Chip log
Encyclopedia
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.
A chip log consists of a wood
en board attached to a line (the log-line). The log-line has a number of knot
s 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.
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.
Bartolomeu Crescêncio, who designed it in the end of 15th century or in the beginning of 16th century. 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. 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.
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 fathom
s 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.
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.
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.
Navigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...
tool used by mariners to estimate the speed
Speed
In kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
of a vessel through water.
Construction
Wood
Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...
en board attached to a line (the log-line). The log-line has a number of knot
Knot
A knot is a method of fastening or securing linear material such as rope by tying or interweaving. It may consist of a length of one or several segments of rope, string, webbing, twine, strap, or even chain interwoven such that the line can bind to itself or to some other object—the "load"...
s 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
Circle
A circle is a simple shape of Euclidean geometry consisting of those points in a plane that are a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius....
, or quadrant
Cartesian coordinate system
A Cartesian coordinate system specifies each point uniquely in a plane by a pair of numerical coordinates, which are the signed distances from the point to two fixed perpendicular directed lines, measured in the same unit of length...
, 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
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
. 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 drogueDrogue
A drogue is a device external to the boat, attached to the stern used to slow a boat down in a storm and to keep the hull perpendicular to the waves. The boat will not speed excessively down the slope of a wave and crash into the next one nor will it broach. By slowing the vessel in heavy...
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 PortuguesePortugal
Portugal , officially the Portuguese Republic is a country situated in southwestern Europe on the Iberian Peninsula. Portugal is the westernmost country of Europe, and is bordered by the Atlantic Ocean to the West and South and by Spain to the North and East. The Atlantic archipelagos of the...
Bartolomeu Crescêncio, who designed it in the end of 15th century or in the beginning of 16th century. An object that would float was thrown overboard and the time required to pass between two points on deck was measured with a sandglass
Hourglass
An hourglass measures the passage of a few minutes or an hour of time. It has two connected vertical glass bulbs allowing a regulated trickle of material from the top to the bottom. Once the top bulb is empty, it can be inverted to begin timing again. The name hourglass comes from historically...
. However, the first reference to a Dutchman's log is in 1623, after the ship log. 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.
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
William Bourne (mathematician)
William Bourne was an English mathematician, innkeeper and former Royal Navy gunner who invented the first navigable submarine and wrote important navigational manuals...
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
Nautical mile
The nautical mile is a unit of length that is about one minute of arc of latitude along any meridian, but is approximately one minute of arc of longitude only at the equator...
as a standard unit of measure
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...
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 fathom
Fathom
A fathom is a unit of length in the imperial and the U.S. customary systems, used especially for measuring the depth of water.There are 2 yards in an imperial or U.S. fathom...
s or 42 feet used with a sandglass
Hourglass
An hourglass measures the passage of a few minutes or an hour of time. It has two connected vertical glass bulbs allowing a regulated trickle of material from the top to the bottom. Once the top bulb is empty, it can be inverted to begin timing again. The name hourglass comes from historically...
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
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...
knot, for nautical mile
Nautical mile
The nautical mile is a unit of length that is about one minute of arc of latitude along any meridian, but is approximately one minute of arc of longitude only at the equator...
per hour
Hour
The hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...
, 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:- the amount of following seaFollowing seaIn boating, a following sea refers to a wave direction that matches the heading of the boat. For example, if the waves of the body of water are heading in the same direction as the sailor, then the water is "following" the sailor's boat...
- the effect of currentOcean currentAn ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...
s - stretch of the line
- inaccuracies in the measurement of elapsed time. Sandglasses were affected by ambient temperatureTemperatureTemperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, humidityHumidityHumidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
, and sea stateSea stateIn oceanography, a sea state is the general condition of the free surface on a large body of water—with respect to wind waves and swell—at a certain location and moment. A sea state is characterized by statistics, including the wave height, period, and power spectrum. The sea state varies with...
.
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
Odometer
An odometer or odograph is an instrument that indicates distance traveled by a vehicle, such as a bicycle or automobile. The device may be electronic, mechanical, or a combination of the two. The word derives from the Greek words hodós and métron...
, were eventually developed and replaced the traditional chip log.
Today, the most accurate means of maritime speed measurement comes from Doppler
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
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
- Dead reckoningDead reckoningIn navigation, dead reckoning is the process of calculating one's current position by using a previously determined position, or fix, and advancing that position based upon known or estimated speeds over elapsed time, and course...
- a form of navigation which depends on speed measurement - Traverse boardTraverse boardThe traverse board is a memory aid formerly used in dead reckoning navigation to easily record the speeds and directions sailed during a watch. Even an illiterate crew member could use the traverse board....
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.