A fathom is a unit of length in the imperial and the U.S. customary systems equal to 6 feet (1.8288 m), used especially for measuring the depth of water. The fathom is not an International Standard (SI) unit, nor is it accepted internationally as a non-SI unit. However it is historically the most frequently employed maritime measure of depth in the English speaking world.
There are two yards (6 feet) in an imperial fathom. Originally the span of a man's outstretched arms, the size of a fathom has varied slightly depending on whether it was defined as a thousandth of an (Admiralty) nautical mile or as a multiple of the imperial yard. Formerly, the term was used for any of several units of length varying around 5- feet (1.5-1.7 m).
The name derives from the Old English word fæðm, cognate to the Danish (via the Vikings) word "favn" meaning embracing arms or a pair of outstretched arms. Cognate maybe also via the Old High German word "fadum" of the same meaning. In Middle English it was fathme.
The Ancient Greek measure known as the orguia (Greek: , orgyiá, lit."outstretched") is usually translated as "fathom". By the Byzantine period, this unit came in two forms: a "simple orguia" (? , hapl? orguiá) roughly equivalent to the old Greek fathom (6 Byzantine feet, c. 1.87m) and an "imperial" (, basilik?) or "geometric orguia" (? , ge?metrik? orguiá) that was one-eighth longer (6 feet and a span, c. 2.10m).
One fathom is equal to:
In the international yard and pound agreement of 1959 the United States, Australia, Canada, New Zealand, South Africa, and the United Kingdom defined the length of the international yard to be exactly 0.9144 metre. In 1959 United States kept the US survey foot as definition for the fathom.
In October 2019, U.S. National Geodetic Survey and National Institute of Standards and Technology announced their joint intent to retire the U.S. survey foot, with effect from the end of 2022. The fathom in U.S. Customary units is thereafter defined based on the International 1959 foot, giving the length of the fathom as exact 1.8288 meters in the United States as well.
The British Admiralty defined a fathom to be a thousandth of an imperial nautical mile (which was 6080 ft) or 6.08 feet (1.85 m). In practice the "warship fathom" of exactly 6 feet (1.8 m) was used in Britain and the United States. No conflict in the real world existed as depths on Imperial nautical charts were indicated in feet if less than 30 feet (9.1 m) and in fathoms for depths greater than that. Until the 19th century in England, the length of the fathom was more variable: from feet on merchant vessels to either 5 or 7 feet (1.5 or 2.1 m) on fishing vessels (from 1.7 to 1.5 or 2.1 m).
At one time, a quarter meant one-quarter of a fathom.
A cable length, based on the length of a ship's cable, has been variously reckoned as equal to 100 or 120 fathoms.
To measure the depth of shallow waters, boatmen used a sounding line containing fathom points, some marked and others in between, called deeps, unmarked but estimated by the user. Water near the coast and not too deep to be fathomed by a hand sounding line was referred to as in soundings or on soundings. The area offshore beyond the 100 fathom line, too deep to be fathomed by a hand sounding line, was referred to as out of soundings or off soundings. A deep-sea lead, the heaviest of sounding leads, was used in water exceeding 100 fathoms in depth.
This technique has been superseded by sonic depth finders for measuring mechanically the depth of water beneath a ship, one version of which is the Fathometer (trademark). The record made by such a device is a fathogram. A fathom line or fathom curve, a usually sinuous line on a nautical chart, joins all points having the same depth of water, thereby indicating the contour of the ocean floor.
The components of a commercial fisherman's setline were measured in fathoms. The rope called a groundline, used to form the main line of a setline, was usually provided in bundles of 300 fathoms. A single 50-fathom (300 ft; 91 m) skein of this rope was referred to as a line. Especially in Pacific coast fisheries the setline was composed of units called "skates", each consisting of several hundred fathoms of groundline, with gangions and hooks attached. A tuck seine or tuck net about 70 fathoms (420 ft; 130 m) long, and very deep in the middle, was used to take fish from a larger seine.
A line attached to a whaling harpoon was about 150 fathoms (900 ft; 270 m). A forerunner -- a piece of cloth tied on a ship's log line some fathoms from the outboard end -- marked the limit of drift line. A kite was a drag, towed under water at any depth up to about 40 fathoms (240 ft; 73 m), which upon striking bottom, was upset and rose to the surface.
A shot, one of the forged lengths of chain joined by shackles to form an anchor cable, was usually 15 fathoms (90 ft; 27 m).
A burial at sea (where the body is weighted to force it to the bottom) requires a minimum of six fathoms of water. This is the origin of the phrase "to deep six" as meaning to discard, or dispose of.
Until early in the 20th century, it was the unit used to measure the depth of mines (mineral extraction) in the United Kingdom. Miners also use it as a unit of area equal to 6 square feet (0.56 m2) in the plane of a vein. In Britain, it can mean the quantity of wood in a pile of any length measuring 6 feet (1.8 m) square in cross section. In Central Europe, the Klafter was the corresponding unit of comparable length, as was the Toise in France. In Hungary the square fathom ("négyszögöl") is still in use as an unofficial measure of land area, primarily for small lots suitable for construction.[clarification needed]
As opposed to drifting, a piece of fishing gear is considered set when it is anchored or attached to the bottom or shore so that it is not free to move about with water or wind currents. By contrast, a drift line or net has no such attachment to the bottom or shore and is therefore free to drift or move with any currents.