How they work, when they lie, and why professional bridges never trust only one
Contents
Use the links below to jump to any section:
- Why Compasses Still Matter on Modern Bridges
- The Magnetic Compass: Fundamental Principles
- Strengths of the Magnetic Compass
- Weaknesses and Limitations of the Magnetic Compass
- The Gyro Compass: Fundamental Principles
- Strengths of the Gyro Compass
- Weaknesses and Failure Modes of the Gyro Compass
- Why Ships Must Carry Both
- Gyro Repeaters, Feed Chains, and Error Propagation
- When Compasses Disagree: What That Really Means
- Professional Compass Philosophy on the Bridge
1. Why Compasses Still Matter on Modern Bridges
Despite ECDIS, GNSS, AIS, and integrated bridge systems, compasses remain the legal and practical foundation of navigation.
Every electronic position, vector, and track ultimately depends on heading reference.
If heading is wrong:
- radar vectors are wrong
- ARPA CPA/TCPA is wrong
- ECDIS course-up displays are misleading
- autopilot steering becomes dangerous
Compasses do not just tell you where you are pointing —
they tell you whether everything else can be trusted.
2. The Magnetic Compass: Fundamental Principles
The magnetic compass is the simplest and most reliable directional reference carried at sea.
It aligns itself with the Earth’s magnetic field, pointing toward magnetic north, not true north.
Key characteristics:
- entirely passive
- requires no electrical power
- responds directly to Earth’s magnetism
The compass card aligns itself along magnetic field lines, with the north-seeking end attracted toward the Earth’s magnetic north pole.
This simplicity is the magnetic compass’s greatest strength — and its greatest limitation.
3. Strengths of the Magnetic Compass
The magnetic compass is:
- independent of power supply
- immune to software failure
- unaffected by satellite loss
- physically simple and robust
From a safety perspective, it is:
- the last surviving reference in total blackout
- legally required as a standard compass
- trusted as a verification tool
In accident investigations, the phrase
“the magnetic compass was available but not consulted”
appears far more often than
“the magnetic compass was inaccurate.”
4. Weaknesses and Limitations of the Magnetic Compass
The magnetic compass has real, unavoidable limitations:
- affected by variation (Earth-based error)
- affected by deviation (ship-based error)
- sluggish in heavy seas
- affected by nearby ferrous materials
- difficult to read accurately at high speed or yaw
It also suffers near the magnetic poles, where horizontal magnetic force weakens.
The magnetic compass is honest but imperfect — it tells the truth, but not always the whole truth.
This is why correction and cross-checking are essential.
5. The Gyro Compass: Fundamental Principles
The gyro compass does not rely on magnetism.
It uses:
- a rapidly spinning gyroscope
- Earth’s rotation
- gravity and damping forces
to seek true north, not magnetic north.
Through a combination of rigidity in space and Earth rotation effects, the gyro aligns itself with the Earth’s axis of rotation.
This makes the gyro compass:
- a true north reference
- stable in most sea conditions
- ideal for feeding repeaters, radars, ECDIS, and autopilot
6. Strengths of the Gyro Compass
The gyro compass offers major operational advantages:
- indicates true heading directly
- stable and easy to read
- feeds multiple bridge systems
- integrates with modern navigation equipment
Because of this, the gyro often becomes the primary steering and reference compass on the bridge.
That convenience is also the danger.
7. Weaknesses and Failure Modes of the Gyro Compass
Gyro compasses do fail, and often subtly.
Common gyro issues include:
- latitude error
- speed error
- ballistic deflection during manoeuvres
- power supply disturbances
- incorrect settling after blackout
- misalignment after maintenance
Crucially, gyro errors often:
- develop slowly
- remain within “reasonable-looking” values
- propagate silently into all connected systems
A gyro can be wrong while appearing perfectly stable.
8. Why Ships Must Carry Both
SOLAS requires ships to carry:
- a gyro compass
- a magnetic compass
This is not redundancy for convenience — it is redundancy for error detection.
The two compasses are based on completely different physical principles.
If both agree, confidence increases.
If they disagree, investigation is mandatory.
Professional navigation does not ask:
“Which compass do I prefer?”
It asks:
“Why are these two telling me different things?”
9. Gyro Repeaters, Feed Chains, and Error Propagation
On modern ships, the gyro does not live alone.
It feeds:
- radar
- ECDIS
- ARPA
- AIS
- autopilot
- conning displays
If the gyro is wrong, everything downstream is wrong in the same direction.
This creates a dangerous illusion of agreement.
Multiple screens showing the same wrong heading do not confirm accuracy — they confirm common-source dependency.
The magnetic compass breaks that illusion.
10. When Compasses Disagree: What That Really Means
Disagreement between magnetic and gyro compasses is not unusual.
Possible causes include:
- known deviation on the magnetic compass
- changing gyro error due to latitude or speed
- recent manoeuvres
- heavy weather
The danger is not disagreement.
The danger is ignoring disagreement.
Every disagreement is a question being asked by the ship:
“Which reference is lying — and why?”
11. Professional Compass Philosophy on the Bridge
A professional bridge team treats compasses as:
- independent witnesses
- not authorities to be obeyed blindly
Key principles:
- never trust a single heading source
- cross-check regularly
- expect gyro error to change
- expect magnetic deviation to vary
- treat agreement as reassurance, not proof
Compasses are not background instruments.
They are active safety tools.
Tags
magnetic compass · gyro compass · ship navigation · heading reference · bridge watchkeeping · compass errors · maritime safety