You may ask why we continue to use dead reckoning (DR) and paper charts in an age of electronic positioning systems (such as LORAN, GPS, INS and dGPS which combines INS and GPS to produce a three dimesional plot, even when GPS reception is not possible, and ECDIS-N, which takes LORAN, GPS and INS plots and places them on a digital chart).
The first step to understanding DR navigation is to understand the historical necessity of DR sailing. An excellent writeup on this history exists, as written by Blue Dragon. The key difference between historical and modern DR must be emphasized: modern DR is graphical rather than mathematical, and is used as an aid to a total navigational picture rather than the principal means of navigation. However, the DR plot remains essential for three reasons:
First, it standardizes plotting procedures so that anyone familiar with navigation can pick up your chart and plot and immediately have a mental picture of the navigation situation (what and where are the next hazards, what evolutions will need to occur in the near future, what options are available in case of an emergency, etc.) Second, DR ensures the position of the ship is constantly updated, as actual fix intervals may be separated by a large period of time (especially for offshore cruising or racing). It also ensures that the position of the ship is known even into the future, providing the skipper, afterguard, or captain with a higher level of situational awareness. Finally, it supersedes electronic navigation as no ship relying solely on one aid to navigation has ever been found innocent in a court of admiralty. DR establishes a historical record for legal purposes, and shows seaman-like actions taken in advance of the collision, should one occur.
To perform DR navigation, begin as soon as you establish a fix on the chart. Label this fix with a circle (1/4") around the fix if it was achieved by visual bearings, an equilateral triangle with 1/4" sides if it was achieved by one or more electronic sources (radar soundings, fathometer soundings or GPS coordinates, for example) or a combination of electronic and visual means. Use a square with 1/4" sides if the position was established by inspection or estimation (for example, you are tied to the southern point on a pier shown on the chart, or if you estimate a position based on the current). Label the fix with the four-digit 24-hour time. Then, immediately draw a ray from the fix on your course. Label this line with your course and speed by writing "C(COURSE)" above and "S(SPEED)" where (COURSE) is your three digit heading, and (SPEED) is your speed in knots. We are now ready to start taking DR positions. A DR position is labeled by a small semi-circle, about the same size as your visual label. It is labelled by only the last two digits of the time, unless the time is the top of the hour, in which case you write all four digits. Follow the six rules of DR to know when and how to draw DR points:
The Six Rules of DR
- Every hour, on the hour.
- At the time of every course change. Label the new course line, drawn from the DR point, with "C(COURSE)"
- When motoring, at the time of every speed change. Label the old course line at the DR point with "S(SPEED)"
- At the time of obtaining a single line of position by either visual means, a visual range where two objects align, or by electronic bearings such as RACON beacons.
- At the time of obtaining any fix or running fix.
- The instructions for drawing course lines and DR points: a new course line is drawn at any fix or running fix, but never for an estimated position or a single LOP. When drawing DR points, always draw two DR points for the next two fix intervals ahead of the last fix or running fix.
The following is an example of how to draw a visual fix at 0952, with an easterly course, at a speed of 11.4 knots.
58 1000 04
0952 C090 ∩ ∩ ∩
O--------------------------------------
S11.4
Note that the semicircles should touch the course line and there should be a dot in the middle of the O, something I can't simulate with
ASCII.
Got it? It looks like a lot to memorize, but once you do this for a living for a while it comes naturally. The fastest way to make it natural is to force yourself to do it every time you set sail.
Now that you've mastered when to place a DR, you need to know where to place the DR. This is where we must be able to calculate speed, distance and time. Simply use the calculation that distance {nautical miles} = (speed {knots})(time {hours}). This equation will always work, however sometimes it is inconvenient. That is where the sixty minute rule, six minute rule and three minute rule come into effect. Let's assume that a nautical mile is 2000 yards- it's pretty close: the error created by assuming a 1:2000 ratio is much smaller than the error created by the width of your pencil lead on most charts.
- Three Minute Rule: The distance in yards travelled in three minutes is equal to speed in knots multiplied by 100.
- Six Minute Rule: The distance in nautical miles travelled in six minutes is equal to speed in knots divided by 10.
- Sixty Minute Rule: The distance in nautical miles travelled in sixty minutes is equal to the speed in knots.
As you can see, with these three rules we can make many distance calculations in our heads quite easily. For example, fifteen minutes is five three minute rules: speed in knots times 100 times 5. Once we have a distance, we simply place a DR mark on the course line that distance from the last fix.
For more information on how Dead Reckoning is used in professional navigation, consider reading about currrent sailing, precise nautical piloting and celestial navigation.