Posted on
Tuesday, August 19, 2025
Boaters often encounter two primary units for measuring speed: miles per hour (MPH) and knots. Both are often utilized, but each serves a different purpose depending on how and where you’re navigating. MPH is familiar. It’s the standard unit on roads and in everyday conversation, making it intuitive when talking about speed with friends or when cruising close to shore. Knots, on the other hand, are designed for navigation. One knot equals 1.15078 MPH, but more importantly, it equals one nautical mile per hour. Because nautical miles directly correspond to the Earth’s geometry (latitude and longitude), knots integrate seamlessly with GPS systems, marine charts, and traditional methods of plotting a course.
A nautical mile equals one minute of latitude, roughly 6,076.12 feet or 1,852 meters. This alignment simplifies distance measurement and course planning on a globe-based coordinate system. Whether calculating arrival times, estimating fuel needs, or planning tidal transits, knots make navigation math more accurate and consistent. Regardless of whether a country uses metric or imperial units on land, knots are the global standard for maritime navigation.
Before satellites, chartplotters, and digital instruments, sailors measured speed using a chip log: a weighted wooden board attached to a rope knotted at regular intervals. Tossed over the stern, the log created drag while a sandglass measured time.
Counting the knots that passed through the sailor’s hands provided speed in nautical miles per hour. This centuries-old method is the origin of the term “knots” and highlights how deeply connected these units are to traditional navigation.
MPH is straightforward and intuitive for casual use, while knots are tightly integrated with marine navigation systems and remain essential for precise course plotting. For modern boaters, understanding both units allows you to move confidently between casual cruising and serious passage planning.
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