A fathom is a unit of length in the imperial and the U.S. customary systems equal to, used especially for measuring the depth of water. The fathom is not an International Standard 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 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 nautical mile or as a multiple of the imperial yard. Formerly, the term was used for any of several units of length varying around.
Name
The name derives from the Old English word fæðm, cognate to the Danish 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.
Forms
Ancient fathoms
The Ancient Greek measure known as the orguia is usually translated as "fathom". By the Byzantine period, this unit came in two forms: a "simple orguia" roughly equivalent to the old Greek fathom and an "imperial" or "geometric orguia" that was one-eighth longer.
The British Admiralty defined a fathom to be a thousandth of an imperial nautical mile or. In practice the "warship fathom" of exactly 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 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 on fishing vessels.
Derived units
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.
Use of the fathom
Water depth
Most modern nautical charts indicate depth in metres. However, the U.S. Hydrographic Office uses feet and fathoms. A nautical chart will always explicitly indicate the units of depth used. 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. 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. Some extensive flat areas of the sea bottom with constant depth are known by their fathom number, like the Broad Fourteens or the Long Forties, both in the North Sea.
Line length
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, was usually provided in bundles of 300 fathoms. A single 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 and hooks attached. A tuck seine or tuck net about long, and very deep in the middle, was used to take fish from a larger seine. A line attached to a whalingharpoon was about. 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, 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. A shackle, a length of cable or chain equal to. In 1949, the British navy redefined the shackle to be. In Finland, fathom is sometimes, albeit seldom, used as a maritime unit, nautical mile or cable length.
Burial
A burial at sea 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. The phrase is echoed in Shakespeare's The Tempest, where Ariel tells Ferdinand, "Full fathom five thy father lies".
On land
Until early in the 20th century, it was the unit used to measure the depth of mines in the United Kingdom. Miners also use it as a unit of area equal to in the plane of a vein. In Britain, it can mean the quantity of wood in a pile of any length measuring 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 is still in use as an unofficial measure of land area, primarily for small lots suitable for construction.
