Most sailors today rely on their GPS for navigation offshore. The system not only provides an exact position on demand, it can also display cross-track error, drift, current and leeway, speed over the ground, course, and other information, all superimposed over a chart of the region.
It would be foolish to not take advantage of this marvelous aid; but it is also important not to become dependent on it. At any moment, the device could malfunction, or fail altogether, for a variety of reasons. And even if it is working perfectly, if you become disoriented by darkness, bad weather, or just become confused, you can suddenly lose confidence in its output. And as any pilot can tell you, failing to trust your instruments can have deadly consequences. It is absolutely essential that you adopt a backup navigation technique, not just in case your GPS should fail, but to provide a check on its operation, to give you the confidence in your system you will need if say, suddenly, unexpectedly, you sail into a fog bank.
This is what the dead reckoning track, or DR, does for you. Long before GPS, mariners were keeping a DR to summarize their navigational situation and put in one place all the data needed to navigate the ship in a systematic fashion that any seaman could easily interpret. The DR track is simply a line on the chart showing the ship's anticipated course and marked with ticks indicating where it is expected to be at specific times in the future. The track will alert the seaman to potential hazards and to navigational aids or landmarks in the vicinity of the track, and when he can expect to be near these places. He will not only know whether or not there is a reef, or lighthouse up ahead, he will also know (roughly) in which direction to look, and how far away. And he will also know how much time he has before he gets there.
A good chart will also provide valuable information along the track, both of navigational interest and of potential hazard, such as water depths, reefs, aids to navigation, lights and their flashing characteristics, land masses, shipping lanes, safe anchorages and no-anchor zones and so on. The seaman will know ahead of time what he is heading into, what to look forward to and look out for. As the voyage progresses and the skipper plots new positions obtained by piloting (or GPS) on the chart, he will be able to see how his ship is being affected by drift or leeway, current or wind.
We'd love to assume our boat is on rails, its course and speed dictating precisely where it will be at any time in the future, but we know better. The plotting of a fix by the DR lets us know how good our estimates were, and what unknown forces are acting on the boat. And if a really big contradiction arises, we know someone has made a mistake somewhere along the line and we have to get to the bottom of it immediately. So if you misplot a lat-long from the GPS, misread your compass or log, or take a bearing on the wrong light, the mistake will be immediately evident. The DR is your guard against operator error, human SNAFU.
Normally, you lay out a DR on the chart from the position of your last good fix. You carefully plot the fix as a little triangle with a dot in it, label it with the time, and draw a line bearing to your destination along the course you will be steering. At intervals along the DR, you mark with ticks the positions you expect to be at with circles, so they won't be confused with fixes. The time interval can be minutes, hours, or even days, depending on whether the DR is in a harbor, coastal voyage, or ocean crossing. So if you start from a known location at 0817, you mark it with a triangle and draw your DR from that point. At 0900, 1000, 1100, etc you draw estimated positions (EPs) as little labeled circles, on the track. Along the DR, place the course and an arrow indicating direction (like this: 045 ->). Even if you strike below when you go off watch, your relief will be able to glance at the chart and quickly grasp the navigational status. Use your parallel rulers and the compass rose on the chart to plot the angle of the course, and a nautical slide rule (or calculator, or pencil and paper) to calculate the distances traveled., and dividers to mark them on the chart. Remember, distance = speed times time, so starting at your 0817 departure posit, your 0900 DR EP will be 43 minutes later. so if your estimated speed is 6 knots, then 6kt x 43/60 hr = 4.3 nautical miles. The remaining EPs will be spaced 6 nautical miles apart. Weems and Plath makes an excellent nautical slide rule, the '105'. Get it.
As you proceed, take every possible opportunity to take frequent fixes, and plot them on the chart. If they agree well with your DR, you will get a nice, warm, fuzzy feeling inside. If not, you need to figure out why. Sooner or later, the fixes will start drifting off your DR, or you will be hitting your EPs early or late. Try to determine why, and from your last good fix, lay out a new DR, which may necessitate a change in course. Check for hazards and potential aids to navigation, make a note of emergency harbors or alternate routes you may need to take advantage of. You do this periodically, even if you know exactly where you are and everything is going down by the numbers. If something should go wrong and all you have to rely on is your DR, at least it will be a fresh one and it can be depended on for a while and you will have some understanding about how your boat's actual performance is comparing to your estimates. This is very important. If you have a perfectly working means to navigate, you don't need a DR. You need the DR to tell you when its not perfect, to give you confidence when it is, or for when it quits working at all.
With an up-to-date DR on your chart, you will always have situational awareness, because the very act of maintaining and referring to it will force you to be familiar with your surroundings. This is something staring at a GPS will not do for you. The DR will also allow you to do planning, to set up turn bearings or danger bearings, to consider alternate routes, safe harbors, escape routes, emergency maneuvers and other Plan Bs. You can rehearse these emergencies while the chart is in front of you and all the data you need is clearly laid out. This is something else a GPS can't do for you. Sure, you can punch in an emergency waypoint, but if a shoal is in the way the satellites won't tell you. If you need help, or need to change your plans, or divert to help another boat in trouble, the chart and the DR will suggest alternatives.
And remember, a DR on your chart is considered evidence in a court of law.
In the open ocean, a chart is of little use, and not always available; DRs are kept on "plotting sheets" with only lat/lon lines on them. Here is one much like I often worked on in the Navy, keeping track of the maneuvers of a ship conducting normal patrol operations. Every course and speed change is marked and a new DR laid out. You will note the fixes at 1120 and 1535 have not been marked by little triangles, to distinguish them from estimated positions. If I had done that I would have been gigged for it.http://www.bwsailing.com/bw/wp-content/uploads/2014/08/DB-Jan-2012-443x776.jpg