Magnetic compass
Navigation
The steering compass needs no power and shows your heading instantly, even when every electronic box has failed. It's only as good as its correction: iron, magnets and live wiring near it cause errors called deviation that change as the boat turns.
How it works
The compass card lines up with the earth's magnetic field. Variation is the difference between magnetic and true north where you are, shown on the chart's compass rose. Deviation is the extra error caused by your own boat's magnetism, and it changes with heading. Small built-in adjusting magnets (compensators) reduce deviation; what's left is written on a deviation card.
Using it
- Steer by the compass rather than GPS course over ground; COG is worked out from past positions and is unreliable when you're slow or turning.
- Allow for variation (from the chart) and deviation (from your card) when converting between compass and true courses.
- Check deviation on several headings using ranges (two charted objects in line) or bearings of distant charted objects, and write the results on a deviation card.
- Keep phones, handheld radios, speakers, tools and anything magnetic away from the compass; switched-on wiring nearby can also cause deviation.
- Read the right lubber line: a flat-card compass is read at the far side and a direct-reading compass at the near side, so a heading can look 180 degrees out.
Looking after it
- The bowl should be completely full of fluid. One maker says a bubble means repair or replacement; Bowditch notes a small leak can sometimes be topped up. Either way, have it looked at.
- Recheck deviation after adding electronics, speakers or steel near the helm, or after major work on the boat.
- Built-in compensators have limits; one maker says its compasses correct about 15 degrees. Beyond that, move the magnetic source or call a professional compass adjuster.
- Fuse the compass light; one maker says a 1-amp fuse is enough for its lights.
Troubleshooting
Compass and GPS disagree
- Some difference is normal: current and leeway set you off, and COG is poor at slow speed.
- Apply your deviation card for that heading.
- Look for anything magnetic newly placed near the compass.
Compass was fine on the bench but is wrong once installed
- Something near it is magnetic: speakers, microphones, steel, or current-carrying wires.
- Move the source if you can, then compensate with the built-in adjusters following the maker's instructions.
Bubble in the compass
- Have it repaired or replaced; the maker's service departments and authorised stations repair most compasses.
Buying a boat that has one
- Look for a deviation card and ask when it was made; if electronics have been added near the helm since, it's out of date.
- Check for bubbles, cloudy fluid and a crazed dome.
- Note what has been mounted close to the compass since it was installed.
- Many compasses can be repaired, and parts are often still available from the maker.
Sources (2)
General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.