Magnetic Compass Adjustment for Boats (Illustrated) for ocean-going vessels between 9 and 18 meters (30 to 60 feet).
Below, you’ll find the PDF GUIDE with four different illustrated methods at the bottom of this page.
Note on Terminology:
The terms compensation and calibration are often used in English in the context of magnetic compass work. Sometimes, they refer to both techniques: compass adjustment and the measurements used to establish a deviation card. Other times, they may refer only to one of these two techniques. The word calibration is also frequently used in relation to the correction of electronic compasses.
The definitions of the terms adjustment and compensation as provided by the Defense Mapping Agency Hydrographic/Topographic Center (Washington, D.C., 1980), which states:
“In this handbook, the term compass adjustment refers to any changes of permanent magnet or soft iron correctors whereby normal compass errors are reduced.
The term compass compensation refers to any change in the current supplied to compass compensating coils whereby the errors due to degaussing are reduced.”
These complex techniques, used on large steel vessels, are beyond this guide’s scope.
For adjustment, this guide only addresses corrections for permanent magnetic errors, which induce compass errors.
These are the main errors typically encountered on ocean-going vessels between 9 and 18 meters (30 to 60 feet).
Once again, the words adjustment and compensation are often mixed up, but the intended meaning is usually easy to understand from the context.
Magnetic Compass Adjustment for Boats (Illustrated): introduction
If you’re planning to sail offshore — beyond sheltered coastal waters — you should carry a properly adjusted magnetic compass.
As a result, this essential safety item is required by many national and international regulations.
Moreover, ensuring compass adjustment and keeping a current deviation card onboard reflects good seamanship
Still vital in navigation, the magnetic compass complements the sextant—explored further at easysextant.com.
It’s simple, reliable, and works without electronics—providing extra safety during electrical failure.
Despite competition from modern GPS systems, the compass remains irreplaceable for traditional navigation.
OBJECTIVE OF THIS GUIDE
To correct the deviation of the magnetic steering compass on board and to measure residual deviation errors.
Two separate guides: one presents four different adjustment (compensation) methods, and the other outlines three methods for establishing the deviation card (or curve). All are illustrated and adapted for 30- to 60-foot cruising sailboats.
Additionally, on my website easysextant.com, you’ll find a more general explanation of magnetic variation and deviation.
Here, however, we focus exclusively on the phenomenon of deviation, caused by the permanent magnetic influences on board vessels.
Note: in maritime tradition, the term “magnetic variation” is often used instead of “magnetic declination.”
Place the compass at least 50 cm from any metallic source of interference.
If a significant magnetic influence is detected, either move the interfering object or relocate the compass.
Magnetic influence follows the inverse square law: when the distance between a magnetic source and the compass is doubled, the effect on the compass is reduced to one quarter.
Electrical currents in nearby wires can disturb the compass.
The installer should twist all circuit wires into tight pairs, especially near the compass and panel.
Check your compass before setting off on a passage to ensure its errors remain within acceptable limits.
The navigator should inspect the compass to ensure the card turns freely.
In particular, have there been any changes in nearby metallic objects?
Also, is the compass properly aligned with the ship’s fore-and-aft axis?
Technical Characteristics of the Magnetic Compass
Recommended Compass Card Diameter by Vessel Size:
- Boats 16 to 30 feet (4.9 to 9.1 meters):
→ 2.75 to 3.5 inches (70 to 90 mm) diameter. - Boats 30 to 40 feet (9.1 to 12.2 meters):
→ At least 4 inches (100 mm) diameter. - Vessels over 40 feet (12.2 meters and above):
→ Minimum 5 inches (125 mm) diameter.
On small boats, provided there are no significant metallic masses on board, compass adjustment can be replaced by simply comparing the accuracy of the main compass with that of a secondary hand-bearing compass on different headings to ensure they are more or less in agreement.
This simplified method, though less rigorous than standard procedures, often provides sufficient accuracy for safe navigation without requiring complex adjustments.
In addition, the skipper should record the date, location, sea state, and individuals involved in the logbook.
In theory, a gyrocompass — almost perfectly accurate — would be the best tool for checking a magnetic compass.
But since gyrocompasses are usually only found on big ships or professionally operated vessels, this guide sticks to methods that make sense for regular cruising boats.
Prerequisites for compass adjustment
To begin with, prerequisite conditions include a calm sea without swell and, preferably, minimal tidal current. Slack tide periods are ideal.
Such ideal conditions are rare—just a few days a year—but they offer the perfect opportunity to enjoy a peaceful day at sea. Therefore, allow a full morning for compass adjustment.
All electrical equipment should be switched on to replicate the magnetic fields present under normal navigation conditions. However, it may also be useful to perform a second check with all systems turned off, in order to compare and assess the influence of onboard electronics on the compass.·
Make sure that anyone standing near the compass has emptied their pockets of any items that could create magnetic interference, such as mobile phones, watches, key chains, or knives. Also, only use non-magnetic screwdrivers for compass adjustments.
Helmsman’s Tool: The Deviation Card Explained
Accuracy depends on the quality of the compass and the care taken during adjustment. Make sure the compass’s lubber line is properly aligned with the vessel’s centerline! There should be no air bubbles in the bowl.
Even compasses that come with built-in may still require an optional adjustment unit for accurate calibration.
Because compass compensation is never flawless, a residual deviation always remains. This deviation is measured relative to the vessel’s heading during the process of establishing the deviation card.
This deviation card will then be used by the helmsmen to correct the known error and steer by the true heading.
When the compass is temporarily deflected during testing with a magnet, it should return to its original alignment within a standard minimum time—neither too quickly nor too slowly.”
Reminder: Compass adjustment is never permanent or universally reliable. Changes in onboard equipment, cargo, or structural modifications can affect deviation. It should be checked regularly and adjusted when necessary.
Note: In traditional navigation, the Compass Heading (CH) and True Heading (TH) are the most frequently used. The Magnetic Heading (MH), on the other hand, plays a crucial role during compass adjustment and measurements for creating a Deviation Card
The illustrated guide
A. Introduction
B. Adjustment of a Magnetic Steering Compass Equipped with Integrated Correctors.
B.1 Adjustment of a Magnetic Steering Compass with Integrated Correctors by Comparing the Steering Compass with the Bearing Compass on Four Magnetic Headings 90° Apart, Following the Instructions. P
B.2 Adjustment of a Magnetic Steering Compass with Integrated Correctors Using Two Bearing Lines: North/South and East/West. P
B.3 Adjustment of a Magnetic Steering Compass with Integrated Correctors by Comparing the Steering Compass to the GPS Course on Four Headings, Roughly North/South and East/West, Spaced 90° Apart.
C. Adjustment of a Magnetic Steering Compass with External Compensators.
C.1 Introduction to Adjustment Using Magnets Placed and/or Moved Outside the Compass. PC.
2 Illustrated Example of Adjustment Using Magnets Placed and/or Moved Outside the Compass.P