All you need to know about nautical miles.

Q. Why the velocity of ship and aircraft is measured in nautical miles? explain with all facts and figures.

Sea stories, fictions and other adventure are often having terrible instances that due to storms ships being stranded at sea without knowing the right direction. It’s correct that, unlike on land, navigation in the open ocean or in the air is significantly difficult. Velocity of moving objects in water and in the air is measured in knot.

Another name for the unit of knot is International Nautical Mile. The reason for using a unit other than miles for a ship is that the globe is divided into 360⁰ longitude, and 1 knot is equal to the distance of 60th part of 1⁰ longitude. This makes it easy to calculate how many knot a ship travels per hour and how much geographical distance it travels. For example, a ship traveling at a speed of 20 knot; it means that ship is travelling at 20 nautical miles per hour. One nautical mile (knot) = 1.1508 miles of land i.e., 6076 feet.

After knowing the reason for adopting knot as a unit of velocity, let’s understand the method of such velocity. 

Once upon a time there were no mechanical speedometers that could tell the speed of a ship in the Mediterranean, so a very simple method was adopted. The sailors used a rope wrapped around a spinning wheel, on which pieces of wood tied at every 47 feet and 3 inches apart which formed a chain hundreds of feet long. Each peace has a thick, rounded plaque with a heavy metal strip attached to the curved edge, so that the piece of plaque stood upright and floated. 

As the ship moved forward, the spindle on the rear deck was untied and the pieces of wood attached to the rope by the knot went into the water in turn. 

A mariner, who used a precise watch (hour glass) counted how many pieces went in 28 seconds. The ratio of 28 seconds with 1 hour is equal to the ratio of 6067 feet with 47 feet and 3 inches, so in 28 seconds if 10 wooden pieces of rope went into the water, the speed of the ship would known to be 10 knot. This method was known as a Dutchman’s (chip) log. (see the 18th century chip log picture below)

 

Knot continued to be used as a velocity measurement even after the invention of mechanical speedometer in 19th century. The basic reason was that the geographical distance as well as the locations were clearly expressed in the longitude so, knot is the most suitable other then miles or kilometers.

The knot unit was considered suitable for the aircraft and space navigation for the same reason.

FYI- For a quite a number of years, disagreement among various nations prevails regarding the exact measurement of a nautical mile; primarily, based on the Earth’s circumference. In 1929, the international nautical mile was standardized at 6,076 feet; and subsequently adopted by United States in mid fifties. Obviously, a nautical mile is different from a mile on land because it is based on walking distance. Did you know that -- It was the Romans who first defined a land mile as 1,000 paces or pairs of steps; then it was set at its current measurement of 5,280 feet by Queen Elizabeth I of England in 1593. One more thing….

1 knot = 1.85 Kilometers per hour.