Why is a Ship’s Speed Measured in Knots?

September 7, 2015

ships-701596_1280When we drive a car or ride a bike, our speed is measured in miles per hour.  But when you get in a boat, those measurements change and the term knot is used to measure how fast you’re going.  Why are knots used for speed on the water?  It actually comes from something that was done long ago.

Crews of sailing ships far back in history didn’t have the advantage of GPS or modern maps.  They had to use what they had around them to determine where they were going, and how long it was going to take get there.  Knowing the speed of a ship’s travel was vital.  Without it, the crews of these ships could find themselves so far of course that it would prove deadly.  That’s why knowing exactly how fast a ship was going was imperative for navigation and to know when a ship would arrive at its destination.

The knot sounds like it has to do with rope, and that would be correct.  The term, knot, originated in the 17th century and is based on the length of the nautical mile.  A nautical mile is based on the circumference of the earth, and it is equal to one minute of latitude.  The mile we are familiar with on land is the statute mile, and a nautical mile is slightly longer (1 nautical mile = 1.1508 statute miles).

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A Common Log    Remi Kaupp/Wikimedia

Sailors from the 17th century would measure the speed of the ship they were on using a seemingly simple device called a “common log”.  It was a piece of rope attached to a piece of wood shaped like a wedge.  Based on the length of a nautical mile, knots were tied at specific intervals along the length of rope, at 14.4 meters along the rope to be exact.  One end of the rope was attached to the ship, and the wooden wedge was thrown in the water.  Using a sand hourglass that acted as a timer, the sailors would time how much of the line unspooled, and they would count the number of knots on the rope that passed through their fingers.  After a specified amount of time had passed, the number of knots that had been counted would indicate how fast the ship was traveling at that time.

The sailors didn’t have the convenience of having an indicator to tell them the speed of the ship all the time, and they would calculate the speed throughout the day to get an average of how fast they were traveling.  This allowed them to then calculate their position more accurately, and the time it would take to get them where they wanted to go.

In today’s modern age, the instruments to measure speed are still called a log, but the way it is measured is much different.   By using a Doppler measurement of ultrasonic sensors, the speed of a ship is accurately measured all of the time.  But one thing hasn’t changed, the nautical mile is still used for distance, and it’s also used with aeronautics.  Airplanes measure their speed in knots and distance in nautical miles since they are based on the earth’s circumference.

So the next time you hear the term knot used for speed, you’ll realize that it came from a simple device devised by sailors long ago who were using a lot of ingenuity.

About the author 

Daniel Ganninger - The writer, editor, and chief lackey of Knowledge Stew, the author of the Knowledge Stew line of great trivia books, and editor of Fact World and the Knowledge Stew sister site on Medium. I hope you find things here to annoy those around you with your new found knowledge.

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