The deviation table records how much your ship's compass departs from the magnetic direction on each heading — caused by iron, electronics and stray currents on board. It is the basis for turning the mwK into a steerable compass course (MgK). This guide takes you from compensating the compass to a well-kept table in NauticCalc.
Prerequisites
- Magnetic compass on board (compensatable)
- Current variation for your area (Variation)
- Quiet berth or calm sea for compensation
- A helper at the helm is useful so you can read off in peace
Step 1: Prepare compensation
Read your compass manufacturer's instructions first. You need access to the compensation screws and enough open water to run eight cardinal/intercardinal headings (0°, 45°, 90° …) cleanly. Clear away magnetic disturbers beforehand: phone, tools, cans — anything that does not normally live in this spot will distort the measurement.
Step 2: Run courses and record
Run the headings one after another and hold each course steady. For each heading:
- rwK from the chart (or from the passage plan)
- Apply variation → mwK
- Read MgK on the compass
- Deviation = difference between mwK and MgK — keep the sign (east/west or plus/minus) consistent across the whole table
Record everything straight away — on paper or directly in NauticCalc. The more evenly your boat runs, the more reliable the values.
Step 3: Maintain the table in NauticCalc
In the app under course correction / deviation table:
- Enter course ranges or individual headings
- Check the values for plausibility — on a well-compensated compass only a few degrees usually remain
- Save; the table is then used automatically in course correction
Step 4: Update regularly
Deviation changes when the magnetic environment changes. After refits (electronics, speakers, engine work) or when you notice discrepancies while under way: re-compensate or adjust the table. A date in the table helps place the current state.
Common mistakes
- Confusing variation and deviation — one comes from the earth's magnetic field, the other from the boat
- Measuring only a single heading and applying it to all
- Keeping an old table unchanged after changes on board
- Noting the sign (east/west) inconsistently
Read next: Course correction guide · Variation