Interesting fact:

One of the more innovative forms of navigation in the past is known as dead reckoning. The idea was simple: A stretch of rope with knots tied at regular distances was attached to a log and then thrown overboard. By counting the number of knots that went over the side before pulling the log back in, sailors could estimate their speed! They would then note this every day in logbooks to tell how far they were traveling over long distances. Ever wondered why a ship’s speed was measured in knots? Well, now ya know!