Nav Tip: Dead Reckoning - Sail Magazine

Nav Tip: Dead Reckoning

There aren’t many sinking sensations to compare with the one you get when your GPS decides to take an unscheduled break, especially if you’ve been relying on an electronic chartplotter. One minute you know exactly where you are. The next you’re surrounded by a trackless sea, feeling distinctly insecure.
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deadreckoning2

There aren’t many sinking sensations to compare with the one you get when your GPS decides to take an unscheduled break, especially if you’ve been relying on an electronic chartplotter. One minute you know exactly where you are. The next you’re surrounded by a trackless sea, feeling distinctly insecure. It’s no good saying, “This will never happen in 2012,” because there are a number of reasons why it very well might. Your first priority is to find out where you are, a fairly straightforward job if you’ve taken a few basic precautions. Here’s what you need to do.

• Always carry a paper chart. You don’t have to have it open if you’re using a good plotter (although it’s a must for passage planning), but when the plotter goes off-watch, that chart is your lifeline. 

• Maintain an up-to-date ship’s logbook on paper. This doesn’t have to be fancy. All it needs is some columns for time, position, course and the distance-log reading. You may choose to enter more data, and all credit to you if you do, but those basics will get you home.

• Make a log entry once an hour, or whenever you pass a significant mark, such as an offshore buoy.

Armed with this data, you can look an electronic meltdown in the eye without missing a heartbeat. 

deadreckoning

Dead Reckoning

Because you’ve logged your GPS positions, you know exactly where you were an hour ago. With luck it’ll be a good bit less time than that. Plot this on the chart as your “last known position.” 

Starting from there, using parallel rulers or a chart protractor, plot a line in the direction you’ve been steering. Unless there’s been serious current, you’re somewhere on this course line, or pretty near it. Label it with, “C 065M,” for example, if you’ve been steering 065 on the compass— the “M” is a good idea because it denotes a magnetic heading to make sure you don’t confuse it with a true one. If you’re motoring, you might note the speed as well–“S 5.5” perhaps.

Now read the distance log. The difference between now and what you noted at the last known position is how far you’ve come down the course line. Mark this off on the latitude scale using dividers, then transfer it to the course line by pricking one leg onto the last known position. The position at which the second leg lands on the course line is your “dead reckoning,” or DR, position. Mark it with a dot and a tiny semi-circle, label it with the time, and then log it with distance run. 

Note the soundings on the chart for this position and see if it stacks up with the reading on your depthsounder. If it does, the chances are that you’re not far away from the DR. To be sure you’re safe, the next little task consitutes the bedrock of all good navigation—double-checking. Along the coast, the best confirmation is to take a fix. 

Fixing your position

First have a good look around. You may well find that some easily recognizable object, such as a buoy the DR suggested was a mile away, is really only 100 yards off. If the depth corroborates this, you won’t find a much better fix than that. Log your position and set a course for home. If you aren’t close enough to anything immediately recognizable, you’ll have to deduce where you are by using lines of position, or LOPs.

An LOP is a line drawn on the chart in an observed direction from a known object. There are many ways of producing these, but the universal favorite is a magnetic bearing taken with a handbearing compass or the steering compass if it’s convenient. Plot the line onto the chart, noting the time on its upper edge. You know you are somewhere along it; the question is where. To nail this down, find another LOP and plot that as well. You’re now on two lines, so your only possible location is where they meet. 

If you can only produce two LOPs, reference the intersection with a time label and log it as a fix with distance run and the rest. You now have a “last known position” from which to start your next DR plot. All that’s left is to get going. If you can see a third object offering another useful LOP, grab that as well. The compass on a rocking, rolling boat is an imperfect instrument. The extra line will either confirm the accuracy of the first two, or cast some honest doubt on the proceedings so you can exercise seamanlike caution.

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