Two pirates were working together at the gunwale. One of them tossed a wood plank with a rope around it into the sea while the other counted the time. Once the time was up, the pirate stopped the rope and calculated the length of rope that was being released into the sea.
"Current speed is 10 knots!"
Most ships from western countries in the 18th century used this method to calculate their speed. Of course, it was not 100% accurate because the sea itself was fluid and was constantly flowing all the time. However, it was more than adequate. A more precise method to calculate speed wouldn't invented until the 20th century.
