mark
One of the bits of leather or coloured bunting placed upon a sounding line at intervals of from two to five fathoms. (The unmarked fathoms are called "deeps".)
mark: depth marker on a sounding line
A mark is a physical indicator fastened to a sounding line at measured intervals to help a sailor read water depth quickly without counting knots or fathoms by hand. Traditional marks were small pieces of leather, canvas, or coloured bunting sewn or tied at intervals of 2 to 5 fathoms apart. When the line touched bottom, the mark nearest the water surface told the leadsman the approximate depth in fathoms.
The marking system follows a pattern. Marks appear at regular fathom intervals, typically at 2, 3, 5, 7, 10, 13, 15, 17, and 20 fathoms, though the exact spacing varied between shipping companies and naval traditions. The unmarked intervals between marks are called deeps. A leadsman could feel or see a mark in darkness or fog, making it useful for night navigation or in poor visibility when precise sounding was critical before modern electronic depth finders.
Marks and the Wider Sounding Practice
Sounding lines were standard equipment on sailing and steam vessels until electronic sonar became reliable in the mid-twentieth century. The line itself was marked in two ways: by the knots or leather tags (marks) at known depths, and by the weight of the lead sinker at the end, which was heavy enough to sink through water column and push into soft seabed long enough to pick up samples of mud, sand, or shell. This dual system gave the sailor both depth and seabed type information.
Marks reduced error in the dangerous work of taking soundings in coastal waters, estuaries, and harbour approaches where rocks, shoals, and uncharted features posed collision risk. A tired or hurried leadsman could miscount, but marks provided a physical reference that was harder to misread. The practice remained in use on smaller vessels and in training long after large ships switched to electronic systems.
Different materials and colours were used to make marks distinctive. Leather was durable and could be shaped easily; bunting or cloth was lighter and more visible from a distance. Some traditions used knots of different types or spacing rather than attached objects. The term mark entered ship language as the standard name for any such indicator, regardless of material, and remains in maritime historical texts and terminology.