| fathom | |
|---|---|
 Standard units inRegensburg: the metal rods are (from left to right) a fathom (klafter),foot (schuch, Modern GermanSchuh "shoe") andell (öln, Modern GermanElle). | |
| General information | |
| Unit system | imperial/US units |
| Unit of | length |
| Conversions | |
| 1 fathomin ... | ... is equal to ... |
| imperial/US units | 6 ft |
| SI unit equivalent | 1.8288 m |
Afathom is aunit oflength in theimperial and theU.S. customary systems equal to 6 feet (1.8288 m), used especially formeasuring the depth of water.[1] The fathom is neither aninternational standard (SI) unit, nor an internationally accepted non-SI unit. Historically it was the maritime measure of depth in the English-speaking world but, apart from within the US, charts now use metres.
There are twoyards (6feet) in an imperial fathom.[1] Originally the span of a man'soutstretched 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 imperialyard. Formerly, the term was used for any of several units of length varying around 5–5+1⁄2 feet (1.5–1.7 m).
The term (pronounced/ˈfæðəm/) derives (viaMiddle Englishfathme) from theOld Englishfæðm, which iscognate with the Danish wordfavn (via the Vikings) and means "embracing arms" or "pair of outstretched arms". It is maybe also cognate with theOld High German word "fadum", which has the same meaning and also means "yarn (originally stretching between the outstretched fingertips)".[2][3][4][5][6]
TheAncient Greek measure known as theorguia (Ancient Greek:ὀργυιά,orgyiá,lit. "outstretched") is usually translated as "fathom".[7] By theByzantine period, thisunit came in two forms: a "simple orguia" (ἁπλὴ ὀργυιά,haplē orguiá) roughly equivalent to the old Greek fathom (6Byzantine 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 aspan,c. 2.10 m).[8][9]
One international fathom is equal to:
TheBritish Admiralty defined a fathom to be a thousandth of animperial 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.[11] No conflict between the definitions existed in practice,[citation needed] since 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: from5+1⁄2 feet on merchant vessels to either 5 or 7 feet (1.5 or 2.1 m) on fishing vessels (from1.7 to 1.5 or 2.1 m).[11]
Other definitions of fathom include:
One metre is about 0.5468 fathoms
In theinternational yard and pound agreement of 1959 theUnited States,Australia,Canada,New Zealand,South Africa, and theUnited Kingdom defined the length of the international yard to be exactly 0.9144metre. In 1959 United States kept the US survey foot as definition for the fathom.
In October 2019, theU.S. National Geodetic Survey and theNational 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 exactly 1.8288 metres in the United States as well.[13][14]
At one time, aquarter meant one-quarter of a fathom.[citation needed]
Acable length, based on the length of a ship's cable, has been variously reckoned as equal to 100 or 120 fathoms.[citation needed]
Most modern nautical charts indicate depth in metres. However, theU.S. Hydrographic Office uses feet and fathoms.[16] Anautical chart will always explicitly indicate the units of depth used.[17]
To measure the depth of shallow waters, boatmen used asounding line containing fathom points, some marked and others in between, calleddeeps, unmarked but estimated by the user.[18] Water near the coast and not too deep to be fathomed by a hand sounding line was referred to asin soundings oron soundings.[19] The area offshore beyond the 100 fathom line, too deep to be fathomed by a hand sounding line, was referred to asout of soundings oroff soundings.[20] Adeep-sea lead, the heaviest of sounding leads, was used in water exceeding 100 fathoms in depth.[21]
This technique has been superseded by sonicdepth finders for measuring mechanically the depth of water beneath a ship, one version of which is theFathometer (trademark).[22] The record made by such a device is afathogram.[23] Afathom line orfathom curve, a usually sinuous line on a nautical chart, joins all points having the same depth of water, thereby indicating thecontour of the ocean floor.[24]
Some extensive flat areas of the sea bottom with constant depth are known by their fathom number, like theBroad Fourteens or theLong Forties, both in theNorth Sea.
The components of a commercial fisherman'ssetline were measured in fathoms. The rope called agroundline, 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 aline. Especially in Pacific coast fisheries the setline was composed of units calledskates, each consisting of several hundred fathoms of groundline, withgangions and hooks attached. Atuck seine ortuck net about 70 fathoms (420 ft; 130 m) long, and very deep in the middle, was used to take fish from a largerseine.
Aline attached to awhalingharpoon was about 150 fathoms (900 ft; 270 m). Aforerunner — a piece of cloth tied on a ship's log line some fathoms from the outboard end — marked the limit of drift line.[25] Akite 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.
Ashot, one of the forged lengths of chain joined by shackles to form an anchor cable, was usually 15 fathoms (90 ft; 27 m).[26]
Ashackle, a length of cable or chain equal to12+1⁄2 fathoms (75 ft; 22.9 m).[27] In 1949, the British navy redefined the shackle to be 15 fathoms (90 ft; 27 m).[28]
TheFinnish fathom (syli) is occasionally used:1⁄1000nautical mile or1⁄100cable length.
Aburial 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.[29]
The phrase is echoed in Shakespeare'sThe Tempest, whereAriel tellsFerdinand, "Full fathom five thy father lies".
Until early in the 20th century, it was the unit used to measure the depth ofmines (mineral extraction) in theUnited Kingdom.[30] Miners also use it as a unit of area equal to 6 feet square (3.34 m2) in the plane of a vein.[2] 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.[2] InCentral Europe, theklafter was the corresponding unit of comparable length, as was thetoise inFrance. InHungary the square fathom ("négyszögöl") is still in use as an unofficial measure of land area, primarily for small lots suitable for construction.[clarification needed]
The scale of a paper or raster chart is shown near the title, as are the units used for soundings (these are normally metres or fathoms, though occasionally they may be in feet). Make sure you take notice of this or you'll have some surprises.
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.
