Why are boats measured in knots?

SDStaff Larasaurus & SDStaff Rngrjeff, Straight Dope Science Advisory Board replies:

That’s a good question, Gabriela. It’s funny how good, short questions have long answers. You might want to get a cup of coffee before you read on.

Before modern instrumentation, the speed of a ship was measured by a crewman tossing a “log-line” off the ship. Here’s what “A Sea of Words: A Lexicon & Companion for Patrick O’Brian’s Seafaring Tales” has to say about log-lines: “A line of 100 fathoms or more attached at one end to the log-ship (a wooden apparatus in the shape of a pie wedge, attached to the log-line … weighted with lead along the arc, it floated point up in the water) which was released overboard. Knots tied in the log-line were counted as sand ran out of a log-glass (a half minute sandglass). With a 28 second glass, the line was divided into lengths of 47.33 feet; with a 30 second glass, 50.75 feet.” So, by counting the “knots” that passed in the given amount of time, you got your speed.

As more accurate instrumentation was developed, “knot” came to represent “nautical miles per hour.” I’m not a mind reader, but I bet you’re wondering how long a nautical mile is. The Merriam-Webster online dictionary says a NM is an international unit equal to 6076.115 feet (1852 meters) used officially in the U.S. since July 1, 1959.

Why have a different standard for statute (land) miles and nautical miles? Well, one nautical mile is equal to one minute of arc around the earth, assuming the earth were a perfect sphere. What’s a minute of arc? Remember that a circle has 360 degrees. Each degree is divided into 60 minutes, and each minute has 60 seconds. Degrees, minutes, and seconds of arc are used to express latitude and longitude. If a mariner sees on her navigational chart that she has to sail due north from 45 degrees north latitude to 46 degrees N, she knows she has to go 60 nautical miles (1 degree = 60 minutes). An easy way for a navigator to remember this is the saying: “A minute is a mile.”

Hope this answer satisfies your curiosity!

SDStaff Larasaurus & SDStaff Rngrjeff, Straight Dope Science Advisory Board

Send questions to Cecil via cecil@straightdope.com.

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