Dead Reckoning Navigation

Dead reckoning navigation or DR is a technique for marine navigation that helps to determine a prior position using the ship's course, wind, sea conditions and speed. It's one of the oldest methods of navigation developed by Mediterranean navigators, as might imagine, this technique helps you determine your current position according to an already determined previous position, or a future position from the calculate current position, so on.

Some call it ded (from deduced) reckoning, to work in DR navigation, it's important to have a previously determined position (fix) and follow these principles:

  • Start from a fix, and take the time.
  • Label the course in degrees.
  • Use speed on the water and the adequate formula to calculate the distance.
  • It's important to register all positions, at least one very hour and when the course is changing.
  • And remember to plot the DR position at the following situations:
    • At least every hour.
    • After any change of course or speed.
    • After every fix
    • After plotting a line of position.

You can identify the new position or the DR position, and as we said before, it's calculated based on course and speed. This position might vary for leeway, current effects, and steering error, after calculating these values, we get the new estimated position EP.

However, the dead reckoning navigation is not only useful to calculate positions, the metrics one can get help to predict sunset, sunrise, rainfall, and sighting lights, and other hazards during the navigation. One can know of successful experiences using this method since ancient times like the trips that realized Columbus to the New World after discovering it.

The dead reckoning also helps in predicting which stars are going to be available to observation, predict arrival times.

In earlier times, navigators started to use a daily log to register all DR and current positions on the trip, including the speed and it was also recorded in regular intervals. Nowadays, the method of Dead reckoning navigation is still used because in case of failure of other equipment, such as GPS, it comes necessary.

It was and is still a great way to navigate, but the basic problem with dead reckoning is that the results you can get always will depend on the water and as you might see it is moving almost all the time respect the relative to the water and the water can be (usually is) moving.

As you might know, there is some theory you must know to calculate the DR, and it's the currents of the ocean, that horizontal movement of the sea surface by meteorological, oceanographic or topographical effects, you must consider mainly:

  • Cross currents will affect the motion of the vessel but not in the direction.
  • Along-track currents will make speed relative to water ("knots") different from speed relative to ocean bottom.

If you're learning how to sail, remember that this is a very useful technique, especially when other technological equipment could fail during a trip, so you don't get lost; that's the importance to have some knowledge on this process.