Fathom

[fath-uhm]

A fathom is a unit of length in the Imperial system (and the derived U.S. customary units), used especially for measuring the depth of water.

There are 2 yards (6 feet) in a fathom. Based on the distance between the fingertips of a man's outstretched arms, its size 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 and 5 1/2 feet.

The name derives from the Old English word fæðm meaning embracing arms or a pair of outstretched arms. In Middle English it was fathme, cognate to the Latin patēre to be open, pandere to spread or unfold, and the Greek petannynai to spread out. Hence its earlier meaning, now obsolete: a full stretch of the arms in a straight line, and consequently grasp or reach.

A brass was a unit of length equal to 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. At one time, a quarter meant a fourth of a fathom.

Abbreviations: f, fath, fm, fth, fthm.

International Fathom

One fathom is equal to:

1.8288 metres (1 metre is about 0.5468 fathoms)
2 yards (1 yard is exactly 0.5 fathoms)
4 cubits (1 cubit is exactly 0.25 fathoms)
6 feet (1 foot is about 0.1667 fathoms)
18 hands
72 inches

In 1954 the United States and countries of the Commonwealth of Nations defined the length of the international yard to be exactly 0.9144 metres. With the adoption of the metric SI system the use of fathoms declined.

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. In practice the fathom was always regarded as exactly 6 feet. No conflict in the real world existed as depths on Imperial nautical charts were indicated in feet if less than 30 feet and in fathoms for depths above that.

Use of the fathom

Most nautical charts produced by hydrographic offices worldwide, with the notable except of those produced by the U.S. Hydrographic Office, which uses feet and fathoms, now indicate depths in metres. Nevertheless, most English-speaking mariners are familiar with the unit. A nautical chart will always explicitly indicate the units of depth used.

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 in the plane of a vein. In Britain, it can mean the quantity of wood in a pile of any length measuring 6 feet square in cross section.

Historically, though, the fathom has been used to define depths and layers of the sea and lengths of line (rope).

Water depth

Sounding

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 offsoundings or out of 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 a 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.

Ocean zones

For oceanographers, the belt or region of shallow water adjoining the seacoast is the neritic zone, usually considered to extend from low-tide mark to a depth of 100 fathoms. The fauna and flora of the sea bottom in this zone constitute the epibenthos, while the epiplankton occurs from the surface of the sea to a depth of about 100 fathoms. Off the Norwegian and New England coasts and on the Grand Banks of Newfoundland, cod congregate at certain seasons in water of 20 to 40 fathoms' depth. A depth of six fathoms or less at low water was part of the definition of a reef or a shoal.

The fauna and flora of the sea bottom between 100 and 500 fathoms are the mesobenthos. One denizen of this region is the oilfish (Ruvettus pretiosus), which lives at a depth of from 100 to 400 fathoms in the Mediterranean, middle Atlantic and throughout the southern seas.

The deeper parts of the ocean, especially between 100 and 1000 fathoms, are the bathyal zone or bathyal district — the slope from the continental shelf at 100 fathoms to the abyssal zone at 1000 fathoms. One feature on the ocean bottom of this district is a dome, a rounded isolated elevation found at depths greater than 100 fathoms.

In the deep sea, the water has a depth of 1000 fathoms or more. In a deep, a generally long and narrow area in the ocean, the depth generally exceeds 3000 fathoms.

A waterproof watch, its movement enclosed in a case in which the openings for the winding and cover are sealed with gaskets, should be able to withstand pressures equal to several fathoms of submersion.

Line length

The components of a commercial fisherman’s setline were measured in fathoms. The rope called groundline, used to form the main line of a setline, was usually provided in bundles of 300 fathoms. A single 50-fathom 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 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 long. 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, that on 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.

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, meaning to discard, or dispose of.

Other fathoms and similar units of length

Units of length similar to the size of the fathom can be found in many cultures. Some are listed below.

Culture	Name	Length in metres
Croatian	hvat	1.896484
Czech	sáh	1.7928
Danish	favn	1.883124
Dutch	vadem, vaam	1.883679
Estonian	süld	2.1336
Finnish	syli	n/a
French	toise (circa 1150), brasse (1409)	~1.949
German	Klafter, Faden = 6 Fuß	n/a resp. 1.7
Ancient Greek	orguia	1.8542
Hungarian	öl	1.8964838 (Viennese)
India (State of Manipur)	Sana lamjel	n/a
Italian	braccio	~1.65
Japanese	hiro (尋)	~1.818
Maltese	qasba	~2.096
Norwegian	favn	1.882
Polish	sążeń	1.728
Portuguese	braça	2.2
Russian	morskaya sazhen (морская сажень)	1.852
Turkish	kulaç	1.83
Sanskrit	vyama	n/a
Serbian	хват	n/a
Slovak	siaha	n/a
Spanish	braza	1.6718
Swedish	famn	1.7814