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Why You Need A Magnetic Compass

Why You Need A Magnetic Compass

A bit of history: the magnetic compass or a simple version of the compass was used during the Chinese Han Dynasty and the Song Dynasty over 2000 years ago. Since then, mariners have navigated the oceans of the world utilising this very basic instrument! It has proven to be a reliable navigational tool right up to the present day.

During the last century, other instruments have become more user-friendly, such as the gyro compass which points to true north, and recently the satellite compass, which relies on satellites for information and also points to true north. The magnetic fluxgate compass, which uses the earth’s magnetic field for direction, is also used with automatic pilots and works in a very limited magnetic field.

The gyro, the satellite and fluxgate compasses are reliant on electrical power to make them work. This, among other reasons, is where the magnetic compass has the advantage; it does not rely on an external power sauce.

The magnetic compass does have a disadvantage: it points to magnetic north, not true north. If any magnetic material or instrument is placed within an area of around 2.0 ft (60 cm) from its location, it will effect the instrument’s accuracy, depending on what the material or instrument is. On one hand, aluminium, fibreglass and wooden boats are less affected by the earth’s magnetic field passing through them and the local magnetism caused by the boats’ own magnetic effect. On the other hand, steel and reinforced concrete vessels are more liable to have greater errors to compensate for and therefore require extra compensators to correct the compass successfully.

How do you fix this problem? One has only to apply a small calculation for it to read true north. To make the magnetic compass accurate and perform properly, it has to first be placed in the fore and aft line of the boat. If it can be mounted on the centreline of the boat, it can be aligned with the bow of the vessel. This is the preferred position. If this is not possible, then the compass should be placed either side of the centreline but must be parallel to the centreline of the vessel. This alignment is most important as the compass will have a permanent error in it if it is not aligned properly in the fore and aft line of the boat.

Final adjustment to the magnetic compass can be achieved by built-in corrector magnets (compensators) being adjusted. This can be done by the owner, or preferably, by a licensed compass adjuster who will check, adjust and supply a certificate of deviations for the vessel. The only errors to be applied to the magnetic compass are (A) variation and (B) deviation. These are combined to give the total correction to obtain the true course.

Both of the above errors can be easily dealt with. Firstly, variation is depicted on the chart for your area and is reasonably constant for the Gold Coast, which is around 11 degrees “east”, which is a plus(+). If the compass has no deviation, then the magnetic course is what the compass is showing, and the true course is + 11 degrees. Simple! If, however, your compass has “known deviations” (a table of deviations for every 10 degrees of heading), this additional error is applied, plus or minus, to the variation, and then applied to the magnetic course to provide the true course.

All the above sounds very off-putting, but if the compass is compensated correctly to within 1 or 2 degrees it will not be necessary to make any mathematical calculations unless you are working off a chart. If your chart plotter is set to magnetic, then your compass should be reading close to that heading remembering that the plotter is giving “course over the ground” and your compass is giving the “direction of the boat’s head”.

What is the difference between the two, one may ask. The course over the ground is the result of the effect of wind and current that is acting on the boat, which isnot necessarily the way the bow of the boat is pointing.

One of the most common instruments used today on all types of recreational boats is the chart plotter. Unfortunately, it does not give one the direction of the boat’s fore and aft line, contrary to what most boat owners think. A chart plotter will provide the vessel’s direction over the ground or “course made good”, but this is not necessarily the direction of the boat’s head or fore and aft line. It is only the compass input of a compass that will do this, and a magnetic compass is preferred. (Unless the plotter has the ability to use an input from more than one GPS, such plotters are very expensive!)

The compass shows the “direction” of the boat’s fore and aft line at all times, even at anchor. And this is a very good reason for having a reliable magnetic compass on one’s vessel.




The cheapest and most reliable is the magnetic compass. (See picture above.) 

The normal modern boat compass can be purchased from $100 to around $1000 depending on the size and type required for the vessel for which it is intended. This is a whole lot cheaper than a gyro compass ($6000-$20,000) or the latest satellite compasses ($5000-$7000+). 

By Capt Paul Matthews