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.
Understanding Ships and Submarines: Knots and the Log
The link between nautical distance and Earth comes from latitude. Imagine moving north or south along a line of longitude. A small angle of one minute on Earth’s surface corresponds very closely to one nautical mile.
This made chart work practical long before satellite navigation. A navigator could measure a distance with dividers, compare it with the latitude scale at the chart edge, and use the same unit for the ship’s movement. Regular land miles do not fit this angular system as neatly.
Knowing speed is only useful when the speed is measured against the right reference. A ship can move through the water at a steady rate while a current carries it sideways or pushes it forward. The speed through water affects how the hull, propeller, and rudder perform.
The speed over the ground tells navigators how quickly the vessel’s position changes on the chart. These values can differ by several knots in a strong tide or ocean current. Wind can add another effect by pushing a vessel off its intended direction.
The old chip log was a simple but clever instrument. Sailors threw a weighted wooden chip into the sea so it stayed nearly still compared with the water. A rope attached to it ran out from a reel.
Knots were tied into the rope at equal spaces, while a sailor used a sandglass to time the run. The number of rope markers that passed in the chosen interval gave an estimate of speed.
The method depended on careful timing, a rope that had not stretched, and a ship moving cleanly past the chip. Rough seas and human error could make the result less reliable.
Modern vessels use electronic logs, but they still need checking. A paddle wheel log counts water flowing past the hull. A pressure log compares pressures created by moving water.
Doppler logs use sound waves reflected from particles in the water or from the seabed. GPS gives speed over the ground by tracking changing position. Officers compare these readings because each method has limits.
Shallow water, air bubbles, marine growth on a sensor, and weak satellite signals can affect results. Students should pay attention to the difference between a measured value and the real situation.
A speed reading is not enough by itself. Direction, current, time, and location decide where the ship will actually be.
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.