Why Nautical Mile and Knot Are The Units Used at Sea?  (2)

By Shamseer Mambra  

 …..History of Maritime Navigation

However, centuries passed before the use of a standard method to measure the distance and speed during navigation at sea. A number of new techniques and methods were experimented from time to time, making marine navigation more meaningful. Until the fifteenth century, coastal navigation was mostly in practice, since the open sea voyages were limited to regions of predictable winds and currents. On the open sea, mariners mostly depended on dead reckoning— the process of calculating one’s current position using a ship’s last position, speed, and direction.  Further ventures by the sailors were enabled by the development of scientific and mathematically-based methods and tools in the following years.The invention of the sextant, the chip log and Chronometers, etc. made the calculation of latitude and longitude possible and much easier. And, the modern era saw the replacement of ancient navigational tools with electronic and technological equivalents and also the determination of standard measures including Prime Meridian. With the help of new technologies, from Gyroscopic Compass to GPS, now marine navigation has become more systematic and easy.

History of Nautical Mile and Knot

Years after the use of several techniques to determine the position and speed of a vessel, British mathematician Edmund Gunter succeeded in enhancing navigational tools including a new quadrant to define latitude at sea. Gunter claimed that the lines of latitude can be used as the basis for a unit of measurement for distance. Eratosthenes and his successors had already assessed the circumference of the Earth, helping other mathematicians to build on.  Gunter proposed the nautical mile as one minute or one-sixtieth (1/60) of one degree of latitude (one degree is 1/360 of a circle, one minute of arc is 1/21600 of a circle). Using the circumference of the Earth assessed by Dutch scientist WillebrordSnellius aka Snell- who assessed it at 24,630 Roman miles or 24,024 statute miles -Gunter defined a nautical mile as 6,080 feet (1853 meters), i.e. the length of one minute of arc at 48 degrees latitude.

Even decades after these developments, there was no standard definition of a nautical mile and different countries had different definitions to follow until 1929. It was in 1929, at the First International Extraordinary Hydrographic Conference in Monaco, the international nautical mile was accepted as exactly as 6,076 feet (1,852 meters). Currently, this is the standard definition of a nautical mile in use and is accepted by the International Hydrographic Organization and the International Bureau of Weights and Measures.The United States measurements were based on the Clarke 1866 Ellipsoid and based on this calculation, a nautical mile was 6080.20 feet (1,853 meters).  Similarly, the United Kingdom was defining the nautical mile based on the knot- unit of speed measured from dragging pieces of knotted string. According to this, one knot was defined as one nautical mile and one nautical mile represented 6,080 feet (1853.18 meters).  However, both the US and the UK abandoned their own definitions in 1954 and 1970 respectively and accepted the international measure of a nautical mile.

On the other hand, the term knot can be traced back to the 17th century, during which period sailors used a device called common log to measure the speed of the ship. The common log was a device consisting of a wedge-shaped piece of wood and a coil of rope with uniformly spaced knots attached to the piece of wood. During the sail, the piece of wood was allowed to float for a specific time after lowering from the back of the vessel, and also the line was allowed to play out freely from the coil as the wood floats. After some time, the line was pulled in and the sailors used to count the number of knots on the rope between the ship and the wood to measure the speed of the ship. Sailors concluded the speed of the vessel according to the average of frequent measurements taken throughout the day. At present, with the help of advanced technologies, the knot measurements are determined using methods such as mechanical tow, Doppler radar, and/or GPS.

Calculation of Nautical Mile and knot:

The nautical chart turns to be the one of the important element abroad the vessel once it sets sail. The fixed relationship between distance, speed and time helps sailors calculate the distance the vessel expected to travel in a given time. The formula being used by sailors is 60 x D = S x T which is expressed as 60D = ST.