When you’re driving on land, speed is measured in either miles per hour (mph) or kilometers per hour (km/h). These measurements are straightforward because you can see stationary landmarks and estimate how far and how fast you’ve traveled. Out at sea, however, landmarks are often nonexistent, making it impractical for ships to rely on the same land-based travel systems. This is why vessels use knots instead of miles per hour to measure speed.
A knot is defined as one nautical mile per hour, and a nautical mile is slightly longer than a standard mile. Specifically, a nautical mile equals 1.1508 miles (1,852 meters), and it’s based on the Earth’s geometry; one nautical mile corresponds to one minute of latitude on the globe. This system makes nautical miles and knots more useful for navigation because they align with global coordinates used on nautical charts.
The term “knot” has historical roots that date back hundreds of years. Sailors once measured speed using a rope with evenly spaced knots tied into it. The rope was tossed overboard, and as the ship moved forward, sailors counted how many knots passed through their hands in a set amount of time, hence the term “knots” for speed at sea.
By using knots, seafarers gain a practical, universally understood measure of speed that connects directly to navigation and charting, something that miles per hour simply can’t provide in the open ocean.
The history of knots dates back to as early as the 15th century
Historians debate the exact period when sailors first began measuring speed in knots, but most agree the practice emerged sometime between the 16th and 17th centuries. Before this innovation, sea voyages were perilous and unpredictable. Without reliable methods to gauge speed or distance, sailors struggled to calculate how long provisions would last. This uncertainty often led to starvation, dehydration, and diseases like scurvy during long journeys.
The breakthrough came with the invention of the chip log, a simple but effective tool for measuring speed. Sailors would use a long rope wound around a spool, with evenly spaced knots tied at intervals of roughly 47.25 feet (14.4 meters). At the end of the rope was a wooden board, or “chip,” that dragged behind the ship. When thrown overboard, the rope was allowed to run out while sailors measured time using a 28-second hourglass.
This system worked because it tied directly to the nautical mile, a unit based on the Earth’s circumference. A nautical mile equals about 1.15 miles (1.85 kilometers), making it ideal for navigation since it aligns with degrees of latitude. By design, one knot corresponds to traveling one nautical mile per hour, a standard still used in maritime and aviation navigation today.
Knots are now measured using electronic equipment
Although we no longer use ropes to measure distance at sea, knots remain the dominant unit for gauging speed over water, alongside nautical miles. Even the largest and most luxurious cruise ships report their speed in knots. The system took time to achieve global acceptance: the nautical mile wasn’t internationally standardized until 1929, when it was defined as exactly 1,852 meters. The United States didn’t adopt that international standard until 1954.
Tools for measuring speed and distance on a boat are still known as “logs,” though they’re no longer made of wood. Today, the most commonly used logs are electronic (electromagnetic) logs and pitometer logs.
An electronic log, often called an electromagnetic log, determines speed using the principles of electromagnetic induction. When seawater flows past the ship’s hull, it passes through a magnetic field generated by the log. This creates a voltage proportional to the water’s speed, which is then converted into a velocity reading.
A pitometer log, by contrast, relies on pressure measurements. It uses a system of tubes beneath the hull, one facing forward to capture dynamic pressure, and others facing sideways to measure static pressure. As the vessel moves, the forward-facing tube records higher pressure than the side tubes, and the difference between them is used to calculate speed.
