Lubber line

The lubber line is the fixed reference mark on a compass bowl, usually a vertical line on the forward inside rim, aligned with the ship’s fore-and-aft axis so the heading reads against the moving compass card. The card stays north-stable while the bowl and lubber line turn with the ship, so the figure under the line is the ship’s head. Correct alignment of the lubber line with the keel line matters: a misaligned lubber line introduces a constant heading error on every course. The same reference principle applies to the gyrocompass repeater and the radar heading marker.