What is course and speed in shipping

navigation

The beginnings of navigation

The art of navigating on the high seas was first passed on orally to subsequent generations. It was empirical knowledge that was fed from observing the sun, moon and stars, but also the wind and the direction of the wind.

In addition, the plumb bob was used not only to measure the water depth, but also to determine the texture of the sandy, rocky or muddy subsoil with a sticky "plumb bob".

Sailing instructions, the so-called peripli, existed as early as ancient times. They contained information about distances, shoals, dangerous currents and conspicuous landmarks that were used by seafarers for orientation.

From these sea route descriptions, the first medieval portolan cards developed from the 13th century. They are considered the forerunners of today's nautical charts and enabled seafarers to determine the course based on a template.

The sea route to India

For a long time, the sea route descriptions were very vague and imprecise. That didn't change until navigation became a science based on Greek astronomy, geography, and cartography.

It was mainly Portuguese and Spanish seafarers who let themselves be carried away by the tempting prospect of lucrative spice trade and therefore looked for a sea route to India. This should succeed in the second half of the 15th century.

These expeditions led to a break with Ptolemy's view of the world, according to which the Indian Ocean was an inland sea and therefore there could be no sea connection from the Atlantic to the Indian Ocean and thus to the coveted Far Eastern Spice Islands.

On behalf of the Portuguese King John II (1455-1495), the Portuguese navigator Bartolomeu Dias was the first European to sail south on the west coast of Africa and finally reached the southernmost headland of Africa.

Because that was seen as a good sign, perhaps to find a sea route to India after all, King John II baptized the discovered headland with the name "Cabo da Boa Esperanca - Cape of Good Hope". The final proof that India could be reached by sea was only provided by Vasco da Gama, who in 1498 was the first European to arrive in Calicut, India with a small fleet.

The compass

Vasco da Gama already had a compass on board when he discovered the sea route to India. Chinese navigators were the first to navigate using magnetic needles. Probably afterwards it was Arab traders who introduced the magnetic compass into the Arab world via the Silk Road in the 10th century.

Initially, the magnetic pointer was floated in a bowl of water. It was later placed on a pen so that the needle could orient itself to the north in the earth's magnetic field. Later, the magnetic needle was placed in the center of the compass rose, which enabled a graduation and thus the direction of travel of the ship or the position of a place in relation to the north direction could be determined.

The Jacob's staff and the sextant

Using the longitude using the compass, navigators of the 15th century were also able to calculate the latitude of their course. The first calculation methods had already been developed in ancient times to determine the angular distance between the horizon and the sun or a solid celestial body such as the Pole Star.

At the beginning of the 14th century, the mathematician and philosopher Levi ben Gerson invented an easy-to-use device with which the angular distance could be represented in a simple manner. The so-called Jacob's staff has been part of the standard equipment of seafarers since the 15th century.

The sextant worked according to the same principle and, in combination with astronomical and nautical tables, was for a long time one of the most important instruments for determining the position of a ship alongside the compass.

The log as a speedometer

The device with which the speed of a ship could be measured was less spectacular. In order to be able to plan the course of the ship as precisely as possible, one had to know how fast a ship was moving on the intended course and how strong the drift was acting, which was caused by the current of the sea and the wind power.

Experienced seafarers used to be able to estimate the speed at which they were traveling on the water from the height of the bow wave and the sound of the wake. Of course, this only gave very inaccurate measurement results. This was achieved more precisely with the log log, which was introduced into shipping at the beginning of the 17th century.

It was a piece of wood that was thrown overboard from the stern of the ship and that was attached to a long line. There were knots in this line at regular intervals that were used to measure speed. An hourglass was used to measure the number of knots on the line in half a minute.

To this day, the term ship knot is the nautical measure for determining the speed of a ship.

The chronometer

In addition to the direction of the compass, the geographical latitude and the speed, the seafarers also needed a method for determining their exact position at sea that could also determine the longitude. On their voyages of discovery in the 15th century, seafarers still used the sidereal time for their position calculations.

The 24-hour rhythm of one day provided the reference time with which the earth rotates around the celestial pole once a day. This natural clock served as a timekeeper for seafarers.

However, since the sidereal time deviates from the solar time by about four minutes every day, appropriate compensation calculations had to be made.

This cumbersome method of calculating the longitude was not made superfluous until 1761 when the English watchmaker John Harrison invented it. Harrison had developed a precision clock in 1759 that worked over long periods of time and in different climatic zones without losing time due to mechanical resistance.

This made it possible to compare the time at sea with the reference time of the chronometer, accurate to the second.

The chronometer showed the local time in Greenwich. The prime meridian runs through Greenwich, i.e. the reference longitude, which forms a semicircle between the north and south poles perpendicular to the equator.

The seafarers orientated themselves on the prime meridian by measuring the time at the highest point of the sun at 12 o'clock and comparing it with the time shown on the chronometer. The longitude could then be determined exactly from the time difference with the help of nautical tables.

When the British navigator James Cook set out on his South Seas expedition in 1772, he had the first replica of a chronometer on board and was able to precisely map the country he had discovered.