Knot
Symbol: knWorldwide (maritime and aviation)
¿Qué es un/una Knot (kn)?
Formal Definition
The knot (symbol: kn) is a unit of speed equal to one nautical mile per hour. Since the international nautical mile is defined as exactly 1,852 meters, one knot equals 1,852 meters per hour, or approximately 0.51444 m/s, 1.852 km/h, or 1.15078 mph. The knot is the standard unit of speed in maritime and aviation navigation worldwide.
The knot's definition is tied directly to the nautical mile, which was originally defined as one minute of arc of latitude on the Earth's surface. This geographic relationship gives the knot a special utility in navigation: a vessel traveling at 1 knot covers 1 minute of latitude per hour, making position calculations on nautical charts straightforward.
Naming Convention
The knot is both a speed unit and its own name — saying "knots per hour" is incorrect and redundant, as a knot already means nautical miles per hour. The correct usage is simply "knots": a ship travels at "20 knots," not "20 knots per hour." This is a common source of confusion among non-mariners.
Etymology
The Chip Log
The word "knot" comes from the historical method of measuring a ship's speed using a chip log (also called a common log). This device, in use from at least the 16th century, consisted of a triangular piece of wood (the chip) attached to a long rope with knots tied at regular intervals. The chip was thrown overboard and the rope allowed to run out freely. A sailor counted how many knots passed through their hands during a fixed time interval (measured by a sandglass), and the number of knots gave the ship's speed.
The original knot spacing was calculated so that the count directly gave speed in nautical miles per hour. With a 28-second sandglass, the knots were spaced approximately 14.4 meters (47 feet 3 inches) apart. If seven knots passed through the sailor's hands in 28 seconds, the ship was moving at 7 nautical miles per hour — 7 knots.
Evolution of the Term
The term "knot" for the speed unit (rather than the rope feature) was well established by the 17th century. Samuel Pepys, the English diarist and naval administrator, recorded ship speeds in knots in the 1660s. The transition from the physical knot-counting method to the abstract speed unit was complete by the 19th century, though chip logs remained in use as backup equipment into the 20th century.
Precise Definition
International Definition
The knot is defined as one international nautical mile (1,852 meters exactly) per hour: 1 kn = 1,852 m/h = 1.852 km/h = 0.514444 m/s. This definition was standardized by the International Hydrographic Organization (IHO) and the International Civil Aviation Organization (ICAO).
Symbol Conventions
The internationally recognized symbol for the knot is "kn." However, "kt" is also commonly used, particularly in aviation (e.g., in METAR weather reports). The ISO 80000-3 standard specifies "kn." The abbreviation "kts" (with an 's' for plural) is sometimes used informally but is not standard. The unit should never be written as "knots/hour" or "kn/h," as the knot already includes the time component.
Relationship to Navigation
The knot's value in navigation derives from its connection to the nautical mile and Earth's geometry. The Earth's circumference is approximately 21,600 nautical miles (360° × 60 minutes per degree). A vessel at the equator, heading due north at 1 knot, changes its latitude by 1 minute (1/60 of a degree) every hour. This direct correspondence between speed and angular distance simplifies dead reckoning and chart-based navigation.
Historia
Age of Sail
During the Age of Sail (15th-19th centuries), accurate speed measurement was critical for navigation. Without GPS or radio, sailors relied on dead reckoning — estimating position by tracking speed and direction from a known starting point. The chip log provided a reasonably accurate speed measurement, and the knot became the universal maritime speed unit among European seafaring nations.
Prior to the chip log, speed was estimated by watching foam or debris pass along the hull — a method called "Dutchman's log." The chip log, introduced in the late 16th century (first documented in William Bourne's "A Regiment for the Sea" in 1574), represented a major improvement in accuracy.
Standardization
As navigation became more scientific in the 18th and 19th centuries, the nautical mile and knot were gradually standardized. Different countries used slightly different nautical mile definitions until the international nautical mile (1,852 meters) was adopted by the International Hydrographic Bureau (now IHO) in 1929. The United States adopted this definition in 1954, and the UK in 1970, making the knot truly international.
Aviation Adoption
When aviation emerged in the early 20th century, the knot was adopted for airspeed because pilots needed to navigate over long distances using the same charts and principles as sailors. ICAO standardized the use of knots for airspeed and nautical miles for distance in international aviation. This convention unifies maritime and aviation navigation under the same speed and distance system.
Modern Measurement
Modern vessels and aircraft measure speed using GPS, Doppler radar, pitot tubes (aircraft), and electromagnetic speed logs (ships). These instruments display speed in knots, maintaining continuity with centuries of maritime tradition while achieving vastly greater accuracy than the original chip log.
Uso actual
Maritime Navigation
Every commercial vessel, naval ship, and ocean-going yacht uses knots as the primary speed unit. Container ships cruise at 12-25 knots. Tankers at 10-16 knots. Naval destroyers can exceed 30 knots. Aircraft carriers operate at 30+ knots. Luxury cruise ships cruise at 18-24 knots. Sailing yachts typically sail at 5-10 knots.
Aviation
All airspeed indicators in aircraft display speed in knots. Air traffic control communicates speeds in knots. Aviation weather reports (METARs, TAFs) report wind speed in knots. Typical airspeeds: light aircraft 90-150 knots, jet airliners 450-550 knots (cruising), military fighters 400-1,200+ knots. Flight plans calculate time en route using groundspeed in knots.
Meteorology
International meteorological organizations (WMO) recommend reporting wind speed in m/s or knots. Marine weather forecasts use knots exclusively. The Beaufort scale, widely used for sea state, defines wind speeds in knots: Force 4 (moderate breeze) is 11-16 knots, Force 8 (gale) is 34-40 knots, and Force 12 (hurricane) is 64+ knots. Storm warnings and small craft advisories are issued with wind speeds in knots.
Ocean Currents
Oceanographers measure current speeds in knots. The Gulf Stream flows at 1-5 knots depending on location. The Kuroshio Current reaches 2-4 knots. Tidal currents in straits and channels can exceed 10 knots. Navigators must account for current speed in knots when planning passages.
Everyday Use
Boating and Sailing
Recreational boaters use knots as their speed reference. A sailboat in light wind makes 3-5 knots. A motorboat cruises at 15-30 knots. A personal watercraft (jet ski) can reach 55-65 knots. Understanding knots is essential for boating safety: speed through water, fuel consumption calculations, and arrival time estimates all use knots.
Fishing
Fishermen use knots for trolling speeds (2-8 knots depending on target species), current speed at fishing grounds, and travel time to fishing areas. Marine GPS units display speed in knots. Wind forecasts in knots help fishermen decide whether conditions are safe for their vessel.
Converting Knots
A useful approximation for quick mental conversion: 1 knot ≈ 1.15 mph ≈ 1.85 km/h. For rough estimates, increase knots by about 15% to get mph. To convert to km/h, roughly double the knots and subtract 10%. For example, 20 knots ≈ 23 mph ≈ 37 km/h.
Flight Tracking
When tracking flights on apps like Flightradar24 or FlightAware, ground speed is typically shown in knots (or converted to km/h or mph by the app). A commercial jet cruising at 35,000 feet might show a ground speed of 450-550 knots. Headwinds or tailwinds can cause significant variations.
In Science & Industry
Physical Oceanography
Ocean current measurements use knots in applied contexts and m/s in research publications. Surface drifter buoys report speeds in knots or cm/s. The Antarctic Circumpolar Current, the largest ocean current, has average speeds of 0.5-1 knot. Mesoscale eddies in the ocean have rotation speeds of 0.5-2 knots.
Marine Biology
Animal swimming speeds are sometimes reported in knots for marine species. A blue whale can cruise at approximately 5 knots and sprint at 20 knots. Dolphins swim at 10-20 knots. A sailfish — the fastest fish — can reach approximately 60 knots (110 km/h) in short bursts.
Naval Architecture
Ship design uses knots as the standard speed unit. Hull speed — the theoretical maximum speed of a displacement hull — is calculated using waterline length and expressed in knots: hull speed (knots) ≈ 1.34 × √(waterline length in feet). The Froude number, a dimensionless parameter relating speed to wave-making resistance, is often calculated with speed input in knots.
Climate Science
Tropical cyclone intensity classification uses knots. The Saffir-Simpson scale was originally defined in knots: Category 1 (64-82 knots), Category 2 (83-95 knots), Category 3 (96-112 knots), Category 4 (113-136 knots), Category 5 (137+ knots). These definitions remain primary in WMO tropical cyclone advisories.
Interesting Facts
The chip log method of measuring speed survived for over 400 years. Royal Navy vessels carried chip logs as backup navigation equipment into the 1950s, long after mechanical and electronic speed measurement was available.
The fastest sailing vessel is the Vestas Sailrocket 2, which reached 65.45 knots (121.1 km/h) in 2012 in Namibia. For context, most recreational sailboats rarely exceed 8 knots.
The Beaufort scale was devised by Rear Admiral Sir Francis Beaufort of the Royal Navy in 1805 and remains in use today. Its original descriptions were based on the amount of sail a warship could carry, not wind speed in knots.
A nautical mile equals approximately 1.151 statute miles. This means that speed in knots is always about 15% higher numerically than speed in mph — a ship at 20 knots is moving at about 23 mph.
USS Blue Ridge, the US Navy's oldest operational warship (commissioned 1970), still has a maximum speed classified simply as '23+ knots' — naval tradition of expressing ship speeds in whole knots persists.
The fastest commercial vessel is the Francisco, an Australian-built high-speed ferry operating in Argentina, which cruises at 50+ knots (93 km/h) using gas turbine engines and a wave-piercing catamaran hull.