fathom
A man's armspan, generally reckoned to be six feet (about 1.8 metres). Later used to measure the depth of water, but now generally replaced by the metre outside American usage.
fathom: a six-foot unit for sounding depth
A fathom is a unit of length equal to six feet, or approximately 1.83 metres. It originated as a practical measure: the distance a man could span with his arms outstretched, used by sailors and fishermen to estimate rope and line lengths. The term comes from the Old English faeth, meaning embrace or the space enclosed by the arms.
In marine work, the fathom became standardized as the primary unit for measuring water depth. A leadsman would cast a sounding line marked at fathom intervals (using knots, leather strips, or cloth tags) to determine if a vessel had safe clearance over the seabed. A typical sounding line carried marks at 2, 3, 5, 7, 10, 13, 15, 17, and 20 fathoms. Charts marked "4 fathoms" meant 24 feet of water, a critical distinction for merchant ships drawing 12 to 16 feet.
The fathom persists in American maritime practice, navigation charts, and fisheries management, where depth readings still appear in fathoms rather than metres. Nautical charts issued by U.S. agencies, port authorities, and fishing grounds continue to use this unit. Fishermen working with trap lines, nets, and echo sounders in North American waters routinely work in fathoms; a common coastal depth might be described as 8 to 12 fathoms.
Outside North America, the metre replaced the fathom during the 20th century, particularly after electronic soundings made logarithmic conversion less burdensome. However, American maritime infrastructure, depth data accumulated over centuries, and established safety margins built on fathom increments meant wholesale conversion remained impractical. Confusion between systems remains a real hazard in international waters; a pilot accustomed to metres reading a fathom-marked chart without conversion risks critical misjudgement.
Modern marine GPS and electronic charts allow software to display depth in either unit, but the fathom remains embedded in nautical culture. Understanding it is essential for anyone reading traditional American charts, interpreting fisheries regulations, or communicating with experienced watermen who think in arm-spans rather than decimal multiples.