currents and navigation
Diederik Willemsen www.sailingissues.com
Currents reflect the horizontal movement of water whereas tides reflect
vertical movements. These currents influence the ship's position and are
therefore important to understand.
The horizontal movement is primarily caused by the gravitational pull of
celestial bodies. But also other factors are in play:
differences in water temperatures caused by heating and cooling due
to the earth's atmosphere;
differences in salinity caused by rain, evaporation and estuaries;
wind induced friction;
the Coriolis force which is a consequence of the earth's rotation.
Prominent features in the map of the major
oceanic surface currents include the subtropical
gyres centred 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 (not shown) are caused primarily by water
density differences and in general return the (now colder) water back
towards the tropics.
To predict the behaviour of major ocean currents several references are
available. The Sailing Directions Planning Guides
contain some information on normal locations and strengths of ocean
currents. Nevertheless, the Pilot Charts are
by far 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 historical 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:
tidal stream or
tidal current: gravitational
current: gravitational, rivers, wind
In order to predict tidal stream one needs to use tide tables in
conjunction with a tidal atlas, or a
Tidal streams are described by drift/rate
and set, in which drift/rate is the speed and
set is the direction of the current.
Tidal stream atlases show the tidal currents for each hour of the tidal
cycle. They comprise a total of 13 tidal charts ranging from 6 hr before
HW till 6 hr after HW.
So, these charts are relative to the time of HW and to use them we must
know the absolute time of HW.
Though several layouts can be used, usually the direction of the tidal
stream is shown by arrows, which are heavier where the tidal streams are
stronger. Figures against the arrows give the mean neap and spring drift
or rate in tenths of knots.
indicates a mean neap drift of 2.1 knots and a mean spring drift of 4.6
Oceanic surface current: A continuous
and regular movement of ocean water flowing along a definable path,
maintaining the earth's thermodynamic balance. Horizontal pressure
gradients, winds and the Coriolis force are three prominent causes of
Deep oceanic current:
Sailing Directions Planning Guides:
Pilot Charts: .
Drift (DFT) or Rate: Speed in knots of