Knot (speed)
Encyclopedia
The knot is a unit of speed
equal to one nautical mile
(which is defined as 1.852 km) per hour, approximately 1.151 mph. The abbreviation kn is preferred by the International Hydrographic Organization
(IHO), which includes every major sea-faring nation; however, the abbreviations kt (singular) and kts (plural) are also widely used. However, use of the abbreviation kt for knot conflicts with the SI symbol for kilotonne
. The knot is a non-SI unit
accepted for use with the International System of Units
(SI). Worldwide, the knot is used in meteorology
, and in maritime
and air
navigation—for example, a vessel travelling at 1 knot along a meridian
travels one minute
of geographic longitude
in one hour. Etymologically, the term knot derives from counting the number of knot
s in the line that unspooled from the reel of a chip log
in a specific time.
1,852 m is the length of the internationally-agreed nautical mile. The U.S. adopted the international definition in 1954, having previously used the U.S. nautical mile (1,853.248 m). The U.K. adopted the international nautical mile definition in 1970, having previously used the U.K. Admiralty
nautical mile (6,080 ft [1,853.184 m]).
The speeds of vessels relative to the fluid
s in which they travel (boat speeds and air speeds) are measured in knots. For consistency, the speeds of navigational fluids (tidal stream
s, river currents and wind speed
s) are also measured in knots. Thus, speed over the ground (SOG) (ground speed
(GS) in aircraft) and rate of progress towards a distant point ("velocity
made good", VMG) are also given in knots.
. This consisted of a wooden panel, weighted on one edge to float upright, and thus present substantial resistance to moving with respect to the water around it, attached by line to a reel. The chip log was "cast" over the stern of the moving vessel and the line allowed to pay out. Knot
s placed at a distance of 47 feet 3 inch
es (14.4018 m) passed through a sailor's fingers, while another sailor used a 30 second sand-glass
(28 second sand-glass is the current accepted timing) to time the operation. The knot count would be reported and used in the sailing master's dead reckoning
and navigation
. This method gives a value for the knot of 20.25 in/s, or 1.85166 km/h. The difference from the modern definition is less than 0.02%.
and the scale varies with latitude. On a chart of the North Atlantic, the scale varies by a factor of two from Florida to Greenland. A single graphic scale
, of the sort on many maps, would therefore be useless on such a chart. Since the length of a nautical mile is, for practical purposes, identical to a minute of latitude, a distance in nautical miles on a chart can easily be measured by using dividers and the latitude scales on the sides of the chart.
Speed is sometimes incorrectly expressed as "knots per hour", which would actually be a measure of acceleration, as in "nautical miles per hour per hour".
specified that distances were to be in statute miles, and speeds in miles per hour. In 1969 these standards were progressively amended to specify that distances were to be in nautical miles, and speeds in knots.
The following abbreviations are used to distinguish between various measurements of airspeed
.
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...
equal to one 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...
(which is defined as 1.852 km) per hour, approximately 1.151 mph. The abbreviation kn is preferred by the International Hydrographic Organization
International Hydrographic Organization
The International Hydrographic Organization is the inter-governmental organisation representing the hydrographic community. It enjoys observer status at the UN and is the recognised competent authority on hydrographic surveying and nautical charting...
(IHO), which includes every major sea-faring nation; however, the abbreviations kt (singular) and kts (plural) are also widely used. However, use of the abbreviation kt for knot conflicts with the SI symbol for kilotonne
Tonne
The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...
. The knot is a non-SI unit
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
accepted for use with the International System of Units
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
(SI). Worldwide, the knot is used in meteorology
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
, and in maritime
Shipping
Shipping has multiple meanings. It can be a physical process of transporting commodities and merchandise goods and cargo, by land, air, and sea. It also can describe the movement of objects by ship.Land or "ground" shipping can be by train or by truck...
and air
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...
navigation—for example, a vessel travelling at 1 knot along a meridian
Meridian (geography)
A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations along it with a given longitude. The position of a point along the meridian is given by its latitude. Each meridian is perpendicular to all circles of latitude...
travels one minute
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....
of geographic longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
in one hour. Etymologically, the term knot derives from counting the 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 in the line that unspooled from the reel of a 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.-Construction:...
in a specific time.
Definitions
1 international knot =- 1 nautical mileNautical mileThe 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 hourHourThe hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...
(by definition), - 1.852 kilometres per hourKilometres per hourThe kilometre per hour is a unit of speed, expressing the number of kilometres traveled in one hour. The unit symbol is km/h or km·h−1....
(exactly), - 0.514 metres per second,
- 1.151 miles per hour (approximately).
1,852 m is the length of the internationally-agreed nautical mile. The U.S. adopted the international definition in 1954, having previously used the U.S. nautical mile (1,853.248 m). The U.K. adopted the international nautical mile definition in 1970, having previously used the U.K. Admiralty
Admiralty
The Admiralty was formerly the authority in the Kingdom of England, and later in the United Kingdom, responsible for the command of the Royal Navy...
nautical mile (6,080 ft [1,853.184 m]).
The speeds of vessels relative to the fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
s in which they travel (boat speeds and air speeds) are measured in knots. For consistency, the speeds of navigational fluids (tidal stream
Tidal stream
A tidal stream can refer to two different phenomena:*in marine science it refers to the currents associated with the tides, generally near a coastline or harbor....
s, river currents and wind speed
Wind speed
Wind speed, or wind velocity, is a fundamental atmospheric rate.Wind speed affects weather forecasting, aircraft and maritime operations, construction projects, growth and metabolism rate of many plant species, and countless other implications....
s) are also measured in knots. Thus, speed over the ground (SOG) (ground speed
Ground speed
Ground speed is the speed of an aircraft relative to the ground. Information displayed to passengers through the entertainment system often gives the aircraft groundspeed rather than airspeed....
(GS) in aircraft) and rate of progress towards a distant point ("velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
made good", VMG) are also given in knots.
Origin
Until the mid-19th century vessel speed at sea was measured using a chip logChip 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.-Construction:...
. This consisted of a wooden panel, weighted on one edge to float upright, and thus present substantial resistance to moving with respect to the water around it, attached by line to a reel. The chip log was "cast" over the stern of the moving vessel and the line allowed to pay out. 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 placed at a distance of 47 feet 3 inch
Inch
An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...
es (14.4018 m) passed through a sailor's fingers, while another sailor used a 30 second sand-glass
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...
(28 second sand-glass is the current accepted timing) to time the operation. The knot count would be reported and used in the sailing master's dead reckoning
Dead reckoning
In 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...
and navigation
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...
. This method gives a value for the knot of 20.25 in/s, or 1.85166 km/h. The difference from the modern definition is less than 0.02%.
Modern use
Although the unit knot does not fit within the primary SI system, its retention for nautical and aviation use is important because standard nautical charts are on the Mercator projectionMercator projection
The Mercator projection is a cylindrical map projection presented by the Belgian geographer and cartographer Gerardus Mercator, in 1569. It became the standard map projection for nautical purposes because of its ability to represent lines of constant course, known as rhumb lines or loxodromes, as...
and the scale varies with latitude. On a chart of the North Atlantic, the scale varies by a factor of two from Florida to Greenland. A single graphic scale
Linear scale
A linear scale, also called a bar scale, scale bar, graphic scale, or graphical scale, is a means of visually showing the scale of a map, nautical chart, engineering drawing, or architectural drawing....
, of the sort on many maps, would therefore be useless on such a chart. Since the length of a nautical mile is, for practical purposes, identical to a minute of latitude, a distance in nautical miles on a chart can easily be measured by using dividers and the latitude scales on the sides of the chart.
Speed is sometimes incorrectly expressed as "knots per hour", which would actually be a measure of acceleration, as in "nautical miles per hour per hour".
Aeronautical terms
Prior to 1969, airworthiness standards for civil aircraft in the United States Federal Aviation RegulationsFederal Aviation Regulations
The Federal Aviation Regulations, or FARs, are rules prescribed by the Federal Aviation Administration governing all aviation activities in the United States. The FARs are part of Title 14 of the Code of Federal Regulations...
specified that distances were to be in statute miles, and speeds in miles per hour. In 1969 these standards were progressively amended to specify that distances were to be in nautical miles, and speeds in knots.
The following abbreviations are used to distinguish between various measurements of airspeed
Airspeed
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....
.
- KTAS is "knots true airspeedTrue airspeedTrue airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. True airspeed is important information for accurate navigation of an aircraft.-Performance:...
", the airspeed of an aircraft relative to undisturbed air. - KIAS is "knots indicated airspeedIndicated airspeedIndicated airspeed is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. IAS is directly related to calibrated airspeed , which is the IAS corrected for instrument and installation errors....
", the speed shown on an aircraft's pitot-staticPitot-static systemA pitot-static system is a system of pressure-sensitive instruments that is most often used in aviation to determine an aircraft's airspeed, Mach number, altitude, and altitude trend. A pitot-static system generally consists of a pitot tube, a static port, and the pitot-static instruments...
airspeed indicatorAirspeed indicatorThe airspeed indicator or airspeed gauge is an instrument used in an aircraft to display the craft's airspeed, typically in knots, to the pilot.- Use :...
. - KCAS is "knots calibrated airspeedCalibrated airspeedCalibrated airspeed is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most civilian EFIS displays also show CAS...
", the indicated airspeed corrected for position errorPosition errorPosition error is one of the errors affecting the systems in an aircraft for measuring airspeed and altitude. It is not practical or necessary for an aircraft to have an airspeed indicating system and an altitude indicating system that are exactly accurate...
and instrument error. - KEAS is "knots equivalent airspeedEquivalent airspeedEquivalent airspeed is the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same as the dynamic pressure at the true airspeed and altitude at which the aircraft is flying. In low-speed flight, it is the speed which would be shown by an airspeed...
", the calibrated airspeed corrected for adiabaticAdiabatic processIn thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero. Such a process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time,...
compressible flowCompressible flowCompressible flow is the area of fluid mechanics that deals with fluids in which the fluid density varies significantly in response to a change in pressure. Compressibility effects are typically considered significant if the Mach number of the flow exceeds 0.3, or if the fluid undergoes very large...
for the particular altitude.
See also
- Beaufort scaleBeaufort scaleThe Beaufort Scale is an empirical measure that relates wind speed to observed conditions at sea or on land. Its full name is the Beaufort Wind Force Scale.-History:...
- Hull speedHull speedHull speed, sometimes referred to as displacement speed, is the speed of a boat at which the bow and stern waves interfere constructively, creating relatively large waves, and thus a relatively large value of wave drag...
, which deals with theoretical estimates of practical maximum speed of displacement hulls. - Metre per secondMetre per secondMetre per second is an SI derived unit of both speed and velocity , defined by distance in metres divided by time in seconds....
- Nautical mileNautical mileThe 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...
- Orders of magnitude (speed)Orders of magnitude (speed)To help compare different orders of magnitude, the following list describes various speed levels between approximately 1 m/s and 3 m/s. Values in bold are exact.- List of orders of magnitude for speed :-See also:...
- Knotted cordKnotted cordA knotted cord was a primitive surveyor's tool for measuring distances. It is literally a length of cord with knots at regular intervals. They were eventually replaced by surveyor's chains, which being made of metal were less prone to stretching and thus were more accurate and consistent.Knotted...
- Rope (unit)Rope (unit)A rope was a unit of measurement, used in Somersetshire in drainage, hedging, and wall building. It is both a unit of length and a unit of area. As a linear measure, used in drainage and hedging, it is equal to 20 feet, i.e. 6.096 m . As a measure of area, used in wall building, it is equal to...