Hi! RonBee
SOS – It was the fault of my teacher because he taught me wrong when I attended nautical school. Anyway, thank for your information. Here is the real marine world - http://www.boatsafe.com/nauticalknowhow/060199tip6.htm

Imperial system – I should have said more precise that it was the British imperial system. One fathom is equal to 6 six feet. About 15 years ago mariners used two systems of nautical charts, i.e. metric system and imperial system. You may ask why using imperial system which is a so complex system. It is because nautical chart companies in Great Britain supply most nautical charts in English worldwide. They need time phasing out the imperial system charts.

Here is the link about imperial system:
http://www.squarerigged.nl/papa/dimen/defin.htm
Please read the paragraph of “THE IMPERIAL SYSTEM”

Another link to a more authoritative website: http://www.bartleby.com/65/en/Englsh-u.html
Please read the paragraph of “Differences between American and British Systems”