A ship's speed is commonly measured in knots because the term "knot" originated as a unit of speed used in maritime navigation. The use of knots dates back to the days of sailing ships when sailors needed a way to measure the speed of their vessels.
In the past, sailors used a device called a "chip log" or "common log" to measure the speed of a ship. The chip log consisted of a wooden board (the "chip") attached to a line with knots at regular intervals. The line was thrown overboard, and the chip log would float behind the ship, trailing in the water.
To measure the ship's speed, the sailors would count the number of knots that passed through their hands in a specific time interval, typically measured using a sandglass or an hourglass. The number of knots counted in a given time interval represented the ship's speed in nautical miles per hour.
A nautical mile is a unit of distance used in maritime navigation, and it is defined as one minute of latitude along a line of longitude. The term "knot" originated from the practice of tying knots in the log line to provide reference points for counting. The speed measured by counting the knots in the log line became known as the ship's speed in knots.
Even though modern ships now use more accurate electronic means to measure speed, such as GPS-based systems or pitot tubes, the use of knots as a unit of speed has persisted in maritime tradition and is still commonly used today. It provides a convenient and familiar way for sailors and maritime professionals to communicate and reference ship speeds.