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.
I am perfectly aware that the majority of Wooden Boat aficionados are sensible folk. However, I need to point out that I am an amateur wooden boat enthusiast simply writing in order to try to help other amateur wooden boat enthusiasts. And while I take every care to ensure that the information in DIY Wood Boat.com is correct, anyone acting on the information on this website does so at their own risk.