why do boats use knots instead of mph ?

Upon hearing the question, I was perplexed as to why boats and planes use knots. Became a never-ending quest for a simple explanation of why, and how in the world to comprehend it without all of the statistics!

Why do Boats  use Knots instead of KMs or Miles? Boats and planes measure speed in knots because one nautical mile is equivalent to one knot. Nautical miles are utilised because they are a unit of measurement for the circumference of the Earth. Because the Earth is circular, the nautical mile accounts for the Earth’s curvature and the distance covered in a minute.

Any measurement of speed is distance / time. In the case of knots the distance term is the nautical mile. This is one of the few non-SI units based an a logical premise, namely one nautical mile is one minute of arc of latitude. This helps simplify navigation.

Where Did The Term “Knots” Come From? Origin of Knots.

While the knot as a measure of speed originated with sailing vessels that used knotted ropes tied to logs thrown overboard and counting the knots as the log extended the rope, it is still used today in both the marine and aviation industries as a distance and speed measurement due to its direct correlation to “Great Circle Navigation.”

If we sliced the globe vertically in half and then divided the half globe into degrees based on a 360 degree circle, we would observe that it is 90 degrees above and 90 degrees below the Equator, with a total of 180 degrees from pole to pole. If we then draw a line around the globe at each degree from the Equator, we will have concentric circles symbolising “Degrees of Latitude.” These latitude lines are similarly spaced apart regardless of the distance between the Equator and the Poles.

Each Degree of Latitude equals 60 Nautical Miles, and each Minute of Latitude equals 1 Nautical Mile when divided by 60. This constant is critical for navigation on the globe and is based on the earth’s circumference. We can measure a distance in any direction on a map and then convert it to a Great Circle distance by vertically aligning the measurement along the Lines of Longitude. The Great Circle distance is determined by the number of degrees, minutes, and seconds measured.

One will notice the use of “Minutes and Seconds,” which relates to the way time is measured and to Global Navigation Practices.

Because neither Statute Miles nor Metric Distances bear this special relationship to Global Great Circle Navigation, they must be converted to Nautical Miles.

While statute miles, kilometres, and other units of measurement have a variety of uses in aviation, the nautical mile continues to be the principal unit of distance and speed because to its direct correlation to Great Circle Navigation.

A vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour. This varied by a greater quantity travelling from the equator to north pole. The Mariners came with a solution of fixing this by making-

1 international knot =1 nautical mile per hour
1852 m is the length of the internationally agreed nautical mile.
The US adopted the international definition in 1954, having previously used the US nautical mile (1853.248 m).
The UK adopted the international nautical mile definition in 1970, having previously used the UK Admiralty nautical mile (6080 ft [1853.184 m]).

It is Neutical Miles not KMS

Because ships and aircraft measure distances in nautical miles rather than kilometres, they use knots to represent speed. This is because they employ mercator projection maps. This is the map that results from projecting the earth’s surface, which is a globe, onto a cylinder.

The wonderful feature of this style of map, and the reason navigators adore it, is that the angles on the map are accurate. This means that an angle measured on the map matches to the angles measured outside using a compass or sextant.

The unit “knot” is based on the nautical mile. The definition of that distance unit is as scientific and rational as the metric system: A nautical mile is based on the circumference of the planet Earth, divided into 360 degrees, which are then is divided into 60 minutes. 

A minute of arc on the planet Earth is 1 nautical mile. This unit of measurement is used by all nations for air and sea travel. If you are traveling at a speed of 1 nautical mile per hour, you are said to be traveling at a speed of 1 knot. Since navigators are having to calculate their position on the basis of the 360 degree system, it makes sense to express distance and speed accordingly.

BTW, the metric system is based on a similar definition, dividing the length of a curve from North Pole to equator (via Paris) by 10,000 to define one kilometre (hence the French spelling).

Only the use of units based on bodily measures or traditional trading units (feet, yard, inches, pounds, bushels, gallons, barrels etc.) is completely arbitrary and bonkers to apply in a modern industrial context.


On the negative side, the fixed relationship between vertical and horizontal scales is obliterated. While the forms remain fairly accurate at the equator, the further north or south you travel, the more stretched the shapes become in the north-south direction, and when you reach the polar circles, the shapes become extremely absurd. As a result, the scale of the map becomes dependent on the region of the world represented. 

To obtain an accurate distance measurement, navigators use the vertical edge of the map, where the latitude is indicated in degrees, minutes, and seconds. And the conventional unit is the nautical mile, which perfectly equates to one minute of arc measured on the meridian, rather than 1.852 kilometres, which would make computations not impossible but excessively complex and prone to error. (Errors that may easily prove fatal) .

Thus, while seafarers measure speed in mph, they do so in nautical miles, which they refer to as mph knots to avoid confusion.

How do you differentiate A Mile, A KiloMeter, And A Nautical Mile?

These are the three most often used methods for measuring distance. Distance and speed are measured in miles in the United States. This system is referred to as the imperial or statutory system.

Kilometers are used practically everywhere else in the world. This is referred to as the metric system. Distance and speed are determined in metric values, which differ somewhat from imperial units.

A mile is approximately one-half the length of a kilometre. A mile is 5,280 feet, or approximately 1,609 metres or 1.6 kilometres. Which is why, when Americans travel to other nations and drive about, they have difficulties. It’s strange for them to see 120 kilometres on the car’s speedometer!

A nautical mile, on the other hand, is a whole distinct concept. A nautical mile is a unit of measurement based on the Earth’s circumference. To truly grasp it, we’ll need to brush up on our knowledge of Longitude and Latitude.

For the time being, though, keep in mind that a nautical mile equals “one minute” of latitude.

Calculating A Nautical Mile

It is not necessary to comprehend this entirely, but it would be beneficial to know if you wish to study it further. For the purposes of this article, we will discuss why boats and planes utilise knots and nautical miles.

Given that we understand how the Earth is graphed to obtain GPS coordinates. We need to understand why nautical miles differ from standard miles or kilometres.

They are distinct due to the Earth’s curvature. Due to the fact that our graph of boxes will somewhat curve when we approach either the North or South poles. There needs to be an adjustment for when we travel over vast swaths of the Earth.

Due to the greater oval shape of the Earth, there are larger parts of the graph as we approach the equator. At the poles, the parts are smaller.
This is where things becomes a little more complicated and perplexing for all of us. Our attention should be drawn to the introduction of the degrees. Due to the Earth’s 360-degree rotation around the starting longitude and returning to it.

That can be reduced to a distance.

Calculate the circumference of the Earth by counting the longitude lines and dividing by “360 degrees.” This is how we obtain the 1 degree distance of 6,080 feet. That was the original nautical mile distance.

When we examine the same thing using simply latitude lines. We see that the distance changes due to the fact that the Earth is not a perfectly circular object. As a result, the distance between the poles and the equator is corrected to 6,110 feet at the poles and 6,050 feet at the equator.

Given the 60-foot gap between the two, the distance is 6,080 feet when halved. The math can get much more sophisticated than that, but that is how I understand it in layman’s terms. This is how it appears to me!

Today, they have it down to a precise distance of 6,076 feet or 1.15 miles.

A Knot is one nautical mile per hour. A nautical mile was originally one minute of latitude (60 nautical miles is one degree of latitude). This made navigation calculations simpler. A nautical mile has since been defined as exactly 1852 meters, which is very close to the mean distance (1852.3 meters) of minutes of latitude.

Considering The World As A Graph

The prime meridian and the equator serve as our beginning points for our graph. Keeping in mind that the equator represents our latitude and the prime meridian represents our longitude. We know the origins of all GPS coordinates.

Consider this: the equator is the horizontal line that circles the graph’s centre. The term latitude is derived from the word ladder. Latitude and ladder both ascend and descend, or rather, you can ascend and descend on the ladder!

Thus, we can denote our latitude by all of the horizontal lines and the equator by the commencement. As you climb the ladder northward, each line is referred to as a “degree” and contains a description of North. There are ninety degrees remaining till we reach the north pole. Thus, if you were on line 50, you would be at a latitude of 50 degrees North.

Likewise, for the South. For the equator, begin at 0 degrees. We descend the degree ladder until we reach 90 degrees South, or the south pole.

Vertical lines begin at zero at Greenwich, England. From there, they count 360 degrees westward until they return to England and the zero beginning position.

These lines intersect to form a variety of boxes that indicate an exact place on the Earth. These places, along with their associated numbers, provide us with the GPS coordinates.

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