A knot is a unit of speed that has its roots in ancient sailing practices. At a time when no one even dreamed of advanced electronic devices, sailors measured the speed of their boats with a rope and an hourglass. How are speed measurements performed today? How many kilometers per hour is 1 knot? Let us explain!
What is a knot — how many km/h is it?
A knot is a unit of speed used in boating and aviation. 1 knot corresponds to 1 INM/h (nautical mile per hour). 1 INM/h is equal to 1,852 meters and corresponds to the length of 1 geographic minute of the great circle, and therefore the surface of the Earth.
To convert 1 knot to km/h, the following formula can be used:
1 knot = 1 INM/h = 1.852 km/h = 0.51(4) m/s
This means that if a ship (or yacht) is moving at 10 knots, it will traverse 10 nautical miles in an hour. Thus, it travels at a speed of 18.52 km/h.
How was speed calculated in the past? The history of the knot and the nautical mile
The term “knot” comes from the traditional method of measuring a ship’s speed with the so-called chip log used in sailing since the 16th century. The basis of this instrument was a rope with evenly spaced knots that acted as distance markers.
And when it comes to the nautical mile, there is a reason why 1 INM equals 1,852 meters. This value corresponds to the exact length of one angular minute on the surface of the globe measured along the meridian. This unit was standardized at an international conference in 1929 to harmonize the different values of the nautical mile used to date.
How is the speed of a boat measured in sailing?
The boat’s speed can be calculated using both modern and traditional “old” methods. According to the definition, it expresses the movement of the vessel relative to the bottom of the body of water and the shore. It is a vector-like physical quantity defined by the derivative of an object’s position in time.
We distinct:
- speed over the ground (SOG) — is distance relative to time,
- speed through the water (STW) — is defined relative to water masses that “move” relative to the sea floor.
“Manual” ways to measure a vessel’s speed
One of the easiest but proven (though nowadays emergency) methods to measure a ship’s speed is the so-called Dutchman’s log. However, it is only applicable if there are no other ways to determine the speed of the unit.
A side log involves releasing any object (such as an empty bottle or piece of wood) in front of the ship’s bow. Starting from the moment the object is at bow, the time it takes to reach the stern line is measured. To measure speed in this way, you do the following calculation:
Length of the ship / time it took the object to pass from bow to stern = speed of the ship
In the past, a simple device known as a chip log was used to measure speed. It consisted of a triangular plank (shaped like a slice of a disk) weighted with lead and with ropes coming out of each of its tops.
In the past, to calculate the speed of sea travel on some ships people would… spit overboard! Observation of the spit and its position helped determine the speed at which the vessel was moving. This type of measurement was referred to as Spanish log.
Modern methods of measuring ship speed
Modern navigation commonly uses Doppler Velocity Logs (DVL) and GPS satellite navigation.
DVL uses the Doppler effect to measure the frequency shift of acoustic waves reflected from the sea floor or water particles. This allows to accurately determine the speed relative to both the water and the bottom of the body of water. In contrast, GPS systems operate somewhat differently. They make it possible to determine the location of a ship and directly measure its speed relative to the bottom.
Other known ways to measure ship speed
The speed of a ship can also be determined by observing the drift. In the absence of a chip log, it can be contingently estimated on the basis of the time required to sail between known reference points (e.g., between buoys or islands). However, the accuracy of such measurement leaves much to be desired.
Why are ship speed measurements so important?
Knot, km/h and INM/h are the units used to determine a ship’s speed at sea. Attempts have been made to measure it for centuries, but only the development of technology has made it possible to determine this value accurately.
In the past, ingenuity and simple methods were what mattered at sea, while today advanced equipment, including Doppler and GPS systems, are used to measure speed. Its exact determination has always been of great importance to the company. Today, it is mandatory to measure speed for various reasons, which we discuss below.
Navigation and security
Knowing the speed of a ship or yacht, it is possible to accurately estimate the time needed to reach potentially dangerous places (for vessels, these include shoals and reefs). This also makes it possible to calculate very accurately the moment of arrival at the next point on the route (e.g., to a marina). Knowing the current speed also helps in making appropriate maneuvering decisions (e.g., when approaching a port, anchoring, mooring).
Crew comfort
Maintaining the right speed can affect the stability of the vessel, resulting in less sway and a more comfortable cruising experience. And when weather conditions worsen, it is important to reduce it in order not to expose the ship or yacht to violent heeling and dangerous waves.
Observing the law
Often regulations specify the maximum speed allowed in port areas, near swimming areas or on certain inland routes. Their compliance is only possible through ongoing measurement.
Advances in measuring ship speed
From simple methods using ropes and hourglasses to today’s sophisticated navigation devices, the way to measure speed has evolved over the centuries. Using the name “knot” is a tribute to ancient methods and practices, but modern ways of determining speed are based on the use of modern, high-tech equipment.
