Ever wondered why ship’s speed is measured in knots and not in km/ hour or metre / sec? Well, read on to know the history behind it.
During earlier days, mariners used to measure how fast their boat was moving by throwing a piece of wood or other floatable object over the vessel’s bow and then by counting the amount of time elapsed before the vessel’s stern passed that piece of wood or the floatable object. This method was known as a Dutchman’s log method.
By the end of the 16th century, sailors started using a chip log to measure the speed. In this method, knots were tied at uniform intervals (14.4018 metres or 47 feet, 3 inches) in a length of rope and then one end of the rope attached to a pie-slice-shape piece of wood, lined with lead on the bottom. Sailors would throw the wood piece into the sea, behind the ship, and the rope would start unwinding from the reel. The faster the ship was moving forward the faster the rope would unwind.
The line of rope was allowed to roll out freely for about 28 seconds, which was tabulated with an hourglass. The number of knots that had gone over the ship’s stern was counted and used in calculating the vessel’s rate of speed.
A knot means one nautical mile per hour. Therefore, a ship travelling at 10 knots would go 10 nautical miles per hour. That is how the term “Knots” came into existence.
Here are two interesting videos to understand the whole process of measuring ship’s speed in knots: