In marine navigation, which unit is used to express speed?

• A. Kilometers per hour
• B. Nautical miles per hour
• C. Meters per second
• D. Knot

Answer: (D)

Speed at sea is expressed in knots. The knot is defined as one nautical mile per hour, which fits perfectly with how distance and time are used in navigation: charts are in nautical miles and latitude/longitude relate to nautical miles. The term comes from the old log-and-tow method, where a line marked with knots was let out behind the boat and the number of knots that paid out in a set time indicated speed. So one knot equals about 1.852 kilometers per hour (roughly 1.151 miles per hour). While you’ll also hear “nautical miles per hour,” the conventional maritime term for that speed is knot. Kilometers per hour and meters per second are more common on land or in general engineering, not the standard for navigation at sea.