Why is speed at sea measured in knots?

Because 15th-century sailors didn’t have GPS…

By Sarah Jensen

Adventure novels and history books are filled with harrowing stories of sailing ships delayed at sea — tales of sailors running low on food and fresh water, dying of scurvy, and getting trapped in the doldrums, or the tropics during storm season. Unless sailors knew how fast they were going, they could end up days off schedule, endangering those on board and worrying loved ones awaiting them in port.

“With no landmarks to gauge their progress across the open sea, sailors couldn’t tell how fast or how far they were traveling,” explains Camila Caballero, an MIT senior and the academic coordinator for Amphibious Achievement, an athletic and academic outreach program for urban youth in Boston. But when the nautical mile — 1.852 kilometers — was introduced in the 15th century, they had a handy standard against which to measure speed and created out of necessity the chip log, the world’s first maritime speedometer. “They used materials they had on hand,” she explains. “A wedge-shaped piece of wood, a small glass timer, and a really long rope.”

But not just any rope would do. Based on the length of the nautical mile, knots were tied along the log line at intervals of 14.4 meters. One end was secured to the ship’s stern and the other was attached to the wooden board, which was dropped into the water. “As one sailor watched the sand empty through the 30-second glass, his shipmate held the line as it played out behind the ship and counted the knots as they passed between his fingers,” says Caballero. Dividing that 14.4 meters by 30 seconds told them that one knot equaled 1.85166 kilometers per hour or one nautical mile. By performing the calculation using the actual number of knots that unspooled, the sailors were able to measure the ship’s speed.

The average of frequent measurements taken throughout the day proved to be a highly accurate reflection of how fast a ship was moving. The data was used to help them navigate by dead reckoning, the method used before the advent of modern instruments.

Today, maritime speed is determined by ultrasonic sensors or Doppler measurement, and the 30-second divisor in the rate equation has been replaced by 28. But the instrument for measuring a vessel’s speed is still called a log, and marine and aeronautical distances are still measured in nautical miles. “Maps used at sea and in the air are based on the earth’s circumference,” says Caballero. “Their scale varies with latitude, and the nautical mile, about 500 feet longer than the land mile, reconciles those differences.”

And in both today’s pilothouse and cockpit, the speed equal to one nautical mile an hour is still called a knot, the term an echo of the days when crewmembers of square-riggers and caravels got creative with a few simple materials and produced an essential and significant little gadget.

For more information about Amphibious Achievement, see the story about them in the June 19, 2012, Boston Globe.

Thanks to S. Venkatesh from Tirunelveli, India, for this question.