DR, Dead Reckoning is one of the most basic and useful Navigational skills.
It is untrue to suggest that traditional methods of dead reckoning are no longer used as that is precisely what most modern inertial navigation systems depend upon, except that they do it electronically.
However, plotting your DR the old fashioned way is a very simple system which, does not rely on any electronics or outside input.
DR positions should be
regular intervals depending on the nature of the passage.
They should be updated between fixes and changes in course or speed.
Of course it's important to remember that any direct reckoning
whether done electronically or manually is only an approximation as it
factor in tidal effects or leeway.
But by plotting the DR manually, as well as fixing your position with GPS, you will have a far better feel for where you are and how you are progressing.
Marine Chart Line Plotting In
my opinion your DR plots should be drawn
directly on to the relevant chart.
Some folk use 'chart
rather than writing directly onto their chart, personally I feel that
too prone to error and anyway it is easy to erase pencil lines provided
don't score them too heavily.
The first job is to draw
This is the preferred course from you starting position towards your
The starting point should be a fix such as the harbor entrance or a
marker which is shown on the chart.
Having drawn this you will now be aware of any possible hazards along
and the course required to avoid them.
It will also allow you
to work out the time
when you need to change course to avoid them.
How often you need to DR
along your rhumb
line will depend on where you are.
The closer to the coast and the more hazards there are, the more often
should DR and compare with the GPS information and if coastal sailing,
fixes taken from known conspicuous marks.
Running the DR along a rhumb line will quickly allow you to see if you are drifting off course.
Marine Chart Plotting Symbols
Traditionally a 'dead
reckoning' plot does not take into account the effect of currents or
A position which is
determined by factoring
in the estimated effects of wind and tide is regarded as estimated
The dead reckoning begins with a known position, or fix.
The line is then drawn
on the chart along the
The distance of the DR position along the line is marked by multiplying
speed through the water by the time to the mark.
The plotting of a DR is
normally separated by
either time intervals or where a fix can be made.
Where a position fix is unavailable the new dead reckoning plot should
from an estimated position EP.
This should be worked
out by taking into
account the effects of wind, tide and leeway; estimated set and
It is important to mark
the positions times
distances etc clearly using the recognised conventions so there is no
and they can be easily read later.
For consistency, and by
convention, use only
True directions on the chart.
Use the latitude scales
on the sides of the
chart for distances in nautical miles (one minute of latitude equals
Record the time using the 24 hour clock.
Boat speed must be the speed through the water, that's the speed taken from the knot meter not the Speed Over Ground (SOG) as taken from the GPS.
All Dead Reckoning calculations are fairly simple provided you use the correct units of measurement.
And course should always be noted using the same preference whether in degrees True or Magnetic.
I prefer to use Magnetic but I always mark it as such so there is no confusion.
|Distance =||Speed X Time|
|Speed =||60 x Distance|
60 x Distance
The further you extend a
DR from a Fix the greater will
be the 'Circle of Error'.
While factoring in Set
and Drift will reduce
the size of the Circle of Error, it should still be taken into
However, the more you practice your Dead Reckoning techniques the sooner it will become second nature and you will not only gain confidence in your navigational ability but you will learn to compensate for the errors.