Colorized drawing of a chip log and hour glass timer. This is a navigation tool used by mariners to estimate the speed of a vessel through water. A chip log consists of a wooden board attached to a line (the log-line). The log-line has a number of knots tied in it at uniform spacings. The log-line is wound on a reel to allow it to be paid out easily in use. Originally, the distance between marks was 7 fathoms or 42 feet used with a sandglass with a 30 second running time. Later the distance was refined to 47 feet, 3 inches for a standard glass of 28 seconds.