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Word of the Month — Knot

By Ian Brown, Editor

May 2021 – Whether you fly using an airspeed measured in knots, miles per hour, or kilometers per hour, you're probably aware of the conversions. 1 kt = 1.15 mph = 1.85 kph.

Up to the 16th century, sailors used an actual log which was tied to a piece of rope and thrown overboard to measure speed. The length of rope that ran out in half a minute (timed using a sandglass) was measured and the speed through the water was estimated, based on a certain length of rope corresponding to a certain speed through the water. It was a bit like watching a leaf float by and measuring how long it took to go a certain distance. In these early days, figuring out speed over the water was essential to knowing where you were. This method was known as "heaving the log."

A refinement of this came along in the 16th century that attempted to standardize the "log." It was a piece of wood of a fixed size, held at a certain orientation under the water, with knots tied into the rope at fixed distances so that the number of knots paid out in half a minute could be recorded. This was known as the Chip Log method.

Remember that the definition of a nautical mile is based on the Earth's circumference at the equator being divided into 360 degrees. Each degree is divided further into 60 minutes, which is use to define a nautical mile. One knot of speed is one nautical mile per hour.

The knot used as a measure of speed is exactly 1,852 meters per hour by internationally recognized standards although both the U.K. and the U.S. had their own measurements that differed slightly prior to 1970.

Interestingly the abbreviation for knots "kts" is specific to aviation, and "kn" is preferred internationally by engineers and seafarers.

If you use miles per hour not knots you're probably not interested in knots, not at all.

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