How to Swing a Compass on a Boat: A Step-by-Step Guide for Sailors

By Owen Palmer

Updated November 2026

Swinging a compass on a boat means rotating the vessel through a series of known headings and recording how far the compass reads off from true. You use that record to build a deviation table, which lets you correct compass readings and steer or plot accurately. The job takes a calm day, a clear bearing reference, and patience, and it should be repeated any time you add metal or electronics near the compass.

What You Need Before You Start

Swinging a compass is easier if you gather your tools first. Work on a day with light winds and flat water, and make sure the boat is away from steel hulls, moorings, and other metal structures.

Shopping for one? See our guide: Top 7 Boat Compass Flush Mount Options for Sailors: Buyer's Guide.

A reliable reference bearing, such as a charted range or a distant landmark whose bearing you know
A hand-bearing compass for taking sights on the reference
A notepad or deviation card and a pencil
A helper at the helm, or an autopilot set to hold a steady heading
Your boat’s current charts and a tide or current table if you are in tidal water

Step-by-Step: How to Swing a Boat Compass

Clear the deck. Remove or stow anything ferrous, including tools, cans, and loose gear. Turn off nearby electrical loads that might affect the compass, such as large winches or engine-driven gear, where practical.
Choose a reference. Pick a charted object or range with a known magnetic or true bearing. Confirm the variation for your location and year on the chart.
Steady the boat on each heading. Start on magnetic north and hold the boat as steady as you can. Let the compass settle for at least 30 seconds before reading it.
Take the reference bearing. From the centerline of the boat, take a hand-bearing sight on your reference object. Compare it to the known bearing to get the error at that heading.
Record the deviation. Note the compass heading and the difference from the known heading. Repeat this every 30 degrees, or every 15 degrees for better resolution.
Complete the circle. Continue around 360 degrees, then check your results by repeating a few headings in the opposite direction.

How to Read and Apply the Results

Each reading gives you a deviation value, shown as east or west. Positive numbers mean the compass reads too high, and negative numbers mean it reads too low, so you add or subtract accordingly. Plot your results on a deviation card or a simple table and keep it at the helm.

Compass heading	Deviation	True heading
000°	2° E	358°
090°	1° W	091°
180°	3° E	177°

If the deviation is large or changes sharply from one heading to the next, look for a nearby source of magnetic interference. Adjusting the compass with its built-in corrector magnets, where your model has them, is usually done after you confirm the source. For a deeper look at troubleshooting, see Why Is My Boat Compass Not Working? 6 Causes and Fixes.

Common Mistakes to Avoid

Swinging the compass while the boat is still rocking or moving through a wake
Forgetting to account for variation, which is separate from deviation
Leaving a steel tool, handheld radio, or phone near the compass during the swing
Reading the compass before the card has settled
Failing to re-swing after installing new electronics or a new mount

Recommended Gear

A hand-bearing compass makes accurate reference sights much easier than trying to judge bearings by eye. If you are working on a dinghy or small boat with limited space, a View on Amazon is a practical choice for taking the sightings you need.

A deviation card holder mounted near the helm keeps your table visible and protected from spray. Search for a View on Amazon to find one that fits your steering position.

FAQ

How often should I swing my boat compass?

Swing the compass once a season and any time you add, remove, or move metal or electrical equipment near it. Recheck after a collision or a lightning strike, since either can shift the deviation.

Can I swing a compass by myself?

Yes, if you use a hand-bearing compass and can hold the boat steady on each heading. An autopilot set to a fixed heading helps a lone sailor keep the boat steady long enough to take each reading.

What is the difference between variation and deviation?

Variation is the angle between true north and magnetic north, which depends on your location and is printed on charts. Deviation is the error caused by your boat’s own magnetic field and equipment, and it changes with heading.

Conclusion

A properly swung compass gives you a deviation table you can trust at the helm and on the chart table. Work calmly, clear metal from the area, and record every heading carefully. Repeat the job whenever the boat’s magnetic environment changes, and keep your results where you can reach them.

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About Owen Palmer

Owen Palmer is part of our editorial team and reviews and edits our sailing gear guides, checking product details against manufacturer specifications and owner feedback.

About the author

Owen Palmer

Owen Palmer writes reviews and buying guides for Sailing The Web.