knot

A knot is a unit of speed equal to one nautical mile per hour, or approximately 1.852 kilometers per hour. It is the standard measure of vessel velocity in maritime operations, navigation, and naval contexts worldwide. One nautical mile equals 6,076 feet, derived from one minute of arc along the Earth's meridian, which makes the knot a natural choice for sea navigation where latitude and longitude grids govern position.

The term originated from the practice of measuring ship speed using a log line: a rope with knots tied at regular intervals, attached to a weighted wooden chip tossed overboard. As the ship moved forward, crew members counted how many knots passed through their hands during a fixed time interval (typically 28 seconds, using a sand glass). The number of knots that passed indicated the ship's speed in nautical miles per hour. Though electronic speed logs replaced this method by the mid-20th century, the terminology persisted.

Modern vessels report speed in knots across all maritime communication: pilot boarding requests, harbor speed restrictions, voyage planning, and weather routing. A 20-knot transit is significantly different from a 20-mile-per-hour transit. This distinction matters because nautical charts, tide tables, and navigational instruments all operate in nautical miles and knots. Confusion between knots and statute miles has been a factor in maritime incidents, particularly when crew from different maritime traditions work together.

Speed over ground (SOG) and speed through water (STW) are distinct measurements. STW is measured by electromagnetic or impeller-type logs that detect motion relative to the surrounding water; SOG is determined by GPS or celestial fixes and accounts for current and drift. The difference between them reveals current strength and direction, essential data for passage planning through straits, rivers, and tidal zones.

Typical operating speeds vary by vessel type: container ships maintain 20 to 25 knots, bulk carriers 13 to 15 knots, and tugs 10 to 12 knots. Speed restrictions in pilotage waters, traffic separation schemes, and environmental zones are always expressed in knots. Fuel consumption curves and engine performance data are charted against knots of propeller slip and ship speed, making the unit inseparable from maritime engineering and logistics.