Fathom

Name

The name derives from the Old English word fæðm, corresponding to the Old High German word "fadum" meaning embracing arms or a pair of outstretched arms. In Middle English it was fathme.

Ancient fathoms

The Ancient Greek measure known as the orguia (Greek: ὀργυιά, orgyiá, lit. "outstretched") is usually translated as "fathom". By the Byzantine period, this unit came in two forms: a "simple orguia" (ἁπλὴ ὀργυιά, haplē orguiá) roughly equivalent to the old Greek fathom (6 Byzantine feet, c. 1.87 m) and an "imperial" (βασιλικὴ, basilikē) or "geometric orguia" (γεωμετρικὴ ὀργυιά, geōmetrikē orguiá) that was one-eighth longer (6 feet and a span, c. 2.10 m).

International fathom

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.

British fathom

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 above that. Until the 19th century in England, the length of the fathom was more variable: from  5 ¹⁄₂ 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).

Derived units

At one time, a quarter meant one-fourth 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.

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 (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.

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 setline, was usually provided in bundles of 300 fathoms. A single 50-fathom skein (300 feet (91.4 m)) 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 long (420 feet (128.0 m)), 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 long (900 feet (274.3 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, 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 long (90 feet (27.4 m)).

A shackle, a length of cable or chain equal to 12.5 fathoms or 75 feet. In 1949, the British navy redefined the shackle to be 15 fathoms.

In Finland, fathom (syli) is sometimes, albeit seldom, used as a maritime unit,  ¹⁄₁₀₀₀ of a nautical mile and  ¹⁄₁₀₀ of cable length.

Burial

It is customary, when burying the dead, to inter the corpse at a fathom's depth, or six feet under. 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.

A similar sentiment is expressed in The Tempest, where a character is told "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 (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 Hungary square fathom ("négyszögöl") is still in use as an unofficial measure of land area, primarily for small lots suitable for construction.