![Speyer Handlog](https://upload.wikimedia.org/wikipedia/commons/a/a8/Speyer_Handlog.jpg)
A chip log, also called common log,
ship log, or just log, is a
navigation
Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...
tool
mariner
A sailor, seaman, mariner, or seafarer is a person who works aboard a watercraft as part of its crew, and may work in any one of a number of different fields that are related to the operation and maintenance of a ship.
The profession of the s ...
s use to estimate the
speed
In everyday use and in kinematics, the speed (commonly referred to as ''v'') of an object is the magnitude of the change of its position over time or the magnitude of the change of its position per unit of time; it is thus a scalar quanti ...
of a vessel through water. The word ''
knot
A knot is an intentional complication in cordage which may be practical or decorative, or both. Practical knots are classified by function, including hitches, bends, loop knots, and splices: a ''hitch'' fastens a rope to another object; a ' ...
'', to mean
nautical mile
A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. Historically, it was defined as the meridian arc length corresponding to one minute ( of a degree) of latitude. Today ...
per
hour
An hour (symbol: h; also abbreviated hr) is a unit of time conventionally reckoned as of a day and scientifically reckoned between 3,599 and 3,601 seconds, depending on the speed of Earth's rotation. There are 60 minutes in an hour, and 24 ho ...
, derives from this measurement method.
History
All nautical instruments that measure the speed of a ship through water are known as logs. This nomenclature dates back to the days of sail, when sailors tossed a log attached to a rope knotted at regular intervals off the
stern
The stern is the back or aft-most part of a ship or boat, technically defined as the area built up over the sternpost, extending upwards from the counter rail to the taffrail. The stern lies opposite the bow, the foremost part of a ship. Ori ...
of a ship. Sailors counted the number of knots that passed through their hands in a given time to determine the ship's speed. Today, sailors and aircraft pilots still express speed in knots.
Construction
A chip log consists of a
wood
Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin th ...
en board attached to a line (the
log-line
A rope is a group of yarns, plies, fibres, or strands that are twisted or braided together into a larger and stronger form. Ropes have tensile strength and so can be used for dragging and lifting. Rope is thicker and stronger than similarly c ...
). The log-line has a number of
knot
A knot is an intentional complication in cordage which may be practical or decorative, or both. Practical knots are classified by function, including hitches, bends, loop knots, and splices: a ''hitch'' fastens a rope to another object; a ' ...
s at uniform intervals. The log-line is wound on a reel so the user can easily
pay it out.
Over time, log construction standardized. The shape is a quarter
circle
A circle is a shape consisting of all points in a plane that are at a given distance from a given point, the centre. Equivalently, it is the curve traced out by a point that moves in a plane so that its distance from a given point is const ...
, or
quadrant with a radius of or ,
[ and thick.][ The log-line attaches to the board with a bridle of three lines that connect to the vertex and to the two ends of the quadrant's arc. To ensure the log submerges and orients correctly in the water, the bottom of the log is weighted with ]lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
.[ This provides more resistance in the water, and a more accurate and repeatable reading. The bridle attaches in such a way that a strong tug on the log-line makes one or two of the bridle's lines release, enabling a sailor to retrieve the log.
]
Use
A navigator who needed to know the speed of the vessel had a sailor drop the log over the ship's stern. The log acted as a drogue
A drogue (also known as a storm drogue) is a device trailed behind a boat on a long line attached to the stern. A drogue is used to slow the boat down in a storm and to prevent the hull (watercraft), hull from becoming side-on to the water waves ...
, remaining roughly in place while the vessel moved away. The sailor let the log-line run out for a fixed time while counting the knots that passed over. The length of log-line passing (the number of knots) determined the reading.
Origins
The first known device that measured speed is often claimed to be the Dutchman's log. This invention is attributed to the Portuguese
Portuguese may refer to:
* anything of, from, or related to the country and nation of Portugal
** Portuguese cuisine, traditional foods
** Portuguese language, a Romance language
*** Portuguese dialects, variants of the Portuguese language
** Portu ...
Bartolomeu Crescêncio Bartolomeu is a given name of Portuguese people, Portuguese, Galician language, Galician or Romanians, Romanian origin. It is a cognate of Bartholomew (name), Bartholomew. Notable people with this name include:
* Bartolomeu Anania – a Romanian Or ...
, who designed it in the end of the 15th century or in the beginning of the 16th century. A sailor threw a floating object overboard and used a sandglass to measure the time it took to pass between two points on deck. The first reference to a Dutchman's log is in 1623—later than 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.[Turner, Gerard L'E., ''Antique Scientific Instruments'', Blandford Press Ltd. 1980 ]
Mariner
A sailor, seaman, mariner, or seafarer is a person who works aboard a watercraft as part of its crew, and may work in any one of a number of different fields that are related to the operation and maintenance of a ship.
The profession of the s ...
s have used the log for a long time. The first known description of the device in print is in ''A Regiment for the Sea'' by William Bourne, in 1574. Bourne devised a half-minute sandglass for timing. At the time, a mile was reckoned as 5,000 feet, so in 30 seconds at one mile per hour, a ship would travel about 42 feet:
:
Initially, the log-line was not knotted and sailors measured the length directly on the line. With the introduction of the nautical mile
A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. Historically, it was defined as the meridian arc length corresponding to one minute ( of a degree) of latitude. Today ...
as a standard unit of measure
A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can be expressed as a multip ...
at sea in the 15th century, they began to mark the line at equal intervals proportional to the nautical mile and to the time interval used for measurement. Initially, the markings were simply knots in the line. Later, sailors worked knotted cords into the log-line. Many ships used knots spaced 8 fathom
A fathom is a unit of length in the imperial and the U.S. customary systems equal to , used especially for measuring the depth of water. The fathom is neither an International Standard (SI) unit, nor an internationally-accepted non-SI unit. Hi ...
s (48 feet
The foot ( : feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg made ...
or 14.6 meter
The metre (British spelling) or meter (American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its prefi ...
s) apart, while other ships used the 7-fathom prescription. The time interval needs to be adjusted according to the distance between knots. Substituting 6,000 feet for 1 mile, the above formula yields 28.8 seconds for a distance of 8 fathoms. In fact, 28-second and 14-second glasses used to be common among navigation equipment.
Accuracy and considerations by the navigator
Use of a log did not give an exact speed measure. The sailor had to incorporate a number of considerations:
*The amount of following sea
A following sea refers to a wave direction that is similar to the heading of a waterborne vessel under way. The word "sea" in this context refers to open water wind waves.
In the strict sense, a following sea has a direction of propagation betwee ...
*The effect of current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
s
*Stretch of the line
*Inaccuracy in the time measurement—because ambient temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
, humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
, and sea state
In oceanography, 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, p ...
affected the sandglass
Frequent measurements helped mitigate 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, the movement of the water itself—could not be corrected. Navigators relied on position fixes to correct for these errors. Modern navigation tools, such as GPS, report speed over ground, and in general do not give the same result as a log when currents are present.
More modern logs and replacements
*
Mechanical speed logs called patent logs[ or taffrail logs, operating on physical principles in a manner similar to a car's ]odometer
An odometer or odograph is an instrument used for measuring the distance traveled by a vehicle, such as a bicycle or car. The device may be electronic, mechanical, or a combination of the two (electromechanical). The noun derives from ancient Gr ...
by towing a vane or rotor from the stern (or taffrail
In naval architecture, a taffrail is the handrail around the open deck area toward the stern of a ship or boat. The rear deck of a ship is often called the afterdeck or poop deck. Not all ships have an afterdeck or poop deck. Sometimes taffrail r ...
) by a long line, were developed in the eighteenth century (or earlier) but became practical in the nineteenth century and replaced the traditional chip log. Examples of taffrail logs include those made by Thomas Walker & Son. Since the second half of the twentieth century, sailors continue to use more modern mechanical and electro-mechanical versions based upon a small impeller or paddle wheel attached to (or through) the bottom of the hull, especially on smaller yachts.
* Most commonly used however (especially on merchant ships) is the electromagnetic log An Electromagnetic Log, sometimes called an "EM log", is an electronic sensor which measures the speed of a vessel through sea water. Like many other technologies, its name derives from the traditional chip log. It makes use of Faraday's law of ind ...
, more reliable as there is no mechanism subject to breakdown.
* In recent years ultrasonic speed sensors have become available. These use two ultrasonic transducers—one forward, one aft—that send ultrasonic pulses through the water flowing past the hull. By calculating the time differential in pulse propagation from one sensor to the other, the device calculates the speed of the hull through the water.
* Doppler log: Today, a very accurate maritime speed measurement comes from Doppler measurement, either derived acoustically with Doppler-Sonar —or with radio interferometrically by Doppler measurement of satellite signals, such as those from the Global Positioning System (GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
). However, most commercial GPS systems are not configured to operate in this mode.
See also
*Pitometer log
Pitometer logs (also known as pit logs) are devices used to measure a ship's speed relative to the water. They are used on both surface ships and submarines. Data from the pitometer log is usually fed directly into the ship's navigation system.
H ...
(pit log) – a more modern form of log operating on the principle of differential pressure
*Dead reckoning
In navigation, dead reckoning is the process of calculating current position of some moving object by using a previously determined position, or fix, and then incorporating estimates of speed, heading direction, and course over elapsed time. ...
– a form of navigation that depends on speed measurement
*Traverse board
The traverse board is a tool formerly used in dead reckoning navigation to easily record the speeds and directions sailed during a watch. Even crew members who could not read or write could use the traverse board.
As the mathematician William Bou ...
– a traditional tool used aboard ship to keep track of speed and heading
References
*''The Oxford Companion to Ships and the Sea'', Peter Kemp ed., 1976 pp 192–193.
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 feet, but part C of "Construction" uses the modern distance of {{fraction, 47, 1, 4 feet.''
Measuring instruments
Navigational equipment
Portuguese inventions