A piece of leather, cotton or bunting woven into a hand lead line at specific intervals to measure water depth. Traditional sounding lines use marks at 2, 3, 5, 7, 10, 13, 15, 17 and 20 fathoms, each shaped or textured differently so a leadsman can identify the depth by touch in the dark. The depths between these marked intervals are known as deeps.
Heard on the job
By the mark five, captain, and the bottom is shoaling fast on this heading.
Insider tip
A leadsman calls out marks directly (By the mark seven) but estimates the unmarked deeps (By the deep six).