What it really proves, what it does not, and why “in date” does not mean “correct”
Contents
Use the links below to jump to any section:
- What Compass Swinging Actually Is
- Why Ships Are Swung
- When a Compass Must Be Swung
- Who Carries Responsibility for Swinging
- How Compass Swinging Is Carried Out
- Deviation Cards and Their Limitations
- Acceptable Errors and Tolerances
- What Calibration Cannot Fix
- The Myth of the “Perfectly Swung” Compass
- Compass Swinging in the Age of Gyros and ECDIS
- Professional Expectations on the Bridge
1. What Compass Swinging Actually Is
Compass swinging is the controlled process of determining a ship’s magnetic deviation on a range of known headings. It does not correct the Earth’s magnetic field, and it does not make the compass “accurate” in an absolute sense.
What it does is map how the ship’s structure and equipment distort the magnetic field around the compass. The result is a deviation curve, recorded and presented so that bridge officers can apply corrections during navigation.
Swinging a compass does not remove deviation. It measures it.
2. Why Ships Are Swung
Every steel ship distorts the Earth’s magnetic field. That distortion changes with:
- heading
- loading condition
- electrical state
- structural changes
Without swinging, deviation is unknown. Unknown deviation is far more dangerous than known deviation, even if the known value is relatively large.
Compass swinging exists to replace assumption with data.
3. When a Compass Must Be Swung
Compass swinging is required whenever conditions exist that could materially affect magnetic behaviour. This includes newbuild delivery, dry docking, structural modifications, major electrical changes, collision damage, or when deviation values appear unreliable.
It is also required at periodic intervals defined by flag, class, or company procedures.
Crucially, swinging is also required when doubt exists. If bridge officers cannot reconcile compass readings with observed bearings, the compass is effectively out of calibration — regardless of certificate dates.
4. Who Carries Responsibility for Swinging
While compass adjusters or shore technicians may physically conduct the swing, responsibility does not end with them.
The Master remains responsible for ensuring:
- the compass has been swung when required
- deviation data is available and legible
- officers understand how to use it
Compass calibration is a management responsibility, not a technical curiosity.
5. How Compass Swinging Is Carried Out
In practice, a ship is taken through a series of steady headings, usually at known reference points or ranges. The observed compass heading is compared against a known true or magnetic reference, and the difference is recorded.
This is repeated through a full circle, typically every 15 or 30 degrees.
The resulting data is used to either adjust correctors to reduce deviation, or to produce a deviation card showing remaining errors.
The key operational point is that the ship must be steady. Poor helm control or environmental interference invalidates results.
6. Deviation Cards and Their Limitations
A deviation card is not a guarantee. It is a snapshot.
It reflects the ship’s magnetic condition at the time of swinging, under those specific circumstances.
Deviation cards do not account for:
- cargo changes
- temporary ferrous items
- electrical load variations
- structural aging
They assume stability. Ships rarely provide it.
This is why deviation cards must be used with judgement, not blind trust.
7. Acceptable Errors and Tolerances
Regulations allow a certain amount of residual deviation after swinging. Zero deviation is neither expected nor realistic.
What matters operationally is that deviation is:
- known
- consistent
- documented
- stable across headings
A compass with small, predictable deviation is safer than one with unknown behaviour, even if the latter appears closer to correct on a single heading.
8. What Calibration Cannot Fix
Compass swinging cannot correct:
- poor compass siting
- magnetic interference from nearby equipment
- bad bridge discipline
- failure to cross-check
- misunderstanding of variation and deviation
It is not a substitute for good watchkeeping.
A well-swung compass can still be dangerously misused.
9. The Myth of the “Perfectly Swung” Compass
One of the most persistent myths on ships is that once a compass has been swung, it can be trusted indefinitely.
This belief has led directly to incidents.
Ships are dynamic systems. Their magnetic characteristics change gradually and sometimes suddenly. Swinging reduces uncertainty — it does not eliminate it.
Professional bridges treat compass swinging as maintenance, not certification of infallibility.
10. Compass Swinging in the Age of Gyros and ECDIS
Modern bridges often regard compass swinging as a formality because the gyro is used for steering and navigation.
This is backwards thinking.
The magnetic compass exists precisely because gyros can fail, drift, or misalign. A magnetic compass that has not been properly swung is not a backup — it is decoration.
If the gyro fails at sea, the ship will fall back on the magnetic compass immediately. At that moment, the deviation card becomes operationally critical.
11. Professional Expectations on the Bridge
Professional officers understand that compass swinging:
- establishes a baseline, not certainty
- supports decision-making, not replaces it
- requires periodic validation by observation
A swung compass must still be checked using transits, bearings, and cross-references during routine navigation.
If the observed error differs materially from the deviation card, the compass is no longer “in tolerance,” regardless of paperwork.
That recognition is what separates compliance from seamanship.
Tags
compass swinging · compass calibration · deviation card · magnetic compass · bridge watchkeeping · navigation accuracy · maritime safety