What is “dead reckoning” (DR)

What is “dead reckoning” (DR)

A Dead Reckoning position is simplest form of estimating your position as it does not take into account the effects of the tidal stream, current, wind or other factor affecting the position of the boat. Basically, a DR position is ‘worked up’ using direction and distance data. By carefully recording in our log book the direction and how far we have travelled, we can ‘dead reckon’ (DR) our position.

Vessel using dead reakoning.
Fig 7.0 – A simple DR position represented by our water track, a line with one arrow, terminating in a small line representing our DR position with time and log reading.

We use pencil lines on the chart to represent the vessels direction of travel. In Fig 7.0, a vessel has headed north for one hour. In that hour it has logged 6 nautical miles as it travelled through the water. When we plot this information on the chart we draw a line form our fixed or last known position, and represent graphically the direction and distance the the vessel has travelled on the chart. You can see that the line is drawn from a small circle with a dot in the middle, this is the symbol we use for a confirmed position or ‘fix’. If we have estimated this position we would use a small triangle with a dot to represent an estimated position which will be looking at further on in this module. The line representing the distance and direction travelled is called our water track or heading and has a small direction arrow on it to represent the direction and terminates at a small line represent the dead reckoning position.

It is good practice and necessary that whenever you plot a fix or a line on a chart to write the Time and Log reading for the vessel. The differences between the time and log readings provide us with vital information and in Fig 7.0 shows us that the boat travelled 6 miles though the water over 1 hour, and therefore had boat speed of 6 knots.

So the water track is a representation of the boats movement through the water. It is the direction the boat is heading in as is moves on the surface of the water. Remember, the heading or water track is the reading you get from the ships steering compass and the direction the vessel is pointing at any given moment in time.