Ships and submarines measure speed in knots because navigation is based on Earth’s shape and latitude and longitude. A knot is one nautical mile per hour, not one regular mile per hour. This unit helps sailors connect speed, time, and distance directly on nautical charts.
Understanding knots makes it easier to plan routes, estimate arrival times, and compare marine speeds.
Key Facts
- 1 knot = 1 nautical mile per hour
- 1 nautical mile = 1852 m = 1.852 km
- 1 knot ≈ 1.151 miles per hour
- distance = speed × time
- If speed is in knots and time is in hours, distance is in nautical miles
- A chip log estimated speed by counting knots in a rope that ran out during a timed interval
Vocabulary
- Knot
- A knot is a unit of speed equal to one nautical mile per hour.
- Nautical mile
- A nautical mile is a distance of 1852 meters, based on the size of Earth and used in marine and air navigation.
- Chip log
- A chip log is an old speed measuring tool that used a floating wooden board, a knotted line, and a timer.
- Sonar
- Sonar is a system that uses sound waves underwater to detect objects, measure depth, or help track motion.
- Log line
- A log line is the rope attached to a chip log, often tied with knots at regular spacing for measuring speed.
Common Mistakes to Avoid
- Treating knots as miles per hour is wrong because 1 knot is about 1.151 miles per hour, so the values are close but not equal.
- Using regular miles on a nautical chart is wrong because marine charts are designed around nautical miles, latitude, and longitude.
- Forgetting to convert minutes to hours is wrong because speed in knots means nautical miles per hour, so time must be in hours when using distance = speed × time.
- Thinking submarines use chip logs underwater is wrong because modern submarines use instruments such as sonar, inertial navigation, GPS at the surface, and speed sensors rather than trailing a rope.
Practice Questions
- 1 A ship travels at 12 knots for 3 hours. How many nautical miles does it travel?
- 2 A submarine covers 46 nautical miles in 2 hours. What is its average speed in knots?
- 3 Explain why nautical miles and knots are especially useful for navigation on Earth compared with ordinary miles per hour.