POSITION – DR
In navigation, Ded Reckoning (DR) is the process of calculating your position by using a previously determined fix. You advance your first fix based upon known or estimated speeds over elapsed time and course.
You must maintain the same speed, time and course for the duration (note length of time) or calculate an average. Of course, set and drift (effects of current) need to be accounted for at times to provide an Estimated Position.
To rely on a DR position, you must ensure your first fix is correct. As the new calculation is based on this position, if it is wrong the errors will accumulate.
You have obtained a good fix and for an hour you have been sailing at 5 knots. During this time, you have travelled five miles through the water. If your course has been the same, simply extend your course line and mark off five miles (measure miles off the latitude scale nearest to your location). This is assuming no set and drift.
The following example is a vessel travelling at 6 knots for one and a half hours (you will need to remember the speed, distance, time formula for more complex calculations).
In one hour at 6 knots you would travel six miles, in another half an hour you would travel another three miles, therefore over an hour-and-a-half you would have travelled a total of nine nautical miles.
The DR position is marked with an X and the time noted. Remember, this example does not allow for set and drift. Estimated Position (EP) is DR with estimated set and drift applied.
Position – Estimated Position (EP)
This is a two-stage process:
- Plot your DR position on your chart.
- From your DR position, lay off the estimated set and drift of the current (direction in degrees and distance in nautical miles). Remember if you do your plotting over an hour it is easier (travelling at 4 knots over an hour means you travel for 4 nautical miles).
TIP: If your chat is large scale and you need to work your calculations over thirty minutes, just remember to half everything. When travelling at 4 knots, in half an hour you will have travelled 2 nautical miles. It is the same for the drift, if the drift rate is 2 knots, for half an hour the drift rate will be one knot.
In navigation, set is the bearing of the current and drift is the speed (in knots).
If the current is running to the north at 2 knots and you have worked your DR course over one and a half hours, then plot the EP from your DR position. This will be 2 (knots of current) x 1.5 (hrs), giving you a distance of three miles. From your DR position, draw a line heading north (direction of current) and the distance of three nautical miles will be your EP. See diagram below.
Note: The conventions and more details on the Set and Drift aspect are in the Set and Drift module. It may be easier to complete that first to really see how simple these (the above) calculations are.