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Currents

 

How Are Currents Measured? 1 2 3 4 5 6

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What is a “knot”?

The term “knot”, in reference to currents, is defined as one nautical mile per hour and is used to measure speed. A nautical mile is slightly more than a standard mile.

1 nautical mile = 1.15 miles = 1.85 kilometers
1 knot = 1.15 miles per hour = 1.85 kilometers per hour
1 knot = 20.251969 inches per second = 51.44 centimeters per second

 

animation of how knots are measured

The term knot dates from the 17th Century, when sailors measured the speed of their ship by the use of a device called a “common log.”  This device was a coil of rope with uniformly spaced knots tied in it, 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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