Compass Use Guide

Why Compasses Still Matter in the GPS Age

Every modern boat has GPS, so why bother with a compass? Because electronics fail. Batteries die, screens crack, GPS signals drop, and electrical systems short out—usually at the worst possible moment. A magnetic compass requires no power, no satellites, and no software updates. It’s the ultimate backup navigation tool, and knowing how to use one could save your life.

Beyond emergencies, compasses provide instant heading information without looking away from the water. Good seamanship means understanding traditional navigation, even if you primarily rely on electronics.

Understanding Magnetic vs True North

Your compass doesn’t point to the North Pole (true north). It points to magnetic north, which is currently located in northern Canada, about 1,000 miles from the geographic pole. This difference is called variation or declination.

Variation changes depending on your location. On the U.S. East Coast, it’s about 10-15°W. On the West Coast, it’s 10-15°E. In the Midwest, it’s near zero. Navigation charts show variation with compass roses that display both magnetic and true bearings.

The Rule: Add westerly variation, subtract easterly variation when converting from true to magnetic bearings. Most recreational boaters work exclusively in magnetic bearings to avoid conversion errors.

Deviation: Your Boat’s Magnetic Personality

Metal, electronics, speakers, and electrical wiring on your boat create localized magnetic fields that deflect the compass needle. This error is called deviation, and unlike variation, it changes with your heading.

You must compensate for deviation by creating a deviation card—a table showing the correction needed on each heading. Professional compass adjusters can minimize deviation to ±2°, but residual errors still require correction.

Creating a Deviation Card

Find a location with clear visibility to a known distant landmark. Using a chart, determine the magnetic bearing from your position to that landmark. Motor your boat slowly in a circle, recording the compass reading every 30° while pointing at the landmark. The difference between the actual bearing and your compass reading is the deviation for that heading.

Record these values on a card and mount it near your compass. When steering a course, apply the deviation correction: If your deviation card says “+3°” on heading 090°, and you want to make good 090° magnetic, steer 087° on your compass.

Types of Marine Compasses

Binnacle Compass (Pedestal Mounted)
Traditional, mounted on a pedestal in the cockpit. Excellent visibility, easy adjustment, and large cards for precise reading. Ideal for sailboats and larger powerboats.

Bulkhead Compass (Flush Mounted)
Mounted on the helm console or cabin bulkhead. Common on powerboats where space is limited. Requires careful positioning for comfortable viewing.

Bracket Compass
Attaches to rails or vertical surfaces. Useful for small boats where permanent mounting isn’t practical.

Hand-Bearing Compass
Portable compass used for taking bearings to landmarks, navigation aids, or other vessels. Essential for coastal piloting and collision avoidance. Quality models have built-in sights and illumination.

Reading and Using Your Compass

Marine compasses display the full 360° circle, with north at 000°/360°, east at 090°, south at 180°, and west at 270°. The lubber line—a fixed mark on the compass housing—indicates your heading. Align the bow of your boat with the desired course shown on the compass card.

Steering a Compass Course:

1. Determine your desired magnetic course from the chart
2. Apply deviation correction from your deviation card
3. Steer to keep the corrected heading aligned with the lubber line
4. Check actual progress periodically using GPS or landmarks (current and wind cause drift)

Taking Bearings for Position Fixing

A bearing is the direction from your boat to an object. Using hand-bearing compasses or sighting over your main compass, you can determine your position through triangulation.

The Process:

1. Identify 2-3 charted landmarks or navigation aids
2. Take compass bearings to each (write them down immediately)
3. Apply variation and deviation to convert to magnetic bearings
4. Plot the reciprocal bearings on your chart from each landmark
5. Your position is where the lines intersect (or form a small triangle)

This three-bearing fix is a fundamental piloting technique that works even when GPS fails.

Compass Adjustment and Compensation

Professional compass adjusters use small magnets and soft iron correctors inside the compass housing to minimize deviation. This process involves aligning the boat on known magnetic headings and adjusting internal compensators until deviation is minimized.

DIY adjustment is possible but tricky. If deviation exceeds ±5° after installation, hire a professional. The $200-400 cost is worthwhile—an improperly adjusted compass is worse than no compass at all.

Compass Maintenance

Marine compasses require minimal maintenance but deserve attention:

• Check fluid levels: The compass card floats in dampening fluid (mineral spirits or kerosene). If bubbles appear, the seal has failed. Small bubbles may work out; large bubbles require professional repair.
• Protect from UV: Cover your compass when not in use. UV degrades the fluid and fades the card.
• Avoid magnetic items: Keep phones, tablets, speakers, tools, and other magnetic objects at least 2 feet away from the compass.
• Clean the dome: Use mild soap and water only. Solvents damage acrylic domes.
• Verify calibration: Periodically check known bearings to ensure your deviation card remains accurate. Adding electronics or moving equipment affects deviation.

Common Compass Errors and How to Avoid Them

Heeling Error (Sailboats): When heeled, compasses can show significant error due to the vertical component of the earth’s magnetic field. Some compasses include heeling magnets for correction. Alternatively, note typical heeling errors and compensate mentally.

Acceleration/Deceleration Error: Rapid speed changes cause temporary compass swings. Wait for the card to settle before reading.

Electronic Interference: VHF radios, radar, autopilots, and even LED lights create magnetic fields. Test for interference by turning equipment on/off while checking compass stability.

Practicing Compass Navigation

Don’t wait for your GPS to fail to learn compass work. Practice these skills regularly:

• Steer by compass on short trips, comparing your track on GPS afterward
• Take hand-bearing fixes in familiar waters and verify positions against GPS
• Update your deviation card annually or after installing new electronics
• Navigate short distances using only chart, compass, and time/speed calculations

The best time to discover compass problems is on a calm Sunday afternoon—not in fog, at night, or during equipment failure offshore. Master your compass now, and it will serve you reliably for decades.