ftm
fathom
Lengthexact by definition
A fathom is exactly six feet, 1.8288 metres. It is the unit of water depth on charts, of rope and anchor cable, and of mining shaft depth. Charts published in fathoms are still in service, and a sounding of "five fathoms" is thirty feet of water.
Watch out: Hydrographic offices have been converting charts to metres for decades, and a chart's depth unit is printed in its title block. Mixing a fathom chart with a metre depth sounder is a grounding waiting to happen.
| 1 ftm | 1.8288 m |
Where the unit came from
From the Old English fæðm, the span of a man's outstretched arms, which is how a leadsman actually measured line as he hauled it in. The lead line was marked at set fathoms with leather, cloth and cord so it could be read by feel in the dark.
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