Magnetic Compass

The magnetic compass indicates a direction at sea. However there are some precautions we should take in using it because the North the magnetic compass indicates is different from Magnetic North and True North, because it is subject to:

Earth's magnetic field - which results in a difference between the geographical (true) north and the magnetic north. This is called Magnetic Variation (D);
Magnetic field of the ship - which results in a difference between the magnetic north and the north of the magnetic compass.
This is called Magnetic Deviation (δ).
The sum of the fields referred above is referred to just as Variation (V). The Variation is the difference between the geographic north and the north of the magnetic compass and can be calculated algebraically by the formula:
V = D + δ.

Magnetic Compass Compensation

For correct use of the magnetic compass this must be compensated and the respective deviation’s table elaborated.

Compensation shall be made every three years for vessels of 150 gross tonnage or more and whenever deviation exceed 5 °.

Using the Table of Deviations

The deviation’s table can be elaborated in two ways:

Direct - deviation’s shown are due to the bow of the compass;
Inverse – deviation’s are shown as a function of the magnetic bow.

The direct table is the most common and indicates us, depending on the bow to which we navigate, what value to correct in order to obtain the true bow. The examples presented are calculated according to this type of table.

Examples:

1. Our ship sails at 150, magnetic compass. From the table it is obtained that the deviation for this bow is 2°E. The magnetic bow would be 152.
2. Our ship sails at240, magnetic compass. From the table it is obtained that the deviation for this bow is of 1,5°W. The magnetic bow would be 238.5.
3. Our ship is sailing 2m south of Sesimbra on January 1, 2017. From the navigation chart of the area we obtain the value of the Magnetic Variation for the year 2017 = 2,5°W.
We find ourselves navigating at 300 magnetic compass and we want to calculate our geographic (true) bow.We can calculate the Variation by the formula described above:
V = D + δ, where V = -2,5° + 2°; V = -0,5°.
The geographic bow would be 299,5°.