What Are Nautical Knots? Ship Speed Measurement Explained

What are nautical knots? A knot is a unit of speed equal to one nautical mile per hour. The nautical mile is defined as exactly 1,852 meters. One knot is therefore 1.852 kilometers per hour, about 1.151 statute miles per hour, or 0.5144 meters per second. Ships and aircraft use it because the nautical mile is tied to the geometry of the earth.

One nautical mile is very close to one minute of latitude, so distance comes straight off the latitude scale at the side of a chart. Charterparty speed clauses on North Atlantic and transpacific trades are written in knots, and USCG and MCA examiners expect nothing else.

The unit is written as knots, abbreviated kn or kt. Speed is never expressed as knots per hour, because the hour is already inside the definition. That mistake appears often enough in reports and charter correspondence to be worth correcting.

Where the knot came from

The name is literal. From the sixteenth century, mariners measured speed with a chip log: a weighted wooden quadrant on a line, thrown over the stern so that it stayed roughly stationary in the water while the ship ran on. The line was marked with knots at regular intervals, and a sandglass timed the run. The number of knots that ran out in the glass gave the speed directly in nautical miles per hour.

The common arrangement used a 28 second glass with knots spaced around 47 feet (14.3 m) apart, which makes the arithmetic come out to nautical miles per hour. The earliest known printed description of the device appears in William Bourne's A Regiment for the Sea in 1574. The modern value of 1,852 meters for the nautical mile was agreed at the First International Extraordinary Hydrographic Conference at Monaco in 1929.

Conversions worth memorising

• 1 knot = 1.852 km/h = 1.151 mph = 0.514 m/s
• 10 knots = 18.5 km/h = 11.5 mph
• 20 knots = 37.0 km/h = 23.0 mph
• 1 nautical mile = 1,852 m = 1.151 statute miles = 6,076 ft
• Rough conversion at sea: knots to km/h, add 85 per cent; knots to mph, add 15 per cent.

Speed through water and speed over ground

Two different speeds are displayed on every modern bridge, and confusing them causes real errors.

Speed through the water (STW) is what the ship's log measures: motion relative to the surrounding water. Electromagnetic logs and Doppler logs in water-track mode give this figure. It is the speed that matters for engine performance, propeller slip, fuel consumption analysis and charter party speed warranties.

Speed over ground (SOG) is what GNSS measures: motion relative to the seabed. It is the figure that governs ETA, passage planning and collision avoidance work on the radar in ground-stabilised mode. In a four knot tidal stream, the two readings can differ by more than the ship's own margin of maneuver.

A Doppler log in bottom-track mode gives SOG directly in shallow water, which is why it is used for docking, where the difference between the two speeds decides whether the ship kisses or hits the berth.

Working with knots on watch

The reason knots survive as the unit of choice is arithmetic. Distance in nautical miles divided by speed in knots gives time in hours with no conversion factor, and the latitude scale on the chart is the distance scale. Two shortcuts follow from that:

• The six minute rule. In six minutes a ship covers one tenth of its speed in nautical miles. At 14 knots that is 1.4 nautical miles, which makes six minute plots on the radar a fast way to estimate range rates.
• The three minute rule. In three minutes a ship covers its speed in hundreds of yards, a shortcut still used in pilotage and naval work.

Speed also drives commercial exposure. Time charter parties usually contain a speed and consumption warranty describing what the ship will do in defined good weather conditions, and performance claims are calculated against the measured speed through water in those conditions.

Slow steaming decisions, weather routing advice and voyage economics all live in the same arithmetic: a vessel that reduces speed from 14 to 12 knots cuts fuel consumption sharply, because propulsion power varies roughly with the cube of speed, but adds hours to every leg.

The other kind of knot

Because the word does double duty, a search for nautical knots often returns rope work: bowlines, clove hitches, sheet bends and round turns with two half hitches. Those are knots in the seamanship sense, tied in line, and they share nothing with the speed unit beyond the name and a shared ancestry in the knotted log line. If you are looking for the rope work, the term to search is marlinspike seamanship.

What to check on your own bridge

Compare the log reading with GNSS speed over ground in calm conditions with no significant current, and note the difference. A persistent discrepancy is either an uncalibrated log or a fouled sensor, and it will distort every performance report the ship submits. The passage planning and voyage performance material in the Marine Insight 360 Knowledge Base sets out how those figures are used once they leave the bridge.

The speed log is one of the quieter sources of error on a bridge. An electromagnetic sensor fouled by marine growth reads low and drifts, a Doppler log loses bottom track once the water deepens and switches to water track without anyone noticing, and a damaged sensor or a blocked sea valve leaves the display frozen on a plausible number. None of that trips an obvious alarm.

The consequences run in two directions. A wrong log input feeds the radar, so true vectors and the CPA on each target are wrong while the display looks normal. Commercially, an uncalibrated log distorts the noon reports, and speed and consumption claims under a time charter are argued on exactly those figures. Charterers' consultants check log data against weather routing records, and the discrepancy is where the dispute starts.

Sources and further reading

• NOAA National Ocean Service: What is a knot?
• Royal Museums Greenwich: Knots, measuring speed at sea
• Britannica: knot, speed unit and definition

What to read next

• Browse related maritime guides