Functions and areas of use

Get to grips with magnetic compasses, roll compensation, deviation and declination

Free turning space with magnetic needle

If you have a recreational boat, you should have a magnetic compass on board. A magnetic compass consists of a capsule filled with compass fluid and a magnetic needle with free swing space resting on a pin. Unless the needle is disturbed by magnetic fields, it points in the same direction as the component of the earth's magnetic field, that is, one end of the compass needle (the pole) points towards the north pole and the other end towards the south pole.

True or geographic north
When navigating with a map and compass, it is important to remember that true or geographic north (the North Pole) is not the same as magnetic north. The angle, or difference between geographic and magnetic north is called declination and is either east or west of the magnetic north pole, depending on where on Earth you are.

Eastern declination means that the earth's magnetic field locally points in a direction that passes east of the North Pole. Western declination means that the magnetic field passes west of the North Pole. In many parts of the world the declination is so small that there is no reason to take it into account. In other places, however, the declination is strong and must always be taken into account in order to navigate correctly.

The declination angle changes over time
The declination, or deviation, is indicated in the chart so that you can take it into account when you calculate your compass course. But keep in mind that the declination angle changes over time when the Earth's magnetic field changes, and it is therefore important to use current maps and charts.

Built-in compensator

The compass can be affected by interference from magnetic objects on board or in the surrounding environment and then give a false reading with the compass needle. This is called deviation. Therefore, your marine compass should be placed far away from, for example, metal objects, antennas or windshield wipers. Silva marine compasses have a built-in compensator that allows you to adjust the compass needle if it is disturbed by magnetic fields. The compass's manual specifies a recommended distance from, for example, speakers and other electronics.

Vibrations and heeling

On board your boat, you need a compass that works in proper sea conditions and despite vibrations and heeling. Silva's compasses have compass housings that are filled with liquid, which makes the needle more stable at sea, and because it is suspended on a sapphire, it also becomes more accurate. Remember that an illuminated compass is especially important when navigating in the dark!

The right compass for the right boat

Whether your newly purchased boat comes with a boat compass depends on the manufacturer, but regardless, you must 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 even at high speed. A sailboat instead needs a compass with a movable compass rose that can compensate for heavy heeling and still show the direction. Your choice of compass is also affected by the size of the boat, as well as where on board or where on the control panel you intend to place the compass.

The size of the compass

Larger compasses are usually recommended for larger boats, but let the distance between the helmsman or driver and the compass determine the choice of size. The compass must be placed so that the helmsman or driver can quickly and easily read the course, while it must also be placed well protected. Most often, the compass is placed on a bulkhead, the instrument panel, on a podium, on the roof or on the mast.

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