MAG.COMPASS: The magnetic compass and sextant remain vital for offshore navigation. Along with paper charts and dead reckoning, they provide essential redundancy in case modern systems fail.
On this page, you will find a description of the compass used on ocean-going sailing vessels.
A dedicated page on compass error explains both magnetic variation and deviation.
Another page guides you through creating a deviation card or curve, with a free illustrated PDF guide presenting three different methods.
Finally, the last page focuses on correcting or adjusting the compass using integrated deviation rods or external magnets, and also includes a free illustrated PDF guide with four different adjustment methods.
Mag.Compass: the simplified description of a magnetic compass:
In addition to the use of damping liquids, there are numerous ways to stabilize the compass rose card or the compass itself, including gimbal suspension and systems of weights and counterweights.
In fact, the lubber line is a fixed mark on the compass housing that aligns with the ship’s fore-and-aft axis to show the vessel’s heading on the compass card.
The compass rose:
1 compass point = 11.25°
2 compass points = 22.5°
4 compass points = 45° (1/8 of a circle)
8 compass points = 90° (1/4 of a circle)
Different magnetic compasses:
In the first case, the magnetic needle moves in front of a graduated circle, whereas in the second case, a dial attached to the magnetic needle moves in front of a fixed reference mark.
First case:
Here the magnetic needle moves in front of a graduated circle.
If the compass is unaffected by external magnetic fields other than the Earth’s magnetic field, it will point to magnetic north.
Second case:
When the needle is fixed to the compass card, it does not move relative to the vessel. In reality, it is the vessel that turns beneath the card, giving the impression that the needle is moving.
Magnetic steering compass
Fixed or removable, it is used to determine the boat’s heading.
It can be mounted on a horizontal or vertical plane, on a bracket, a stand, or a mast.
Manufacturers design them for sailboats to handle heel and for motorboats to resist vibrations.
Of course, there are many types of magnetic compasses available, ranging from inexpensive to high-end models.
However, it’s important to remember that compass deviation is not caused by the quality or price of the compass itself.
Instead, deviation arises from magnetic influences within the vessel, and this error varies depending on the ship’s heading.
That’s why you cannot buy a pre-adjusted (compensated) compass—every compass must be adjusted on board the specific vessel where it will be used.
The hand-bearing compass:
In particular, the hand-bearing compass allows navigators to measure angles relative to landmarks such as lighthouses, buoys, and buildings.
It is also used to prevent collisions and serves as a secondary compass, allowing comparisons with readings from the steering compass.
Mag.compass: The Importance of the Bearing Compass
Although the bearing compass was long used to determine a position by taking bearings on two or more fixed points (such as a lighthouse, buoy, or peak) and plotting them on a nautical chart to draw intersecting lines, the advent of GPS and electronic charts has largely replaced this instrument for such tasks.
However, the bearing compass remains essential for assessing collision risks, particularly when crossing paths with another vessel.
* COLREG, short for the International Regulations for Preventing Collisions at Sea, is an international maritime convention established by the International Maritime Organization (IMO). These rules provide guidance to ships and other vessels on how to avoid collisions and navigate safely in all waters.
Furthermore, bearing compasses play a crucial role in verifying and correcting deviations in the magnetic compass.
Mag.compass : compliance with ISO and SOLAS standards:
The compass manufacturer can confirm whether it meets the standards:
ISO: Ensures high manufacturing standards (quality, accuracy, durability).
SOLAS: Ensures proper use and maintenance for maritime safety.
In summary, ISO pertains to manufacturing, while SOLAS ensures safety at sea.
Mag compass: Size of the Compass Rose:
1 Boats from 5 to 9 meters: Diameter of 70 to 90 mm.
2 Boats from 9 to 12 meters: Minimum diameter of 100 mm.
3 Boats over 12 meters: Compass rose of at least 130 mm.
A Boats from 16 to 29 feet: Diameter of 2.75 to 3.54 inches.
B Boats from 30 to 39 feet: Minimum diameter of 3.94 inches.
C Boats over 39 feet: Compass rose of at least 5.12 inches.
Mag.compass : The Lubber Line:
It is essential that the lubber line is properly aligned during compass installation, meaning its marker must be strictly parallel to the ship’s axis. This ensures there are no permanent errors in the readings.
If the compass is installed on a bulkhead that is not perfectly perpendicular to the boat’s central axis, a compensation block must be used.
Mag.compass : Shape of the compass rose:
1. Traditional Flat Dial:
In fact, the lubber line shows the heading at the rear of the compass and can be read from above.
2. Conical Dial with Direct Reading:
This type of dial can sometimes be mistaken for a reverse reading at 180°. For accurate reading, you must observe the front of the dial, not the back. Positioned at eye level, the compass displays the lubber line pointing toward the direction you are facing.
3. Combination of the Two Previous Types:
This versatile version allows the lubber line to be read from either the front or the back of the compass.
Mounting Types:
Flush-mounted (horizontal or vertical).
On a pedestal or mini-pedestal for wheel steering.
Bracket or support-mounted for greater flexibility.
Useful Options:
Anti-theft, protection, waterproofing, and UV resistance for enhanced durability.
Integrated lighting for night navigation.
Magnetic N/S and E/W Compensators:
Consequently, correcting deviations from environmental magnetic interference is essential for accurate navigation.
Magnetic Compass Zones: A Matter of Balance
Compass zoning arises from variations in the Earth’s magnetic field depending on latitude. The Earth is divided into magnetic zones because the field’s inclination changes: it is nearly horizontal near the equator (1) and almost vertical near the poles (2).
A compass designed for a specific magnetic zone has a “balanced” needle to compensate for the local magnetic inclination (or “dip”). Without this adjustment, the compass card — to which the needle is attached — could tilt excessively, causing it to touch the bottom of the compass bowl and stick. This would result in inaccurate readings. For this reason, compasses are specially balanced for equatorial, temperate, or polar zones.
