Section 8: Principles of Magnetic Compass

Question 1

As per SOLAS, what are the general carriage requirements for a magnetic compass?

Answer 1

• SOLAS Chap V Reg 19.2.1
• All ships
• Magnetic compass
• Compass card within the housing independent of power supply.
• Means of correcting heading and bearings to true at all times. (Deviation table + Var on the compass rose / charts)

Ship <150 gross tonnage and passenger ships, in addition, shall be fitted with:
- A spare magnetic compass, interchangeable with the magnetic compass.

Question 2

What is meant by retentivity?

Answer 2

• A measure of how much magnetism remains in a material after the inducing force has been removed.
• Hard iron hangs on to a considerable amount of magnetism after the inducing force is removed.
• Soft iron retains little or no magnetism after the inducing force is removed.

Question 3

What is meant by coercivity?

Answer 3

• A measure of a material’s resistance to being magnetised.
• Hard iron resists being magnetised and once magnetised, holds on longer. Hence hard iron has a high retentivity and high coercivity.
• Soft iron is readily magnetised and once magnetised, does not hold long. Hence, soft iron has low retentivity and low coerctivity.

Question 4

What are the properties of ‘hard iron’?

Answer 4

• High coerctivity.
• High retentivity.

Question 5

What are the properties of ‘soft iron’?

Answer 5

• Low coerctivity.
• Low retentivity.

Question 6

How do you prepare a deviation card?

Answer 6

• Prepare the compass and azimuth ring.
• Identify a distant prominent object or well-defined transit bearings on an updated navigation chart/ECDIS.
• Locate a safe area where you can turn the vessel around and steady up on 8 points of the compass. Usually done at anchorage, using bow thruster or short kicks on the engine.
• Take bearings accurately while maintaining a set of steady headings on each of the 8 points and the average value is noted to minimise observation errors.
• The difference gives the compass error on that heading. When you take off variation from the compass error, it gives you the deviation on that heading.
• Draw up a fresh table of residual deviations, noting the exact location and orientation of all soft iron correctors and magnets in the binnacle.

Question 7

What is meant by a ‘magnetic safe distance’?

Answer 7

To eliminate interference this is the minimum distance between a magnetic compass and a magnetic or electrical equipment.

Question 8

What is the purpose of magnetic screening?

Answer 8

An enclosure made of magnetic materials which surrounds the magnetic compass to reduce external interference on the compass.

Question 9

What is the main installation error for a magnetic compass?

Answer 9

• Misalignment of the lubber line with the centre line of the ship.

Question 10

What is meant by magnetic variation?

Answer 10

• Magnetic variation / declination is the angular difference between True North and Magnetic North.
• It is due to the Earth’s magnetic field, which travels from South to North magnetic poles.
• Compass will be accurate when it aligns with the magnetic field. (N/S direction)

Question 11

What is meant by magnetic deviation?

How do you apply variation and deviation to find True heading or bearing?

Answer 11

• Compass deviation is the difference between Magnetic North and the direction in which the compass is pointing.
• TVMDC

Question 12

What are the causes of magnetic deviation?

Answer 12

Induced/ Soft magnetism:
- Vessel’s magnetic fields.
- Changes with orientation and location.

Permanent / Hard magnetism:
- Type of cargo.
- Electronic instruments.
- Machinery onboard.
- Built-in, stays constant.

The compass adjuster nullifies the effect of the unwanted magnetic fields by placing correctors (magnets and soft iron) adjacent to the compass.

Question 13

What is meant by ‘swinging the compass’ or ‘swinging the ship’?

Answer 13

• Swinging of the ship around the compass card in open waters.
• With the vessel steady on each of the eight primary compass points. The difference being actual and magnetic bearings is the deviation.
• Ship’s permanent/ hard iron magnetism are reduced or eliminated during the adjustment.
• Magnets are aligned fore and aft and athwartships to compensate for the permanent horizontal components of the ship’s magnetism.
• 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.
• Heeling error magnets compensate for the vertical component of the ship’s magnetism.

Question 14

How is deviation determined while ‘swinging the compass’ or ‘swinging the ship’? What are the precautions during this process to ensure accuracy?

Answer 14

The deviation can be determined by several methods: the sun’s azimuth or known bearings of distant objects, such as a mountain peak or lighthouse are considered the most accurate. In certain circumstances, such as poor visibility, 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.
GPS compasses are normally accurate to within a degree or so with the vessel on a steady heading but are often far less accurate on a swinging vessel. All navigation instruments, whether portable or fixed, including GPS compasses, should themselves be checked for error each time they are used for calibrating a magnetic compass.

Question 15

Which flag is to be hoisted when ‘swinging the compass’ or ‘swinging the ship’?

Answer 15

Signal flags OSCAR over QUEBEC - Denotes swinging the ship

Question 16

Why can’t the compass be swung with the ship alongside?

Answer 16

• Deviation needs to be obardsed with different ship headings.
• A possible source of magnetic interference, such as cranes, steel piles, reinforced concrete jetties, etc.

Question 17

Why does a boat compass card sometimes tilt?

Answer 17

The earth’s magnetic field travels from the Magnetic South Pole to the Magnetic North Pole. (Magnetic Dip)

In the south, the magnetic field comes up, out of the earth and in the north, it goes back down into the earth.

NH: Compass + counterweight = Prevent needle from pointing downwards.
SH: Compass + counterweight = Prevent needle from pointing upwards.

Other reasons for compass card tilt:
* Heeling error magnets (if fitted) require adjustment.
* Damaged card float chamber.
* Damaged jewel pivot.
* Card dislodged from the pivot.
* Low liquid/fluid level.

Question 18

What is meant by degaussing?

Answer 18

Degaussing is to reduce the strength or eliminate the magnetic field to create a magnetic camouflage.

Question 19

Q. What is the necessity to engage a professional, qualified compass adjuster to correct the compass?

Answer 19

A compass swing requires an understanding of the earth’s and ship’s magnetic fields and an ability to differentiate between the permanent magnetism of the ship’s hard iron and the induced magnetism of the ship’s soft iron.

Master might need to adjust, with shorebased assistance.

Question 20

How do you remove the ‘bubble’ in the compass bowl?

Answer 20

• Locate the filling screw on the side of the compass and position the filling screw upwards.
• Remove the filling screw and gently bring the trapped air bubbles to the fill hole.
• Insert the syringe and inject enough compass oil to fill it.
• Check to ensure no more trapped air bubbles before closing up the compass.

Question 21

How do you obtain the compass liquid? How do you know if the top-up liquid is compatible? What are the common ingredients of compass liquid?

Answer 21

Finding the correct liquid/fluid for the compass can be a problem. It can be one or a mixture of several ingredients. Different manufacturers use different ingredients and some are not compatible with others. Some are not compatible with the compass and can remove the paint and markings from the compass card or cause other damage. Some are oil-based, some are water/spirit-based.

The safest option is to obtain the correct liquid from the manufacturer. Unfortunately, this can be difficult. Some chandlers will stock ‘compass liquid’ but the ingredients of this are often unknown. If the required ingredients can be determined, it may be possible to obtain suitable liquid from local sources at a much cheaper rate.

To check compatibility, draw some existing liquid out of the bowl with a syringe and mix it with a small amount of the new liquid. It will often be immediately obvious if it is not compatible.

The following are some of the main types of compass liquid ingredients:

Ethyl alcohol (ethanol) / distilled water.
Isopropanol (rubbing alcohol) / distilled water.
Kerosene (paraffin oil).
Silicon oil.
Mineral oil.

Question 22

Q. Why is it necessary to swing the compass regularly?

Answer 22

Over a period of time and after certain events, the addition or removal of equipment or the carriage of magnetic cargo, such as iron ore, the vessel’s magnetic fields may change, altering the residual deviation of the compass. In some circumstances, the changes can be quite dramatic.

New steel vessels will have their compass adjusted when first commissioned. It has been known for a one or two-year-old vessel to record deviation as high as 30 to 40 degrees as the residual magnetic fields created during the building process gradually dissipate.

Navigators of seagoing vessels are required to observe and record compass errors daily whilst on passage. These observations are important not only for safe navigation but also to assist the compass adjuster in making an accurate analysis of the causes of deviation should the compass require adjustment.

National marine agencies specify that commercial vessels have their compass adjusted only by a person qualified and authorised to do so. International standards for magnetic compasses and compass adjusting are governed by the International Organization for Standardization (ISO) and the International Maritime Organization (IMO) SOLAS 74 Convention and HSC Code (for high-speed craft).

Question 23

Q. What are the requirements of the International Organization for Standardization (ISO) regarding deviation card? Are you aware of any other local regulation in this regard?

Answer 23

ISO 25862: 2009 (E): States that all SOLAS vessels should have their compass swung/adjusted and a new deviation card issued at maximum two-yearly intervals. When a new vessel is commissioned, compass deviation on any heading should be no more than 3°. Thereafter, the deviation on any heading should be 5° or less.

Vessels transiting the Panama Canal are required by the canal authorities to have had a valid compass deviation card issued within the previous 12 months.

Many maritime authorities and organisations stipulate that the magnetic compass is to be swung and adjusted annually. Prudent mariners and vessel operators will always ensure that the compass is regularly checked and properly adjusted.

Question 24

Q. What are the requirements of the Australian Uniform Shipping Laws Code (USL code) regarding compass adjustment on Non-SOLAS vessels in Australia?

Answer 24

In Australia, any Non-SOLAS vessel operating under state survey (USL Code) is required to have its magnetic compass examined and adjusted by an approved compass adjuster at maximum three-yearly intervals or maximum four-yearly intervals if operating under the recently introduced NSCV.

In addition to regular routine checking of the compass for deviation, and adjustment for survey compliance, all seagoing vessels should have their compass inspected, swung and adjusted, and a new deviation card issued when any of the following apply:

On a new vessel.
After periods of lay-up.
When a new compass is installed.
When deviation exceeds 5 degrees on any heading.
After trauma, such as lightning strike, grounding, fire, etc.
When compass performance is unsatisfactory or unreliable.
When a record of compass deviation has not been maintained.
After alterations and additions to the vessel’s structure and equipment.
After repairs involving welding, cutting, grinding, etc. which may affect the compass.
When electrical or magnetic equipment close to the compass is added, removed or altered.
When compass deviation does not appear to correspond with that shown on the deviation card.
When the validity period of the deviation card set by the national or state marine authority is due to expire.