soundingline

A sounding line is a nautical instrument used to measure the depth of water beneath a vessel. It consists of a rope or cord with a weighted end, called a sounding lead, which is lowered over the stern or bow until it touches the bottom. The length of line that has run out is then read to determine the depth. In addition to depth, many leads carried a small amount of tallow or soft material to gather a seabed sample, allowing mariners to identify bottom type (such as mud, sand, or gravel).

History and usage have roots in ancient seafaring, with widespread use during the age of sail. Measurements were often expressed in fathoms and feet, though meters later became common in some regions. The line was usually marked at regular intervals to facilitate quick readings, and the lead could include a hollow space to hold a small amount of tallow for bottom sampling. While primarily a depth-measuring tool, the sounding line also aided navigation by confirming water depth in shallow or unfamiliar waters.

Modern context and limitations: echo sounding and multibeam sonar have largely supplanted traditional sounding lines on large ships, providing continuous, high-resolution depth data. Nevertheless, sounding lines remain in use for small boats, training, and certain shallow-water operations, where simple, low-tech depth checks are valuable. Readings can be affected by wind, current, line stretch, or snagging, and measurements reflect depth directly beneath the vessel at the time of sounding.