Over the plate, from deck to water, runs a tarred log line, and down it, one after another, each at its own pace, slide the tied knots; below, a wooden chip rocks on the swell and keeps the line taut. On the left the sand runs in the glass: the upper half empties, the lower fills, and single grains keep falling in between. Three lines of waves travel in different directions with mismatched periods, and along the measuring rule at the bottom a mark rides from edge to edge to the reading taken.
Unit passport
Non-SI · accepted for navigation sheet 1/1 · rev. 2026-09Knot kn · kt
Non-SI unit · quantity: speed · 1 nautical mile per hour
A unit named after a rope
A wooden chip was thrown over the stern, weighted with lead so that it stood upright in the water and barely moved, and from it a line paid out with knots tied at equal intervals. A sailor let the line run through his fingers and counted the knots aloud while a second man watched the sandglass, and when the sand ran out the count was cut off: as many knots as were counted, that much speed the ship was making.
The whole trick is that the interval between the knots and the time in the glass are matched to each other, so the count gave the speed in miles per hour at once, with no arithmetic on a wet deck. The device left ships more than a century ago; the log became mechanical, then electromagnetic, and now satellite-based altogether, while the name of the unit survived and now lies at the foundation of the international rules for preventing collisions at sea.
Today the definition of the knot mentions neither line nor sand: it is exactly one nautical mile per hour, that is, one thousand eight hundred and fifty-two metres divided by three thousand six hundred seconds. The number comes out uneven — zero point five one four metres per second and change — but the convenience remains: a distance in miles divided by a speed in knots immediately gives hours.
International documents write the symbol as two lower-case Latin letters — kn — while aviation and weather reports settled on kt, and old English tables also show kts. The word “knot” takes a plural, the symbol does not; a non-breaking space goes after the number.
The hour already sits inside the unit, so there is no such thing as “knots per hour”, only a change of speed, which is measured in knots per minute. A knot is not a nautical mile and not a distance at all; besides, speed through the water and speed over the ground differ by the current, and in a strait they diverge by a good ten per cent.
01 · Definition
line and glass · a mile per hour · through the water and over the groundA knot equals one nautical mile per hour, and since 1929 the mile has been exactly one thousand eight hundred and fifty-two metres, so the knot is expressed exactly too: zero point five one four four four four metres per second, rounded to the sixth decimal. The definition is purely arithmetic, and the unit has no standard of its own, because it inherits the standard of length and the definition of the second entirely.
The historical tie between the knot and the instrument rested on a proportion. If the interval between knots on the line equals the distance a ship covers in the time of the glass at one mile per hour, then the number of knots gone over the side is the speed in those very miles. For a twenty-eight-second glass the interval comes to fourteen metres forty centimetres, that is, forty-seven feet three inches, and the English fleet held that marking as the standard.
The trouble was that different ships marked their lines differently and their glasses ran unevenly, so the error was not random but systematic: it did not average out over a watch but accumulated in the reckoning. A navigator whose line was marked short logged a speed higher than the real one day after day and pushed his plot ahead along the course, and came up on the coast earlier than he thought — a plot that fills eighteenth-century shipwreck reports.
Finally, the knot is measured in two ways, and the difference still matters today. A log of any design — from the wooden chip to the electromagnetic sensor in the hull — gives speed relative to the water, whereas a satellite receiver counts movement relative to the seabed, and the difference between them is the current. In the Strait of Gibraltar or off Cape Horn it reaches several knots, and so the bridge keeps both figures side by side.
Where forty-seven feet three inches come from
On the left the ship makes her way, and a line pays out astern: the knots run faster the higher the chosen speed, and each one that passes the sailor’s hand at the gunwale adds one to the count. On the right the glass runs, and as soon as its upper half is empty the count is cut off — two bars show what was counted and what really was. Move the glass time: the line is marked for one duration of it, and take another and the reading starts to lie in one and the same direction, while the bottom line works out how many miles a day of such reckoning will push the plot.
02 · Conversion
speeds · wind and wave · passage timeOne speed in three conversations
The first tab spreads the entered speed over everyday units. The second reads the same number as wind and shows what the sea does in such a wind. The third works out time and the day’s run, that is, it answers the question for which the knot was invented in the first place.
A rule to remember: a knot is roughly half a metre per second and just under two kilometres per hour, and the other way round — a metre per second is almost exactly two knots.
Weather services give wind now in knots, now in metres per second, and the difference between the figures is almost twofold, so twenty in a forecast means either a fresh breeze or a real gale. Before comparing forecasts from different services, make sure both speak in the same unit.
| Quantity | Value | Note |
|---|---|---|
| {k} | {v} | {note} |
{text}
04 · Measuring instruments
chip log · rotator · electromagnetic · DopplerFour ways to catch speed
The instrument that gave the unit its name
An oak chip shod with lead was thrown over the stern, where it stood upright in the water and stayed almost still while the ship moved ahead and drew the line out. The first few fathoms up to a mark were not counted, letting the chip clear the wake, and then the knots were counted aloud until the cry of “stop” over the empty glass. The accuracy was about half a knot, which was quite enough for reckoning, since wind and current added far more uncertainty.
A rotator that counts turns
In the mid-nineteenth century the line was replaced by a towed rotator with slanted blades, which spun the faster the faster the ship went and, through a long line, turned a register on the taffrail. The instrument no longer counted speed but distance run; speed was derived by dividing the increment by time, which made the readings steadier: the random jerks of the waves averaged out by themselves. Walker’s log stayed aboard ships for almost a century and in lifeboat kits outlived even the arrival of radio navigation.
A sensor with no moving parts
Seawater conducts electricity, so as it flows past a coil in the hull it induces a potential difference across the electrodes proportional to the speed of the flow. Such a log does not wear out and does not tangle in weed, but it shows the speed of the water in the boundary layer right at the plating, which differs slightly from the speed of the free stream, and so it is calibrated on a measured mile with a correction factor. The reading still remains speed through the water, not over the ground.
Speed counted from the bottom
The slanted beams of the acoustic log strike the seabed, and from the frequency shift of the reflected signal the instrument finds the movement relative to the bottom — that very speed over the ground which no log working through the water knows. When berthing this lets one see way in fractions of a knot and even sideways drift, and in deep water, where the bottom echo is lost, the instrument switches to scattering in the water column and goes back to measuring through the water, which it honestly reports with a note on the screen.
05 · Writing rules
kn and kt · the hour inside · through the water or over the groundThe hour is already inside the word
There are few mistakes here, and all of them come from forgetfulness: the unit hides both distance and time within itself, and so it does not tolerate adding them again from outside.
The first mistake is the best known, and it amounts to saying “kilometres per hour per hour”. The second passes off a distance as a speed, whereas in a watch one covers miles and only miles. The third is dangerous in forecasts, where some services give wind in knots and others in metres per second, so a bare number means nothing. The fourth skimps on precision where it cannot be skimped: rounding to one point eight gives an error of almost three per cent, that is, an extra hour on a transatlantic passage. The fifth leaves the reader guessing whether the speed was measured by log or by satellite, though in a strait the difference between them can be a quarter of the speed. The sixth simply mangles a symbol that exists in that form in no standard.
06 · Neighbouring units
nautical mile · metre per second · kilometre per hourThe knot rests on the nautical mile as the kilometre per hour rests on the kilometre, and so the whole family of speeds differs only in which measure of length is laid at its base. The difference between them is purely a convention, and the knot’s staying power rests on the mile being equal to a minute of arc and read straight off the chart border.
The basis of the knot: the distance corresponding to one minute of arc of a meridian, set in 1929 at a round number of metres.
The unit of the system in which most of the world’s meteorologists speak; converting to it and back is almost a doubling, which is used for quick estimates.
The everyday measure of speed, linked to the knot by the exact factor 1.852, since both measures of length in this ratio are set as whole numbers of metres.
Among the Simetrium data sheets, standing nearby are nautical mile, from which the knot took its substance, and the speed of light — the other extreme of the same row, where the unit is not fixed by an instrument but, on the contrary, declared exact and used to define the metre.
07 · Historical section
archive · from rope to factorHow a rope became a unit
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A forty-two-foot line cut the interval by one ninth and so overstated the reading by about twelve per cent, which on a long passage pushed the dead-reckoning position ahead along the course by tens of miles.
Two speeds of one ship
Any log, from the wooden chip to the coil in the hull, measures the ship’s speed relative to the water, since it is the water that flows past the hull and spins the rotator. Yet the ship moves not along the water but along the chart, and the navigator needs speed relative to the bottom, and the difference between these two quantities is the current, which in narrow straits and off continental coasts can be comparable to the speed itself.
Before satellite receivers appeared, the current had to be taken from atlases and tide tables, adding to the log reading a correction whose reliability depended on the age of the edition. Now the receiver gives speed over the ground directly, and so the bridge keeps both figures: the log tells how the ship behaves in the water and whether the engine has enough revolutions, and the satellite tells when the ship will arrive.
Hence the peculiarity of the unit: the knot has no standard of its own and cannot have one, and what is verified is not the unit itself but the instrument, by running the ship over a measured mile of known length in both directions to cancel the current by averaging. All that is left of the old log line is the habit of checking the reading against something external, since no log knows what the water around it is doing.
Catalogue of quantities
speed, way and navigationSI base units
the metre and the second are highlighted — the knot is a ratio of length to timeWho goes at what speed
passage time is worked out from the same formula| Vessel or object | Knots | km/h | Per day | |
|---|---|---|---|---|
| {name} | {kn} | {kmh} | {day} | {time} |
The day’s-run column shows the main convenience of the unit: to learn how many miles will lie astern by this same hour tomorrow, it is enough to multiply the speed by twenty-four. The last column turns speed into time the same way, and it is this simplicity that kept the knot on the bridge, even though inside the receiver counts metres per second.