A Dutchman’s log is an early speed measuring device that used a buoyant object — usually a large piece of wood — tossed overboard near the bow of the vessel and assumed to be “dead” in the water. The time it took for the boat to move past the object over a measured distance — based on marks near the bow and near the stern on the vessel — was timed. From there, the speed of the vessel was calculated. As this was an early speed measuring device, it didn’t take into account the effects of wind and currents on the calculated position of the vessel.
Captain John Smith — he of Pocahontas fame — wrote a book entitled, “An Accidence or The Pathway to Experience Necessary for all Young Sea-men, or those that are desirous to goe to Sea” that was first published in London in 1626. In this book, he makes mention of the Dutchman’s log.
According to the “Houghton Mifflin Guide to Science & Technology” the term Dutchman’s log was in use in 1575. Two years later, Humphry Cole invented the ship’s log which kept track of a ship’s speed with respect to the water. This invention was known as the log-and-line and consisted of a float attached to a line that was a specific length and that was paid out for a specific length of time.
In the end Idiomation tracked down that the Dutchman’s log was invented by Portuguese inventor, Bartolomeu Crescêncio, near the end of 15th century putting in the late 1400s.
Historically Speaking 