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Magnetic Compass

The magnetic compass. Its importance and declination

The quality of the magnetic compass on board is very important as well as its installation. Today, the quality is not a problem. There are many brands on the market that offer different models with excellent quality and all charter sailboats or motorboats are equipped with the compasses of the best brands.

In addition to its technical qualities of sensitivity and stability, it must be very easy to read and be weather resistant, principally to withstand the high temperatures caused by the sun on long summer days.

The first mariners had to sail close to the coast, using the references provided by the landscape. Eventually they learned to look at the stars for orientation at night, but during the day and on offshore waters such reading became impossible. However, the accurate data on the boat’s location and the destination to reach were essential for the voyage completion. Thanks to the human ingenuity in the late thirteenth century appeared an instrument in the Mediterranean that would revolutionize navigation. It was the magnetic needle or magnetic compass.


Magnetic Compass on a charter yacht
The magnetic compass should be located in such a way that it is clearly visible from the steering position of a charter yacht.

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The magnetic field that exists anywhere on the globe is the result of a vertical force and a horizontal one. The vertical component acts on the compass rose’s horizontal one and makes it tilt to the North or South. The intensity of this component varies depending on the geographic location. It means that a balanced compass in Valencia won’t keep its horizontal rose in Melbourne.

The horizontal component acts on the directionality of the rose. The compass environment on board and the various metal parts create a local magnetic field that is specific to each boat and is different from the Earth’s magnetic field. The indicated North will not be the magnetic North.

The difference of the direction in degrees between compass North and magnetic North is called declination or variation and its value can be positive or negative. To decrease the declination’s value we have to choose a location for our compass which is as isolated as possible from the objects that cause magnetic fields, such as another compass, a fire extinguisher, speakers, cameras, watches, computers, mobile phones, etc.

It is important to know the values of our compass declination that must be represented by a graph curve on the variation chart and must be updated every year. The existence of the updated variation chart aboard is required if we navigate with A and B categories.

In an upcoming article we will explain how to create the graph curve on the variation chart.

Fitting it with magnets that allow the compensation is suitable.

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