Chip log

Chip log

A chip log, also called common log, ship log or just log, is a navigation tool used by mariners to estimate the speed of a vessel through water.

Contents

Construction

Diagram of a chip log
Ship log and associated kit. The reel of log-line is clearly seen. The timing sandglass is in the upper left and the log in the lower left. The small light-coloured wooden pin and plug form a release mechanism for two lines of the bridle.
From the Musée de la Marine, Paris.

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.

Over time, the log was standardized in construction. The shape is a quarter circle, or quadrant, and the log-line is attached to the board with a bridle of three lines connected to the vertex and to the two ends of the quadrant's arc. In order to ensure that the log submerges and is oriented correctly, the bottom of the log is weighted with lead. This provides for more resistance in the water and a more accurate and repeatable reading of speed. The bridle is attached in such a way that a strong tug on the log-line results in one or two of the bridle's lines releasing, allowing the log to be retrieved with relative ease.

Usage

When the navigator wished to determine the speed of his vessel, a sailor dropped the log over the stern of the ship. The log would act as a drogue and remain roughly in place while the vessel moved away. The log-line was allowed to run out for a fixed period of time. The speed of the ship was indicated by the length of log-line passing over the stern during that time.

Origins

The first known device to measure speed is often claimed to be the Dutchman's Log. However this invention is attributed to the Portuguese Bartolomeu Crescêncio, who designed it in the end of 15th century or in the beginning of 16th century.[1] An object that would float was thrown overboard and the time required to pass between two points on deck was measured with a sandglass. However, the first reference to a Dutchman's log is in 1623, after the ship log.[2] The Dutchman's log could be used with a brass tobacco box, rectangular with rounded ends. This box had tables on it to convert log timing to speed.[3][4]

The log has been used by mariners for a long time. The first occurrence of a description of the device in print was in A Regiment for the Sea by William Bourne in 1574.

Initially, the log-line was not knotted and the length was measured directly on the line. With the introduction of the nautical mile as a standard unit of measure at sea in the 15th century, the line began to be marked at equal intervals proportional to the nautical mile and to the time interval used for measurement. Initially, the markings were in the form of knots in the line. Later, knotted cords were worked into the log-line.

Originally, the distance between marks was 7 fathoms or 42 feet used with a sandglass with a 30 second running time. Later refinements in the length of the nautical mile caused the distance between knots to be changed. Eventually, the distance was 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.

Accuracy and considerations by the navigator

The use of a log did not give an exact measure of speed. A number of considerations had to be taken into account, for example:

Frequent measurements helped in mitigating some of these inaccuracies by averaging out individual errors, and experienced navigators could determine their speed through the water with a fair degree of accuracy.

Because a log measures the speed through the water, some errors, especially the effect of currents, that is the movement of the water itself, can not be corrected for. Navigators rely on position fixes to correct for these errors.

Modern navigation tools such as GPS report speed over ground and in general will not give the same results as a log when currents are present.

More modern logs and replacements

Modern version of a mechanical log

Mechanical chip logs, operating on physical principles in a manner similar to a car's odometer, were eventually developed and replaced the traditional chip log.

Today, the most accurate means of maritime speed measurement comes from Doppler measurement, either derived acoustically by use of Doppler Sonar or radio interferometrically by Doppler measurement of satellite signals such as those from Global Positioning System (GPS). Most commercial GPS systems are not configured to operate in this mode, however.

See also

References

  1. ^ URL:http://www.cienciaviva.pt/projectos/concluidos/inventions/instrumentos.asp Agência Nacional para a Cultura Científica, Ciência Viva, Descobertas e Invenções Portuguesas (in Portuguese)
  2. ^ May, William Edward, A History of Marine Navigation, G. T. Foulis & Co. Ltd., Henley-on-Thames, Oxfordshire, 1973, ISBN 0-85429-143-1
  3. ^ Turner, Gerard L'E., Antique Scientific Instruments, Blandford Press Ltd. 1980 ISBN 0-7137-1068-3
  4. ^ The ‘Dutchman’s Log’ or the Seaman’s Tobacco Box of Pieter Holm
  • The Oxford Companion to Ships and the Sea, Peter Kemp ed., 1976 pp 192–193. ISBN 0-586-08308-1

External links

  • Chip Log pattern on the webpage of the Navy & Marine Living History Association, Inc. note: the distance given in the materials list for this pattern is 33 1/3 feet, but part C of "Construction" uses the modern distance of 47 1/4 feet.

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Look at other dictionaries:

  • chip log — noun : the common log consisting of a chip and log line * * * Naut. a log for measuring the speed of a vessel. Also called hand log. [1825 35] * * * chip log, a device for measuring a ship s speed, consisting of a cord with carefully spaced knots …   Useful english dictionary

  • chip log — /ˈtʃɪp lɒg/ (say chip log) noun → log (def. 3b) …  

  • chip log — Naut. a log for measuring the speed of a vessel. Also called hand log. [1825 35] * * * …   Universalium

  • chip log — noun A simple device for measuring the speed of a vessel, consisting of a wooden board (drag) attached to a line on a reel. Syn: hand log …   Wiktionary

  • Log-chip — n. (Naut.) A thin, flat piece of board in the form of a quadrant of a circle attached to the log line; called also {log ship}. See 2d {Log}, n., 2. [1913 Webster] …   The Collaborative International Dictionary of English

  • log-ship — Log chip Log chip , n. (Naut.) A thin, flat piece of board in the form of a quadrant of a circle attached to the log line; called also {log ship}. See 2d {Log}, n., 2. [1913 Webster] …   The Collaborative International Dictionary of English

  • Log — may refer to: *Chip log, a device used in navigation to estimate the speed of a vessel through water. *Data log, a record of sequential data *Laplacian of Gaussian * Log (journal) , an architectural magazine *Log (village), a former urban type… …   Wikipedia

  • log chip — noun also log ship : a thin flat usually triangular piece of wood that is typically like a quadrant of a circle and that usually has a radius of 5 or 6 inches and is loaded with lead on the arc so as to float point up and that is attached to a… …   Useful english dictionary

  • log — log1 loggish, adj. /lawg, log/, n., v., logged, logging. n. 1. a portion or length of the trunk or of a large limb of a felled tree. 2. something inert, heavy, or not sentient. 3. Naut. any of various devices for determining the speed of a ship,… …   Universalium

  • log — /lɒg / (say log) noun 1. an unhewn portion or length of the trunk or a large limb of a felled tree. 2. something inert or heavy. 3. Nautical a. a device for determining the speed of and distance covered by a ship. b. Also, chip log. a device… …  

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