What is the purpose of the lubber's line on a marine compass?

• A. It marks true north on the compass
• B. It is used to obtain readings based on magnetic north by aligning with the keel
• C. It helps determine the ship's speed
• D. It indicates the horizon line

Answer: (B)

The lubber line is a fixed reference on the compass that aligns with the ship’s fore-and-aft axis. When you set the ship’s keel to match this line, you read the compass card to determine the magnetic heading the vessel is pointing toward. In short, the line provides a reference to read bearings relative to magnetic north, based on the ship’s forward direction. It’s not about marking true north, speed, or the horizon line.