How did ships know how fast they were going?
What is a ship's log?
How were knots invented for speed?
How long is a nautical mile?
Watch this old footage for the answers.
(Duration: 5:25 minutes)
Ship's Logs & Knots
Oceanservice.noaa.gov explains it this way:
'A knot is one nautical mile per hour (1 knot = 1.15 miles per hour). The term knot dates from the 17th century, when sailors measured the speed of their ship by using a device called a "common log." This device was a coil of rope with uniformly spaced knots, attached to a piece of wood shaped like a slice of pie. The piece of wood was lowered from the back of the ship and allowed to float behind it. The line was allowed to pay out freely from the coil as the piece of wood fell behind the ship for a specific amount of time. When the specified time had passed, the line was pulled in and the number of knots on the rope between the ship and the wood were counted. The speed of the ship was said to be the number of knots counted (Bowditch, 1984).'
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