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.
Author:
Publish date:
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.

Related

albintoilet

Gear: Albin Pump Marine Toilet

Head Start Is there room for a new marine toilet? Albin Pump Marine thinks so, having just introduced its line of Swedish-built heads—ranging from compact to full-size models—to the American market. The toilets feature vitreous porcelain bowls and either wooden or thermoplastic ...read more

07n_45R2699

Multihull Sailor: Classic Cats

If you’re looking for a decent sub-40ft cruising cat, you have few choices when it comes to new-boat offerings. It is a well-known fact that the multihull market has taken off in a way very few could have predicted. Despite Hurricane Irma’s recent destruction of a large part of ...read more

TOTW_PromoSite

SAIL's Tip of the Week

Presented by Vetus-Maxwell. Got a tip? Send it to sailmail@sailmagazine.com Thanks a bunch  This scene is very calm and seamanlike. No frantic rope throwing or shouting. As he passes the line to the gent on the dock, the crew on the boat says, quietly and clearly, “Would you ...read more

mcarthy-and-mouse

Experience: McCarthy and the Mouse

Sitting at the helm in a light breeze, my arms crusted with a fine rime of salt, my skin so dry I’d lost my fingerprints, I heard a clatter and a curse from below. There were only three of us a thousand miles from shore and only one on watch at a time. Usually, the off watch lay ...read more

2018-giftGuide

2018 Holiday Gift Guide

Brass Yacht Lamp Does someone on your gift list spend the whole winter missing the warm days on the water? Let them bring a little bit of nautical atmosphere home with this new lamp from Weems & Plath. The glass enclosure means the flame cannot be blown out even by ...read more

image001

Opinion: On Not Giving Up Sailing

E.B. White was 64 when he wrote his now-famous essay “The Sea and the Wind That Blows,” which begins as a romantic paean to sailing and then drifts, as if spun around by a pessimistic eddy of thought, into a reflection on selling his boat. Does an aging sailor quit while he’s ...read more

1812-JeanneaueNewsVideo

Jeanneau Sun Odyssey 410

Designed by Marc Lombard, the Sun Odyssey 410 shares much in common with her older siblings including of course, the walk-around deck. Other features that set the 410 apart from other models being introduced this year include the 410’s “negative bow” shape allowing for a longer ...read more