The knotty truth

Ever wonder what a knot is, and why we use them in aviation? I did.

The term actually began with real knots tied into a rope, and like much of our aviation lingo, comes from the maritime world. While the exact beginning is difficult to determine, it is believed that the knot methodology began in the late 1500’s, and that it was in widespread use by the 1600’s. In order to measure speed, sailors would tie knots in a rope every42 feet (six feet per fathom, so there were seven fathoms between each knot). The rope was attached to a piece of wood called a “chip log,” which would be cast in the water to the rear of the ship, where it would essentially remain stationary as the rope paid out.

Using a small hour-glass, the sailors would time for approximately 30 seconds and count the number of knots that would pass through their hands and into the water. Over time, the standards were revised, and if you were try this at home your chip log would need knots every 47.3 feet, and you need an hourglass that can time for 28 seconds instead of 30.

At first the United States stuck with the statue measurement of miles per hour in aviation, but in 1969 the FAA began transitioning the industry to the nautical standards. The logic for this is that aeronautical charts, like marine charts, are printed in the nautical scale. By converting our references from miles per hour to knots, we can be sure that we are all speaking the same language.

In case you did not know, the nautical mile is equal to 1.15 statute miles; one statue mile is equal to 0.868 nautical miles. It also brought the American aviation industry into ICAO standards.

I don’t suggest that you start extending rope behind your airplane to measure speed, but if you do at least some of us will understand why.

–Chip Wright

4 Responses to “The knotty truth”

  1. Tom says:

    However, you miss the main reason it is a standard- a minute of lattitude is one nautical mile, and it is a universal standard in part due to the fact its length is tied to the size of the earth. The history gives it its name; its current length is more defined by the earth, and who is to say that the orignal unit may have been so as well, lost in time? Why else would someone have chosen knots so spaced on a rope, except it was easy to link the resultant speed with distance on charts?

  2. Thomas Boyle says:

    Chip,

    You’ve explained where the term “knots” came from, but not where the unit itself came from, nor why it’s preferred in navigation to either the mph or the kph.

    The reason, of course, is that the north-south distance covered by 1 minute of latitude is a nautical mile (and the knot is 1 nautical mile per hour). Minutes of longitude vary depending on how far one is from the poles, and are close to a nautical mile somewhere not far from the equator, but minutes of latitude are approximately the same distance everywhere. Thus, the nautical mile is a “relevant” distance for the navigator.

    Interestingly, the meter was originally defined as 1/10,000,000 of the average distance from the pole to the equator. Since that is also 90 x 60 = 5,400 nm, there would be 1,852 meters in a nautical mile – and that is the modern definition of a nautical mile.

  3. My students faces always light up when they realize the relationship between knots, nautical miles, and minutes of latitude when dealing with the sectional chart. There is almost a sense of pride instead of dreading learning a new unit of measurement!

    Thank you for the great piece and comments!

  4. David Consbruck, CFII says:

    Yes, the nautical mile is 1 minuted of latitude. That is so easy to remember. It also why I use the vertical scale markers on a Sectional Chart for measuring distance, even if I am plotting a course that is not exactly N-S.

    The horizontal scale is skewed based on location and is therefore almost never completely accurate. One will notice it varies in length from the vertical, in an attempt to show the distances more accurately. Does all this matter? Not really that much, but it sure is nice to know.

    Thanks to Chip, Tom, and Thomas for sharing their knowledge. Understanding is an important factor in developing that key ingredient in being a good pilot, “judgement”.

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