Measuring Speed at Sea

The Common Log shown here (accession number 1934.9)was used to measure the distance a ship traveled through the water, thus allowing the calculation of speed. The log consists of a reel with line marked at set lengths with knots and a wooden "chip" attached. Additionally, a log glass, or half-minute glass similar in shape and function to an hour glass, is pictured. The process by which the log is used is called "heaving the log". It is usually a three-man operation with one person holding the reel, another the glass and the third the line with the chip. The chip, which is weighted on the bottom to hold it perpendicular in the water, is heaved over the lee side at the stern by the third man and at the appropriate time he commands the glass to be turned. The line runs off the reel, pulled by the resistance of the chip in the water, and after the glass is empty the line is stopped from paying out any further. The number of "knots" on the line that has run out indicates the speed of the vessel. For a more complete description, see the following link to the Museum's digital copy of William Falconer's 1815 book, A New Universal Dictionary of the Marine.