How does a boat measure speed?

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Knots one nautical mile per hour there are 1 1/8 of a nautical mile in a statute mile

Originally measured by throwing a piece of wood attached to a piece of rope over the back of a boat and counting how many knots went past in a given time.

These days it is more normally measured by:

* a) Using a pitot static tube which measure difference in pressure and uses Bernoulli's equation to find the velocity.
* b) Some form of propeller (technically impeller) which is suspended under the boat. The passing water turns, the speed of rotation is measured, and this gives you the speed of the boat.
* c) Using ultrasonic to measure the speed, that small bubbles of air in the water passing under the boat go past.
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