Node (unit)

Article

May 28, 2022

Knot is a unit of speed measurement equivalent to one nautical mile per hour, or 1.852 km/h. Although not a unit of the International System of Units (SI), it is often used, although not encouraged, in direct relation with the units of the international system. The discouragement is due to the need for mathematical adjustments due to the curvature of the Earth. This is due to the fact that the concept of the Knot is directly related to the dimensions of the Earth, with a nautical mile corresponding to the length of 1' (one minute) of the great circle.

Definition

The conventional value of the node (0.514 m/s), corresponding to a nautical mile of 1 852 m (the "International Nautical Mile"), was adopted at the First International Extraordinary Hydrographic Conference, held in Monaco in 1929. The knot is thus a unit derived from the nautical mile, or nautical mile: 1 knot 1 nautical mile/hour 1,852 meters/hour 1,852 kilometers/hour .Since the knot is, by definition, a measure of speed, that is, of distance traveled as a function of time, it is incorrect to say knots per hour, which would actually mean an acceleration, since we would be talking about the change in velocity with time. This solecism is however frequent in common parlance when referring to the speed of a vessel.

Conversion of units

1 node is equivalent to: 0.51444 m/s 1.852 km/h 1.00 nautical mile/h 1.15 land mile/h10 knots correspond to 18.52 km/h; 20 knots at 37.04 km/h; 30 knots at 55.56 km/h; 50 knots at 92.6 km/h. 1 Mach, at atmospheric temperature and pressure normal to the Earth's surface, corresponds to 644,384 knots.

Origin

The knot has its origin in the practices used on ships to estimate speed. One of the most common ways was to launch a float with a calibrated shape from the stern of the ship, which due to friction with the water was relatively immobile on the surface, connected to the ship by a cable that had knots made at regular distances. Measuring the number of knots that were dropped during a given time to allow the float not to be dragged, i.e. counting the number of knots released from the ship in the measurement period, gave an estimate of the speed. One of the most used equipment for this purpose was the boat, whose invention, at the end of the 15th century or the beginning of the 16th century, is attributed to the Portuguese Bartolomeu Crescêncio. The boat consisted of a wooden float of triangular section, the barge, whose arched edge was ballasted so that it remained vertical in the water when towed. The boat was tied by a two-legged crow's foot to a cord of dough, knotted at suitable distances, which was wound on a wooden spool. Once the boat was launched, the knots left were counted during the time it took for an hourglass to empty. To facilitate the calculation, an appropriate combination of the distances between the knots in the cable and the measurement times was used, which allowed the direct value of the speed to be obtained without calculations. A common combination was to use an hourglass that took 28 seconds to empty and a cable with knots spaced 47 feet 3 inches apart. With this combination, each launched node corresponds to 14.40 m/28 s, that is, to 1,851.66 m/h, which for a nautical mile of 1,852 m gives an estimation error of only 0.02%, value much lower than the errors made in the observation and the error induced by the drift effect of the float. The distance between nodes can be calibrated as a function of the float, to compensate for its drift and the error induced by the speed of the water.

Terms used in aviation

KTAS stands for "knots true airspeed", a measure of an aircraft's actual speed through the air. KIAS stands for "knots indicated airspeed", it is airspeed in