I'm just a retired simple sailor who held a PPL,from my Principles of Navigation schooling I seem to recollect that a nautical mile was assumed to be 6080 feet to account for the irregularities in the shape of the Earth.For practical purposes 6000',2000yards was used for simplicity,this equated to 10 cables or 0.1 of a N.M.This term was also used for the length of an anchor cable/chain although I can't remember an anchor chain longer than 9 shackles,which equates to 15 fathoms per shackle which doesn't fit the mould.
Colin
