log Book;
(d) the Maintenance Manual, and from the compass itself.
What is the ship’s multiplier?
Answer: The difference between the directional force at the ship’s position (with spheres in place) and the directional force ashore is called the ‘ships multiplier’, represented by the symbol 2. It is used to adjust weight on the vertical force instrument (VFI) needle when correcting for heeling error.
How would you find the value of the magnetic bearing of a chosen target?
Answer: Take the bearing of the fixed object when the ship is on 8 different headings (N, NE, E, SE, S, SW, W, NW). Add the values together and divide by ‘8’.
What certificates cover the magnetic compass?
Answer: The magnetic compass is covered by the Safety Equipment Certificate and is one of the navigational instruments listed in the record of inspection. Additionally, it should be noted that the Safety Ship Construction Certificate will not be issued unless the vessel is equipped with a ‘type tested compass’.
What conditions would you require for swinging the compass?
Answer: Prior to swinging the compass the ship should be noted to be in an upright condition, with all derricks/cranes, lifeboats, etc., stowed in a seagoing position. The sited position of the compass would be observed and no electrical instruments or influences are inside the
specified position of the binnacle (any electrical equipment to close could give rise to an effecting magnetic field). The foundation, retaining deck bolts, should be sighted to be firm so as to prevent any
unsolicited movement.
Weather conditions should be good and the intended area of swing should be free of magnetic anomalies and clear of traffic focal points.
The alignment of the ‘lubber line’ and the azimuth mirror should be checked for correctness, prior to commencing any swing.
The overall condition of the compass bowl would be inspected andbany air bubble eliminated. The gimbal arrangement should be seen to be free and to have clean bearings. The condition of the pivot would be checked by use of a magnet at the side to induce a movement and observed return to the heading.
The soft iron correctors should be inspected to ensure they are clear of retained magnetism, then placed correctly within the order of correction.
Where should the bucket be positioned when correcting for heeling error when the ship is at the magnetic equator?
Answer: The bucket for heeling error magnets should be at the bottom
of the tube, when correction is taking place at the magnetic equator.
When carrying out a compass swing how far away from a chosen target should the vessel be?
Answer: The distance from target should be at least 15 miles. This will reduce bearing parallax to a minimum.
What reference would you use to gain information of compass standards, and required checks and inspections?
Answer: Standards and checks on both magnetic and gyro compasses can be found in Safety of Life at Sea (Convention) (SOLAS), Chapter 5, and Annex 13.
Why would you employ the services of a compass adjuster?
Answer: To use his experience and make use of his local knowledge.
What is the purpose of the ‘vertical force instrument’
(VFI)?
Answer: To correct for heeling error with the vessel in the upright position.
Describe the method of a full compass adjustment by a compass adjuster?
Answer: Prior to boarding, level the VFI while ashore. On boarding the vessel, carry out pre-swing checks. Place the Flinders bar and position spheres in mid-bracket positions. Take bowl out and position VFI to correct for heeling error. At the swinging ground establish a suitable
target. Commence swing and correct coefficients. Swing again and check
residuals. Produce ‘deviation card’ and complete documentation.