Equipment Guide · Magnetic Compass, Variation & Deviation
A magnetised card on a pivot in a bowl of liquid — no power, no signal, no software — which is exactly why it survives every blackout that kills the gyro. The price is two errors: variation from the Earth (off the chart) and deviation from the ship's own steel (off the deviation card, different on every heading). Tap a step or press "Play" to follow the story from the Earth's field, through the ship's steel and the binnacle correctors, up the CDMVT ladder from compass course to true — then take the helm in the simulator and steer a buoyed channel by magnetic compass alone.
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Steering 218° by compass, the card says deviation is 3°E on this heading: east is added, so the magnetic course is 221°. The chart rose gives variation 5°W: west is subtracted, so the true course made good is 216°. Going the other way — from the planned true course to the course you order the helmsman — you walk back down the same ladder with every sign flipped. The net error here is only 2°W, but apply it the wrong way and the ship tracks 4° off the planned line: at 12 knots that walks you roughly 0.8 nm sideways every hour, which is the difference between the fairway and the bank.
Press Start to take the helm — dial the compass course so C + D + V lands on the planned true course, or get a sign backwards and watch the cross-track build.
About the simulator
The gyro toppled in a blackout and you are steering a buoyed channel on the magnetic compass alone. The plan says 090°T up the middle, limits ±1.0 nm either side. Deviation comes off the card above for the heading you actually steer; variation is what the chart gives you. Dial the compass course to steer so that C + D + V lands on the planned true course — get a sign backwards and you will be off by double the error, and a few degrees held for minutes becomes miles.
Channel limits, marked by the buoy lines — cross ±1.0 nm and you are in shoal water · within ±1° the helmsman's steering absorbs the wander · XTE grows at speed × sin(error).
The ship tracks wherever C + D + V actually points, not where you meant it to point. Deviation changes as you change the compass heading — the card row moves with you — and variation keeps its own sign regardless. Hold an error of a few degrees and the cross-track distance builds tick by tick until the buoy line goes past and the echo sounder starts counting down.
Working principle — explained in our own plain words with our own schematic from general marine knowledge (not copied from any OEM manual). Conceptual; varies by ship/model. Reviewed and verified by a Master Mariner.