Most people have heard that ships and aircraft measure speed in knots, but few know why. Unlike cars that use kilometres per hour or miles per hour, the maritime world relies on this seemingly unusual unit. The answer lies in centuries of seafaring tradition, practical navigation, and a clever use of rope and time. Understanding why ship speed is measured in knots reveals a fascinating story of innovation on the high seas.
Why ship speed is measured in knots. The origins and reasons
The term “knot” dates back to the 16th and 17th centuries, when sailors needed a consistent method to estimate speed over water. Back then, no digital instruments or mechanical speedometers existed. Mariners developed a simple but ingenious tool: the log line.
How sailors measured speed using rope and knots
The log line was a long rope with evenly spaced knots tied at specific intervals. Sailors tied one end to a piece of wood (called the “log”) and threw it overboard, letting it trail behind the ship. As the log floated, the rope unwound. Using a sandglass—typically timed to 28 seconds—they counted how many knots passed through their hands during that period. Each knot that slipped through represented one nautical mile per hour.
This method, while primitive, was remarkably effective for open-sea navigation. It standardised speed estimation long before the age of steam or GPS.
Why one knot equals one nautical mile per hour
To this day, 1 knot = 1 nautical mile per hour, which equals approximately 1.852 km/h. Nautical miles are based on the Earth’s circumference, making them ideal for global navigation. Since 1 nautical mile equals one minute of latitude, using knots simplifies chart plotting and distance calculation on maritime maps.
How the knot supports global navigation systems
Unlike kilometres or miles, nautical miles and knots directly relate to the Earth’s geometry. This makes them essential for accurate navigation using latitude and longitude. Pilots and ship captains can measure distance and speed without having to constantly convert units.
For example, a ship travelling at 20 knots will cover 20 nautical miles in one hour. That means it moves through 20 minutes of latitude, which matches chart scales exactly. This precision is critical in both oceanic and aerial navigation.
Why knots are still used in aviation and meteorology
Although knots originated at sea, the unit has also been adopted in aviation and meteorology. Aircraft report speed in knots, as do weather forecasts—especially for wind speeds. This consistency across fields maintains compatibility with nautical maps and flight charts.
Why ships do not use kilometres per hour or miles per hour
Switching to standard land-based units would require rewriting global navigation systems, reworking maps and retraining professionals worldwide. The knot, despite being old-fashioned, remains the most efficient and logical unit for maritime travel. It is also internationally recognised by organisations such as the International Maritime Organization (IMO) and the International Civil Aviation Organization (ICAO).
How the log line evolved into modern technology
Today’s ships use doppler radar, GPS, and speed sensors to calculate speed with far more precision. However, the term “knot” still persists, both for tradition and for practical reasons.
In modern navigation systems, digital displays show speed in knots by default. On large merchant vessels, cruise liners, and naval ships, the unit is embedded in every dashboard, control panel and route planner. Even software for route simulation and maritime forecasting defaults to knots.
The lasting legacy of knots in maritime navigation
The continued use of knots symbolises the link between old-world seafaring and today’s high-tech shipping. What started as sailors tossing rope overboard became a global standard. The humble knot is not just a measurement—it’s a reminder of maritime history.
