If you have a leisure boat, you should have a magnetic compass on board. A magnetic compass consists of a capsule filled with liquid and a free-floating magnet needle resting on a pin. When the needle is not disturbed by magnetic fields, it aligns itself with the direction of the earth's magnetic field component. This means that one end of the needle points towards the north and the other end to the south.
Geographic north or magnetic north
When you navigate with a map and compass it is important to take into account that the geographical north (the North Pole) is not always the same as the magnetic north. This deviation is an angle called declination and is either east or west of the magnetic north, depending on where on earth you are.
Declination is measured in degrees east or west of the geographic north. If the angle of declination is east it means that the magnetic field is east of the North Pole. In many parts of the world the angle of declination is so small that you will not need to take it into account. However, in other parts the angle of declination is greater and you need to consider this in order to navigate correctly.
Declination changes over time
The declination is indicated on the chart so that you can take it into account when calculating your compass course. However, have in mind that Earth's magnetic field varies over time thus the declination also changes. Therefore it is important to use maps and charts that are up to date.
The compass is also affected by interference from magnetic objects on board. This is called deviation. Therefore, your marine compass should be placed far away from metal objects, antennas or windscreen wipers, for example. Silva’s marine compasses have a built-in compensator, and the user manual indicates the recommended distance from, for example, loudspeakers and other electronics.
On board your boat, you need a compass that works both in proper swells and despite vibrations and heeling. Silva's compasses have liquid-filled housings, making the needle more stable when sailing because it's suspended on a sapphire, which makes it more accurate. Keep in mind that an illuminated compass is especially important when navigating in the dark!
Whether your newly purchased boat comes with a marine compass depends on the manufacturer, but regardless, make sure you have the right compass for your boat. Vibrating motorboats require a compass with proper damping so that you can read the compass rose. A sailboat, on the other hand, needs a compass with a sturdy rose that can withstand heavy heeling. Your choice will also be influenced by the size of your boat and the location of the compass.
Larger compasses are usually recommended for larger boats but let the distance between the helmsman and the compass determine the choice of size. The compass should be well protected and positioned so that the helmsman can read it quickly and easily. Usually, compasses are placed on a bulkhead, dashboard, podium, ceiling or mast.
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