Tidal Stream Vectors
for Estimating Position and Course to Steer
Stream is one of the complicating
factors affecting navigating on water.
Understanding how the horizontal movement of the water will affect your
necessary if you are to plot EPs and arrive safely at
Set and Drift.
of the more interesting aspects of navigating at sea is that we are
on or in a medium which is itself moving in three dimensions.
Set and drift are the
expressions we use to
describe the effect that the horizontal movement of the tide or tidal
as well as currents and leeway have on a boats progress.
this example Alice in her dinghy is travelling at 5 knots through the
against the tide, Bill is also doing 5 knots but with the
Assuming that there is 2
knots of tide
running then Alice will be travelling at 5-2=3 knots towards her
so, 3 knots is her speed made good or speed over the ground.
Bill has the tidal
stream with him so his
speed made good will is 5+2=7 knots.
This is assuming that
their progress is not
being affected by other factors such as the wind.
In this instance both
vessels are traveling
along the direction of the tidal stream so, the calculations are
When a boat is pointing at an angle across the tidal stream this will
the direction that the vessel actually travels over the
And the effect on the
boat speed made good,
will be dependent on both the angles and speeds involved.
The simplest way to work the progress made good by a vessel is by using
Back to Top
Tidal Stream Vectors.
prefer to draw their tidal
vectors directly on the chart.
to keep their charts free
from lots of construction lines and do the vectoring on a separate
sheet (this is just some plain paper).
certain conventions used when
drawing vectors, which if kept to will help avoid any
As well as
using the standard notations
Estimated positions and
time and log reading of the time it was plotted.
Buy Boat Books on-line
The use of
arrows will not only
differentiate between the different track lines but also indicate the
The water track
or course through the
water of the vessel is shown with one arrow pointing in the direction
track or course made good is
normally shown with two arrows pointing in the direction of travel.
And the tidal
vector is drawn with three
arrows pointing in the direction in which the tide is
bearings and distances must
use the same units of measurements.
normally in degrees
if working directly on to
on the chart must be
measured against the adjacent latitude
When working on a plotting sheet any convenient distance scale
can be used
so long as the same scale is used throughout.
And avoid confusion by always designating the top of plotting
Back to Top
To obtain an Estimated Position (EP) from a
(DR) you will need to ascertain the appropriate tide speed
This information can be found in your nautical
almanac or from the
Tidal Diamonds on the chart.
The tidal flow and direction for the specific time and
location of your
passage can then be added to your DR in the form of a vector
In this sketch example the dead reckoning for one hour at a speed of 4
and a bearing of 90 deg. True was extended from the Fix at A to the DR
The tidal stream for the time of the passage was running at
1.75 knot on a
bearing of 330 deg.True.
The tidal vector line was then drawn from C in the direction
This is where it is important to include
the arrows to the
line as a
reminder of the set of the tidal stream.
Tide flow is always from the DR towards the EP.
As this diagram represents one hour we can now draw the tide
the bearing 330 deg. and mark the EP at B 1.75 units from C.
The ground track AB is not normally drawn in when plotting an
However, if you wish you can draw the line AB this will
represent the actual
course and speed over the ground or made good.
In this case the bearing made good was 065 deg.
True and the distance covered in the hour was 3.5 nautical
miles so the
speed over the ground was 3.5 knots.
The same principle can be used to determine the course and
speed required to
reach a destination.
However as you are working the vector backwards you have to
start by using
an assumed speed over the ground to arrive at a speed and bearing
The vectors are always worked out for an hour because it
calculations as all the speeds, boat and tide, are given in
So, even if you want to establish an EP 45mins or even 1hour
leaving A you still work the speed and bearing out for one
A sailing yacht tacking to windward can plot the water tracks
for each leg
then plot an aggregate tidal stream for the whole period at the end.
Remember that an Estimated Position is not a Fix but a best
And the further you travel from a fix the greater will be the
And as well as tide set and drift you may need to add into the
allowance for leeway, depending on the nature of you boat Leeway will
greatest for a sailing yacht, but can also be affect motor craft
the wind strength direction and the hull shape.
Back to Top
The culmination of every successful passage
is the entry into
a harbor or an
During the final moment of approach to the coast the helmsman
will have no
time to work out tidal vectors.
Besides, tide tables and diamonds only give overall average
cannot account for local variations close to the shore.
During the final moments of approach, allowance for the tide
will have to be
done by eyeball.
Many harbours and channels where there are cross currents will
Markers set up to
make pilotage easier and safer.
However in the absence of such marks the helmsman can pick out
markers from shore based elements.
The best transits
require a marker which is
as far away as possible and one which is as close as possible both
be fixed objects.
In the little sketch above it would be tempting to use one of
boats in the harbor but could you be sure that they are tied up and
The spire in the distance is an obvious back mark that could
be aligned with
the center of the entrance.
Or the left hand end of the harbor wall could be lined up with
the trees to
the left of the spire.
The tidal stream, as I've shown it, is flowing from port to
assuming we are in a boat approaching the entrance.
If we point the boat directly at the entrance the tide will
push us to
starboard, and if we are using the spire as our marker it will appear
to starboard as well.
Or order to counteract the tide flow we need to point the boat
the port hand side.
Using the markers, when the back marker appears to move to
starboard we need
to steer the boat more to port.
And if the back marker seems to move to port of the front
marker we need to
steer more to starboard.
These are simple techniques, which once appreciated will make
it easy to
deal with those complicating tidal stream.
in every process of navigation, be it on the sea or
generally, to plot a successful course you need to know where you are
Back to Top
To comment as a Guest just enter your name (nickname) and then add your Comment.