The sea transport relates to all the human activities of circulation on the seas and Océan S. One speaks about ocean navigation when the ship sails in open sea (out of sight of ground) and of coastal navigation (or Cabotage , but this term is restricted with the carried on activity) when it sails for the coasts.
The Navigation also indicates the whole of the techniques and methods which allow the ship:
- to determine its position,
- to calculate the route to follow to go to its destination, in full safety.
The person in charge on a ship of navigation is the navigator . He uses various methods and techniques, the most adapted to the context and according to the equipment of the ship.
To determine its position
There exists an important distinction between navigation out of sight of ground and the coastal navigation. With traditional means:- out of sight of ground, navigation consists in being placed in geographical coordinates (latitude and longitude). The method used is then the Navigation with the regard; the point thus “considered as” “is readjusted” more or less frequently thanks to astronomical points and, at one time, with the possible use of terrestrial radioelectric systems (Consol, LORAN, extinct Oméga… today all);
- the Coastal navigation is “relative”: the navigator places himself in Azimut and outdistances compared to a Amer, by optical triangulation (not by three raisings) ou/et by means of his Radar of navigation.
Today, with the development and the generalization of the satellite systems of positioning (GPS, Galileo soon…), it is possible to permanently obtain a precise position geographical, which thus makes theoretically superfluous the behavior of a regard. For ground, the traditional methods are always used. However, a receiver GPS, associated with electronic sea charts (ECDIS), makes it possible to replace them in full safety. Failing this, it is necessary to defer its road envisaged on its receiver ( waypoints ) and to supervise any variation well.
Coastal navigation
As soon as it could take the sea, the man developed techniques of navigation resting on the observation of the coast: the techniques of navigation for ground today are always used when the ship is close to the ground. It is a question of identifying points remarkable along the coasts - Amer S natural or artificial, Balise S and of night fires of the headlights and the beacons - to raise to them Azimut with a Amplitude compass, and to defer these statements on a Sea chart. The statement of three land-marks (triangulation) makes it possible to the navigator to know his position and to correct its course consequently.
Navigation with the regard
Navigation with the regard is the method which consists in deducing its position from its road and the distance covered since the last known position. Traditionally, this method rests on the instruments measuring its course (compass), its speed (log, tachometer, airspeed indicator…) and time (stop watch) like with the possible estimate (or the calculation) of the influence of the environment (running, wind) on its walk.
Astronomical navigation
Very early, the man noted that the movement of the stars (sun, stars, the moon) could be given in advance and provide precise indications on the position of the observer. The development of several instruments able to record the height of a star above the horizon, of which more succeeded is the Sextant, made it possible to the navigator to determine his latitude. Progress in the clock industry, by getting to the navigator a stable time reference frame, made it possible to calculate with an increasing accuracy, longitude. Astronomical navigation requires knowledge, to identify the stars, to use a Sextant, to consult éphémérides and to carry out calculations which make it possible to readjust the estimated position of the ship.
The radioelectric use of the navigation systems
To determine its road
To sail also consists in determining a road, a route to go from a point to another, by avoiding the dangers and, possibly, by benefitting from the weather environment (winds) and oceanographical (currents). This road is traced, beforehand, on a Sea chart (and/or on a receiver GPS); during navigation, the navigator endeavors to hold this road of which it could deviate under the influence from the wind and the current or for a Manœuvre (the anti-collision one for example).
Ocean navigation
In the absence of any obstacle, the shortest route of a point with another, on the terrestrial sphere, is an arc of large circle, called Orthodromie or orthodromic road. On a chart Mercator, this road is not a straight line. The straight line, or road with constant course, is called Loxodromie.
The profit of an orthodromic road on a loxodromic road is appreciable only at long distances and with the high latitudes.
A detailed study of the climatological, oceanographical and weather conditions makes it possible, in addition, to choose the optimal road which will make it possible to benefit from the wind and the current and to avoid the disturbances likely to slow down walk. This study, the maritime Routing , is, of course, of essential importance, for the sailing, but it is also useful for the merchant navy.
Coastal navigation
The principal concern in coastal navigation is to avoid the dangers represented by the coast, the reefs and the weak funds.The road, prepared and traced on the chart (or receiver GPS), thus takes account of its elements, the security distances which one agrees, the depth registered on the chart and possibly of the calculation of the Marée and depth of minimal water of safety that one wishes to preserve under the skittle in all circumstances (called “ Pied of pilot ”).
These security distances depend on the characteristics of the ship (draft, speed and, out of tightened water, length and radius of gyration) and also of the environment (particular weather conditions and of visibility, beaconing, land-marks). The foot of pilot also depends on the characteristics of the ship, of the conditions of sea and swell, of reliability to be granted to the statements of probe of the chart. A Sondeur makes it possible to control the effective depth under the skittle.
The coastal navigation is based on the many helps represented by the maritime Signalisation (especially abundant on the coasts of the developed countries), the Balisage cardinal at sea, side in the channels. The land-marks on the coast in addition make it possible to trace on the chart of the raisings of guard, being thus ensured if it is not crossed not to arrive in a danger zone. Two aligned land-marks allow a Navigation by alignments, particularly practical and sure in a channel.
The calculation of the tides
In the water subjected to the tide, a Calcul of tide is carried out by the navigator to determine where and when a road or a passage is practicable in full safety.The seas subjected to the tide also comprise current variable which can accelerate or slow down the boat and in certain places tightened to raise a dangerous sea. This phenomenon must be integrated in calculations of the navigator.
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