What is the dead reckoning position? Yes , everyone should more or less be able to find their position by dead reckoning, we however do it somewhat more sophisticated.

Upon sailing from a port one can determine ones position with great certainty. Once the port is out of sight we start calculating with the course we are heading ( reading the compass ) the miles we sail (reading the log; to be regarded as the shipboard milometer ), the current (averages from found in the nautical publications ) and drift. The ship is not always making the way it is heading and especially a sailing vessel is affected by the wind pushing the ship sideways. The angle the vessel is making relative to straight ahead we estimate. One could use the wake ( the trail through the water ) of the vessel as a guidance. If we add up all these variables, we end up somewhere where we assume is our position.

Alas, we cannot avoid some errors. The compass course is an average of the best attempt of the helmsmen to steer the charted course line. The current may be differing from the averages posted and the drift is just an estimate. The longer we continue doing so, the greater the difference between the position plotted in the chart and our actual position. The outcome remains a sum of errors.

When using celestial navigation ( using a sextant ) however, we need this dead reckoning position in order to correct it to a position nearer to our actual position. One can calculate the dead reckoning by means of some goniometry, but we can also construct the elements of course, current, and drift in the charts. The chart we are currently use is called an ocean passage chart and it covers a very large piece of ocean. Every pencil point is as large as hundreds of soccer fields and thus we cannot construct these lines in the chart with much accuracy. For that purpose we have a so called plotting sheet, a piece of chart of useable scale free from details. After all there is only sea here. Using this sheet we are able to deduce our location and read a position.

We plot our position in the sea chart once a day, at noon. This has to be done using a very sharp pencil in order not to coincide with the one of the previous day.

What a humongous pool of water….

Richard Slootweg
Captain Clipper Stad Amsterdam