Currents reflect the horizontal movement of water and tides reflect vertical movements. These currents influence the your vessel's position.
The horizontal movement is caused by the gravitational pull of celestial bodies. But there are other factors:
1. Differences in water temperatures caused by heating and cooling due to the earth's atmosphere.
2. Differences in salinity caused by rain, evaporation and estuaries.
3. Wind induced friction.
4. The Coriolis force which is a consequence of the earth's rotation.
Major oceanic surface currents include the subtropical gyres centered on 30 degrees latitude in each of the major ocean basins. The earth's rotation (origin of the Coriolis force) and the change in wind direction with latitude from the east in the tropics and from the west at mid-latitudes cause the circulation of the gyres to be clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. The well-known Gulf Stream in the Atlantic and its counterpart in the Pacific, the Kuroshio Current, are strong currents that carry heat northward from the tropics. The deep oceanic currents are caused primarily by water density differences and in general return the water back towards the tropics.
To predict the behavior of major ocean currents, references are available. The Sailing Directions Planning Guides contain some information on normal locations and strengths of ocean currents. The Pilot Charts are the best reference for predicting the direction and speed of these currents. On these charts, arrows indicate the direction of the prevailing current, a number printed above the arrow indicates the average speed. Since this information is based upon averages, it won't predict the actual ocean current encountered with 100% accuracy.
Ocean surface currents need not be considered in coastal areas. Usually, when close to the continental shelf, the horizontal movement of water is defined by two terms:
1. Tidal stream or tidal current gravitational.
2. Current rivers, wind.
In order to predict tidal stream one needs to use tide tables.Tidal streams are described by drift / rate and set, in which drift / rate is the speed and set is the direction of the current.
