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| THE MAGNETIC COMPASS IN THE ELECTRONIC AGE |
Despite the modern tendency to rely heavily on electronic navigation aids, the magnetic compass remains a primary navigation instrument on any vessel, and continues to operate independently, in the not uncommon event of an electronics failure or malfunction.
Vessels are required to be equipped with a means of determining direction and heading, readable from the steering position and independent of any power supply. A correctly installed and adjusted magnetic compass, of a size and type suitable for the vessel, fulfills this requirement.
Other uses for a magnetic compass
GLOBAL NAVIGATION SATELITE SYSTEMS (GNSS), such as GPS, can contribute to safer and easier navigation, and can be interfaced with AIS, electronic charts (ECDIS) and radar. However, they should not be seen as magnetic compass replacements. GPS will only indicate the track of a moving vessel. In other words, where she has been and not necessarily where she is heading; and with the vessel stationary, other methods have to be employed.
GPS is currently the only fully operational GNSS. It is owned and controlled by the U.S. military. Signals are subject to periodic and regional loss and error, intentional and unintentional. Some areas of the world have problematic or no coverage.
A SATELLITE or GPS COMPASS, comprising 2 or preferably 3 antennas aligned symetrically fore and aft, will indicate the direction of the ship's head in either true or magnetic and is normally accurate to within one degree. It is however, subject to signal error and reliant on a supply of electricity. As it displays in digital form on a screen it cannot be used for taking bearings.
An electronic FLUXGATE COMPASS uses a number of electrical coils to detect magnetic north. It will also detect any other magnetic fields around it and is therefore as susceptible to deviation as the standard compass.
A GYRO COMPASS, usually fitted on larger vessels, points true north instead of magnetic north. It is nomally accurate to one or two degrees. Modern fibre optic gyro compasses are continuously corrected by computers, updated from GPS. It can take several hours for a gyro compass to operate correctly from the time it is switched on, or switched back on, after a power outage.
STATE OF THE ART TECHNOLOGY can be a great asset to the modern seafarer - when it works properly. As we all know, it sometimes doesn't, and then things can very quickly turn pear shaped. User error, due to inadequate training, fatigue, etc, can also contribute to innacuracies and misinterpretation of data. Over reliance on electronic navigation aids leads to complacency and sometimes to disaster.
In recent years, there have been numerous well documented occasions, particularly involving electronic failure whilst navigating in resticted visibilty, on which a reliable magnetic compass was the difference between continuing the voyage safely and a major marine incident. |
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| COMPASS LOCATION AND INSTALLATION |
On most large vessels the standard compass is installed on the "monkey island", i.e. above the wheelhouse. It is usually viewed from the helm via a periscope. Often, electronic repeaters are installed so that compass headings can also be viewed at various places around the wheelhouse. Being installed on the highest deck of the ship enables it to be used for taking bearings and keeps it as far away from magnetic interference as possible.
Ideally, the compass should be installed on the vessel's centre line so that deviating magnetic forces are mostly symetrical around the compass. On certain vessels, such as aircraft carriers, some fishing vessels and some modern container ships, the compass is offset, and this can create interesting challenges for compass adjusters.
On smaller vessels the compass is usually located in front of the helm position. Care should be taken to ensure the compass is installed far enough away from instruments, such as radios, speakers, engine rev counters (tachometers), etc, which produce strong magnetic fields. A few inches one way or the other can sometimes be the difference between major and minor deviation.
It should be installed so it is easily readable from the helm and also accessible for adjusting. A great many modern vessels, particularly luxury motor yachts, have not been designed with this in mind. On one particular sleek, multi-million dollar super yacht, it was found that, in order to access the integral correctors of the flush fitting compass, either the console would need to be partially demolished or the raked wheelhouse windscreen would have to be removed.
Ideally, the compass should be sited so that bearings of objects and other vessels may be taken. This is not always practicable, particularly on smaller vessels, in which case other means of taking bearings should be provided. It should not be forgotten that the compass is a valuable tool in collision avoidance.
Suffice to say, all fastenings used to install the compass should be of non-ferrous, non-magnetic material, e.g. bronze or marine grade stainless steel. |
| COMPASS DEVIATION AND CORRECTION |
All vessels have their own magnetic fields which produce a distinct 'magnetic signature'. Some of these fields are permanently built into the vessel and some are 'induced': influenced by the earth's own magnetic field, cargo carried, electronic instruments, position of machinery and equipment, etc. These magnetic fields can combine to cause the compass needle to point away, or deviate , from magnetic north. The amount of deviation can vary considerably from heading to heading.
The aim of the compass adjuster is to nullify the effect of the unwanted fields by placing special compensating magnets and soft iron correctors around the compass to enable it to operate correctly and point towards magnetic north.
Over a period of time, or after certain events, 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 launched. It is not unusual for a year old vessel to record deviation of 30 to 40 degrees as the residual magnetic fields created by the building process gradually dissipate.
In addition to regular routine checking of the compass for deviation, all sea going vessels should have their compass inspected, corrected for deviation, and a new deviation card issued, when any of the following apply:
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Checking the compass for deviation involves taking the vessel to a suitable location, turning the vessel through 360 degrees and, with the vessel steady on the primary cardinal and intercardinal compass points, comparing existing compass headings or bearings with what we know the actual magnetic headings or bearings should be. This process is known as swinging the compass.
During the process, deviation is reduced or, if possible, eliminated, using the compensating magnets and soft iron correctors. The timing of this operation is often governed by the tide, the weather and other vessels in the vicinity.
Deviation can be determined by a number of methods; the sun's azimuth or known bearings of distant objects, such as a mountain peak or lighthouse are considered most accurate. In certain circumstances, such as poor visibilty, comparisons with other navigation instruments, such as a gyro compass, are sometimes made.
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.
Serious discrepancies have been found with readings from portable GPS compasses. This is possibly due either to incorrect positioning of the antenna, which are highly vulnerable to signal interference, to external signal error, or to over-optimistic predictions of accuracy by manufacturers.
THE TIME IT TAKES TO SWING AND ADJUST THE COMPASS is influenced by the condition and accessibility of the compass and correctors, the manoeuvrability of the vessel, the complexity of the magnetic fields involved and the skill of the helmsman.
On successful completion of compass swing, a table recording any remaining residual deviation and a statement as to the good working order of the compass will be issued. A current deviation card is a legal requirement on all sea going commercial vessels.
It should be noted that the compass cannot be adjusted with the vessel alongside, as it is necessary to turn the vessel through 360 degrees, in open water, clear of other vessels, away from magnetic interferences such as cranes, steel piles, reinforced concrete jetties, etc.
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| WHY ENGAGE A PROFESSIONAL LICENSED COMPASS ADJUSTER? |
Effective correction, or compensation, of the marine compass for any deviation error found during the compass swing requires an understanding of the earth's and ship's magnetic fields and an ability to differentiate between permanent and induced magnetism.
It is necessary to recognise the effect the various magnetic fields have on the ship's compass and to have a practical knowlege of the workings of the marine compass and its correctors. Simply reducing or eliminating compass deviation on a vessel in one location can actually make it worse when the vessel travels to another location, particularly when substantial changes in latitude are involved.
Whilst amateur or DIY compass adjusting is not a completely outrageous concept on pleasure craft, it has been known to transform a relatively simple problem into a fairly complex one, particularly on steel vessels.
Most licensed compass adjusters are highly skilled technicians, professional seafarers and qualified navigators who have undertaken rigorous and comprehensive training to meet national and international standards.
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. |
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From time to time an air bubble may appear in the damping liquid in the bowl of a marine compass. This is usually a result of leakage around the seal between the glass and the bowl. Sometimes it indicates damage to the bowl or diaphram. A small bubble will not in itself affect the performance of the compass but may partially obscure the compass card. A larger bubble can have an adverse effect on performance.
Removing the bubble requires some patience as it is necessary to replace the air with liquid. Some modern, cheaper compasses are sealed units and cannot be refilled. If the compass is refillable and is leaking a lot of liquid, an attempt at repairing should be made before refilling. Often, particularly in the case of small cheaper compasses, purchasing a new compass is found to be the most economical option.
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 with 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 |
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SOLAS CHAPTER V, REGULATION 19.2.1:
"All ships irrespective of size shall have:
- a properly adjusted standard magnetic compass, or other means, independent of any power supply, to determine the ship's heading and display the reading at the main steering position.
- a pelorus or compass bearing device, or other means, independent of any power supply, to take bearings over an arc of the horizon of 360°".
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 Standarazation (ISO) and the International Maritime Organization (IMO) SOLAS 74 Convention and HSC Code (for high speed craft).
IN AUSTRALIA, a vessel under 35 metres (LOA), operating under Australian state survey (Uniform Shipping Laws Code / National Standard for Commercial Vessels) or, in Queensland only, a Commercial Certificate of Registration issued by Maritime Safety Queensland (MSQ), is required to have its magnetic compass examined and adjusted by an approved compass adjuster. Maximum intervals between examinations are not to exceed three years.
Holders of a Commonwealth of Australia / Australian Maritime Safety Authority unrestricted compass adjuster's licence are permitted to adjust the magnetic compasses of vessels of all sizes. Holders of a compass adjuster's licence, issued by an Australian state or territory, are permitted to adjust the magnetic compasses of vessels under 35 metres (LOA).
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International Convention for the Safety Of Life At Sea (SOLAS) Chapter V (International Maritime Organization)
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International Regulations for Preventing Collisions At Sea (COLREGS) (International Maritime Organization)
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National Standard for Commercial Vessels Part C Section 7 (Australia)
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Uniform Shipping Laws (USL) Code Section 13 (Australia)
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Marine Order 21 (Australia)
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Merchant Marine Circular No. 138 (Panama)
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Marine Guidance Notice (MGN) 57 (M+F) (United Kingdom)
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Maritime Rule Part 45 - Navigational Equipment (New Zealand)
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Merchant Shipping (Safety) (Navigational Equipment) Regulation 12 (Hong Kong)
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