Navigation By Dead Reckoning

Dead Reckoning is basically using known factors combined with known formulas to create estimated positions on a chart while making way.

We must use the term “estimated position (EP)” because there are variables in each course line which we cannot know with absolute certainty if we have no electronic navigation to assist us.

[Of course, if you can use a sextant and have a chronograph, your accuracy is increased significantly]

Accuracy of using dead reckoning requires knowing your starting point with some level of certainty; all other locations are calculated off of the starting point. If the starting point is not accurate, every way-point following will be inaccurate as well.

Dead reckoning follows the idea that  if you know where you start and you know how fast you travel and in what direction, you can determine where you are at any point along your line of travel.

As a very unrealistic but basic example:

​If you start at point A and cruise for 4 hours on one heading at 10 knots, then turn 90 degrees to port and cruise for another 2 hours at 10 knots, then turn 90 degrees to starboard and cruise for another 4 hours at t0 knots we end up at estimated point D.

This sounds easy. But the variables are the key to how close your estimated point is to your actual point.

When you consider your speed is rarely a constant, and your speed and direction are affected by drift, current, and tides, etc., you make multiple heading changes while along your route, and the difficulty ever steering in a straight line on any body of water; your dead reckoning becomes more of a best-guess than a true representation of your location.

This is why practice and using other cross-checks are important for good navigation technique.

It is important that you never mix nautical miles per hour (kts) with statute mile per hour (mph) or your calculation will not be correct. Likewise, because nautical miles (NM) and statute miles (SM) are not the same distance they must not be interspersed when dead reckoning. It does not matter which set you use, but you must be consistent for all your dead reckoning calculations. Assume they are not interchangeable while navigating underway. It is always best to use nautical measurements when plotting on a marine chart, however. 

DEAD RECKONING REQUIRES YOU HAVE READILY AVAILABLE:

  1. A paper chart
  2. A stop watch or similar time piece
  3. A set of dividers
  4. A pencil and eraser
  5. A set of parallel dividers
  6. Basic math skills or a calculator

The basic formulas we use in dead reckoning (you will find additional formulas on their respective pages) are:

Time x Speed = Distance

Distance / Speed = Time

Distance / Time = Speed

CLICK HERE FOR A DISCUSSION ABOUT COMPASS ACCURACY, HEADINGS AND BEARINGS

CLICK HERE FOR DISCUSSION ABOUT TIDES AND CURRENT

Once you understand the information provided on reading and adjusting a compass, how to follow a heading, adjusting for tide, current, and drift set, you follow the steps below to navigate.

STEPS FOR DEAD RECKONING NAVIGATION:

To navigate a predetermined course (CLICK HERE TO REVIEW COURSE PLANNING)

  1. Record the exact time you pass your known point, making good on your intended heading
  2. Start a stopwatch as you cross your known starting point.
  3. Look at your speed
  4. Refer to the corresponding leg on your chart and record your speed next to this line
  5. Use the formula Distance/Speed = Time and determine the estimated time at the current speed and heading until your next navigation point
  6. Monitor your heading, speed, and the time on the stopwatch and try not to deviate from any planned variable.
  7. As the stopwatch passes the calculated time from step 5, make your helm correction to the new heading and repeat steps 1-7 for each leg of your journey.

To navigate when all your electronics fail you

  1. If you know your current position prior to electronic failure go to step 3.
  2. If you do not know your current position, CLICK HERE TO REVIEW USING TRIANGULATION
    • If you do not have sight of the land and have no estimate of your position, you are lost and this section will not help.
  3. Using your chart, determine a suitable course from your your position to your intended point of arrival
  4. Place a navigation point at each of the positions that require a change of direction along your route
  5. Draw a course line, use your parallel rulers to determine magnetic heading of each leg and record next to the course line
  6. Use your dividers to measure distance of each leg and record next to each course line
  7. Go to step 1 above in “Navigate a pre-determined course” and follow 1-7 until arriving at your destination (or close by…)
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