Navigation and Seamanship

3.2.2.1. Operation of chip log

3.2.2.1. Operation of chip log or ship log

When the sailor wanted to determine speed, the log chip was thrown overboard where it floated vertically due to its ballasting. A considerable length of log line was let out to get the log chip into undisturbed water astern, where its resistance to forward motion keeps it essentially stationary in water (the wooded plank stay at the same place in water). As the ship moved forward, the line was pulled off the reel. This was usually a 30 second timer and gave far more accurate results. Markings were made on the log line which represented the distance travelled by the ship. A sand glass was also used to measure the time taken by the ship to move away from the chip log. According to the length of the log line released over the stern and the markings on the line, the speed of the ship was calculated. Originally, the distance between marks was 7 fathoms (42 feet) used with a sandglass with a 30 second running time. Later refinements in the length and distance were set to 47 feet , 3 inches (14.4 meters ) for a standard glass of 28 seconds. The name of the unit knot , for nautical mile per hour, was derived from this method of measurement.

Various factors related to the sea condition such as speed and direction of currents, stretching of the log line and inaccuracy of measured time. Two men were required to use the log, one to hold the reel and other to throw the wooden piece. In a improved version ship log, the wooden chip was replaced the with a propeller like rotator. The revolutions of the propeller were recorded, revealing both the distance covered and the speed at any given time. Apart from the traditional chip logs, various mechanical chip logs were also invented and used in the later years which worked more on the basis of physical properties and thus provided accuracy. Today, chip logs have been substituted by the ultra modern GPS systems