Why Is Speed at Sea Measured in Knots?

The Difference Between Nautical Miles and Knots

The reason is ancient, and an evolution of different seafaring cultures, culminating in knots on a rope thrown to different depths over the side, as will be discussed. On land, speed is expressed in miles per hour (mph) or kilometers per hour (kph). However, measurements change when you’re at sea. You’ve probably heard the term “knot” being used to describe speed on the open sea, but what is a knot, and where did it come from?

Have you ever wondered: “Why speed at sea is measured in knots?” In this article, we’ll take a look at the history of nautical travel to provide you with the answer to this question and discuss why it is still used today.

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Early Methods Used to Measure Speed at Sea

Tales of sailors running out of basic necessities, dying of scurvy, and becoming stranded during hurricane season are commonplace in adventure stories and historical accounts of sailing vessels detained at sea.

Without knowing their speed, seafarers could run days behind schedule, endangering people on board and unsettling their loved ones waiting for them. Sailors were unable to determine their speed or distance because there were no landmarks to measure their progress on the open sea.

This is why they needed to come up with a reliable way to measure the speed of their vessels and the distance traveled. Admiralty law mandated a uniform system at some point.

The nautical mile (the equivalent of 1.1508 land-measured miles) was initially used as a unit of measurement for speed in the 15th century, and as a result, the chip log, which was the very first marine speedometer designed to measure speed, was born.

The Dutchman’s Log

Tossing a piece of wood over the vessel’s bow and timing how long it took for the stern to reach the object allowed sailors to determine how quickly their ship was traveling. This was one of the earliest forms of measurement and was the precursor for the chip log method that measured speed in knots. Dutchman’s log was the name used to describe this technique.

Chip Log Technique

By the 15th century, mariners began to use what they called the “chip log” or “common log” method to measure boat speed. This technique involved knotting a piece of rope at regular intervals. One end was attached to the ship’s stern, and the other would be attached to a pie-slice-shape piece of wood (or “chip”).

The rope, along with the wood shaped like a pie, was tossed into the sea and allowed to unroll freely as the ship advanced for a certain period, usually measured in hours (which they measured using an hourglass). After that, the number of knots that passed the ship’s stern was counted to determine how fast the vessel moved forward. The speed was expressed in knots per hour.

Where Does the Term “Knot” Come from?

Ancient mariners didn’t have access to the technology we have today. This means they had to use the materials they had on hand to measure a vessel’s speed. Once the seamen had determined the number of knots that crossed the ship’s stern using the chip log method, they used this to calculate the speed of the ship in knots per hour.

The nautical mile measures the distance from one knot to another. This is how one nautical mile per hour came to be known as a knot.

Therefore, if a ship’s speed measured 15 knots, this means that it was traveling at 15 nautical miles per hour. The day’s average of numerous readings showed to be a remarkably accurate representation of how quickly a ship was going. They used the information to navigate using dead reckoning, which was the standard practice before the development of modern technology.

Speed Measurements Today

Today, Doppler or ultrasonic sensors are used to calculate nautical speed, and the 30-second denominator has been changed to 28 in the rate calculation.

However, the term “log” still refers to the device used to gauge a ship’s speed, and nautical miles continue to be used to describe both marine and aviation distances.

The Nautical Mile Is Based on the Earth’s Circumference

Maps used in the air and sea are based on the earth’s circumference. Moreover, one nautical mile, which is about 500 feet greater than a land mile, balances out the scale variations with latitude.

The International Nautical Mile

Different nations disagreed on its measurement for several years. The international nautical mile was set at 6,076 feet in 1929, and the US accepted it in 1954. A nautical mile differs from a land mile, which is calculated based on walking distance.

Sailors and Pilots Today Still Remember the Common Log Method

Today sailors and pilots still refer to the speed of one nautical mile per hour as a knot, a remnant from the time when the crews of ships in adventure tales made do with some basic materials.

What’s the Difference Between Nautical Miles and Knots?

At this point, you may be confused about nautical miles and knots. In the section below, we’ll discuss the difference between these two terms.

Nautical Miles Measure Distance

The distance covered by water is expressed in nautical miles. As we’ve mentioned before, a nautical mile is 1.1508 land miles long, which is somewhat greater than a mile on land. Moreover, the nautical mile is based on latitude and longitude, and a nautical mile is equivalent to one minute of latitude.

Knots Measure Speed

On the other hand, the term knot is utilized to denote speed. One nautical mile per hour, or about 1.15 land miles per hour, is equal to one knot.

Final Thoughts

Ancient mariners used whatever they had to measure nautical speed, and their innovative techniques have shaped maritime measurements today. Next time you hear the word “knot” being used at sea, you’ll know what it means and how it came to be a monumental measurement.

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