A lead line is a traditional depth-sounding device: a weighted lead attached to a marked line, lowered until it reaches the seabed to give a direct depth reading.
How It Works
The line is marked at set intervals so the depth can be read directly once the lead touches bottom and the line goes slack. The bottom of the lead often has a hollow cavity filled with tallow or a similar substance (‘arming the lead’), which picks up a sample of the seabed material — sand, mud, shingle — giving additional information useful for confirming position against a chart’s bottom composition notations.
Why It’s Still Carried
Despite being centuries-old technology, a lead line remains required backup equipment on many vessels, specifically for situations where the echo sounder fails or its readings are in doubt — particularly valuable in shallow or unfamiliar waters where an accurate depth reading is genuinely safety-critical.
Lead Line vs Echo Sounder
An echo sounder gives continuous, real-time digital depth readings without requiring physical deployment, making it the standard modern tool — but it depends on functioning electronics and correct calibration. A lead line is slower and requires manual deployment but works independent of any electronic system, which is exactly why it remains valuable as a backup.
Exam Relevance
Lead line use, its role as echo sounder backup, and traditional depth-sounding methods are covered within Navigation and Ship Knowledge topics in deck officer competency exams.