By the way, nautical miles are not equal to american mile. It is a meridian length of one arc minute size. Useful for those who navigate over the globe. But hard to understand to surface people. 
Nevertheless I prefer kilometers too.
That's the modern definition, but you can't possibly believe that in the days of sailing ships they had sufficient instrumentation to calculate how many arc-minutes a ship had travelled or was moving at.
A seaman would throw a log over the side, tied to a rope with regularly spaced knots, the earliest version being every 100 feet. (After all, who wants to divide 5280 by 60 and then pace off that amount of rope? The calculation is just as sticky in meters.) The sailor counted how many knots when through his fingers in 1 minute, and that was the ship's speed. X knots per minute = X hundred feet per minute, or X times 6000 feet per hour, or X Knots.
Sometime later (undoubtedly AFTER someone was able to accurately calculate the length of an arc-minute) a knot was standardized at 1852 meters or 6076 feet. Interestingly enough, the US and the UK were among the last countries to accept the international definition, in 1954 and 1970 respectively.
