A fathom is 6 feet, the length of rope a man can extend from open arm to open arm. The rope was lowered into the sea to measure depth. A cable length is the length of a ship’s cable, about 600 feet. A nautical mile is 10 cable lengths, or 6,076 feet. 1 nautical mile = 1.1508 miles A knot is the measure of speed on water. One knot is 1 nautical mile per hour.