Here are some interesting things about fathoms as related to the distance of ones outstretched arms. Your reach, fingertip to fingertip is very nearly the same as your height.
Since the fathom is a very old measurement based on this reach one might conclude that either people were indeed six feet tall in ancient times as well as today or that people were just as big at exaggeration.
In use this is not a practical measurement unless you knot a rope as described because, as any sailor knows, to measure a rope with no knots you don't count by your fingertips since you are grasping and coiling but by something a bit less counting off your coils and multiplying.
My own reach is about 5' 9", larger than my 5' 7" height but my grasp is nominally only 5' which is actually more convenient when measuring off how much line to play out for anchor rodes and such.