The use of knots to measure speed in watercraft dates back to the days of sailing ships. The term "knot" refers to a unit of speed equivalent to one nautical mile per hour. There are historical reasons why this unit became widely adopted in the maritime industry:
Nautical mile: A nautical mile is a unit of distance that is based on the circumference of the Earth. It is defined as one minute of latitude, which is approximately equal to one nautical mile. The use of nautical miles for measuring distances at sea allows for more accurate navigation, especially when using charts and instruments based on the Earth's coordinate system.
Timekeeping: In the past, determining the speed of a ship relied on timekeeping methods like the chip log. A chip log was a device that involved a log line with knots tied at regular intervals. The line was thrown overboard, and the number of knots that passed through a sailor's hands in a specific time period was counted. This count was then converted to the ship's speed in knots. The use of knots in this context was simply a practical method of measurement that sailors could easily understand and use.
Tradition and standardization: Over time, the practice of measuring speed in knots became standardized and deeply ingrained in maritime traditions. This standardization ensures consistency across different vessels and enables effective communication between maritime professionals. It has been widely adopted by naval and maritime organizations worldwide.
While knots are the primary unit for measuring speed at sea, miles per hour (mph) is still used in some cases, particularly in contexts where land-based measurements are involved or for non-maritime purposes. However, for consistency and compatibility with existing practices and navigation systems, the maritime industry continues to use knots as the preferred unit for measuring speed on watercraft.